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This chapter explains how scientists think and how they "do" science. It describes how scientific theories develop and how scientists investigate questions to advance scientific knowledge. The chapter also explains how science may be misused and how and why human subjects are protected in scientific research.
• 1.1: Case Study: Why Should You Learn About Science?
Elena and Daris are expecting their first child. They are excited for the baby to arrive, but they are nervous as well. Will the baby be healthy?
• 1.2: What Is Science?
You may think of science as a large and detailed body of knowledge, but science is actually more of a process than a set of facts. The real focus of science is the accumulation and revision of scientific knowledge. Science is a special way of gaining knowledge that relies on evidence and logic. Evidence is used to continuously test ideas. Through time, with repeated evidence gathering and testing, scientific knowledge advances.
• 1.3: The Nature of Science
Science is a distinctive way of gaining knowledge about the natural world that starts with a question and then tries to answer the question with evidence and logic. Science is an exciting exploration of all the whys and hows that any curious person might have about the world. You can be part of that exploration. Besides your curiosity, all you need is a basic understanding of how scientists think and how science is done. In this concept, you'll learn how to think like a scientist.
• 1.4: Theories in Science
A scientific theory is a broad explanation of events that is widely accepted by the scientific community. To become a theory, an explanation must be strongly supported by a great deal of evidence. People commonly use the word theory to describe a guess or hunch about how or why something happens. For example, you might say, "I think a woodchuck dug this hole in the ground, but it's just a theory." Using the word theory in this way is different from the way it is used in science.
• 1.5: Scientific Investigations
Science is more about doing than knowing. Scientists are always trying to learn more and gain a better understanding of the natural world. There are basic methods of gaining knowledge that is common to all of science. At the heart of science is the scientific investigation. A scientific investigation is a plan for asking questions and testing possible answers in order to advance scientific knowledge.
• 1.6: Scientific Experiments
An experiment is a special type of scientific investigation that is performed under controlled conditions. Like all investigations, an experiment generates evidence to test a hypothesis. But unlike some other types of investigations, an experiment involves manipulating some factor in a system in order to see how it affects the outcome. Ideally, experiments also involve controlling as many other factors as possible in order to isolate the cause of the experimental results.
• 1.7: Extrapolations of Scientific Investigations
Many questions in human biology are investigated with observational as opposed to experimental studies. An observational study measures characteristics in a sample but does not attempt to manipulate variables of interest. A simple example of an observational study is a political poll. A sample of adults might be asked how old they are and which of two candidates they favor. The study provides a snapshot in time of potential voters' opinions and how they differ by age of the respondent.
• 1.8: Case Study Conclusion: Shot and Chapter Summary
New mother Elena left her pediatrician’s office still unsure whether to vaccinate baby Juan. Dr. Rodriguez gave her a list of reputable sources where she could look up information about the safety of vaccines for herself, such as the Centers for Disease Control and Prevention (CDC).
Thumbnail: This image describes the Scientific Method as a cyclic/iterative process of continuous improvement. (Public Domain).
01: The Nature and Process of Science
Case Study: To Give a Shot or Not
Elena and Daris are expecting their first child. They are excited for the baby to arrive, but they are nervous as well. Will the baby be healthy? Will they be good parents? In addition to these big concerns, it seems like there are a million decisions to be made. Will Elena breastfeed or will they use formula? Will they buy a crib or let the baby sleep in their bed?
Elena goes online to try to find some answers. She finds a website from an author who writes books on parenting. On this site, she reads an article that argues that children should not be given many of the standard childhood vaccines, including the measles, mumps, and rubella (MMR) vaccine.
The article claims that the MMR vaccine has been proven to cause autism and gives examples of three children who came down with autism-like symptoms shortly after their first MMR vaccination at one year of age. The author believes that the recent increase in the incidence of children diagnosed with autism spectrum disorders is due to the fact that the number of vaccinations given in childhood has increased.
Elena is concerned. She does not want to create lifelong challenges for their child. Besides, aren’t diseases like measles, mumps, and rubella basically eradicated by now? Why should they risk the health of their baby by injecting them with vaccines for diseases that are a thing of the past?
Once baby Juan is born, Elena brings them to the pediatrician’s office. Dr. Rodriguez says Juan needs some shots. Elena is reluctant and shares what she has read online. Dr. Rodriguez assures Elena that the study that originally claimed a link between the MMR vaccine and autism has been found to be fraudulent and that vaccines have repeatedly been demonstrated to be safe and effective in peer-reviewed studies.
Although Elena trusts their doctor, she is not fully convinced. What about the increase in the number of children with autism and the cases where symptoms of autism appeared after MMR vaccination? Elena has a tough decision to make, but a better understanding of science can help her. In this chapter, you will learn about what science is (and what it is not), how it works, and how it relates to human health.
Chapter Overview: The Nature and Process of Science
In the rest of the chapter, you'll learn much more about science, including how scientists think and how they advance scientific knowledge. Specifically, you'll learn that:
• Science is a distinctive way of gaining knowledge about the natural world that is based on evidence and logic. Scientists assume that nature can be understood with systematic study; that scientific ideas are open to revision, although sound scientific ideas can withstand repeated testing; and that science is limited in the types of questions it can answer.
• A scientific theory is at the pinnacle of explanations in science. A theory is a broad explanation for many phenomena that is widely accepted because it is supported by a great deal of evidence. An example of a theory in human biology is the germ theory of disease. It took more than two centuries of research to provide enough evidence that microorganisms ("germs") cause disease for this explanation to become widely accepted and attain the status of a theory.
• The process of science is epitomized by scientific investigation. This is a procedure for gathering evidence to test a hypothesis. A scientific investigation typically involves steps such as asking a question based on observations and formulating a hypothesis as a testable answer to the question. It also generally involves collecting data as evidence for or against the hypothesis, drawing conclusions, and communicating results. In reality, the process of science is not simple and straightforward. The process actually tends to be nonlinear, iterative, creative, and unpredictable. "Doing" science can be very exciting!
• Scientific experiments are a special type of scientific investigation, in which variables are manipulated by the researcher to test expected outcomes. Experiments are performed under controlled conditions to mitigate the effects of other variables on the outcome variable. Experiments provide the best evidence that one variable causes another variable in scientific research. An example of an experiment in human biology is the astounding public health experiment to test Salk's polio vaccine that was undertaken in 1953. Some 600,000 children received a vaccine injection; another 600,000 received a placebo injection of useless salt water. The vaccine group had a significant drop in polio cases relative to the placebo group, providing support for the hypothesis that the vaccine prevented the disease.
• Many questions in human biology are not amenable to experimental research. Consider the question: "Does smoking cause lung cancer?" It would not be ethical to deliberately experiment with human subjects by exposing them to harmful tobacco smoke in order to see whether they develop lung cancer. For questions like this, observational studies are done to look for correlations between variables. For example, Doll and Hill gathered information on past smoking habits from a large sample of lung cancer patients and another large sample of controls without lung cancer. Smoking and lung cancer were found to be correlated. Correlation does not imply causation, but it can be a big hint!
• Research involving human subjects presents special challenges to scientists. Until the 1970s, there were few ethical guidelines for researchers to follow when studying human subjects. A shamefully unethical syphilis study called the Tuskegee study changed all that. The Tuskegee study was conducted on African-American men in Alabama from 1932 to 1972. This study was done to see the progression of syphilis. In this study, the control group with the disease was not treated for syphilis. When details of the study were leaked to the media, the public was outraged and the U.S. Congress got involved. In 1974, Congress passed important legislation to protect human subjects in scientific research projects. Chief among the protections was the necessity of informed consent.
As you read this chapter, think about the following questions:
1. What do you think about the quality of Elena’s online source of information about vaccines compared to Dr. Rodriguez’s sources?
2. Do you think the arguments presented here that claim that the MMR vaccine causes autism are scientifically valid? Could there be alternative explanations for the observations?
3. Why do you think diseases like measles, polio, and mumps are rare these days, and why are we still vaccinating for these diseases?
Attributions
1. Pregnant woman by Petar Milošević licenced CC BY-SA 4.0 via Wikimedia Commons
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/01%3A_The_Nature_and_Process_of_Science/1.1%3A_Case_Study_-_Why_Should_You_Learn_About_Science.txt |
Ouch!
This individual in Figure \(1\) is getting a flu vaccine. You probably know that getting a vaccine can hurt, but it's usually worth it. A vaccine contains dead or altered forms of "germs" that normally cause a disease, such as flu or measles. The germs in vaccines have been inactivated or weakened so they can no longer cause illness, but they are still "noticed" by the immune system. They stimulate the immune system to produce chemicals that can kill the actual germs if they enter the body, thus preventing future disease. How was such an ingenious way to prevent disease discovered? The short answer is more than two centuries of science.
Science as Process
You may think of science as a large and detailed body of knowledge, but science is actually more of a process than a set of facts. The real focus of science is the accumulation and revision of scientific knowledge. Science is a special way of gaining knowledge that relies on evidence and logic. Evidence is used to continuously test ideas. Through time, with repeated evidence gathering and testing, scientific knowledge advances.
We've been accumulating knowledge of vaccines for more than two centuries. The discovery of the first vaccine, as well as the process of vaccination, dates back to 1796. An English doctor named Edward Jenner observed that people who became infected with cowpox did not get sick from smallpox, a similar but much more virulent disease (Figure \(2\)). Jenner decided to transmit cowpox to a young child to see if it would protect them from smallpox. He gave the child cowpox by scratching liquid from cowpox sores into the child's skin. Then, six weeks later, he scratched liquid from smallpox sores into the child's skin. As Jenner predicted, the child did not get sick from smallpox. Jenner had discovered the first vaccine, although additional testing was needed to show that it really was effective.
Almost a century passed before the next vaccine was discovered, a vaccine for cholera in 1879. Around the same time, French chemist Louis Pasteur found convincing evidence that many human diseases are caused by germs. This earned Pasteur the title of "father of germ theory." Since Pasteur's time, vaccines have been discovered for scores of additional diseases caused by "germs," and scientists are currently researching vaccines for many others.
Benefits of Science
Medical advances such as the discovery of vaccines are one of the most important benefits of science, but science and scientific knowledge are also crucial for most other human endeavors. Science is needed to design safe cars, predict storms, control global warming, develop new technologies of many kinds, help couples have children, and put humans on the moon! Clearly, the diversity of applications of scientific knowledge is vast!
Review
1. Explain why science is more accurately considered a process than a body of knowledge.
2. State three specific examples of human endeavors that are based on scientific knowledge.
3. Jenner used a young boy as a research subject in his smallpox vaccine research. Today, scientists must follow strict guidelines when using human subjects in their research. What unique concerns do you think might arise when human beings are used as research subjects?
4. What gave Jenner the idea to develop a vaccine for smallpox?
5. Why do you think almost a century passed between the development of the first vaccine (for smallpox) and the development of the next vaccine (for cholera)?
6. How does science influence your daily life?
Explore More
bio.libretexts.org/link?16712#Explore_More
Check out this video to learn more about the smallpox vaccine:
Attributions
1. Nurse administers a vaccine by Rhoda Baer for National Cancer Institute, public domain via Wikimedia Commons
2. Child with smallpox by CDC/James Hicks, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/01%3A_The_Nature_and_Process_of_Science/1.2%3A_What_Is_Science.txt |
Why Does a Moose Have Antlers?
Did you ever wonder why a moose, like the one in Figure \(1\), grows large antlers? The antlers may grow as wide as 1.8 m (6 ft) from tip to tip! The antlers use up a lot of energy to grow and carry around. They can even get caught in brush and trees. In these ways, they would seem to be more of a detriment than a help, so what purpose do the antlers serve? And why do only male moose grow them? If you've ever asked questions such as these about the natural world, then you were thinking like a scientist.
Defining Science
Science is a distinctive way of gaining knowledge about the natural world that starts with a question and then tries to answer the question with evidence and logic. Science is an exciting exploration of all the whys and hows that any curious person might have about the world. You can be part of that exploration. Besides your curiosity, all you need is a basic understanding of how scientists think and how science is done. In this section, you'll learn how to think like a scientist.
Thinking Like a Scientist
Thinking like a scientist rests on certain underlying assumptions. Scientists assume that:
• Nature can be understood through systematic study.
• Scientific ideas are open to revision.
• Sound scientific ideas withstand the test of time.
• Science cannot provide answers to all questions.
Nature is Understandable
Scientists think of nature as a single system controlled by natural laws. By discovering natural laws, scientists strive to increase their understanding of the natural world. Laws of nature are expressed as scientific laws. A scientific law is a statement that describes what always happens under certain conditions in nature.
Examples of scientific laws include Mendel's Laws of Inheritance. These laws were discovered by an Austrian Monk, named Gregor Mendel (Figure \(2\)), in the mid-1800s. The laws describe how certain traits are inherited from parents by their offspring. Although Mendel discovered his laws of inheritance by experimenting with pea plants, we now know that the laws apply to many other organisms, including human beings. The laws describe how we inherit relatively simple genetic traits, such as blood type, from our parents. For example, if you know the blood types of your parents, you can use Mendel's laws to predict your chances of having a particular blood type.
Barbara McClintock (Figure \(2\)) added to our understanding of inheritance in the 1950s by discovering how chromosomes exchange information during meiosis. Meiosis is how organisms produce reproductive cells (such as egg or sperm). McClintock worked with corn and, using the color traits in the kernels demonstrated how crossing-over is used to exchange information between chromosomes. An understanding of how crossing-over works is essential to our understanding of inheritance because it explains why using Mendelian rules of inheritance does not always produce the correct ratios.
Scientific Ideas are Open to Change
Science is more of a process than a set body of knowledge. Scientists are always testing and revising their ideas, and as new observations are made, existing ideas may be challenged. Ideas may be replaced with new ideas that better fit the facts, but more often existing ideas are simply revised. For example, when scientists discovered how genes control genetic traits, they didn't throw out Mendel's laws of inheritance. The new discoveries helped to explain why Mendel's laws applied to certain traits but not others. They showed that Mendel's laws are part of a bigger picture. Through many new discoveries over time, scientists gradually build an increasingly accurate and detailed understanding of the natural world.
Occasionally, scientific ideas change radically. Radical changes in scientific ideas were given the name paradigm shifts by the philosopher Thomas Kuhn in 1962. Kuhn agreed that scientific knowledge typically accumulates gradually, as new details are added to established theories. However, Kuhn also argued that from time to time, a scientific revolution occurs in which current theories are abandoned and completely new ideas take their place.
Although there is debate among scientists as to what constitutes a paradigm shift, the theory of evolution is widely accepted as a good example in biology. In fact, some scientists argue that it is the only example of a paradigm shift in biology. Prior to Charles Darwin's publication of his theory of evolution in the 1860s, most scientists believed that God had created living species and that the species on Earth had not changed since they were created. Drawing on a great deal of evidence and logical arguments, Darwin demonstrated that species could change and that new species could arise from pre-existing ones. This was such a radical change in scientific thinking that Darwin was reluctant to publish his ideas for fear of a backlash from other scientists and the public. Indeed, Darwin was at first ridiculed for his evolutionary theory, but in time, it was widely accepted and became a cornerstone of all life sciences.
Scientific Knowledge May Be Long Lasting
Many scientific ideas have withstood the test of time. For example, about 200 years ago, the scientist John Dalton proposed atomic theory — the theory that all matter is made of tiny particles called atoms. This theory is still valid today. During the two centuries since the theory was first proposed, a great deal more has been learned about atoms and the even smaller particles of which they are composed. Nonetheless, the idea that all matter consists of atoms remains valid. There are many other examples of basic scientific ideas that have been tested repeatedly and found to be sound. You will learn about many of them as you study human biology.
Not All Questions Can be Answered by Science
Science rests on evidence and logic, and evidence comes from observations. Therefore, science deals only with things that can be observed. An observation is anything that is detected through human senses or with instruments and measuring devices that extend human senses. Things that cannot be observed or measured by current means — such as supernatural beings or events — are outside the bounds of science. Consider these two questions about life on Earth:
• Did life on Earth evolve over time?
• Was life on Earth created by a supernatural deity?
The first question can be answered by science on the basis of scientific evidence such as fossils and logical arguments. The second question could be a matter of belief but no evidence can be gathered to support or refute it. Therefore, it is outside the realm of science.
Feature: Human Biology in the News
Scientific research is often reported in the popular media. In fact, that's how most people learn about new scientific findings. Informing the public about scientific research is a valuable media service, but the types of scientific investigations that are reported may lead to a distorted public perception of what science is and how reliable its results are. Why? There are actually two types of science, often referred to as consensus science and frontier science. The latter type of science is the type that usually makes the news, but the media generally do not distinguish between the two types. Therefore, many people may infer that what they read about frontier science is typical of all science.
• Consensus science refers to scientific ideas that have been researched for a long period of time and for which a great deal of evidence has accumulated. This type of research generally fits well within current scientific paradigms. A good example of consensus science is global climate change. Data showing the impact of increasing levels of atmospheric carbon dioxide, due to human activities, on global warming have been accumulating for many decades. Today, virtually all climate scientists agree that global warming is occurring and that human actions are largely responsible for it. However, the few scientists — and many politicians — who do not agree with the consensus view receive greater media attention because the consensus view is "old" news. The findings have been coming in for years, and new research in the area keeps finding similar results.
• Frontier science, in contrast, refers to scientific ideas that are relatively new and have not yet been supported by years of scientific evidence. Frontier research takes place at the frontiers of knowledge in a particular field. A good example of frontier science is research into the presumed link between cholesterol in the diet and cholesterol in the blood. The consensus view for many years was that a diet high in cholesterol increases blood levels of cholesterol, which may lead, in turn, to cardiovascular disease. Recent research challenging this accepted view found that genes play a more significant role than diet in blood levels of cholesterol and risk of cardiovascular disease.
The media tend to focus on frontier science because it seems controversial and may lead to major new scientific breakthroughs. With more research, ideas in frontier science may be supported by more evidence, gain wider acceptance, and become consensus science. In some cases, frontier science that is at odds with a current paradigm may even lead to a paradigm shift. However, the opposite may happen instead. Additional research may undermine the initial findings of frontier research so that the new and exciting ideas are rejected. Unfortunately, when frontier science is later shown to be mistaken, people may infer that all science, including consensus science, is unreliable.
Review
1. Define science.
2. What is the general goal of science?
3. Identify four basic assumptions that scientists make when they study the natural world.
4. Explain why science cannot provide answers to all questions.
5. Do observations in science have to be made by the naked eye? Can you think of a way in which scientists might be able to make observations about something they cannot directly see?
6. If something cannot be observed, can it be tested scientifically?
7. What do you think would be more susceptible to being disproved — conclusions drawn from frontier science or consensus science? Explain your reasoning.
8. Scientific knowledge builds upon itself. Give an example of a scientific idea from the reading where the initial idea became extended as science advanced.
9. What is a dramatic change in scientific understanding is called?
10. Discuss this statement: “Scientific ideas are always changing, so they can't be trusted.” Do you think this is true?
11. True or False: Science is a process.
12. True or False: A scientific law describes what happens most of the time under certain conditions.
13. What is one piece of evidence that life on Earth evolved over time?
14. Why do you think that as technology advances, scientific knowledge expands?
Explore More
Attributions
1. Moose Superior by USDA Forest Service, public domain via Wikimedia Commons
2. Gregor Mendel by Hugo Iltis via the Wellcome Library, London, public domain via Wikimedia Commons
1. Barbara McClintock by mithsonian Institution/Science Service; Restored by Adam Cuerden, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/01%3A_The_Nature_and_Process_of_Science/1.3%3A_The_Nature_of_Science.txt |
Oh Vapor So Foul!
An individual in this sketch is holding his nose to avoid breathing in the miasma. Miasma refers to a toxic vapor that people believed for centuries was a cause of many diseases, including cholera and plague. The idea that miasma causes diseases was first proposed in the second century B.C.E. by a prominent Greek physician named Galen. They believed that miasma, which is identifiable by its foul smell, emanates from rotting organic matter and sickens people who live close enough to inhale it. Miasma was the predominant explanation for disease transmission from the time of Galen until the germ theory of disease became widely accepted in the late 1800s.
What Is a Scientific Theory?
Germ theory, which is described in detail below, is one of several scientific theories you will read about in human biology. A scientific theory is a broad explanation of events that is widely accepted by the scientific community. To become a theory, an explanation must be strongly supported by a great deal of evidence.
People commonly use the word theory to describe a guess or hunch about how or why something happens. For example, you might say, "I think a woodchuck dug this hole in the ground, but it's just a theory." Using the word theory in this way is different from the way it is used in science. A scientific theory is not just a guess or hunch that may or may not be true. In science, a theory is an explanation that has a high likelihood of being correct because it is so well supported by evidence.
Germ Theory: A Human Biology Example
The germ theory of disease states that contagious diseases are caused by "germs," or microorganisms, which are organisms that are too small to be seen without magnification. Microorganisms that cause disease are called pathogens. Human pathogens include bacteria and viruses, among other microscopic entities. When pathogens invade humans or other living hosts, they grow, reproduce, and make their hosts sick. Diseases caused by germs are contagious because the microorganisms that cause them can spread from person to person.
First Statement of Germ Theory
Germ theory was first clearly stated by an Italian physician named Girolamo Fracastoro in the mid-1500s. Fracastoro proposed that contagious diseases are caused by transferable "seed-like entities," which we now call germs. According to Fracastoro, germs spread through populations, making many people sick, through direct or indirect contact between individuals.
Fracastoro's idea, though essentially correct, was disregarded by other physicians. Instead, Galen's idea of miasma remained the accepted explanation for the spread of disease for another 300 years. However, evidence for Fracastoro's idea accumulated during that time. Some of the earliest evidence was provided by the Dutch lens and microscope maker Anton van Leeuwenhoek, who discovered microorganisms. By the 1670s, van Leeuwenhoek had directly observed many different types of microorganisms, including bacteria.
Evidence from Puerperal Fever
One of the first physicians to demonstrate that a microorganism is the cause of a specific human disease was the Hungarian obstetrician Ignaz Semmelweis in the 1840s. The disease was puerperal fever, an often-fatal infection of the female reproductive organs. Puerperal fever is also called childbed fever because it usually affects women who have just given birth.
Semmelweis observed that deaths from puerperal fever occurred much more often when women had been attended by doctors at his hospital than by midwives at home. Semmelweis also noticed that doctors often came directly from autopsies to the beds of women about to give birth. From his observations, Semmelweis inferred that puerperal fever was a contagious disease caused by some type of matter carried to pregnant patients on the hands of doctors from autopsied bodies. As a consequence, Semmelweis urged doctors and medical students at his hospital to wash their hands with chlorinated lime water before examining pregnant women. After this change, the hospital's death rate for women who had just given birth fell from 18 to 2 percent, which was a 90 percent reduction. Some of Semmelweis' findings are presented in Figure \(3\).
Semmelweis published his results, but they were derided by the medical profession. The idea that doctors themselves were the carriers of a fatal disease was taken as a personal affront by his fellow physicians. One of Semmelweis' peers protested indignantly that doctors are gentlemen and that gentlemen's hands are always clean. As a result of attitudes such as this, Semmelweis became the target of a vicious smear campaign. Eventually, Semmelweis had a mental breakdown and was committed to a mental hospital, where he died.
Discovering Microbes
Throughout the later 1800s, more formal investigations were conducted on the relationship between germs and disease. Some of the most important was undertaken by Louis Pasteur (pictured in his lab in Figure \(4\)). Pasteur was a French chemist who did careful experiments to show that fermentation, food spoilage, and certain diseases are caused by microorganisms.
He discovered the cause of puerperal fever in 1879 and determined it was an infection caused by the bacterium Streptococcus pyogenes (Figure \(5\)). Although Pasteur was not the first person to propose germ theory, his investigations clearly supported it. He also became a strong proponent of the theory and managed to convince most of the scientific community of its validity.
Emerging Diseases
Scientific theories are not static and neither is the world around us. While we have been studying disease for hundreds of years, there is always more to learn. One reason for this is that organisms (such as those that cause disease) are always changing. This evolution of organisms leads to new diseases such as the COVID-19 pandemic. This resulted from a novel coronavirus (SARS-CoV-2) which is a descendent of coronaviruses that did not infect humans. Scientists are continually developing new strategies for learning about and curing emerging diseases.
Review
1. Define scientific theory.
2. Contrast how the word theory is used in science and in everyday language.
3. What is the germ theory of disease? How did it develop?
4. Explain why Pasteur, rather than Fracastoro or Semmelweis, is called the father of germ theory.
5. Galen and Fracastoro may have come up with different explanations for how a disease is spread, but what observations do you think they made that were similar?
6. Use the explanation of Semmelweis’ research and the graph in Figure \(2\) to answer the following questions.
1. What was Semmelweis’ observation that led him to undertake this study? What question was he trying to answer?
2. What was the hypothesis (i.e. proposed answer for a scientific question) that Semmelweis was testing?
3. Why did Semmelweis track death rates from puerperal fever at Dublin Maternity Hospital where autopsies were not performed?
4. What were the two pieces of evidence shown in the graph that supported Semmelweis’ hypothesis?
5. Why do you think it was important that Semmelweis compared Dublin Maternity Hospital and Wien Maternity Clinic over the same years?
7. What is the difference between a microorganism and a pathogen?
8. Explain why the development of the microscope lent support to the germ theory of disease.
9. Does the observation of microorganisms alone conclusively prove that germ theory is correct? Why or why not?
10. Who do you think was using more scientific reasoning - Semmelweis or the physicians that derided his results? Explain your answer.
Explore More
How is Semmelweis's work relevant to us today? Listen to this story from RadioLab to find out how his work gave us a powerful weapon against modern global pandemics. [
How does a scientific theory differ from a scientific law? Watch this excellent TED animation to find out.
Attributions
1. Man holding his nose by Wellcome Collection gallery, CC BY 4.0 via Wikimedia Commons
2. Yearly mortality rates by Power.corrupts, public domain via Wikimedia Commons
3. Louis Pasteur public domain via Wikimedia Common
1. Streptococcus pyogenes by CDC, public domain via Wikimedia Commons
4. Microbiologist by Governor Tom Wolf, licensed CC-BY 2.0 via Flickr
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/01%3A_The_Nature_and_Process_of_Science/1.4%3A_Theories_in_Science.txt |
What Turned the Water Orange?
If you were walking in the woods and saw this stream, you probably would wonder what made the water turn orange. Is the water orange because of something growing in it? Is it polluted with some kind of chemicals? To answer these questions, you might do a little research. For example, you might ask local people if they know why the water is orange, or you might try to learn more about it online. If you still haven't found answers, you could undertake a scientific investigation. In short, you could "do" science.
"Doing" Science
Science is more about doing than knowing. Scientists are always trying to learn more and gain a better understanding of the natural world. There are basic methods of gaining knowledge that is common to all of science. At the heart of science is the scientific investigation. A scientific investigation is a plan for asking questions and testing possible answers in order to advance scientific knowledge.
Figure \(2\) outlines the steps of the scientific method. Science textbooks often present this simple, linear "recipe" for a scientific investigation. This is an oversimplification of how science is actually done, but it does highlight the basic plan and purpose of any scientific investigation: testing ideas with evidence. We will use this flowchart to help explain the overall format for scientific inquiry.
Science is actually a complex endeavor that cannot be reduced to a single, linear sequence of steps, like the instructions on a package of cake mix. Real science is nonlinear, iterative (repetitive), creative, unpredictable, and exciting. Scientists often undertake the steps of an investigation in a different sequence, or they repeat the same steps many times as they gain more information and develop new ideas. Scientific investigations often raise new questions as old ones are answered. Successive investigations may address the same questions but at ever-deeper levels. Alternatively, an investigation might lead to an unexpected observation that sparks a new question and takes the research in a completely different direction.
Knowing how scientists "do" science can help you in your everyday life, even if you aren't a scientist. Some steps of the scientific process — such as asking questions and evaluating evidence — can be applied to answering real-life questions and solving practical problems.
Making Observations
A scientific investigation typically begins with observations. An observation is anything that is detected through human senses or with instruments and measuring devices that enhance human senses. We usually think of observations as things we see with our eyes, but we can also make observations with our sense of touch, smell, taste, or hearing. In addition, we can extend and improve our own senses with instruments such as thermometers and microscopes. Other instruments can be used to sense things that human senses cannot detect at all, such as ultraviolet light or radio waves.
Sometimes chance observations lead to important scientific discoveries. One such observation was made by the Scottish biologist Alexander Fleming (Figure \(3\)) in the 1920s. Fleming's name may sound familiar to you because he is famous for the discovery in question. Fleming had been growing a certain type of bacteria on glass plates in his lab when he noticed that one of the plates had been contaminated with mold. On closer examination, Fleming observed that the area around the mold was free of bacteria.
Asking Questions
Observations often lead to interesting questions. This is especially true if the observer is thinking like a scientist. Having scientific training and knowledge is also useful. Relevant background knowledge and logical thinking help make sense of observations so the observer can form particularly salient questions. Fleming, for example, wondered whether the mold — or some substance it produced — had killed bacteria on the plate. Fortunately for us, Fleming didn't just throw out the mold-contaminated plate. Instead, he investigated his question and in so doing, discovered the antibiotic penicillin.
Hypothesis Formation
To find the answer to a question, the next step in a scientific investigation typically is to form a hypothesis. A hypothesis is a possible answer to a scientific question. But it isn’t just any answer. A hypothesis must be based on scientific knowledge. In other words, it shouldn't be at odds with what is already known about the natural world. A hypothesis also must be logical, and it is beneficial if the hypothesis is relatively simple. In addition, to be useful in science, a hypothesis must be testable and falsifiable. In other words, it must be possible to subject the hypothesis to a test that generates evidence for or against it, and it must be possible to make observations that would disprove the hypothesis if it really is false.
A hypothesis is often expressed in the form of prediction: If the hypothesis is true, then B will happen to the dependent variable. Fleming's hypothesis might have been: "If a certain type of mold is introduced to a particular kind of bacteria growing on a plate, the bacteria will die." Is this a good and useful hypothesis? The hypothesis is logical and based directly on observations. The hypothesis is also simple, involving just one type each of mold and bacteria growing on a glass plate. This makes it easy to test. In addition, the hypothesis is falsifiable. If bacteria were to grow in the presence of the mold, it would disprove the hypothesis if it really is false.
Hypothesis Testing
Hypothesis testing is at the heart of a scientific investigation. How would Fleming test his hypothesis? He would gather relevant data as evidence. Evidence is any type of data that may be used to test a hypothesis. Data (singular, datum) are essentially just observations. The observations may be measurements in an experiment or just something the researcher notices. Testing a hypothesis then involves using the data to answer two basic questions:
1. If my hypothesis is true, what would I expect to observe?
2. Does what I actually observe match what predicted?
A hypothesis is supported if the actual observations (data) match the expected observations. A hypothesis is refuted if the actual observations differ from the expected observations.
Testing Fleming's Hypothesis
To test his hypothesis that the mold kills bacteria, Fleming grew colonies of bacteria on several glass plates and introduced mold to just some of the plates. He subjected all of the plates to the same conditions except for the introduction of mold. Any differences in the growth of bacteria on the two groups of plates could then be reasonably attributed to the presence/absence of mold. Fleming's data might have included actual measurements of bacterial colony size, like the data shown in the data table below, or they might have been just an indication of the presence or absence of bacteria growing near the mold. Data like the former, which can be expressed numerically, are called quantitative data. Data like the latter, which can only be expressed in words, such as present or absent, are called qualitative data.
Table \(1\): Hypothetical data of bacterial growth on plates with and without mold introduction.
Bacterial Plate Identification Number Introduction of Mold to Plate? Total Area of Bacterial Growth on Plate after 1 Week (mm2)
1 yes 48
2 yes 57
3 yes 54
4 yes 59
5 yes 62
6 no 66
7 no 75
8 no 71
9 no 69
10 no 68
Analyzing and Interpreting Data
The data scientists gather in their investigations are raw data. These are the actual measurements or other observations that are made in an investigation, like the measurements of bacterial growth shown in the data table above. Raw data usually must be analyzed and interpreted before they become evidence to test a hypothesis. To make sense of raw data and decide whether they support a hypothesis, scientists generally use statistics.
There are two basic types of statistics: descriptive statistics and inferential statistics. Both types are important in scientific investigations.
• Descriptive statistics describe and summarize the data. They include values such as the mean, or average, value in the data. Another basic descriptive statistic is the standard deviation, which gives an idea of the spread of data values around the mean value. Descriptive statistics make it easier to use and discuss the data and also to spot trends or patterns in the data.
• Inferential statistics help interpret data to test hypotheses. They determine how likely it is that the actual results obtained in an investigation occurred just by chance rather than for the reason posited by the hypothesis. For example, if inferential statistics show that the results of an investigation would happen by chance only 5 percent of the time, then the hypothesis has a 95 percent chance of being correctly supported by the results. An example of a statistical hypothesis test is a t-test. It can be used to compare the mean value of the actual data with the expected value predicted by the hypothesis. Alternatively, a t-test can be used to compare the mean value of one group of data with the mean value of another group to determine whether the mean values are significantly different or just different by chance.
Assume that Fleming obtained the raw data shown in the data table above. We could use a descriptive statistic such as the mean area of bacterial growth to describe the raw data. Based on these data, the mean area of bacterial growth for plates with mold is 56 mm2, and the mean area for plates without mold is 69 mm2. Is this difference in bacterial growth significant? In other words, does it provide convincing evidence that bacteria are killed by the mold or something produced by the mold? Or could the difference in mean values between the two groups of plates be due to chance alone? What is the likelihood that this outcome could have occurred even if mold or one of its products does not kill bacteria? A t-test could be done to answer this question. The p-value for the t-test analysis of the data above is less than 0.05. This means that one can say with 95% confidence that the means of the above data are statistically different.
Drawing Conclusions
A statistical analysis of Fleming's evidence showed that it did indeed support his hypothesis. Does this mean that the hypothesis is true? No, not necessarily. That's because a hypothesis can never be proven conclusively to be true. Scientists can never examine all of the possible evidence, and someday evidence might be found that disproves the hypothesis. In addition, other hypotheses, as yet unformed, may be supported by the same evidence. For example, in Fleming's investigation, something else introduced onto the plates with the mold might have been responsible for the death of the bacteria. Although a hypothesis cannot be proven true without a shadow of a doubt, the more evidence that supports a hypothesis, the more likely the hypothesis is to be correct. Similarly, the better the match between actual observations and expected observations, the more likely a hypothesis is to be true.
Many times, competing hypotheses are supported by evidence. When that occurs, how do scientists conclude which hypothesis is better? There are several criteria that may be used to judge competing hypotheses. For example, scientists are more likely to accept a hypothesis that:
• explains a wider variety of observations.
• explains observations that were previously unexplained.
• generates more expectations and is thus more testable.
• is more consistent with well-established theories.
• is more parsimonious, that is, is a simpler and less convoluted explanation.
Correlation-Causation Fallacy
Many statistical tests used in scientific research calculate correlations between variables. Correlation refers to how closely related two data sets are, which may be a useful starting point for further investigation. However, correlation is also one of the most misused types of evidence, primarily because of the logical fallacy that correlation implies causation. In reality, just because two variables are correlated does not necessarily mean that either variable causes the other.
A simple example can be used to demonstrate the correlation-causation fallacy. Assume a study found that both ice cream sales and burglaries are correlated; that is, rates of both events increase together. If correlation really did imply causation, then you could conclude that ice cream sales cause burglaries or vice versa. It is more likely, however, that a third variable, such as the weather, influences rates of both ice cream sales and burglaries. Both might increase when the weather is sunny.
An actual example of the correlation-causation fallacy occurred during the latter half of the 20th century. Numerous studies showed that women taking hormone replacement therapy (HRT) to treat menopausal symptoms also had a lower-than-average incidence of coronary heart disease (CHD). This correlation was misinterpreted as evidence that HRT protects women against CHD. Subsequent studies that controlled other factors related to CHD disproved this presumed causal connection. The studies found that women taking HRT were more likely to come from higher socio-economic groups, with better-than-average diets and exercise regimens. Rather than HRT causing lower CHD incidence, these studies concluded that HRT and lower CHD were both effects of higher socioeconomic status and related lifestyle factors.
Communicating Results
The last step in a scientific investigation is communicating the results to other scientists. This is a very important step because it allows other scientists to try to repeat the investigation and see if they can produce the same results. If other researchers get the same results, it adds support to the hypothesis. If they get different results, it may disprove the hypothesis. When scientists communicate their results, they should describe their methods and point out any possible problems with the investigation. This allows other researchers to identify any flaws in the method or think of ways to avoid possible problems in future studies.
Repeating a scientific investigation and reproducing the same results is called replication. It is a cornerstone of scientific research. Replication is not required for every investigation in science, but it is highly recommended for those that produce surprising or particularly consequential results. In some scientific fields, scientists routinely try to replicate their own investigations to ensure the reproducibility of the results before they communicate them.
Scientists may communicate their results in a variety of ways. The most rigorous way is to write up the investigation and results in the form of an article and submit it to a peer-reviewed scientific journal for publication. The editor of the journal provides copies of the article to several other scientists who work in the same field. These are the peers in the peer-review process. The reviewers study the article and tell the editor whether they think it should be published, based on the validity of the methods and significance of the study. The article may be rejected outright, or it may be accepted, either as is or with revisions. Only articles that meet high scientific standards are ultimately published.
Review
1. Outline the steps of a typical scientific investigation.
2. What is a scientific hypothesis? What characteristics must a hypothesis have to be useful in science?
3. Explain how you could do a scientific investigation to answer this question: Which of the following surfaces in my home has the most bacteria: the house phone, TV remote, bathroom sink faucet, or outside door handle? Form a hypothesis and state what results would support it and what results would refute it.
4. Use the Table \(1\) above that shows data on the effect of mold on bacterial growth to answer the following questions
1. Look at the areas of bacterial growth for the plates in just one group – either with mold (plates 1-5) or without mold (plates 6-10). Is there a variation within the group? What do you think could be possible sources of variation within the group?
2. Compare the area of bacterial growth for plate 1 vs. plate 7. Does this appear to be more of a difference between the mold group vs. the no mold group than if you compared plate 5 vs. plate 6? Using these differences among the individual data points, explain why it is important to find the mean of each group when analyzing the data.
3. Why do you think it would be important for other researchers to try to replicate the findings in this study?
5. A scientist is performing a study to test the effects of an anti-cancer drug in mice with tumors. They look in the cages and observes that the mice that received the drug for two weeks appear more energetic than those that did not receive the drug. At the end of the study, the scientist performs surgery on the mice to determine whether their tumors have shrunk. Answer the following questions about the experiment.
1. Is the energy level of the mice treated with the drug a qualitative or quantitative observation?
2. At the end of the study, the scientist measures the size of the tumors. Is this qualitative or quantitative data?
3. Would the size of each tumor be considered raw data or descriptive statistics?
4. The scientist determines the average decrease in tumor size for the drug-treated group. Is this raw data, descriptive statistics, or inferential statistics?
5. The average decrease in tumor size in the drug-treated group is larger than the average decrease in the untreated group. Can the scientist assume that the drug shrinks tumors? If not, what do they need to do next?
6. Do you think results published in a peer-reviewed scientific journal are more or less likely to be scientifically valid than those in a self-published article or book? Why or why not
7. Explain why real science is usually “nonlinear”?
Explore More
Watch this TED talk for a lively discussion of why the standard scientific method is an inadequate model of how science is really done.
Attributions
1. Rio Tinto River by Carol Stoker, NASA, public domain via Wikimedia Commons
2. Scientific Method by OpenStax, licensed CC BY 4.0
3. Alexander Flemming by Ministry of Information Photo Division Photographer, public domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/01%3A_The_Nature_and_Process_of_Science/1.5%3A_Scientific_Investigations.txt |
Seeing Spots
The spots on this child's tongue are an early sign of vitamin C deficiency, which is also called scurvy. This disorder, which may be fatal, is uncommon today because foods high in vitamin C are readily available. They include tomatoes, peppers, and citrus fruits such as oranges, lemons, and limes. However, scurvy was a well-known problem on navy ships in the 1700s. It was said that scurvy caused more deaths in the British fleet than French and Spanish arms. At that time, the cause of scurvy was unknown and vitamins had not yet been discovered. Anecdotal evidence suggested that eating citrus fruits might cure scurvy. However, no one knew for certain until 1747, when a Scottish naval physician named John Lind did an experiment to test the idea. Lind's experiment was one of the first clinical experiments in the history of medicine.
What Is an Experiment?
An experiment is a special type of scientific investigation that is performed under controlled conditions. Like all investigations, an experiment generates evidence to test a hypothesis. But unlike some other types of investigations, an experiment involves manipulating some factors in a system in order to see how it affects the outcome. Ideally, experiments also involve controlling as many other factors as possible in order to isolate the cause of the experimental results.
An experiment generally tests how one particular variable is affected by some other specific variable. The affected variable is called the dependent variable or outcome variable. The variable that affects the dependent variable is called the independent variable. It is also called the manipulated variable because this is the variable that is manipulated by the researcher. Any other variables (control variable) that might also affect the dependent variable are held constant, so the effects of the independent variable alone are measured.
Lind's Scurvy Experiment
Lind began his scurvy experiment onboard a British ship after it had been at sea for two months and sailors had started showing signs of scurvy. He chose a group of 12 sailors with scurvy and divided the group into 6 pairs. All 12 sailors received the same diet, but each pair also received a different daily supplement to the diet (Table \(1\)).
Table \(1\): Lind's Scurvy Experiment
Pair of Subjects Daily Supplement to the Diet Received by this Pair
1 1 quart of cider
2 5 drops of sulfuric acid
3 6 spoons of vinegar
4 1 cup of seawater
5 2 oranges and 1 lemon
6 spicy paste and a drink of barley water
Lind's experiment ended after just five days when the fresh citrus fruits ran out for pair 5. However, the two sailors in this pair had already fully recovered or greatly improved. The sailors in pair 1 (receiving the quart of cider) also showed some improvement, but sailors in the other pairs showed none.
Can you identify the independent and dependent variables in Lind's experiment? The independent variable is the daily supplement received by the pairs. The dependent variable is the improvement/no improvement in scurvy symptoms. Lind's results supported the citrus fruit cure for scurvy, and it was soon adopted by the British navy with good results. However, the fact that scurvy is caused by a vitamin C deficiency was not discovered until almost 200 years later.
Sampling
Lind's scurvy experiment included just 12 subjects. This is a very small sample by modern scientific standards. The sample in an experiment or other investigation consists of the individuals or events that are actually studied. It rarely includes the entire population because doing so would likely be impractical or even impossible.
There are two types of errors that may occur by studying a sample instead of the entire population: chance error and bias.
• A chance error occurs if the sample is too small. The smaller the sample is, the greater the chance that it does not fairly represent the whole population. Chance error is mitigated by using a larger sample.
• Bias occurs if the sample is not selected randomly with respect to a variable in the study. This problem is mitigated by taking care to choose a randomized sample.
A reliable experiment must be designed to minimize both of these potential sources of error. You can see how the sources of error were addressed in another landmark experiment: Jonas Salk's famous 1953 trial of his newly developed polio vaccine. Salk's massive experiment has been called the "greatest public health experiment in history."
Salk's Polio Vaccine Experiment
Imagine a nationwide epidemic of a contagious flu-like illness that attacks mainly children and often causes paralysis. That's exactly what happened in the U.S. during the first half of the 20th century. Starting in the early 1900s, there were repeated cycles of polio epidemics, and each seemed to be stronger than the one before. Many children ended up on life support in so-called "iron lungs" (see photo below) because their breathing muscles were paralyzed by the disease.
Polio is caused by a virus, and there is still no cure for this potentially devastating illness. Fortunately, it can now be prevented with vaccines. The first polio vaccine was discovered by Jonas Salk in 1952. After testing the vaccine on himself and his family members to assess its safety, Salk undertook a nationwide experiment to test the effectiveness of the vaccine using more than a million schoolchildren as subjects. It's hard to imagine a nationwide trial of an experimental vaccine using children as "guinea pigs." It would never happen today. However, in 1953, polio struck such fear in the hearts of parents that they accepted Salk's word that the vaccine was safe and gladly permitted their children to participate in the study.
Salk's experiment was very well designed. First, it included two very large, random samples of children — 600,000 in the treatment group, called the experimental group, and 600,000 in the untreated group, called the control group. Using very large and randomized samples reduced the potential for chance error and bias in the experiment. Children in the experimental group were injected with the experimental polio vaccine. Children in the control group were injected with a harmless saline (saltwater) solution. The saline injection was a placebo. A placebo is a "fake" treatment that actually has no effect on health. It is included in trials of vaccines and other medical treatments, so subjects will not know in which group (control or experimental) they have been placed. The use of a placebo helps researchers control for the placebo effect. This is a psychologically-based reaction to a treatment that occurs just because the subject is treated, even if the treatment has no real effect.
Experiments in which a placebo is used are generally blind experiments because the subjects are "blind" to their experimental group. This helps prevent bias in the experiment. Often, even the researchers do not know which subjects are in each group. This type of experiment is called a double-blind experiment because both subjects and researchers are "blind" to which subjects are in each group. Salk's vaccine trial was a double-blind experiment, and double-blind experiments are now considered the gold standard of clinical trials of vaccines, therapeutic drugs, and other medical treatments.
Salk's polio vaccine proved to be highly successful. Analysis of data from his study revealed that the vaccine was 80 to 90 percent effective in preventing polio. Almost overnight, Salk was hailed as a national hero. He appeared on the cover of Time magazine and was invited to the White House. Within a few years, millions of children had received the polio vaccine. By 1961, the incidence of polio in the U.S. had been reduced by 96 percent.
Limits on Experimentation
Well-done experiments are generally the most rigorous and reliable scientific investigations. However, their hallmark feature of manipulating variables to test outcomes is not possible, practical, or ethical in all investigations. As a result, many ideas cannot be tested through experimentation. For example, experiments cannot be used to test ideas about what our ancestors ate millions of years ago or how long-term cigarette smoking contributes to lung cancer. In the case of our ancestors, it is impossible to study them directly. Researchers must rely instead on indirect evidence, such as detailed observations of their fossilized teeth. In the case of smoking, it is unethical to expose human subjects to harmful cigarette smoke. Instead, researchers may use large observational studies of people who are already smokers, with nonsmokers as controls, to look for correlations between smoking habits and lung cancer.
Feature: Human Biology in the News
Lind undertook his experiment to test the effects of citrus fruits on scurvy at a time when seamen were dying by the thousands from this nutritional disease as he explored the world. Today's explorers are astronauts in space, and their nutrition is also crucial to the success of their missions. However, maintaining good nutrition in astronauts in space can be challenging. One problem is that astronauts tend to eat less while in space. Not only are they very busy on their missions, but they may also get tired of the space food rations. The environment of space is another problem. Factors such as microgravity and higher radiation exposure can have major effects on human health and require nutritional adjustments to help counteract them. A novel way of studying astronaut nutrition and health is provided by identical twin astronauts Scott and Mark Kelly (Figure \(3\)).
The Kellys are the first identical twin astronauts, but twin studies are nothing new. Scientists have used identical (homozygotic) twins as research subjects for many decades. Identical twins have the same genes, so any differences between them generally can be attributed to environmental influences rather than genetic causes. Mark Kelly spent almost a full year on the International Space Station (ISS) between 2015 and 2016, while his twin, Scott Kelly, stayed on the ground, serving as a control in the experiment. You may have noticed a lot of media coverage of Mark Kelly's return to Earth in March 2016 because his continuous sojourn in space was the longest of any American astronaut at that time. NASA is learning a great deal about the effects of long-term space travel on the human body by measuring and comparing nutritional indicators and other health data in the twins.
Review
1. How do experiments differ from other types of scientific investigations?
2. Identify the independent and dependent variables in Salk's nationwide polio vaccine trial.
3. Compare and contrast chance error and bias in sampling. How can each type of error be minimized?
4. What is the placebo effect? Explain how Salk's experimental design controlled for it.
5. Fill in the blanks. The _____________ variable is manipulated to see the effects on the ___________ variable.
6. True or False. In studies of identical twins, the independent variable is their genetics.
7. True or False. Experiments cannot be done on humans.
8. True or False. Larger sample sizes are generally better than smaller ones in scientific experiments.
9. Answer the following questions about Lind’s scurvy experiment.
1. Why do you think it was important that the sailors’ diets were all kept the same, other than the daily supplement?
2. Can you think of some factors other than diet that could have potentially been different between the sailors that might have affected the outcome of the experiment?
3. Why do you think the sailors who drank cider had some improvement in their scurvy symptoms?
10. Explain why double-blind experiments are considered to be more rigorous than regular blind experiments.
11. Why are studies using identical twins so useful?
12. Do you think it is necessary to include a placebo (such as an injection with saline in a drug testing experiment) in experiments that use animals? Why or why not?
Explore More
Watch this entertaining TED talk, in which biochemist, Kary Mullis, talks about the experiment as the basis of modern science.
Check out this video to learn more about conducting scientific experiments:
Attributions
1. Scorbutic tongue by CDC, public domain via Wikimedia Commons
2. Iron lung ward by Food and Drug Administration, public domain via Wikimedia Commons
3. Mark and Scott Kelly by NASA/Robert Markowitz, public domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/01%3A_The_Nature_and_Process_of_Science/1.6%3A_Scientific_Experiments.txt |
Up in Smoke
You've probably seen this warning label dozens of times. It's been required on cigarette packs in the U.S. since 1965, one year after the U.S. Surgeon General first issued a report linking cigarette smoking with diseases such as lung cancer. The report was based on thousands of research articles, including important research results published by British scientists Richard Doll and Austin Bradford Hill. Starting in 1950, Doll and Hill conducted large-scale, long-term observational studies on smoking and lung cancer and demonstrated a strong correlation between the two.
Observational Studies
Many questions in human biology are investigated with observational as opposed to experimental studies. An observational study measures characteristics in a sample but does not attempt to manipulate variables of interest. A simple example of an observational study is a political poll. A sample of adults might be asked how old they are and which of two candidates they favor. The study provides a snapshot in time of potential voters' opinions and how they differ by age of the respondent. Whether the results of the study apply to the population as a whole depends mainly on how large and random the sample is.
How is an observational study different from an experiment — the gold standard of scientific research studies? The main difference is how subjects are treated. In an observational study, no attempt is made to influence the subjects in any way. In an experiment, in contrast, the researcher applies a treatment to a group of subjects and attempts to isolate the effects of the treatment on an outcome variable by comparing the experimental group with a control group. For example, in 1954, Jonas Salk did an experimental trial of his newly discovered polio vaccine by giving it to a very large sample of children. Children in an equally large control group were given a harmless injection of a saline solution but no vaccine. Salk then compared the two groups of children and determined that the vaccine was 80 to 90 percent effective in preventing polio.
Types of Observational Studies
There are three different types of observational studies: cross-sectional, case-control, and cohort studies. All three types have pros and cons.
Cross-sectional Studies
A cross-sectional study is a type of observational study that collects data from a sample of subjects just once at a certain point in time. The political poll described above is a simple example of a cross-sectional study. A possible link between smoking and lung cancer was also first suggested by cross-sectional studies. Researchers found a higher rate of lung cancer in people who smoked than in those who did not smoke at the time of the study. In other words, the two variables seemed to be associated.
Cross-sectional studies are relatively cheap and easy to do, but their results are weak, so they are rarely used alone. More often, a researcher uses a cross-sectional study to find variables that may be linked and then does a case-control or cohort study to further investigate a possible relationship between the two variables.
Case-Control Studies
A case-control study is a type of observational study that compares a group of subjects having a trait of interest (cases) with a group of similar subjects not having the trait (controls). This type of study is retrospective. Subjects are asked to report their behaviors in the past in an attempt to find correlations between specific past behaviors and current status. The retrospective nature of case-control studies is their main weakness. Subjects' responses may be inaccurate because they forget or are dishonest about past habits.
A classic example of a case-control study is the early research on smoking and lung cancer carried out by Doll and Hill (Figure \(2\)). In 1950, the two scientists interviewed 700 lung cancer patients (cases) and 700 people without lung cancer (controls). They gathered information on past smoking habits and other characteristics of people in the two groups. When they compared the two groups, they found a strong association between past smoking behavior and current lung cancer status.
Cohort Studies
A cohort study is an observational study in which a group of similar subjects (the cohort) is selected at the start of the study and then followed over time. This type of study is prospective. The researchers collect data on the cohort periodically for months or even years into the future. Because the researchers collect the information directly, the data are likely to be more accurate than the self-reported recall data in case-control studies. Prospective data also allow researchers to establish the sequence of progression of disease states or other conditions of interest. On the other hand, cohort studies are the most costly and difficult observational studies to undertake.
One of the largest-ever cohort studies was undertaken by Doll and Hill in 1951. It was based on their earlier case-control study and further investigated the link between smoking and lung cancer. The cohort that began the study included almost 50,000 British male physicians, and they were followed by the researchers over the next 50 years. Initial findings of the study were first reported in 1954, and then updated results were reported periodically after that. The last report was published in 2004, and it reflected on the previous 50 years of research findings. This study provided even stronger evidence for the correlation between smoking and lung cancer.
Numerous other research studies, including experimental studies, have shown conclusively that smoking causes lung cancer, among many other health problems. Figure \(3\) shows some of the ill effects that have since been demonstrated to be caused by smoking.
Correlation vs. Causation in Observational Studies
Observational studies can generally establish correlation but not necessarily causation. Correlation is an association between two variables in which a change in one variable is associated with a change in the other variable. Correlation may be strong or weak. It can also be positive or negative.
• If two variables are shown to have a positive correlation, both variables change in the same direction. For example, an observational study might find that more smoking is correlated with a higher risk of lung cancer. In other words, as smoking goes up, so does lung cancer.
• If two variables are shown to have a negative correlation, they change in opposite directions. For example, an observational study might find that people who exercise more are less likely to develop lung cancer. In other words, as exercise increases, lung cancer decreases.
One of the main differences between observational studies and experiments is the issue of correlation vs. causation. Because observational studies do not control all variables, any correlations they show between variables cannot be interpreted as one variable causes another. In experiments, in contrast, all possible variables are controlled, making it safer to conclude that changes in one variable cause changes in another. Unfortunately, when observational studies are reported in the news media, this distinction is not often made. Instead, a variable that is correlated with another in an observational study may be reported incorrectly as causing changes in the other variable.
In observational studies, it is always possible that some other variable affects both of the variables of interest and explains the correlation. An example of the confusion of correlation and causation in observational studies is the case of the health effects of coffee. Many early observational studies of coffee consumption and health found a positive correlation between drinking coffee and health problems such as heart disease and cancer. Does this mean that drinking coffee causes these health problems? Not necessarily, although news media have reported this conclusion. Looking more deeply into the issue reveals that coffee drinking is also associated with a less health-conscious lifestyle. People who drink coffee tend to practice other behaviors that may negatively impact their health, such as smoking cigarettes or drinking alcohol. Larger observational studies in which such lifestyle differences were taken into account have found no correlation between coffee consumption and health problems. In fact, they have found that moderate coffee consumption may actually have some health benefits.
Rationale for Observational Studies
If observational studies cannot establish causation, why are they done? Why aren't all research questions investigated experimentally? There are several important reasons to do observational studies:
• An observational study may be the only type of study that is feasible for certain research questions because experiments are impossible, impractical, or unethical to undertake. For example, it would be unethical to do an experiment on smoking and health in which subjects in the smoking sample are deliberately exposed to tobacco smoke and then observed to see if they develop lung cancer.
• An observational study is generally cheaper and easier to conduct than an experimental study.
• An observational study usually can study more subjects and obtain a larger set of data than an experimental study.
Models
Another way to gain scientific knowledge without experimentation is with modeling. A model is a representation of part of the real world. Did you ever build a model car or airplane? Scientific models are something like that. They represent the real world but are simpler. This is one reason that models are especially useful for investigating complex systems. By studying a much simpler model, it is easier to learn how the real system works.
As a hypothesis, a model must be evaluated. It is assessed by criteria such as how well it represents the real world, what limitations it has, and how useful it is. The usefulness of a model depends on how well its predictions match observations of the real world. Keep in mind that even when a model's predictions match real-world observations, it doesn't prove that the model is correct or that it is the only model that works.
Modeling Biological Systems
Many phenomena in biology occur as part of a complex system, whether the system is a cell, a human organ such as the brain, or an entire ecosystem. Models of biological systems can range from simple two-dimensional diagrams to complex computer simulations. Figure \(3\) depicts a model of nicotine's effect on cells in the nervous system.
Model Organisms
Using other organisms as models of the human body is another way models are used in human biology research. A model organism is a nonhuman species that is extensively studied to understand particular biological phenomena. The expectation is that discoveries made in the model organism will provide insights into the workings of the human organism. In researching human diseases, for example, model organisms allow for a better understanding of the disease process without the added risk of harming actual human beings. The model species chosen should react to the disease or its treatment in a way that resembles human physiology. Although biological activity in a model organism does not ensure the same effect in humans, many drugs, treatments, and cures for human diseases are developed in part with the guidance of model organisms.
Model organisms that have been used in human biology research range from bacteria such as E. coli to nonhuman primates such as chimpanzees. The mouse Mus musculus, pictured below, is a commonly used model organism in human medical research. For example, it has been widely used to study diet-induced obesity and related health problems. In fact, the mouse model of diet-induced obesity has become one of the most important tools for understanding the interplay of high-fat Western diets and the development of obesity.
Feature: Reliable Sources
You may get most of your news from the Internet. You probably also research personal questions and term paper topics online. Unlike the information in newspapers and most television news broadcasts, information on the Internet is not regulated for quality or accuracy. Almost anybody can publish almost anything they wish on the web. The responsibility is on the user to evaluate Internet resources. How do you know if the resources you find online are reliable? The questions below will help you assess their reliability.
1. How did you find the web page? If you just "googled" a topic or question, the search results may or may not be reliable. More likely to be trustworthy are web pages recommended by a faculty member, cited in an academic source, or linked with a reputable website.
2. What is the website's domain? If its URL includes .edu, it is affiliated with a college or university. If it includes .gov, it is affiliated with the federal government, and if it includes .org it is affiliated with a nonprofit organization. Such websites are generally more trustworthy sources of information than .com websites, which are commercial or business websites.
3. Who is the author of the web page? Is the author affiliated with a recognized organization or institution? Are the author's credentials listed, and are they relevant to the information on the page? Is current contact information for the author provided?
4. Is the information trustworthy? Are sources cited for facts and figures? Is a bibliography provided? Does there seem to be a particular bias or point of view presented, or does the information seem fair and balanced? Does the page contain advertising that might impact the content of information that is included?
5. Is the information current? When was the page created and last updated? Are the links on the page current and functional?
Put this advice into practice. Go online and find several web pages that provide information on the topic of smoking and lung cancer. Which websites do you think provide the most reliable information? Why?
Review
1. Explain why observational studies cannot establish causation. Describe an example to illustrate your explanation.
2. Compare and contrast the three types of observational studies described above.
3. Identify three possible reasons for doing an observational study.
4. Why are models commonly used in human biology research?
5. Multiple answers: What kind of a study involves the recall of variables that occurred in the past? What kind involves the observation of variables from the beginning?
1. positive correlation; negative correlation
2. negative correlation; positive correlation
3. retrospective; prospective
4. prospective; retrospective
6. True or False. A positive correlation means there are health benefits to the variable under investigation.
7. True or False. A cohort is a group of subjects of different ages, weights, genders, and health statuses.
8. A study is done to investigate whether soda consumption influences the development of diabetes. The subjects are individuals recently diagnosed with diabetes compared to controls who do not have diabetes. All of the respondents are asked how many times a week they drank soda over the last two years. Answer the following questions about this scientific investigation.
1. What type of observational study is this?
2. The subjects with diabetes are “matched” to the controls, meaning that the researchers tried to minimize the effect of other variables outside of the variable of interest (i.e. soda consumption). What do you think some of those other variables could be?
3. Do you think the data about soda consumption will be accurate? Why or why not?
4. How could you change the study to get more accurate data on whether there is a relationship between soda consumption and diabetes? Explain why your new study would be more accurate.
9. Do you think that computer simulation models of biological systems can be accurate without observations or experiments on actual living organisms or tissues?
10. Explain why both observational and experimental investigations are useful in science.
Explore More
Learn more about the Blue Brain Project by watching this TED talk.
Attributions
1. Tobacco package warning by CDC/ Debora Cartagena, public domain via Wikimedia Commons
2. Sir Austin Bradford Hill by Wellcome Collection gallery, licensed CC BY 4.0 via Wikimedia Commons
3. Adverse effects of tobacco by Mikael Häggström, released into the public domain via Wikimedia Commons
4. Nicotine increases dopamine by National Institute of Health, public domain via Wikimedia Commons
5. Mouse by US government, public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/01%3A_The_Nature_and_Process_of_Science/1.7%3A_Extrapolations_of_Scientific_Investigations.txt |
Case Study Conclusion: To Give a Shot or Not
New mother Elena left her pediatrician’s office still unsure whether to vaccinate baby Juan. Dr. Rodriguez gave Elena a list of reputable sources where she could look up information about the safety of vaccines herself, such as the Centers for Disease Control and Prevention (CDC). Elena reads that the consensus within the scientific community is that there is no link between vaccines and autism. She finds a long list of studies published in peer-reviewed scientific journals that disprove any link. Additionally, some of the studies are “meta-analyses” that analyzed the findings from many individual studies. Elena is reassured by the fact that many different researchers, using a large number of subjects in numerous well-controlled and well-reviewed studies, all came to the same conclusion.
Elena also went back to the author’s website that originally scared her about the safety of vaccines. She found that the author was not a medical doctor or scientific researcher, but rather was a self-proclaimed “child wellness expert.” Also, the doctor sold books and advertising on their site, some of which were related to claims of vaccine injury. Elena realized that the doctor was both an unqualified and potentially biased source of information.
Also, Elena realized that some of the doctor's arguments were based on correlations between autism and vaccines, but, as the saying goes, “correlation does not imply causation.” For instance, the recent rise in autism rates may have occurred during the same time period as an increase in the number of vaccines given in childhood, but Elena could think of many other environmental and social factors that have also changed during this time period. There are just too many variables to come to the conclusion that vaccines, or anything else, are the cause of the rise in autism rates based on that type of argument alone. Also, Elena learned that the age of onset of autism symptoms happens to typically be around the time that the MMR vaccine is first given, so the apparent association in the timing may just be a coincidence.
Public health, sanitation, and the use of antibiotics and vaccines have lessened the impact of infectious disease on human populations. Through vaccination programs, better nutrition, and vector control (carriers of disease), international agencies have significantly reduced the global infectious disease burden. Reported cases of measles in the United States dropped from around 700,000 a year in the 1950s to practically zero by the late 1990s and declared eradicated by the year 2000 (Figure \(2\)). Globally, measles fell 60 percent from an estimated 873,000 deaths in 1999 to 164,000 in 2008. This advance is attributed entirely to a comprehensive vaccination program.
However, Elena came across news about a measles outbreak that originated in California in 2014, 2015, and the latest outbreak of 2019 (Figure \(3\)). Measles wasn’t just a disease of the past as she had thought! She learned that measles and whooping cough, which had previously been rare thanks to widespread vaccinations, are now on the rise, and that people choosing not to vaccinate their children seems to be one of the contributing factors. Elena realized that it is important to vaccinate their baby against these diseases, not only to protect the baby from their potentially deadly effects but to also protect others in the population.
In her reading, Elena learns that scientists do not yet know the causes of autism, but she feels reassured by the abundance of data that disproves any link with vaccines. She thinks that the potential benefit of protecting their baby’s health against deadly diseases outweighs any unsubstantiated claims about vaccines. She will be making an appointment to get baby Juan their shots soon.
Chapter Summary
• Science is a distinctive way of gaining knowledge about the natural world that is based on the use of evidence to logically test ideas. As such, science is more of a process than a body of knowledge.
• A scientific theory, such as the germ theory of disease, is the highest level of explanation in science. A theory is a broad explanation for many phenomena that is widely accepted because it is supported by a great deal of evidence.
• The scientific investigation is the cornerstone of science as a process. An investigation is a procedure for gathering evidence to test a hypothesis.
• A scientific experiment is a type of scientific investigation in which the researcher manipulates variables under controlled conditions to test expected outcomes. Experiments are the gold standard for scientific investigations and can establish causation between variables.
• Nonexperimental scientific investigations such as observational studies and modeling may be undertaken when experiments are impractical, unethical, or impossible. Observational studies generally can establish correlation but not causation between variables.
Chapter Summary Review
1. Which of the following is the best example of “doing science?”
1. memorizing the processes of the water cycle
2. learning how to identify trees from their leaves
3. learning the names of all the bones in the human body
4. making observations of wildlife while hiking in the woods
2. A scientist develops a new idea based on their observations of nature. What should they do next?
1. think of a way to test the idea
2. claim that they have discovered a new theory
3. reject any evidence that conflicts with the idea
4. look only for evidence that supports the idea
3. Which of the following is defined as a possible answer to a scientific question?
1. an observation
2. data
3. a hypothesis
4. statistics
4. Do scientists usually come up with a hypothesis in the absence of any observations? Explain your answer.
5. Why does a good hypothesis have to be falsifiable?
6. Name one scientific law.
7. Name one scientific theory.
8. Give an example of a scientific idea that was later discredited.
9. Would the idea that the Earth revolves around the Sun be considered consensus science or frontier science?
10. True or False. A scientific investigation always follows the same sequence of steps in a linear fashion.
11. True or False. Data that does not support a hypothesis is not useful.
12. True or False. Experimentation is the only valid type of scientific investigation.
13. True or False. Correlation does not imply causation.
14. Explain why science is considered an iterative process.
15. A statistical measurement called a P-value is often used in science to determine whether or not a difference between two groups is actually significant or simply due to chance. A P-value of 0.03 means that there is a 3% chance that the difference is due to chance alone. Do you think a P-value of 0.03 would indicate that the difference is likely to be significant? Why or why not?
16. a. Why is it important that scientists communicate their findings to others? How do they usually do this?
17. What is a “control group” in science?
18. In a scientific experiment, why is it important to only change one variable at a time?
19. Which is the dependent variable – the variable that is manipulated or the variable that is being affected by the change?
20. Which is most likely to show or disprove causation between two variables?
1. a controlled experiment
2. an observational study
3. the development of a hypothesis
4. an observation
21. You see an ad for a “miracle supplement” called NQP3 that claims the supplement will reduce belly fat. They say it works by reducing the hormone cortisol and by providing your body with missing unspecified “nutrients”, but they do not cite any peer-reviewed clinical studies. They show photographs of three people who appear slimmer after taking the product. A board-certified plastic surgeon endorses the product on television. Answer the following questions about this product.
1. Do you think that because a doctor endorsed the product, it really works? Explain your answer.
2. Do you think the photographs are good evidence that the product works? Why or why not?
Attributions
1. Sad mom by dirvish, licensed CC BY 2.0 via Flickr
2. Measles by 2over0, released into the public domain via Wikimedia Commons
3. Measles cases by CDC, public domain
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/01%3A_The_Nature_and_Process_of_Science/1.8%3A__Case_Study_Conclusion%3A__Shot_and_Chapter_Summary.txt |
This chapter introduces the human species. It identifies traits we share with all other living things and basic principles of biology that apply to us as well as to all other life. The chapter also describes the diversity of species on Earth, similarities we share with our closest relatives in the animal kingdom, and traits that make us unique.
• 2.1: Case Study: Why Should You Study Human Biology?
Human biology is the scientific study of the human species that includes the fascinating story of human evolution and a detailed accounting of our genetics, anatomy, physiology, and ecology. In short, human biology focuses on how we got here, how we function, and the role we play in the natural world. Importantly, this helps us to better understand human health – how to stay healthy and how diseases and injuries can be treated.
• 2.2: Shared Traits of All Living Things
You've probably seen this famous statue created by the French sculptor Auguste Rodin. Rodin's skill as a sculptor is evident because the statue looks so lifelike. In fact, the statue is made of rock so its only resemblance to life is how it appears.
• 2.3: Diversity of Life
The collage above shows a single species in each of the six kingdoms into which all of Earth's living things are commonly classified.
• 2.4: The Human Animal
Relative to all animals, this child and monkey are practically "cousins." From genes to morphology to behavior, they are similar in many ways. That's because both of them are primates, and they share an evolutionary past.
• 2.5: Case Study Conclusion: Inhabitants and Chapter Summary
As you may recall, Wajiha’s strep throat was caused by Streptococcus pyogenes bacteria, the species shown in the photomicrograph above. She took antibiotics to kill the S. pyogenes, but this also killed her "good" bacteria, throwing off the balance of microorganisms living inside of her, which resulted in diarrhea and a yeast infection.
Thumbnail: The human body in action (Unsplash License; Sid Suratia via Unsplash)
02: Introduction to Human Biology
Case Study: Our Invisible Inhabitants
Wajiha is suffering from a fever, body aches, and a painful sore throat that gets worse when she swallow. She visits her doctor who examines her and performs a throat culture. When the results come back, the doctor tells Wajiha she has strep throat, which is caused by the bacteria Streptococcus pyogenes. The doctor prescribes an antibiotic to kill the bacteria and advises Wajiha to take the full course of the treatment even if she is feeling better earlier because stopping early can cause an increase in bacteria that are resistant to antibiotics.
Wajiha takes the antibiotic as prescribed. Towards the end of the course, her throat is feeling much better but she can’t say the same for other parts of her body! She has developed diarrhea and an itchy vaginal yeast infection. Wajiha calls her doctor, who suspects that the antibiotic treatment has caused both her digestive distress and her yeast infection. The doctor explains that our bodies are home to many different kinds of microorganisms, some of which are actually beneficial to our bodies by helping us digest our food or keeping the population of harmful microorganisms down. When we take an antibiotic, many of these “good” bacteria are killed along with the “bad” disease-causing bacteria, which can result in diarrhea and yeast infections.
The doctor prescribes an antifungal medication for Wajiha’s yeast infection. The doctor also recommends that Wajiha eat yogurt with “live cultures” to try to help replace the beneficial bacteria in her gut. Clearly, our bodies contain a delicate balance of inhabitants that are invisible without a microscope, and changes in that balance can cause unpleasant health effects.
What Is Human Biology?
As you read the rest of this book, you'll learn more amazing facts about the human organism and how biology relates to your health. Human biology is the scientific study of the human species that includes the fascinating story of human evolution and a detailed accounting of our genetics, anatomy, physiology, and ecology. In short, human biology focuses on how we got here, how we function, and the role we play in the natural world. Importantly, this helps us to better understand human health – how to stay healthy and how diseases and injuries can be treated. This is probably of personal interest to you in terms of your own health and the health of your friends and family, and also has broader implications for society and the human species as a whole.
As you read this book, think about what you want to learn about your own human body. What questions or concerns do you have? Make a list of them and use the list to guide your study of human biology. You can revisit the list throughout the course to see if your questions have been answered. If not, you'll have the tools to find the answers. You will have learned how to find sources of information about human biology and how to judge which sources are most reliable.
Chapter Overview: Introduction to Human Biology
In the rest of this chapter, you'll learn about the traits shared by all living things, the basic principles that underlie all of biology, the vast diversity of living organisms, what it means to be human, and our place in the animal kingdom. Specifically, you'll learn:
• The seven traits shared by all living things including the maintenance of a more-or-less constant internal environment, called homeostasis; multiple levels of organization consisting of one or more cells; using energy and exhibiting metabolism; the ability to grow and develop; the ability to evolve adaptations to the environment; the ability to detect and respond to environmental stimuli; and the ability to reproduce.
• The diversity of life, including the different kinds of biodiversity, the definition of a species, the classification and naming systems for living organisms, and how evolutionary relationships can be represented through diagrams such as phylogenetic trees.
• How the human species is classified, our close relatives and ancestors, and some ways in which we evolved.
• The traits humans share with other primates including physical characteristics and social behaviors.
As you read this chapter, think about the following questions about Wajiha’s situation:
1. What do single-celled organisms, such as the bacteria and yeast living in and on Wajiha, have in common with humans?
2. How are bacteria, yeast (a fungus), and humans classified?
3. How do the concepts of homeostasis and biodiversity apply to Wajiha’s situation?
4. Why can stopping antibiotics early cause the development of antibiotic-resistant bacteria?
Attributions
1. Capt. Wan Mun Chin examines a patient by U.S. Navy photo by Journalist 1st Class Jeremy L. Wood., public domain via Wikimedia Commons
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/02%3A_Introduction_to_Human_Biology/2.1%3A_Case_Study_-_Why_Should_You_Study_Human_Biology.txt |
The Thinker
You've probably seen this famous statue created by the French sculptor Auguste Rodin. Rodin's skill as a sculptor is evident because the statue looks so lifelike. In fact, the statue is made of rock so its only resemblance to life is how it appears. How does a statue made of rock differ from a living, breathing human being or other living organisms? What is life? What does it mean to be alive? Science has answers to these questions.
Characteristics of Living Things
To be classified as a living thing, most scientists agree that an object must have all seven of the following traits. These are traits that human beings share with other living things.
1. homeostasis
2. organization
3. metabolism
4. growth
5. adaptation
6. response to stimuli
7. reproduction
Homeostasis
All living things are able to maintain a more-or-less constant internal environment. They keep things relatively stable on the inside regardless of the conditions around them. The condition in which a system is maintained in a more-or-less steady state is called homeostasis. Human beings, for example, maintain stable internal body temperature. If you go outside when the air temperature is below freezing, your body doesn't freeze. Instead, by shivering and other means, it maintains a stable internal temperature.
Organization
Living things have multiple levels of organization. Their molecules are organized into one or more cells. A cell is the basic unit of the structure and function of living things. Cells are the building blocks of living organisms. An average adult human being, for example, consists of trillions of cells. Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells and onion cells in the figure below. What similarities do you see?
Metabolism
All living things can use energy. Their cells have the "machinery" of metabolism, which is the building up and breaking down of chemical compounds. Living things can transform energy by converting chemicals and energy into cellular components. This form of metabolism is called anabolism. They can also break down, or decompose, organic matter, which is called catabolism. Living things require energy to maintain internal conditions (homeostasis), for growth, and other life processes.
Growth
All living things have the capacity for growth. Growth is an increase in size that occurs when there is a higher rate of anabolism than catabolism. For example, a human infant has changed dramatically in size by the time it reaches adulthood, as is apparent from the image below. In what other ways do we change as we grow from infancy to adulthood?
Adaptations and Evolution
An adaptation is a characteristic of populations. Individuals of a population carry a variety of genes. When the environment changes, some individuals of the population can withstand the changed conditions and reproduce more than the individuals who cannot live in the given environment. A change in the allele frequencies and makeup of the populations over time is called evolution. It comes about through the process of natural selection.
Response to Stimuli
All living things detect changes in their environment and respond to them. A response can take many forms, from the movement of a unicellular organism in response to external chemicals (called chemotaxis), to complex reactions involving all the senses of a multicellular organism. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (called phototropism).
Reproduction
All living things are capable of reproduction. Reproduction is the process by which living things give rise to offspring. Reproduction may be as simple as a single cell dividing into two cells. This is how bacteria reproduce. Reproduction in human beings and many other organisms is much more complicated. Nonetheless, whether a living thing is a human being or a bacterium, it is normally capable of reproduction.
Feature: Myth vs. Reality
Myth: Viruses are living things.
Reality: The traditional scientific view of viruses is that they originated from bits of DNA or RNA that were shed from the cells of living things but that they are not living things themselves. Scientists have long argued that viruses are not living things because they do not have most of the defining traits of living organisms. A single virus called a virion, consists of a set of genes (DNA or RNA) inside a protective protein coat, called a capsid. Viruses have an organization, but they are not cells and do not possess the cellular "machinery" that living things use to carry out life processes. As a result, viruses cannot undertake metabolism, maintain homeostasis, or grow. They do not seem to respond to their environment, and they can reproduce only by invading and using "tools" inside host cells to produce more virions. The only traits viruses seem to share with living things is the ability to evolve adaptations to their environment. In fact, some viruses evolve so quickly that it is difficult to design drugs and vaccines against them. That's why maintaining protection from the viral disease influenza, for example, requires a new flu vaccine each year.
Within the last decade, new discoveries in virology, the study of viruses, suggest that this traditional view about viruses may be incorrect and the "myth" that viruses are living things may be the reality. Researchers have discovered giant viruses that contain more genes than cellular life forms such as bacteria. Some of the genes code for proteins needed to build new viruses, suggesting that these giant viruses may be able — or were once able — to reproduce without a host cell. Some of the strongest evidence that viruses are living things comes from studies of their proteins, which show that viruses and cellular life share a common ancestor in the distant past. Viruses may have once existed as primitive cells but at some point lost their cellular nature to become modern viruses that require host cells to reproduce. This idea is not so far-fetched when you consider that many other species require a host to complete their life cycle.
Review
1. Identify seven traits that most scientists agree are shared by all living things.
2. What is homeostasis? What is one way humans fulfill this criterion of living things?
3. Define reproduction, and describe an example.
4. Assume that you found an object that looks like a dead twig. You wonder if it might be a stick insect. How could you determine if it is a living thing?
5. Describe viruses and what traits they do and do not share with living things. Do you think viruses should be considered living things? Why or why not?
6. People who are biologically unable to reproduce are certainly still considered to be alive! Discuss why this situation does not invalidate the criteria that living things must be capable of reproduction.
7. What are the two types of metabolism described here and what are their differences?
8. What are some similarities between cells of different organisms? If you are not familiar with the specifics of cells, simply describe the similarities you see in the pictures above.
9. What are two processes that use energy in a living thing?
10. Give an example of a response to stimuli in humans.
11. Do unicellular organisms, such as bacteria, have an internal environment that they maintain through homeostasis?
12. Evolution occurs through ___________ ____________ .
13. If alien life is found on other planets, do you think they will necessarily have cells? Discuss your answer.
14. Movement in response to an external chemical is called ___________, while movement towards light is called ___________ .
Attributions
1. The Thinker by innoxiuss, Licensed CC BY 2.0 via Wikimedia Commons
2. Human cheek cells by Krishna satya 333, CC BY-SA 4.0 via Wikimedia Commons
1. Onion cells by kaibara87, CC BY 2.0 via Wikimedia Commons
3. Baby, public domain via Nappy
4. Basic scheme of a virus by DEXi, dedicated CC0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/02%3A_Introduction_to_Human_Biology/2.2%3A_Shared_Traits_of_All_Living_Things.txt |
So Many Species!
The collage below shows six kingdoms into which all of Earth's living things are commonly classified. How many species are there in each kingdom? In a word, millions. A total of almost 2 million living species have already been identified, and new species are being discovered all the time. Scientists estimate that there may be as many as 30 million different species alive on Earth today! Clearly, there is a tremendous variety of life on Earth.
What Is Biodiversity?
Biological diversity, or biodiversity, refers to all of the variety of life that exists on Earth. Biodiversity can be described and measured at three different levels: species, genetic, and ecosystem diversity.
• Species diversity refers to the number of different species in an ecosystem or on Earth as a whole. This is the commonest way to measure biodiversity. Current estimates for Earth's total number of living species range from 5 to 30 million species.
• Genetic diversity refers to the variation in genes within all these species.
• Ecosystem diversity refers to the variety of ecosystems on Earth. An ecosystem is a system formed by populations of many different species interacting with each other and their environment.
Defining a Species
Biodiversity is most often measured by counting species, but what is a species? The answer to that question is not as straightforward as you might think. The formal biological definition of species is a group of actually or potentially interbreeding organisms. This means that members of the same species are similar enough to each other to produce fertile offspring together. By this definition of species, all human beings alive today belong to one species, Homo sapiens. All humans can potentially interbreed with each other but not with members of any other species.
In the real world, it isn't always possible to make the observations needed to determine whether different organisms can interbreed. For one thing, many species reproduce asexually, so individuals never interbreed even with members of their own species. When studying extinct species represented by fossils, it is usually impossible to know whether different organisms could interbreed. Therefore, in practice, many biologists and virtually all paleontologists generally define species on the basis of morphology, rather than breeding behavior. Morphology refers to the form and structure of organisms. For classification purposes, it generally refers to relatively obvious physical traits. Typically, the more similar to one another different organisms appear, the greater the chance that they will be classified in the same species.
Classifying Living Things
People have been trying to classify the tremendous diversity of life on Earth for more than two thousand years. The science of classifying organisms is called taxonomy. Classification is an important step in understanding the present diversity and past evolutionary history of life on Earth. It helps make sense of the overwhelming diversity of living things.
Linnaean Classification
All modern classification systems have their roots in the Linnaean classification system. It was developed by Swedish botanist Carolus Linnaeus in the 1700s. He tried to classify all living things that were known at his time. He grouped together organisms that shared obvious morphological traits, such as the number of legs or shape of leaves.
The Linnaean system of classification consists of a hierarchy of groupings, called taxa (singular, taxon). Figure \(2\) shows an expanded version of Linnaeus's original classification system. In the original system, taxa range from the kingdom to the species. The kingdom is the largest and most inclusive grouping. It consists of organisms that share just a few basic similarities. Examples are the plant and animal kingdoms. The species is the smallest and most exclusive grouping. Ideally, it consists of organisms that are similar enough to interbreed, as discussed above. Similar species are classified together in the same genus (plural, genera), similar genera are classified together in the same family, and so on all the way up to the kingdom.
Binomial Nomenclature
Perhaps the single greatest contribution Linnaeus made to science was his method of naming species. This method, called binomial nomenclature, gives each species a unique, two-word Latin name consisting of the genus name followed by a specific species identifier. An example is Homo sapiens, the two-word Latin name for humans. It literally means “wise human.” This is a reference to our big brains.
Why is having two names so important? It is similar to people having a first and a last name. You may know several people with the first name Michael, but adding Michael’s last name usually pins down exactly who you mean. In the same way, having two names uniquely identifies a species.
Revisions in the Linnaean Classification
Linnaeus published his classification system in the 1700s. Since then, many new species have been discovered. Scientists can also now classify organisms on the basis of their biochemical and genetic similarities and differences rather than just their outward morphology. These changes have led to revisions in the original Linnaean system of classification.
A major change to the Linnaean system is the addition of a new taxon called the domain. The domain is a taxon that is larger and more inclusive than the kingdom, as shown in Figure \(2\). Most biologists agree that there are three domains of life on Earth: Bacteria, Archaea, and Eukarya (Figure \(3\)). Both the Bacteria and the Archaea domains consist of single-celled organisms that lack a nucleus. This means that their genetic material is not enclosed within a membrane inside the cell. The Eukarya domain, in contrast, consists of all organisms whose cells have a nucleus. In other words, their genetic material is enclosed within a membrane inside the cell. The Eukarya domain is made up of both single-celled and multicellular organisms. This domain includes several kingdoms, including the animal, plant, fungus, and protist kingdoms.
Phylogenetic Classification
Linnaeus classified organisms based on morphology. Basically, organisms were grouped together if they looked alike. After Darwin published his theory of evolution in the 1800s, scientists looked for a way to classify organisms that took into account phylogeny. Phylogeny is the evolutionary history of a group of related organisms. It is represented by a phylogenetic tree, or some other tree-like diagram, like the one in Figure \(3\) for the three domains. A phylogenetic tree shows how closely related different groups of organisms are to one another. Each branching point represents a common ancestor of the branching groups. Figure \(3\), for example, shows that the Eukarya shared a more recent common ancestor with the Archaea than they did with the Bacteria. This is based on comparisons of important similarities and differences between the three domains.
Review
1. What is biodiversity? Identify three ways that biodiversity may be measured.
2. Define biological species. Why is this definition often difficult to apply?
3. Explain why it is important to classify living things and outline the Linnaean system of classification.
4. What is binomial nomenclature? Give an example.
5. Contrast Linnaean and phylogenetic systems of classification.
6. Describe the taxon called the domain, and compare the three widely recognized domains of living things.
7. True or False. Humans have identified all of the species on Earth.
8. True or False. In the binomial nomenclature for humans, Homo is the genus and sapiens refers to the specific species.
9. A kingdom is a:
1. domain
2. taxon
3. genera
4. phylogeny
10. In Linnaean classification, similar classes together make up a ___________ .
11. Based on the phylogenetic tree for the three domains of life above, explain whether you think Bacteria are more closely related to Archaea or Eukarya.
12. A scientist discovers a new single-celled organism. Answer the following questions about this discovery.
1. If this is all you know, can you place the organism into a particular domain? If so, what is the domain and if not, why not?
2. What is one type of information that could help the scientist classify the organism?
13. Define morphology. Give an example of a morphological trait in humans.
14. Which type of biodiversity is represented by the differences between humans?
15. Why do you think it is important for the definition of a species that members of a species can produce fertile offspring?
Attributions
1. Tree of living organisms by Maulucioni y Doridí, licensed CC BY-SA 3.0 via Wikimedia Commons
2. Biological classification by Peter Halasz, released into the public domain via Wikimedia Commons
3. Domain Trees by Crion, CC BY-SA 4.0 via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/02%3A_Introduction_to_Human_Biology/2.3%3A_Diversity_of_Life.txt |
"Cousins"
Relative to all animals, this child and monkey are practically "cousins." From genes to morphology to behavior, they are similar in many ways. That's because both of them are primates, and they share an evolutionary past.
How Humans Are Classified
You probably know that modern humans belong to the species Homo sapiens. But what is our place in nature? How are our species classified? A simple classification is represented in Figure \(2\). Humans can move on their own and are placed in the animal kingdom. Further, humans belong to the animal phylum known as chordates because we have a backbone. The human animal has hair and milk glands, so we are placed in the class of mammals. Within the mammal class, humans are placed in the primate order.
Humans as Primates
Living members of the primate order include monkeys, apes, and humans; and any member of this order of mammals is called a primate. At some point in the distant past, we shared ape-like ancestors with all these modern groups of primates. We share between 93 percent and almost 99 percent of our DNA sequences with them, providing hard evidence that we have relatively recent common ancestors. Besides genes, what traits do we share with other primates? Primates are considered generalists among mammals. A generalist is an organism that can thrive in a wide variety of environmental conditions and make use of a variety of different resources, such as consuming many different types of food. Although primates exhibit a wide range of characteristics, there are several traits that are shared by most primates.
Primate Traits
Primates have five digits (fingers or toes) on each extremity (hand or foot). The fingers and toes have nails instead of claws and are covered with sensitive tactile pads. The thumbs (and in many species the big toes as well) are opposable, meaning they can be brought into opposition with the other digits, allowing both a power grasp and a precision grip. You can see these features of the primate extremities in the capuchin monkey pictured below.
The primate body is generally semi-erect or erect, and primates have one of several modes of locomotion, including walking on all four legs (quadrupedalism), vertical clinging and leaping, swinging from branch to branch in trees (brachiation), or walking on two legs (bipedalism), the last of which applies only to our own species today. The primate shoulder girdle has a collar bone (clavicle), which is associated with a wide range of motion of the upper limbs.
Relative to other mammals, primates rely less on their sense of smell. They have a reduced snout and relatively small area in the brain for processing olfactory (odor) information. Primates rely more on their sense of vision, which shows several improvements over that of other mammals. Most primates can see in color. Primates also tend to have large eyes with forward-facing placement in a relatively flat face. This results in an overlap of the visual fields of the two eyes, allowing stereoscopic vision, or three-dimensional, vision. Other indications of the importance of vision to primates are the protection given to the eyes by a complete bony eye socket and the large size of the occipital lobe of the brain where visual information is processed.
Primates are noted for their relatively large brains, high degree of intelligence, and complex behaviors. The part of the brain that is especially enlarged in primates is the cerebrum, which analyzes and synthesizes sensory information and transforms it into motor behaviors appropriate to the environment. Primates tend to have longer lifespans than most other mammals. In particular, there is a lengthening of the prenatal period and the postnatal period of dependency of infants on adults, providing an extended opportunity for learning in juveniles. Most primates live in social groups. In fact, primates are among the most social of animals. Depending on the species, adult nonhuman primates may live in mated pairs or in groups of up to hundreds of members.
Life in the Trees
Scientists think that many primate traits are adaptations to an arboreal, or tree-dwelling, lifestyle. Primates are thought to have evolved in trees, and the majority of primates still live in trees. For life in the trees, the sense of vision trumps the sense of smell, and three-dimensional vision is especially important for grasping the next branch or limb. Having mobile limbs, a good grip, and manual dexterity are matters of life and death when one lives high above the ground. While some modern primates are mainly terrestrial (ground-dwelling) rather than arboreal, all primates possess adaptations for life in the trees.
Figure \(4\) shows the present distribution of nonhuman primates around the world. Tropical forests in Central and South America are home to many species of monkeys, including the capuchin monkey pictured above. Old World tropical forests in Africa and Asia are home to many other species of monkeys, including the crab-eating macaque pictured above, as well as all modern apes.
Humans as Hominids
Who are our closest relatives in the primate order? We are placed in the family called Hominidae. Any member of this family is called a hominid. Hominids include four living genera: chimpanzees, gorillas, orangutans, and humans. Among these four genera are just seven living species: two in each genera except humans, with our sole living species, Homo sapiens. The Orangutan mother pictured in figure \(5\) cradling her child shows how similar these hominids are to us.
Hominids are relatively large, tailless primates, ranging in size from the bonobo, or pygmy chimpanzee, which may weigh as little as 30 kg (66 lb), to the eastern gorilla, which may weigh over 200 kg (440 lb). Most modern humans fall somewhere in between that range. In all species of hominids, males are somewhat larger and stronger, on average, than females, but the differences may not be great. Except for humans, hominids are mainly quadrupedal, although they can get around bipedally if need be to gather food or nesting materials. Humans are the only habitually bipedal species of living hominids.
The Human Genus
Within the hominid family, our species is placed in the genus Homo. Our species, Homo sapiens, is the only living species in this genus. Several earlier species of Homo existed but have since gone extinct, including the species Homo erectus. An artist's reconstruction of a Homo erectus individual is shown in figure \(6\).
By about 2.8 million years ago, early Homo species such as Homo erectus were probably nearly as efficient at bipedal locomotion as modern humans. Relative to quadrupedal primates, they had a broader pelvis, longer legs, and arched feet. However, from the neck up, they were still quite different from us. They typically had bigger jaws and teeth, a sloping forehead, and a relatively small brain.
Homo sapiens
During the roughly 2.8 million years of the evolution of the Homo genus, the remaining features of Homo sapiens evolved. These features include:
• small front teeth (incisors and canines) with relatively large molars, at least compared to other primates.
• a decrease in the size of the jaws and face, and an increase in the size of the cranium, forming a nearly vertical forehead.
• a tremendous enlargement of the brain, especially in the cerebrum, which is the site of higher intellectual functions.
The increase in brain size occurred very rapidly as far as evolutionary change goes, between about 800,000 and 100,000 years ago. During this period, the size of the brain increased from about 600 cm3 to about 1400 cm3 and the earliest Homo sapiens appeared. This was also a period of rapid climate change, and many scientists think that climate change was a major impetus for the evolution of a larger, more complex brain. In this view, as the environment became more unpredictable, bigger, "smarter" brains helped our ancestors survive. Paralleling the biological evolution of the brain was the development of culture and technology as behavioral adaptations for exploiting the environment. These developments, made possible by a big brain, allowed modern humans and their recent ancestors to occupy virtually the entire world and become the dominant land animals.
Our species Homo sapiens is the most recent iteration of the basic primate body plan. Because of our big, complex brain, we clearly have a much greater capacity for abstract thought and technological advances than any other primate, even chimpanzees who are our closest living relatives. However, it is important to recognize that in other ways, we are not as adept as other living hominids around the world. We are physically weaker than gorillas, far less agile orangutans, and arguably less well-mannered than bonobos.
Feature: Human Biology in the News
Imagine squeezing through a seven-inch slit in rock to enter a completely dark cave full of lots and lots of old bones. It might sound like a nightmare to most people, but it was a necessary part of a recent exploration of human origins in South Africa as reported in the New York Times in September 2015. The cave and its bones were actually first discovered by spelunkers in 2013, who reported it to paleontologists. An international research project was soon launched to explore the cave. The researchers would eventually conclude that the cave was a hiding place for the dead of a previously unknown early species of Homo, whom they gave the name Homo naledi. Members of this species lived in South Africa around 2.5 to 2.8 million years ago.
Homo naledi individuals were about 5 feet tall and weighed around 100 pounds, so they probably had no trouble squeezing into the cave. Modern humans are considerably larger on average. In order to retrieve the fossilized bones from the cave, six very slender female researchers had to be found on social media. They were the only ones who could fit through the crack to access the cave. The work was difficult and dangerous but also incredibly exciting. The site constitutes one of the largest samples for any extinct early Homo species anywhere in the world, and the fossils represent a completely new species of that genus. The site also suggests that early members of our genus were intentionally depositing their dead in a remote place. This behavior was previously thought to be limited to later humans.
Like other early Homo species, Homo naledi exhibits a mosaic of old and modern traits. From the neck down, these early hominins were well adapted for upright walking. Their feet were virtually indistinguishable from modern human feet (see image below), and their legs were also long like ours. Homo naledi had relatively small front teeth but also a small brain, no larger than an average orange. Clearly, the spurt in brain growth in Homo did not occur in this species.
Watch the news for more exciting updates about this early species of our genus. Paleotontolgists researching the cave site estimate that there are hundreds if not thousands of fossilized bones still remaining in the cave. There are sure to be many more discoveries reported in the news media about this extinct Homo species.
Review
1. Outline how humans are classified. Name their taxa, starting with the kingdom and ending with the species.
2. List several primate traits. Explain how they are related to life in the trees.
3. What are hominids? Describe how living hominids are classified.
4. Discuss species in the genus Homo.
5. Relate climatic changes to the evolution of the genus Homo within the last million years.
6. What is the significance of the fact that we share 93 to 99 percent of our DNA sequence with other primates?
7. Which species do you think we are more likely to share a greater amount of DNA sequence with — nonprimate mammals or nonmammalian chordates? Explain your answer.
8. What is the relationship between shared DNA and shared traits?
9. Compared to other mammals, primates have a relatively small area of their brain dedicated to olfactory processing. What does this tell you about the sense of smell in primates compared to other mammals? Why?
10. The part of the brain in primates that is specially enlarged is the:
1. cerebrum
2. cerebellum
3. clavicle
4. brainstem
11. Why do you think it is interesting that nonhuman primates can use tools?
12. True or False. All primates are primarily quadrupedal.
13. True or False. Homo erectus was in the same family as modern humans.
14. True or False. Humans are superior in all ways to other primates.
15. Explain why the discovery of Homo naledi was exciting.
Attributions
1. Child and monkey, public domain via piqsels
2. Human taxonomy by Suzanne Wakim dedicated to the public domain is based on biological classification by Peter Halasz, public domain via Wikimedia Commons
3. White-fronted Capuchi Monkey by WolfmanSF, licensed CC BY 2.5 via Wikimedia Commons
4. Non-human primate range by Jackhynes dedicated to the public domain via Wikimedia Commons
5. Orangutan mother and baby by Bonnie U. Gruenberg, CC BY-SA 3.0 via Wikimedia Commons
6. Homo erectus by Ryan Somma, CC BY-SA 2.0 via Flickr
7. Foot of Homo naledi by W. E. H. Harcourt-Smith, Z. Throckmorton, K. A. Congdon, B. Zipfel, A. S. Deane, M. S. M. Drapeau, S. E. Churchill, L. R. Berger & J. M. DeSilva,
8. Licensed CC BY 4.0 via Wikimedia Commons
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/02%3A_Introduction_to_Human_Biology/2.4%3A_The_Human_Animal.txt |
Case Study Conclusion: Our Invisible Inhabitants
As you may recall, Wajiha’s strep throat was caused by Streptococcus pyogenes bacteria, the species shown in the photomicrograph above. Wajiha took antibiotics to kill the S. pyogenes, but this also killed her “good” bacteria, throwing off the balance of microorganisms living inside of her, which resulted in diarrhea and a yeast infection.
After reading this chapter, you should now know that microorganisms such as the bacteria and yeast that live in humans are also similar to us in many ways. They are living organisms and therefore share the traits of homeostasis, organization, metabolism, growth, adaptation, response to stimuli, and reproduction with us. They, like us, contain genes, consist of cells, and have the ability to evolve. Wajiha’s beneficial gut bacteria help digest her food as part of her metabolic processes. Wajiha got a yeast infection likely because the growth and reproductive rates of the yeast living on her body were not held in check by beneficial bacteria after she took antibiotics. You can see that there are many ways in which an understanding of the basic characteristics of life can directly apply to your own.
You also learned how living organisms are classified, from bacteria that are in the Bacteria domain, to yeast (fungus kingdom) and humans (animal kingdom) that are both in the Eukarya domain. You probably now recognize that Streptococcus pyogenes is the binomial nomenclature for this species and the fact that Streptococcus refers to the genus name.
As Wajiha’s doctor told her, there are many different species of microorganisms living in the human digestive system. You should recognize this as a type of biodiversity called species diversity. This diversity is maintained in a balance, or homeostasis, that can be upset when one type of organism is killed — for instance, by antibiotics.
Wajiha’s doctor advised her to complete the entire course of antibiotics because stopping too early would kill the bacteria that are most susceptible to the antibiotic while leaving the bacteria that are more resistant to the antibiotic alive. This difference in susceptibility to antibiotics is an example of genetic diversity. Over time, the surviving antibiotic-resistant bacteria will have increased survival and reproductive rates compared to the more susceptible bacteria, and the trait of antibiotic resistance will become more common in the population. In this way, the bacteria can evolve and become better adapted to their environment — at a major cost to our health because our antibiotics will no longer be effective. This issue of improper use of antibiotics leading to increased antibiotic resistance is a major concern of public health experts.
After reading the last section of this chapter, you know how humans are classified and some characteristics of humans and our near relatives. Beyond our more obvious features of big brains, intelligence, and the ability to walk upright, we also serve as a home to many different organisms that may be invisible to the naked eye but play a big role in maintaining our health.
Chapter Summary
In this chapter, you learned about the basic principles of biology and how humans are situated among other living organisms. Specifically, you learned:
• To be classified as a living thing, most scientists agree that an object must exhibit seven characteristics, including:
• Maintaining a more-or-less constant internal environment, which is called homeostasis.
• Having multiple levels of the organization and consisting of one or more cells.
• Using energy and being capable of metabolism.
• The ability to grow and develop.
• The ability to evolve adaptations to the environment.
• The ability to detect and respond to environmental stimuli.
• The ability to reproduce, which is the process by which living things give rise to offspring.
• Biodiversity refers to the variety of life that exists on Earth. It includes species diversity, genetic diversity within species, and ecosystem diversity.
• The formal biological definition of species is a group of actually or potentially interbreeding organisms. In reality, organisms are often classified into species on the basis of morphology.
• A system for classifying living things was introduced by Linnaeus in the 1700s. It includes taxa from the species (least inclusive) to the kingdom (most inclusive). Linnaeus also introduced a system of naming species, called binomial nomenclature.
• The domain, a taxon higher than the kingdom, was later added to the Linnaean system. Living things are generally grouped into three domains: Bacteria, Archaea, and Eukarya. Humans and other animal species are placed in the Eukarya domain.
• Modern systems of classification take into account phylogenies, or evolutionary histories of related organisms, rather than just morphological similarities and differences. These relationships are often represented by phylogenetic trees or other tree-like diagrams.
• The human species, Homo sapiens, is placed in the primate order of the class of mammals, which are chordates in the animal kingdom.
• Traits humans share with other primates include five digits with nails and opposable thumbs; an excellent sense of vision including the ability to see in color and stereoscopic vision; a large brain, high degree of intelligence, and complex behaviors. Like most other primates, we also live in social groups. Many of our primate traits are adaptations to life in the trees.
• Within the primate order, our species is placed in the hominid family, which also includes chimpanzees, gorillas, and orangutans.
• The genus Homo first evolved about 2.8 million years ago. Early Homo species were fully bipedal but had small brains. All are now extinct.
• During the last 800,000 years, Homo sapiens evolved, with smaller faces, jaws, and front teeth but much bigger brains than earlier Homo species.
Now that you understand the basic principles of biology and some of the characteristics of living organisms, in the next chapter, you will learn about the molecules that make up living organisms and the chemistry that allows organisms to exist and function.
Chapter Summary Review
1. What are the seven traits of life?
2. A scientist is exploring in a remote area with many unidentified species. They find an unknown object that does not appear to be living. What is one way they could tell whether it is a dead organism that was once alive, versus an inanimate object that was never living?
3. Cows are dependent on bacteria living in their digestive systems to help break down cellulose in the plant material that the cows eat. Explain what characteristics these bacteria must have to be considered living organisms themselves, and not just part of the cow.
4. What is the basic unit of structure and function in living things?
5. Give one example of homeostasis that occurs in humans.
6. Can a living thing exist without using energy? Why or why not?
7. True or False. Evolution is a change in the characteristics of living things over time.
8. True or False. Only some living things have genes.
9. Give an example of a response to stimuli that occurs in a unicellular organism.
10. A scientist discovers two types of similar-looking insects that have not been previously identified. Answer the following questions about this discovery.
1. What is one way they can try to determine whether the two types are the same species?
2. If they are not the same species, what are some ways they can try to determine how closely related they are to each other?
3. What is the name for a type of diagram they can create to demonstrate their evolutionary relationship to each other and to other insects?
4. If they determine that the two types are different species but the same genus, create your own names for them using binomial nomenclature. You can be creative and make up the genus and species names, but be sure to put them in the format of binomial nomenclature.
5. If they are the same species but have different colors, what kind of biodiversity does this most likely reflect?
6. If they are the same species, but one type of insect has a better sense of smell for their limited food source than the other type, what do you think will happen over time? Assume the insects will experience natural selection.
11. Put the following taxa in order from the most specific to the most inclusive: phylum; species; kingdom; genus; family; domain; class; order
12. Humans are in the which domain?
13. Monkeys, apes, and humans are all in the:
1. Same genus
2. Same order
3. Same class
4. Both B and C
14. Amphibians, such as frogs, have a backbone but no hair. What is the most specific taxon that they share with humans?
15. Arboreal means:
1. Living on the ground
2. Living in the ocean
3. Living in trees
4. Living on grasslands
16. What is one characteristic of extinct Homo species that was larger than that of modern humans?
17. What is one characteristic of modern humans that is larger than that of extinct Homospecies?
18. True or False. Most primates live in social groups.
19. True or False. Most other mammals have longer lifespans than primates.
20. True or False. Archaea are classified into the Bacteria domain.
21. How is the long period of dependency of infants on adults in primates related to learning?
22. Name one type of primate in the hominid family, other than humans.
23. Why do you think that scientists compare the bones of structures (such as the feet) of extinct Homo species to ours?
24. Some mammals other than primates also have their eyes placed in the front of their face, such as cats. How do you think the vision of a cat compares to that of a mouse, where the eyes are placed more at the sides?
25. Living sponges are animals. Are we in the same kingdom as sponges? Explain your answer.
Attributions
1. Streptococcus pyogenes by CDC, public domain via Wikimedia Commons | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/02%3A_Introduction_to_Human_Biology/2.5%3A_Case_Study_Conclusion%3A__Inhabitants_and_Chapter_Summary.txt |
This chapter provides the chemistry background needed to understand the human body, its functions, and its processes. The chapter describes biochemical compounds and reactions as well as the significance of water to life.
• 3.1: Case Study: Chemistry and Your Life
Mohinder is a college student who has watched his father suffer from complications of type 2 diabetes over the past few years.
• 3.2: Elements and Compounds
An element is a pure substance. It cannot be broken down into other types of substances. Each element is made up of just one type of atom.
• 3.3: Chemical Bonding
When you think of bonding, you may not think of ions. Like most of us, you probably think of bonding between people. Like people, molecules bond -- and some bonds are stronger than others.
• 3.4: Biochemical Compounds
Biochemical compounds make up the cells and other structures of organisms and carry out life processes. Carbon is the basis of all biochemical compounds, so carbon is essential to life on Earth. Contrary to popular belief, carbohydrates are an important part of a healthy diet. They are also one of four major classes of biochemical compounds.
• 3.5: Carbohydrates
Carbohydrates are the most common class of biochemical compounds. They include sugars and starches. Carbohydrates are used to provide or store energy, among other uses. Like most biochemical compounds, carbohydrates are built of small repeating units, or monomers, which form bonds with each other to make larger molecules, called polymers. In the case of carbohydrates, the small repeating units are known as monosaccharides.
• 3.6: Lipids
Lipids are a major class of biochemical compounds that includes oils and fats. Organisms use lipids to store energy and for many other uses. Lipid molecules consist mainly of repeating units called fatty acids. There are two types of fatty acids: saturated and unsaturated. Both types consist mainly of simple chains of carbon atoms bonded to one another and to hydrogen atoms.
• 3.7: Proteins
Protein shakes are popular with people who want to build muscle because muscle tissue consists mainly of protein. Proteins are one of the four major Macromolecules.
• 3.8: Nucleic Acids
DNA and RNA are polynucleotides and categorized under Nucleic acids, a type of Macromolecule. They are built of small monomers called nucleotides.
• 3.9: Energy in Chemical Reactions
These old iron chains give off a small amount of heat as they rust. The rusting of iron is a chemical process. It occurs when iron and oxygen go through a chemical reaction similar to burning, or combustion.
• 3.10: Chemical Reactions in Living Things
We stay alive because millions of different chemical reactions are taking place inside our bodies all the time.
• 3.11: Biochemical Properties of Water
It's often called the "water planet," or "the blue marble." You probably just call it "home." Almost three-quarters of our home planet is covered by water. Water, like carbon, has a special role in living things.
• 3.12: Acids and Bases
Strong acids can hurt you if they come into contact with your skin or eyes. Therefore, it may surprise you to learn that your life depends on acids.
• 3.13: Case Study Conclusion: Diet and Chapter Summary
After reading this chapter, you should be able to see numerous connections between chemistry, human life, and health.
Thumbnail: 3D model of L-tryptophan. (Public Domain; Benjah-bmm27).
03: Chemistry of Life
Case Study: Diet Dilemma
Mohinder is a college student who has watched their father suffer from complications of type 2 diabetes over the past few years. Mohinder likes to use gender-neutral pronouns, such as they, them, and their's. In type 2 diabetes, the hormone insulin does not transmit its signal sufficiently. Because insulin normally removes sugar from the bloodstream and brings it into the body’s cells, diabetes causes blood sugar levels to not be regulated properly. This can cause damage to the cells of the body.
Diabetes can be treated with insulin injections, as shown above, as well as dietary modifications, but sometimes complications can still occur. Mohinder’s father has some nerve damage, or neuropathy, in his feet due to his diabetes. This made his feet numb and so he didn’t notice when he got minor injuries to his feet, which led to some serious infections.
Mohinder is obese and knows that their weight plus a family history of diabetes increases their risk of getting diabetes themselves. They want to avoid the health issues that their father has suffered. Mohinder begins walking every day for exercise and starts to lose some weight. They also want to improve their diet in order to lose more weight, lower their risk of diabetes, and improve their general health, but they are overwhelmed with all the different dietary advice they read online and hear from their friends and family.
Mohinder's father tells them to limit refined carbohydrates, such as white bread and rice because that is what he does to help keep his blood sugar at an acceptable level. But Mohinder’s friend tells them that eating a diet high in carbohydrates and low in fat is a good way to lose weight. Mohinder reads online that “eating clean” by eating whole, unprocessed foods and avoiding food with “chemicals” can help with weight loss. One piece of advice that everyone seems to agree on is that drinking enough water is good for overall health.
All of this dietary advice may sound confusing, but you can better understand health conditions such as diabetes and the role of diet and nutrition by understanding chemistry. Chemistry is so much more than reactions in test tubes in a lab — it is the atoms, molecules, and reactions that make us who we are and keep us alive and functioning properly. Our diets are one of the main ways our bodies take in raw materials that are needed for the important chemical reactions that take place inside of us.
Chapter Overview: Chemistry of Life
As you read this chapter, you will learn more about how chemistry relates to our lives, health, and the foods we eat. Specifically, you will learn:
• The nature of chemical substances, including elements and compounds and their component atoms and molecules.
• The types and mechanisms of the formation of chemical bonds.
• The structures and functions of biochemical compounds including carbohydrates, lipids, proteins, and nucleic acids such as DNA and RNA.
• What chemical reactions are, how energy is involved in chemical reactions, how enzymes assist in chemical reactions, and what some types of biochemical reactions in living organisms are.
• Properties of water and the importance of water for most biochemical processes.
• What pH is and why maintaining a proper pH in the body is important for biochemical reactions.
As you read the chapter, think about the following questions regarding Mohinder’s situation and how diabetes and diet relate to the chemistry of life.
1. Why do you think Mohinder’s father having diabetes increases his risk of getting diabetes?
2. What is the difference between refined (simple) carbohydrates and complex carbohydrates? Why are refined carbohydrates particularly problematic for people with diabetes?
3. Insulin is a peptide hormone. In which class of biochemical compounds would you categorize insulin?
4. Why is drinking enough water important for overall health? Can you drink too much water?
5. Sometimes “eating clean” is described as avoiding “chemicals” in food. Think about the definition of “chemicals” and how it relates to what we eat.
Attributions
1. Diabetes Detritus by Alden Chadwick, licensed CC BY 2.0 via Flickr
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.01%3A_Case_Study%3A_Chemistry_and_Your_Life.txt |
What Are You Made of?
If you look at your hand, what do you see? Of course, you see skin, which consists of cells. But what are skin cells made of? Like all living cells, they are made of matter. In fact, all things are made of matter. Matter is anything that takes up space and has mass. Matter, in turn, is made up of chemical substances. A chemical substance is a matter that has a definite composition and the same composition throughout. A chemical substance may be either an element or a compound.
Elements and Atoms
An element is a pure substance. It cannot be broken down into other types of substances. Each element is made up of just one type of atom.
Periodic Table of the Elements
There are almost 120 known elements. As you can see in the Periodic Table of the Elements shown in Figure \(3\), the majority of elements are metals. Examples of metals are iron (Fe) and copper (Cu). Metals are shiny and good conductors of electricity and heat. Nonmetal elements are far fewer in number. They include hydrogen (H) and oxygen (O). They lack the properties of metals. The element most important to life is Carbon (C). Find carbon in the table. What type of element is it, metal or nonmetal?
Structure of an Atom
An atom is the smallest particle of an element that still has the properties of that element. Every substance is composed of atoms. Atoms are extremely small, typically about a ten-billionth of a meter in diameter. However, atoms do not have well-defined boundaries, as suggested by the atomic model shown in figure \(2\). An atom is composed of many subatomic particles. We will only discuss protons, neutrons, and electrons.
Table \(1\): Subatomic Particles
Particle Proton Neutron Electron
Electric Charge +1 0 -1
Location Nucleus Nucleus Outside the nucleus
Mass 1 amu 1 amu ~0 amu
If the number of protons and electrons in an atom are equal, then an atom is electrically neutral because the positive and negative charges cancel out. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion.
The negatively charged electrons of an atom are attracted to the positively charged protons in the nucleus by a force called electromagnetic force, for which opposite charges attract. Electromagnetic force between protons in the nucleus causes these subatomic particles to repel each other because they have the same charge. However, the protons and neutrons in the nucleus are attracted to each other by a different force, called nuclear force, which is usually stronger than the electromagnetic force repelling the positively charged protons from each other.
Compounds and Molecules
A compound is a unique substance that consists of two or more elements combined in fixed proportions. This means that the composition of a compound is always the same. The smallest particle of most compounds in living things is called a molecule. Consider water as an example. A molecule of water always contains one atom of oxygen and two atoms of hydrogen. The composition of water is expressed by the chemical formula H2O. A model of a water molecule is shown in Figure \(4\). Notice that molecules can be drawn in different ways, but represent the same molecule. In this case, a molecule made of one oxygen and two hydrogens.
What causes the atoms of a water molecule to “stick” together? The answer is chemical bonds. A chemical bond is a force that holds together the atoms of molecules. Bonds in molecules involve atoms sharing electrons. New chemical bonds form when substances react with one another.
Review
1. What is an element? Give three examples.
2. Define compound. Explain how compounds form.
3. Compare and contrast atoms and molecules.
4. The compound called water can be broken down into its constituent elements by applying an electric current to it. What ratio of elements is produced in this process?
5. Relate ions to elements and atoms.
6. What is the most important element of life?
7. Iron oxide is often known as rust — the reddish substance you might find on corroded metal. The chemical formula for this type of iron oxide is Fe2O3. Answer the following questions about iron oxide and briefly explain each answer.
1. Is iron oxide an element or a compound?
2. Would one particle of iron oxide be considered a molecule or an atom?
3. Describe the relative proportion of atoms in iron oxide.
4. What causes the Fe and O to stick together in iron oxide?
5. Is iron oxide made of metal atoms, metalloid atoms, nonmetal atoms, or a combination of any of these?
8. Explain why ions have a positive or negative charge.
9. Name the three subatomic particles described in this section.
Explore More
Watch the video below to learn more about the size of an atom.
Watch the video below to learn about four newly discovered elements.
Attributions
1. Diversity and unity by Frerieke, licensed CC BY 2.0 via Wikimedia Commons
2. Carbon atom licensed CC BY-SA 3.0 via Ascension Glossary
3. Periodic table by Dmarcus100, CC BY-SA 4.0 via Wikimedia Commons
4. Water molecule by Booyabazooka, released into the public domain via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.02%3A_Elements_and_Compounds.txt |
Chemical Bonding
When you think of bonding, you may not think of ions. Like most of us, you probably think of bonding between people. Like people, molecules bond — and some bonds are stronger than others. It's hard to break up a mother and baby, or a molecule made up of one oxygen and two hydrogen atoms! A chemical bond is a force of attraction between atoms or ions. Bonds form when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom that may be involved in chemical interactions. Valence electrons are the basis of all chemical bonds.
Why Bonds Form
To understand why chemical bonds form, consider the common compound known as water, or H2O. It consists of two hydrogen (H) atoms and one oxygen (O) atom. As you can see in the on the left side of the Figure \(2\) below, each hydrogen atom has just one electron, which is also its sole valence electron. The oxygen atom has six valence electrons. These are the electrons in the outer energy level of the oxygen atom.
In the water molecule on the right in Figure \(2\), each hydrogen atom shares a pair of electrons with the oxygen atom. By sharing electrons, each atom has electrons available to fill its sole or outer energy level. The hydrogen atoms each have a pair of shared electrons, so their first and only energy level is full. The oxygen atom has a total of eight valence electrons, so its outer energy level is full. A full outer energy level is the most stable possible arrangement of electrons. It explains why elements form chemical bonds with each other.
Types of Chemical Bonds
Not all chemical bonds form in the same way as the bonds in water. There are actually four different types of chemical bonds that we will discuss here are non-polar covalent, polar covalent, hydrogen, and ionic bonding. Each type of bond is described below.
Non-polar Covalent Bonds
For methane (CH4) in Figure \(3\), the carbon atom (with four electrons in its outermost valence energy shell) shares a single electron from each of the four hydrogens. Hydrogen has one valence electron in its first energy shell. Covalent bonding is prevalent in organic compounds. In fact, your body is held together by electrons shared by carbons and hydrogens! The electrons are equally shared in all directions; therefore, this type of covalent bond is referred to as non-polar.
Polar Covalent Bonds and Hydrogen Bonds
A covalent bond is the force of attraction that holds together two nonmetal atoms that share a pair of electrons. One electron is provided by each atom, and the pair of electrons is attracted to the positive nuclei of both atoms. The water molecule represented in Figure \(4\) contains polar covalent bonds.
The attractive force between water molecules is a dipole interaction. The hydrogen atoms are bound to the highly electronegative oxygen atom (which also possesses two lone pair sets of electrons, making for a very polar bond. The partially positive hydrogen atom of one molecule is then attracted to the partially negative oxygen atom of a nearby water molecule as shown in Figure \(4\) ).
A hydrogen bond is an intermolecular and intramolecular attractive force in which a hydrogen atom that is covalently bonded to a highly electronegative atom is attracted to a lone pair of electrons on an atom or a partially negative atom in a neighboring polar molecule. Hydrogen bonds are also found intramolecularly in the tertiary and quaternary structures of protein and DNA strands.
Hydrogen bonding occurs only in molecules where hydrogen is covalently bonded to one of three elements: fluorine, oxygen, or nitrogen. These three elements are so electronegative that they withdraw the majority of the electron density in the covalent bond with hydrogen, leaving the H atom very electron-deficient. The H atom nearly acts as a bare proton, leaving it very attracted to lone pair electrons on a nearby atom.
The hydrogen bonding that occurs in water leads to some unusual, but very important properties. Most molecular compounds that have a mass similar to water are gases at room temperature. Because of the strong hydrogen bonds, water molecules are able to stay condensed in the liquid state. Figure \(5\) shows how the bent shape and two hydrogen atoms per molecule allow each water molecule to be able to hydrogen bond to two other molecules.
In the liquid state, the hydrogen bonds of water can break and reform as the molecules flow from one place to another. When water is cooled, the molecules begin to slow down. Eventually, when water is frozen to ice, the hydrogen bonds form a very specific network shown on the right side of Figure \(6\). When water is liquid, the molecules are more motile and don't produce this rigid structure.
Ionic bonds
Electrons are transferred between atoms. An ion will give one or more electrons to another ion. Table salt, sodium chloride (NaCl), is a common example of an ionic compound. Note that sodium is on the left side of the periodic table and that chlorine is on the right side of the periodic table. In Figure \(7\), an atom of lithium donates an electron to an atom of fluorine to form an ionic compound. This happens to full fill their outermost valence shell. The transfer of the electron gives the lithium ion a net charge of +1, and the fluorine ion a net charge of -1. These ions bond because they experience an attractive force due to the difference in sign of their charges.
Review
1. How is a covalent bond different from an ionic bond?
2. Why is a hydrogen bond a relatively weak bond?
3. Diagram the polarity of a water molecule.
4. What is a chemical bond?
5. Explain why hydrogen and oxygen atoms are more stable when they form bonds in a water molecule.
6. How many valence electrons does sodium have? How many valence electrons does chlorine have? How does a chlorine atom bonds with sodium? What is the charge on a sodium ion? What about the chlorine ion?
7. When does covalent bonding occur? How does it work?
8. How many valence electrons does oxygen have?
Attributions
1. Mother & daughter by Lyd235, CC BY-SA 4.0 via Wikimedia Commons
2. Water molecule by CNX OpenStax, licensed CC BY 4.0 via Wikimedia Commons
3. Covalent bond by DynaBlast, licensed CC BY-SA 2.5 via Wikimedia Commons
4. Hydrogen bonding in water, public domain via Wikimedia Commons
5. 3D model hydrogen bonds by Michal Maňas, public domain via Wikimedia Commons
6. Liquid water and ice by P99am, CC BY-SA 3.0 via Wikimedia Commons
7. NaF by Wdcf, CC BY-SA 3.0 via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.03%3A_Chemical_Bonding.txt |
Carbs Galore
What do all these foods have in common? All of them consist mainly of large compounds called carbohydrates, often referred to as "carbs." Contrary to popular belief, carbohydrates are an important part of a healthy diet. They are also one of four major classes of biochemical compounds.
Chemical Compounds in Living Things
The compounds found in living things are known as biochemical compounds. Biochemical compounds make up the cells and other structures of organisms and carry out life processes. Carbon is the basis of all biochemical compounds, so carbon is essential to life on Earth. Without carbon, life as we know it could not exist.
Why is carbon so basic to life? The reason is carbon’s ability to form stable bonds with many elements, including itself. This property allows carbon to form a huge variety of very large and complex molecules. In fact, there are nearly 10 million carbon-based compounds in living things!
Most biochemical compounds are very large molecules called polymers. A polymer is built of repeating units of smaller compounds called monomers. Monomers are like the individual beads on a string of beads, and the whole string is the polymer. The strings of beads pictured below are simple models of polymers in biochemical compounds.
Classes of Biochemical Compounds
Although there are millions of different biochemical compounds in Earth's living things, all biochemical compounds contain the elements carbon, hydrogen, and oxygen. Some contain only these elements; others contain additional elements as well. The vast number of biochemical compounds can be grouped into just four major classes: carbohydrates, lipids, proteins, and nucleic acids.
Carbohydrates
Carbohydrates include sugars and starches. These compounds contain only the elements carbon, hydrogen, and oxygen. Functions of carbohydrates in living things include providing energy to cells, storing energy, and forming certain structures, such as the cell walls of plants. The monomer that makes up large carbohydrate compounds is called a monosaccharide. The sugar glucose, represented by the chemical model below, is a monosaccharide. It contains six carbon atoms (C) and several atoms of hydrogen (H) and oxygen (O). Thousands of glucose molecules can join together to form a polysaccharide such as starch.
Lipids
Lipids include fats and oils. They contain primarily the elements carbon, hydrogen, and oxygen, although some lipids contain additional elements such as phosphorus. Functions of lipids in living things include storing energy, forming cell membranes, and carrying messages. Lipids consist of repeating units that join together to form chains called fatty acids. Most naturally occurring fatty acids have an unbranched chain of an even number (generally from 4 to 28) of carbon atoms.
Proteins
Proteins include enzymes, antibodies, and many other important compounds in living things. They contain the elements carbon, hydrogen, oxygen, nitrogen, and sulfur. The functions of proteins are very numerous. They include helping cells keep their shape, making up muscles, speeding up chemical reactions, and carrying messages and materials. The monomers that make up large protein compounds are called amino acids. There are 23 different amino acids that combine into long chains (called polypeptides) to form the building blocks of a vast array of proteins in living things.
Nucleic Acids
Nucleic acids include the molecules DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). They contain the elements carbon, hydrogen, oxygen, nitrogen, and phosphorus. Their functions in living things are to encode instructions for making proteins, to help make proteins, and to pass the instructions from parents to offspring. The monomer that makes up nucleic acids is the nucleotide. All nucleotides are the same except for a component called a nitrogen base. There are four different nitrogen bases, and each nucleotide contains one of these four bases. The sequence of nitrogen bases in the chains of nucleotides in DNA and RNA makes up the code for protein synthesis, called the genetic code. The animation below represents the very complex structure of DNA, which consists of two chains of nucleotides.
Review
1. Why is carbon so important to life on Earth?
2. What are the biochemical compounds?
3. Describe the diversity of biochemical compounds, and explain how they are classified.
4. Identify two types of carbohydrates. What are the main functions of this class of biochemical compounds?
5. What roles are played by lipids in living things?
6. The enzyme amylase is found in saliva. It helps break down starches in foods into simpler sugar molecules. What type of biochemical compound do you think amylase is?
7. Explain how DNA and RNA contain the genetic code.
8. What are the three elements present in every class of biochemical compound?
9. For each of the following terms (nucleic acid; amino acid; monosaccharide; protein; nucleotide; polysaccharide)
1. Determine whether it is a monomer or a polymer.
2. Match each monomer with its correct polymer.
3. Identify which class of biochemical compound is represented by each monomer/polymer pair.
10. Is glucose a monomer or a polymer? Explain your answer.
11. What is one element contained in proteins and nucleic acids, but not in carbohydrates?
12. Describe the relationship between proteins and nucleic acids.
13. Why do you think it is important to eat a diet that contains a balance of carbohydrates, proteins, and fats?
Explore More
The video below discusses the importance of the element carbon.
Watch the video below to learn more about polymers and monomers.
Attributions
1. Grain products by Scott Bauer USDA, public domain via Wikimedia Commons
2. Fio de conta by Toluaye, released into the public domain via Wikimedia Commons
3. Glucose by Ben; Yikrazuul, public domain via Wikimedia Commons
4. DNA cropped by Spiffistan, released into the public domain via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.04%3A_Biochemical_Compounds.txt |
The Cellulose of Our Lives
Where would we be without our jeans? They have been the go-to pants for many people for decades, and they are still as popular as ever. Jeans are made of denim, a type of cotton fabric. Cotton is a soft, fluffy fiber that grows in a protective case around the seeds of cotton plants. The fiber is almost pure cellulose. Cellulose is the single most abundant biochemical compound found in Earth's living things and one of several types of carbohydrates.
What Are Carbohydrates?
Carbohydrates are the most common class of biochemical compounds. They include sugars and starches. Carbohydrates are used to provide or store energy, among other uses. Like most biochemical compounds, carbohydrates are built of small repeating units, or monomers, which form bonds with each other to make larger molecules, called polymers. In the case of carbohydrates, the small repeating units are known as monosaccharides. Each monosaccharide consists of six carbon atoms, as shown in the model of the monosaccharide glucose below.
Sugars
Sugars are the general name for sweet, short-chain, soluble carbohydrates, which are found in many foods. Their function in living things is to provide energy. The simplest sugars consist of a single monosaccharide. They include glucose, fructose, and galactose. Glucose is a simple sugar that is used for energy by the cells of living things. Fructose is a simple sugar found in fruits, and galactose is a simple sugar found in milk.
Other sugars contain two monosaccharide molecules and are called disaccharides. An example is sucrose or table sugar. It is composed of one fructose molecule and one glucose molecule. Other disaccharides include maltose (two glucose molecules) and lactose (one glucose molecule and one galactose molecule). Lactose occurs naturally in milk. Some people can't digest lactose. If they drink milk, it causes gas, cramps, and other unpleasant symptoms unless the milk has been processed to remove the lactose.
Complex Carbohydrates
The simple sugars form the foundation of more complex carbohydrates. The cyclic forms of two sugars can be linked together by means of a condensation reaction. The figure below shows how a glucose molecule and a fructose molecule combine to form a sucrose molecule. A hydrogen atom from one molecule and a hydroxyl group from the other molecule are eliminated as water, with a resulting covalent bond linking the two sugars together at that point.
Glucose and fructose combine to produce the disaccharide sucrose in a condensation reaction as shown in Figure \(3\). Sucrose, commonly known as table sugar, is an example of a disaccharide.
A disaccharide is a carbohydrate formed by the joining of two monosaccharides. Other common disaccharides include lactose and maltose. Lactose, a component of milk, is formed from glucose and galactose, while maltose formed from two glucose molecules. During digestion, these disaccharides are hydrolyzed in the small intestine to form the component monosaccharides, which are then absorbed across the intestinal wall and into the bloodstream to be transported to the cells.
Some carbohydrates consist of hundreds or even thousands of monosaccharides bonded together in long chains. These carbohydrates are called polysaccharides ("many saccharides"). Polysaccharides are also referred to as complex carbohydrates. Complex carbohydrates that are found in living things include starch, glycogen, cellulose, and chitin. Each type of complex carbohydrate has different functions in living organisms but they generally either store energy or make up certain structures of living things.
Starch
Starch is a complex carbohydrate that is made by plants to store energy. For example, the potatoes pictured below are packed full of starches that consist mainly of repeating units of glucose and other simple sugars. The leaves of potato plants make sugars by photosynthesis, and the sugars are carried to underground tubers where they are stored as starch. When we eat starchy foods such as potatoes, the starches are broken down by our digestive system to sugars, which provide our cells with energy. Starches are easily and quickly digested with the help of digestive enzymes such as amylase, which is found in the saliva. If you chew a starchy saltine cracker for several minutes, you may start to taste the sugars released as the starch is digested.
Glycogen
Animals do not store energy as starch. Instead, animals store the extra energy as the complex carbohydrate glycogen. Glycogen is a polysaccharide of glucose. It serves as a form of energy storage in fungi as well as animals and is the main storage form of glucose in the human body. In humans, glycogen is made and stored primarily in the cells of the liver and the muscles. When energy is needed from either storage depot, the glycogen is broken down to glucose for use by cells. Muscle glycogen is converted to glucose for use by muscle cells, and liver glycogen is converted to glucose for use throughout the rest of the body. Glycogen forms an energy reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of lipids, which are the primary form of energy storage in animals.
Glycogen plays a critical part in the homeostasis of glucose levels in the blood. When blood glucose levels rise too high, excess glucose can be stored in the liver by converting it to glycogen. When glucose levels in the blood fall too low, glycogen in the liver can be broken down into glucose and released into the blood.
Cellulose
Cellulose is a polysaccharide consisting of a linear chain of several hundred to many thousands of linked glucose units. Cellulose is an important structural component of the cell walls of plants and many algae. Human uses of cellulose include the production of cardboard and paper, which consist mostly of cellulose from wood and cotton. The cotton fibers pictured below are about 90 percent cellulose.
Certain animals, including termites and ruminants such as cows, can digest cellulose with the help of microorganisms that live in their gut. Humans cannot digest cellulose, but it nonetheless plays an important role in our diet. It acts as a water-attracting bulking agent for feces in the digestive tract and is often referred to as "dietary fiber."
Chitin
Chitin is a long-chain polymer of a derivative of glucose. It is found in many living things. For example, it is a component of the cell walls of fungi, the exoskeletons of arthropods such as crustaceans and insects (including the beetle pictured in Figure \(7\)), and the beaks and internal shells of animals such as squids and octopuses. The structure of chitin is similar to that of cellulose.
Feature: My Human Biology
You probably know that you should eat plenty of fiber, but do you know how much fiber you need, how fiber contributes to good health, or which foods are good sources of fiber? Dietary fiber consists mainly of cellulose, so it is found primarily in plant-based foods, including fruits, vegetables, whole grains, and legumes. Dietary fiber can't be broken down and absorbed by your digestive system. Instead, it passes relatively unchanged through your gastrointestinal tract and is excreted in feces. That's how it helps keep you healthy.
The fiber in food is commonly classified as either soluble or insoluble fiber.
• Soluble fiber dissolves in water to form a gel-like substance as it passes through the gastrointestinal tract. Its health benefits include lowering blood levels of cholesterol and glucose. Good sources of soluble fiber include whole oats, peas, beans, and apples.
• Insoluble fiber does not dissolve in water. This type of fiber increases the bulk of feces in the large intestine and helps keep food wastes moving through, which may help prevent or correct constipation. Good sources of insoluble fiber include whole wheat, wheat bran, beans, and potatoes.
How much fiber do you need for good health? That depends on your age and gender. The Institute of Medicine recommends the daily fiber intake for adults shown in the table below. Most dietitians further recommend a ratio of about 3 parts insoluble fiber to 1 part soluble fiber each day. Most fiber-rich foods contain both types of fiber, so it usually isn't necessary to keep track of the two types of fiber as long as your overall fiber intake is adequate.
Use food labels and online fiber counters to find out how much total fiber you eat in a typical day. Are you consuming enough fiber for good health? If not, consider ways to increase your intake of this important substance. For example, substitute whole grains for refined grains, eat more legumes such as beans, and try to consume at least five servings of fruits and vegetables each day.
Table \(1\): Recommended Daily Fiber Intake for Males and Females
Gender Age 50 or Younger Age 51 or Older
Male 38 grams 30 grams
Female 25 grams 21 grams
Summary
• Carbohydrates are the most common class of biochemical compounds. The basic building block of carbohydrates is the monosaccharide, which consists of six carbon atoms.
• Sugars are sweet, short-chain, soluble carbohydrates that are found in many foods and supply us with energy. Simple sugars, such as glucose, consist of just one monosaccharide. Some sugars, such as sucrose, or table sugar, consist of two monosaccharides and are called disaccharides.
• Complex carbohydrates, or polysaccharides, consist of hundreds or even thousands of monosaccharides. They include starch, glycogen, cellulose, and chitin. They generally either store energy or form structures, such as cell walls, in living things.
• Starch is a complex carbohydrate that is made by plants to store energy. Potatoes are a good food source of dietary starch, which is readily broken down to its component sugars during digestion.
• Glycogen is a complex carbohydrate that is made by animals and fungi to store energy. Glycogen plays a critical part in the homeostasis of blood glucose levels in humans.
• Cellulose is the single most common biochemical compound in living things. It forms the cell walls of plants and certain algae. Like most other animals, humans cannot digest cellulose, but it makes up most of the crucial dietary fiber in the human diet.
• Chitin is a complex carbohydrate, similar to cellulose, that makes up organic structures such as the cell walls of fungi and the exoskeletons of insects and other arthropods.
Review
1. What are carbohydrates? Describe their structure.
2. Compare and contrast sugars and complex carbohydrates.
3. Identify the four main types of complex carbohydrates and their functions.
4. If you chew on a starchy food such as a saltine cracker for several minutes, it may start to taste sweet. Explain why.
5. True or False. Glucose is mainly stored by lipids in the human body.
6. Put the following carbohydrates in order from smallest to largest: cellulose; fructose; sucrose
7. Name three carbohydrates that contain glucose as a monomer.
8. Jeans are made of tough, durable cotton. Explain how you think this fabric gets its tough qualities, based on what you know about the structure of carbohydrates.
9. Which do you think is faster to digest — simple sugars or complex carbohydrates? Explain your answer.
10. True or False. Cellulose is broken down in the human digestive system into glucose molecules.
11. Which type of fiber dissolves in water? Which type does not dissolve in water?
12. What are the similarities and differences between muscle glycogen and liver glycogen?
13. Which carbohydrate is used directly by the cells of living things for energy?
14. Which of the following is not a complex carbohydrate?
1. chitin
2. starch
3. disaccharide
4. none of the above
Explore More
Watch the video below to learn about the health impacts of carbohydrates.
Attributions
1. Body paint by Cuerpos Pintados, licensed CC BY 2.0 via Wikimedia Commons
2. Glucose public domain via Wikimedia Commons
3. Sucrose by Christopher Auyeung and Joy Sheng, CC BY-NC 3.0, via CK-12
4. Potatoes by Elza Fiuza/ABr, licensed CC BY 3.0 via Wikimedia Commons Brazil
1. Cotton by KoS, released into the public domain via Wikimedia Commons
2. Ten-lined June beetle by Junkyardsparkle, dedicated CC0 via Wikimedia Commons
5. Three Polysaccharides by OpenStax College, licensed CC BY 3.0 via Wikimedia Commons Brazil
6. Beans by Charles Brooking, released into the public domain via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.05%3A_Carbohydrates.txt |
Yum!
It glistens with fat, from the cheese to the steak. You may never have visited Philadelphia, but you probably know about its famous gastronomic delight, the Philly cheesesteak, pictured here. Both cheese and steak are typically high-fat foods, so this sandwich is definitely not recommended if you are following a low-fat diet. We need some fats in our diet for good health, but too much of a good thing can be harmful to our health, no matter how good it tastes. What are fats? And why do we have such a love-hate relationship with them? Read on to find out.
Lipids and Fatty Acids
Fats are actually a type of lipid. Lipids are a major class of biochemical compounds that includes oils as well as fats. Organisms use lipids to store energy and for many other uses.
Lipid molecules consist mainly of repeating units called fatty acids. There are two types of fatty acids: saturated fatty acids and unsaturated fatty acids. Both types consist mainly of simple chains of carbon atoms bonded to one another and to hydrogen atoms. The two types of fatty acids differ in how many hydrogen atoms they contain.
Saturated Fatty Acids
In saturated fatty acids, carbon atoms are bonded to as many hydrogen atoms as possible. All the carbon-to-carbon atoms share just single bonds between them. This causes the molecules to form straight chains, as shown in Figure \(2\). The straight chains can be packed together very tightly, allowing them to store energy in a compact form. Saturated fatty acids have relatively high melting points, explaining why they are solids at room temperature. Animals use saturated fatty acids to store energy.
Unsaturated Fatty Acids
In unsaturated fatty acids, some carbon atoms are not bonded to as many hydrogen atoms as possible. Instead, they form double or even triple bonds with other carbon atoms. This causes the chains to bend (see Figure \(2\)). The bent chains cannot be packed together very tightly. Unsaturated fatty acids have relatively low melting points, which explains why they are liquids at room temperature. Plants use unsaturated fatty acids to store energy.
Monounsaturated fatty acids contain one less hydrogen atom than the same-length saturated fatty acid chain. Monounsaturated fatty acids are liquids at room temperature but start to solidify at refrigerator temperatures. Good food sources of monounsaturated fats include olive and peanut oils and avocados.
Polyunsaturated fatty acids contain at least two fewer hydrogen atoms than the same-length saturated fatty acid chain. Polyunsaturated fatty acids are liquids at room temperature and remain in the liquid state in the refrigerator. Good food sources of polyunsaturated fats include safflower and soybean oils and many nuts and seeds.
Types of Lipids
Lipids may consist of fatty acids alone, or they may contain other molecules as well. For example, some lipids contain alcohol or phosphate groups. Types of lipids include triglycerides, phospholipids, and steroids. Each type has different functions in living things.
Triglycerides
Triglycerides are formed by combining a molecule of glycerol with three fatty acid molecules. Glycerol (also called glycerine) is a simple compound known as a sugar alcohol. It is a colorless, odorless liquid that is sweet tasting and nontoxic. Triglycerides are the main constituent of body fat in humans and other animals. They are also found in fats derived from plants. There are many different types of triglycerides, with the main division being between those that contain saturated fatty acids and those that contain unsaturated fatty acids.
In the human bloodstream, triglycerides play an important role in metabolism as energy sources and transporters of dietary fat. They contain more than twice as much energy as carbohydrates, the other major source of energy in the diet. When you eat, your body converts any calories it doesn't need to use right away into triglycerides, which are stored in your fat cells. When you need energy between meals, hormones trigger the release of some of these stored triglycerides back into the bloodstream.
Phospholipids
Phospholipids are a major component of the cell membranes of all living things. Each phospholipid molecule has a "tail" consisting of two long fatty acids and a "head" consisting of a phosphate group and glycerol molecule (see diagram below). The phosphate group is a small negatively charged molecule. The phospholipid head is hydrophilic or attracted to water. The fatty acid tail of the phospholipid is hydrophobic or repelled by water. These properties allow phospholipids to form a two-layer, or bilayer, cell membrane.
As shown in the diagram below, a phospholipid bilayer forms when many phospholipid molecules line up tail to tail, forming an inner and outer surface of hydrophilic heads. The hydrophilic heads point toward both the watery extracellular space and the watery intracellular space (lumen) of the cell.
Steroids
Steroids are lipids with a ring structure. Each steroid has a core of seventeen carbon atoms arranged in four rings of five or six carbons each (see model pictured below). Steroids vary by the other components attached to this four-ring core. Hundreds of steroids are found in plants, animals, and fungi, but most steroids have one of just two principal biological functions: some steroids, such as cholesterol, are important components of cell membranes; many other steroids are hormones, which are messenger molecules. In humans, steroid hormones include cortisone, a fight-or-flight hormone; and the sex hormones estrogen and testosterone.
Feature: My Human Body
During a routine checkup with your family doctor, your blood was collected for a lipid profile. The results are back, and your triglyceride level is 180 mg/dL. Your doctor says this is a little high. A blood triglyceride level of 150 mg/dL or lower is considered normal. Higher levels of triglycerides in the blood have been linked to increased risk of atherosclerosis, heart disease, and stroke.
If a blood test reveals that you have high triglycerides, the levels can be lowered through healthy lifestyle choices and/or prescription medications. Healthy lifestyle choices to control triglyceride levels include:
• losing weight. If you are overweight, losing even 5 or 10 pounds may help lower your triglyceride level.
• cutting back on calories. Extra calories are converted to triglycerides and stored as fat, so reducing your calories should also reduce your triglyceride level.
• avoiding sugary and refined foods. Simple carbohydrates, such as sugars and foods made with white flour, can increase triglyceride levels.
• choosing healthier fats. Trade saturated fats found in animal foods for healthier unsaturated fats found in plants and oily fish. For example, substitute olive oil for butter and salmon for red meat.
• limiting alcohol consumption. Alcohol is high in calories and sugar and has a strong effect on triglyceride levels.
• exercising regularly. Aim for at least 30 minutes of physical activity on most or all days of the week to lower triglyceride levels.
If healthy lifestyle changes aren't enough to bring down high triglyceride levels, drugs prescribed by your doctor are likely to help.
Review
1. What are lipids?
2. Compare and contrast saturated and unsaturated fatty acids.
3. Identify three major types of lipids, and describe differences in their structures.
4. How do triglycerides play an important role in human metabolism?
5. Explain how phospholipids form cell membranes.
6. What is cholesterol, and what is its major function?
7. Give three examples of steroid hormones in humans.
8. Which type of fatty acid do you think is predominant in the steak and cheese of the cheesesteak shown above? Explain your answer.
9. Which type of fat would be the most likely to stay liquid in colder temperatures — bacon fat, olive oil, or soybean oil? Explain your answer.
10. Why do you think that the shape of the different types of fatty acid molecules affects how easily they solidify?
11. High cholesterol levels in the bloodstream can cause negative health effects but explain why we wouldn’t want to get rid of all the cholesterol in our bodies.
12. Name two types of lipids that are part of the cell membrane.
13. True or False. Fatty acids are made up of triglycerides.
14. Which type of lipid often functions as chemical messenger molecules?
Explore More
Watch the video below to learn more about triglycerides and the difference between saturated and unsaturated fatty acids.
Attributions
1. Philly cheesesteak by jeffreyw, licensed CC BY 2.0 via Wikimedia Commons
2. Fatty acids by Mariana Ruiz Villarreal (LadyofHats), CC BY-NC 3.0 for CK-12 foundation
3. Triglyceride by Wolfgang Schaefer, released into the public domain via Wikimedia Commons
4. Phospholipid bilayer by OpenStax, CC BY 4.0 via Wikimedia Commons
5. Steroid by Jynto, dedicated CC0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.06%3A_Lipids.txt |
Protein Shake
Drinks like this shake contain a lot of protein. Such drinks are popular with people who want to build muscle because muscle tissue consists mainly of protein. Making up muscles is just one of a plethora of functions of this amazingly diverse class of biochemicals.
What Are Proteins?
Proteins are organic compounds that contain carbon, hydrogen, oxygen, nitrogen, and, in some cases, sulfur. These compounds have many essential functions within the cell (see below). Proteins are made of smaller units called amino acids. There are 20 different common amino acids needed to make proteins. All amino acids have the same basic structure, which is shown in Figure \(3\). Only the side chain (labeled R in the figure) differs from one amino acid to another. These side chains can vary in size from just one hydrogen atom in glycine to a large heterocyclic group in tryptophan. The variable side chain gives each amino acid unique properties. The side chains can also characterize the amino acid as (1) nonpolar or hydrophobic, (2) neutral (uncharged) but polar, (3) acidic, with a net negative charge, and (4) basic, with a net positive charge at neutral pH.
Proteins can differ from one another in the number and sequence (order) of amino acids. It is because of the side chains of the amino acids that proteins with different amino acid sequences have different shapes and different chemical properties. Small proteins can contain just a few hundred amino acids. Yeast proteins average 466 amino acids. The largest known proteins are the titins, found in muscle, which are composed of over 27,000 amino acids.
Protein Structure
Amino acids join together to form a molecule called a dipeptide. The –OH from the carboxyl group of one amino acid combines with a hydrogen atom from the amino group of the other amino acid to produce water. This is called a condensation reaction - a reaction in which two molecules combine to form a single molecule with a release of water. Figure \(3\)) shows this process. The top part of the image shows two amino acids; note the -OH in amino acid 1 and the -H in amino acid two are highlighted. These are the atoms that will be removed from the amino acids to form water. This allows a covalent bond forms between the carboxyl carbon of one amino acid and the amine nitrogen of the second amino acid. This reaction forms a molecule called a dipeptide and the carbon-nitrogen covalent bond is called a peptide bond. When repeated numerous times, a lengthy molecule called a polypeptide is eventually produced. Very lengthy polypeptides with functional configuration are called proteins.
Proteins may have up to four levels of structure, from primary to quaternary, as described and shown in the diagram below, giving them the potential for tremendous diversity:
• A protein’s primary structure is the sequence of amino acids in its polypeptide chain(s). This sequence of amino acids determines the higher levels of protein structure and is encoded in genes.
• A protein's secondary structure consists of regularly repeating local structures stabilized by hydrogen bonding between the carboxylic and amino groups of the backbone. The most common secondary structures include the alpha-helix and beta-sheet. Because secondary structures are local, many regions of different secondary structures can be present in the same protein molecule.
• A protein's tertiary structure refers to the overall three-dimensional shape of a single protein molecule. It is determined by the spatial relationship of non-covalent and covalent bonds between the "R" groups of distant amino acids in a polypeptide. The tertiary structure is what controls the basic function of the protein.
• Not all proteins have a final, quaternary structure. This is a structure formed by several protein molecules that function together as a single protein complex.
Functions of Proteins
The diversity of protein structures explains how this class of biochemical compounds can play so many important roles in living things. What are the roles of proteins?
• Some proteins have structural functions. They may help cells keep their shape or make up muscle tissues.
• Many proteins are enzymes that speed up chemical reactions in cells. Enzymes are usually highly specific and accelerate only one or a few chemical reactions. Thousands of different biochemical reactions are known to be catalyzed by enzymes, including most of the reactions involved in metabolism. A reaction without an enzyme might take millions of years to complete, whereas, with the proper enzyme, it may take just a few milliseconds!
• Other proteins are antibodies. These are proteins that bind to specific foreign substances, such as proteins on the surface of bacterial cells. This targets the cells for destruction.
• Still, other proteins carry messages or materials. For example, a protein called myoglobin is an oxygen-binding protein found in the muscle tissues of most mammals including humans. You can see a model of the tertiary structure of myoglobin in the figure below.
The chief characteristic of proteins that allows their diverse set of functions is their ability to bind other molecules specifically and tightly. For example, myoglobin can bind specifically and tightly with oxygen. The region of a protein responsible for binding with another molecule is known as the binding site. This site is often a depression on the molecular surface, determined largely by the tertiary structure of the protein.
Protein Consumption, Digestion, and Synthesis
Proteins are necessary for the diets of humans and other animals. We cannot make all the different amino acids we need, so we must obtain some of them from the foods we consume. Through the process of digestion, we break down the proteins in food into free amino acids that can then be used to synthesize our own proteins. Protein synthesis from amino acid monomers takes place in all cells and is controlled by genes. Once new proteins are synthesized, they generally do not last very long before they are degraded and their amino acids are recycled. A protein's lifespan is generally just a day or two in mammalian cells.
Review
1. What are proteins?
2. How do two amino acids combine together to make a dipeptide?
3. Outline the four levels of protein structure.
4. Identify four functions of proteins.
5. Explain why proteins can take on so many different functions in living things.
6. What is the role of proteins in the human diet?
7. Can you have a protein with both an alpha helix and a beta-sheet? Why or why not?
8. If there is a mutation in a gene that causes a different amino acid to be encoded than the one that is usually encoded in that position within the protein, would that affect:
1. The primary structure of the protein? Explain your answer.
2. The higher structures (secondary, tertiary, quaternary) of the protein? Explain your answer.
3. The function of the protein? Explain your answer.
9. What is the region of a protein responsible for binding to another molecule called? Which level/s of protein structure create this region?
10. Arrange the following in order from the smallest to the largest level of organization:
11. peptide; protein; amino acid; polypeptide
12. True or False. You can tell the function of all proteins from their quaternary structure.
13. Explain what the reading means when it says that amino acids are “recycled.”
Watch the video below to learn about how proteomics, the study of proteins, can be used in cancer research.
Attributions
1. Protein Shake by Sandstein, licensed CC BY 3.0 via Wikimedia Commons
2. Amino acid by YassineMrabet, public domain via Wikimedia Commons
3. Peptide formation by YassineMrabet, public domain via Wikimedia Commons
4. Peptide bond by OpenStax, CC BY 3.0 via Wikimedia Commons
5. Myoglobin by AzaToth, public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.07%3A_Proteins.txt |
Who's Who?
Identical twins show clearly the importance of genes in making us who we are. Genes, in turn, would not be possible without nucleic acids.
Nucleic acids are the class of biochemical compounds that includes DNA and RNA. These molecules are built of small monomers called nucleotides. Many nucleotides bind together to form a chain called a polynucleotide. The nucleic acid DNA (deoxyribonucleic acid) consists of two polynucleotide chains. The nucleic acid RNA (ribonucleic acid) consists of just one polynucleotide chain.
Structure of Nucleic Acids
Each nucleotide consists of three smaller molecules:
1. a sugar molecule (the sugar deoxyribose in DNA and the sugar ribose in RNA).
2. a phosphate group.
3. a nitrogenous base.
Nucleotides are connected to form DNA as shown in Figure \(3\). The sugar molecule of one nucleotide binds to the phosphate group of the next nucleotide. These two molecules alternate to form the backbone of the nucleotide chain. The nitrogen bases in a nucleic acid stick out from the backbone. There are four different nitrogenous bases: cytosine, adenine, guanine, and either thymine (in DNA) or uracil (in RNA). In DNA, hydrogen bonds form between bases on the two nucleotide chains and hold the chains together. Each type of base binds with just one other type of base: cytosine always bonds with guanine, and adenine always bonds with thymine. These pairs of bases are called complementary base pairs.
The hydrogen bonding of complementary bases causes DNA molecules automatically to take their well-known shape, called a double helix, which is shown in the animation in Figure \(4\). A double helix is like a spiral staircase. The double helix shape forms naturally and is very strong, making the two polynucleotide chains difficult to break apart.
Roles of Nucleic Acids
The DNA of cells is organized into structures called chromosomes as shown in Figure \(5\). The letters A, T, G, and C stand for the bases adenine, thymine, guanine, and cytosine. The sequence of these four bases in DNA is a code that carries instructions for making proteins. The DNA helix is wrapped around proteins called histones to form nucleosomes. These are then further structured into chromatin and, finally, chromosomes. Human cells have 46 chromosomes; other organisms have different numbers of chromosomes.
DNA makes up genes, and the sequence of bases in DNA makes up the genetic code. Between “starts” and “stops,” the code carries instructions for the correct sequence of amino acids in a protein. The information in DNA is passed from parent cells to daughter cells whenever cells divide. The information in DNA is also passed from parents to offspring when organisms reproduce. This is how inherited characteristics are passed from one generation to the next.
Feature: Human Biology in the News
Look at the Neanderthals in Figure \(6\). The image is an artist's reconstruction of these close human relatives, who seem to have disappeared from Europe some 50,000 years ago. The consensus that Neandertals were brutish and went extinct when overtaken by modern humans is undergoing revision as we learn more about these interesting members of the genus Homo.
Several years ago, scientists were able to extract DNA from fossilized bones of Neanderthals (see Figure \(7\)). When the Neanderthal DNA was compared with modern human DNA, researchers discovered similarities in the DNA of Neanderthals and modern European-derived peoples that suggest modern humans mated with Neanderthals. Some experts now think that Neanderthals didn’t go extinct but were simply incorporated into the much larger population of Homo sapiens.
New research published in Science early in 2016 shows that our inherited Neanderthal DNA may be more than just an interesting curiosity or useful evidence of our evolutionary past. These bits of DNA may actually be affecting our health today. In the research reported in Science, scientists looked for Neanderthal DNA sequences in the DNA from an electronic database compiled from health records of almost 30,000 modern American adults. The scientists found that certain segments of Neanderthal DNA are especially common in people who have particular medical conditions, such as depression and increased amounts of blood clotting. Other bits of Neanderthal DNA seems to boost the immune response to certain parasites and other pathogens.
Most of the Neanderthal DNA segments that have persisted into our modern gene pool were probably beneficial in prehistoric times. Now, however, they may increase the risk of disease because our lifestyles and environments have changed so much since then. For example, an increase in blood clotting would have helped prevent life-threatening bleeding from injuries or childbirth in the past, but today it may increase the risk of blood clots and strokes in older people with sedentary lifestyles. Even immune-boosting bits of Neanderthal DNA may now do more harm than good for Americans who live in environments where there are far fewer parasites. They may make our immune systems overactive and cause allergies and autoimmune disorders.
Review
1. What are the nucleic acids?
2. How does RNA differ in structure from DNA?
3. Describe a nucleotide. Explain how nucleotides bind together to form a polynucleotide.
4. What role do nitrogen bases in nucleotides play in the structure and function of DNA?
5. What is the role of RNA?
6. Explain why Mark and Scott Kelly look so similar, using what you learned about nucleic acids in this article.
7. True or False. A, C, G, and T represent the bases in RNA.
8. True or False. The two polynucleotide chains of RNA twist into a double helix shape.
9. True or False. Cytosine always binds to guanine in DNA.
10. If part of a chain of DNA has the sequence of bases: ATTG, what is the corresponding sequence of bases that it binds to on the other chain?
11. Arrange the following in order from the smallest to the largest level of organization: DNA; nucleotide; polynucleotide
12. As part of the DNA replication process, the two polynucleotide chains are separated from each other, but each individual chain remains intact. Which bonds are broken in this process?
1. Bonds between adjacent sugars and phosphate groups
2. Bonds within nucleotides
3. Bonds between complementary bases
4. Bonds between adenine and guanine
13. Adenine, guanine, cytosine, and thymine are:
1. Nucleotides
2. Nitrogenous bases
3. Sugars in DNA and RNA
4. Phosphate groups
14. Some diseases and disorders are caused by genes. Explain why these genetic disorders can be passed down from parents to their children.
Attributions
1. Twins by Peter Voerman, licensed CC BY-NC 2.0 via Flickr.com
2. DNA nucleotides by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
3. DNA nucleotides by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
4. DNA cropped by Jahobr, released into the public domain via Wikimedia Commons
5. DNA macrostructure by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
6. Le Moustier, public domain via Wikimedia Commons
7. Neanderthal DNA extraction by Max Planck Institute for Evolutionary Anthropology, public domain via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.08%3A_Nucleic_Acids.txt |
Slow Burn
These old iron chains give off a small amount of heat as they rust. The rusting of iron is a chemical process. It occurs when iron and oxygen go through a chemical reaction similar to burning, or combustion. The chemical reaction that occurs when something burns obviously gives off energy. You can feel the heat, and you may be able to see the light of flames. The rusting of iron is a much slower process, but it still gives off energy. It's just that it releases energy so slowly you can't detect a change in temperature.
What Is a Chemical Reaction?
A chemical reaction is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a reactant, and a substance that forms as a result of a chemical reaction is called a product. During the reaction, the reactants are used up to create the products.
Another example of a chemical reaction is the burning of methane gas, shown in Figure $2$. In this chemical reaction, the reactants are methane (CH4) and oxygen (O2), and the products are carbon dioxide (CO2) and water (H2O). As this example shows, a chemical reaction involves the breaking and forming of chemical bonds. Chemical bonds are forces that hold together the atoms of a molecule. Bonds occur when atoms share electrons. When methane burns, for example, bonds break within the methane and oxygen molecules, and new bonds form in the molecules of carbon dioxide and water.
Chemical Equations
Chemical reactions can be represented by chemical equations. A chemical equation is a symbolic way of showing what happens during a chemical reaction. For example, the burning of methane can be represented by the chemical equation:
$\ce{CH_4 + 2O_2 \rightarrow CO_2 + 2 H_2O}$
The arrow in a chemical equation separates the reactants from the products and shows the direction in which the reaction proceeds. If the reaction could occur in the opposite direction as well, two arrows pointing in opposite directions would be used. The number 2 in front of O2 and H2O shows that two oxygen molecules and two water molecules are involved in the reaction. If just one molecule is involved, no number is placed in front of the chemical symbol.
Role of Energy in Chemical Reactions
Matter rusting or burning are common examples of chemical changes. Chemical changes involve chemical reactions, in which some substances, called reactants, change at the molecular level to form new substances, called products. All chemical reactions involve energy. However, not all chemical reactions release energy, as rusting and burning do. In some chemical reactions, energy is absorbed rather than released.
Exergonic Reactions
A chemical reaction that releases energy is called an exergonic reaction. This type of reaction can be represented by a general chemical equation:
$\mathrm{Reactants \rightarrow Products + Energy}$
Besides rusting and burning, examples of exothermic reactions include chlorine combining with sodium to form table salt. The decomposition of organic matter also releases energy because of exergonic reactions. Sometimes on a chilly morning, you can see steam rising from a compost pile because of these chemical reactions (see Figure $3$). Exergonic chemical reactions also take place in the cells of living things. In a chemical process similar to combustion, called cellular respiration, the sugar glucose is "burned" to provide cells with energy.
Endergonic Reactions
A chemical reaction that absorbs energy is called an endergonic reaction. This type of reaction can also be represented by a general chemical equation:
$\mathrm{Reactants + Energy \rightarrow Products}$
Did you ever use a chemical cold pack like the one in the picture below? The pack cools down because of an endergonic reaction. When a tube inside the pack is broken, it releases a chemical that reacts with water inside the pack. This reaction absorbs heat energy and quickly cools down the contents of the pack.
Many other chemical processes involve endergonic reactions. For example, most cooking and baking involves the use of energy to produce chemical reactions. You can't bake a cake or cook an egg without adding heat energy. Arguably, the most important endergonic reactions occur during photosynthesis. When plants produce sugar by photosynthesis, they take in light energy to power the necessary endergonic reactions. The sugar they produce provides plants and virtually all other living things with glucose for cellular respiration.
Activation Energy
All chemical reactions need energy to get started. Even reactions that release energy need a boost of energy in order to begin. The energy needed to start a chemical reaction is called activation energy. Activation energy is like the push a child needs to start going down a playground slide. The push gives the child enough energy to start moving, but once she starts, she keeps moving without being pushed again. Activation energy is illustrated in Figure $5$.
Why do all chemical reactions need energy to get started? In order for reactions to begin, reactant molecules must bump into each other, so they must be moving, and movement requires energy. When reactant molecules bump together, they may repel each other because of intermolecular forces pushing them apart. Overcoming these forces so the molecules can come together and react also takes energy.
Review
1. What is a chemical reaction?
2. Identify reactants and products in a chemical reaction.
3. List three examples of common changes that involve chemical reactions.
4. Define a chemical bond.
5. What is a chemical equation? Give an example.
6. Our cells use glucose (C6H12O6) to obtain energy in a chemical reaction called cellular respiration. In this reaction, six oxygen molecules (O2) react with one glucose molecule. Answer the following questions about this reaction.
1. How many oxygen atoms are in one molecule of glucose?
2. Write out what the reactant side of this equation would look like.
3. How many oxygen atoms are in the reactants in total? Explain how you calculated your answer.
4. How many oxygen atoms are in the products in total? Is it possible to answer this question without knowing what the products are? Why or why not?
7. Answer the following questions about the equation you saw above: CH4+ 2O2 → CO2 + 2H2O
1. Can carbon dioxide (CO2) become transformed into methane (CH4) and oxygen (O2) in this reaction? Why or why not?
2. How many molecules of carbon dioxide (CO2) are produced in this reaction?
8. Is the evaporation of liquid water into water vapor a chemical reaction? Why or why not
9. Why do bonds break in the reactants during a chemical reaction?
10. Contrast endergonic and exergonic chemical reactions. Give an example of each.
11. Define activation energy.
12. Explain why all chemical reactions require activation energy.
13. Heat is a form of ____________ .
14. In which type of reaction is heat added to the reactants?
15. In which type of reaction is heat produced?
16. If there was no heat energy added to an endothermic reaction, would that reaction occur? Why or why not?
17. If there was no heat energy added to an exothermic reaction, would that reaction occur? Why or why not?
18. Explain why a chemical cold pack feels cold when activated.
19. Explain why cellular respiration and photosynthesis are “opposites” of each other.
20. Explain how the sun indirectly gives our cells energy.
Explore More
Watch the video below to learn more about activation energy.
Attributions
1. Chaîne by Daplaza, licensed CC BY-SA 3.0 via Wikimedia Commons
2. Gas Stove Burner Blue Flame by Federico Cardoner, licensed CC BY 2.0 via Flickr
3. Compost steaming by Lucabon, CC BY-SA 4.0 via Wikimedia Commons
4. Cooler pack by Julie Magro, licensed CC BY 2.0 via Flickr
5. Activation energy by Hana Zavadska for CK-12 licensed CC BY-NC 3.0
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.09%3A_Energy_in_Chemical_Reactions.txt |
Assembly Line
We stay alive because millions of different chemical reactions are taking place inside our bodies all the time. Each of our cells is like the busy auto assembly line pictured here. Raw materials, half-finished products, and waste materials are constantly being used, produced, transported, and excreted. The "workers" on the cellular assembly line are mainly enzymes. These are the proteins that make biochemical reactions happen.
What Are Biochemical Reactions?
Chemical reactions that take place inside living things are called biochemical reactions. The sum of all the biochemical reactions in an organism is referred to as metabolism. Metabolism includes both exothermic (heat-releasing) chemical reactions and endothermic (heat-absorbing) chemical reactions.
Catabolic Reactions
Exergonic reactions in organisms are called catabolic reactions. These reactions break down molecules into smaller units and release energy. An example of a catabolic reaction is the breakdown of glucose during cellular respiration, which releases energy that cells need to carry out life processes.
Anabolic Reactions
Endergonic reactions in organisms are called anabolic reactions. These reactions absorb energy and build bigger molecules from smaller ones. An example of an anabolic reaction is the joining of amino acids to form a protein. Which type of reactions — catabolic or anabolic — do you think occur when your body digests food?
Enzymes
Most biochemical reactions in organisms need help in order to take place. Why is this the case? For one thing, temperatures are usually too low inside living things for biochemical reactions to occur quickly enough to maintain life. The concentrations of reactants may also be too low for them to come together and react. Where do the biochemical reactions get the help they need to proceed? The help comes from enzymes.
An enzyme is a protein that speeds up a biochemical reaction. It is a biological catalyst. An enzyme generally works by reducing the amount of activation energy needed to start the reaction. Figure \(2\) shows the activation energy needed for glucose to combine with oxygen to produce carbon dioxide and water. The overall reaction releases energy, but an initial activation energy is needed to start the process. The activation energy without an enzyme is much higher than the activation energy when an enzyme is used.
How Well Enzymes Work
Enzymes are involved in most biochemical reactions, and they do their jobs extremely well. A typical biochemical reaction that would take several days or even several centuries to occur without an enzyme is likely to occur in just a split second with the proper enzyme! Without enzymes to speed up biochemical reactions, most organisms could not survive. Enzymes are substrate-specific. The substrate of an enzyme is the specific substance it affects (Figure \(3\)). Each enzyme works only with a particular substrate, which explains why there are so many different enzymes. In addition, for an enzyme to work, it requires specific conditions, such as just the right temperature and pH. Some enzymes work best under acidic conditions, for example, while others work best in neutral environments.
Enzyme-Deficiency Disorders
There are hundreds of known inherited metabolic disorders in humans. In most of them, a single enzyme is either not produced by the body at all or is produced in a form that doesn't work. The missing or defective enzyme is like an absentee worker on the cell's assembly line. The absence of the normal enzyme means that toxic chemicals build-up or an essential product isn't made. Generally, the normal enzyme is missing because the individual with the disorder inherited two copies of a gene mutation, which may have occurred originally many generations in the past.
Any given inherited metabolic disorder is generally quite rare in the general population. However, there are so many different metabolic disorders that a total of 1 in 1,000 to 2,500 newborns can be expected to have one. In certain ethnic populations, such as Ashkenazi Jews (Jews of central and eastern European ancestry), the rate of certain inherited metabolic disorders is much higher.
Feature: Reliable Sources
The most common of all known enzyme-deficiency disorders is glucose-6-phosphate-dehydrogenase, or G6PD, deficiency. In the U.S., the disorder occurs most often in African-American males. The enzyme G6PD is needed to prevent the abnormal breakdown of red blood cells. Without the enzyme, red blood cells break down prematurely and anemia results.
Choose one of the following topics about G6PD deficiency:
• genetic basis
• signs and symptoms
• diagnosis and treatment
• worldwide distribution
For the topic, you chose, go online to learn more about it. Find at least three sources of additional information that you think are reliable. Compare the information provided by the different sources, and identify any discrepancies among them. Do additional online research as needed to try to find a reliable consensus view of the discrepant issue.
Review
1. What are biochemical reactions?
2. Define metabolism.
3. Compare and contrast catabolic and anabolic reactions.
4. Explain the role of enzymes in biochemical reactions.
5. What are enzyme-deficiency disorders?
6. True or False. Metabolism is one specific type of catabolism.
7. True or False. Biochemical reactions include catabolic and anabolic reactions.
8. Explain why the relatively low temperature of living things, as well as the low concentration of reactants, would cause biochemical reactions to occur very slowly in the body without enzymes.
9. Answer the following questions about what happens after you eat a sandwich.
1. Pieces of the sandwich go into your stomach, where there are digestive enzymes that break down the food. Which type of metabolic reaction is this? Explain your answer.
2. Through the process of digestion, part of the sandwich is broken down to glucose, which is then further broken down to release energy that your cells can use. Is this an exergonic or endergonic reaction? Explain your answer.
3. The proteins in the cheese, meat, and bread in the sandwich are broken down into their component amino acids. Then your body uses those amino acids to build new proteins. Which kind of metabolic reaction is represented by the building of these new proteins? Explain your answer.
10. Explain why your body doesn’t just use one or two enzymes for all of its biochemical reactions.
11. What is the specific substance that enzyme affects in a biochemical reaction called?
12. An enzyme is a biological
1. catabolism
2. form of activation energy
3. catalyst
4. reactant
Attributions
1. Final Assembly by Brian Snelson, licensed CC BY 2.0 via Wikimedia Commons
2. Enzyme action by Hana Zavadska for CK-12 licensed CC BY-NC 3.0
3. Enzymes by SweetChickaD, licensed CC BY-NC-SA 2.0 via Flickr
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.10%3A_Chemical_Reactions_in_Living_Things.txt |
The Blue Marble
It's often called the "water planet," and it's been given the nickname "the blue marble." You probably just call it "home." Almost three-quarters of our home planet is covered by water, and without it, life as we know it could not exist on Earth. Water, like carbon, has a special role in living things. It is needed by all known forms of life. Although water consists of simple molecules, each containing just three atoms, its structure gives it unique properties that help explain why it is vital to all living organisms.
Chemical Structure and Properties of Water
You are probably already familiar with many of the water’s properties. For example, you no doubt know that water is tasteless, odorless, and transparent. In small quantities, it is also colorless. However, when a large amount of water is observed, as in a lake or the ocean, it is actually light blue in color. The blue hue of the water is an intrinsic property and is caused by selective absorption and scattering of white light. These and other properties of water depend on its chemical structure.
The transparency of water is important for organisms that live in water. Because water is transparent, sunlight can pass through it. Sunlight is needed by water plants and other water organisms for photosynthesis.
Chemical Structure of Water
Each molecule of water consists of one atom of oxygen and two atoms of hydrogen, so it has the chemical formula H2O. The arrangement of atoms in a water molecule, shown in Figure \(2\), explains many of the water’s chemical properties. In each water molecule, the nucleus of the oxygen atom (with 8 positively charged protons) attracts electrons much more strongly than do the hydrogen nuclei (with only one positively charged proton). This results in a negative electrical charge near the oxygen atom (due to the "pull" of the negatively charged electrons toward the oxygen nucleus) and a positive electrical charge near the hydrogen atoms. A difference in electrical charge between different parts of a molecule is called polarity. A polar molecule is a molecule in which part of the molecule is positively charged and part of the molecule is negatively charged.
Water is a good solvent
Water is considered a very good solvent in the biochemical reactions. Figure \(3\) illustrates how water dissolves salts. Table salt (NaCl) consists of a positively charged sodium ion and a negatively charged chloride ion. The oxygen of water is attracted to the positive Na ion. The hydrogens of water are attracted to the negative Cl ion.
Hydrogen Bonding
Opposite electrical charges attract one another. Therefore, the positive part of one water molecule is attracted to the negative parts of other water molecules. Because of this attraction, bonds form between hydrogen and oxygen atoms of adjacent water molecules, as demonstrated in Figure \(4\). This type of bond always involves a hydrogen atom, so it is called a hydrogen bond.
Hydrogen bonds can also form within a single large organic molecule. For example, hydrogen bonds that form between different parts of a protein molecule bend the molecule into a distinctive shape, which is important for the protein’s functions. Hydrogen bonds also hold together the two nucleotide chains of a DNA molecule.
Sticky, Wet Water
Water has some unusual properties due to its hydrogen bonds. One property is cohesion, the tendency for water molecules to stick together. The cohesive forces between water molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently, they cohere more strongly to those directly associated with them on the surface. For example, if you drop a tiny amount of water onto a very smooth surface, the water molecules will stick together and form a droplet, rather than spread out over the surface. The same thing happens when water slowly drips from a leaky faucet. The water doesn't fall from the faucet as individual water molecules but as droplets of water. The tendency of water to stick together in droplets is also illustrated by the dew drops in Figure \(5\).
Another important physical property of water is adhesion. In terms of water, adhesion is the bonding of a water molecule to another substance, such as the sides of a leaf's veins. This process happens because hydrogen bonds are special in that they break and reform with great frequency. This constant rearranging of hydrogen bonds allows a percentage of all the molecules in a given sample to bond to another substance. This grip-like characteristic that water molecules form causes capillary action, the ability of a liquid to flow against gravity in a narrow space. An example of capillary action is when you place a straw into a glass of water. The water seems to climb up the straw before you even place your mouth on the straw. The water has created hydrogen bonds with the surface of the straw, causing the water to adhere to the sides of the straw. As the hydrogen bonds keep interchanging with the straw's surface, the water molecules interchange positions and some begin to ascend the straw.
Adhesion and capillary action are necessary to the survival of most organisms. It is the mechanism that is responsible for water transport in plants through roots and stems, and in animals through small blood vessels.
Hydrogen bonds also explain why water’s boiling point (100°C) is higher than the boiling points of similar substances without hydrogen bonds. Because of water’s relatively high boiling point, most water exists in a liquid state on Earth. Liquid water is needed by all living organisms. Therefore, the availability of liquid water enables life to survive over much of the planet.
Furthermore, water has a high specific heat because it takes a lot of energy to raise or lower the temperature of the water. As a result, water plays a very important role in temperature regulation. Since cells are made up of water, this property helps to maintain homeostasis.
The Density of Ice and Water
The melting point of water is 0°C. Below this temperature, water is a solid (ice). Unlike most chemical substances, water in a solid state has a lower density than water in a liquid state. This is because water expands when it freezes. Again, hydrogen bonding is the reason. Hydrogen bonds cause water molecules to line up less efficiently in ice than in liquid water. As a result, water molecules are spaced farther apart in ice, giving ice a lower density than liquid water. A substance with lower density floats on a substance with higher density. This explains why ice floats on liquid water, whereas many other solids sink to the bottom of liquid water.
In a large body of water, such as a lake or the ocean, the water with the greatest density always sinks to the bottom. Water is most dense at about 4°C. As a result, the water at the bottom of a lake or the ocean usually has a temperature of about 4°C. In climates with cold winters, this layer of 4°C water insulates the bottom of a lake from freezing temperatures. Lake organisms such as fish can survive the winter by staying in this cold, but unfrozen, water at the bottom of the lake.
Review
1. Describe the structure of a water molecule. What is polarity, and why is water polar?
2. Explain how the internal polarity of the water molecule makes it a good solvent?
3. Explain how hydrogen bonds cause molecules of liquid water to stick together.
4. What is capillary action? Give an example.
5. What property of water helps to maintain homeostasis and how?
Attributions
1. Water Planet by NASA/Robert Simmon and Marit Jentoft-Nilsen, public domain via Wikimedia Commons
2. Water by Lumen Learning licensed CC BY 2.0
3. Dissolving salt by Charles Molnar and Jane Gair, licensed CC BY 4.0
4. Hydrogen bonding by Lumen Learning licensed CC BY 2.0
5. Water drops by U.S. Fish and Wildlife Service, public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.11%3A_Biochemical_Properties_of_Water.txt |
Danger! Battery Acid!
You probably know that car batteries like this one contain dangerous chemicals such as strong acids. Strong acids can hurt you if they come into contact with your skin or eyes. Therefore, it may surprise you to learn that your life depends on acids. There are many acids inside your body, and some of them are as strong as battery acid. Acids are needed for digestion and some forms of energy production. Genes are made of nucleic acids, proteins of amino acids, and lipids of fatty acids.
Water and Solutions
Acids such as battery acid are solutions. A solution is a mixture of two or more substances that has the same composition throughout. Many solutions are a mixture of water and some other substance. Not all solutions are acids. Some are bases and some are neither acids nor bases. To understand acids and bases, you need to know more about pure water.
In pure water (such as distilled water), a tiny fraction of water molecules naturally breaks down to form ions. An ion is an electrically charged atom or molecule. The breakdown of water is represented by the chemical equation:
$\mathrm{2H_2O \rightarrow H_3O^+ + OH^-}$
The products of this reaction are a hydronium ion (H3O+) and a hydroxide ion (OH-). The hydroxide ion, which has a negative charge, forms when a water molecule gives up a positively charged hydrogen ion (H+). The hydronium ion, which has a positive charge, forms when another water molecule accepts the hydrogen ion.
Acidity and pH
The concentration of hydronium ions in a solution is known as acidity. In pure water, the concentration of hydronium ions is very low; only about 1 in 10 million water molecules naturally breaks down to form a hydronium ion. As a result, pure water is essentially neutral. Acidity is measured on a scale called pH. Pure water has a pH of 7, so the point of neutrality on the pH scale is 7.
Examples of pH
• Liquid drain cleaner has a pH = 14
• Bleaches, oven cleaner, lye have a pH = 13.5
• Ammonia solution has a pH = 10.5 - 11.5
• Baking soda has a pH = 9.5
• Sea water has a pH = 8
• Blood has a pH = 7.4
• Milk, urine, saliva have a pH = 6.3 - 6.6
• Black coffee has a pH = 5
• Grapefruit juice, soda, tomato juice have a pH = 2.5 - 3.5
• Lemon juice, vinegar have a pH = 2
• Batter acid, hydrochloric acid have a pH = 0
Acids
If a solution has a higher concentration of hydronium ions than pure water, it has a pH lower than 7. A solution with a pH lower than 7 is called an acid. As the hydronium ion concentration increases, the pH value decreases. Therefore, the more acidic a solution is, the lower its pH value is. Did you ever taste vinegar? Like other acids, it tastes sour. Stronger acids can be harmful to organisms. For example, stomach acid would eat through the stomach if it were not lined with a layer of mucus. Strong acids can also damage materials, even hard materials such as glass.
Bases
If a solution has a lower concentration of hydronium ions than pure water, it has a pH higher than 7. A solution with a pH higher than 7 is called a base. Bases, such as baking soda, have a bitter taste. Like strong acids, strong bases can harm organisms and damage materials. For example, lye can burn the skin, and bleach can remove the color from clothing.
Acids, Bases, and Enzymes
Many acids and bases in living things provide the pH that enzymes need. Enzymes are biological catalysts that must work effectively for biochemical reactions to occur. Most enzymes can do their job only at a certain level of acidity. Cells secrete acids and bases to maintain the proper pH for enzymes to do their work.
Every time you digest food, acids and bases are at work in your digestive system. Consider the enzyme pepsin, which helps break down proteins in the stomach. Pepsin needs an acidic environment to do its job. The stomach secretes the strong acid called hydrochloric acid that allows pepsin to work. When stomach contents enter the small intestine, the acid must be neutralized. This is because enzymes in the small intestine need a basic environment in order to work. An organ called the pancreas secretes a base named bicarbonate into the small intestine, and this base neutralizes the acid.
Feature: My Human Body
Do you ever have heartburn? The answer is probably "yes." More than 60 million Americans have heartburn at least once a month, and more than 15 million suffer from it on a daily basis. Knowing more about heartburn may help you prevent it or know when it's time to seek medical treatment.
Heartburn doesn't have anything to do with the heart, but it does cause a burning sensation in the vicinity of that organ. Normally, the acid secreted into the stomach remains in the stomach where it is needed to allow pepsin to do its job of digesting proteins. A long tube called the esophagus carries food from the mouth to the stomach. A sphincter, or valve, between the esophagus and stomach, opens to allow swallowed food to enter the stomach and then closes to prevent stomach contents from back flowing into the esophagus. If this sphincter is weak or relaxes inappropriately, stomach contents flow into the esophagus. Because stomach contents are usually acidic, this causes the burning sensation known as heartburn. People who are prone to heartburn and suffer from it often may be diagnosed with GERD, which stands for gastroesophageal reflux disease.
GERD — as well as occasional heartburn — often can be improved by dietary and other lifestyle changes that decrease the amount and acidity of reflux from the stomach into the esophagus.
• Some foods and beverages seem to contribute to GERD, so these should be avoided. They include chocolate, fatty foods, peppermint, coffee, and alcoholic beverages.
• Decreasing portion size and eating the last meal of the day at least a couple of hours before bedtime may reduce the risk of reflux occurring.
• Smoking tends to weaken the lower esophageal sphincter, so quitting the habit may help control reflux.
• GERD is often associated with being overweight, and losing weight often brings improvement.
• Some people are helped by sleeping with the head of the bed elevated. This allows gravity to help control the backflow of acids into the esophagus from the stomach.
Review
1. What is the solution?
2. Define acidity.
3. Explain how acidity is measured.
4. Compare and contrast acids and bases.
5. Hydrochloric acid is secreted by the stomach to provide an acidic environment for the enzyme pepsin. What is the pH of this acid? How strong of an acid is it compared with other acids?
6. True or False. Strong bases are gentle and cannot hurt you, unlike strong acids.
7. True or False. The lower the pH, the higher the concentration of hydronium ions.
8. Define an ion.
9. Identify the ions in the following equation and explain why they are ions:
$\mathrm{2H_2O \rightarrow H_3O^+ + OH^-}$
10. Explain why the pancreas secretes bicarbonate into the small intestine.
11. Do you think pepsin would work in the small intestine? Why or why not?
12. How does the pH of the stomach compare to the small intestine? It is
1. the same as
2. not as important as the pH of
3. higher than
4. lower than
13. You may have mixed vinegar and baking soda and noticed that they bubble and react with each other.
1. Explain why this happens.
2. Explain what happens to the pH of this solution after you mix the vinegar and baking soda.
14. Pregnancy hormones can cause the lower esophageal sphincter to relax. What effect do you think this has on pregnant women? Explain your answer.
Explore More
Watch the video below to learn more about acids, bases, and pH.
Attributions
1. Battery by dave_7, licensed CC BY 2.0 via Wikimedia Commons
2. pH scale by OpenStax College, licensed CC BY 3.0 via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.12%3A_Acids_and_Bases.txt |
Case Study Conclusion: Diet Dilemma
After reading this chapter, you should be able to see numerous connections between chemistry, human life, and health. In Mohinder’s situation, chemistry is involved in the reasons why their father has diabetes, why their personal risk of getting diabetes is high, and the different dietary changes they are considering.
For instance, type 2 diabetes is caused mainly by a lack of response in the body to the hormone insulin, which causes problems in the regulation of blood sugar, or glucose. Insulin is a peptide hormone, and as you have learned, peptides are chains of amino acids. Therefore, insulin is in the class of biochemical compounds called proteins. Mohinder is at increased risk of diabetes partly because there is a genetic component to the disease. DNA, which is a type of chemical compound called a nucleic acid, is passed down from parents to their offspring and carries the instructions for the production of proteins in units called genes. If there is a problem in a gene (or genes) that contributes to the development of a disease, such as type 2 diabetes, this can get passed down to the offspring and may raise that child’s risk of getting the disease.
But genetics is only part of the reason why Mohinder is at an increased risk of diabetes. Obesity itself is a risk factor and one that can be shared in families due to shared lifestyle factors such as poor diet and lack of exercise, in addition to genetics. Consumption of too many refined carbohydrates, such as white bread and soda, also may contribute to obesity and the development of diabetes. As you probably now know, these simple carbohydrates are more easily and quickly broken down in the digestive system to glucose than larger complex carbohydrate molecules, such as those found in vegetables and whole grains. This can lead to dramatic spikes in blood sugar levels, which is particularly problematic for people with diabetes because they have trouble maintaining their blood sugar at a safe level. You can understand why Mohinder’s father limits the consumption of refined carbohydrates, and in fact, some scientific studies have shown that avoiding refined carbohydrates may actually help reduce the risk of getting diabetes in the first place.
Mohinder’s friend recommended eating a low fat and high carbohydrate diet to lose weight, but you can see that which type of carbohydrate — simple or complex — is an important consideration. Eating lots of white bread and rice may not help Mohinder reduce their risk of diabetes! But a healthy diet that helps them lose weight may lower their risk of diabetes since obesity itself is a factor. Which specific diet will work best to help them lose weight probably depends on a variety of factors including their biology, lifestyle, and food preferences. Mohinder should consult with their doctor about their diet and exercise plan so that their specific situation can be taken into account and monitored by a medical professional.
Drinking enough water is usually good advice for everyone, especially if it replaces sugary drinks like soda. You now know that water is important for many of the chemical reactions that take place in the body.
Finally, you probably now realize that “chemicals” do not have to be scary, toxic substances. All matter consists of chemicals, including you, water, and healthy fresh fruits and vegetables, like the ones pictured above. When people advocate “clean eating” and avoiding “chemicals” in food, they are usually referring to avoiding synthetic, or man-made, chemical additives such as preservatives. This can be a healthy way to eat because it involves eating a variety of whole, fresh, unprocessed foods. But there is no reason to be scared of chemicals in general – they are simply molecules and how they react depends on what they are, what other molecules are present, and the environmental conditions surrounding them.
Chapter Summary
By now, you should have a good understanding of the basics of the chemistry of life. Specifically, you have learned:
• All matter consists of chemical substances. A chemical substance has a definite and consistent composition and may be either an element or a compound.
• An element is a pure substance that cannot be broken down into other types of substances.
• An atom is the smallest particle of an element that still has the properties of that element. Atoms, in turn, are composed of subatomic particles, including negative electrons, positive protons, and neutral neutrons. The number of protons in an atom determines the element it represents.
• Atoms have equal numbers of electrons and protons so they have no charge. Ions are atoms that have lost or gained electrons so they have either a positive or negative charge. Atoms with the same number of protons but different numbers of neutrons are called isotopes.
• There are almost 120 known elements. The majority of the elements are metals. A smaller number are nonmetals, including carbon, hydrogen, and oxygen.
• A compound is a substance that consists of two or more elements in a unique composition. The smallest particle of a compound is called a molecule. Chemical bonds hold together the atoms of molecules. We discussed four types of bonds, polar covalent bond, hydrogen bond, non-polar covalent bond, and ionic bond.
• In an ionic bond, an atom gives away one or more electrons to another atom.
• In a covalent bond, two atoms share one or more electrons. The equal sharing of electrons gives rise to a non- polar covalent bond, and unequal sharing of electrons gives rise to a polar covalent bond.
• The polar molecules make hydrogen bonds between them and within themselves.
• A chemical bond is a force of attraction between atoms or ions. Bonds form when atoms share or transfer valence electrons.
• Atoms form chemical bonds to achieve a full outer energy level, which is the most stable arrangement of electrons.
• Compounds can form only in chemical reactions, and they can break down only in other chemical reactions.
• Biochemical compounds are carbon-based compounds found in living things. They make up cells and other structures of organisms and carry out life processes. Most biochemical compounds are large molecules called polymers that consist of many repeating units of smaller molecules called monomers.
• There are millions of different biochemical compounds, but all of them fall into four major classes: carbohydrates, lipids, proteins, and nucleic acids.
• Carbohydrates are the most common class of biochemical compounds. They provide cells with energy, store energy, and make up organic structures such as the cell walls of plants. The basic building block of carbohydrates is the monosaccharide.
• Sugars are short-chain carbohydrates that supply us with energy. Simple sugars, such as glucose, consist of just one monosaccharide. Some sugars, such as sucrose, or table sugar, consist of two monosaccharides and are called disaccharides. Disaccharides are formed with the condensation reaction.
• Complex carbohydrates, or polysaccharides, consist of hundreds or even thousands of monosaccharides. They include starch, glycogen, cellulose, and chitin.
• Starch is made by plants to store energy and is readily broken down to its component sugars during digestion.
• Glycogen is made by animals and fungi to store energy and plays a critical part in the homeostasis of blood glucose levels in humans.
• Cellulose is the most common biochemical compound in living things. It forms the cell walls of plants and certain algae. Humans cannot digest cellulose, but it makes up most of the crucial dietary fiber in the human diet.
• Chitin makes up organic structures such as the cell walls of fungi and the exoskeletons of insects and other arthropods.
• Lipids include fats and oils. They store energy, form cell membranes, and carry messages.
• Lipid molecules consist mainly of repeating units called fatty acids. Fatty acids may be saturated or unsaturated, depending on the proportion of hydrogen atoms they contain. Animals store fat as saturated fatty acids; plants store fat as unsaturated fatty acids.
• Types of lipids include triglycerides, phospholipids, and steroids.
• Triglycerides contain glycerol (an alcohol) in addition to fatty acids. Humans and other animals store fat as triglycerides in fat cells.
• Phospholipids contain phosphate and glycerol in addition to fatty acids. They are the main component of cell membranes in all living things.
• Steroids are lipids with a four-ring structure. Some steroids, such as cholesterol, are important components of cell membranes. Many other steroids are hormones.
• Proteins include enzymes, antibodies, and numerous other important compounds in living things. They have many functions including helping cells keep their shape, making up muscles, speeding up chemical reactions, and carrying messages and materials.
• Proteins are made up of small monomer molecules called amino acids.
• A peptide bond is formed between two amino acids when they come together in a condensation synthesis reaction. Long chains of amino acids form polypeptides. The sequence of amino acids in polypeptides makes up the primary structure of proteins. Secondary structure refers to configurations such as helices and sheets within polypeptide chains. Tertiary structure is a protein's overall three-dimensional shape, which controls the molecule's basic function. A quaternary structure forms if multiple protein molecules join together and function as a complex.
• The chief characteristic that allows proteins' diverse functions is their ability to bind specifically and tightly with other molecules.
• Nucleic acids include DNA and RNA. They encode instructions for making proteins, helping make proteins, and passing the encoded instructions from parents to offspring.
• Nucleic acids are built of monomers called nucleotides, which bind together in long chains to form polynucleotides. DNA consists of two polynucleotides, and RNA consists of one polynucleotide.
• Each nucleotide consists of a sugar molecule, phosphate group, and a nitrogen base. Sugars and phosphate groups of adjacent nucleotides bind together to form the "backbone" of the polynucleotide. Bonds between complementary bases hold together the two polynucleotide chains of DNA and cause it to take on its characteristic double helix shape.
• DNA makes up genes, and the sequence of nitrogen bases in DNA makes up the genetic code for the synthesis of proteins. RNA helps synthesize proteins in cells. The genetic code in DNA is also passed from parents to offspring during reproduction, explaining how inherited characteristics are passed from one generation to the next.
• A chemical reaction is a process that changes some chemical substances into others. A substance that starts a chemical reaction is called a reactant, and a substance that forms in a chemical reaction is called a product. During the chemical reaction, bonds break in reactants and new bonds form in products.
• Chemical reactions can be represented by chemical equations. According to the law of conservation of mass, mass is always conserved in a chemical reaction, so a chemical equation must be balanced, with the same number of atoms of each type of element in the products as in the reactants.
• Many chemical reactions occur all around us each day, such as iron rusting and organic matter rotting, but not all changes are chemical processes. Some changes, such as ice melting or paper being torn into smaller pieces, are physical processes that do not involve chemical reactions and the formation of new substances.
• All chemical reactions involve energy and need activation energy to begin. Exergonic reactions release energy. Endergonic reactions absorb energy.
• Biochemical reactions are chemical reactions that take place inside living things. The sum of all the biochemical reactions in an organism is referred to as metabolism. Metabolism includes catabolic reactions, which are exothermic reactions, and anabolic reactions, which are endothermic reactions.
• Most biochemical reactions need a biological catalyst called an enzyme to speed up the reaction by reducing the amount of activation energy needed for the reaction to begin. Most enzymes are proteins that affect just one specific substance, called the enzyme's substrate.
• Water is a polar molecule; therefore, water molecules make hydrogen bonds between them. Due to this property water exists as a liquid over a wide range of temperatures and dissolves many substances. These properties depend on water's polarity, which causes water molecules to "stick" together.
• Organisms need water to dissolve many substances and for most biochemical processes, including photosynthesis and cellular respiration.
• A solution is a mixture of two or more substances that has the same composition throughout. Many solutions consist of water and one or more dissolved substances.
• Acidity is a measure of the hydronium ion concentration in a solution. Pure water has a very low concentration and a pH of 7, which is the point of neutrality on the pH scale. Acids have a higher hydronium ion concentration than pure water and a pH lower than 7. Bases have a lower hydronium ion concentration than pure water and a pH higher than 7.
• Many acids and bases in living things are secreted to provide the proper pH for enzymes to work properly.
Now that you understand the chemistry of the molecules that make up living things, in the next chapter you will learn how these molecules make up the basic unit of structure and function in living organisms — cells — and will be able to understand some of the crucial chemical reactions that occur within cells.
Chapter Summary Review
1. The four major classes of biochemical compounds are carbohydrates, lipids, proteins, and nucleic acids. For each of the substances below, identify which of these classes includes the substance.
1. Enzymes
2. Fructose
3. DNA
4. RNA
5. Steroids
2. The chemical formula for the complex carbohydrate glycogen is C24H42O21.
1. What are the elements in glycogen?
2. How many atoms are in one molecule of glycogen?
3. Is glycogen an ion? Why or why not?
4. Is glycogen a monosaccharide or polysaccharide? Besides memorizing this fact, how would you know this based on the information in the question?
5. What is the function of glycogen in the human body?
3. What is the difference between an ion and a polar molecule? Give an example of each in your explanation.
4. Define monomer and polymer.
5. For each of the following polymers, identify the name of its monomers.
1. RNA
2. Protein
3. Complex carbohydrate
6. What is the difference between a protein and a polypeptide?
7. People with diabetes have trouble controlling the level of glucose in their bloodstream. Knowing this, why do you think it is often recommended that people with diabetes limit their consumption of carbohydrates?
8. Identify each of the following reactions as endergonic or exergonic.
1. Cellular respiration
2. Photosynthesis
3. Catabolic reactions
4. Anabolic reactions
9. Pepsin is an enzyme in the stomach that helps us digest protein. Answer the following questions about pepsin.
1. What is the substrate for pepsin?
2. How does pepsin work to speed the reaction of protein digestion?
3. Given what you know about the structure of proteins, what do you think are some of the products of the reaction that pepsin catalyzes?
4. The stomach is normally acidic. What do you think would happen to the activity of pepsin and the effect on protein digestion if the pH is raised significantly?
10. What defines a neutral pH? What is the numerical value of a neutral pH?
11. True or False. Unsaturated fatty acids have straight chains.
12. True or False. The DNA code carries instructions for the correct sequence of nucleic acids in a protein.
13. True or False. Phospholipids make up cell membranes.
14. The function of proteins can include
1. helping cells keep their shape
2. helping to destroy foreign substances
3. speeding up biochemical reactions
4. all of the above
15. Which of the following is not part of a nucleotide?
1. nitrogen base
2. cellulose molecule
3. sugar molecule
4. phosphate group
16. The “push” needed to start a chemical reaction is the
1. enzymatic energy
2. endothermic energy
3. activation energy
4. reactant energy
Attributions
1. Soy whey protein diet by Peggy Greb, USDA ARS, public domain via Wikimedia Commons
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/03%3A_Chemistry_of_Life/3.13%3A_Case_Study_Conclusion%3A_Diet_and_Chapter_Summary.txt |
This chapter describes nutrients, nutrient needs, and healthy eating to achieve good nutrition. It also discusses eating disorders, problems of obesity and malnutrition, and causes and prevention of foodborne diseases.
• 4.1: Case Study: Fueling Our Bodies Properly
What does this nutritional information mean? As you read this chapter, you will learn about the nutrients your body needs to function and stay healthy, and how eating too much or too little of certain nutrients can wreak havoc on your health. You will learn how to interpret the tables above, and will better understand the health consequences of a diet that is heavy in typical fast food items.
• 4.2: Nutrients
Nutrients are substances the body needs for energy, building materials, and control of body processes. There are six major classes of nutrients based on biochemical properties: carbohydrates, proteins, lipids, water, vitamins, and minerals. Fiber, which consists largely of nondigestible carbohydrates, is sometimes added as the seventh class of nutrients.
• 4.3: Healthy Eating
Healthy eating is a panacea for many human ailments. A healthy diet reduces risk of obesity, cardiovascular disease, type 2 diabetes, osteoporosis, and cancer. Not surprisingly, it also extends the length of life. In fact, an unhealthy diet is one of the leading preventable causes of death. A healthy diet also has mental health benefits. It may stall or reduce the risk of dementia and have a positive effect on memory.
• 4.4: Eating Disorders
Eating disorders are mental health disorders defined by abnormal eating habits that adversely affect health. Eating disorders typically begin during late childhood, adolescence, or early adulthood. In developed countries such as the United States, they occur in about 4 percent of people and are much more common in females than males. In developing countries, they are less common but increasing in frequency. Eating disorders are serious diseases and can even be fatal.
• 4.5: Obesity
Obesity is a disease in which excess body fat has accumulated to the extent that it is likely to have negative effects on health. Obesity is commonly diagnosed on the basis of the body mass index (BMI). BMI is an estimate of body fatness based on a person's weight relative to his or her height. BMI is calculated by dividing a person's weight (in kilograms) by the square of the person's height (in meters).
• 4.6: Undernutrition
Undernutrition is defined as insufficient intake of nutritious foods. People who are undernourished are likely to have low body fat reserves, so one indicator of undernutrition in individuals is a low body mass index (BMI). Adults are considered underweight if their body mass index (BMI) is less than 18.5 kg/m2. Children are considered underweight if their BMI is less than the 5th percentile of the reference values for children of the same age.
• 4.7: Foodborne Diseases
Foodborne disease, commonly called food poisoning, is any disease that is transmitted via food. Picnic foods create a heightened risk of foodborne disease mainly because of problems with temperature control. If hot foods are not kept hot enough or cold foods are not kept cold enough, foods may enter a temperature range in which microorganisms such as bacteria can thrive.
• 4.8: Case Study Conclusion: Fast Food and Chapter Summary
What is wrong with fast food? That is the question that Carlos, who you read about in the beginning of the chapter, asked himself after learning that his friend Kevin eats it five or six times a week, and thinks that this diet is not necessarily that bad for him.
04: Nutrition
Case Study: What's Wrong with Fast Food?
Like many Americans, 20-year-old Abdul eats fast food several times a week. After a long day of classes and work, it’s easy for him to pick up fast food for dinner from a drive-through window on his way home. He also often has fast food for lunch on his short break. He knows that fast food probably isn’t the healthiest choice, but it is convenient and he likes it. Besides, he is young and only slightly overweight, with no major health problems, so he is not too concerned about it affecting his health.
One day, Abdul gives his friend Carlos a ride home and suggests they pick up some fast food on the way. Carlos says, “Nah, I don’t eat that stuff very often. It’s not good for you.” Abdul feels a little defensive and asks Carlos what exactly is wrong with it. Carlos says, “Well, it has a lot of calories and it’s not exactly fresh food.” Abdul says he doesn't think it has any more calories than other types of meals, and he eats some fresh fruit and vegetables at other times — is it really that bad for his health to eat fast food five or six times a week?
Carlos thinks about this. He has heard many times that fast food is not good for your health, but he is not sure of the exact reasons. When he gets home, he decides to do some research. He visits the website of Abdul’s favorite fast food restaurant and looks up the nutritional information for Abdul's typical meal of a cheeseburger, large fries, and a large soda. Some of the information he finds is shown in Table \(1\) and Table \(2\).
Table \(1\): Nutritional Information for a Typical Fast Food Meal
Food Calories Total Fat (%DV) Saturated Fat (%DV) Trans Fat Carbohydrates (%DV)
Burger 540 43% 49% 1 g 15%
Fries 510 37% 17% 0 g 22%
Soda 300 0% 0% 0 g 27%
Total 1,350 80% 66% 1 g 64%
Table \(2\): Percentage of the adult recommended daily value (%DV) for each nutrient, based on a 2,000 Calorie a day diet.
Food Sodium (%DV) Iron (%DV) Vitamin A (%DV) Vitamin C (%DV) Calcium (%DV)
Burger 40% 25% 10% 2% 15%
Fries 15% 6% 0% 30% 2%
Soda 1% 0% 0% 0% 0%
Total 56% 31% 10% 32% 17%
What does this nutritional information mean? How can it help Carlos understand the potential health impact of Abdul frequently eating meals like this?
Chapter Overview: Nutrition
In this chapter, you will learn about nutrients, proper nutrition, and the negative health consequences of bad nutrition and improperly prepared food. Specifically, you will learn about:
• The six major classes of nutrients — carbohydrates, proteins, lipids, water, vitamins, and minerals — are substances the body needs for energy, building materials, and body processes.
• Essential nutrients, which must be obtained from food, and nonessential nutrients, which can be synthesized by the body.
• Macronutrients, which the body needs in relatively large quantities, and micronutrients, which the body needs in relatively small quantities.
• The functions of specific nutrients in the body and sources of these nutrients.
• Phytochemicals and their potential role in maintaining normal body functions and good health.
• Guidelines for healthy eating and good nutrition, and why a healthy diet can reduce the risk of many diseases.
• Energy homeostasis, which is the balance between calories consumed and those that are used by the body.
• Types of malnutrition, including undernutrition, overnutrition, and unbalanced nutrition.
• Nutrient and energy density and how knowledge of these factors can be used to make healthier food choices.
• How appetite is regulated.
• Eating disorders including anorexia nervosa, bulimia nervosa, and binge eating disorder and their causes, health effects, and treatments.
• Obesity and how it is defined, its causes, health consequences, ways to prevent and treat it, and the impact on public health.
• Undernutrition and how it is defined, its causes, specific undernutrition syndromes, and the often irreversible effects on children.
• The impact of undernutrition around the world, including richer nations, and public health approaches to treat and prevent undernutrition.
• The causes of foodborne diseases, including microorganisms and toxins; symptoms of the foodborne diseases; and ways to prevent foodborne disease including good hygiene and proper food preparation and storage.
As you read this chapter, think about the following questions related to the tables above that contain nutritional information for Abdul’s typical fast food meal:
1. Which nutrients might Abdul consume too much of if he eats meals like this frequently? Why would these nutrients be a concern? What health issues could be caused by consuming them in excess?
2. Which nutrients might Abdul not get enough of if he eats meals like this frequently? What health issues could this cause?
3. What are some ways Abdul can make better food choices, even at a fast-food restaurant? Why would these choices improve his diet and health?
Attributions
1. Costco menu by Quazie, licensed CC BY 2.0 via Flickr
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/04%3A_Nutrition/4.1%3A_Case_Study%3A_Fueling_Our_Bodies_Properly.txt |
Fighting Phytochemicals
Many wars have been fought to acquire these spices from India. Chemicals and oils in the spices infuse specific smells and tastes in Indian cuisine. Food and culture are intertwined, and people bring their culture with them when they settle in a foreign country. Sometimes their culture is accepted, and sometimes it becomes a cause of discrimination that people have to face for embracing their culture.
This colorful display of Indian spices is not just pretty to look at, the items pictured are also rich in phytochemicals. Phytochemicals are a large group of recently discovered chemicals, such as oils and colors, that occur naturally in plants. Many of them are known to protect plants by fighting off insect attacks and infectious diseases. Phytochemicals in the food we eat may also be needed to help keep us healthy. If so, some nutritionists think they should be classified as nutrients.
What Are Nutrients?
Nutrients are substances the body needs for energy, building materials, and control of body processes. There are six major classes of nutrients based on biochemical properties: carbohydrates, proteins, lipids, water, vitamins, and minerals. Fiber, which consists largely of nondigestible carbohydrates, is sometimes added as the seventh class of nutrients.
Besides the biochemical classification of nutrients, nutrients are also categorized as either essential or nonessential nutrients. Essential nutrients cannot be synthesized by the human body, at least not in sufficient amounts for normal functioning, so these nutrients must be obtained from food. Nonessential nutrients, in contrast, can be synthesized in the body in sufficient quantities for normal functioning, although they are generally obtained from food as well. Except for dietary fiber, all dietary carbohydrates are considered nonessential. Every other major class of nutrients contains multiple essential compounds. For example, there are nine essential amino acids, at least two essential fatty acids, and many essential vitamins and minerals. Water and fiber are also essential nutrients.
The major classes of nutrients are also categorized as macronutrients or micronutrients depending on how much of them the body needs.
Macronutrients
Macronutrients are nutrients that the body needs in relatively large amounts. They include carbohydrates, proteins, lipids, and water. All macronutrients except water are used by the body for energy, although this is not their sole physiological function. The energy provided by macronutrients in food is measured in kilocalories, commonly called Calories, where 1 Calorie is the amount of energy needed to raise 1 kilogram of water by 1 degree Celsius.
Carbohydrates
Carbohydrates are organic compounds made up of simple sugars (as in the cotton candy pictured in Figure \(2\)). Carbohydrates are classified by the number of sugars they contain as monosaccharides (one sugar), such as glucose and fructose; disaccharides (two sugars), such as sucrose and lactose; and polysaccharides (three or more sugars), including starch, glycogen, and cellulose (the main component of dietary fiber). Dietary carbohydrates come mainly from grains, fruits, and vegetables. All digestible carbohydrates in the diet are used by the body for energy. One gram of dietary carbohydrates provides 4 Calories of energy. Fiber, such as the cellulose in plant foods, cannot be digested by the human digestive system, so most of it just passes through the digestive tract. Although it does not provide energy as other carbohydrates do, it is nonetheless considered an essential nutrient for its physiological roles. There are two types of fiber in many plant foods: soluble fiber and insoluble fiber.
Soluble fiber consists of nondigestible complex plant carbohydrates that dissolve in water, forming a gel. This type of dietary fiber thickens and slows the movement of chyme through the small intestine and thereby slows the absorption of glucose into the blood. The consistency of food after it has been mechanically digested in the stomach is referred to as chyme. This may lessen insulin spikes and the risk of type 2 diabetes. Soluble fiber can also help lower blood cholesterol. Good dietary sources of soluble fiber include oats, apples, and beans.
Insoluble fiber consists mainly of cellulose and does not dissolve in water. As insoluble fiber moves through the large intestine, it stimulates peristalsis. Peristalsis is the involuntary constriction of the smooth muscle of the GI tract that pushes the food content in the tract. This keeps food wastes moving and helps prevent constipation. The insoluble fiber in the diet may also lessen the risk of colon cancer. Good dietary sources of insoluble fiber include cabbage, bell peppers, and grapes.
Proteins
Proteins are organic compounds made up of amino acids. You may think of meat and fish as major sources of dietary proteins — and they are — but there are many good plant sources as well, including soybeans (see the figure below) and other legumes. Proteins in food are broken down during digestion to provide the amino acids needed for protein synthesis. Proteins in the human body are the basis of many body structures, including muscles and skin. Proteins also function as enzymes that catalyze biochemical reactions, hormones that regulate body functions in other ways, and antibodies that help fight pathogens. Any amino acids from food that are not needed for these purposes are excreted in the urine, converted to glucose for energy, or stored as fat. One gram of protein provides 4 Calories of energy.
The most important aspect of protein structure from a nutritional standpoint is amino acid composition. About 20 amino acids are commonly found in the human body, of which about 11 are nonessential because they can be synthesized internally. The other 9 amino acids are essential amino acids that must be obtained from dietary sources. Essential amino acids are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. Animal proteins such as meat and fish are concentrated sources of all 9 essential amino acids, whereas plant proteins may have only trace amounts of one or more essential amino acids.
Lipids
Lipids, commonly called fats, are organic compounds made up mainly of fatty acids. Fats in foods (Figure \(4\)), as well as fats in the human body, are typically triglycerides (three fatty acids attached to a molecule of glycerol). Fats provide the body with energy and serve other vital functions, including helping to make and maintain cell membranes and functioning as hormones. When used for energy, one gram of fat provides 9 Calories of energy.
Saturated vs Unsaturated Fats
Fats are classified as either saturated or unsaturated depending on the type of bonds in their fatty acids.
• In saturated fats, carbon atoms share only single bonds, so each carbon atom is bonded to as many hydrogen atoms as possible. Saturated fats tend to be solids at room temperature. Most saturated fat in the diet comes from animal foods, such as meat and butter.
• In unsaturated fats, at least one pair of carbon atoms share a double bond, so these carbon atoms are not bonded to as many hydrogen atoms as possible. Unsaturated fats with just one double bond are called monounsaturated fats. Those with multiple double bonds are called polyunsaturated fats. Unsaturated fats tend to be liquids at room temperature. Unsaturated fats in the diet come mainly from certain fish such as salmon and from plant foods such as seeds and nuts.
Essential Fatty Acids
Most fatty acids are not essential. The body can make them as needed, generally from other fatty acids, although this takes energy. Only two fatty acids are known to be essential, called omega-3 and omega-6 fatty acids. They cannot be synthesized in the body, so they must be obtained from food. The most commonly used cooking oils in processed foods are rich in omega-6 fatty acids, so most people get plenty of these fatty acids in their diet. Omega-3 fatty acids are not as prevalent in foods, and most people do not get enough of them in food. Good food sources of omega-3 fatty acids include oily fish such as salmon, walnuts, and flax seeds.
Trans Fats
Trans fats are unsaturated fats that contain types of bonds that are rare in nature. Trans fats are typically created in an industrial process called partial hydrogenation. They may be used in a variety of processed foods (such as those shown in Figure \(5\)) because they tend to have a longer shelf life without going rancid. Trans fats are known to be detrimental to human health.
Water
Water is essential to life because biochemical reactions take place in water. Water is continuously lost from the body in multiple ways, including in urine and feces, during sweating, and as water vapor in exhaled breath. This constant loss of water makes water an essential nutrient that must be replenished often.
Too little water is called dehydration. It can cause weakness, dizziness, and heart palpitations. Severe dehydration can lead to death. It is easy to become dehydrated in hot weather, especially when exercising. It is more difficult to consume too much water, but overhydration is also possible. It can result in water intoxication, a serious and potentially fatal condition.
Micronutrients
Micronutrients are nutrients the body needs in relatively small amounts. Micronutrients do not provide energy. Instead, they are necessary for the biochemical reactions of metabolism, among other vital functions. They include vitamins, minerals, and possibly phytochemicals as well.
Vitamins
Vitamins are organic compounds that generally function as coenzymes. A coenzyme is a “helper” molecule that is required for a protein enzyme to work. In this capacity, vitamins play many roles in good health, ranging from maintaining normal vision (vitamin A) to help the blood clot (vitamin K). Some functions of these and several other vitamins are listed in the table below. Most vitamins are essential nutrients and must be obtained from food. Fruits, vegetables, meat, and fish are all high in one or more essential vitamins. There are only a few nonessential vitamins. Vitamins B7 and K are produced by bacteria in the large intestine, and vitamin D is synthesized in the skin when it is exposed to UV light
Table \(1\): Selected Vitamins and Some of Their Functions
Vitamin Function
A normal vision
B1 (thiamin) production of cellular energy from food
B3(niacin) cardiovascular health
B7 (biotin) support of carbohydrate, protein, and fat metabolism
B9 (folic acid) fetal health and development
B12 normal nerve function and production of red blood cells
C making connective tissue
D healthy bones and teeth
E normal cell membranes
K blood clotting
Minerals
Minerals are inorganic chemical elements that are necessary for normal body processes and good health. Because they are inorganic and not synthesized biologically, all nutrient minerals are considered essential nutrients.
Several minerals are needed in relatively large quantities (> 150 mg/day), so they are sometimes referred to as macrominerals or bulk minerals. They include:
• calcium, which is needed for bone strength, neutralizing acidity in the digestive tract, and nerve and cell membrane functions. Dairy products are good sources of calcium.
• magnesium, which is needed for strong bones, maintaining pH, processing ATP, and other functions. Green leafy vegetables, bran, and almonds are high in magnesium.
• phosphorus, which is needed for bone strength, energy processing, pH regulation, and phospholipids in cell membranes. Milk and meat are good sources of phosphorus.
• sodium, which is needed to regulate blood volume, blood pressure, water balance, and pH. Most processed foods have added sodium. A salt shaker is another common source of sodium.
• chloride, which is needed for the production of hydrochloric acid in the stomach and for cell membrane transport. Chloride in table salt added to processed foods provides plenty of chloride in most diets.
• potassium, which is needed for the proper functioning of the heart and nerves, water balance, and pH. Many fruits and vegetables are high in potassium.
• sulfur, which is needed for the synthesis of many proteins. Meat and fish are good sources of sulfur.
Other minerals are needed in much smaller quantities (≤150 mg/day), so they are often referred to as trace minerals. The table below lists several trace minerals and some of their functions. Good dietary sources of trace minerals include whole grains, seafood, fruits, vegetables, nuts, and legumes.
Table \(2\): Selected Trace Minerals and Some of Their Functions
Trace Mineral Function
Cobalt synthesis of vitamin B12 by gut bacteria
Copper component of many enzymes
Chromium metabolism of sugar
Iodine synthesis of thyroid hormones
Iron component of hemoglobin and many enzymes
Manganese processing of oxygen
Molybdenum component of several enzymes
Selenium component of oxidases (antioxidants)
Zinc component of several enzymes
Phytochemicals
The naturally occurring, disease- and pest-fighting plant chemicals known as phytochemicals are commonly consumed in plant foods, particularly spices and fresh vegetables and fruits. Besides fighting attacks on plants, many phytochemicals give plants their distinctive colors and characteristic flavors and aromas. Phytochemicals are the reason that blueberries are blue (Figure \(6\)) and that garlic has its characteristically strong, pungent taste and smell. There are known to be as many as 4,000 different phytochemicals in plants. Preliminary evidence suggests that certain phytochemicals in the diet help protect human health. For example, some phytochemicals may act as antioxidants that counter cancer-causing free radicals. Research on phytochemicals is still relatively young, so time will tell whether they will eventually be classified as micronutrients.
Review
1. What are the nutrients?
2. List the six major classes of nutrients based on biochemical properties.
3. Compare and contrast essential and nonessential nutrients.
4. Identify macronutrients.
5. Which nutrients are classified as micronutrients? Why?
6. Describe carbohydrates, state how much energy they provide, and list good food sources of carbohydrates.
7. If fiber in food cannot be digested, why is it considered a nutrient?
8. Describe proteins, state their general uses in the human body, and identify food sources that are high in proteins. How much energy do proteins provide?
9. Describe lipids, identify how much energy they provide, and state their general uses in the human body.
10. Distinguish between saturated, unsaturated, and trans fats.
11. Water provides no energy or materials the body needs for building or controlling body processes. Why is it considered a nutrient?
12. What are vitamins? What is the general role of most vitamins? Which vitamins are not essential nutrients? Why?
13. What are the dietary minerals? Give examples of macrominerals and trace minerals.
14. What are phytochemicals? What are good food sources of phytochemicals?
15. Which of the following are inorganic substances?
1. Vitamins
2. Minerals
3. All micronutrients
4. A and B
Explore More
Dietary intake of the bioactive components within fruits and vegetables has been shown to have chemopreventative effects. Many flavonoids have been found to be cytotoxic to cancer cells. Learn more here:
Attributions
1. Indian Spices by Joe mon bkk, CC BY-SA 4.0 via Wikimedia Commons
2. Cotton candy fan by college.library, licensed CC BY 2.0 via Wikimedia Commons
3. Soybean Composition Infographic by United Soybean Board, licensed CC BY 2.0 via Wikimedia Commons
4. Butter and oil by National Cancer Institute, public domain via Wikimedia Commons
5. Avoiding trans fat by The U.S. Food and Drug Administration, public domain via Wikimedia Commons
6. Weather tomorrow - sunny with plentiful blueberries by Gordana Adamovic-Mladenovic, licensed CC BY 2.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/04%3A_Nutrition/4.2%3A_Nutrients.txt |
Balancing Act
If you practice yoga, then you know that yoga positions such as the headstand demonstrated here can help you develop a good balance. Having good balance, in turn, can reduce your risk of falls and injuries. Another kind of balance is important to ensure that you have good health and prevent disease, and that kind of balance is a balance in your diet. Achieving a dietary balance requires healthy eating.
Benefits of Healthy Eating
It sounds like something a snake-oil salesman would say, but it’s true: healthy eating is a panacea for many human ailments. A healthy diet reduces risk of obesity, cardiovascular disease, type 2 diabetes, osteoporosis, and cancer. Not surprisingly, it also extends the length of life. In fact, an unhealthy diet is one of the leading preventable causes of death. A healthy diet also has mental health benefits. It may stall or reduce the risk of dementia and have a positive effect on memory.
Diet and Nutrition
If you adopt healthy eating habits and get enough exercise, you are likely to have good nutrition. Nutrition is the process of taking in nutrients in food and using them for growth, metabolism, and repair. Good nutrition requires eating foods rich in nutrients with the right amount of food energy (Calories) to balance energy use.
Nutrient Balance and Nutrient Density
Eating a wide range of foods, especially fruits and vegetables, is the basis of healthy eating. This helps ensure that you are eating a wide range of nutrients. However, there is only a limited amount of food you can eat in a single day without consuming too many Calories. In order to maximize the number of nutrients you take in, you need to spend your “Calorie budget” wisely by choosing foods that have high nutrient density. Nutrient density refers to how much of a given nutrient is provided by a particular food, relative to the mass of the food or the number of Calories it provides. Consider vitamin K as an example. The recommended daily value of vitamin K for adults is 90 µg. Both kale and iceberg lettuce provide vitamin K. A cup of iceberg lettuce provides about 50 µg vitamin K; a cup of kale provides more than 1000 µg of vitamin K. Therefore, kale has about 20 times the nutrient density for vitamin K as lettuce. Black beans, pictured below, are another good example of nutrient-dense food.
Energy Homeostasis and Energy Density
Good nutrition also requires achieving energy homeostasis. Energy homeostasis is a balance between the energy consumed in food and the energy expended in metabolism and physical activity. If more energy is taken in as food than is used for metabolism and activity, then the extra energy is stored as fat. An extra 3,500 Calories of food energy results in the storage of almost half a kilogram (1 lb) of body fat. If less energy is taken in than is used, then stored fat may be used for energy. The human brain, particularly the hypothalamus, plays a central role in regulating energy homeostasis. Based on biochemical signals from the body, the hypothalamus generates a sense of hunger or satiation as needed to maintain energy balance.
Energy homeostasis depends on more than hunger and satiation. It also depends on dietary choices, eating habits, and activity levels. To achieve energy balance, it is important to consider the energy density of the food. Energy density refers to the number of Calories a food provides per gram (or ounce). Foods high in carbohydrates or proteins are generally less energy-dense than foods high in lipids. Carbohydrates and proteins provide 4 Calories of energy per gram, whereas lipids provide 9 Calories of energy per gram. However, within nutrient classes, there is considerable variation in the energy density of foods. For example, fruits are high in carbohydrates that the body uses for energy. Both casaba melons and figs are fruits and provide energy, but an ounce of casaba melon provides only about 8 Calories of energy, whereas an ounce of figs provides about 80 Calories of energy. This means that figs have 10 times the energy density of casaba melons.
Malnutrition
Bad nutrition is referred to as malnutrition. The word malnutrition may make you think of starving children in Africa who do not have enough food to eat. This type of malnutrition is called undernutrition, and it is a major nutritional problem in developing countries. Undernutrition is typically caused by inadequate energy intake, often coupled with frequent bouts of infectious disease. It usually results in people being underweight for their height, and it commonly leads to growth failure in children.
Undernutrition is just one type of malnutrition. Excessive food intake can also cause malnutrition, in this case, overnutrition. This is the major nutritional problem in developed countries. Overnutrition is typically caused by excessive energy intake coupled with inadequate energy expenditure in physical activity. Overnutrition usually leads to people becoming overweight or obese (see Figure \(3\)). Obesity is associated with a host of health problems and diseases, including metabolic syndrome, cardiovascular disease, type 2 diabetes, and some types of cancer, among others.
Unbalanced nutrition is another type of malnutrition. In this case, the diet contains too much or not enough of specific nutrients other than energy. This type of malnutrition often occurs with undernutrition. However, a person doesn’t have to be undernourished to lack specific nutrients. People with adequate food intake and even people with overnutrition may have unbalanced nutrition. Getting either too much or not enough of particular nutrients may cause diseases or other health problems. For example, inadequate vitamin A intake may cause blindness, whereas too much vitamin A can be toxic. Likewise, dietary calcium deficiency may contribute to osteoporosis, whereas too much calcium can cause kidney stones.
Nutrient Needs
The goal of healthy eating is to take in the proper amount of each nutrient to meet nutrient and energy needs. The FDA identifies the recommended adult daily values (DV) for a wide variety of nutrients, based on a 2,000 Calorie daily diet. The values in the table are average values. The exact amount of each nutrient that a given individual needs may differ, depending on factors, such as age and gender. Different stages of adulthood have different nutrient needs for several nutrients, and males have somewhat higher needs for many nutrients than do females at most ages, mainly because of gender differences in body size. Other factors that influence individual nutrient needs include health status and activity level. People in poor health may need some nutrients in greater quantities. People who are very active need to obtain more energy from macronutrients in their diet.
Tools and Tips for Healthy Eating
There are so many nutrients and daily values. Eating to meet all these nutrient needs may seem like an overwhelming challenge. Do you need to keep track of the nutrient content of everything you eat to guarantee that you are meeting your nutrient needs? Fortunately, the answer is no. Healthy eating is much easier than that. Just use MyPlate and nutrition facts labels and follow the tips below. This approach will help ensure that you are meeting your nutrient needs.
MyPlate
One of the most useful tools for healthy eating is MyPlate, which is shown below. This is a visual guide to healthy eating that was developed by the United States Department of Agriculture (USDA) in 2011. MyPlate replaces the previous MyPyramid guide from the USDA and is easier to apply to daily eating. MyPlate depicts the relative proportions of different types of foods you should eat at each meal (or cumulatively by the end of the day). The foods are selected from five different food groups: vegetables, fruits, grains (such as cereal, bread, or pasta), proteins (such as meat, fish, or legumes), and dairy (such as milk, cheese, or yogurt).
According to MyPlate, about half of the food on your plate should be vegetables and fruits, and the other half should be grains and proteins. A serving of a dairy product should also accompany each meal. A meal based on MyPlate might include a serving of kale, an apple, a turkey sandwich, and a glass of milk. Think about some of your own typical meals. Do they match MyPlate? If not, how could you modify them to get the right proportions of foods from each food group?
Nutrition Facts Labels
If you are like most Americans, you rely heavily on packaged and processed foods. Although limiting these foods in your diet is a good aim, in the meantime, make use of the nutrition facts labels on these foods. A nutrition facts label, like the one shown above, gives the nutrient content and ingredients in food and makes it easy to choose the best options. A quick look at the nutrition facts can help you choose foods that are high in nutrients you are likely to need more of (such as fiber and protein) and low in nutrients you probably need less of (such as sodium and sugar). Checking the ingredients list on labels can help you choose food items that contain the most nutritious ingredients, such as whole grains instead of processed grains.
The sample nutrition label in Figure \(5\) indicates that a serving of this food is 55 g (with about 8 servings in the package). Each serving contains:
• 230 calories (with 40 calories from fat)
• 8 g total fat (making up 12% of the recommended fats per day)
• 1 g of this total fat is saturated fat (making up 5% of the daily value)
• 0 g is trans fat
• 0 mg cholesterol (0% of the daily value)
• 160 mg sodium (7% of the daily value)
• 37 g total carbohydrates (12% of daily value)
• 4 g of that is dietary fiber (16% of daily value)
• 1 g sugars
• 3 g proteins
• 10% of the recommended daily value of vitamin A
• 8% of the recommended daily value of vitamin C
• 20% of the recommended daily value of calcium
• 45% of the recommended daily value of Iron
Tips for Healthy Eating
The following tips can help you attain energy homeostasis while meeting your nutrient needs.
• Eat several smaller meals throughout the day rather than a few larger meals. Eating more frequently keeps energy, blood glucose, and insulin levels stable.
• Make healthy food choices. Try to eat whole foods rather than processed foods. Whole foods have more nutrients than processed foods. Raw foods also generally retain more nutrients than cooked foods. Overall, try to eat more plant foods and fewer animal foods.
• Make healthy grain choices. Try to make at least half your grains whole grains. Choose food items with whole grains listed as the main ingredient. Avoid foods that contain mainly or only processed grains, such as white flour or white rice. Include a variety of grains, such as rice, oats, and wheat.
• Make healthy fruit and vegetable choices. Include a variety of green, yellow, red, and orange fruits and vegetables, like those pictured below. These foods are high in vitamins and phytochemicals. Consume whole fruits instead of juices. Whole fruits are higher in fiber and more filling and may also have less sugar. The highest fiber fruits include plums and prunes.
• Combine amino acids in plant foods. The right combinations, such as beans and rice, make complete proteins with all nine essential amino acids. The two foods do not have to be eaten at the same meal to be used together by the body.
• Limit sugar and salt intake. Fresh foods generally have less of these two nutrients than processed foods. For packaged foods, read nutrition facts labels and choose options that are lower in sodium and sugars. Keep in mind that sugar may come in many forms, including high fructose corn syrup. Put away the salt shaker and sugar bowl so you won’t be tempted to add extra sodium or sugar at the table.
• Limit saturated fats. Eat more fish and legumes and less red meat. Use nut and vegetable oils instead of butter or other fats derived from animals.
• Always check for trans fats on nutrition facts labels. Try to avoid these harmful artificial fats completely.
• Increase omega-3 fatty acids. Foods that contain these essential fatty acids include salmon, walnuts, flax seeds, and canola oil.
• Stay hydrated. Eat foods high in water, such as fruits and vegetables. Also, try to drink 2 liters (about 8 cups) of fluids each day. Choose water or unsweetened beverages such as tea or coffee instead of sweetened beverages. Sweetened drinks such as soft drinks contain no nutrients except sugar. Frequent consumption of sweetened beverages is a major risk factor for metabolic syndrome, obesity, type 2 diabetes, and cardiovascular disease.
• Engage in at least 30 minutes of physical activity most days of the week. Besides all the other benefits of exercise to human health, such as strong bones and muscles, exercise uses energy that helps to balance the Calories in food.
Feature: My Human Biology
The Zone Diet, Blood Type Diet, Paleo Diet, grapefruit diet — no doubt, you’ve heard of them all. Such diets — often referred to as fad diets — certainly feature prominently in the media, and many people try them. If you want to lose weight, you may be thinking about trying a fad diet yourself. And why not? Fad diets are certainly appealing. They often promise quick weight loss without exercise. They also generally spell out exactly what you can and cannot eat. This makes it easier for some people to consume less in the short term. But do fad diets really work, and are they healthy ways to lose weight?
In reality, most fad diets do not lead to significant, long-term weight loss. People may lose a lot of weight initially, but the weight loss is likely to be due to loss of water rather than fat. In addition, many fad diets are unhealthy because they are unbalanced. They typically restrict or eliminate foods — such as fruits, dairy, or whole grains — that should be the basis of a healthy eating plan because they are dense in critical nutrients. At the same time, such diets may recommend overconsumption of certain nutrients that can actually compromise health when eaten in excess. For example, high-protein diets can put a strain on the kidneys and potentially lead to kidney stones and gout. Fad diets that are restricted to certain foods also quickly become boring and difficult to stick with. They generally are not intended to become a healthy, lifetime eating plan. Once the weight is lost (if it is), dieters usually go back to their old ways of eating and regain the weight.
How can you tell if a diet is a fad diet? Ask these questions about it:
1. Does the diet promise rapid weight loss (> 2 pounds a week)?
2. Does the diet claim that the weight loss will be from certain parts of the body (such as “stubborn belly fat”)?
3. Does the diet claim to work even without exercise?
4. Does the diet help sell a product, such as prepackaged meals, pills, or books?
5. Does the diet lack well-validated scientific evidence to back its claims?
6. Does the diet use “before and after” photos or testimonials from individual dieters to “prove” that the diet works?
7. Does the diet identify “bad” foods and “good” foods?
8. Does the diet require following a rigid menu or meal plan?
9. Does the diet sound too good to be true?
If you can answer “yes” to even one of these questions, then the diet is likely to be a fad diet. If you are serious about losing weight safely and permanently, avoid fad dieting. Instead, follow the general eating and physical activity recommendations made in this concept as well as the following specific tips.
• Practice portion control. This means knowing serving sizes, which are generally smaller than most people think. You can use nutrition facts labels and the table below to avoid super-sizing your food.
Table \(1\): Serving Size Comparisons for Selected Types of Food
Type of Food Serving Size Comparison
Raw leafy vegetables
Baked potato
small fist
Milk or yogurt
Fresh fruit
baseball
Cereal
Bread
hockey puck
Meat
Poultry
computer mouse
Fish checkbook
Cheese six stacked dice
Margarine
Butter
one die
• Eat breakfast every day and do not skip meals. This will keep your metabolism fired up so you use more energy. If you go long periods without eating, your body goes into starvation mode and starts “hoarding” Calories.
• Get off the couch. Choose physical activities that you enjoy so you will do them regularly. The only exercise that works is the one you will actually do. Also, include more physical activity throughout each day. Park farther from your destination and walk the rest of the way. Take the stairs whenever you can. Wearing a pedometer may challenge you to reach the recommended 10,000 steps a day.
• Follow MyPlate but use a smaller plate. If you use a salad plate instead of a dinner plate, for example, the same-sized serving of food will look larger. You will eat less without feeling deprived.
Following the healthy-eating guidelines in this concept and the specific tips above should lead to a slow but steady weight loss of a pound or two a week. Losing weight slowly may be frustrating, but it is more likely to stay off than weight that is lost rapidly. It took you time to gain excess weight so it will take time to lose it. The only way to keep it off is to establish a healthy pattern of eating and physical activity that you can live with lifelong.
Review
1. Why is healthy eating important for good health?
2. Define nutrition. What does good nutrition depend on?
3. Define malnutrition, and identify types of malnutrition.
4. Briefly describe what healthy eating means.
5. What is nutrient density? How is it related to nutritious food choices?
6. Define energy homeostasis, and identify factors that regulate or influence energy homeostasis.
7. What is energy density, and how is it related to energy homeostasis?
8. What are the recommended daily values of nutrients?
9. List factors that influence nutrient needs of individuals.
10. Describe MyPlate. What food groups is it based on?
11. What information do nutrition facts labels provide? How can the information be used to choose the most nutritious food options?
12. Give two tips for healthy eating.
13. If there are 100 Calories per 100 grams of a baked potato, and 5.5 Calories per 1 gram of potato chips, which has a higher energy density — a baked potato or potato chips? Explain your answer.
14. When comparing two multivitamins in the store, you see that one has 500% of the daily value for a particular vitamin, while the other has 100% of the daily value for that vitamin. Is the one with 500% of the daily value necessarily better for your health? Explain your answer.
15. Explain why it is better for your health to eat whole fruits instead of drinking fruit juices.
Explore More
Many people feel passionate about only eating natural, healthy foods and seek dietary advice from many different sources. In today's time it can be hard to distinguish fact from fiction, learn about consuming cholesterol here:
Attributions
1. Yoga by YogawithAmit via Pixabay license
2. Corn and black bean salad by National Cancer Institute, public domain via Wikimedia Commons
3. BMI by Adriana Arcaia, Janet Woolen, and Suzanne Bakken (Article: A Systematic Method for Exploring Data Attributes in Preparation for Designing Tailored Infographics of Patient Reported Outcomes), CC BY 4.0 via Research Gate
4. My plate infographic by United States Department of Agriculture, public domain via Wikimedia Commons
5. Nutrition label by USFDA, public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/04%3A_Nutrition/4.3%3A_Healthy_Eating.txt |
Before and After
The child on the left in this sketch is in the throes of a serious disorder. They are extremely emaciated and on the brink of death. The same child is depicted on the right after being treated for their disorder. They have gained weight and appear to be healthy. What disease wreaked such havoc on this child’s health? Was it cancer? Some terrible parasitic infection? The answer may surprise you. The disease that caused such serious health consequences is a mental health disorder, specifically, the eating disorder anorexia nervosa.
What Are Eating Disorders?
Eating disorders are mental health disorders defined by abnormal eating habits that adversely affect health. Eating disorders typically begin during late childhood, adolescence, or early adulthood. In developed countries such as the United States, they occur in about 4 percent of people. In a study of 9,713 participants showed that about 5.5% of young males (adolescents and young adults from 12 colleges and universities) manifested elevated eating disorder risk in the United States (Mariusz Jaworski et al., 2019). In developing countries, they are less common but increasing in frequency. Eating disorders are serious diseases and can even be fatal. In fact, they result in about 7,000 deaths a year in the United States, making them the mental disorders with the highest mortality rate.
Major Eating Disorders
Common eating disorders in the United States include anorexia nervosa, bulimia nervosa, and binge eating disorder. They differ in the patterns of disordered eating that characterize them, but all of them can have life-threatening health consequences. They may also have similar causes.
Anorexia Nervosa
Anorexia nervosa is an eating disorder in which people consistently eat very little even though they may be obsessed with food. They typically have an obsessive fear of gaining weight. They also usually have an unrealistic perception of their own low body weight and see themselves as fat even when they are very thin. This misperception of one’s true body size and shape, along with dissatisfaction with that misperception, is called body dysmorphic disorder. It is very common in people with eating disorders such as anorexia nervosa. In fact, the majority of people with body dysmorphic disorder also have an eating disorder.
The food restriction in anorexia nervosa results in excessive weight loss and often amenorrhea (cessation of menses) in females. Other serious consequences of this pattern of eating include loss of bone mass; weakening of the heart and other muscles; abnormally low heart rate and blood pressure; and weakness, dizziness, and fainting. Because of such consequences, there is a significant risk of heart failure, and this can result in death.
Bulimia Nervosa
Bulimia nervosa is an eating disorder in which people recurrently binge on large amounts of food. Because of the extreme fear of gaining weight, each binge is usually followed by trying to purge the food from the body. This may be done — more or less successfully — by vomiting, using laxatives or enemas, taking diuretics, or exercising excessively.
People with bulimia nervosa may or may not have an abnormal weight, but they are likely to develop an electrolyte imbalance due to the repeated binging and purging. This imbalance may cause an irregular heartbeat, which can lead to heart failure and death. Frequent vomiting can also cause rupture of the stomach or esophagus, which can be fatal, as well as erode dental enamel.
Binge Eating Disorder
Binge eating disorder is an eating disorder in which people repeatedly binge on large amounts of food, and each binge is followed by feelings of guilt but not by purging. Adverse health impacts of binge eating disorder include excessive weight gain, obesity, high blood pressure, and high cholesterol. People with binge eating disorder are also at increased risk of gallbladder disease, cardiovascular disease, and type 2 diabetes.
Causes of Eating Disorders
The causes of eating disorders are not fully understood and are likely to vary among individuals. However, in virtually all cases, both biological and environmental factors appear to play a role.
Biological Factors
Genes are likely to be involved in the development of eating disorders because having a close biological relative with an eating disorder increases one’s own risk tenfold or more. At a biochemical level, eating disorders are thought to be caused in part by the deregulation of neurotransmitters such as serotonin and dopamine. Serotonin normally has an inhibitory effect on eating and dopamine regulates the rewarding property of food. Imbalance in these neurotransmitters is likely to affect appetite and eating behavior. Deregulation of the hormones leptin and ghrelin may also be involved in eating disorders. These two hormones normally help maintain the body’s energy balance by increasing or decreasing food intake. This occurs through the regulation of appetite and eating behavior. Leptin is produced mainly by fat cells in the body. It normally inhibits appetite by inducing a feeling of satiety. Ghrelin is produced in the stomach and small intestine. Its normal role is to stimulate the appetite. If these hormones are out of balance, the imbalance will affect appetite and may lead to disordered eating.
Environmental Factors
A number of environmental factors have also been shown to increase the risk of developing eating disorders. One of the most salient is abuse suffered as a child, including physical, psychological, or sexual abuse. Child abuse has been shown to triple the risk of developing an eating disorder. Parental pressure to control a child’s eating habits can also increase the risk, as can having a fragile sense of self-identity. In older individuals, social isolation increases the risk of eating disorders.
For women, cultural ideal relates to slenderness, and for men to the musculature that is thought to be a major contributing cause of anorexia and bulimia nervosa. Dancers (like the one pictured below), jockeys, and athletes such as gymnasts are the groups of young individuals who may feel exceptional pressure to be thin. Up to 12 percent of dancers develop anorexia or bulimia, compared with about 2 percent of individuals in the general population.
Treatment and Recovery
Treatment of eating disorders varies according to the type and severity of the eating disorder. Usually, more than one treatment option is used. Treatment typically includes mental health counseling, which can take place in a variety of settings, such as a community program, private practice, or hospital. Treatment may also include the use of antidepressants or other medications because many people with eating disorders also suffer from depression or other mental health disorders. Nutritional counseling is often recommended as well. Hospitalization is occasionally required, in many cases to treat the adverse physical health consequences of the disordered eating.
The goal of treatment is recovery, including gaining control of eating, adopting normal eating habits, and attaining a normal weight. About 50 to 85 percent of people with eating disorders recover with treatment. However, some may have to struggle to maintain normal eating behaviors throughout the rest of their life.
Feature: Reliable Sources
People with anorexia nervosa, as with many other health problems, may seek information and advice online before or instead of contacting a healthcare professional. The web offers a plethora of useful information on eating disorders, including anorexia nervosa, but some websites, blogs, and social media pages actually have the agenda of promoting disordered eating. The term pro-ana (from “pro-anorexia”) refers to organizations, websites, and other sources that promote anorexia nervosa. Their mission is to normalize or even glamorize anorexia nervosa. They defend it as a lifestyle choice and an accomplishment of self-control rather than as a mental disorder. Research has shown that visiting pro-ana sites can have a negative impact on eating behavior in people both with and without eating disorders. After visiting such sites, people tend to decrease their Caloric intake, although most of them do not actually perceive that they have reduced their intake of Calories.
Following a 2001 episode of the Oprah Winfrey Show that focused on pro-ana, the mainstream press started covering the issue. Pressure from the public and pro-recovery organizations led to some social media and other websites adopting policies of blocking pro-ana pages or labeling them with warning messages. As a result, many pro-ana groups have taken steps to conceal themselves. For example, they may claim that they are simply providing a nonjudgmental forum for people with anorexia nervosa to discuss their disorder. They may also claim that they exist in part to provide support for those who choose to enter recovery.
Some clues that a website or page may be pro-ana include providing information on topics such as:
• crash dieting techniques and recipes.
• socially acceptable pretexts for refusing food, such as veganism.
• ways to hide weight loss from parents and doctors.
• reducing the adverse health effects of anorexia.
• ways to ignore or suppress hunger pangs.
Do you think you can tell the difference between pro-ana websites and legitimate pro-recovery websites, which are designed to encourage the development and maintenance of healthy behaviors and cognition? Go online and try to find at least one pro-ana website and at least one pro-recovery website. Then write a brief explanation of how you made your choices.
Review
1. What are eating disorders? How serious are they?
2. What demographic group is most likely to be diagnosed with eating disorders?
3. Describe anorexia nervosa and its adverse effects on health.
4. What is bulimia nervosa? How does it affect health?
5. Define binge eating disorder, and identify its health consequences.
6. Why are genes likely to be involved in the development of eating disorders?
7. Explain how the deregulation of biochemicals may be involved in eating disorders.
8. Discuss environmental factors that may increase the risk of eating disorders.
9. Identify types of treatment for eating disorders. How effective is the treatment likely to be
10. What is a common ultimate cause of death in people with anorexia nervosa and bulimia nervosa?
11. True or False. Someone who is a normal weight cannot have an eating disorder.
12. True or False. The neurotransmitter serotonin normally has an inhibitory effect on eating.
13. When you are feeling hungry, what do you think are the relative levels of your ghrelin and leptin hormones? Explain your answer.
14. Which disorder is most likely to affect teeth enamel?
1. Anorexia nervosa
2. Binge eating disorder
3. Bulimia nervosa
4. None of the above
1. Female dancers are how many times as likely to develop anorexia or bulimia than women in the general population.
2. half
3. just
4. two times
5. six times
Explore More
In the following video, the speaker touches on the fact that eating disorders are about control in a very sober and balanced way.
America has a cultural obsession with thinness and its correlation with beauty. Watch this video by underwear model Cameron Russell and learn her take on why looks aren't everything:
Attributions
1. Anorexia by Wellcome Gallery licensed CC BY 4.0 via Wikimedia Commons
2. David Motta by Alexey Yakovlev licensed CC BY-SA 4.0 via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/04%3A_Nutrition/4.4%3A_Eating_Disorders.txt |
Too Much of a Good Thing
Everybody needs food energy just to stay alive, but too much energy consumption, coupled with too little energy use, is too much of a good thing. People who consistently consume more food energy than they use may become obese like the woman pictured here.
What Is Obesity?
Obesity is a disease in which excess body fat has accumulated to the extent that it is likely to have negative effects on health. Obesity is commonly diagnosed on the basis of the body mass index (BMI). BMI is an estimate of body fatness based on a person’s weight relative to their height. BMI is calculated by dividing a person’s weight (in kilograms) by the square of the person’s height (in meters). The graph below shows how BMI is used to categorize obesity and other body mass classes for adults. A BMI value of 18.5 to 25 kg/m2 is considered normal. A BMI below 18.5 kg/m2 is considered underweight. A BMI between 25 and 30 kg/m2 places a person in the overweight range. A BMI greater than 30 kg/m2 classifies a person as obese.
Obesity in children and adolescents is generally defined not simply by BMI but by comparison of BMI with reference values. Obesity is diagnosed when a child has a BMI that is greater than the 95th percentile of the reference values for children of the same age. This means that the child’s BMI is higher than that of 95 percent of same-aged children in the reference sample. The reference values are based on large samples of children from the mid-to-late 20th century, before the recent rise in childhood obesity.
Categories of Obesity
The medical profession breaks down obesity into additional categories, although the exact delineation of the categories is not universally agreed upon. Commonly accepted categories include:
• severe obesity, which is diagnosed when a person has a BMI ≥ 35 kg/m2.
• morbid obesity, which is diagnosed when a person has a BMI ≥ 35 kg/m2 and obesity-related health problems, or when a person has a BMI ≥ 40 kg/m2 but < 45 kg/m2.
• super obesity, which is diagnosed when a person has a BMI ≥ 45 kg/m2
Fat Distribution in Obesity
Obesity can also be categorized in terms of fat distribution, as measured by waist-to-hip ratio (waist circumference divided by hip circumference). A waist-to-hip ratio greater than 0.85 for women or 0.90 for men is diagnostic of central obesity, in which most of the excess fat is stored in the abdomen. This type of fat distribution gives a person an apple shape, like the man pictured above. People with central obesity are at greater risk of the adverse health consequences of obesity than people who store most of their excess fat around the hips (giving them a pear shape). Because it accounts for the elevated risks associated with central obesity, waist-to-hip ratio is a better predictor than BMI of mortality in older obese patients
Causes of Obesity
Like many other diseases, most cases of obesity are the result of an interplay between genetic and environmental factors. Obesity is most commonly caused by a combination of excessive food intake, inadequate physical activity, and genetic susceptibility.
Genetic Influences on Obesity
Various genes that control appetite and metabolism predispose people to develop obesity when sufficient food energy is present. It is likely that dozens of such genes exist. Family studies reveal the strength of the genetic influence on obesity. When both parents are obese, 80 percent of their offspring will also be obese. For comparison, when both parents are of normal weight, less than 10 percent of their offspring will be obese.
Diet and Obesity
From 1971 to 2000 in the United States, the average amount of food consumed by women actually increased by 335 Calories per day, and by men by 168 Calories per day. During the same period, the rate of obesity in U.S. adults increased from about 15 to 31 percent. Most of the extra food energy came from an increase in carbohydrate consumption. Primary sources of these extra carbohydrates were sugar-sweetened beverages, like those pictured below. Sugary beverages include not only soft drinks but also fruit drinks, sweetened iced tea and coffee, and energy and vitamin water drinks. Such drinks now account for almost 25 percent of daily food energy in young adults in the United States. This is an alarming statistic, given that these drinks provide no other nutrients except energy.
Activity Levels and Obesity
A sedentary lifestyle plays a significant role in obesity. Worldwide, there has been a large shift toward less physically demanding work. There has also been an increased reliance on cars and labor-saving devices at home. Currently, an estimated 30 percent of the world’s population gets insufficient exercise.
Other Causes of Obesity
A minority of cases of obesity are caused by certain medications or by other diseases. Medications that may increase the risk of obesity include antidepressant and antipsychotic drugs, steroids such as prednisone, and some forms of hormonal contraception, among others. Diseases that increase the risk of obesity include hypothyroidism, Cushing’s disease, binge eating disorder, and Prader-Willi syndrome.
Consider Prader-Willi syndrome as an example. A young child with this syndrome is shown in Figure \(3\). The syndrome occurs due to the loss of function of specific genes on chromosome 15. Symptoms of the syndrome include constant hunger, which typically leads to severe obesity in childhood. Prader-Willi syndrome is caused by genetic defects but it is not generally inherited. Instead, the genetic changes happen during the formation of the egg or sperm or during embryonic development.
Pathophysiology of Obesity
A recently proposed physiological mechanism for the development of obesity is leptin resistance. Leptin is called the satiety hormone. It is secreted by fat cells and helps to regulate appetite based on the body’s fat reserves. When fat reserves are high, more leptin is secreted and appetite is inhibited, so you eat less. The opposite occurs when fat reserves are low. In obesity, decreased sensitivity to leptin occurs, resulting in an inability to detect satiety despite high-fat reserves. As a consequence, people with leptin resistance never feel satiated and are likely to overeat and gain more weight.
Health Consequences of Obesity
As shown in Figure \(4\), obesity increases the risk of many health problems and diseases, including:
• sleep apnea
• lung disease, asthma, pulmonary blood clots
• liver disease, fatty liver, cirrhosis
• gallstones
• cancer of breast, uterus, colon, esophagus, pancreas, kidney, prostate
• stroke
• heart disease, abnormal lipid profile, high blood pressure
• diabetes
• pancreatitis
• abnormal periods and infertility
• arthritis
• inflamed veins often with blood clots
• gout
The health consequences of obesity are mainly due to the effects of either increased fat mass or increased numbers of fat cells. Extra weight from excess body fat places a lot of stress on the body and its organ systems, causing diseases such as osteoarthritis and obstructive sleep apnea. An increased number of fat cells increases inflammation and the risk of blood clots. It also changes the body’s metabolism, altering the body’s response to insulin and potentially leading to insulin resistance and type 2 diabetes. This explains why the link between obesity and type 2 diabetes is so strong. Obesity is thought to be the root cause of 64 percent of cases of type 2 diabetes in men and 77 percent of cases in women.
Not surprisingly, obesity has been found to reduce life expectancy. On average, obesity reduces life expectancy by six or seven years. Super obesity reduces life expectancy by as much as ten years.
Treating and Preventing Obesity
Most cases of obesity are treatable or preventable through changes in diet and physical activity that restore energy balance to the body. In fact, obesity is one of the leading preventable causes of disease and death worldwide. The amount of energy provided by the diet can be reduced by decreasing consumption of energy-dense (high-Calorie) foods, such as foods high in fat and sugar, and increasing consumption of high-fiber foods. The fiber in the diet cannot be digested, so it adds bulk and a feeling of fullness without adding Calories. All types of low-carbohydrate and low-fat diets appear equally beneficial in reducing obesity and its health risks. In some cases, medications may be prescribed to help control obesity by reducing appetite or fat absorption.
Bariatric Surgery
If changes in diet and exercise and even medications are not effective, bariatric surgery may be recommended as a treatment for obesity. Bariatric surgery is the single most effective medical treatment for obesity. There are several different types of bariatric surgery, one of which is illustrated in figure \(5\). In this particular type of surgery, the size of the stomach is greatly reduced so less food can be eaten at a time. The length of the small intestine is also reduced so fewer nutrients can be absorbed from food.
Public Health Approaches
Public health approaches to the problem of obesity include efforts to understand and correct the environmental factors responsible for increasing obesity rates. The goals are to reduce food energy consumption and promote energy expenditure in physical activity. Efforts to reduce energy consumption include promoting healthy meals and limiting access to sugary beverages and junk foods in schools. Efforts to promote physical activity include increasing access to parks and developing pedestrian routes in urban environments.
Review
1. Define obesity.
2. How is obesity generally diagnosed in adults? In children?
3. Compare and contrast severe obesity, morbid obesity, and super obesity.
4. What is central obesity? What is its relationship to the adverse health consequences of obesity?
5. Give examples of medications and disorders that may cause obesity.
6. Discuss factors that cause most cases of obesity.
7. What is leptin resistance, and what is its connection with obesity?
8. Identify some of the health consequences of obesity.
9. Describe types of treatments available for obesity.
10. Describe public health approaches to treating and preventing obesity.
11. Which is likely to be worse for a person’s health — having their hip circumference be larger than their waist circumference, or vice versa? Explain your answer.
12. What factors is BMI based on?
13. Why is the recent increase in childhood obesity a public health concern?
Explore More
Watch this video to learn which is better for losing weight, diet or exercise:
Check out this video to learn about body mass index (BMI) and why it might not be the best measurement for obesity:
Attributions
1. Obesidad en Mexico by Mallinaltzin, CC BY 3.0 via Wikimedia Commons
2. Body mass index chart by InvictaHOG, public domain via Wikimedia Commons
3. PWS8 by Fanny Cortés M1, M. Angélica Alliende R1,a, Andrés Barrios R1,2, Bianca Curotto L1,b, Lorena Santa María V1,c, Ximena Barraza O3, Ledia Troncoso A2, Cecilia Mellado S4,6, Rosa Pardo V, CC BY 4.0 via Wikimedia Commons
4. Medical complications of obesity by CDC, public domain via Wikimedia Commons
5. Roux-En-Y by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/04%3A_Nutrition/4.5%3A_Obesity.txt |
Then and Now
This photo was taken in the late 1960s. It shows an American volunteer nurse and their patients at an orphanage in Nigeria. The children’s distended bellies are signs of kwashiorkor, a severe form of undernutrition caused by inadequate Calorie and protein intake, usually exacerbated by frequent bouts of infectious disease. Kwashiorkor and other forms of undernutrition still occur at high rates in Nigeria and many other places in the world. Today, one out of every seven people on our planet goes to bed hungry, and 25,000 people a day die of hunger-related diseases. Clearly, undernutrition is still a major problem now as it was then.
Defining Undernutrition
Undernutrition is defined as an insufficient intake of nutritious foods. People who are undernourished are likely to have low body fat reserves, so one indicator of undernutrition in individuals is a low body mass index (BMI). Adults are considered underweight if their body mass index (BMI) is less than 18.5 kg/m2. Children are considered underweight if their BMI is less than the 5th percentile of the reference values for children of the same age.
Undernutrition in Children
The effects of undernutrition are particularly important during childhood when energy and other nutrients are needed for normal growth and development. Children are more likely than adults to become severely undernourished as well as to suffer permanent effects from undernutrition. They may become dangerously thin, with loss of muscle as well as fat. This is called wasting (see figure \(2\)). If they lack adequate energy for growth, they will stop growing. If they are chronically undernourished, the growth deficits may cause them to be too short for their age. This is called stunting. Unless adequate nutrition becomes available later so they can make up their growth deficits, stunted children will end up shorter than their genetic potential for height by the time they are adults.
Undernutrition and Infection
Undernutrition and infectious diseases in children have a positive synergistic relationship. Each increases the risk of the other and makes the other worse. Children who are undernourished may be weakened and have a less than robust immune system. This makes them more susceptible to infectious diseases and likely to become sicker when they have infectious diseases. Children who are sick with infectious diseases may need more nutrients to defend against infection. At the same time, they may have reduced intake or absorption of nutrients due to symptoms such as vomiting and diarrhea. In these ways, infectious disease increases the risk of undernutrition or makes existing undernutrition worse.
Undernutrition and Low Birthweight
Many children are born with the disadvantage of low birth weight (< 2.5 kg, or 5.5 lb.) caused by maternal undernutrition and intrauterine growth restriction. Babies with low birth weight are more susceptible to disease and more likely to die in infancy. In children that survive infancy, low birthweight may result in slow growth and developmental delays throughout early childhood.
Undernutrition Syndromes
Severe cases of undernutrition may develop into life-threatening syndromes such as kwashiorkor, as in the Nigerian orphans pictured above. Another common severe undernutrition syndrome is called marasmus.
Kwashiorkor
Kwashiorkor was first described in the medical literature in the 1930s. The name comes from a West African word meaning “disease of the deposed child.” The original meaning of the term is a clue to the cause of this syndrome. If a young child is weaned from the breast so a new baby can be breastfed, the “deposed child” is likely to go from a mostly breastmilk diet, which is high in protein, to a mostly plant-food diet, which is low in protein. Although Kwashiorkor may occur in a child who lacks protein but not Calories, it occurs more often when the diet is also deficient in Calories. That’s why kwashiorkor is commonly called protein-Calorie malnutrition.
The defining sign of kwashiorkor in an undernourished child is edema (swelling) of the ankles and feet and often a distended belly, both of which are clearly visible in the child pictured below. The lack of protein causes osmotic imbalances that prevent tissue fluids from being returned to the bloodstream. This mechanism accounts for the accumulation of fluid in the ankles, feet, and abdomen. Other common signs of kwashiorkor include enlarged liver (which contributes to the abdominal distension), thinning hair, loss of teeth, skin rash, and skin and hair depigmentation. Children with kwashiorkor may also be irritable and lose their appetite. Kwashiorkor can have a long-term impact on a child's physical and mental development and frequently leads to death without treatment.
Marasmus
Marasmus comes from a Greek word meaning “wasting away.” It is a severe undernutrition syndrome caused by extremely low intakes of food energy. Signs and symptoms of marasmus include wasting (as depicted above, low body temperature, anemia, dehydration, weak pulse, and cold extremities. Without treatment, marasmus is often fatal, although it generally has a better prognosis than untreated kwashiorkor.
Micronutrient Deficiencies
Besides deficiencies in food energy and protein, many undernourished people suffer from deficiencies of specific vitamins or minerals. Some of the most common micronutrient deficiencies worldwide are iron, vitamin A, and iodine deficiencies.
Iron Deficiency
Iron deficiency is the single most common micronutrient deficiency worldwide, affecting about 2 billion people. Iron deficiency, in turn, causes anemia, which is especially common in women and children under the age of five years. Anemia can lead to increased mortality in infancy and poor cognitive and motor development in early childhood. The problems caused by iron-deficiency anemia in childhood cannot be reversed.
Vitamin A Deficiency
Vitamin A deficiency is also very common in developing countries. In young children, vitamin A plays an essential role in the development of the immune system, so vitamin A deficiency adversely affects the ability of the immune system to fight off infections. Vitamin A deficiency also contributes to anemia and causes visual impairments, ranging from night blindness (inability to see well at low light levels) to total blindness.
Iodine Deficiency
Since the early 1900s, iodine has been added to salt in many countries, including the United States and most of Europe, virtually eliminating iodine deficiency in these countries. However, inadequate iodine intake is still a public health problem in dozens of countries, and about 30 percent of the world’s people are iodine deficient.
Iodine is needed for thyroid hormone production. In adults, iodine deficiency causes reversible signs and symptoms of inadequate thyroid hormone. These may include an enlarged thyroid gland, called a goiter (see the photo above), and a sluggish metabolism. In children, iodine deficiency is much more serious. It causes permanent intellectual disability because thyroid hormone is needed for normal brain growth and development, from the fetal stage through early childhood. Iodine deficiency is the most important cause of preventable intellectual disability in the world.
Causes of Undernutrition
A small percentage of undernutrition occurs because of diseases such as cancer, anorexia nervosa, celiac disease, and cystic fibrosis (all of which you can read about in other concepts). However, the vast majority of undernutrition globally occurs because people simply don’t have enough nutritious food to eat. They take in less energy than the minimum daily energy requirement so they are underweight, and they are likely to have other nutritional deficiencies as well.
Worldwide food supplies are adequate to provide food to all if the food supplies were equally distributed and accessible to everyone. Unfortunately, that is not the case. Adequate food is not available to people over large areas of Africa and Asia. Even if food supplies were equally distributed, most undernutrition would still occur in these areas because of the inability of many people to access food due to poverty. Poverty is a consequence as well as a cause of undernutrition, and the two form a self-perpetuating cycle. Impoverished individuals are less likely to have access to enough nutritious food for good health and for normal growth and development. As a result, they are more likely to be undernourished. Undernutrition, in turn, makes them less likely to attend or perform well in school and as adults to be less productive workers, thus limiting their income.
Although undernutrition is not as common in the richer nations of the world as it is elsewhere, it still occurs in significant proportions of people. Even in a land of plenty like the United States, socioeconomic disparities result in some people being undernourished due to lack of access to sufficient nutritious food. The photo below, taken in New York City, shows a woman searching for edible food in a garbage dumpster. Besides lacking adequate money to buy nutritious foods, many poor people in the United States live in areas that are considered “food deserts,” defined as areas with limited access to nutritious foods. In these food deserts, supermarkets with fresh produce and other nutritious food choices are typically too far away for people to utilize. Instead, they must rely on small neighborhood stores that stock mainly over-priced processed foods or on fast food restaurants that offer primarily high-fat and high-sugar food options. With social inequalities on the rise in the United States and some other rich nations, problems of food access and undernutrition may worsen.
Treatment and Prevention of Undernutrition
Treating and preventing undernutrition is a huge and complex problem requiring multifaceted approaches. Potential solutions must target both individual and public health. Meeting individual needs is generally achieved through direct nutrition interventions (like the one illustrated in the photo above). In such interventions, the health-care sector typically delivers nutritional supplements directly to acutely malnourished people who might otherwise require hospitalization. Public health interventions may focus on improvements in agriculture, water, sanitation, or education, among other public health targets. Some of the most successful public health interventions have been those aimed at eliminating specific micronutrient deficiencies. For example, in the early 1990s, iodine deficiency was addressed by a global campaign to iodize salt. This campaign reduced the rate of iodine deficiency from about 70 percent to 30 percent.
Review
1. Define undernutrition.
2. How is underweight status determined in adults? How is it determined in children?
3. Why is undernutrition a more significant problem in children than adults?
4. What are wasting and stunting?
5. Describe kwashiorkor.
6. What is marasmus?
7. List three of the most common micronutrient deficiencies worldwide. Describe how each deficiency affects health.
8. Why do the vast majority of cases of undernutrition occur?
9. Explain how undernutrition and poverty are related.
10. Why does undernutrition occur even in the richer nations of the world?
11. How should the problem of global undernutrition be tackled?
12. Which best describes the relationship between undernutrition and infectious disease?
1. Undernutrition can increase the risk of infectious disease.
2. Infectious disease can increase the risk of undernutrition.
3. Undernutrition and infectious disease are independent of one another.
4. A and B
13. True or False. A diet that has enough energy but is deficient in protein can result in a type of undernutrition syndrome.
14. True or False. The distended belly seen in kwashiorkor is due primarily to the stomach expanding in response to lack of food in it.
Attributions
1. Kwashiorkor by CDC, public domain via Wikimedia Commons
2. Worldvision muac by World Vision Deutschland, licensed CC BY 3.0 via Wikimedia Commons
3. Starved girl by Dr. Lyle Conrad for CDC, public domain via Wikimedia Commons
4. Girl affected by famine by Fridtjof Nansen, public domain via Wikimedia Commons
5. Woman with goiter by Martin Finborud, public domain via Wikimedia Commons
6. Hunger map by Allice Hunter, licensed CC BY-SA 4.0 via Wikimedia Commons
7. Royal food NY by Carlos. A. Martinez, licensed CC BY 2.0 via Wikimedia Commons
8. Children receive PlumpyNut nutritional aid by USAID Africa Bureau, public domain via Wikimedia Commons
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/04%3A_Nutrition/4.6%3A_Undernutrition.txt |
Picnic Fun
Picnics like this one can be a lot of fun. Food always seems to taste better when eaten outdoors. Ants and other insects can be attracted to picnic foods and be annoying. However, a greater potential hazard may lurk within the picnic foods themselves: microorganisms that cause foodborne disease.
What Is Foodborne Disease?
Foodborne disease, commonly called food poisoning, is any disease that is transmitted via food. Picnic foods create a heightened risk of foodborne disease mainly because of problems with temperature control. If hot foods are not kept hot enough or cold foods are not kept cold enough, foods may enter a temperature range in which microorganisms such as bacteria can thrive.
Many people do not think about food safety until a foodborne disease affects them or a family member. While the food supply in the United States is one of the safest in the world, the CDC estimates that 76 million Americans a year get a foodborne disease, of whom more than 300,000 are hospitalized and 5,000 die. Preventing foodborne disease remains a major public health challenge.
Causes of Foodborne Disease
Most foodborne diseases are caused by microorganisms in food. Some are caused by toxins in food or adulteration of food by foreign bodies.
Microorganisms
Microorganisms that cause foodborne diseases include bacteria, viruses, parasites, and prions. The four most common foodborne pathogens in the United States are a virus called norovirus and three genera of bacteria: Salmonella species (such as Salmonella typhimurium, pictured below), Clostridium perfringens, and Campylobacter jejune. Although norovirus causes many more cases of foodborne disease, Salmonella species are the pathogens in food that are most likely to be deadly. Parasites that cause human foodborne diseases are mostly zoonoses — animal infections that can be transmitted to humans. Parasites such as pork tapeworm (Taenia solium) are ingested when people eat inadequately cooked infected animal tissue. The prions that cause mad-cow disease have been transmitted to people through the ingestion of contaminated beef.
Toxins
Toxins are another common cause of foodborne disease. Toxins may come from a variety of sources. Foods may be contaminated with toxins in the environment. Pesticides applied to farm fields are common examples of environmental food toxins. Toxins may be produced by microorganisms in food. An example is botulism toxin that is produced by the bacterium Clostridium botulinum. Some toxins occur naturally in certain plants and fungi. A common example is mushrooms. Dozens of species are poisonous and some are deadly, like the aptly named death-cap mushroom pictured below. Many deadly mushrooms look similar to edible species, making them even more dangerous. Food plants can also be infected with fungi that make people sick when they eat the plants. Fungi in the genus Aspergillus are frequently found in nuts, maize, and corn. They produce a toxin called aflatoxin, which targets the liver, potentially causing cirrhosis of the liver and liver cancer.
Adulteration by Foreign Bodies
Another potential cause of the foodborne disease is the adulteration of foods by foreign bodies. Foreign bodies refer to any substances or particles that are not meant to be foods. They can include pests such as insects, animal feces such as mouse droppings, hairs (human or nonhuman), cigarette butts, and wood chips, to name just a few. Some foods are at risk of contamination with lead or other toxic chemicals because they are stored or cooked in unsafe containers, such as ceramic pots with lead-based glaze.
Characteristics of Foodborne Diseases
Foodborne diseases differ in specific characteristics but they share some commonalities, often including similar symptoms.
Symptoms and Incubation Period
Foodborne diseases commonly cause gastrointestinal symptoms such as vomiting and diarrhea. They also frequently cause fevers, aches, and pains. The length of time between the consumption of contaminated food and the first appearance of symptoms is called the incubation period. This concept is illustrated in the figure below. The incubation period for a foodborne disease can range from a few hours to several days or even longer, depending on the cause of the disease. Toxins generally cause symptoms sooner than microorganisms. When symptoms do not appear for days, it is difficult to connect them with the agent that caused them.
During the incubation period, microbes generally pass through the stomach and into the small intestine. Once in the small intestine, they attach to cells lining the intestinal walls and begin to multiply. Some types of microbes stay in place in the intestine, although they may produce toxins that are absorbed into the bloodstream and carried to cells throughout the body. Other types of microbes directly invade deeper body tissues.
Infectious Dose
Whether a person becomes ill from a microbe or a toxin depends on how much of the agent was consumed. The amount that must be consumed to cause disease is called the infectious dose. It varies by disease agent and also by host factors, such as age and overall health.
Sporadic Cases vs. Outbreaks
The vast majority of reported cases of foodborne disease occur as sporadic cases in individuals. The origin of most sporadic cases is never determined. Only a small number of foodborne disease cases happen as part of disease outbreaks. An outbreak of a foodborne disease occurs when two or more people experience the same disease after consuming food from a common source. The majority of foodborne disease outbreaks originate in restaurants, but they also originate in nursing homes, hospitals, schools, and summer camps.
An example of a foodborne disease outbreak in the United States is the Salmonella outbreak shown in Figure \(5\). The CDC map below shows where most of the cases occurred. The reported cases began in July and were traced back to onions produced in California. Within 2 weeks the onions were recalled. The outbreak was over by October. Overall, a total of 1,127 people across 48 states, were infected with the outbreak strain of Salmonella Newport. There were 167 hospitalizations and no deaths reported.
Factors that Increase the Risk of Food Contamination
The foodborne disease usually arises from food contamination through improper handling, preparation, or storage of food. Food can become contaminated at any stage from the farmer’s field to the consumer’s plate.
Poor Hygiene
Many foods become contaminated by microorganisms because of poor hygienic practices, such as handling or preparing foods with unwashed hands. Consider norovirus, the leading cause of foodborne disease in the United States. The virus can easily contaminate food because it is very tiny and highly infective. People sick with the virus shed billions of virus particles. Unfortunately, It takes fewer than 20 virus particles to make someone else sick. Food can become contaminated with virus particles when infected people get stool or vomit on their hands and then fail to wash their hands before handling food. People who consume food can ingest the virus particles and get sick.
Cross-Contamination
Another major way that foods become contaminated is through cross-contamination. This occurs when microbes are transferred from one food to another. Some raw foods commonly contain bacteria such as Salmonella, including eggs, poultry, and meat. These foods should never come into contact with ready-to-eat foods, such as raw fruits and vegetables or bread. If a cutting board, knife, or counter-top is used to prepare contaminated foods, it should not be used to prepare other foods without proper cleaning in between.
Failure of Temperature Control
Foods contaminated with bacteria or other microorganisms may become even more dangerous if failure of temperature control allows the rapid multiplication of microorganisms. Bacteria generally multiply most rapidly at temperatures between about 4 and 60 degrees C (40 and 140 degrees F). Perishable foods that remain within that temperature range for more than two hours may become dangerous to eat because of rapid bacterial growth.
Prevention of Foodborne Disease
Preventing foodborne disease is both a personal and a society-wide problem. Both governments and individuals must work to solve it.
The Government’s Role
In the United States, the prevention of foodborne disease is mainly the role of government agencies such as the Food and Drug Administration and local departments of health. Such government agencies are responsible for setting and enforcing strict rules of hygiene in food handling in stores and restaurants (see the sign below). Government agencies are also responsible for enforcing safety regulations in food production, from the way foods are grown and processed to the way they are shipped and stored. Government regulations require that food to be traceable to their point of origin and date of processing. This helps epidemiologists identify the source of foodborne disease outbreaks.
Food Safety at Home
At home, the prevention of foodborne disease depends mainly on good food safety practices.
• Regular handwashing is one of the most effective defenses against the spread of foodborne diseases. Always wash hands before and after handling or preparing food and before eating.
• Rotate food in your pantry so older items are used first. Make sure foods have not expired before you consume them. Be aware that perishable foods such as unpreserved meats and dairy products have a relatively short storage life, usually just a few days in the refrigerator.
• Rinse fresh produce before eating. This is especially important if the produce is to be eaten raw. Even if you do not plan to eat the outer skin or rind, wash it because microbes or toxins on the surface can contaminate the inside when the food is cut open or peeled.
• Many bacteria in food can be killed by thorough cooking, but food must reach an internal temperature of at least 74 degrees C (165 degrees F) to kill any bacteria the food contains. Use a cooking thermometer like the one pictured below to ensure food gets hot enough to make it safe to eat.
• Foods meant to be eaten hot should be kept hot until served, and foods meant to be eaten cold should be kept refrigerated until served. Perishable leftovers should be refrigerated as soon as possible. Any perishable foods left at a temperature between 4 and 60 degrees C (40 and 140 degrees F) for more than two hours should be thrown out.
• Make sure the temperature in the refrigerator is kept at or below 4 degrees C (40 degrees F) to inhibit bacterial growth in refrigerated foods. If your refrigerator does not have a built-in thermometer, you can buy one to monitor the temperature. This is especially important in a power outage. If the temperature stays below 40 degrees F until the power comes back on, the food is safe to eat. If the temperature goes above 40 degrees F for two hours or more, the food may no longer be safe and should not be consumed.
• Keep the temperature of the freezer below 18 degrees C (0 degrees F). Foods frozen at this temperature will keep indefinitely, although they may gradually deteriorate in quality.
• Do not thaw foods at room temperature. Freezing foods does not kill microbes; it preserves them. They will become active again as soon as they thaw. Either thaw frozen foods slowly in the refrigerator or thaw them quickly in the microwave, cool water, or while cooking. Never refreeze food once it has thawed.
Feature: Myths vs. Reality
Myths about foodborne diseases abound. Some of the most common myths are debunked below.
Myth: It must have been the mayonnaise.
Reality: Mayonnaise is acidic enough that it does not provide a good medium for the growth of bacteria unless it becomes heavily contaminated by a dirty utensil or is mixed with other foods that decrease its acidity. Mayo may have gotten a bad rap because it is often consumed at picnics, where temperature control may be poor and lead to bacterial growth in other, non-acidic foods.
Myth: Foodborne disease is caused by food that has “gone bad.”
Reality: Eating spoiled or rotten food is seldom the cause of foodborne disease. Most cases of foodborne disease are caused by contamination of food by unwashed hands or cross-contamination of food by unwashed utensils or cutting boards.
Myth: Foodborne disease is caused by eating restaurant foods.
Reality: Foodborne disease is caused by contamination of foods in the home as well as in restaurants. Restaurant kitchens must be regularly inspected to ensure sanitary conditions for food preparation. There are no such inspections of home kitchens.
Myth: Foodborne disease is caused by the last food eaten.
Reality: Symptoms of the foodborne disease may not strike for several hours to several days following infection, so the last meal eaten may not be the culprit. This makes it very difficult to know which food caused the symptoms.
Review
1. What is a foodborne disease?
2. How common are foodborne diseases in the United States?
3. What are the main causes of foodborne disease? Give examples of each cause.
4. Define the incubation period and infectious dose.
5. Discuss similarities and differences among foodborne diseases.
6. Compare and contrast sporadic cases and disease outbreaks of foodborne disease.
7. What are the three main ways that food becomes contaminated?
8. List three food safety practices that can help prevent transmission of foodborne disease in the home.
9. If you store cooked leftovers at room temperature (about 68 degrees F) for more than two hours, are they safe to eat if you heat them up well first? Explain your answer.
10. True or False. There is no need to wash a melon before cutting it because you will not be eating the rind.
11. True or False. Foodborne diseases can sometimes cause a form of cancer.
12. Explain why it can be hard to trace the source of a foodborne disease if it has a long incubation period.
13. Which are a bacterial species that can cause foodborne disease?
1. Clostridium perfringens
2. Norovirus
3. Taenia solium
4. All of the above
14. Why do you think the incubation period for a foodborne disease is generally shorter when the agent is a toxin compared to a microorganism?
15. Why do you think it is often recommended to rapidly cool a large quantity of homemade soup by putting the pot in an ice water bath before storing it in the refrigerator?
Attributions
1. Picnic by Dylan Lake, licensed CC BY 2.0 via Wikimedia Commons
2. Salmonella by US gov, public domain via Wikimedia Commons
3. Amanita phalloides by George Chernilevsky, public domain via Wikimedia Commons
4. Concept of incubation period by Patilsaurabhr, public domain via Wikimedia Commons
5. Salmonella outbreak by CDC, public domain
6. Clean hands guardians of health by CDC/ Minnesota Department of Health, R.N. Barr Library; Librarians Melissa Rethlefsen and Marie Jones, public domain via Wikimedia Commons
7. Pork thermometer by USDA, public domain via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/04%3A_Nutrition/4.7%3A_Foodborne_Diseases.txt |
Case Study Conclusion: What’s Wrong with Fast Food?
What is wrong with fast food? That is the question that Carlos, who you read about at the beginning of the chapter, asked after learning that his friend Abdul eats it five or six times a week and thinks that this diet is not necessarily that bad. In order to find some answers, Carlos went to the website of Abdul’s favorite fast food restaurant and found nutritional information for Abdul's typical meal of a cheeseburger, large fries, and a large soda. Some of the information that Carlos found is shown in the tables below. Knowing what you now know about nutrition, what aspects of this meal could potentially be harmful to Abdul’s health if he eats like this frequently?
As Carlos already said to Abdul, fast food meals are often very high in calories. This meal has 1,350 Calories. A typical adult should consume around 2,000 Calories a day, so this single meal has more than half the calories typically needed by a person in one day. Some fast-food meals have even more calories. The cheeseburger in this meal has 540 Calories, which is typical for a moderately-sized fast-food cheeseburger. But some larger fast-food burgers, or burgers with more toppings, can have over 1,000 Calories! As you can see, it can be quite easy to exceed your calorie recommendation for the day if you eat a typical fast food meal, considering that you will probably eat two other meals that day as well.
What is the problem with consuming excess calories? As you have learned, it is important to maintain energy homeostasis — that is, a balance between the energy you consume and what your body uses. If you eat more calories than your body needs, you will store that extra food energy as fat, which can cause you to become obese. Obesity raises the risk of many diseases and health problems, including cardiovascular disease, type 2 diabetes, stroke, liver disease, cancer, pancreatitis, osteoarthritis, sleep apnea, and asthma. Many of these medical conditions can be deadly, which is why obesity can shorten a person’s lifespan. Although Abdul is only slightly overweight at this point, if he regularly consumes more calories than he uses (which is likely with a diet high in fast food) he will gain excess body fat, raising his risk of obesity and its associated diseases.
Table \(1\): Nutritional Information for a Typical Fast Food Meal
Food Calories Total Fat (%DV) Saturated Fat (%DV) Trans Fat Carbohydrates (%DV)
Burger 540 43% 49% 1 g 15%
Fries 510 37% 17% 0 g 22%
Soda 300 0% 0% 0 g 27%
Total 1,350 80% 66% 1 g 64%
Food Sodium (%DV) Iron (%DV) Vitamin A (%DV) Vitamin C (%DV) Calcium (%DV)
Burger 40% 25% 10% 2% 15%
Fries 15% 6% 0% 30% 2%
Soda 1% 0% 0% 0% 0%
Total 56% 31% 10% 32% 17%
% DV = percentage of the adult recommended daily value (DV) for each nutrient, based on a 2,000 Calorie a day diet.
Why do typical fast-food meals have so many calories even if they don’t appear to be particularly large? For one, these foods are typically high in fat. Notice that this meal contains 80% of the recommended daily value (DV) of total fat — close to the limit for the entire day! As you have learned, fat is energy-dense. One gram of fat has nine Calories, while one gram of protein or carbohydrate has only four Calories. This means that meals high in fat, like this one, will generally have more calories than a lower fat meal of equivalent size. A large amount of fat in the burger and fries contributes to the high energy density of this meal.
But fat isn’t the only reason this meal is so high in calories. The soda contains 300 Calories — about the same number of calories as three apples! For most people, three apples would be more satiating than a soda. This is in part because apples have fiber, which is filling. As you have learned, sodas and other sugary beverages generally have no other nutrients besides carbohydrates. You can see from the tables that the soda is the largest contributor of carbohydrates to this meal, with very few other nutrients. If Abdul is frequently drinking large sodas, he is getting a significant percentage of his calories from a substance that is not giving him a feeling of fullness, which may cause him to consume more calories overall. In fact, many scientists think that the increase in consumption of sugary beverages has been a major contributor to the obesity epidemic.
Besides excess calories, what nutrients in this meal could cause health problems? This meal has both a high percentage of saturated fat (66% DV) as well as some trans fat. The American Heart Association recommends that people limit their consumption of saturated fat since it has been shown to raise the risk of heart disease. Trans fats are particularly dangerous and can increase the risk of heart disease, stroke, and type 2 diabetes. In fact, in 2015 the U.S. Food and Drug Administration (FDA) ruled that trans fats have not been shown to be safe for human consumption, and ordered food producers to remove them from the food supply by 2018. While some fast-food restaurants voluntarily removed trans fats from their food prior to this time, as of early 2017 some restaurants still had items containing trans fats on their menus —like the burger from Abdul’s favorite restaurant.
Another nutrient that fast food meals tend to have too much of is sodium. This meal has over half the sodium you should eat in a day, mostly from the burger. And this burger isn’t the worst one around — some fast food burgers have double the recommended DV for sodium! Burgers with bacon are particularly high in sodium. Consumption of excess sodium can lead to high blood pressure, cardiovascular disease, and stroke.
Consumption of excess nutrients is not the only concern when a person frequently eats fast food. As you can see from the tables, this meal is relatively low in some vitamins and minerals such as vitamin A (10% DV) and calcium (17% DV). As you have learned, vitamin A is important for maintaining normal vision and, in young children, the development of the immune system, among other functions. Calcium is a macromineral needed for bone strength, neutralizing acidity in the digestive tract, and nerve and cell membrane functions. Eating a diet low in specific nutrients can cause a form of malnutrition called unbalanced nutrition. If Abdul eats meals like this frequently, he would have to make sure to get plenty of essential nutrients from other sources in order to maintain his health, which may be difficult if fast food takes the place of healthier foods in his diet. Carlos was correct to be concerned about the lack of fresh food in most fast food meals. Fresh fruits and vegetables contain fiber, phytochemicals, and many vitamins and minerals that are important for maintaining health.
But as Abdul brought up, is fast food worse than other types of food? While it tends to be particularly high in calories, fat, and sodium, it is not very nutrient-dense. The same is true for many other types of meals eaten outside the home. Many chain restaurants have nutritional information listed on their website — you can look up some of your favorites. You might be surprised to learn that some restaurant entrees contain more than 2,000 Calories for a single meal, combined with an excessive amount of saturated fat and sodium. These items are just as bad or worse for your health than some fast food meals.
The keys to healthy eating are to know what you are consuming and to make good choices. Preparing fresh food at home is usually healthier than eating out, but most restaurants have some healthier options. After Carlos tells Abdul what Carlos found out about Abdul’s favorite meal, Abdul decides to make some changes. Abdul does not want to face a future of obesity and potentially life-threatening health conditions. He decides to pack a healthy lunch to take with him during the day and will eat more dinners at home. When he does occasionally eat fast food, he will make better choices. Skipping the soda will easily save them 300 Calories. Abdul loves fries but he realizes that if he orders small fries instead of large, he can save 280 Calories and 20% DV of total fat. If he orders a smaller cheeseburger, he can save an additional 240 Calories and 25% DV total fat. Then if he is still hungry, he can add a piece of fruit from home for additional nutrients. He will also try other options at fast-food restaurants, such as salads or grilled chicken sandwiches, which may be healthier. However, he should check the nutritional information first, since some seemingly healthy options can still be high in calories, fat, and salt due to added dressings, sauces, and cheese. Healthy eating and good nutrition don’t have to be difficult if you are armed with information and make good choices with your long-term health in mind.
Chapter Summary
In this chapter, you learned how nutrition relates to the functioning of your body and your health. Specifically, you learned that:
• Nutrients are substances the body needs for energy, building materials, and control of body processes. There are six major classes of nutrients: carbohydrates, proteins, lipids, water, vitamins, and minerals.
• Essential nutrients cannot be synthesized by the human body, so they must be consumed in food. Nonessential nutrients can be synthesized by the human body, so they need not be obtained directly from food.
• Macronutrients are nutrients that are needed in relatively large amounts. They include carbohydrates, proteins, lipids, and water. All macronutrients except water provide energy, which is measured in Calories. Micronutrients are nutrients that are needed in relatively small amounts. They do not provide energy. They include vitamins and minerals.
• Carbohydrates are organic compounds made of simple sugars. Besides sugars, they include starches, glycogen, and cellulose. Dietary carbohydrates come mainly from grains, fruits, and vegetables. They are used for energy, and one gram of carbohydrates provides 4 Calories of energy. Fiber consists of nondigestible carbohydrates that help control blood glucose and cholesterol (soluble fiber) or that stimulate peristalsis and prevent constipation (insoluble fiber).
• Proteins are organic compounds made of amino acids. Dietary proteins come from sources such as meat, fish, and legumes. Amino acids from foods that are not needed for synthesizing new proteins by the body may be used for energy. One gram of proteins provides 4 Calories of energy. Of the 20 amino acids the human body needs, 9 amino acids are essential.
• Lipids are organic compounds made of fatty acids. Fatty acids are needed by the body for energy, cell membranes, and other functions. One gram of lipids provides 9 Calories of energy. Only two fatty acids (omega-3 and omega-6) are essential in the diet. Animal fats are mainly saturated fats, whereas plant fats are mainly unsaturated fats. Artificial trans fats are added to many foods and are known to be harmful to human health.
• Water is essential to life. It is continuously lost from the body in urine, sweat, and exhaled breath, so it must be replenished often. Too little or too much water consumption can be dangerous to health.
• Vitamins are organic compounds that generally function as coenzymes. As such, they are needed for a wide range of normal body functions and necessary for good health. Most vitamins are essential. Exceptions include vitamins B7 and K, which are made by intestinal bacteria; and vitamin D, which is made in the skin when it is exposed to UV light.
• Minerals are inorganic chemical elements that are necessary for many body processes and needed for good health. Minerals are not synthesized biologically, so they are essential nutrients. Macrominerals, which are needed in relatively large quantities, include calcium, magnesium, phosphorus, and sodium. Trace minerals, which are needed in much smaller quantities, include cobalt, iodine, iron, and zinc.
• Healthy eating is fundamentally important for good health. A healthy diet reduces the risk of obesity, cardiovascular disease, cancer, and many other diseases. It also extends life.
• Nutrition refers to the process of taking in nutrients in food and using them for growth, metabolism, and repair. Good nutrition depends on meeting nutrient needs while maintaining energy balance, called energy homeostasis.
• The opposite of good nutrition is malnutrition. Malnutrition includes undernutrition, in which there is inadequate energy intake; overnutrition, in which there is excessive energy intake; and unbalanced nutrition, in which there is too much or not enough of specific nutrients, such as vitamin A or calcium.
• Good nutrition requires healthy eating. This means eating a wide range of nutritious foods that provide the correct balance of nutrients. It also means taking in the correct amount of food energy to balance energy use.
• Nutrient density refers to how much of a given nutrient a particular food provides, relative to the mass of the food or the number of Calories it provides. Foods vary greatly in nutrient density — making informed food choices is important for achieving nutrient balance.
• Energy homeostasis is regulated by the hypothalamus, which controls appetite and satiation, but it also depends on dietary choices and activity levels. Energy density refers to the amount of energy a food provides per unit of mass or volume. Choosing foods with lower or higher energy density as needed to balance energy expenditure can help maintain energy homeostasis.
• Recommended daily values of nutrients can be used as a general guide to nutrient needs. At the level of individuals, requirements for many nutrients may vary based on age, gender, health status, activity level, and other factors.
• Tools such as MyPlate and nutrition facts labels are invaluable for healthy eating. MyPlate is a visual guide to the relative proportions of foods in five different food groups (vegetables, fruits, grains, protein, and dairy) that you should eat at each meal. Nutrition facts labels give the nutrient content and ingredients in packaged foods, which can help you choose the most nutritious options.
• Eating disorders are mental health disorders defined by abnormal eating habits that adversely affect health. They generally begin by young adulthood and are much more common in females than males. Eating disorders are mental disorders with the highest mortality rate.
• Anorexia nervosa is an eating disorder in which people consistently eat very little and become extremely thin. They may also develop amenorrhea and other serious health problems. People with anorexia nervosa often fail to appreciate how thin they are and how severe their illness is.
• Bulimia nervosa is an eating disorder in which people recurrently binge on large amounts of food, followed by purging the food from the body through vomiting, using laxatives, exercising excessively, or other methods. People with bulimia nervosa may have normal weight but often have serious health problems such as electrolyte imbalances and irregular heartbeat.
• Binge eating disorder is an eating disorder in which people repeatedly binge on large amounts of food, followed by feelings of guilt but not by purging. This generally leads to excessive weight gain, obesity, and other serious disorders.
• Genes are likely to be involved in the development of eating disorders because eating disorders tend to “run in families.” At a biochemical level, eating disorders may be caused in part by dysregulation of neurotransmitters or the hormones leptin and ghrelin, which normally help maintain the body’s energy homeostasis.
• Environmental factors that increase the risk of eating disorders include being abused as a child, tight parental control over eating habits, fragile sense of self-identity, and social isolation. Cultural idealization of thinness in females may be a major cause of anorexia nervosa and bulimia nervosa in particular.
• Treatment of an eating disorder depends on the type and severity of the disorder. Treatment options include mental health counseling, medications, nutritional counseling, and hospitalization. The majority of people with eating disorders recover with treatment.
• Obesity is a disease in which excess body fat has accumulated to the extent that it is likely to have negative effects on health. Obesity is diagnosed in adults when their body mass index (BMI), which is an estimate of body fatness, is greater than 30 kg/m2. Obesity is diagnosed in children when their BMI is greater than the 95th percentile for children of that age.
• Obesity may be further categorized by the medical profession as severe obesity, morbid obesity, and super obesity. Obese people who store most of their excess fat in the abdomen have central obesity, putting them at greater risk of adverse health consequences of obesity.
• Dozens of genes that control appetite and metabolism may predispose people to develop obesity when sufficient food energy is present. While rates of obesity have risen, diets have increased in Calories, mainly from excess carbohydrates (often in the form of sugary drinks), and activity levels have declined due to changes in work and technology.
• Leptin resistance has been proposed as a physiological mechanism underlying obesity. A decreased sensitivity to leptin results in an inability to detect satiety despite high-fat reserves. This causes people to never feel satiated and to overeat and gain weight.
• Obesity increases the risk of many other health problems and diseases, including cardiovascular disease, type 2 diabetes, certain types of cancer, osteoarthritis, and obstructive sleep apnea. The health consequences of obesity are due to the effects of either increased fat mass or increased numbers of fat cells.
• Most cases of obesity are treatable or preventable through changes in diet and physical activity that restore energy balance to the body. All types of low-carbohydrate and low-fat diets appear equally beneficial in reducing obesity and its health risks. Other treatments may include medications to control appetite or reduce nutrient absorption and bariatric surgery to modify the digestive tract in ways that limit the intake of food and absorption of nutrients from food.
• Undernutrition is defined as an insufficient intake of nutritious foods. People who are undernourished are usually underweight. Adults are considered underweight if their BMI is less than 18.5 kg/m2. Children are considered underweight if their BMI is less than the 5thpercentile of the reference values for children of the same age.
• Undernutrition is a more significant problem in children who need nutrients for growth and development. They may become dangerously thin (called wasting) or stop growing so they are too short for their age (stunting). Growth deficits often begin in utero due to maternal undernutrition, resulting in low birthweight and its associated risks.
• Severe undernutrition may develop into life-threatening syndromes, such as kwashiorkor or marasmus, both of which can be fatal without treatment. Kwashiorkor occurs when the diet is especially deficient in protein, causing edema and other characteristic signs of the syndrome. Marasmus occurs when the diet is especially deficient in food energy, causing extreme emaciation and other abnormalities.
• Some of the most common micronutrient deficiencies worldwide are iron, vitamin A, and iodine deficiencies. Iron deficiency causes anemia, which in childhood can lead to permanent cognitive and motor deficits. Vitamin A deficiency can weaken the immune system, contribute to anemia, and cause blindness. Iodine deficiency leads to inadequate thyroid hormone, causing goiter and hypothyroidism in adults and intellectual disability in children.
• The vast majority of undernutrition globally occurs because people do not have enough nutritious food to eat. Although there is enough food to meet the needs of the global human population, the food is unevenly distributed and for many people inaccessible because of poverty. Caused by poverty, undernutrition also contributes to poverty because of its effects on health, growth, development, and ultimately on the ability to work and earn income.
• Undernutrition is less common in the richer nations than it is elsewhere, but it still occurs because of wealth inequalities and the existence of food deserts, which are areas with limited access to nutritious foods.
• Treating and preventing undernutrition is a huge and complex problem requiring multifaceted approaches. They include direct nutritional interventions, generally provided through the health-care sector to people who are acutely malnourished, as well as public health interventions that focus on improvements in agriculture, water, sanitation, education, or the like. The most successful interventions have been those that address deficiencies of specific micronutrients such as iodine.
• Foodborne disease is any disease that is transmitted via food. As many as 76 million Americans a year get a foodborne disease, and thousands of them die from it.
• Foodborne diseases are caused by microorganisms, toxins, or adulteration of food by foreign bodies. Norovirus and several genera of bacteria cause most foodborne diseases. Toxins that cause foodborne disease may come from the environment or from microorganisms in food. Alternatively, they may be consumed in toxic plants or fungi. Foreign bodies such as cigarette butts and insects can accidentally get into food at any stage.
• Many foodborne diseases share some of the same symptoms, such as vomiting and diarrhea, but they are quite variable in other ways. The incubation period (time from infection to first symptoms) of foodborne disease can range from a few hours to many days. The infectious dose (an amount that must be consumed to cause disease) can vary greatly depending on the agent of disease.
• The vast majority of reported cases of foodborne disease occur as sporadic cases in individuals. Only a minority of cases occur as part of a disease outbreak, in which two or more people get the same foodborne disease from a common source, such as the same restaurant.
• The foodborne disease usually arises from food contamination through improper handling, preparation, or storage of food. The main ways food becomes contaminated are through poor hygiene, cross-contamination, and failure of temperature control.
• Government agencies such as the Food and Drug Administration are responsible for keeping the food supply safe. Food safety at home depends mainly on following good food safety practices. These range from regular handwashing to maintaining the correct refrigerator temperature.
Chapter Summary Review
1. Imagine you are a nurse who is assessing the BMI of patients. For each of the patients below, identify whether they are underweight, normal weight, overweight, or obese (and the subcategories of severe obesity, morbid obesity, and super obesity). If you cannot determine BMI category based on the information given, explain why. Then discuss whether each patient may have a health/nutritional concern, or whether this cannot be determined from the information given. Finally, if they do have a health/nutritional concern, list some ways they may be able to improve their health. Patients:
1. An adult with a BMI of 41 kg/m2
2. An adult with a BMI of 24 kg/m2
3. An adult with a BMI of 17 kg/m2
4. A child with a BMI of 27 kg/m2
2. Which nutrients provide energy for the body?
1. Vitamins
2. Minerals
3. Proteins
4. All of the above
5. None of the above
3. For each of the statements below, choose whether it applies to soluble fiber, insoluble fiber, both, or neither.
1. Stimulates movement of food wastes through the large intestine
2. Slows movement of chyme through the small intestine
3. Classified as a carbohydrate
4. Nonessential nutrient
4. Sometimes one type of nutrient can be converted to another type of nutrient in the body. Give one example of this, and describe when it occurs.
5. True or False. Trans fat is a type of unsaturated fat.
6. True or False. Bulimia nervosa always involves vomiting to purge food from the body.
7. Which is the best definition of essential nutrients?
1. Nutrients that are needed in large quantities by the body.
2. Nutrients that must be obtained from food.
3. Nutrients that provide energy for the body.
8. Which typically has the highest energy density?
1. Proteins
2. Carbohydrates
3. Sugars
4. Lipids
9. If you are reading the nutrition facts label on a food item and see that “partially hydrogenated” oil is one of the ingredients, what type of fat is likely to be present? Is this fat a healthy choice? Why or why not?
10. Identify two ways in which processed foods are typically less healthy than whole foods.
11. The table below contains nutritional information listed on a bag of tortilla chips. Read the table and then answer the following questions.
1. If you eat 16 of these tortilla chips, how many Calories have you consumed?
2. A palm-sized (4 oz) serving of grilled, boneless, skinless chicken breast has about 190 Calories. How does this compare to the Calories in 16 tortilla chips? Which is more energy-dense? Explain your answer.
3. What is the percentage DV of fat, sodium, and dietary fiber in 16 of these chips? If you wanted to maintain a healthy diet, following the DV recommendations, what percentage DV of fat, sodium, and fiber would you have left for the rest of the day after eating a snack of 16 of these chips?
Table \(2\): nutritional information listed on a bag of tortilla chips
Serving size 1 oz (28 g) About 8 chips
Calories per serving 140
Total fat per serving 7 g 10% Daily Value (DV)
Sodium per serving 115 mg 5% DV
Dietary fiber per serving 1 g 5% DV
12. Can you eat too many calories but still be malnourished? Why or why not?
13. Explain why one person with a BMI of 35 kg/m2 could be considered severely obese, while another person with the same BMI could be considered morbidly obese.
14. Explain how leptin normally regulates appetite, in the absence of leptin resistance.
15. Central obesity refers to:
1. obesity that is due to genetic causes.
2. obesity that occurs in the middle of the U.S.
3. a body fat distribution where most of the excess body fat is stored in the abdomen as opposed to the hips.
4. obesity that has resulted in type 2 diabetes.
16. Name the demographic group that is more likely to be obese and have eating disorders.
17. Match each of the statements below with the eating disorder that best matches it. Eating disorder choices are anorexia nervosa, bulima nervosa, or binge eating disorder. Each disorder is used only once.
1. People with this disorder often develop an electrolyte imbalance.
2. People with this disorder typically eat very little.
3. People with this disorder are at risk of developing type 2 diabetes.
18. If an undernourished child has a distended abdomen, are they more likely to have kwashiorkor or marasmus? Explain your answer and describe the nutritional deficits that likely caused this undernutrition syndrome.
19. Do you think foodborne disease can exacerbate or even cause undernutrition? Explain your answer.
20. True or False. Micronutrient deficiencies in children can cause long-term cognitive deficits.
21. True or False. The risk of getting a foodborne disease can be eliminated by adopting a vegetarian diet.
Attributions
1. Man eats burger by Nick Taylor, licensed CC BY 2.0 via Flickr | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/04%3A_Nutrition/4.8%3A_Case_Study_Conclusion%3A_Fast_Food_and_Chapter_Summary.txt |
This chapter outlines the discovery of cells and cell theory. It identifies ways in which all cells are alike and ways in which they vary. The chapter describes in detail important cell structures and their functions; and it explains how cells obtain energy, grow, and divide.
• 5.1: Case Study: The Importance of Cells
We all get tired sometimes, especially if we have been doing a lot of physical activity like these hikers. But for Jasmin, a 34 year old former high school track star who is now a recreational runner, his tiredness was going far beyond what he thought should be normal for someone who is generally in good physical shape.
• 5.2: Discovery of Cells and Cell Theory
Cells are the basic units of the structure and function of living things. All organisms are made up of one or more cells, and all cells have many of the same structures and carry out the same basic life processes.
• 5.3: Variation in Cells
Although all living cells have certain things in common, different types of cells, even within the same organism, may have their unique structures and functions. Cells with different functions generally have different shapes that suit them for their particular job.
• 5.4: Plasma Membrane
The plasma membrane is a structure that forms a barrier between the cytoplasm inside the cell and the environment outside the cell. The membrane protects and supports the cell and controls everything that enters and leaves it.
• 5.5: Cytoplasm and Cytoskeleton
The cytoplasm is a thick, usually colorless solution that fills each cell and is enclosed by the cell membrane. Sometimes cytoplasm acts like a watery solution, and sometimes it takes on a more gel-like consistency. A framework of protein scaffolds called the cytoskeleton provides the cytoplasm and the cell with structure.
• 5.6: Cell Organelles
An organelle is a structure within the cytoplasm of a eukaryotic cell that is enclosed within a membrane and performs a specific job. Organelles in animal cells include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and vacuoles.
• 5.7: Cell Transport
If a cell were a house, the plasma membrane would be walls with windows and doors. Moving things in and out of the cell is an important role of the plasma membrane. It controls everything that enters and leaves the cell. There are two basic ways that substances can cross the plasma membrane: passive transport, which requires no energy; and active transport, which requires energy.
• 5.8: Active Transport and Homeostasis
When substances require energy to cross a plasma membrane often because they are moving from an area of a lower concentration to an area of a higher concentration, the process is called active transport.
• 5.9: Cellular Respiration
Energy is required to break down and build up molecules and to transport many molecules across plasma membranes. A lot of energy is lost to the environment as heat. The story of life is a story of energy flow - its capture, its change of form, its use for work, and its loss as heat. The cells of living things power their activities with the energy-carrying molecule ATP. The cells of most living things make ATP from glucose in the process of cellular respiration. This process occurs in three sta
• 5.10: Fermentation
An important way of making ATP without oxygen is fermentation. Fermentation starts with glycolysis, which does not require oxygen, but it does not involve the latter two stages of aerobic cellular respiration (the Krebs cycle and electron transport). There are two types of fermentation, called alcoholic fermentation and lactic acid fermentation.
• 5.11: Case Study Conclusion: Tired and Chapter Summary
Jasmin discovered that his extreme fatigue, muscle pain, vision problems, and vomiting were due to issues in his mitochondria, an organelle. Mitochondria create energy for the cells of the body.
05: Cells
Case Study: More Than Just Tired
We all get tired sometimes, especially if we have been doing a lot of physical activity. But for Jasmin, a 34-year-old former high school track star who is now a recreational runner, her tiredness was going far beyond what she thought should be normal for someone who is generally in good physical shape. She was experiencing extreme fatigue after her runs, as well as muscle cramping, spasms, and an unusual sense of heaviness in her legs. At first, she chalked it up to getting older, but her exhaustion and pain worsened to the point where this former athlete could no longer run for more than a few minutes at a time. She also began to experience other unusual symptoms, such as blurry vision and vomiting for no apparent reason.
Concerned, she went to her doctor. Her doctor ran many tests and consulted with several specialists. After several months, Jasmin is finally diagnosed with a mitochondrial disease. Jasmin is surprised. She has an 8-year-old niece with a mitochondrial disease, but her niece’s symptoms started when she was very young, and included seizures and learning disabilities. How can Jasmin have the same disease but different symptoms? Why did she not have problems until adulthood while her niece had symptoms at an early age? And what are mitochondria anyway?
Chapter Overview: Cells
As you will learn in this chapter, mitochondria are important structures within our cells. This chapter will describe cells, which are the basic unit of structure and function in all living organisms. Specifically, you will learn:
• How cells were discovered, their common structures, and the principles of cell theory.
• The importance of size and shape in the functions of cells.
• The differences between eukaryotic cells (such as those in humans and other animals) and prokaryotic cells (such as bacteria).
• The structures and functions of parts of cells including mitochondria, the plasma membrane, cytoplasm, cytoskeleton, nucleus, ribosomes, Golgi apparatus, endoplasmic reticulum, vesicles, and vacuoles.
• How the processes of passive and active transport move substances into and out of cells and help maintain homeostasis.
• How organisms obtain the energy needed for life, including how the sugar glucose is broken down to produce ATP through the processes of aerobic cellular respiration and anaerobic respiration.
As you read this chapter, think about the following questions related to Jasmin’s disease:
1. What are mitochondria? What is their structure, function, and where did they come from during evolution?
2. Why are fatigue and “exercise intolerance,” such as Jasmin’s extreme exhaustion after running, common symptoms of mitochondrial diseases?
3. Why do you think Jasmin has symptoms that affect so many different parts of her body including her legs, eyes, and digestive system?
Attributions
1. Tired by Dace Kiršpile licensed CC BY 2.0 via Flickr
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.01%3A_Case_Study%3A_The_Importance_of_Cells.txt |
A Big Blue Cell
What is this incredible object? Would it surprise you to learn that it is a human cell? The cell is actually too small to see with the unaided eye. It is visible here in such detail because it is being viewed with a very powerful microscope. Cells may be small in size, but they are extremely important for life. Like all other living things, you are made of cells. Cells are the basis of life, and without cells, life as we know it would not exist. You will learn more about these amazing building blocks of life when you read this section.
If you look at a living matter with a microscope — even a simple light microscope — you will see that it consists of cells. Cells are the basic units of the structure and function of living things. They are the smallest units that can carry out the processes of life. All organisms are made up of one or more cells, and all cells have many of the same structures and carry out the same basic life processes. Knowing the structure of cells and the processes they carry out is necessary to understanding life itself.
Discovery of Cells
The first time the word cell was used to refer to these tiny units of life was in 1665 by a British scientist named Robert Hooke. Hooke was one of the earliest scientists to study living things under a microscope. The microscopes of his day were not very strong, but Hooke was still able to make an important discovery. When he looked at a thin slice of cork under his microscope, he was surprised to see what looked like a honeycomb. Hooke made the drawing in the figure below to show what he saw. As you can see, the cork was made up of many tiny units, which Hooke called cells.
Soon after Robert Hooke discovered cells in cork, Anton van Leeuwenhoek in Holland made other important discoveries using a microscope. Leeuwenhoek made his own microscope lenses, and he was so good at it that his microscope was more powerful than other microscopes of his day. In fact, Leeuwenhoek’s microscope was almost as strong as modern light microscopes. Using his microscope, Leeuwenhoek was the first person to observe human cells and bacteria.
Cell Theory
By the early 1800s, scientists had observed the cells of many different organisms. These observations led two German scientists, named Theodor Schwann and Matthias Jakob Schleiden, to propose that cells are the basic building blocks of all living things. Around 1850, a German doctor named Rudolf Virchow was studying cells under a microscope when he happened to see them dividing and forming new cells. He realized that living cells produce new cells through division. Based on this realization, Virchow proposed that living cells arise only from other living cells.
The ideas of all three scientists — Schwann, Schleiden, and Virchow — led to cell theory, which is one of the fundamental theories unifying all of biology. Cell theory states that:
• All organisms are made of one or more cells.
• All the life functions of organisms occur within cells.
• All cells come from already existing cells.
Seeing Inside Cells
Starting with Robert Hooke in the 1600s, the microscope opened up an amazing new world — the world of life at the level of the cell. As microscopes continued to improve, more discoveries were made about the cells of living things. However, by the late 1800s, light microscopes had reached their limit. Objects much smaller than cells, including the structures inside cells, were too small to be seen with even the strongest light microscope.
Then, in the 1950s, a new type of the microscope was invented. Called the electron microscope, it used a beam of electrons instead of light to observe extremely small objects. With an electron microscope, scientists could finally see the tiny structures inside cells. In fact, they could even see individual molecules and atoms. The electron microscope had a huge impact on biology. It allowed scientists to study organisms at the level of their molecules and led to the emergence of the field of cell biology. With the electron microscope, many more cell discoveries were made. Figure \(3\) shows how the cell structures called organelles appear when scanned by an electron microscope.
Structures Shared By All Cells
Although cells are diverse, all cells have certain parts in common. These parts include a plasma membrane, cytoplasm, ribosomes, and DNA.
1. The plasma membrane (also called the cell membrane) is a thin coat of phospholipids that surrounds a cell. It forms the physical boundary between the cell and its environment, so you can think of it as the “skin” of the cell.
2. Cytoplasm refers to all of the cellular material inside the plasma membrane. The Cytoplasm is made up of a watery substance called cytosol and contains other cell structures such as ribosomes.
3. Ribosomes are structures in the cytoplasm where proteins are made.
4. DNA is a nucleic acid found in cells. It contains the genetic instructions that cells need to make proteins.
These parts are common to all cells, from organisms as different as bacteria and human beings. How did all known organisms come to have such similar cells? The similarities show that all life on Earth has a common evolutionary history.
Review
1. Describe cells.
2. Explain how cells were discovered.
3. Outline how cell theory developed.
4. Identify structures shared by all cells.
5. True or False. Cork is not a living organism.
6. True or False. Some organisms are made of only one cell.
7. True or False. Ribosomes are found outside of the cytoplasm of a cell.
8. Proteins are made on _____________ .
9. What are the differences between a light microscope and an electron microscope?
10. The first microscopes were made around
1. 1965
2. 1665
3. 1950
4. 1776
11. Which of these scientists made each of the following discoveries? (Anton van Leeuwenhoek; Robert Hooke; Rudolf Virchow)
1. Observed some of the first cells and first used the term “cell”
2. Observed the first human cells
3. Observed cells dividing
12. Robert Hooke sketched what looked like honeycombs, or repeated circular or square units when he observed plant cells under a microscope.
1. What is each unit?
2. Of the shared parts of all cells, what makes up the outer surface of each unit?
3. Of the shared parts of all cells, what makes up the inside of each unit?
Explore More
To learn more about cell theory, and its history, watch the video below.
Attributions
1. Healthy human T-cell by NIAID Flickr's photostream, public domain via Wikimedia Commons
2. Cork Micrograph by Robert Hook, public domain via Wikimedia Commons
3. Chlamydomonas by Dartmouth Electron Microscope Facility, Dartmouth College, released into the public domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.02%3A_Discovery_of_Cells_and_Cell_Theory.txt |
Bacteria Attack!
Figure $1$ shows a bacterial cell (colored green) attacking human red blood cells. The bacterium causes a disease called relapsing fever. The bacterial and human cells look very different in size and shape. Although all living cells have certain things in common — such as a plasma membrane and cytoplasm — different types of cells, even within the same organism, may have their own unique structures and functions. Cells with different functions generally have different shapes that suit them for their particular job. Cells vary not only in shape but also in size, as this example shows. In most organisms, however, even the largest cells are no bigger than the period at the end of this sentence. Why are cells so small?
Explaining Cell Size
Table $1$: Characteristics of small and large cubes
Characteristic Small Cube Large Cube
sides (S) $1 cm$ $3 cm$
Surface Area (SA) $6 S^2 = 6 \times 1^2 = 6 cm^2$ $6 S^2 = 6 \times 3^2 = 54 cm^2$
Volume (V) $S^3 = 1^3 = 1 cm^3$ $S^3 = 3^3 = 27 cm^3$
SA:V $SA/V = 6/1 = 6$ $SA/V = 54/27 = 2$
Most organisms, even very large ones, have microscopic cells. Why don't cells get bigger instead of remaining tiny and multiplying? What limits cell size?
The answers to these questions are clear once you know how a cell functions. To carry out life processes, a cell must be able to quickly pass substances into and out of the cell. For example, it must be able to pass nutrients and oxygen into the cell and waste products out of the cell. Anything that enters or leaves a cell must cross its outer surface. It is this need to pass substances across the surface that limits how large a cell can be.
Look at the two cubes in Figure $2$. As this figure and table show, a larger cube has less surface area relative to its volume than a smaller cube. This relationship also applies to cells; a larger cell has less surface area relative to its volume than a smaller cell. A cell with a larger volume also needs more nutrients and oxygen and produces more wastes. Because all of these substances must pass through the surface of the cell, a cell with a large volume will not have enough surface area to allow it to meet its needs. The larger the cell is, the smaller its ratio of surface area to volume, and the harder it will be for the cell to get rid of its wastes and take in necessary substances. This is what limits the size of the cell.
Cell Form and Function
Cells with different functions often have different shapes. The cells in Figure $3$ - Figure $5$ are just a few examples of the many different shapes that human cells may have. Each type of cell in the figure has characteristics that help it do its job. For example, the job of the nerve cell is to carry messages to other cells. The nerve cell has many long extensions that reach out in all directions, allowing it to pass messages to many other cells at once. Do you see the tail of each tiny sperm cell? Its tail helps a sperm cell "swim" through fluids in the female reproductive tract in order to reach an egg cell. The white blood cell has the job of destroying bacteria and other pathogens. Figure $5$ shows the large white blood cell (in yellow) engulfing and destroying bacteria (in orange).
Cells With and Without a Nucleus
There is a basic cell structure that is present in many but not all living cells: the nucleus. The nucleus of a cell is a structure in the cytoplasm that is surrounded by a membrane (the nuclear membrane) and contains DNA. Based on whether or not they have a nucleus, there are two basic types of cells: prokaryotic cells and eukaryotic cells.
Prokaryotic Cells
Prokaryotic cells are cells without a nucleus. The DNA in prokaryotic cells is in the cytoplasm rather than enclosed within a nuclear membrane. Prokaryotic cells are found in single-celled organisms, such as the bacterium represented by the model below. Organisms with prokaryotic cells are called prokaryotes. They were the first type of organisms to evolve and are still the most common organisms today.
Table $2$: Prokaryotic cell structures
Cell Structure Description
Flagellum Long projection(s) outside of the cell in some bacteria; aids in the motility
Pili Small projections outside of the cell; aid in attachment
Capsule A thick protective layer outside the cell wall of some bacteria
Cell wall Outer layer of bacterial cells; more chemically complex than eukaryotic cell walls
Plasma Membrane Phospholipid bilayer marking the outside of the cytoplasm
Cytoplasm The fluid portion of the cell
Ribosome Involved in protein synthesis
Nucleoid Circular DNA found in the cytoplasm
Plasmid Small loops of DNA found in some bacteria
Eukaryotic Cells
Eukaryotic cells are cells that contain a nucleus. A typical eukaryotic cell is represented by the model below. Eukaryotic cells are usually larger than prokaryotic cells. They are found in some single-celled and all multicellular organisms. Organisms with eukaryotic cells are called eukaryotes, and they range from fungi to people.
Besides a nucleus, eukaryotic cells also contain other organelles. An organelle is a structure within the cytoplasm that performs a specific job in the cell. Organelles called mitochondria, for example, provide energy to the cell, and organelles called vacuoles store substances in the cell. Organelles allow eukaryotic cells to carry out more functions than prokaryotic cells can.
Table $3$: Eukaryotic cell structures
Structure Location Description
Flagellum Outside the cell A projection used for locomotion in some eukaryotic cells
Plasma Membrane Outer layer of cell Phospholipid bilayer enclosing the cytoplasm
Cytoplasm Bound by the plasma membrane Entire region between the plasma membrane and the nuclear envelope, consisting of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals
Golgi Vesicles
(Golgi Apparatus)
Cytoplasm A series of stacked membranes that sorts, tags, and packages lipids and proteins for distribution
Ribosomes free-floating or on rough ER Involved in protein synthesis
Rough Endoplasmic Reticulum Cytoplasm Interconnected membranous structures that are studded with ribosomes and engage in protein modification and phospholipid synthesis
Smooth Endoplasmic Reticulum Cytoplasm Interconnected membranous structures that have few or no ribosomes on its cytoplasmic surface and synthesize carbohydrates, lipids, and steroid hormones; detoxifies certain chemicals (like pesticides, preservatives, medications, and environmental pollutants), and stores calcium ions
Mitochondria Cytoplasm (singular = mitochondrion) cellular organelles responsible for carrying out cellular respiration, resulting in producing ATP, the cell’s main energy-carrying molecule
Peroxisome Cytoplasm The small, round organelle that contains hydrogen peroxide, and detoxifies many poisons
Lysosome Cytoplasm Organelle in an animal cell that functions as the cell’s digestive component; it breaks down proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles
Secretory Vesicle Cytoplasm Small, membrane-bound sac that functions in cellular storage and transport; its membrane is capable of fusing with the plasma membrane and the membranes of the endoplasmic reticulum and Golgi apparatus
Centrosome
(with 2 centrioles)
Cytoplasm Region in animal cells made of two centrioles that serve as an organizing center for microtubules
Actin Filaments Cytoskeleton The cytoskeleton's narrowest element; it provides rigidity and shape to the cell and enables cellular movements
Intermediate Filaments Cytoskeleton Cytoskeletal component, comprised of several fibrous protein intertwined strands, that bears tension, supports cell-cell junctions, and anchors cells to extracellular structures
Microtubules Cytoskeleton The cytoskeleton’s widest element; provides a track along which vesicles move through the cell, pulls replicated chromosomes to opposite ends of a dividing cell, and is the structural element of centrioles
Cytoskeleton Throughout cell Protein fiber network that collectively maintains the cell’s shape, secures some organelles in specific positions and allows cytoplasm and vesicles to move within the cell
Nucleus Cytoplasm Cell organelle that houses the cell’s DNA and directs ribosome and protein synthesis
Nuclear Pore Nucleus Pores in the nuclear envelope allow substances to enter and exit the nucleus.
Nuclear Envelope Nucleus Double-membrane structure that constitutes the nucleus’ outermost portion
Chromatin Nucleus Protein-DNA complex that serves as the chromosomes’ building material
Nucleolus Nucleus Darkly staining body within the nucleus that is responsible for assembling ribosome subunits
Review
1. Explain why most cells are very small.
2. Discuss variations in the form and function of cells.
3. Compare and contrast prokaryotic and eukaryotic cells.
4. True or False. Prokaryotic cells do not have mitochondria.
5. True or False. Prokaryotic cells do not have DNA.
6. True or False. All single-celled organisms are prokaryotes.
7. Which was the first type of organism to evolve – eukaryotes or prokaryotes? Based on their structures, does this make sense to you? Explain your answer.
8. Do human cells have organelles? Explain your answer.
9. Which are usually larger – prokaryotic or eukaryotic cells? What do you think this means for their relative ability to take in needed substances and release wastes? Discuss your answer.
10. DNA in eukaryotes is enclosed within the _______ ________.
11. Name three different types of cells in humans.
12. Which organelle provides energy in eukaryotic cells?
13. What is the function of a vacuole in a cell?
Explore More
The video below explains why scientists believe endosymbiosis is the basis for complex cells.
Attributions
1. Borrelia hermsii Bacteria by NIAI, public domain via Wikimedia Commons
2. Cubes by Hana Zavadska; licensed CC BY-NC 3.0 via CK-12 Foundation
3. Neuron by Wei-Chung Allen Le, et. al. from the PLOS article Dynamic Remodeling of Dendritic Arbors in GABAergic Interneurons of Adult Visual Cortex, CC BY 2.5 via Wikimedia Commons
4. Spermatozoa public domain via Wikimedia Commons
5. Neutrophil with anthrax by Volker Brinkmann, CC BY 2.5 via Wikimedia Commons
6. Average prokaryote by LadyofHats, released into the public domain via Wikimedia Commons
7. Animal cell by LadyofHats, released into the public domain via Wikimedia Commons
8. Cell structures, endomembrane system, and cytoskeleton; for cell table adapted from Biology by BC Campus, CC BY 4.0
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.03%3A_Variation_in_Cells.txt |
A Bag Full of Jell-O
This simple, cut-away model of an animal cell (Figure \(1\)) shows that a cell resembles a plastic bag full of Jell-O. Its basic structure is a plasma membrane filled with cytoplasm. Like Jell-O containing mixed fruit, the cytoplasm of the cell also contains various structures, such as a nucleus and other organelles. Your body is made up of trillions of cells, but all of them perform the same basic life functions. They all obtain and use energy, respond to the environment, and reproduce. How do your cells carry out these basic functions and keep themselves — and you — alive? To answer these questions, you need to know more about the structures that make up cells, starting with the plasma membrane.
The plasma membrane is a structure that forms a barrier between the cytoplasm inside the cell and the environment outside the cell. Without the plasma membrane, there would be no cell. The membrane also protects and supports the cell and controls everything that enters and leaves it. It allows only certain substances to pass through while keeping others in or out. To understand how the plasma membrane controls what passes into or out of the cell, you need to know its basic structure.
Phospholipid Bilayer
The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer, with a hydrophobic, or water-hating, interior and a hydrophilic, or water-loving, exterior. Each phospholipid molecule has a head and two tails. The head “loves” water (hydrophilic) and the tails “fear” water (hydrophobic). The water-fearing tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell. The polar head group and fatty acid chains are attached by a 3-carbon glycerol unit. Figure \(2\) shows a single phospholipid next to a phospholipid bilayer.
Molecules that are hydrophobic can easily pass through the plasma membrane if they are small enough because they are water-hating like the interior of the membrane. Molecules that are hydrophilic, on the other hand, cannot pass through the plasma membrane — at least not without help — because they are water-loving like the exterior of the membrane.
Other Molecules in the Plasma Membrane
The plasma membrane also contains other molecules, primarily other lipids and proteins. The green molecules in Figure \(2\), for example, are the lipid cholesterol. Molecules of the steroid lipid cholesterol help the plasma membrane keep its shape. (Figure \(3\)) shows the cholesterol molecules as yellow structures within the center of the phospholipid bilayer. Other structures shown in (Figure \(3\)):
• Protein channels. These span the full membrane and have a space within them because they are used to transport materials into or out of the cell.
• Transmembrane proteins. The root "trans" explains that these span (go "across") the membrane. Transmembrane proteins can have a variety of functions.
• Peripheral proteins. These are found only on one side of the membrane. They can be found on either the cytoplasmic side or the outside of the membrane.
• Glycoproteins. These consist of a protein in the plasma membrane with chains of carbohydrates projecting out of the cell.
• Glycolipids. These are chains of carbohydrates attached directly to a lipid in the membrane. Both glycoproteins and glycolipids act as labels to identify the cell.
• Filaments of cytoskeleton are found along the cytoplasmic side of the membrane and provide a scaffolding for the membrane.
Additional Functions of the Plasma Membrane
The plasma membrane may have extensions, such as whip-like flagella or brush-like cilia, that give it other functions. In single-celled organisms, like those shown below, these membrane extensions may help the organisms move. In multicellular organisms, the extensions have different functions. For example, the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose.
Feature: My Human Body
If you smoke and need another reason to quit, here's a good one. We usually think of lung cancer as a major disease caused by smoking. But smoking can have devastating effects on the body's ability to protect itself from repeated, serious respiratory infections, such as bronchitis and pneumonia.
Cilia are microscopic, hair-like projects on cells that line the respiratory, reproductive, and digestive systems. Cilia in the respiratory system line most of your airways where they have the job of trapping and removing dust, germs, and other foreign particles before they can make you sick. Cilia secrete mucus that traps particles, and they move in a continuous wave-like motion that sweeps the mucus and particles upward toward the throat, where they can be expelled from the body. When you are sick and cough up phlegm, that's what you are doing.
Smoking prevents cilia from performing these important functions. Chemicals in tobacco smoke paralyze the cilia so they can't sweep mucus out of the airways and they also inhibit the cilia from producing mucus. Fortunately, these effects start to wear off soon after the last exposure to tobacco smoke. If you stop smoking, your cilia will return to normal. Even if prolonged smoking has destroyed cilia, they will regrow and resume functioning in a matter of months after you stop smoking.
Review
1. What are the general functions of the plasma membrane?
2. Describe the phospholipid bilayer of the plasma membrane.
3. Identify other molecules in the plasma membrane, and state their functions.
4. Why do some cells have plasma membrane extensions such as flagella and cilia?
5. Explain why hydrophilic molecules cannot easily pass through the cell membrane. What type of molecule in the cell membrane might help hydrophilic molecules pass through it?
6. Which part of a phospholipid molecule in the plasma membrane is made of fatty acid chains? Is this part hydrophobic or hydrophilic?
7. The two layers of phospholipids in the plasma membrane are called a phospholipid ____________.
8. True or False. The flagella on your lung cells sweep foreign particles and mucus toward your mouth and nose.
9. True or False. Small hydrophobic molecules can easily pass through the plasma membrane.
10. True or False. The side of the cell membrane that faces the cytoplasm is hydrophilic.
11. Steroid hormones can pass directly through cell membranes. Why do you think this is the case?
12. Some antibiotics work by making holes in the plasma membrane of bacterial cells. How do you think this kills the cells?
13. What is the name of the long, whip-like extensions of the plasma membrane that helps some single-celled organisms move?
Explore More
Watch the video below to learn the history of the discovery of cell membranes' structure.
Attributions
1. Animal cell model by Kevin Song, dedicated CC0 via Wikimedia Commons
2. Phospholipid bilayer by LadyofHats, CC BY-NC 3.0 for CK-12 Foundation
3. Plasma membrane by CNX OpenStax, licensed CC BY 4.0 via Wikimedia Commons
4. Giardia by CDC/ Dr. Stan Erlandsen, public domain via Wikimedia Commons
1. Bronchial cells by Charles Daghlian, released into the public domain via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.04%3A_Plasma_Membrane.txt |
A Peek Inside the Cell
Figure \(1\) may look like a colorful work of abstract art or maybe an ultra-modern carpet design, but it's neither. It is actually a model of the interior of a cell. It's an artist's representation of what you might see if you could take a peek inside one of these basic building blocks of living things. A cell's interior is obviously a crowded and busy space. It contains cytoplasm, dissolved substances, and many structures; and it's a hive of countless biochemical activities all going on at once.
Cytoplasm
The cytoplasm is a thick, usually colorless solution that fills each cell and is enclosed by the cell membrane. Cytoplasm presses against the cell membrane, filling out the cell and giving it its shape. Sometimes cytoplasm acts like a watery solution and sometimes it takes on a more gel-like consistency. In eukaryotic cells, the cytoplasm includes all of the material inside the cell but outside the nucleus, which contains its own watery substance called nucleoplasm. All of the organelles in eukaryotic cells, such as the endoplasmic reticulum and mitochondria, are located in the cytoplasm. The cytoplasm helps to keep them in place. It is also the site of most metabolic activities in the cell, and it allows materials to pass easily throughout the cell.
The portion of the cytoplasm surrounding organelles is called cytosol, which is the liquid part of the cytoplasm. It is composed of about 80 percent water and also contains dissolved salts, fatty acids, sugars, amino acids, and proteins such as enzymes. These dissolved substances are needed to keep the cell alive and carry out metabolic processes. For example, enzymes dissolved in cytosol break down larger molecules into smaller products that can then be used by organelles of the cell. Waste products are also dissolved in the cytosol before they are taken in by vacuoles or expelled from the cell.
Though prokaryotic cells do not have organelles (they do have ribosomes), they still have cytoplasm. It is within the cytoplasm that most cellular activities occur, including the many metabolic pathways that occur within organelles, such as photosynthesis and aerobic respiration.
Cytoskeleton
Although cytoplasm may appear to have no form or structure, it is actually highly organized. A framework of protein scaffolds called the cytoskeleton provides the cytoplasm and the cell with structure. The cytoskeleton consists of thread-like filaments and tubules that criss-cross the cytoplasm. You can see these filaments and tubules in the cells in Figure \(2\). As its name suggests, the cytoskeleton is like a cellular “skeleton.” It helps the cell maintain its shape and also helps to hold cell structures such as organelles in place within the cytoplasm.
The eukaryotic cytoskeleton is made up of a network of long, thin protein fibers. These threadlike proteins continually rebuild to adapt to the cell's constantly changing needs. Three main kinds of cytoskeleton fibers are microtubules, intermediate filaments, and microfilaments (Table \(1\)).
• Microtubules are the thickest of the cytoskeleton structures. They are most commonly made of filaments which are polymers of alpha and beta-tubulin and radiate outwards from an area near the nucleus called the centrosome. Two forms of tubulin form dimers (pairs) which come together to form the hollow cylinders. The cylinders are twisted around each other to form the microtubules. Microtubules help the cell keep its shape. They hold organelles in place and allow them to move around the cell, and they form the mitotic spindle during cell division. Microtubules also make up parts of cilia and flagella, the organelles that help a cell move.
• Microfilaments are made of two thin actin chains that are twisted around one another. Microfilaments are mostly concentrated just beneath the cell membrane, where they support the cell and help the cell keep its shape. Microfilaments form cytoplasmatic extensions, such as microvilli and pseudopodia, which allow certain cells to move. The actin and myosin protein interact to cause a contraction in muscle cells. Microfilaments are found in almost every cell and are numerous in muscle cells and in cells that move by changing shape, such as phagocytes (white blood cells that search the body for bacteria and other invaders).
• Intermediate filaments (IF) differ in make-up from one cell type to another. The IF may be composed of vimentin, keratin, desmin, or lamin. Each cell type can have a unique combination of IFs. For example, intermediate filaments made of keratin are found in skin, hair, and nail cells. IFs organize the inside structure of the cell by holding organelles and providing strength. They are also structural components of the nuclear envelope. Intermediate filaments made of the protein keratin are found in skin, hair, and nail cells.
Table \(1\): Cytoskeleton Structure
Characteristic Microtubules Intermediate Filaments Microfilaments
Fiber Diameter About 25 nm 8 to 11 nm Around 7 nm
Protein Composition Tubulin with two subunits, alpha, and beta-tubulin One of the different types of proteins such as lamin, vimentin, desmin, and keratin Actin
Shape Hollow cylinders made of two protein chains twisted around each other Protein fiber coils twisted into each other Two actin chains twisted around one another
Main Functions Organelle and vesicle movement; form mitotic spindles during cell reproduction; cell motility (in cilia and flagella) Organize cell shape; positions organelles in cytoplasm structural support of the nuclear envelope and sarcomeres; involved in cell-to-cell and cell-to-matrix junctions Keep cellular shape; allows movement of certain cells by forming cytoplasmatic extensions or contraction of actin fibers; involved in some cell-to-cell or cell-to-matrix junctions
Feature: Human Biology in the News
News about an important study of the cytoplasm of eukaryotic cells appeared early in 2016. Researchers in Dresden, Germany discovered that when cells are deprived of adequate nutrients, they may essentially shut down and become dormant. Specifically, when cells do not get enough nutrients, they shut down their metabolism, their energy level drops, and the pH of their cytoplasm decreases. Their normally liquid cytoplasm also assumes a solid state. The cells appear dead and as though a kind of rigor mortis has set in. The researchers think that these changes protect the sensitive structures inside the cells and allow the cells to survive difficult conditions. If nutrients are returned to the cells, they can emerge from their dormant state unharmed. They will continue to grow and multiply when conditions improve.
This important basic science research was carried out on a nonhuman organism: one-celled fungi called yeasts. Nonetheless, it may have important implications for humans because yeasts have eukaryotic cells with many of the same structures as human cells. Yeast cells appear to be able to "trick" death by shutting down all life processes in a controlled way. Researchers hope to learn with the continued research on whether human cells can be taught this "trick" as well.
Review
1. Describe the composition of cytoplasm.
2. What are some of the functions of cytoplasm?
3. Outline the structure and functions of the cytoskeleton.
4. Is the cytoplasm made of cells? Why or why not?
5. Name two types of cytoskeletal structures.
6. True or False. The cytoplasm is usually green.
7. True or False. The nucleus of a cell is filled with cytoplasm.
8. In Figure \(2\) of the different cytoskeletal structures above (shown in red and green), what do you notice about these different structures?
9. Describe one example of a metabolic process that occurs in the cytosol.
10. In eukaryotic cells, all of the material inside of the cell but outside of the nucleus is called the ___________.
11. What is the liquid part of the cytoplasm called?
12. What chemical substance makes up most of the cytosol?
13. When yeast cells deprived of nutrients go dormant, their cytoplasm assumes a solid-state. What effect do you think a solid cytoplasm would have on normal cellular processes? Explain your answer.
14. What is the difference between cytoplasm and cytosol?
15. Name the three main parts of the cytoskeleton.
16. List two functions of the eukaryotic cytoskeleton
Explore More
Watch the video below to learn about motor proteins, which transport cellular material by the cytoskeleton.
Attributions
1. Crowded cytosol by TimVickers, released into the public domain via Wikimedia Commons
2. Fluorescent cells by NIH, released into the public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.05%3A_Cytoplasm_and_Cytoskeleton.txt |
Ribosome Review
Figure \(1\) represents an important structure in living cells. It is a component of a ribosome, the cell structure where proteins are synthesized. Large ribosomal subunit (50S) of Haloarcula marismortui, facing the 30S subunit. The ribosomal proteins are shown in blue, the rRNA in ochre (a shade of brown and yellow), the active site in red. All living cells contain ribosomes, whether they are prokaryotic or eukaryotic cells. However, only eukaryotic cells also contain a nucleus and several other types of organelles.
An organelle is a structure within the cytoplasm of a eukaryotic cell that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and vacuoles. Ribosomes are not enclosed within a membrane but are still commonly referred to as organelles in eukaryotic cells.
The Nucleus
The nucleus is the largest organelle in a eukaryotic cell and is considered to be the cell’s control center. It contains most of the cell’s DNA, which makes up chromosomes and is encoded with the genetic instructions for making proteins. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called nucleoplasm that is similar in composition to the cytosol found in the cytoplasm outside the nucleus (Figure \(2\)). Most eukaryotic cells contain just a single nucleus, but some types of cells, such as red blood cells, contain no nucleus. A few other types of cells, such as muscle cells, contain multiple nuclei.
As you can see from the model in Figure \(2\), the membrane enclosing the nucleus is called the nuclear envelope. This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes, called nuclear pores, allow large molecules to pass through the nuclear envelope with the help of special proteins. Large proteins and RNA molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model below is mainly involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they are involved in the synthesis of proteins.
Mitochondria
The mitochondrion (plural, mitochondria) is an organelle that makes energy available to the cell (Figure \(3\)). This is why mitochondria are sometimes referred to as the power plants of the cell. They use energy from organic compounds such as glucose to make molecules of ATP (adenosine triphosphate), an energy-carrying molecule that is used almost universally inside cells for energy.
Scientists think that mitochondria were once free-living organisms because they contain their own DNA. They theorize that ancient prokaryotes infected (or were engulfed by) larger prokaryotic cells, and the two organisms evolved a symbiotic relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra energy from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles inside them. This theory is called the endosymbiotic theory, and it is widely accepted by biologists today
Mitochondrial Compartments
The double membrane nature of the mitochondria results in five distinct compartments, each with an important role in cellular respiration. These compartments are:
1. the outer mitochondrial membrane,
2. the intermembrane space (the space between the outer and inner membranes),
3. the inner mitochondrial membrane,
4. the cristae (formed by infoldings of the inner membrane), and
5. the matrix (space within the inner membrane).
Endoplasmic Reticulum
The endoplasmic reticulum (ER) (plural, reticuli) is a network of phospholipid membranes that form hollow tubes, flattened sheets, and round sacs. These flattened, hollow folds and sacs are called cisternae. The ER has two major functions:
• Transport: Molecules, such as proteins, can move from place to place inside the ER, much like on an intracellular highway.
• Synthesis: Ribosomes that are attached to the ER, similar to unattached ribosomes, make proteins. Lipids are also produced in the ER.
There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER):
• Rough endoplasmic reticulum is studded with ribosomes, which gives it a “rough” appearance. These ribosomes make proteins that are then transported from the ER in small sacs called transport vesicles. The transport vesicles pinch off the ends of the ER. The rough endoplasmic reticulum works with the Golgi apparatus to move new proteins to their proper destinations in the cell. The membrane of the RER is continuous with the outer layer of the nuclear envelope.
• Smooth endoplasmic reticulum does not have any ribosomes attached to it, and so it has a smooth appearance. SER has many different functions, some of which include lipid synthesis, calcium ion storage, and drug detoxification. The smooth endoplasmic reticulum is found in both animal and plant cells and it serves different functions in each. The SER is made up of tubules and vesicles that branch out to form a network. In some cells, there are dilated areas like the sacs of RER. Smooth endoplasmic reticulum and RER form an interconnected network.
Golgi Apparatus
The Golgi apparatus (Figure \(5\)) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It was identified in 1898 by the Italian physician Camillo Golgi. The Golgi apparatus modifies, sorts, and packages different substances for secretion out of the cell, or for use within the cell. The Golgi apparatus is found close to the nucleus of the cell where it modifies proteins that have been delivered in transport vesicles from the Rough Endoplasmic Reticulum. It is also involved in the transport of lipids around the cell. Pieces of the Golgi membrane pinch off to form vesicles that transport molecules around the cell. The Golgi apparatus can be thought of as similar to a post office; it packages and labels "items" and then sends them to different parts of the cell. The Golgi apparatus tends to be larger and more numerous in cells that synthesize and secrete large quantities of materials; for example, the plasma B cells and the antibody-secreting cells of the immune system have prominent Golgi complexes.
The Golgi apparatus manipulates products from the Rough Endoplasmic Reticulum (ER) and also produces new organelles called lysosomes. Proteins and other products of the ER are sent to the Golgi apparatus, which organizes, modifies, packages, and tags them. Some of these products are transported to other areas of the cell and some are exported from the cell through exocytosis. Enzymatic proteins are packaged as new lysosomes.
The stack of cisternae has four functional regions: the cis-Golgi network, medial-Golgi, endo-Golgi, and trans-Golgi network. Vesicles from the ER fuse with the network and subsequently progress through the stack from the cis- to the trans-Golgi network, where they are packaged and sent to their destination. Each cisterna includes special Golgi enzymes which modify or help to modify proteins that travel through it. Proteins may be modified by the addition of a carbohydrate group (glycosylation) or phosphate group (phosphorylation). These modifications may form a signal sequence on the protein, which determines the final destination of the protein. For example, the addition of mannose-6-phosphate signals the protein for lysosomes.
Vesicles and Vacuoles
Both vesicles and vacuoles are sac-like organelles that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein and lipid molecules. You can see an example of this type of transport vesicle in the figure above. Some vesicles are used as chambers for biochemical reactions. Other vesicles include:
• Lysosomes, which use enzymes to break down foreign matter and dead cells.
• Peroxisomes, which use oxygen to break down poisons.
• Transport vesicles, transport contents between organelle as well as between cell exterior and interior.
Centrioles
Centrioles are organelles involved in cell division. The function of centrioles is to help organize the chromosomes before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells and are located near the nucleus. Each centriole is made mainly of a protein named tubulin. The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured below.
Ribosomes
Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome is formed of two subunits, like the one pictured at the top of this section. Both subunits consist of proteins and RNA. RNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in RNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm as well as on the RER.
Review
1. Define organelle.
2. Describe the structure and function of the nucleus.
3. Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.
4. Why are mitochondria referred to as the power plants of the cell?
5. What roles are played by vesicles and vacuoles?
6. Why do all cells need ribosomes, even prokaryotic cells that lack a nucleus and other cell organelles?
7. Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?
8. Lysosomes and peroxisomes are types of:
1. A. Organelles
2. B. Vesicles
3. C. Vacuoles
4. D. Both A and B
9. Which of the following organelles fits best with each description of function? Choose only one organelle for each answer: Golgi apparatus, centrioles, nucleolus, nucleus, rough endoplasmic reticulum
1. a. Contains the genetic instructions for the production of proteins
2. b. Organizes chromosomes before cell division
3. c. Provides a framework for ribosomes
4. d. Packages and labels proteins
5. e. Assembles ribosomes
10. True or False. All eukaryotic cells have a nucleus.
11. True or False. The outer surface of the nucleus of a eukaryotic cell is not completely solid.
Attributions
1. 50S-subunit of the ribosome by Yikrazuul, licensed CC BY-SA 3.0 via Wikimedia Commons
2. Cell nucleus by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
3. Animal mitochondrion by LadyofHats, released into the public domain via Wikimedia Commons
4. Endoplasmic reticulum by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
5. Golgi Apparatus by Openstax, licensed CC BY 4.0 via Wikimedia Commons
6. Centrioles by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.06%3A_Cell_Organelles.txt |
Letting in the Light
Look at the big windows and glass doors in this house. Imagine all the light they must let in on a sunny day. Now imagine living in a house that has walls without any windows or doors. Nothing could enter or leave. Or imagine living in a house with holes in the walls instead of windows and doors. Things could enter or leave, but you couldn’t control what came in or went out. Only if a house has walls with windows and doors that can be opened or closed you can control what enters or leaves. For example, windows and doors allow you to let in light and the family dog and keep out rain and bugs.
Transport Across Membranes
If a cell were a house, the plasma membrane would be walls with windows and doors. Moving things in and out of the cell is an important role of the plasma membrane. It controls everything that enters and leaves the cell. There are two basic ways that substances can cross the plasma membrane: passive transport, which requires no energy; and active transport, which requires energy. Passive transport is explained in this section and Active transport is explained in the next section, Active Transport and Homeostasis. Various types of cell transport are summarized in the concept map in Figure \(2\).
Transport Without Energy
Passive transport occurs when substances cross the plasma membrane without any input of energy from the cell. No energy is needed because the substances are moving from an area where they have a higher concentration to an area where they have a lower concentration. Water solutions are very important in biology. When water is mixed with other molecules this mixture is called a solution. Water is the solvent and the dissolved substance is the solute. A solution is characterized by the solute. For example, water and sugar would be characterized as a sugar solution. More the particles of a solute in a given volume, the higher the concentration. The particles of solute always move from an area where it is more concentrated to an area where it is less concentrated. It’s a little like a ball rolling down a hill. It goes by itself without any input of extra energy.
The different categories of cell transport are outlined in Figure \(2\). Cell transport can be classified as follows:
• Passive Transport which includes
• Simple Diffusion
• Osmosis
• Facilitated Diffusion
• Active Transport can involve either a pump or a vesicle
• Pump Transport can be
• primary
• secondary
• Vesicle Transport can involve
• Exocytosis
• Endocytosis which includes
• Pinocytosis
• Phagocytosis
• Receptor-Mediated Endocytosis
Simple Diffusion
Diffusion Although you may not know what diffusion is, you have experienced the process. Can you remember walking into the front door of your home and smelling a pleasant aroma coming from the kitchen? It was the diffusion of particles from the kitchen to the front door of the house that allowed you to detect the odors. Diffusion is defined as the net movement of particles from an area of greater concentration to an area of lesser concentration.
The molecules in a gas, a liquid, or a solid are in constant motion due to their kinetic energy. Molecules are in constant movement and collide with each other. These collisions cause the molecules to move in random directions. Over time, however, more molecules will be propelled into the less concentrated area. Thus, the net movement of molecules is always from more tightly packed areas to less tightly packed areas. Many things can diffuse. Odors diffuse through the air, salt diffuses through water and nutrients diffuse from the blood to the body tissues. This spread of particles through the random motion from an area of high concentration to an area of lower concentration is known as diffusion. This unequal distribution of molecules is called a concentration gradient. Once the molecules become uniformly distributed, a dynamic equilibrium exists. The equilibrium is said to be dynamic because molecules continue to move, but despite this change, there is no net change in concentration over time. Both living and nonliving systems experience the process of diffusion. In living systems, diffusion is responsible for the movement of a large number of substances, such as gases and small uncharged molecules, into and out of cells.
Osmosis
Osmosis is a specific type of diffusion; it is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Water moves in or out of a cell until its concentration is the same on both sides of the plasma membrane.
Semi-permeable membranes are very thin layers of material that allow some things to pass through them but prevent other things from passing through. Cell membranes are an example of semi-permeable membranes. Cell membranes allow small molecules such as oxygen, water carbon dioxide, and oxygen to pass through but do not allow larger molecules like glucose, sucrose, proteins, and starch to enter the cell directly.
The classic example used to demonstrate osmosis and osmotic pressure is to immerse cells into sugar solutions of various concentrations. There are three possible relationships that cells can encounter when placed into a sugar solution. Figure \(4\) shows what happens in osmosis through the semi-permeable membrane of the cells.
1. The concentration of solute in the solution can be greater than the concentration of solute in the cells. This cell is described as being in a hypertonic solution (hyper = greater than normal). The net flow or water will be out of the cell.
2. The concentration of solute in the solution can be equal to the concentration of solute in cells. In this situation, the cell is in an isotonic solution (iso = equal or the same as normal). The amount of water entering the cell is the same as the amount leaving the cell.
3. The concentration of solute in the solution can be less than the concentration of solute in the cells. This cell is in a hypotonic solution (hypo = less than normal). The net flow of water will be into the cell.
Figure \(5\) demonstrates the specific outcomes of osmosis in red blood cells.
1. Hypertonic solution. The red blood cell will appear to shrink as the water flows out of the cell and into the surrounding environment.
2. Isotonic solution. The red blood cell will retain its normal shape in this environment as the amount of water entering the cell is the same as the amount leaving the cell.
3. Hypotonic solution. The red blood cell in this environment will become visibly swollen and potentially rupture as water rushes into the cell.
Facilitated Diffusion
Water and many other substances cannot simply diffuse across a membrane. Hydrophilic molecules, charged ions, and relatively large molecules such as glucose all need help with diffusion. The help comes from special proteins in the membrane known as transport proteins. Diffusion with the help of transport proteins is called facilitated diffusion. There are several types of transport proteins, including channel proteins and carrier proteins (Figure \(6\))
• Channel proteins form pores, or tiny holes, in the membrane. This allows water molecules and small ions to pass through the membrane without coming into contact with the hydrophobic tails of the lipid molecules in the interior of the membrane.
• Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane.
Review
1. What is the main difference between passive and active transport?
2. Summarize three different ways that passive transport can occur, and give an example of a substance that is transported in each way.
3. Explain how transport across the plasma membrane is related to the homeostasis of the cell.
4. Why can generally only very small, hydrophobic molecules across the cell membrane by simple diffusion?
5. Explain how facilitated diffusion assists in osmosis in cells. Be sure to define osmosis and facilitated diffusion in your answer.
6. Imagine a hypothetical cell with a higher concentration of glucose inside the cell than outside. Answer the following questions about this cell, assuming all transport across the membrane is passive, not active.
1. Can the glucose simply diffuse across the cell membrane? Why or why not?
2. Assuming that there are glucose transport proteins in the cell membrane, which way would glucose flow – into or out of the cell? Explain your answer.
3. If the concentration of glucose was equal inside and outside of the cell, do you think there would be a net flow of glucose across the cell membrane in one direction or the other? Explain your answer.
7. What are the similarities and differences between channel proteins and carrier proteins?
8. True or False. Only active transport, not passive transport, involves transport proteins.
9. True or False. Oxygen and carbon dioxide can squeeze between the lipid molecules in the plasma membrane.
10. True or False. Ions easily diffuse across the cell membrane by simple diffusion.
11. Controlling what enters and leaves the cell is an important function of the:
1. nucleus
2. vesicle
3. plasma membrane
4. Golgi apparatus
Explore More
Check out this video to learn more about osmosis and tonicity:
Attributions
1. House by Moyan Brenn from Italy, CC BY 2.0 via Wikimedia Commons
2. Flowchart by Mandeep Grewal, CC BY-NC 3.0
3. Simple diffusion by LadyofHats Mariana Ruiz Villarreal released into the public domain via Wikimedia Commons
4. Tonicity by CNX OpenStax, CC BY 4.0 via Wikimedia Commons
5. Osmotic pressure on blood cells by LadyofHats Mariana Ruiz Villarreal released into the public domain via Wikimedia Commons
6. Facilitated diffusion by LadyofHats Mariana Ruiz Villarreal released into the public domain via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.07%3A_Cell_Transport.txt |
Like Pushing a Humvee Uphill
You can tell by their faces that these airmen are expending a lot of energy trying to push this Humvee up a slope. The men are participating in a competition that tests their brute strength against that of other teams. The Humvee weighs about 13,000 pounds, so it takes every ounce of energy they can muster to move it uphill against the force of gravity. Transport of some substances across a plasma membrane is a little like pushing a Humvee uphill — it can't be done without adding energy.
What Is Active Transport?
Some substances can pass into or out of a cell across the plasma membrane without any energy required because they are moving from an area of higher concentration to an area of lower concentration. This type of transport is called passive transport as you learned in the last section. Other substances require energy to cross a plasma membrane often because they are moving from an area of lower concentration to an area of higher concentration. This type of transport is called active transport. The energy for active transport comes from the energy-carrying molecule called ATP (adenosine triphosphate). Active transport may also require transport proteins, such as carrier proteins, which are embedded in the plasma membrane. Two types of active transport are pump and vesicle transport.
Pump
Two pump mechanisms (primary and secondary active transports) exist for the transport of small-molecular weight material and macromolecules. The primary active transport moves ions across a membrane and creates a difference in charge across that membrane. The primary active transport system uses ATP to move a substance, such as an ion, into the cell, and often at the same time, a second substance is moved out of the cell. The sodium-potassium pump is a mechanism of active transport that moves sodium ions out of the cell and potassium ions into the cells — in all the trillions of cells in the body! Both ions are moved from areas of lower to higher concentration, so energy is needed for this "uphill" process. The energy is provided by ATP. The sodium-potassium pump also requires carrier proteins. Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane. Figure \(2\) shows in greater detail how the sodium-potassium pump works and the specific roles played by carrier proteins in this process.
To appreciate the importance of the sodium-potassium pump, you need to know more about the roles of sodium and potassium in the body. Both are essential dietary minerals, meaning you have to obtain them in the foods you eat. Both sodium and potassium are also electrolytes, meaning that they dissociate into ions (charged particles) in solution, which allows them to conduct electricity. Normal body functions require a very narrow range of concentrations of sodium and potassium ions in body fluids, both inside and outside of cells.
• Sodium is the principal ion in the fluid outside of cells. Normal sodium concentrations are about 10 times higher outside than inside of cells.
• Potassium is the principal ion in the fluid inside of cells. Normal potassium concentrations are about 30 times higher inside than outside of cells.
These differences in concentration create an electrical gradient across the cell membrane, called the membrane potential. the secondary active transport describes the movement of material using the energy of the electrochemical gradient established by the primary active transport. Using the energy of the electrochemical gradient created by the primary active transport system, other substances such as amino acids and glucose can be brought into the cell through membrane channels. ATP itself is formed through secondary active transport using a hydrogen ion gradient in the mitochondrion. Tightly controlling the membrane potential is critical for vital body functions, including the transmission of nerve impulses and the contraction of muscles. A large percentage of the body's energy goes to maintaining this potential across the membranes of its trillions of cells with the sodium-potassium pump.
Vesicle Transport
Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell. Very large molecules cross the plasma membrane with a different sort of help, called vesicle transport. Vesicle transport requires energy, so it is also a form of active transport. There are two types of vesicle transport: endocytosis and exocytosis.
Endocytosis
Endocytosis is a type of vesicle transport that moves a substance into the cell. The plasma membrane completely engulfs the substance, a vesicle pinches off from the membrane, and the vesicle carries the substance into the cell. It is used by all cells of the body because most substances important to them are polar and consist of big molecules, and thus cannot pass through the hydrophobic plasma membrane. When an entire cell or other solid particle is engulfed, the process is called phagocytosis. When fluid is engulfed, the process is called pinocytosis. When the content is taken in specifically with the help of receptors on the plasma membrane, it is called receptor-mediated endocytosis.
A targeted variation of endocytosis employs binding proteins in the plasma membrane that are specific for certain substances. The particles bind to the proteins and the plasma membrane invaginates, bringing the substance and the proteins into the cell. If passage across the membrane of the target of receptor-mediated endocytosis is ineffective, it will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by a failure of receptor-mediated endocytosis. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood because their cells cannot clear the chemical from their blood.
Exocytosis
Exocytosis is a type of vesicle transport that moves a substance out of the cell. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then, the vesicle membrane fuses with the cell membrane, and the substance is released outside the cell.
Homeostasis and Cell Function
For a cell to function normally, a stable state must be maintained inside the cell. For example, the concentration of salts, nutrients, and other substances must be kept within a certain range. The process of maintaining stable conditions inside a cell (or an entire organism) is homeostasis. Homeostasis requires constant adjustments because conditions are always changing both inside and outside the cell. The processes described in this and previous lessons play important roles in homeostasis. By moving substances into and out of cells, they keep conditions within normal ranges inside the cells and the organism as a whole.
Feature:Feature: My Human Body
Maintaining the proper balance of sodium and potassium in body fluids by active transport is necessary for life itself, so it's no surprise that getting the right balance of sodium and potassium in the diet is important for good health. Imbalances may increase the risk of high blood pressure, heart disease, diabetes, and other disorders.
If you are like the majority of Americans, sodium and potassium are out of balance in your diet. You are likely to consume too much sodium and too little potassium. Follow these guidelines to help ensure that these minerals are in balance in the foods you eat:
• Total sodium intake should be less than 2300 mg/day. Most salt in the diet is found in processed foods or added with a salt shaker. Stop adding salt and start checking food labels for sodium content. Foods considered low in sodium have less than 140 mg/serving (or 5% daily value).
• Total potassium intake should be 4700 mg/day. It's easy to add potassium to the diet by choosing the right foods, and there are plenty of choices. Most fruits and vegetables are high in potassium, but especially potatoes, bananas, oranges, apricots, plums, leafy greens, tomatoes, lima beans, and avocado. Other foods with substantial amounts of potassium are fish, meat, poultry, and whole grains.
Review
1. Define active transport.
2. What is the sodium-potassium pump? Why is it so important?
3. Name two types of vesicle transport. Which type moves substances out of the cell?
4. What are the similarities and differences between phagocytosis and pinocytosis?
5. The sodium-potassium pump is a:
1. Phospholipid
2. Protein
3. Carbohydrate
4. Ion
6. What is the functional significance of the shape change of the carrier protein in the sodium-potassium pump after the sodium ions bind?
7. A potentially deadly poison derived from plants called ouabain blocks the sodium-potassium pump and prevents it from working. What do you think this does to the sodium and potassium balance in cells? Explain your answer.
8. True or False. The sodium-potassium pump uses one protein to pump both sodium and potassium.
9. True or False. Vesicles are made of the nuclear membrane.
10. What is an electrical gradient across the cell membrane called?
11. Chemical signaling molecules called neurotransmitters are released from nerve cells (neurons) through vesicles. This is an example of:
1. Pinocytosis
2. Phagocytosis
3. Endocytosis
4. Exocytosis
12. The energy for active transport comes from
1. ATP
2. RNA
3. Carrier proteins
4. Sodium ions
13. Transport proteins that move substances into and out of a cell are located in which structure?
Attributions
1. Competition by Airman 1st Class Collin Schmidt, public domain via Wikimedia Commons
2. Sodium-potassium pump by LadyofHats Mariana Ruiz Villarreal, released into the public domain via Wikimedia Commons
3. Endocytosis types by LadyofHats Mariana Ruiz Villarreal, released into the public domain via Wikimedia Commons
4. Exocytosis by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
6. Some text is adapted from Concepts of Biology by Open Stax, licensed CC BY 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.08%3A_Active_Transport_and_Homeostasis.txt |
Bring on the S'mores!
This inviting campfire can be used for both heat and light. Heat and light are two forms of energy that are released when a fuel like wood is burned. The cells of living things also get energy by "burning." They "burn" glucose in the process called cellular respiration.
Inside every cell of all living things, energy is needed to carry out life processes. Energy is required to break down and build up molecules and to transport many molecules across plasma membranes. All of life’s work needs energy. A lot of energy is also simply lost to the environment as heat. The story of life is a story of energy flow — its capture, its change of form, its use for work, and its loss as heat. Energy, unlike matter, cannot be recycled, so organisms require a constant input of energy. Life runs on chemical energy. Where do living organisms get this chemical energy?
Where do organisms get energy from?
The chemical energy that organisms need comes from food. Food consists of organic molecules that store energy in their chemical bonds. Glucose is a simple carbohydrate with the chemical formula $\mathrm{C_6H_{12}O_6}$. It stores chemical energy in a concentrated, stable form. In your body, glucose is the form of energy that is carried in your blood and taken up by each of your trillions of cells. Cells do cellular respiration to extract energy from the bonds of glucose and other food molecules. Cells can store the extracted energy in the form of ATP (adenosine triphosphate).
What is ATP?
Let’s take a closer look at a molecule of ATP, shown in the figure $2$. Although it carries less energy than glucose, its structure is more complex. “A” in ATP refers to the majority of the molecule – adenosine – a combination of a nitrogenous base and a five-carbon sugar. “T” and “P” indicate the three phosphates, linked by bonds that hold the energy actually used by cells. Usually, only the outermost bond breaks to release or spend energy for cellular work.
An ATP molecule is like a rechargeable battery: its energy can be used by the cell when it breaks apart into ADP (adenosine diphosphate) and phosphate, and then the “worn-out battery” ADP can be recharged using new energy to attach a new phosphate and rebuild ATP. The materials are recyclable, but recall that energy is not! ADP can be further reduced to AMP (adenosine monophosphate and phosphate, releasing additional energy. As with ADT "recharged" to ATP, AMP can be recharged to ADP.
How much energy does it cost to do your body’s work? A single cell uses about 10 million ATP molecules per second and recycles all of its ATP molecules about every 20-30 seconds.
What Is Cellular Respiration?
Some organisms can make their own food, whereas others cannot. An autotroph is an organism that can produce its own food. The Greek roots of the word autotroph mean “self” (auto) “feeder” (troph). Plants are the best-known autotrophs, but others exist, including certain types of bacteria and algae. Oceanic algae contribute enormous quantities of food and oxygen to global food chains. Plants are also photoautotrophs, a type of autotroph that uses sunlight and carbon from carbon dioxide to synthesize chemical energy in the form of carbohydrates. Heterotrophs are organisms incapable of photosynthesis that must therefore obtain energy and carbon from food by consuming other organisms. The Greek roots of the word heterotroph mean “other” (hetero) “feeder” (troph), meaning that their food comes from other organisms. Even if the food organism is another animal, this food traces its origins back to autotrophs and the process of photosynthesis. Humans are heterotrophs, as are all animals. Heterotrophs depend on autotrophs, either directly or indirectly.
Cellular respiration is the process by which individual cells break down food molecules, such as glucose and release energy. The process is similar to burning, although it doesn’t produce light or intense heat as a campfire does. This is because cellular respiration releases the energy in glucose slowly, in many small steps. It uses the energy that is released to form molecules of ATP, the energy-carrying molecules that cells use to power biochemical processes. Cellular respiration involves many chemical reactions, but they can all be summed up with this chemical equation:
$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy} \nonumber$
where the energy that is released is in chemical energy in ATP (vs. thermal energy as heat). The equation above shows that glucose ($\ce{C6H12O6}$) and oxygen ($\ce{O_2}$) react to form carbon dioxide ($\ce{CO_2}$) and water $\ce{H_2O}$, releasing energy in the process. Because oxygen is required for cellular respiration, it is an aerobic process.
Cellular respiration occurs in the cells of all living things, both autotrophs and heterotrophs. All of them catabolize glucose to form ATP. The reactions of cellular respiration can be grouped into three main stages and an intermediate stage: glycolysis, Transformation of pyruvate, the Krebs cycle (also called the citric acid cycle), and Oxidative Phosphorylation. Figure $3$ gives an overview of these three stages, which are also described in detail below.
Glycolysis
The first stage of cellular respiration is glycolysis. This process is shown in the top box in Figure $3$ showing a 6-carbon molecule being broken down into two 3-carbon pyruvate molecules. ATP is produced in this process which takes place in the cytosol of the cytoplasm.
Splitting Glucose
The word glycolysis means “glucose splitting,” which is exactly what happens in this stage. Enzymes split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as shown in figure $4$. Glucose is first split into glyceraldehyde 3-phosphate (a molecule containing 3 carbons and a phosphate group). This process uses 2 ATP. Next, each glyceraldehyde 3-phosphate is converted into pyruvate (a 3-carbon molecule). this produces two 4 ATP and 2 NADH.
Results of Glycolysis
Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules. These two molecules go on to stage II of cellular respiration. The energy to split glucose is provided by two molecules of ATP. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP. As a result, there is a net gain of two ATP molecules during glycolysis. high-energy electrons are also transferred to energy-carrying molecules called electron carriers through the process
known as reduction. The electron carrier of glycolysis is NAD+(nicotinamide adenine diphosphate). Electrons are transferred to 2 NAD+ to produce two molecules of NADH. The energy stored in NADH is used in stage III of cellular respiration to make more ATP. At the end of glycolysis, the following has been produced:
• 2 molecules of NADH
• 2 net molecules of ATP
Transformation of Pyruvate into Acetyl-CoA
In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are sites of cellular respiration. If oxygen is available, aerobic respiration will go forward. In mitochondria, pyruvate will be transformed into a two-carbon acetyl group (by removing a molecule of carbon dioxide) that will be picked up by a carrier compound called coenzyme A (CoA), which is made from vitamin B5. The resulting compound is called acetyl CoA and its production is frequently called the oxidation or the Transformation of Pyruvate (see Figure $5$. Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next pathway step, the Citric Acid Cycle.
Citric Acid Cycle
Before you read about the last two stages of cellular respiration, you need to review the structure of the mitochondrion, where these two stages take place. As you can see from Figure $6$, a mitochondrion has an inner and outer membrane. The space between the inner and outer membrane is called the intermembrane space. The space enclosed by the inner membrane is called the matrix. The second stage of cellular respiration, the Krebs cycle, takes place in the matrix. The third stage, electron transport, takes place on the inner membrane.
Recall that glycolysis produces two molecules of pyruvate (pyruvic acid). Pyruvate, which has three carbon atoms, is split apart and combined with CoA, which stands for coenzyme A. The product of this reaction is acetyl-CoA. These molecules enter the matrix of a mitochondrion, where they start the Citric Acid Cycle. The third carbon from pyruvate combines with oxygen to form carbon dioxide, which is released as a waste product. High-energy electrons are also released and captured in NADH. The reactions that occur next are shown in Figure $7$.
Steps of the Citric Acid (Krebs) Cycle
The Citric Acid Cycle begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate; see the lower panel of Figure $7$). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle. After citric acid forms, it goes through a series of reactions that release energy. This energy is captured in molecules of ATP and electron carriers. The Krebs cycle has two types of energy-carrying electron carriers: NAD+ and FAD. The transfer of electrons to FAD during the Kreb’s Cycle produces a molecule of FADH2. Carbon dioxide is also released as a waste product of these reactions. The final step of the Krebs cycle regenerates OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvate molecules when it splits glucose.
Results of the Citric Acid Cycle
After the second turn through the Citric Acid Cycle, the original glucose molecule has been broken down completely. All six of its carbon atoms have combined with oxygen to form carbon dioxide. The energy from its chemical bonds has been stored in a total of 16 energy-carrier molecules. These molecules are:
• 2 ATP
• 8 NADH
• 2 FADH$_2$
• 6 CO$_2$: 2 CO$_2$ from Transformation of Acetyl CoA and 4 CO$_2$ from Citric Acid Cycle.
Oxidative phosphorylation
Oxidative phosphorylation is the final stage of aerobic cellular respiration. There are two substages of oxidative phosphorylation, Electron transport chain and Chemiosmosis. In these stages, energy from NADH and FADH2, which result from the previous stages of cellular respiration, is used to create ATP.
Electron Transport Chain (ETC)
During this stage, high-energy electrons are released from NADH and FADH2, and they move along electron-transport chains found in the inner membrane of the mitochondrion. An electron-transport chain is a series of molecules that transfer electrons from molecule to molecule by chemical reactions. These molecules are found making up the three complexes of the electron transport chain (red structures in the inner membrane in Figure $8$). As electrons flow through these molecules, some of the energy from the electrons is used to pump hydrogen ions (H+) across the inner membrane, from the matrix into the intermembrane space. This ion transfer creates an electrochemical gradient that drives the synthesis of ATP. The electrons from the final protein of the ETC are gained by the oxygen molecule, and it is reduced to water in the matrix of the mitochondrion.
Chemiosmosis
The pumping of hydrogen ions across the inner membrane creates a greater concentration of these ions in the intermembrane space than in the matrix – producing an electrochemical gradient. This gradient causes the ions to flow back across the membrane into the matrix, where their concentration is lower. The flow of these ions occurs through a protein complex, known as the ATP synthase complex (see blue structure in the inner membrane in Figure $8$. The ATP synthase acts as a channel protein, helping the hydrogen ions across the membrane. The flow of protons through ATP synthase is considered chemiosmosis. ATP synthase also acts as an enzyme, forming ATP from ADP and inorganic phosphate. It is the flow of hydrogen ions through ATP synthase that gives the energy for ATP synthesis. After passing through the electron-transport chain, the low-energy electrons combine with oxygen to form water.
How Much ATP?
You have seen how the three stages of aerobic respiration use the energy in glucose to make ATP. How much ATP is produced in all three stages combined? Glycolysis produces 2 ATP molecules, and the Krebs cycle produces 2 more. Electron transport from the molecules of NADH and FADH2 made from glycolysis, the transformation of pyruvate, and the Krebs cycle creates as many as 32 more ATP molecules. Therefore, a total of up to 36 molecules of ATP can be made from just one molecule of glucose in the process of cellular respiration.
Review
1. What is the purpose of cellular respiration? Provide a concise summary of the process.
2. Draw and explain the structure of ATP (Adenosine Tri-Phosphate).
3. State what happens during glycolysis.
4. Describe the structure of a mitochondrion.
5. Outline the steps of the Krebs cycle.
6. What happens during the electron transport stage of cellular respiration?
7. How many molecules of ATP can be produced from one molecule of glucose during all three stages of cellular respiration combined?
8. Do plants undergo cellular respiration? Why or why not?
9. Explain why the process of cellular respiration described in this section is considered aerobic.
10. Name three energy-carrying molecules involved in cellular respiration.
11. Energy is stored within chemical _________ within a glucose molecule.
12. True or False. During cellular respiration, NADH and ATP are used to make glucose.
13. True or False. ATP synthase acts as both an enzyme and a channel protein.
14. True or False. The carbons from glucose end up in ATP molecules at the end of cellular respiration.
15. Which stage of aerobic cellular respiration produces the most ATP?
Explore More
Watch the video below for a detailed overview of cellular respiration.
Attributions
1. Campfire by Jon Sullivan, public domain via Wikimedia Commons
2. ATP structure by Mysid, public domain via Wikimedia Commons
3. Cellular Respiration by OpenStax College, licensed CC BY 4.0 via Wikimedia Commons
4. Glycolysis by Lumen Learning, CC BY 4.0
5. Citric Acid Cycle by Lumen Learning, CC BY 4.0
6. Mitochondria by Mariana Ruiz Villarreal LadyofHats, released into the public domain via Wikimedia Commons
7. Krebs Cycle by OpenStax College, licensed CC BY 4.0 via Wikimedia Commons
8. Electron Transport Chain by OpenStax College, licensed CC BY 4.0 via Wikimedia Commons
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0
10. Some text is adapted from Concepts of Biology by OpenStax licensed CC BY 4.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.09%3A_Cellular_Respiration.txt |
Fast and Furious
The muscles of this sprinter will need a lot of energy to complete their short race because they will be running at top speed. The action won't last long, but it will be very intense. The energy the sprinter needs can't be provided quickly enough by aerobic cellular respiration. Instead, a different process must be used by their muscle cells to power their activity.
Making ATP Without Oxygen
The cells of living things power their activities with the energy-carrying molecule ATP (adenosine triphosphate). The cells of most living things make ATP from glucose in the process of cellular respiration. This process occurs in three major stages, and one intermediate stage: glycolysis, oxidation of pyruvate, the Krebs cycle, and electron transport. The latter two stages require oxygen, making cellular respiration an aerobic process. There are also other ways of making ATP from glucose without oxygen, such as anaerobic respiration and fermentation, of making ATP from glucose without oxygen. Our cells do not perform anaerobic respiration. Therefore, we will only focus on fermentation in this section.
Fermentation
Fermentation starts with glycolysis, but it does not involve the latter two stages of aerobic cellular respiration (the Krebs cycle and oxidative phosphorylation). During glycolysis, two NAD+ electron carriers are reduced to two NADH molecules and 2 net ATPs are produced. The NADH must be oxidized back so that glycolysis can continue and cells can continue making 2 ATPs. The cells cannot make more than 2 ATP in fermentation because oxidative phosphorylation does not happen due to a lack of oxygen. There are two types of fermentation, alcoholic fermentation and lactic acid fermentation. Our cells can only perform lactic acid fermentation; however, we make use of both types of fermentation using other organisms.
Alcoholic Fermentation
Alcoholic fermentation The process by which this happens is summarized in Figure \(2\). The two pyruvate molecules are shown in this diagram come from the splitting of glucose through glycolysis. This process also produces 2 molecules of ATP. Continued breakdown of pyruvate produces acetaldehyde, carbon dioxide, and eventually ethanol. Alcoholic fermentation requires the electrons from NADH and results in the generation of NAD+.
Yeast in bread dough also uses alcoholic fermentation for energy and produces carbon dioxide gas as a waste product. The carbon dioxide that is released causes bubbles in the dough and explains why the dough rises. Do you see the small holes in the bread in Figure \(3\)? The holes were formed by bubbles of carbon dioxide gas.
Lactic Acid Fermentation
Lactic acid fermentation is carried out by certain bacteria, including the bacteria in yogurt. It is also carried out by your muscle cells when you work them hard and fast. This is how the muscles of the sprinter in Figure \(1\)get energy for their short-duration but intense activity. The process by which this happens is summarized in Figure \(2\). Again, two pyruvate and two ATP molecules result from glycolysis. Reduction of pyruvate using the electrons carried by NADH produces lactate (i.e. lactic acid). While this is similar to alcoholic fermentation, there is no carbon dioxide produced in this process.
Did you ever run a race, lift heavy weights, or participate in some other intense activity and notice that your muscles start to feel a burning sensation? This may occur when your muscle cells use lactic acid fermentation to provide ATP for energy. The buildup of lactic acid in the muscles causes the feeling of burning. The painful sensation is useful if it gets you to stop overworking your muscles and allow them a recovery period during which cells can eliminate the lactic acid.
Pros and Cons of Fermentation
With oxygen, organisms can use aerobic cellular respiration to produce up to 36 molecules of ATP from just one molecule of glucose. Without oxygen, some human cells must use fermentation to produce ATP, and this process produces only two molecules of ATP per molecule of glucose. Although fermentation produces less ATP, it has the advantage of doing so very quickly. It allows your muscles, for example, to get the energy they need for short bursts of intense activity. Aerobic cellular respiration, in contrast, produces ATP more slowly.
Feature: Myth vs. Reality
Myth: lactic acid build-up can cause muscle fatigue and a burning sensation in muscles. The soreness is thought to be due to microscopic damage to the muscle fibers.
Reality: The statement about lactic acid causing the burn in the muscle has no solid experimental proof. Alternate hypotheses suggest that through the production of lactic acid, the internal pH of the muscle decreases, triggering contraction in muscle due to the activation of motor neurons.
Review
1. State the main difference between aerobic cellular respiration and fermentation.
2. What is fermentation?
3. Compare and contrast alcoholic and lactic acid fermentation.
4. Identify the major pro and the major con of fermentation relative to aerobic cellular respiration.
1. What process is shared between aerobic cellular respiration and fermentation? Describe the process briefly.
2. Why is this process able to occur in fermentation as well as aerobic respiration?
5. Which type of metabolic process occurs in the human body?
1. Aerobic cellular respiration
2. Alcoholic fermentation
3. Lactic acid fermentation
4. Both A and C
6. True or False. Lactic acid fermentation produces carbon dioxide.
7. True or False. Types of bacteria can carry out alcoholic fermentation and lactic acid fermentation.
8. True or False. No ATP is produced by fermentation.
9. Both lactic acid fermentation and alcoholic fermentation use which acid molecules to make their final products?
10. Which type of process is used in the making of bread and wine?
1. Alcoholic fermentation
2. Lactic acid fermentation
3. Aerobic cellular respiration
4. Prokaryotic respiration
11. Is fermentation an aerobic or anaerobic process?
12. What is the reactant, or starting material, shared by aerobic respiration and both types of fermentation?
Explore More
While many people think that Brewers are artisans for their production of beer, in actuality, the true craft and process of beer making are due to anaerobic glycolysis from yeast. Learn more here:
Attributions
1. Jimmy Vicaut by Marie-Lan Nguyen, licensed CC BY 3.0 via Wikimedia Commons
2. Alcohol fermentation by Vtvu, licensed CC BY-SA 3.0 via Wikimedia Commons
3. Bread by National Cancer Institute, public domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.10%3A_Fermentation.txt |
Case Study Conclusion: More Than Just Tired
Jasmin discovered that her extreme fatigue, muscle pain, vision problems, and vomiting were due to problems in her mitochondria. Mitochondria are small membrane-bound organelles found in eukaryotic cells that provide energy for the cells of the body. They do this by carrying out the final two steps of aerobic cellular respiration, the Krebs cycle, and electron transport, which is the major way that the human body breaks down the sugar glucose from food into a form of energy cells can use, namely the molecule ATP.
Because mitochondria provide energy for cells, you can probably easily see why Jasmin was experiencing extreme fatigue, particularly after running. Her damaged mitochondria could not keep up with her need for energy, particularly after intense exercise which requires a lot of additional energy. What is perhaps not so obvious are the reasons for her other symptoms, such as blurry vision, muscle spasms, and vomiting. But all the cells in the body require energy in order to function properly. Mitochondrial diseases can cause problems in mitochondria in any cell of the body, including muscle cells and cells of the nervous system, which includes the brain and nerves. The nervous system and muscles work together to control vision and digestive system functions, such as vomiting, so when they are not functioning properly, a variety of symptoms can emerge. This also explains why Jasmin’s niece, who also has mitochondrial disease, has symptoms related to brain function, such as seizures and learning disabilities. Our cells are microscopic and mitochondria are even tinier, but they are essential for the proper functioning of our bodies and when they are damaged, serious health effects can occur.
A seemingly confusing aspect of mitochondrial diseases is that the type of symptoms, severity of symptoms, and age of onset can vary wildly between people — even within the same family! In Jasmin’s case, she did not notice symptoms until adulthood, while her niece had more severe symptoms starting at a much younger age. However, this makes sense when you know more about how mitochondrial diseases work.
Inherited mitochondrial diseases can be due to damage in either the DNA in the nucleus of cells or the DNA in the mitochondria themselves. Recall that mitochondria are thought to have evolved from prokaryotic organisms that were once free-living, but then infected or were engulfed by larger cells. One of the pieces of evidence that supports this endosymbiotic theory is that mitochondria have their own, separate DNA. When the mitochondrial DNA is damaged or mutated, it can result in some types of mitochondrial diseases. However, these mutations do not typically affect all of the mitochondria in a cell. During cell division, organelles such as mitochondria are replicated and passed down to the new daughter cells. If some of the mitochondria are damaged, and others are not, the daughter cells can have different amounts of damaged mitochondria. This helps explain the wide range of symptoms in people with mitochondrial diseases, even ones in the same family because different cells in their bodies are affected to different extents. Jasmin’s niece was affected strongly and her symptoms were noticed early, while Jasmin’s symptoms were milder and did not become apparent until adulthood.
There is still much more that needs to be discovered about the different types of mitochondrial diseases. But by learning about cells, their organelles, how they obtain energy, and how they divide, you should now have a better understanding of the biology behind these diseases. Apply your understanding of cells to your own life — can you think of other diseases that affect cellular structures or functions, maybe that even affect people you know? Since your entire body is made of cells, when they are damaged or not functioning properly it can cause a wide variety of health problems.
Chapter Summary
In this chapter, you learned many facts about cells. Specifically, you learned that:
• Cells are the basic units of structure and function of living things.
• The first cells, from cork, were observed by Hooke in the 1600s. Soon after, van Leeuwenhoek observed other living cells.
• In the early 1800s, Schwann and Schleiden theorized that cells are the basic building blocks of all living things. Around 1850, Virchow saw cells dividing and added that living cells arise only from other living cells. These ideas led to cell theory, which states that all organisms are made of cells, all life functions occur in cells, and all cells come from other cells.
• The invention of the electron microscope in the 1950s allowed scientists to see organelles and other structures inside cells for the first time.
• There is variation in cells, but all cells have a plasma membrane, cytoplasm, ribosomes, and DNA.
• The plasma membrane is composed mainly of a bilayer of phospholipid molecules and forms a barrier between the cytoplasm inside the cell and the environment outside the cell. It allows only certain substances to pass in or out of the cell. Some cells have extensions of their plasma membrane with other functions, such as flagella or cilia.
• The cytoplasm is a thick solution that fills a cell and is enclosed by the cell membrane. It helps give the cell shape, holds organelles, and provides a site for many of the biochemical reactions inside the cell. The liquid part of the cytoplasm is called cytosol.
• Ribosomes are small structures where proteins are made.
• Cells are usually very small so they have a large enough surface-area-to-volume ratio to maintain normal cell processes. Cells with different functions often have different shapes.
• Prokaryotic cells do not have a nucleus. Eukaryotic cells have a nucleus as well as other organelles. An organelle is a structure within the cytoplasm of a cell that is enclosed within a membrane and performs a specific job.
• The cytoskeleton is a highly organized framework of protein filaments and tubules that criss-cross the cytoplasm of a cell. It gives the cell structure and helps to hold cell structures such as organelles in place.
• The nucleus is the largest organelle in a eukaryotic cell and is considered to be the cell's control center. It contains DNA and controls gene expression, including which proteins the cell makes.
• The mitochondrion is an organelle that makes energy available to cells. According to the widely accepted endosymbiotic theory, mitochondria evolved from prokaryotic cells that were once free-living organisms that infected or were engulfed by larger prokaryotic cells.
• The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. Rough endoplasmic reticulum (RER) is studded with ribosomes. Smooth endoplasmic reticulum (SER) has no ribosomes.
• The Golgi apparatus is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It is also involved in the transport of lipids around the cell.
• Vesicles and vacuoles are sac-like organelles that may be used to store and transport materials in the cell or as chambers for biochemical reactions. Lysosomes and peroxisomes are vesicles that break down foreign matter, dead cells, or poisons.
• Centrioles are organelles located near the nucleus that help organize the chromosomes before cell division so each daughter cell receives the correct number of chromosomes.
• There are two basic ways that substances can cross the cell’s plasma membrane: passive transport, which requires no energy; and active transport, which requires energy.
• No energy is needed for passive transport because it occurs when substances move naturally from an area of higher concentration to an area of lower concentration. Types of passive transport in cells include:
• Simple diffusion, which is the movement of a substance due to differences in concentration without any help from other molecules. This is how very small, hydrophobic molecules, such as oxygen and carbon dioxide, enter and leave the cell.
• Osmosis, which is the diffusion of water molecules across the membrane.
• Facilitated diffusion, which is the movement of a substance across a membrane due to differences in concentration but only with the help of transport proteins in the membrane, such as channel proteins or carrier proteins. This is how large or hydrophilic molecules and charged ions enter and leave the cell.
• Active transport requires energy to move substances across the plasma membrane, often because the substances are moving from an area of lower concentration to an area of higher concentration or because of their large size. Two examples of active transport are the sodium-potassium pump and vesicle transport.
• The sodium-potassium pump moves sodium ions out of the cell and potassium ions into the cell, both against a concentration gradient, in order to maintain the proper concentrations of both ions inside and outside the cell and to thereby control membrane potential.
• Vesicle transport uses vesicles to move large molecules into or out of cells.
• Energy is the ability to do work and is needed by every living cell to carry out life processes.
• The form of energy that living things need is chemical energy, and it comes from food. Food consists of organic molecules that store energy in their chemical bonds.
• Organisms mainly use glucose and ATP for energy. Glucose is the compact, stable form of energy that is carried in the blood and taken up by cells. ATP contains less energy and is used to power cellular processes.
• Cellular respiration is the aerobic process by which living cells break down glucose molecules, release energy, and form molecules of ATP. This process involves Glycolysis, Transformation of Pyruvate, Krebs Cycle, and Oxidative phosphorylation. Overall, in this process, glucose and oxygen react to form carbon dioxide and water.
• The first stage of cellular respiration, called glycolysis, takes place in the cytoplasm. In this step, enzymes split a molecule of glucose into two molecules of pyruvate, which releases energy that is transferred to ATP.
• Pyruvate is transformed to Acetyl CoA in the intermediate stage
• The second major stage of cellular respiration, called the Krebs cycle, takes place in the matrix of a mitochondrion. During this stage, two turns through the cycle result in all of the carbon atoms from the two pyruvate molecules forming carbon dioxide and the energy from their chemical bonds being stored in a total of 16 energy-carrying molecules (including 4 from glycolysis).
• The third stage of cellular respiration, Oxidative Phosphorylation, takes place on the inner membrane of the mitochondrion. Electrons are transported from molecule to molecule down an electron-transport chain. Some of the energy from the electrons is used to pump hydrogen ions across the membrane, creating an electrochemical gradient that drives the synthesis of many more molecules of ATP.
• In all three stages of aerobic cellular respiration combined, as many as 36 molecules of ATP are produced from just one molecule of glucose.
• Some organisms can produce ATP from glucose by anaerobic respiration, which does not require oxygen. Many human cells perform fermentation that also does not require oxygen. It is performed to recycle NADH back into NAD+. There are two types: alcoholic fermentation and lactic acid fermentation. Both start with glycolysis.
• Alcoholic fermentation is carried out by single-celled organisms including yeasts and some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine.
• Lactic acid fermentation is undertaken by certain bacteria, including the bacteria in yogurt, and also by our muscle cells when they are worked hard and fast.
• Anaerobic respiration produces far less ATP than does aerobic cellular respiration, but it has the advantage of being much faster.
Chapter Summary Review
1. For the following questions, choose whether the description applies to eukaryotic cells, prokaryotic cells, or both.
1. Has a nuclear membrane
2. Has a plasma membrane made of a phospholipid bilayer
3. Can be in a multicellular organism
4. Has ribosomes
5. Has an endoplasmic reticulum
6. Its DNA replicates before cell division
7. Has a single circular chromosome
8. Has cytoplasm that splits into two daughter cells during cell division
9. Has a cell cycle that includes interphase and mitosis
10. The type of cell that most likely evolved to become mitochondria
2. Name one example of a prokaryotic organism and one example of a eukaryotic organism.
3. Neurons are cells in the nervous system that transmit messages. They use energy to maintain the balance of sodium and potassium ions inside and outside of them, which is critical for their ability to send messages.
1. What kind of transport is this maintenance of sodium and potassium ion concentrations – active or passive? Explain your reasoning.
2. What creates the barrier between the inside and the outside of these cells?
3. What molecule uses energy to maintain the balance of sodium and potassium ions inside and outside of neurons? Describe two reasons why such a molecule is required.
4. What form of energy is used in this process?
5. Briefly explain how the energy in the food you eat gets there and provides energy for your neurons in the form necessary to power this process.
4. Explain why the inside of the plasma membrane, the side that faces the cytoplasm of the cell, must be hydrophilic.
5. True or False. Anaerobic and aerobic cellular respiration both produce ATP.
6. True or False. The cell membrane can also be called the plasma membrane.
7. True or False. Each phospholipid molecule in the cell membrane has two heads and a tail.
8. True or False. For cells, a smaller size is generally more efficient.
9. True or False. DNA is located in the nucleus of prokaryotic cells.
10. True or False. Cilia and flagella stick out of the cell membrane but are not made of cell membrane themselves.
11. Which statement about the cell membrane is false?
1. It encloses the cytoplasm
2. It protects and supports the cell
3. It keeps all external substances out of the cell
4. None of the above
12. During diffusion, substances move from an area of X? concentration to an area of Y? concentration.
1. higher, lower
2. lower, higher
3. higher, equal
4. lower, equal
13. Which type of respiration involves electron transport?
1. Where does this electron transport occur within the cell?
2. Energy from electron transport is used to pump hydrogen ions across a membrane. Is this active or passive transport of hydrogen ions? Explain your answer.
3. After the process described in part B, hydrogen ions then flow from a region of higher concentration to a region of lower concentration. Is this active or passive transport of hydrogen ions? Explain your answer.
Attributions
1. Mitochondrion byLadyofHats, Public domain, via Wikimedia Commons
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/05%3A_Cells/5.11%3A_Case_Study_Conclusion%3A_Tired_and_Chapter_Summary.txt |
This chapter contains information on DNA discovery, the central dogma of biology, DNA replication, transcription, and how proteins are synthesized through the process of translation. Additionally, the chapter highlights gene regulation. This chapter also discusses the types of mutations and their causes. The other topics of this chapter include biotechnology and the Human Genome Project. In order to understand personalized medicine, we need to know what genes do, how they interact and learn all the differences in DNA between people. As you read this chapter, think about how an understanding of the human genome and genetics is essential for discovering how medicines may affect each of us individually.
• 6.1: Case Study: Why do we need to sequence everybody's genome?
Pharmacogenomics is based on a special kind of genetic testing. It looks for small genetic variations that influence a person's ability to activate and deactivate drugs. Results of the tests can help doctors choose the best drug and most effective dose for a given patient. Many drugs need to be activated by the patient's own enzymes, and inherited variations in enzymes may affect how quickly or efficiently this happens.
• 6.2: DNA and RNA
This young person has naturally red hair. Why is this hair red instead of some other color? And, in general, what causes specific traits to occur? There is a molecule in human beings and most other living things that is largely responsible for their traits. The molecule is large and has a spiral structure in eukaryotes. What molecule is it? With these hints, you probably know that the molecule is DNA.
• 6.3: Chromosomes and Genes
Chromosomes are coiled structures made of DNA and proteins. Chromosomes are encoded with genetic instructions for making proteins. These instructions are organized into units called genes. Most genes contain the instructions for a single protein. There may be hundreds or even thousands of genes on a single chromosome.
• 6.4: Protein Synthesis
Your DNA, or deoxyribonucleic acid, contains the genes that determine who you are. How can this organic molecule control your characteristics? DNA contains instructions for all the proteins your body makes. Proteins, in turn, determine the structure and function of all your cells. What determines a protein's structure? It begins with the sequence of amino acids that make up the protein. Instructions for making proteins with the correct sequence of amino acids are encoded in DNA.
• 6.5: Genetic Code
The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The bases are adenine (A), cytosine (C), guanine (G), and thymine (T) (or uracil, U, in RNA). The four bases make up the "letters" of the genetic code. The letters are combined in groups of three to form code "words," called codons. Each codon stands for (encodes) one amino acid, unless it codes for a start or stop signal. There are 20 common amino acids in proteins.
• 6.6: Mutations
Mutations are random changes in the sequence of bases in DNA or RNA. The word mutation may make you think of Ninja Turtles, but that's a misrepresentation of how most mutations work. First of all, everyone has mutations. In fact, most people have dozens or even hundreds of mutations in their DNA. Secondly, from an evolutionary perspective, mutations are essential. They are needed for evolution to occur because they are the ultimate source of all new genetic variation in any species.
• 6.7: Regulation of Gene Expression
Using a gene to make a protein is called gene expression. It includes the synthesis of the protein by the processes of transcription of DNA and translation of mRNA. It may also include further processing of the protein after synthesis. Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any point in the expression of a gene.
• 6.8: Biotechnology
Biotechnology is the use of technology to change the genetic makeup of living things for human purposes. Generally, the goal of biotechnology is to modify organisms so they are more useful to humans. For example, biotechnology may be used to create crops that yield more food or resist insect pests or viruses, such as the virus-resistant potatoes pictured above. Research is also underway to use biotechnology to cure human genetic disorders with gene therapy.
• 6.9: The Human Genome
The human genome refers to all the DNA of the human species. Human DNA consists of 3.3 billion base pairs and is divided into more than 20,000 genes on 23 pairs of chromosomes. The human genome also includes noncoding sequences (e.g. intergenic region) of DNA.
• 6.10: Case Study Conclusion: Parmacogenomics and Chapter Summary
Arya asked their doctor about Pharmacogenomics. The doctor explains to Arya that Pharmacogenomics is the tailoring of drug treatments to people's genetic makeup, a form of 'personalized medicine'.
Thumbnail: DNA double helix. (public domain; NIH - Genome Research Institute).
06: DNA and Protein Synthesis
Case Study: Pharmacogenomics, a personalized medicine
Arya is 50-year-old and morbidly obese. Arya uses gender-neutral pronouns, such as they, them, and theirs. They have high blood pressure and cardiovascular disease. Recently, they lost 10 pounds of weight in a month without trying. They also get thirsty very easily and make frequent visits to the restroom. Their doctor diagnosed them with insulin-dependent type 2 diabetes after some physical and blood tests. Type 2 diabetes, also called diabetes mellitus, is a condition in which either the beta cells of a person’s pancreas stop secreting insulin due to the high demand of insulin by an overweight person, or the body cells become insensitive to insulin. Insulin is a hormone that activates all the cells of the body to uptake glucose from the bloodstream. Cells need glucose to acquire energy (ATP) through cellular respiration to perform various metabolic activities. High levels of blood glucose in the absence of insulin may lead to high blood glucose and eventually may lead to the symptoms that Arya is experiencing.
Arya’s doctor prescribed gliclazide. Gliclazide belongs to the sulfonylurea category of drugs. Sulfonylureas stimulate the beta cells of the pancreas to secrete insulin. Arya started this treatment and experienced an adverse reaction after taking their second dose. They experienced feelings of hunger, sweating, shakiness, and weakness a few minutes after taking the medication. They called 911. When they recovered, they went back to their doctor. Their doctor told them that they had experienced hypoglycemia, which is one of the major side effects of sulfonylurea-based medicines. The doctor noted that due to the other complications that Arya has, such as cardiovascular disease, gliclazide was the best choice. The doctor explained that not everyone responds to medications in the same way. A drug that works well for one person may not be effective for another. The dose of a drug that cures a disease in one individual may be inadequate for someone else. Some people may experience side effects from a given medication, whereas other people do not. This variation in responses to medications can be due to differences in our genes. That’s where the field of pharmacogenetics comes in. News media have hailed it as the "new frontier in medicine." It certainly seems to hold promise for improving the pharmaceutical treatment of patients.
Pharmacogenomics is based on a special kind of genetic testing. It looks for small genetic variations that influence a person’s ability to activate and deactivate drugs. Results of the tests can help doctors choose the best drug and most effective dose for a given patient. Many drugs need to be activated by the patient’s own enzymes, and inherited variations in enzymes may affect how quickly or efficiently this happens. For example, if a patient’s enzymes break down a particular drug too slowly, then standard doses of the drug may not work very well for that patient. Drugs also must be deactivated to reduce their effects on healthy cells. If a patient’s enzymes deactivate a drug too slowly, then the drug may remain at high levels and cause side effects. Arya experienced a high release of insulin due to the variations in their genotype.
The doctor recommended that Arya goes through genetic testing for a better treatment plan. One of the main benefits of pharmacogenomics is greater patient safety. Pharmacogenomic testing may help identify patients who are likely to experience adverse reactions to drugs so that different, safer drugs can be prescribed. Another benefit of pharmacogenomics is eliminating the trial-and-error approach that is often used to find appropriate medications and doses for a given patient. This saves time and money as well as improving patient outcomes. This is more like a personalized medicine as demonstrated in the picture above.
Because pharmacogenomics is a new field, some insurance companies do not cover it, and it can be very expensive. Also, not all of the genetic tests are yet widely available. In addition, there may be ethical and legal issues associated with genetic testing, including concerns about privacy issues. Because Arya is concerned, they have many questions for their doctor.
In order to understand personalized medicine, we need to know what genes do, how they interact, and learn all the differences in DNA between people. As you read this chapter, think about how an understanding of the human genome and genetics is essential for discovering how medicines may affect each of us individually.
As you read this chapter, try to answer the following questions:
• What is a gene?
• Enzymes are proteins. How are enzymes synthesized?
• What is the relationship between an enzyme and DNA?
• Why do people differ genetically?
• How are the genes sequenced?
Chapter overview: in this chapter, you will learn the following:
• How genes, and their different alleles, are located on chromosomes.
• The 23 pairs of human chromosomes, which include autosomal and sex chromosomes.
• How DNA was discovered to be the inherited genetic material.
• The structure of DNA and how DNA replication occurs.
• The central dogma of molecular biology, which describes how DNA is transcribed into RNA, and then translated into proteins.
• The structure, functions, and possible evolutionary history of RNA.
• How genes code for proteins using codons made of the sequence of nitrogen bases within RNA and DNA.
• How proteins are synthesized through the transcription of RNA from DNA and the translation of protein from RNA, including how RNA and proteins can be modified, and the roles of the different types of RNA.
• What mutations are, what causes them, different specific types of mutations, and the importance of mutations in evolution and to human health.
• How the expression of genes into proteins is regulated and why problems in this process can cause diseases such as cancer.
• What is Biotechnology and how it is applied?
• What is Pharmacogenomics?
Attributions
1. Personalized Medicine by Mark Scrimshire, licensed CC BY 2.0 via Flickr
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.01%3A_Case_Study%3A_Why_do_we_need_to_sequence_everybody%27s_genome.txt |
What Makes You...You?
This person has naturally red hair. Why is this hair red instead of some other color? And, in general, what causes specific traits to occur? There is a molecule in human beings and most other living things that is largely responsible for their traits. The molecule is large and has a spiral structure in eukaryotes. What molecule is it? With these hints, you probably know that the molecule is DNA.
Introducing DNA
Today, it is commonly known that DNA is the genetic material that is passed from parents to offspring and determines our traits. For a long time, scientists knew such molecules existed, that is, they were aware that genetic information is contained within biochemical molecules. However, they didn’t know which molecules play this role. In fact, for many decades, scientists thought that proteins were the molecules that contain genetic information.
Discovery that DNA is the Genetic Material
Determining that DNA is the genetic material was an important milestone in biology. It took many scientists undertaking creative experiments over several decades to show with certainty that DNA is the molecule that determines the traits of organisms. This research began in the early part of the 20th century.
Griffith's Experiments with Mice
The first important discovery was made in the 1920s. An American scientist named Frederick Griffith was studying mice and two different strains of a bacterium called R (rough) strain and S (smooth) strain. He injected the two bacterial strains into mice. The S strain was virulent and killed the mice, whereas the R strain was not virulent and did not kill the mice. You can see these details in Figure \(2\). Griffith also injected mice with S-strain bacteria that had been killed by heat. As expected, the dead bacteria did not harm the mice. However, when the dead S-strain bacteria were mixed with live R-strain bacteria and injected, the mice died.
Based on his observations, Griffith deduced that something in the dead S-strain was transferred to the previously harmless R-strain, making the R-strain deadly. What was this "something?" What type of substance could change the characteristics of the organism that received it?
Avery and His Colleagues Make a Major Contribution
In the early 1940s, a team of scientists led by Oswald Avery tried to answer the question raised by Griffith’s research results. First, they inactivated various substances in the S-strain bacteria. Then they killed the S-strain bacteria and mixed the remains with live R-strain bacteria. (Keep in mind that the R-strain bacteria normally did not harm the mice.) When they inactivated proteins, the R-strain was deadly to the injected mice. This ruled out proteins as genetic material. Why? Even without the S-strain proteins, the R-strain was changed or transformed into a deadly strain. However, when the researchers inactivated DNA in the S-strain, the R-strain remained harmless. This led to the conclusion that DNA — and not protein — is the substance that controls the characteristics of organisms. In other words, DNA is the genetic material.
Hershey and Chase Confirm the Results
The conclusion that DNA is the genetic material was not widely accepted until it was confirmed by additional research. In the 1950s, Alfred Hershey and Martha Chase did experiments with viruses and bacteria. Viruses are not cells. Instead, they are basically DNA (or RNA) inside a protein coat. To reproduce, a virus must insert its own genetic material into a cell (such as a bacterium). Then it uses the cell’s machinery to make more viruses. The researchers used different radioactive elements to label the DNA and proteins in DNA viruses. This allowed them to identify which molecule the viruses inserted into bacterial cells. DNA was the molecule they identified. This confirmed that DNA is the genetic material.
Chargaff Focuses on DNA Bases
Erwin Chargaff (1905-2002), an Austrian-American biochemist from Columbia University, analyzed the base composition of the DNA of various species. This led him to propose two main rules that have been appropriately named Chargaff's rules.
Rule 1
Chargaff determined that in DNA, the amount of one base, a purine, always approximately equals the amount of a particular second base, a pyrimidine. Specifically, in any double-stranded DNA, the number of guanine units equals approximately the number of cytosine units and the number of adenine units equals approximately the number of thymine units.
Human DNA is 30.9% A and 29.4% T, 19.9% G and 19.8% C. The rule constitutes the basis of base pairs in the DNA double helix: A always pairs with T, and G always pairs with C. He also demonstrated that the number of purines (A+G) always approximates the number of pyrimidines (T+C), an obvious consequence of the base-pairing nature of the DNA double helix.
Rule 2
In 1947 Chargaff showed that the composition of DNA, in terms of the relative amounts of the A, C, G, and T bases, varied from one species to another. This molecular diversity added to the evidence that DNA could be the genetic material.
Discovery of the Double Helix
After DNA was shown to be the genetic material, scientists wanted to learn more about it, including its structure. James Watson and Francis Crick are usually given credit for discovering that DNA has a double-helix shape like a spiral staircase, as shown in Figure \(4\). In fact, Watson and Crick's discovery of the double helix depended heavily on the prior work of Rosalind Franklin and other scientists, who had used X-rays to learn more about DNA’s structure. Unfortunately, Franklin and these other scientists have not usually been given credit for their important contributions to the discovery of the double helix.
The double-helix shape of DNA, together with Chargaff’s rules, led to a better understanding of DNA. As a nucleic acid, DNA is made from nucleotide monomers. Long chains of nucleotides form polynucleotides, and the DNA double helix consists of two polynucleotide chains. Each nucleotide consists of a sugar (deoxyribose), a phosphate group, and one of the four bases (adenine, cytosine, guanine, or thymine). The sugar and phosphate molecules in adjacent nucleotides bond together and form the "backbone" of each polynucleotide chain.
Scientists concluded that bonds between the bases hold together the two polynucleotide chains of DNA. Moreover, adenine always bonds with thymine, and cytosine always bonds with guanine. That's why these pairs of bases are called complementary base pairs. If you look at the nitrogen bases in Figure \(3\), you will see why the bases bond together only in these pairings. Adenine and guanine have a two-ring structure, whereas cytosine and thymine have just one ring. If adenine were to bond with guanine as well as thymine, for example, the distance between the two DNA chains would be variable. However, when a one-ring molecule (such as thymine) always bonds with a two-ring molecule (such as adenine), the distance between the two chains remains constant. This maintains the uniform shape of the DNA double helix. The bonded base pairs (A-T and G-C) stick into the middle of the double helix, forming, in essence, the steps of the spiral staircase.
DNA Replication
Knowledge of DNA’s structure helped scientists understand how DNA replicates. DNA replication is the process in which DNA is copied. It occurs during the synthesis (S) phase of the eukaryotic cell cycle. DNA must be copied so that, after cell division occurs, each daughter cell will have a complete set of chromosomes.
DNA replication begins when an enzyme breaks the bonds between complementary bases in the molecule. This exposes the bases inside the molecule so they can be “read” by another enzyme and used to build two new DNA strands with complementary bases. The two daughter molecules that result each contain one strand from the parent molecule and one new strand that is complementary to it. As a result, the two daughter molecules are both identical to the parent molecule. This is a semi-conservative process (see Figure \(5\).)
Helicase and Polymerase
DNA replication begins as an enzyme, DNA helicase, breaks the hydrogen bonds holding the two strands together and forms a replication fork (see Figure \(6\). The resulting structure has two branching strands of DNA backbone with exposed bases. These exposed bases allow the DNA to be “read” by another enzyme, DNA polymerase, which then builds the complementary DNA strand. As DNA helicase continues to open the double helix, the replication fork grows.
Leading and Lagging Strands
Two DNA polymerase enzymes work at a Replication fork. This enzyme can only build new DNA in the 5' → 3' direction. It also needs a primer built by primase to start building DNA. Therefore, the two new strands, the leading strand and the lagging strand, of DNA are “built” in opposite directions. The leading strand is the DNA strand that DNA polymerase constructs in the 5' → 3' direction. This strand of DNA is made in a continuous manner, moving as the replication fork grows. The "lagging” strand is synthesized in short segments known as Okazaki fragments. On the lagging strand, primase builds a short RNA primer. DNA polymerase is then able to use the free 3'-OH group on the RNA primer to make DNA in the 5' → 3' direction till it reaches to end of the template strand. DNA polymerase of the lagging strand then jumps to go further into the replication fork to make another Okazaki fragment. The RNA fragments are then degraded and new DNA nucleotides are added to fill the gaps where the RNA was present. Another enzyme, DNA ligase, is then able to attach (ligate) the DNA nucleotides together, completing the synthesis of the lagging strand (Figure \(6\)).
What is RNA?
RNA structure differs from the DNA structure in three specific ways. Both are nucleic acids and made out of nucleotides; however, RNA is single-stranded while DNA is double-stranded. RNA nucleotides, like those from DNA, have three parts: a 5-carbon sugar, a phosphate group, and a base. RNA contains the 5-carbon sugar ribose, whereas, in DNA, the sugar is deoxyribose. The difference between ribose and deoxyribose is the lack of a hydroxyl group attached to the pentose ring in the 2' position of deoxyribose (see figure Figure \(7\).
Table \(1\): comparison of RNA and DNA
RNA DNA
Strands single stranded double stranded
Specific Base contains uracil contains thymine
Sugar ribose deoxyribose
Size relatively small big (chromosomes)
Location moves to cytoplasm stays in nucleus
Types 3 types: mRNA, tRNA, rRNA generally 1 type
Though both RNA and DNA contain the nitrogenous bases adenine, guanine, and cytosine, RNA contains the nitrogenous base uracil instead of thymine. Uracil pairs with adenine in RNA, just as thymine pairs with adenine in DNA. Uracil and thymine have very similar structures; uracil is an unmethylated form of thymine.
The nucleotide sequence of RNA, which is complementary to the DNA sequence, allows RNA to encode genetic information. RNA though carries the genetic information of just one gene. Hence, compared to DNA, RNA molecules are relatively small.
Review
1. Outline the discoveries that led to the determination that DNA, and not protein, is the biochemical molecule that contains genetic information.
2. State Chargaff's rules. Explain how the rules are related to the structure of the DNA molecule.
3. Explain how the structure of a DNA molecule is like a spiral staircase. Which parts of the staircase represent the various parts of the molecule?
4. Describe the process of DNA replication.
5. When does DNA replication occur, and why is the process said to be semi-conservative?
6. Why do you think dead S strain bacteria injected into mice does not harm the mice but kills them when mixed with living (and normally harmless) R strain bacteria?
7. In Griffith’s experiment, do you think the heat treatment that killed the bacteria also inactivated the bacterial DNA? Why or why not?
8. Give one example of a specific piece of evidence that helped rule out proteins as the genetic material.
9. True or False. Two-ring bases always bind to each other.
10. True or False. DNA replication involves the breaking of one of the polynucleotide chains into individual nucleotides.
11. True or False. In DNA, each nucleotide has a sugar.
12. What would the complementary strand of this stretch of DNA bases be? GTTAC
13. Which scientists detected labeled DNA that was transferred from one organism to another?
1. Hershey and Chase
2. Chargaff
3. Avery
4. Griffith
14. Which enzyme break the bonds between complementary bases and add new complementary nucleotides to the parental strands during DNA replication?
1. Phosphates
2. Enzymes
3. Viruses
4. RNA molecules
15. Describe the differences between DNA and RNA.
16. How is DNA replicated? Why is DNA replication called a "semi-conservative" process?
17. What are the roles of the following enzymes?
1. DNA polymerase
2. DNA helicase
3. DNA ligase
4. primase
Explore More
Rosalind Franklin was a British scientist who helped discover the structure of DNA. To learn more, check this out:
Attributions
1. Rood by Zoë Cleeren, public domain via Wikimedia Commons
2. Griffith experiment by Madprime, dedicated CC0 via Wikimedia Commons
3. DNA Nucleotide by OpenStax College, licensed CC BY 3.0 via Wikimedia Commons
4. DNA structure and bases by MesserWoland, licensed CC BY-SA .30 via Wikimedia Commons
5. DNA replication by Madprime, dedicated CC0 via Wikimedia Commons
6. DNA replication by LadyofHats Mariana Ruiz, released into the public domain via Wikimedia Commons
7. Difference DNA and RNA by Roland1952 licensed CC BY-SA .30 via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.02%3A_DNA_and_RNA.txt |
Identical Twins, Identical Genes
You probably can tell by their close resemblance that these two individuals are identical twins. Identical twins develop from the same fertilized egg, so they inherited copies of the same chromosomes and have all the same genes. Unless you have an identical twin, no one else in the world has exactly the same genes as you. What are genes? How are they related to chromosomes? And how do genes make you the person you are?
Chromosomes are coiled structures made of DNA and proteins. Chromosomes are encoded with genetic instructions for making proteins. These instructions are organized into units called genes. Most genes contain the instructions for a single protein. There may be hundreds or even thousands of genes on a single chromosome.
Human Chromosomes
Each species is characterized by a set number of chromosomes. The human number is 23. Human cells normally have two sets of chromosomes in each of their cells, one set inherited from each parent. There are 23 chromosomes in each set, for a total of 46 chromosomes per cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are actually 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape, and they also contain the same genes. The chromosomes in a pair are known as homologous chromosomes. As see in Figure \(2\), there are two types of chromosomes, autosomal and sex chromosomes. Read more about this in the genetics chapter.
Human Genes
Humans have an estimated 25,000 genes. This may sound like a lot, but it really isn’t. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Only about 25 percent of the nitrogen base pairs of DNA in human chromosomes make up genes and their regulatory elements. Out of this 25 percent, only two percent code for genes. The functions of many of the other base pairs are still unclear.
The majority of human genes have two or more possible versions, called alleles. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA base pairs within alleles.
Linkage
Genes that are located on the same chromosome are called linked genes. Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair color and eye color are linked, so certain hair and eye colors tend to be inherited together, such as blonde hair with blue eyes and brown hair with brown eyes. Can you think of other human traits that seem to occur together? Do you think they might be controlled by linked genes?
Genes located on the sex chromosomes are called sex-linked genes. Most sex-linked genes are on the X chromosome because the Y chromosome has relatively few genes. Strictly speaking, genes on the X chromosome are X-linked genes, but the term sex-linked is often used to refer to them. Figure \(3\) is called a linkage map. A linkage map shows the locations of specific genes on a chromosome. It shows the locations of a few of the genes on the human X chromosome, such as a blood group protein gene, Lethyosis (a skin disease gene), ocular albinism gene, and many more.
Review
1. What are chromosomes and genes, and how are the two related?
2. Describe human chromosomes and genes.
3. Explain the difference between autosomes and sex chromosomes.
4. What are linked genes, and what does a linkage map show?
5. Explain why females are considered the default sex in humans.
6. True or False. Humans have 46 pairs of chromosomes.
7. True or False. Autosomes refer to any chromosome other than sex chromosomes.
8. True or False. The majority of human DNA does not encode for proteins.
9. Explain the relationship between genes and alleles.
10. Put the following in order of size, from smallest to largest: chromosome; gene; base pair
11. Sex-linked genes are usually found on which chromosome? Explain why these genes are called sex-linked.
12. Which of the following are considered homologous chromosomes?
1. Chromosome 22 and the X chromosome
2. The two copies of chromosome 22 that make up a pair
3. All of the chromosomes in a skin cell and all of the chromosomes in a muscle cell
4. Chromosomes 21 and 22
13. What is the one chromosome that is different between genetic males and females? Explain your answer.
14. Most males and females have two sex chromosomes. Explain why then, do only females have Barr bodies.
Explore More
Watch the video below to learn about sex chromosomal disorders, such as Turner syndrome.
Attributions
1. Marian and Vivian Brown by Cmichel67, licensed CC BY 2.0 via Wikimedia Commons
2. Karyotype by National Human Genome Research Institute, public domain via Wikimedia Commons
3. Gene map by Sam McCabe for CK-12 licensed CC BY-NC 3.0
4. Chromosome by KES47, licensed CC BY 3.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.03%3A_Chromosomes_and_Genes.txt |
The Central Dogma of Biology
Your DNA, or deoxyribonucleic acid, contains the genes that determine who you are. How can this organic molecule control your characteristics? DNA contains instructions for all the proteins your body makes. Proteins, in turn, determine the structure and function of all your cells. What determines a protein’s structure? It begins with the sequence of amino acids that make up the protein. Instructions for making proteins with the correct sequence of amino acids are encoded in DNA.
DNA is found in chromosomes. In eukaryotic cells, chromosomes always remain in the nucleus, but proteins are made at ribosomes in the cytoplasm or on the rough endoplasmic reticulum (RER). How do the instructions in DNA get to the site of protein synthesis outside the nucleus? Another type of nucleic acid is responsible. This nucleic acid is RNA or ribonucleic acid. RNA is a small molecule that can squeeze through pores in the nuclear membrane. It carries the information from DNA in the nucleus to a ribosome in the cytoplasm and then helps assemble the protein. In short:
DNA → RNA → Protein
Discovering this sequence of events was a major milestone in molecular biology. It is called the central dogma of biology. The two processes involved in the central dogma are transcription and translation.
Transcription
Transcription is the first part of the central dogma of molecular biology: DNA → RNA. It is the transfer of genetic instructions in DNA to mRNA. Transcription happens in the nucleus of the cell. During transcription, a strand of mRNA is made that is complementary to a strand of DNA called a gene. A gene can easily be identified from the DNA sequence. A gene contains the basic three regions, promoter, coding sequence (reading frame), and terminator. There are more parts of a gene which are illustrated in Figure \(3\).
Steps of Transcription
Transcription takes place in three steps, called initiation, elongation, and termination. The steps are illustrated in Figure \(4\).
1. Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. This signals the DNA to unwind so the enzyme can “read” the bases in one of the DNA strands. The enzyme is ready to make a strand of mRNA with a complementary sequence of bases. The promoter is not part of the resulting mRNA
2. Elongation is the addition of nucleotides to the mRNA strand.
3. Termination is the ending of transcription. As RNA polymerase transcribes the terminator, it detaches from DNA. The mRNA strand is complete after this step.
Processing mRNA
In eukaryotes, the new mRNA is not yet ready for translation. At this stage, it is called pre-mRNA, and it must go through more processing before it leaves the nucleus as mature mRNA. The processing may include the addition of a 5' cap, splicing, editing, and 3' polyadenylation (poly-A) tail. These processes modify the mRNA in various ways. Such modifications allow a single gene to be used to make more than one protein. See Figure \(5\) as you read below:
• 5' cap protects mRNA in the cytoplasm and helps in the attachment of mRNA with the ribosome for translation.
• Splicing removes introns from the protein-coding sequence of mRNA. Introns are regions that do not code for the protein. The remaining mRNA consists only of regions called exons that do code for the protein.
• Editing changes some of the nucleotides in mRNA. For example, a human protein called APOB, which helps transport lipids in the blood, has two different forms because of editing. One form is smaller than the other because editing adds an earlier stop signal in mRNA.
• Polyadenylation adds a “tail” to the mRNA. The tail consists of a string of As (adenine bases). It signals the end of mRNA. It is also involved in exporting mRNA from the nucleus, and it protects mRNA from enzymes that might break it down.
Translation
The translation is the second part of the central dogma of molecular biology: RNA --> Protein. It is the process in which the genetic code in mRNA is read to make a protein. The translation is illustrated in Figure \(6\). After mRNA leaves the nucleus, it moves to a ribosome, which consists of rRNA and proteins. Translation happens on the ribosomes floating in the cytosol, or on the ribosomes attached to the rough endoplasmic reticulum. The ribosome reads the sequence of codons in mRNA, and molecules of tRNA bring amino acids to the ribosome in the correct sequence.
To understand the role of tRNA, you need to know more about its structure. Each tRNA molecule has an anticodon for the amino acid it carries. An anticodon is complementary to the codon for an amino acid. For example, the amino acid lysine has the codon AAG, so the anticodon is UUC. Therefore, lysine would be carried by a tRNA molecule with the anticodon UUC. Wherever the codon AAG appears in mRNA, a UUC anticodon of tRNA temporarily binds. While bound to mRNA, tRNA gives up its amino acid. With the help of rRNA, bonds form between the amino acids as they are brought one by one to the ribosome, creating a polypeptide chain. The chain of amino acids keeps growing until a stop codon is reached.
Ribosomes, which are just made out of rRNA (ribosomal RNA) and protein, have been classified as ribozymes because the rRNA has enzymatic activity. The rRNA is important for the peptidyl transferase activity that bonds amino acids. Ribosomes have two subunits of rRNA and protein. The large subunit has three active sites called E, P, and A sites. These sites are important in the catalytic activity of ribosomes.
Just as with mRNA synthesis, protein synthesis can be divided into three phases: initiation, elongation, and termination. In addition to the mRNA template, many other molecules contribute to the process of translation, such as ribosomes, tRNAs, and various enzymatic factors
Translation Initiation: The small subunit binds to a site upstream (on the 5' side) of the start of the mRNA. It proceeds to scan the mRNA in the 5'-->3' direction until it encounters the START codon (AUG). The large subunit attaches and the initiator tRNA, which carries methionine (Met), binds to the P site on the ribosome.
Translation Elongation: The ribosome shifts one codon at a time, catalyzing each process that occurs in the three sites. With each step, a charged tRNA enters the complex, the polypeptide becomes one amino acid longer, and an uncharged tRNA departs. The energy for each bond between amino acids is derived from GTP, a molecule similar to ATP. Briefly, the ribosomes interact with other RNA molecules to make chains of amino acids called polypeptide chains, due to the peptide bond that forms between individual amino acids. Inside the ribosome, three sites participate in the translation process, the A, P, and E sites. Amazingly, the E. coli translation apparatus takes only 0.05 seconds to add each amino acid, meaning that a 200-amino acid polypeptide could be translated in just 10 seconds.
Translation Termination: Termination of translation occurs when a stop codon (UAA, UAG, or UGA) is encountered (see Figure \(7\). When the ribosome encounters the stop codon, the growing polypeptide is released with the help of various releasing factors and the ribosome subunits dissociate and leave the mRNA. After many ribosomes have completed translation, the mRNA is degraded so the nucleotides can be reused in another transcription reaction.
What Happens Next?
After a polypeptide chain is synthesized, it may undergo additional processes. For example, it may assume a folded tertiary shape due to interactions among its amino acids. It may also bind with other polypeptides or with different types of molecules, such as lipids or carbohydrates. Many proteins travel to the Golgi apparatus within the cytoplasm to be modified for the specific job they will do.
Review
1. Relate protein synthesis and its two major phases to the central dogma of molecular biology.
2. Identify the steps of transcription, and summarize what happens during each step.
3. Explain how mRNA is processed before it leaves the nucleus.
4. Describe what happens during the translation phase of protein synthesis.
5. What additional processes may a polypeptide chain undergo after it is synthesized?
6. Where does transcription take place in eukaryotes?
7. Where does translation take place?
8. Which type of RNA (mRNA, rRNA, or tRNA) best fits each of the statements below? Choose only one type for each.
1. Contains the codons
2. Contains the anticodons
3. Makes up the ribosome, along with proteins
9. If the DNA has a triplet code of CAG in one strand (the strand used as a template for transcription),
1. What is the complementary sequence on the other DNA strand?
2. What is the complementary sequence in the mRNA? What is this sequence called?
3. @hat is the resulting sequence in the tRNA? What is this sequence called? What do you notice about this sequence compared to the original DNA triplet on the template strand?
10. The promoter is a region located in the:
1. DNA
2. mRNA
3. tRNA
4. Both A and B
11. True or False. Introns in mRNA bind to tRNA at the ribosome.
12. True or False. tRNAs can be thought of as the link between amino acids and codons in the mRNA.
Explore More
Messenger RNA molecules are "spliced" in order to create the mRNA involved in protein synthesis. Learn the process here:
Attributions
1. How proteins are made by Nicolle Rager, National Science Foundation, public domain via Wikimedia Commons
2. Gene structure eukaryote by Thomas Shafee, licensed CC BY 4.0 via Wikimedia Commons
3. Components of a gene by Mandeep Grewal, CC BY 4.0
4. Transcription by Calibuon, released into the public domain via Wikimedia Commons
5. Transcript and splicing by Ganeshmanohar, CC BY-SA 4.0 via Wikimedia Commons
6. Initiation and elongation by Jordan Nguyen, CC BY-SA 4.0 via Wikimedia Commons
7. Protein synthesis by OpenStax, CC BY 4.0
8. Gene regulation by OpenStax, CC BY 4.0
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.04%3A_Protein_Synthesis.txt |
Can You Code?
If someone asks you whether you can code, you probably assume they are referring to computer code. The image in Figure \(1\) represents an important code that you use all the time but not with a computer. It's the genetic code, and it is used by your cells to store information and make proteins.
What Is the Genetic Code?
The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The bases are adenine (A), cytosine (C), guanine (G), and thymine (T) (or uracil, U, in RNA). The four bases make up the “letters” of the genetic code. The letters are combined in groups of three to form code “words,” called codons. Each codon stands for (encodes) one amino acid unless it codes for a start or stop signal. There are 20 common amino acids in proteins. With four bases forming three-base codons, there are 64 possible codons. 61 codons are more than enough to code for the 20 amino acids, thus more than one codon codes for a single amino acid. Please find genetic codes in Table \(1\) or in appendix 1.
Table \(1\): Codon Chart. To find the amino acid for a particular codon, find the cell in the table for the first, second, and third bases of the codon. Once you have found the codon, you can find the corresponding amino acid in the adjacent cell on the right side of the codon cell. For example CUG codes for leucine (Leu), AAG codes for lysine (Lys), and GGG codes for glycine (Gly). Text only version of the codon chart.
Second base U Amino acid Second base C Amino acid Second base A Amino acid Second base G Amino acid
First base U UUU Phe UCU Ser UAU Tyr UGU Cys Third base U
First base U UUC Phe UCC Ser UAC Tyr UGC Cys Third base C
First base U UUA Leu UCA Ser UAA (stop) no amino acid UGA (stop) no amino acid Third base A
First base U UUA Leu UCG Ser UAG (stop) no amino acid UGG Trp Third base G
First base C CUU Leu CCU Pro CAU His CGU Arg Third base U
First base C CUC Leu CCC Pro CAC His CGC Arg Third base C
First base C CUA Leu CCA Pro CAA Gln CGA Arg Third base A
First base C CUG Leu CCG Pro CAG Gln CGG Arg Third base G
First base A AUU Ile ACU Thr AAU Asn AGU Ser Third base U
First base A AUC Ile ACC Thr AAC Asn AGC Ser Third base C
First base A AUA Ile ACA Thr AAA Lys AGA Arg Third base A
First base A AUG Met (start) ACG Thr AAG Lys AGG Arg Third base G
First base G GUU Val GCU Ala GAU Asp GGU Gly Third base U
First base G GUC Val GCC Ala GAC Asp GGC Gly Third base C
First base G GUA Val GCA Ala GAA Glu GGA Gly Third base A
First base G GUG Val GCG Ala GAG Glu GGG Gly Third base G
Reading the Genetic Code
If you find the codon AUG in Table \(1\), you will see that it codes for the amino acid methionine. This codon is also the start codon that establishes the reading frame of the code. The reading frame is the way the bases are divided into codons. It is illustrated in Figure \(2\). After the AUG start codon (not shown in the image), the next three bases are read as the second codon. The next three bases after that are read as the third codon, and so on. The sequence of bases is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all the stop codons. They do not code for any amino acids.
Characteristics of the Genetic Code
The genetic code has a number of important characteristics:
• The genetic code is universal. All known living things have the same genetic code. This shows that all organisms share a common evolutionary history.
• The genetic code is unambiguous. This means that each codon codes for just one amino acid (or start or stop). This is necessary so there is no question about which amino acid is the correct one.
• The genetic code is redundant. This means that each amino acid is encoded by more than one codon. For example, in the table above, four codons code for the amino acid threonine. Redundancy in the code helps prevent errors in protein synthesis. If a base in codon changes by accident, there is a good chance that it will still code for the same amino acid.
Review
1. Describe the genetic code.
2. Explain how the genetic code is read.
3. Identify three important characteristics of the genetic code.
4. Summarize how the genetic code was deciphered.
5. Use the table entitled The Genetic Code, shown above, to answer the following questions.
1. Is the code depicted in the table from DNA or RNA? Explain your reasoning.
2. Which amino acid does the codon CAA code for?
3. Does UGA code for an amino acid? Why or why not? If so, which one?
4. Look at the codons that code for the amino acid glycine. How many of them are there? What are their similarities and differences from each other?
5. Imagine that you are doing an experiment similar to the one performed by Nirenberg and Matthaei with 20 test tubes, each containing bacterial cell contents and all 20 amino acids, with one type of amino acid labeled in each tube. If you added synthetic RNA containing only the base cytosine, a polypeptide chain consisting of which amino acid would be produced? Explain your answer.
6. True or False. One codon can encode for more than one amino acid.
7. True or False. The codons for tyrosine in plants are the same as ones that encode for tyrosine humans.
8. True or False. The start codon encodes for an amino acid, in addition to its function establishing where the reading frame starts.
9. How many possible codons are there?
1. 64
2. 20
3. 3
4. It depends on the species
10. How many common amino acids are there in proteins?
1. 64
2. 20
3. 3
4. 4
Explore More
Comparing DNA sequences is vital to understanding evolutionary relationships between organisms. Check out more here:
Attributions:
1. Genetic Code logo by Bas E. Dutilh, et al, licensed CC BY 2.5 via Wikimedia Commons
2. Genetic code by Madprime, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.05%3A_Genetic_Code.txt |
Superhero
Can a mutation really turn a person into a superhero? Of course not, but mutations can sometimes result in drastic changes in living things.
What Are Mutations?
Mutations are random changes in the sequence of bases in DNA. The word mutation may make you think of Ninja Turtles, but that's a misrepresentation of how most mutations work. First of all, everyone has mutations. In fact, most people have dozens or even hundreds of mutations in their DNA. Secondly, from an evolutionary perspective, mutations are essential. They are needed for evolution to occur because they are the ultimate source of all new genetic variation in any species.
Causes of Mutations
Is it possible for mutations to occur spontaneously, or does there have to be a cause of the mutation? Well, the answer is that both are possible. Mutagenesis is a process by which the genetic information of an organism is changed in a stable manner, resulting in a mutation. In nature, mutagenesis can lead to changes that are beneficial or harmful or have no effect. Harmful mutations can lead to cancer and various heritable diseases, but beneficial mutations are the driving force of evolution. In 1927, Hermann Muller first demonstrated the effects of mutations with observable changes in chromosomes. He induced mutagenesis by irradiating fruit flies with X-rays,
Mutagenesis may occur spontaneously or be induced. A spontaneous mutation can just happen. These mutations are not caused by an environmental factor but occur during normal cellular processes. A spontaneous mutation may be due to a mistake during DNA replication. Mutations may also occur during mitosis and meiosis. A mutation caused by an environmental factor, or mutagen, is known as an induced mutation. Typical mutagens include chemicals, like those inhaled while smoking, and radiation, such as X-rays, ultraviolet light, and nuclear radiation. Different mutagens have different modes of damaging DNA. For example, UV radiation may cause bonding between the adjacent nucleotides on one strand of DNA molecule (Figure \(2\)). This prohibits normal bonding between complementary nucleotides of the opposing strand. This causes a bulge in the DNA double helix. If this damage is not repaired, it leads to mutation. Thus, DNA does not replicate, transcribe, and translate properly.
Types of Mutations
There are a variety of types of mutations. Two major categories of mutations are germline mutations and somatic mutations.
• Germline mutations occur in gametes, the sex cells, such as eggs and sperm. These mutations are especially significant because they can be transmitted to offspring and every cell in the offspring will have the mutations.
• Somatic mutations occur in other cells of the body. These mutations may have little effect on the organism because they are confined to just one cell and its daughter cells. Somatic mutations also cannot be passed on to offspring.
Mutations also differ in the way that the genetic material is changed. Mutations may change an entire chromosome or just one or a few nucleotides.
Chromosomal Alterations
Chromosomal alterations are mutations that change chromosome structure or number. They occur when a section of a chromosome breaks off and rejoins incorrectly or does not rejoin at all. Possible ways these mutations can occur are illustrated in Figure \(3\). Chromosomal alterations are very serious. They often result in the death of the organism in which they occur. If the organism survives, it may be affected in multiple ways. An example of a human chromosomal alteration is the mutation that causes Down Syndrome. It is a duplication mutation that leads to developmental delays and other abnormalities. It occurs when the individual inherits an extra copy of chromosome 21. It is also called trisomy ("three-chromosome") 21.
A point mutation is a change in a single nucleotide in DNA. This type of mutation is usually less serious than a chromosomal alteration. An example of a point mutation is a mutation that changes the codon UUU to the codon UCU. Point mutations can be silent, missense, or nonsense mutations, as shown in Table \(1\). The effects of point mutations depend on how they change the genetic code.
Table \(1\): Point Mutation Types
Type Description Example Effect
Silent mutated codon codes for the same amino acid CAA (glutamine) → CAG (glutamine) none
Missense mutated codon codes for a different amino acid CAA (glutamine) → CCA (proline) variable
Nonsense a mutated codon is a premature stop codon CAA (glutamine) → UAA (stop) usually serious
Frameshift Mutations
A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:
AUG-AAU-ACG-GCU = methionine-asparagine-threonine-alanine
Now assume that an insertion occurs in this sequence. Let’s say an A nucleotide is inserted after the start codon AUG. Then the sequence of bases becomes:
AUG-AAA-UAC-GGC-U = methionine-lysine-tyrosine-glycine
Even though the rest of the sequence is unchanged, this insertion changes the reading frame and thus all of the codons that follow it. As this example shows, a frameshift mutation can dramatically change how the codons in mRNA are read. This can have a drastic effect on the protein product. Another example of the frameshift mutation due to the deletion of a nucleotide is illustrated in Figure \(4\). In this example, a premature stop codon is created by the mutation.
Effects of Mutations
The majority of mutations have neither negative nor positive effects on the organism in which they occur. These mutations are called neutral mutations. Examples include silent point mutations, which are neutral because they do not change the amino acids in the proteins they encode.
Many other DNA damages or errors have no effects on the organism because they are repaired before protein synthesis occurs. Cells have multiple repair mechanisms to fix errors in DNA.
Beneficial Mutations
Some mutations have a positive effect on the organism in which they occur. They are referred to as beneficial mutations. They generally code for new versions of proteins that help organisms adapt to their environment. If they increase an organism’s chances of surviving or reproducing, the mutations are likely to become more common over time. There are several well-known examples of beneficial mutations. Here are just two:
1. Mutations have occurred in bacteria that allow the bacteria to survive in the presence of antibiotic drugs. The mutations have led to the evolution of antibiotic-resistant strains of bacteria.
2. A unique mutation is found in people in a small town in Italy. The mutation protects them from developing atherosclerosis, which is the dangerous buildup of fatty materials in blood vessels. The individual in which the mutation first appeared has even been identified.
Harmful Mutations
Imagine making a random change in a complicated machine such as a car engine. The chance that the random change would improve the functioning of the car is very small. The change is far more likely to result in a car that does not run well or perhaps does not run at all. By the same token, any random change in a gene's DNA is likely to result in the production of a protein that does not function normally or may not function at all. Such mutations are likely to be harmful. Harmful mutations may cause genetic disorders or cancer.
• A genetic disorder is a disease, syndrome, or other abnormal condition caused by a mutation in one or more genes or by a chromosomal alteration. An example of a genetic disorder is cystic fibrosis. A mutation in a single gene causes the body to produce thick, sticky mucus that clogs the lungs and blocks ducts in digestive organs.
• Cancer is a disease in which cells grow out of control and form abnormal masses of cells called tumors. It is generally caused by mutations in genes that regulate the cell cycle. Because of the mutations, cells with damaged DNA are allowed to divide without restrictions.
Feature: My Human Body
Inherited mutations are thought to play a role in about 5 to 10 percent of all cancers. Specific mutations that cause many of the known hereditary cancers have been identified. Most of the mutations occur in genes that control the growth of cells or the repair of damaged DNA.
Genetic testing can be done to determine whether individuals have inherited specific cancer-causing mutations. Some of the most common inherited cancers for which genetic testing is available hereditary, breast, and ovarian cancer, caused by mutations in genes named BRCA1 and BRCA2. Besides breast and ovarian cancers, mutations in these genes may also cause pancreatic and prostate cancers. Genetic testing is generally done on a small sample of body fluid or tissue, such as blood, saliva, or skin cells. The sample is analyzed by a lab that specializes in genetic testing, and it usually takes at least a few weeks to get the test results.
Should you get genetic testing to find out whether you have inherited a cancer-causing mutation? Such testing is not done routinely just to screen patients for risk of cancer. Instead, the tests are generally done only when the following three criteria are met:
1. The test can determine definitively whether a specific gene is mutation is present. This is the case with the BRCA1 and BRCA2 gene mutations, for example.
2. The test results would be useful to help guide future medical care. For example, if you found out you had a mutation in the BRCA1 or BRCA2 gene, you might get more frequent breast and ovarian cancer screenings than are generally recommended.
3. You have a personal or family history that suggests you are at risk of inherited cancer.
Criterion number 3 is based, in turn, on such factors as:
• diagnosis of cancer at an unusually young age.
• several different cancers occurring independently in the same individual.
• several close genetic relatives having the same type of cancer (such as a maternal grandmother, mother, and sister all having breast cancer).
• cancer occurring in both organs in a set of paired organs (such as both kidneys or both breasts).
If you meet the criteria for genetic testing and are advised to undergo it, genetic counseling is highly recommended. A genetic counselor can help you understand what the results mean and how to make use of them to reduce your risk of developing cancer. For example, a positive test result that shows the presence of a mutation may not necessarily mean that you will develop cancer. It may depend on whether the gene is located on an autosome or sex chromosome and whether the mutation is dominant or recessive. Lifestyle factors may also play a role in cancer risk even for hereditary cancers, and early detection can often be life-saving if cancer does develop. Genetic counseling can also help you assess the chances that any children you may have will inherit the mutation.
Review
1. Define mutation.
2. Identify the causes of mutation.
3. Compare and contrast germline and somatic mutations.
4. Describe chromosomal alterations, point mutations, and frameshift mutations. Identify the potential effects of each type of mutation.
5. Why are many mutations neutral in their effects?
6. Give an example of a beneficial mutation and an example of a harmful mutation.
7. Why do you think that exposure to mutagens, such as cigarette smoke, can cause cancer?
8. True or False. Mutations are always caused by exposure to toxic substances.
9. True or False. Some mutations can make chromosomes longer or shorter.
10. Explain why the insertion or deletion of a single nucleotide can cause a frameshift mutation.
11. Compare and contrast missense and nonsense mutations.
12. A mutation that substitutes one nucleotide for another is called a ___________ mutation.
13. Which type of mutation is trisomy 21, or Down Syndrome?
14. Explain why mutations are important for evolution.
Explore More
Radiation is all around us and a part of everyday life. But what exactly is it and what does it do to your body? Check it out here:
You probably know that smoking kills, but what exactly does smoking do to your body? Learn more here:
Attributions
1. Superhero via Pixabay license
2. DNA UV mutation by NASA/David Herring, public domain via Wikimedia Commons
3. Chromosome mutation public domain via Wikimedia Commons
4. Frameshift mutation by Genomics Education Programme, licensed CC BY 2.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.06%3A_Mutations.txt |
Express Yourself
This sketch in Figure \(1\) illustrates some of the variability in human cells. The shape and other traits that make each type of cell unique depend mainly on the particular proteins that the cell type makes. For example, an ovum is a cell with a large cytoplasm because it nourishes the embryo after fertilization. Sperm is a small cell with essentially just genetic material. The job of the sperm is to transfer genetic material to the ovum. The sketch shows various other cell types. Proteins are encoded in genes. All the cells in an organism have the same genes, so they all have the genetic instructions for the same proteins. Obviously, different types of cells must use, or express, different genes to make different proteins.
What Is Gene Expression?
Using a gene to make a protein is called gene expression. It includes the synthesis of the protein by the processes of transcription of DNA and translation of mRNA. It may also include further processing of the protein after synthesis.
Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any point in the expression of a gene, from the start of the transcription phase of protein synthesis to the processing of a protein after synthesis occurs. The regulation of transcription is one of the most complicated parts of gene regulation in eukaryotic cells and is the focus of this concept.
Regulation of Transcription
As shown in Figure \(2\), transcription is controlled by regulatory proteins or transcription factors. These proteins bind to regions of DNA, called regulatory elements which are located near promoters. The promoter is the region of a gene where RNA polymerase binds to initiate transcription of the DNA to mRNA. After regulatory proteins bind to regulatory elements, the proteins can interact with RNA polymerase. Regulatory proteins are typically either activators or repressors. Activators are regulatory proteins that promote transcription by enhancing the interaction of RNA polymerase with the promoter. Repressors are regulatory proteins that prevent transcription by impeding the progress of RNA polymerase along the DNA strand so the DNA cannot be transcribed to mRNA.
Enhancers
Although regulatory proteins and elements are typically the key players in the regulation of transcription, other factors may also be involved. For example, regulation of transcription may also involve enhancers. Enhancers are distant regions of DNA that can loop back to interact with a gene's promoter and enhance transcription.
Regulation During Development
The regulation of gene expression is extremely important during the early development of an organism. Regulatory proteins must turn on certain genes in particular cells at just the right time so the individual develops normal organs and organ systems. Homeobox genes are a large group of genes that regulate development during the embryonic stage. In humans, there are an estimated 235 functional homeobox genes. They are present on every chromosome and generally grouped in clusters. Homeobox genes contain instructions for making chains of 60 amino acids called homeodomains. Proteins containing homeodomains are transcription factors that bind to and control the activities of other genes. The homeodomain is the part of the protein that binds to the target gene and controls its expression.
Review
1. Define gene expression.
2. Why must gene expression be regulated?
3. Explain how regulatory proteins may activate or repress transcription.
4. Describe homeobox genes and their role in the development of an organism.
5. Discuss the role of regulatory gene mutations in cancer.
6. Explain the relationship between proto-oncogenes and oncogenes.
7. If a newly fertilized egg contained a mutation in a homeobox gene, what effect do you think this might have on the developing embryo? Explain your answer.
8. Which of the following are proteins?
1. Repressors
2. Promoters
3. Regulatory elements
4. All of the above
9. Which of the following is a region of DNA?
1. Homeodomain
2. Activator
3. TATA box
4. Both A and C
10. Compare and contrast enhancers and activators.
11. True or False. Mutations in genes that normally either promote or suppress cell division can both cause cancer.
12. True or False. Gene expression is only regulated at the transcriptional stage.
13. True or False. If RNA polymerase cannot bind to the promoter of a gene, it cannot transcribe that gene into mRNA.
Explore More
Mutations in the regulation of gene expression can lead to uncontrolled cell division, also known as cancer. Learn more here:
Attributions
1. Animal cell variety by Sunshineconnelly, licensed CC BY 3.0 via Wikimedia Commons
2. Eukaryotic Transcription Gene by OpenStax via Lumen Learning, CC BY 4.0
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.07%3A_Regulation_of_Gene_Expression.txt |
Please Pass the Potatoes
You might want to pass on the potato plants pictured on the right in Figure \(1\). They are infected with a virus, which is quickly killing them. The potato plants on the left are healthy and productive. Why aren't they infected with the same virus? The plants on the left have been engineered genetically, using methods of biotechnology, to make them resistant to the virus.
What Is Biotechnology?
Biotechnology is the use of technology to change the genetic makeup of living things for human purposes. Generally, the goal of biotechnology is to modify organisms so they are more useful to humans. For example, biotechnology may be used to create crops that yield more food or resist insect pests or viruses, such as the virus-resistant potatoes pictured above. Research is also underway to use biotechnology to cure human genetic disorders with gene therapy.
Biotechnology Methods
Biotechnology uses a variety of techniques to achieve its aims. Two commonly used techniques are gene cloning and the polymerase chain reaction.
Gene Cloning
Gene cloning is the process of isolating and making copies of a gene. This is useful for many purposes. For example, gene cloning might be used to isolate and make copies of a normal gene for gene therapy. This is done by putting the gene in a bacterial plasmid. Plasmid DNA is circular DNA that is not part of a chromosome and can replicate independently. Gene cloning involves four steps: isolation, ligation, transformation, and selection. Refer to Figure \(2\) as you read below:
1. In the isolation step, a restriction enzyme is used to break DNA at a specific base sequence. This is done to isolate a gene in the foreign DNA. Restriction enzymes are mostly isolated from bacteria and they only cut DNA at a specific sequence in the DNA. That particular site is called the restriction site of that particular enzyme. The foreign DNA and plasmid are cut with the same restriction enzyme. The restriction enzyme creates "stick ends" which are short stretches of single stranded DNA at the cut site.
2. During ligation, the sticky end of the DNA plasmid binds to the sticky end of the isolated gene. The enzyme DNA Ligase glues the annealed fragments together. The DNA that results is called recombinant DNA.
3. In transformation, the recombinant DNA is inserted into a living cell, usually a bacterial cell. Changing an organism in this way is called genetic engineering.
4. Selection involves growing transformed bacteria to make sure they have the recombinant DNA. This is a necessary step because transformation is not always successful. Only bacteria that contain the recombinant DNA are selected for further use. Many different methods are used for selection, such are lacZ and antibiotic resistance genes.
Polymerase Chain Reaction
The polymerase chain reaction (PCR) makes many copies of a gene or other DNA segment. This might be done in order to make large quantities of a gene for genetic testing. PCR involves three steps: denaturing, annealing, and extension. The three steps are illustrated in Figure \(3\). They are repeated many times in a cycle to make large quantities of the gene.
1. Denaturing involves heating DNA to break the bonds holding together the two DNA strands. This yields two single strands of DNA.
2. Annealing involves cooling the single strands of DNA and mixing them with short DNA segments called primers. Primers have base sequences that are complementary to segments of the single DNA strands. As a result, bonds form between the DNA strands and primers.
3. Extension occurs when an enzyme (Taq polymerase or Taq DNA polymerase) adds nucleotides to the primers. This produces new DNA molecules, each incorporating one of the original DNA strands.
Gel Electrophoresis
Gel electrophoresis is an analytical technique used to separate DNA fragments by size and due to the negative charge on DNA. Notice in Figure \(4\) that the "gels" are rectangular in shape. The gels are made of a gelatin-like material of either agarose or polyacrylamide. An electric field, with a positive charge applied at one end of the gel, and a negative charge at the other end, forces the fragments to migrate through the gel. DNA molecules migrate from negative to positive charges due to the net negative charge of the phosphate groups in the DNA backbone. Longer molecules migrate more slowly through the gel matrix. After the separation is completed, DNA fragments of different lengths can be visualized using a fluorescent dye specific for DNA, such as ethidium bromide. The resulting stained gel shows bands correspond to DNA molecules of different lengths, which also correspond to different molecular weights. Band size is usually determined by comparison to DNA ladders containing DNA fragments of known length. Gel electrophoresis can also be used to separate RNA molecules and proteins.
Uses of Biotechnology
Methods of biotechnology can be used for many practical purposes. They are used widely in both medicine and agriculture.
Applications in Medicine
In addition to gene therapy for genetic disorders, biotechnology can be used to transform bacteria so they are able to make human proteins. Proteins made by bacteria are injected into people who cannot produce them because of mutations.
Insulin was the first human protein to be produced in this way. Insulin helps cells take up glucose from the blood. People with type 1 diabetes have a mutation in the gene that normally codes for insulin. Without insulin, their blood glucose rises to harmfully high levels. At present, the only treatment for type 1 diabetes is the injection of insulin from outside sources. Until recently, there was no known way to make human insulin outside the human body. The problem was solved by gene cloning. The human insulin gene was cloned and used to transform bacterial cells, which could then produce large quantities of human insulin.
Applications in Agriculture
Biotechnology has been used to create transgenic crops. Transgenic crops are genetically modified with new genes that code for traits useful to humans. Transgenic crops have been created with a variety of different traits, such as yielding more food, tasting better, surviving drought, tolerating salty soil, and resisting insect pests. Scientists have even created a transgenic purple tomato (Figure \(5\)) that contains high levels of cancer-fighting compounds called antioxidants.
Ethical, Legal, and Social Issues
The use of biotechnology has raised a number of ethical, legal, and social issues. Here are just a few:
• Who owns genetically modified organisms such as bacteria? Can such organisms be patented like inventions?
• Are genetically modified foods safe to eat? Might they have unknown harmful effects on the people who consume them?
• Are genetically engineered crops safe for the environment? Might they harm other organisms or even entire ecosystems?
• Who controls a person’s genetic information? What safeguards ensure that the information is kept private?
• How far should we go to ensure that children are free of mutations? Should a pregnancy be ended if the fetus has a mutation for a serious genetic disorder?
• Can we develop crop species that provide more nutrients and grow in harsher climates? If so, how do we ensure that farmers in impoverished areas have access to these?
• How do we educate the public so they can make well-informed decisions about new technologies?
As a society, we will need to balance the benefits and concerns of new technologies.
Feature: Reliable Sources
Genetically modified foods, or GM foods, are foods produced from genetically modified organisms. These are organisms that have had changes introduced into their DNA using methods of biotechnology. Commercial sale of GM foods began in 1994, with a tomato that had delayed ripening. By 2015, three major crops grown in the U.S. were raised mainly from GM seeds, including field corn, soybeans, and cotton. Many other crops were also raised from GM seeds, ranging from a variety of vegetables to sugar beets. Other sources of GM foods in our diet include meats, eggs, and dairy products from animals that have eaten GM feed, as well as a plethora of food products that contain some form of soy or corn products, such as soybean oil, soybean flour, corn oil, corn starch, and corn syrup. A quick glance at the ingredients list of most processed foods shows that these products are added to many of the items in a typical American diet.
Most scientists think that GM foods are not any riskier to human health than conventional foods. Nonetheless, in many countries, including the U.S., GM foods are given more rigorous evaluations than conventional foods. For example, GM foods are assessed for toxicity, the ability to cause allergic reactions, and the stability of inserted genes. GM crops are also evaluated for possible environmental effects, such as outcrossing, which is the migration of genes from GM plants to conventional crops or wild plant species.
Despite the extra measures used to evaluate GM foods, there is a lot of public concern about them, including whether they are safe to human health, how they are labeled, and their environmental impacts. These concerns are based on a number of factors, such as the worrying belief that scientists are creating entirely new species and a perceived lack of benefits to the consumer of GM foods. People may also doubt the validity of risk assessments, especially with regard to long-term effects. Also, since all the research on safety and usefulness is presented in scientific journals, it can be difficult for the public to be fully informed about the work being done.
Over the past 50 years, there have been many hundreds of studies looking at how these crops affect the environment, the economy, and the health of humans and animals. The results of most of these studies are fairly clear. But, most people don't read the original findings because there are too many and because they can be difficult to understand. The National Academy of Sciences has written a report summarizing the research findings as well as public comments.
They explain the reason for writing the report: "Consumers in the United States and abroad get conflicting information about GM crops. Proponents tout the benefits while opponents emphasize the risks. There was a need for an independent, objective study to examine what had been learned about GM crops, assesses whether initial concerns and promises were realized since their introduction, and investigates new concerns and recent claims."
Review
1. Define biotechnology.
2. What is recombinant DNA?
3. Identify the steps of gene cloning.
4. What is the purpose of the polymerase chain reaction?
5. Make a flow chart outlining the steps involved in creating a transgenic crop.
6. Explain how bacteria can be genetically engineered to produce a human protein.
7. Identify an ethical, legal, or social issue raised by biotechnology. State your view on the issue, and develop a logical argument to support your view.
8. Explain what primers are and what they do in PCR.
9. What is gel electrophoresis?
10. True or False. Transgenic crops can be created using recombinant DNA.
11. True or False. Gene cloning is defined as the creation of an identical copy of an entire organism.
12. The enzyme Taq polymerase was originally identified from bacteria that live in very hot environments, such as hot springs. Why does this fact make Taq polymerase particularly useful in PCR reactions?
13. A circular piece of DNA from bacteria that is often used to create recombinant DNA is called a ________ _.
14. In what ways are crops modified genetically? What traits are introduced, and what methods are used to introduce them?
15. What are the main human safety questions about GM foods? How is the human safety of GM foods assessed?
16. What are the main environmental concerns about GM crops? How is a risk assessment for the environment performed?
17. What are the major pros and cons of GM crops and foods? Who is most affected by these pros and cons? For example, for pros, do growers and marketers receive most of the benefits, or do consumers also reap rewards?
18. Which of the following is a possible use of biotechnology, now or in the future?
1. Curing genetic disorders
2. Creating transgenic crops that are resistant to pests
3. Producing human proteins in non-human cells
4. All of the above
19. Bacteria that contain a recombinant plasmid are said to be:
1. Transformed
2. Translated
3. Transcripted
4. A transgenic crop
Attributions
1. GMO potatoes by CSIRO licensed CC BY 3.0 via Wikimedia Commons
2. Molecular Cloning by OpenStax, licensed CC BY 3.0 via Wikimedia Commons
3. PCR by Enzoklop, CC BY-SA 3.0 via Wikimedia Commons
4. Gel Electrophoresis by Jennifer0328, CC BY-SA 4.0 via Wikimedia Commons
5. Tomatoes, released into the public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.08%3A_Biotechnology.txt |
Vitruvian Man
The drawing in Figure \(2\), named Vitruvian Man, was created by Leonardo da Vinci in 1490. It was meant to show normal human body proportions. Vitruvian Man is used today to represent a different approach to the human body. It symbolizes a scientific research project that began in 1990, exactly 500 years after da Vinci created the drawing. That project, named the Human Genome Project, is the largest collaborative biological research project ever undertaken.
What Is the Human Genome?
The human genome refers to all the DNA of the human species. Human DNA consists of 3.3 billion base pairs and is divided into more than 20,000 genes onto 23 pairs of chromosomes. The human genome also includes noncoding sequences (e.g. intergenic region) of DNA, as shown in Figure \(2\).
Discovering the Human Genome
Scientists now know the sequence of all the DNA base pairs in the entire human genome. This knowledge was attained by the Human Genome Project (HGP), a \$3 billion, international scientific research project that was formally launched in 1990. The project was completed in 2003, two years ahead of its 15-year projected deadline.
Determining the sequence of the billions of base pairs that make up human DNA was the main goal of the HGP. Another goal was mapping the location and determining the function of all the genes in the human genome. There are only about 20,500 genes in human beings.
A Collaborative Effort
Funding for the HGP came from the U.S. Department of Energy and the National Institutes of Health as well as from foreign institutions. The actual research was undertaken by scientists in 20 universities in the U.S., United Kingdom, Australia, France, Germany, Japan, and China. A private U.S. company named Celera also contributed to the effort. Although Celera had hoped to patent some of the genes it discovered, this was later denied.
Reference Genome of the Human Genome Project
In 2003, the HGP published the results of its sequencing of DNA as a human reference genome. Figure \(4\) illustrates the process of DNA sequencing. The details of this image are out of the scope of this concept and book. The sequence of the human DNA is stored in databases available to anyone on the Internet. The U.S. National Center for Biotechnology Information (NCBI), part of the NIH, as well as comparable organizations in Europe and Japan, maintain the genomic sequences in a database known as Genbank. Protein sequences are also maintained in this database. The sequences in these databases are the combined sequences of anonymous donors, and as such do not yet address the individual differences that make us unique. However, the known sequence does lay the foundation to identify the unique differences among all of us. Most of the currently identified variations among individuals will be single nucleotide polymorphisms or SNPs. An SNP (pronounced "snip") is a DNA sequence variation occurring at a single nucleotide in the genome. For example, two sequenced DNA fragments from different individuals, GGATCTA to GGATTTA, contain a difference in a single nucleotide. If this, base change occurs in a gene, the base change then results in two alleles: the C allele and the T allele. Remember an allele is an alternative form of a gene. Almost all common SNPs have only two alleles. The effect of these SNPs on protein structure and function and any effect on the resulting phenotype are an extensive field of study.
Benefits of the Human Genome Project
The sequencing of the human genome holds benefits for many fields, including molecular medicine and human evolution.
• Knowing the human DNA sequence can help us understand many human diseases. For example, it is helping researchers identify mutations linked to different forms of cancer. It is also yielding insights into the genetic basis of cystic fibrosis, liver diseases, blood-clotting disorders, and Alzheimer's disease, among others.
• The human DNA sequence can also help researchers tailor medications to individual genotypes. This is called personalized medicine, and it has led to an entirely new field called pharmacogenomics. Pharmacogenomics, also called pharmacogenetics, is the study of how our genes affect the way we respond to drugs. You can read more about pharmacogenomics in the Feature below.
• The analysis of similarities between DNA sequences from different organisms is opening new avenues in the study of evolution. For example, analyses are expected to shed light on many questions about the similarities and differences between humans and our closest relatives the nonhuman primates.
Ethical, Legal, and Social Issues of the Human Genome Project
From its launch in 1990, the HGP proactively established and funded a separate committee to oversee potential ethical, legal, and social issues associated with the project. A major concern was the possible use of the knowledge generated by the project to discriminate against people. One issue was the fear that employers and health insurance companies would refuse to hire or insure people based on their genetic makeup, for instance, if they had genes that increased their risk of getting certain diseases. In response, in 1996, the U.S. passed the Health Insurance Portability and Accountability Act (HIPAA). It protects against unauthorized, nonconsensual release of individually identifiable health information to any entity not actively engaged in providing healthcare to a patient. This was followed in 2008 by the Genetic Information Nondiscrimination Act (GINA), which specifically prohibits genetic discrimination by health insurance companies and workplaces.
Review
1. Describe the human genome.
2. What is the Human Genome Project?
3. Identify two main goals of the Human Genome Project.
4. What is the reference genome of the Human Genome Project? What is it based on?
5. Explain how knowing the sequence of DNA bases in the human genome is beneficial for molecular medicine.
6. What was one surprising finding of the Human Genome Project?
7. Why do you think scientists didn’t just sequence the DNA from a single person for the Human Genome Project? Along those lines, why do you think it is important to include samples from different ethnic groups and genders in genome sequencing efforts?
8. True or False. The sequenced human genome does not include noncoding regions — it only includes actual genes.
9. True or False. Knowing the sequence of the human genome can give insight into human evolution.
10. What is pharmacogenomics?
1. If a patient were to have pharmacogenomics done to optimize their medication, what do you think the first step would be?
2. List one advantage and one disadvantage of pharmacogenomics.
11. There are approximately 20,000 human
1. base pairs
2. nucleotides
3. alleles
4. genes
12. Explain how the sequencing of the human genome relates to ethical concerns about genetic discrimination.
Explore More
For years, scientists have had the challenge of sequencing the human genome. Learn more about the human genome project here:
Attributions
1. Vitruvian man public domain via Wikimedia Commons
2. Human genome to genes by LoStrangolatore, CC BY 3.0 via Wikimedia Commons
3. Human genome project by National Human Genome Research Institute (NHGRI), licensed CC BY 2.0 via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.09%3A_The_Human_Genome.txt |
Case Study Conclusion: Pharmacogenomics
Arya asked their doctor about Pharmacogenomics. The doctor explains to Arya that Pharmacogenomics is the tailoring of drug treatments to people’s genetic makeup, a form of ‘personalized medicine’.
Figure \(1\) shows a beta cell of the pancreas. As the blood glucose rises, it enters the cell via the GLUT 2 channel. After entering into the cell, it causes the production of ATP that closes the potassium pump. As potassium stops exiting the cell, it causes calcium channels to open and, finally, that causes insulin release from the cells. This process is even more complicated as many enzymes and proteins are skipped in this brief description of the pathway. The sulfonylurea-based drugs force the closing of a potassium pump by attaching it. This causes the release of insulin by skipping many steps. Because many enzymes and other proteins are involved in this complicated process, people respond differently to medicines. Most respond well and their health improves. Some do not gain any benefits from the treatment, and a minority suffer from side effects. After you take a drug, it is processed (metabolized) by your body. How the drug is processed and how you respond to it is determined, in part, by your genes. Understanding how different genetics affect and how a drug is processed can help doctors to more accurately determine which drug and which dose is best for individual patients. In this chapter, you learned what the genome is and how to recognize genes in the genome. In pharmacogenomics, scientists look at the genome of an individual to identify the genetic factors that influence his or her response to a drug. By finding these genes, medical researchers hope to develop genetic tests that will predict how patients will respond to a drug. This is personalized medicine.
The reason people vary in their responses to drug treatments lies in the genetic differences, or variation, between them. Following the Human Genome Project, research has focused on comparing human genomes to understand genetic variation and work out which genetic variants are important in health and in the way we respond to drugs. We also learned in this chapter that two types of variation are common in the human genome: 1) Single nucleotide polymorphisms (SNPs): changes in single nucleotide bases (A, C, G, and T). This was the case in Arya’s physical response to the sulfonylurea. 2) Structural variation: changes affecting chunks of DNA that can consequently alter the structure of the entire chromosome. Structural variation can happen in a number of ways, for example, Copy number variation (CNV): when there is an increase or decrease in the amount of DNA. This can be due to: deletion, where an entire block of DNA is missing; insertion, where a block of DNA is added in duplication; or where there are additional copies of a section of DNA. Inversion: when chromosome breaks in two places and the resulting piece of DNA is reversed and reinserted back into the chromosome (the opposite way round). Translocation: when genetic material is exchanged between two different chromosomes. SNPs are like changing a single letter in the metaphorical 'recipe book of life', while structural variation is the equivalent of whole paragraphs or pages being lost or repeated. Scientists have been aware of SNPs for a long time, but the extent of structural variation was only revealed when it was possible to sequence and compare many genomes. The structural variation appears to be quite common, affecting around 12 percent of the genome. It has been found to cause a variety of genetic conditions.
Finding disease variants
Humans share around 99.5 percent of their genomes. The 0.5 percent that differs between each of us affects our susceptibility to disease and response to drugs. Although this doesn’t sound like a lot, it still means that there are millions of differences between the DNA of two individuals. For example, because SNPs are common in the genome, it is difficult to work out which single letter changes cause disease and which are passengers that have just come along for the ride and have no effect on health.
So how is it possible to know which genetic variants cause disease and which are passengers?
The way scientists look at disease variants is to compare the genetic makeup of a large number of people who have a specific disease with those who do not. This allows scientists to look for genetic variants that are more common in people with a disease compared to people without the disease. For example, if a particular genetic variant is present in 80 percent of patients with the disease but only 20 percent of the healthy population it suggests that this variant is increasing the risk of that disease. However, looking for a disease that is caused by variants in a single gene is the simplest example. There are many complex diseases where variants in many different genes might be involved. As well as the transcriptional and translational regulation of some enzyme production may vary due to the genetic variation in the enhancer and repressors of a gene. So, for this type of comparison to be effective very large groups of people need to be studied, usually in the tens of thousands, to find the variants that have subtle effects on disease risk. Researchers also try to pick individuals with similar phenotypes, in both the diseased and healthy groups, so that the disease genes are easier to identify and study.
Challenges of pharmacogenomics
Although pharmacogenomics is likely to be an important part of future medical care, there are many obstacles to overcome before it becomes routine. It is relatively rare for a particular drug response to be affected by a single genetic variant. A particular genetic variant may increase the likelihood of an adverse reaction but it will not guarantee it.
As a result, some people with the variant may not experience an adverse reaction to a drug. Similarly, if an individual doesn’t have the gene variant, it doesn’t guarantee they won’t experience an adverse reaction. Often, a large number of interacting genetic and environmental factors may influence the response to a drug.
Even when associations between a genetic variant and a drug response have been clearly demonstrated, suitable tests still have to be developed and proven to be effective in clinical trials. A test that has succeeded in a clinical trial still has to be shown to be useful and cost-effective in a healthcare setting. Regulatory agencies will have to consider how they assess and license pharmacogenetic products. Health services will have to adjust to new ways of deciding the best drug to give to an individual.
The behavior of individual doctors will need to change. A lot of side effects are due to patients not taking their drugs as prescribed or to doctors prescribing the wrong dose. Some examples of pharmacogenomics that work effectively, for example, abacavir for HIV, show that these challenges can be overcome. However, in most cases, implementing the findings from pharmacogenomics is likely to be a complicated process.
Chapter Summary
• Determining that DNA is the genetic material was an important milestone in biology.
• In the 1920s, Griffith showed that something in virulent bacteria could be transferred to nonvirulent bacteria and make them virulent as well.
• In the 1940s, Avery and colleagues showed that the "something" Griffith found was DNA and not protein. This result was confirmed by Hershey and Chase, who demonstrated that viruses insert DNA into bacterial cells.
• In the 1950s, Chargaff showed that in DNA, the concentration of adenine is always the same as the concentration of thymine, and the concentration of guanine is always the same as the concentration of cytosine. These observations came to be known as Chargaff's rules.
• In the 1950s, James Watson and Francis Crick, building on the prior X-ray research of Rosalind Franklin and others, discovered the double-helix structure of the DNA molecule.
• Knowledge of DNA's structure helped scientists understand how DNA replicates, which must occur before cell division. DNA replication is semi-conservative because each daughter molecule contains one strand from the parent molecule and one new strand that is complementary to it.
• Genes that are located on the same chromosome are called linked genes. Linkage explains why certain characteristics are frequently inherited together.
• The central dogma of molecular biology can be summed up as DNA → RNA → Protein. This means that the genetic instructions encoded in DNA are transcribed to RNA, and then from RNA, they are translated into a protein.
• RNA is a nucleic acid. Unlike DNA, RNA consists of just one polynucleotide chain instead of two, contains the base uracil instead of thymine, and contains the sugar ribose instead of deoxyribose.
• The main function of RNA is to help make proteins. There are three main types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).
• According to the RNA world hypothesis, RNA was the first type of biochemical molecule to evolve, predating both DNA and proteins.
• The genetic code was cracked in the 1960s by Marshall Nirenberg. It consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The four bases make up the "letters" of the code. The letters are combined in groups of three to form code "words," or codons, each of which encodes for one amino acid or a start or stop signal.
• AUG is the start codon, and it establishes the reading frame of the code. After the start codon, the next three bases are read as the second codon, and so on until a stop codon is reached.
• The genetic code is universal, unambiguous, and redundant.
• Protein synthesis is the process in which cells make proteins. It occurs in two stages: transcription and translation
• Transcription is the transfer of genetic instructions in DNA to mRNA in the nucleus. It includes the steps of initiation, elongation, and termination. After the mRNA is processed, it carries the instructions to a ribosome in the cytoplasm.
• Translation occurs at the ribosome, which consists of rRNA and proteins. In translation, the instructions in mRNA are read, and tRNA brings the correct sequence of amino acids to the ribosome. Then rRNA helps bonds form between the amino acids, producing a polypeptide chain.
• After a polypeptide chain is synthesized, it may undergo additional processing to form the finished protein.
• Mutations are random changes in the sequence of bases in DNA. They are the ultimate source of all new genetic variation in any species
• Mutations may happen spontaneously during DNA replication or transcription. Other mutations are caused by environmental factors called mutagens.
• Germline mutations occur in gametes and may be passed on to offspring. Somatic mutations occur in other cells than gametes and cannot be passed on to offspring.
• Chromosomal alterations are mutations that change chromosome structure or number and usually affect the organism in multiple ways. Down syndrome (trisomy 21) is an example of a chromosomal alteration.
• Point mutations are changes in a single nucleotide. The effects of point mutations depend on how they change the genetic code and may range from no effects to very serious effects.
• Frameshift mutations change the reading frame of the genetic code and are likely to have a drastic effect on the encoded protein.
• Many mutations are neutral and have no effects on the organism in which they occur. Some mutations are beneficial and improve fitness, while others are harmful and decrease fitness.
• Using a gene to make a protein is called gene expression. Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any stage of protein synthesis or processing.
• The regulation of transcription is controlled by regulatory proteins that bind to regions of DNA called regulatory elements, which are usually located near promoters. Most regulatory proteins are either activators that promote transcription or repressors that impede transcription.
• The regulation of gene expression is extremely important during the early development of an organism. Homeobox genes, which encode for chains of amino acids called homeodomains, are important genes that regulate development.
• Some types of cancer occur because of mutations in genes that control the cell cycle. Cancer-causing mutations most often occur in two types of regulatory genes, called tumor-suppressor genes and proto-oncogenes.
• Biotechnology is the use of technology to change the genetic makeup of living things for human purposes.
• Biotechnology methods include gene cloning and the polymerase chain reaction. Gene cloning is the process of isolating and making copies of a DNA segment such as a gene. The polymerase chain reaction makes many copies of a gene or other DNA segment.
• Biotechnology can be used to transform bacteria so they are able to make human proteins, such as insulin. It can also be used to create transgenic crops, such as crops that yield more food or resist insect pests.
• Biotechnology has raised a number of ethical, legal, and social issues including health, environmental, and privacy concerns.
• The human genome refers to all of the DNA of the human species. It consists of more than 3.3 billion base pairs divided into 20,500 genes on 23 pairs of chromosomes.
• The Human Genome Project (HGP) was a multi-billion dollar international research project that began in 1990. By 2003, it had sequenced and mapped the location of all of the DNA base pairs in the human genome. It published the results as a human reference genome that is available to anyone on the Internet.
• The sequencing of the human genome is helping researchers better understand cancer and genetic diseases. It is also helping them tailor medications to individual patients, which is the focus of the new field of pharmacogenomics. In addition, it is helping researchers better understand human evolution.
Review:
1. Put the following units in order from the smallest to the largest:
1. chromosome
2. gene
3. nitrogen base
4. nucleotide
5. codon
2. Put the following processes in the correct order of how a protein is produced, from earliest to latest:
1. tRNA binding to mRNA
2. transcription
3. traveling of mRNA out of the nucleus
4. folding of the polypeptide
3. What are the differences between a sequence of DNA and the sequence of mature mRNA that it produces?
4. Scientists sometimes sequence DNA that they “reverse transcribe” from the mRNA in an organism’s cells, which is called complementary DNA (cDNA). Why do you think this technique might be particularly useful for understanding an organism’s proteins versus sequencing the whole genome (i.e. nuclear DNA) of the organism?
5. What are proteins are made in the cytoplasm on small organelles called?
6. What might happen if codons encoded for more than one amino acid?
7. Explain why a human gene can be inserted into bacteria and can still produce the correct human protein, despite being in a very different organism.
8. True or False. All of your genes are expressed by all the cells of your body.
9. What does The central dogma of molecular biology describe?
Attributions
1. Glucose insulin release by Fred the Oyster licensed CC BY-SA 4.0 via Wikimedia Commons
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/06%3A_DNA_and_Protein_Synthesis/6.10%3A_Case_Study_Conclusion%3A__Parmacogenomics_and_Chapter_Summary.txt |
This chapter introduces two types of cell divisions. First, it explains mitosis and then meiosis. This chapter also explains why cells divide and how the divisions are regulated. The errors in the division may lead to diseases, such as leukemia.
• 7.1: Case Study: Genetic Similarities and Differences
This introduces the concept of mitosis and meiosis in the form of a Leukemia case study.
• 7.2: Cell Cycle and Cell Division
Cell division is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic or eukaryotic. Cell division is simpler in prokaryotes than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no nucleus, and few other organelles. Eukaryotic cells, in contrast, have multiple chromosomes contained within a nucleus.
• 7.3: Mitotic Phase - Mitosis and Cytokinesis
The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. This is shown in the figure below. Mitosis actually occurs in four phases: prophase, metaphase, anaphase, and telophase.
• 7.4: Mutations and Cancer
Your cells may grow and divide without performing their necessary functions, or without fully replicating their DNA, or without copying their organelles. Probably not much good could come of that. So the cell cycle needs to be highly regulated and tightly controlled. And it is.
• 7.5: Sexual Reproduction: Meiosis and gametogenesis
Whereas asexual reproduction produces genetically identical clones, sexual reproduction produces genetically diverse individuals. Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. As both parents contribute half of the new organism's genetic material, the offspring will have traits of both parents, but will not be exactly like either parent.
• 7.6: Genetic Variation
Genetic variation. It is this variation that is the essence of evolution. Without genetic differences among individuals, "survival of the fittest" would not be likely. Either all survive, or all perish.
• 7.7: Mitosis vs. Meiosis and Disorders
Both mitosis and meiosis result in eukaryotic cells dividing. So what is the difference between mitosis and meiosis? The primary difference is the differing goals of each process. The goal of mitosis is to produce two daughter cells that are genetically identical to the parent cell, meaning the new cells have exactly the same DNA as the parent cell. Mitosis happens when you want to grow, for example. You want all your new cells to have the same DNA as the previous cells.
• 7.8: Case Study Conclusion: Genes and Chapter Summary
Humans are much more genetically similar to each other than they are different.
Thumbnail: Image of the mitotic spindle in a human cell showing microtubules in green, chromosomes (DNA) in blue, and kinetochores in red. (Public Domain; Afunguy).
07: Cell Reproduction
Case Study: Abnormal Cell Division
Like the little children in Figure \(1\), seven-year-old Kim is battling leukemia, a type of cancer that affects blood cells. Leukemia usually starts in the bone marrow, where blood cells are produced. It causes the production of abnormal blood cells, most commonly white blood cells, but it can affect other types of blood cells depending on the type of leukemia. The abnormal blood cells replace the patient’s normal blood cells over time. This can lead to symptoms such as fatigue, frequent infections, and easy bruising or bleeding. Leukemia can be fatal, but fortunately, there are some treatment options available that can prolong life and even may cure the disease.
Kim has undergone chemotherapy to kill the cancerous cells, but his doctors have told his parents that it is not enough. Kim needs a bone marrow transplant in order to replace his abnormal bone marrow with healthy bone marrow. His family members are eager to donate bone marrow to him, but first, they must be tested to see if they are a compatible match.
Unlike blood transfusions where it is relatively easy to find a compatible blood donor, bone marrow transplants require much more specific matching between donor and recipient. They must share several of the same types of proteins, called human leukocyte antigens (HLAs), on the surface of their cells. One type of HLA protein is illustrated in Figure \(2\). Different people have different types of HLA proteins, or markers, depending on their specific genes. Typically, eight to ten HLA markers are tested and compared in the potential bone marrow donor and recipient. At least six or seven of these HLA markers need to be identical between them in order for a match to be made.
If the match is not good, the patient’s body could reject the bone marrow transplant, or, conversely, the transplanted bone marrow could produce immune cells that attack the patient’s body. A good match between donor and recipient is critical for bone marrow donation to be safe and effective.
A full sibling frequently provides the best match for bone marrow donation because they share many of the same genes from their parents. Kim’s sister is tested, but unfortunately, she is not a match for him. This is not all that surprising since there is only about a 25% chance that a sibling will be an identical HLA match. His parents and other family members are also tested, but none of them is a match either. Kim must join the 70% of patients that need to look outside of their families for a bone marrow donor.
Read the rest of this chapter to learn more about how cells originate from cells. Why one damaged cell gives rise to more damaged cells which can lead to diseases like cancer. You will also learn why not every cell becomes cancerous and why cancerous cells divide uncontrollably. You will also learn why two siblings are not exact copies of each other.
Chapter Overview: Cell Reproduction
In this chapter, you will learn about:
• The phases of the cell cycle and how cells divide through mitosis.
• How cancer can result from an unregulated cell division due to a mutation.
• Sexual reproduction.
• Differences and similarities between sexual and asexual reproduction.
As you read the chapter, think about the following questions:
• How cancer originates?
• Why every person doesn't have cancer?
• How chemotherapy kills cancerous cells?
• Why Kim's sister and other family members do not have exactly the same HLA markers?
Attributions
1. Pediatric patients by National Cancer Institute, public domain via Wikimedia Commons
2. HLA-DQ by Pdeitiker, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/07%3A_Cell_Reproduction/7.1%3A_Case_Study%3A_Genetic_Similarities_and_Differences.txt |
So Many Cells!
The baby in Figure \(1\) has a lot of growing to do before they are as big as their mom. Most of their growth will be the result of cell division. By the time the baby is an adult, their body will consist of trillions of cells. Cell division is just one of the stages that all cells go through during their life. This includes cells that are harmful, such as cancer cells. Cancer cells divide more often than normal cells and grow out of control. In fact, this is how cancer cells cause illness. In this concept, you will read about how cells divide, what other stages cells go through, and what causes cancer cells to divide out of control and harm the body.
The Cell Cycle
Cell division is the process in which one cell, called the parent cell, divides to form two new cells, referred to as daughter cells. How this happens depends on whether the cell is prokaryotic or eukaryotic. Cell division is simpler in prokaryotes than eukaryotes because prokaryotic cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no nucleus, and few other organelles. Eukaryotic cells, in contrast, have multiple chromosomes contained within a nucleus and many other organelles. All of these cell parts must be duplicated and then separated when the cell divides. Cell division is just one of several stages that a cell goes through during its lifetime. The cell cycle is a repeating series of events that include growth, DNA synthesis, and cell division. The cell cycle in prokaryotes is quite simple: the cell grows, its DNA replicates, and the cell divides. This form of division in prokaryotes is called asexual reproduction. In eukaryotes, the cell cycle is more complicated.
Eukaryotic Cell Cycle
Figure \(2\) represents the cell cycle of a eukaryotic cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below.
Interphase
The Interphase of the eukaryotic cell cycle can be subdivided into the following phases (Figure \(2\)).
• Growth Phase 1 (G1): The cell spends most of its life in the first gap (sometimes referred to as growth) phase, G1. During this phase, a cell undergoes rapid growth and performs its routine functions. During this phase, the biosynthetic and metabolic activities of the cell occur at a high rate. The synthesis of amino acids and hundreds of thousands or millions of proteins that are required by the cell occurs during this phase. Proteins produced include those needed for DNA replication. If a cell is not dividing, the cell enters the G0 phase from this phase.
• G0 phase: The G0 phase is a resting phase where the cell has left the cycle and has stopped dividing. Non-dividing cells in multicellular eukaryotic organisms enter G0 from G1. These cells may remain in G0 for long periods of time, even indefinitely, such as with neurons. Cells that are completely differentiated may also enter G0. Some cells stop dividing when issues of sustainability or viability of their daughter cells arise, such as with DNA damage or degradation, a process called cellular senescence. Cellular senescence occurs when normal diploid cells lose the ability to divide, normally after about 50 cell divisions.
• Synthesis Phase (S): Dividing cells enter the Synthesis (S) phase from G1. For two genetically identical daughter cells to be formed, the cell’s DNA must be copied through DNA replication. When the DNA is replicated, both strands of the double helix are used as templates to produce two new complementary strands. These new strands then hydrogen bond to the template strands and two double helices form. During this phase, the amount of DNA in the cell has effectively doubled, though the cell remains in a diploid state.
• Growth Phase 2 (G2): The second gap (growth) (G2) phase is a shortened growth period in which many organelles are reproduced or manufactured. Parts necessary for mitosis and cell division are made during G2, including microtubules used in the mitotic spindle.
Mitotic Phase
Before a eukaryotic cell divides, all the DNA in the cell’s multiple chromosomes is replicated. Its organelles are also duplicated. This happens in the interphase. Then, when the cell divides (mitotic phase), it occurs in two major steps, called mitosis and cytokinesis, both of which are described in greater detail in the concept Mitotic Phase: Mitosis and Cytokinesis.
• The first step in the mitotic phase of a eukaryotic cell is mitosis, a multi-phase process in which the nucleus of the cell divides. During mitosis, the nuclear envelope (membrane) breaks down and later reforms. The chromosomes are also sorted and separated to ensure that each daughter cell receives a complete set of chromosomes.
• The second major step is cytokinesis. This step, which occurs in prokaryotic cells as well, is when the cytoplasm divides and two daughter cells form.
Table \(2\): Cell Cycle Summary
State Name Description
Quiescent Senescent Resting phase (G0) A resting phase where the cell has left the cycle and has stopped dividing.
Interphase
1st growth phase (G1)
Synthesis phase (S)
2ndgrowth phase (G2)
Cells increase in size in G1. Cells perform their normal activities.
DNA replication occurs during this phase.
The cell will continue to grow and many organelles will divide during their phase.
Cell division Mitosis (M) Cell growth stops at this stage. Mitosis divides the nucleus into two nuclei, followed by cytokinesis which divides the cytoplasm. Two genetically identical daughter cells result.
Control of the Cell Cycle
If the cell cycle occurred without regulation, cells might go from one phase to the next before they were ready. What controls the cell cycle? How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is controlled mainly by regulatory proteins. These proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. Regulatory proteins control the cell cycle at key checkpoints, which are shown in Figure \(3\). There are a number of main checkpoints:
1. The G1 checkpoint: just before entry into the S phase, makes the key decision of whether the cell big enough to divide. If the cell is not big enough, it goes into the resting period (G0)
2. DNA synthesis Checkpoint: The S checkpoint determines if the DNA has been replicated properly.
3. The mitosis checkpoint: This checkpoint ensures that all the chromosomes are properly aligned before the cell is allowed to divide.
Cancer and the Cell Cycle
Cancer is a disease that occurs when the cell cycle is no longer regulated. This happens because a cell’s DNA becomes damaged. This results in mutations in the genes that regulate the cell cycle. Damage can occur due to exposure to hazards such as radiation or toxic chemicals. Cancerous cells generally divide much faster than normal cells. They may form a mass of abnormal cells called a tumor (see Figure \(4\)). The rapidly dividing cells take up nutrients and space that normal cells need. This can damage tissues and organs and eventually lead to death. When uncontrolled cell division happens in the bone marrow, abnormal and nonfunctional blood cells are produced because the division is happening before the cell is ready for division. In these types of cancer, there is not any evident tumor.
Feature: Human Biology in the News
Henrietta Lacks sought treatment for her cancer at Johns Hopkins University Hospital at a time when researchers were trying to grow human cells in the lab for medical testing. Despite many attempts, the cells always died before they had undergone many cell divisions. Mrs. Lacks's doctor took a small sample of cells from her tumor without her knowledge and gave them to a Johns Hopkins researcher, who tried to grow them on a culture plate. For the first time in history, human cells grown on a culture plate kept dividing...and dividing and dividing and dividing. Copies of Henrietta's Lacks cells — called HeLa cells for her name — are still alive today. In fact, there are currently many billions of HeLa cells in laboratories around the world!
Why Henrietta Lacks' cells lived on when other human cells did not is still something of a mystery, but they are clearly extremely hardy and resilient cells. By 1953, when researchers learned of their ability to keep dividing indefinitely, factories were set up to start producing the cells commercially on a large scale for medical research. Since then, HeLa cells have been used in thousands of studies and have made possible hundreds of medical advances. For example, Jonas Salk used the cells in the early 1950s to test his polio vaccine. Over the decades since then, HeLa cells have been used to make important discoveries in the study of cancer, AIDS, and many other diseases. The cells were even sent to space on early space missions to learn how human cells respond to zero gravity. HeLa cells were also the first human cells ever cloned, and their genes were some of the first ever mapped. It is almost impossible to overestimate the profound importance of HeLa cells to human biology and medicine.
You would think that Henrietta Lacks' name would be well known in medical history for her unparalleled contributions to biomedical research. However, until 2010, her story was virtually unknown. That year, a science writer named Rebecca Skloot published a nonfiction book about Henrietta Lacks, named The Immortal Life of Henrietta Lacks. Based on a decade of research, the book is riveting, and it became an almost instantaneous best seller. As of 2016, Oprah Winfrey and collaborators planned to make a movie based on the book, and in recent years, numerous articles about Henrietta Lacks have appeared in the press.
Ironically, Henrietta herself never knew her cells had been taken, and neither did her family. While her cells were making a lot of money and building scientific careers, her children were living in poverty, too poor to afford medical insurance. The story of Henrietta Lacks and her immortal cells raises ethical issues about human tissues and who controls them in biomedical research. However, there is no question that Henrietta Lacks deserves far more recognition for her contribution to the advancement of science and medicine.
Review
1. What are the two main phases of the cell cycle in a eukaryotic cell?
2. Describe the three phases of interphase in a eukaryotic cell.
3. Explain how the cell cycle is controlled.
4. How is cancer-related to the cell cycle?
5. What are the two major steps of cell division in a eukaryotic cell?
6. In which phase of the eukaryotic cell cycle do cells typically spend most of their lives?
7. Which phases of the cell cycle will have cells with twice the amount of DNA? Explain your answer.
8. If there is damage to a gene that encodes for a cell cycle regulatory protein, what do you think might happen? Explain your answer.
9. True or False. Cells go into G0 if they do not pass the G1 checkpoint.
10. In which phase within interphase does the cell make final preparations to divide?
1. Mitosis
2. Cytokinesis
3. G2
4. S
11. What were scientists trying to do when they took tumor cells from Henrietta Lacks? Why did they specifically use tumor cells to try to achieve their goal?
Explore More
The video below discusses the cell cycle and how it relates to cancer.
Attributions
1. Woman holding baby by M.T ElGassier, via Unsplash license
2. Cell cycle by Zephyris, CC BY-SA 3.0 Wikimedia Commons
3. Cell cycle checkpoints by Lumen Learning, CC BY 4.0
4. Colon cancer by Emmanuelm, CC BY 3.0 via Wikimedia Commons
5. HeLa cells by National Institutes of Health (NIH), public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/07%3A_Cell_Reproduction/7.2%3A_Cell_Cycle_and_Cell_Division.txt |
Divide and Split
Can you guess what this colorful image represents? It shows a eukaryotic cell during the process of cell division. In particular, the image shows the nucleus of the cell dividing. In eukaryotic cells, the nucleus divides before the cell itself splits in two; and before the nucleus divides, the cell’s DNA is replicated, or copied. There must be two copies of the DNA so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of genetic material? To answer that question, you first need to know more about DNA and the forms it takes.
The Forms of DNA
Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. Only when a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a chromosome, like the one shown in Figure \(2\). Because DNA has already replicated, each chromosome actually consists of two identical copies. The two copies of a chromosome are called sister chromatids. Sister chromatids are joined together at a region called a centromere.
The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. Mitosis occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase. They are shown in Figure \(3\) and described in detail below.
Prophase
The first and longest phase of mitosis is prophase. During prophase, chromatin condenses into chromosomes, and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the centrioles near the nucleus begin to separate and move to opposite poles of the cell. Centrioles are small organelles found only in eukaryotic cells that help ensure the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The blue spindle, shown in Figure \(4\), consists of fibers made of microtubules.
Metaphase
During metaphase, spindle fibers fully attach to the centromere of each pair of sister chromatids. As you can see in Figure \(5\), the sister chromatids line up at the equator, or center, of the cell. The spindle fibers ensure that sister chromatids will separate and go to different daughter cells when the cell divides. Some spindles do not attach to the kinetochore protein of the centromeres. These spindles are called non-kinetochore spindles that help in the elongation of the cell. This is visible in Figure \(5\).
Anaphase
During anaphase, sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibers. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole (see Figure \(6\)). At the end of anaphase, each pole of the cell has a complete set of chromosomes
Telophase
The chromosomes reach the opposite poles and begin to decondense (unravel), relaxing once again into a stretched-out chromatin configuration. The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area (see Figure \(7\).
Cytokinesis
Cytokinesis is the final stage of cell division in eukaryotes as well as prokaryotes. During cytokinesis, the cytoplasm splits in two and the cell divides. The process is different in plant and animal cells, as you can see in Figure \(8\). In animal cells, the plasma membrane of the parent cell pinches inward along the cell’s equator until two daughter cells form. In the plant cells, a cell plate forms along the equator of the parent cell. Then, a new plasma membrane and cell wall form along each side of the cell plate.
Review
1. Describe the different forms that DNA takes before and during cell division in a eukaryotic cell.
2. Identify the four phases of mitosis in an animal cell, and summarize what happens during each phase.
3. Explain what happens during cytokinesis in an animal cell.
4. What are the main differences between mitosis and cytokinesis?
5. The familiar X-shaped chromosome represents:
1. How DNA always looks in eukaryotic cells
2. How DNA in eukaryotic cells looks once it is replicated and the cell is about to divide
3. Female sex chromosomes only
4. How DNA appears immediately after cytokinesis
6. Which of the following is not part of a chromosome in eukaryotic cells?
1. Centriole
2. Centromere
3. Chromatid
4. DNA
7. What do you think would happen if the sister chromatids of one of the chromosomes did not separate during mitosis?
8. Put the following processes in order of when they occur during cell division, from first to last:
1. separation of sister chromatids
2. DNA replication
3. cytokinesis
4. lining up of chromosomes in the center of the cell
5. condensation and coiling of DNA into a chromosome
9. Why do you think the nuclear envelope breaks down at the start of mitosis?
10. What are the fibers made of microtubules that attach to the centromeres during mitosis are called?
11. True or False. Chromosomes begin to uncoil during anaphase.
12. True or False. During cytokinesis in animal cells, sister chromatids line up along the equator of the cell.
13. True or False. After mitosis, the result is typically two daughter cells with identical DNA to each other.
Explore More
Watch the video below to visualize mitosis.
Attributions
1. Mitosis fluorescent by US government, public domain via Wikimedia Commons
2. Chromosome by Dietzel65, CC BY-SA 3.0 via Wikimedia Commons
3. Mitosis schematic by M3.dahl, CC BY-SA 3.0 via Wikimedia Commons
4. Prometaphase by LadyofHats, Public domain, via Wikimedia Commons
5. Metaphase by Matt, released into the public domain via Wikimedia Commons
6. Anaphase by Matt, released into the public domain via Wikimedia Commons
7. Telophase by Matt, released into the public domain via Wikimedia Commons
8. Cytokinesis by LadyofHats for CK-12 licensed CC BY-NC 3.0
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/07%3A_Cell_Reproduction/7.3%3A_Mitotic_Phase_-_Mitosis_and_Cytokinesis.txt |
What would happen if this cycle proceeds at will?
Your cells may grow and divide without performing their necessary functions, or without fully replicating their DNA, or without copying their organelles. Probably not much good could come of that. So the cell cycle needs to be highly regulated and tightly controlled. And it is.
Control of the Cell Cycle
How does the cell know when to divide? How does the cell know when to replicate its DNA? How does the cell know when to proceed into mitosis or cytokinesis? The answers to these questions have to do with the control of the cell cycle. But how is the cell cycle controlled or regulated? Regulation of the cell cycle involves processes crucial to the survival of a cell. These include the detection and repair of damage to DNA, as well as the prevention of uncontrolled cell division. Uncontrolled cell division can be deadly to an organism; its prevention is critical for survival.
Cyclins and Kinases
The cell cycle is controlled by a number of protein-controlled feedback processes. Two types of proteins involved in the control of the cell cycle are kinases and cyclins. Cyclins activate kinases by binding to them, specifically they activate cyclin-dependent kinases (CDK). Kinases are enzymes that catalyze the transfer of a phosphate group from ATP to another molecule in a cell. They function as a control switch in many cellular functions, turning a function on or off, and regulating other cellular processes. Many times they are involved in activating a cascade of reactions. Cyclins comprise a group of proteins that are rapidly produced at key stages in the cell cycle. Once activated by a cyclin, CDK enzymes activate or inactivate other target molecules through phosphorylation. It is this precise regulation of proteins that triggers advancement through the cell cycle. Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these critical proteins.
What makes a Cell Cancerous?
Cancer is a disease characterized by a population of cells that grow and divide without respect to normal limits. These cancerous cells invade and destroy adjacent tissues, and they may spread throughout the body. The process by which normal cells are transformed into cancer cells is known as carcinogenesis. This process is also known as oncogenesis or tumorigenesis.
Nearly all cancers are caused by mutations in the DNA of abnormal cells. These mutations may be due to the effects of carcinogens, cancer-causing agents such as tobacco smoke, radiation, chemicals, or infectious agents. These carcinogens may act as an environmental “trigger,” stimulating the onset of cancer in certain individuals and not others. Do all people who smoke get cancer? No. Can secondhand smoke increase a nonsmoking person's chance of developing lung cancer? Yes. It also increases a nonsmoking person's chance of developing heart disease.
Complex interactions between carcinogens and an individual’s genome may explain why only some people develop cancer after exposure to an environmental trigger and others do not. Do all cancers need an environmental trigger to develop? No. Cancer-causing mutations may also result from errors incorporated into the DNA during replication, or they may be inherited. Inherited mutations are present in all cells of the organism.
Oncogenes and Tumor Suppressor Genes
Some types of cancer occur because of mutations in genes that control the cell cycle. Cancer-causing mutations most often occur in two types of regulatory genes, called proto-oncogenes and tumor-suppressor genes.
• Proto-oncogenes are genes that normally help cells divide. When a proto-oncogene mutates to become an oncogene, it is continuously active, even when it is not supposed to be. This is like a car's accelerator pedal being stuck at full throttle. The car keeps racing at top speed. In the case of a cell, the cell keeps dividing out of control, which can lead to cancer.
• Tumor suppressor genes are genes that normally slow down or stop cell division. When a mutation occurs in a tumor suppressor gene, it can no longer control cell division. This is like a car without brakes. The car can't be slowed or stopped. In the case of a cell, the cell keeps dividing out of control, which can lead to cancer.
Several Mutations to Cause Cancer
Oncogenes may be growth factors, protein kinases, GTPases or transcription factors. Growth factors are naturally occurring substances, usually a protein or steroid hormone, capable of stimulating cellular growth, proliferation, and differentiation. They are important for regulating a variety of cellular processes. Usually, they must bind to an extracellular or intracellular receptor to initiate a cellular reaction.
Typically, a series of several mutations that constitutively activate oncogenes and inactivate tumor suppressor genes is required to transform a normal cell into a cancer cell (Figure \(2\)). Cells have developed a number of control mechanisms to overcome mutations in proto-oncogenes. Therefore, a cell needs multiple mutations to transform into a cancerous cell. A mutation in one proto-oncogene would not cause cancer, as the effects of the mutation would be masked by the normal control of the cell cycle and the actions of tumor suppressor genes. Similarly, a mutation in one tumor suppressor gene would not cause cancer either, due to the presence of many "backup" genes that duplicate its functions. It is only when enough proto-oncogenes have mutated into oncogenes and enough tumor suppressor genes have been deactivated that the cancerous transformation can begin. Signals for cell growth overwhelm the signals for growth regulation, and the cell quickly spirals out of control. Often, because many of these genes regulate the processes that prevent most damage to the genes themselves, DNA damage accumulates as one ages.
Usually, oncogenes are dominant alleles, as they contain gain-of-function mutations. The actions of the mutant allele gene product, many times resulting in a constitutively activated protein, are dominant to the gene product produced by the "normal" allele. Meanwhile, mutated tumor suppressors are generally recessive alleles, as they contain loss-of-function mutations. A proto-oncogene needs only a mutation in one copy of the gene to generate an oncogene; a tumor suppressor gene needs a mutation in both copies of the gene to render both products defective. There are instances when, however, one mutated allele of a tumor suppressor gene can render the other copy non-functional. These instances result in what is known as a dominant negative effect.
Review
1. Define cancer.
2. What are cyclin-dependent kinases? What is their role?
3. Discuss the role of oncogenes and tumor suppressor genes in carcinogenesis.
4. Why are multiple mutations required for transformation into a cancerous cell?
5. Identify all the categories of oncogenes and describe two categories.
Attributions
1. Cell cycle by WassermanLab, CC BY-SA 4.0, via Wikimedia Commons
2. Cancer requires multiple mutations by National Cancer Institute, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/07%3A_Cell_Reproduction/7.4%3A_Mutations_and_Cancer.txt |
All in the Family
The family photo in Figure \(1\) illustrates an important point. Children in a family resemble their parents and each other, but the children are never exactly the same unless they are identical twins. Each of the children in the photo inherited a unique combination of traits from the parents. In this concept, you will learn how this happens. It all begins with sex — sexual reproduction, that is.
Sexual Reproduction
Why do you look similar to your parents, but not identical? First, it is because you have two parents. Second, it is because of sexual reproduction. Whereas asexual reproduction produces genetically identical clones, sexual reproduction produces genetically diverse individuals. Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. As both parents contribute half of the new organism’s genetic material, the offspring will have traits of both parents, but will not be exactly like either parent.
Organisms that reproduce sexually by joining gametes, a process known as fertilization, must have a mechanism to produce haploid gametes. This mechanism is meiosis, a type of cell division that halves the number of chromosomes. During meiosis, the pairs of chromosomes separate and segregate randomly to produce gametes with one chromosome from each pair. Meiosis involves two nuclear and cell divisions without interphase in between, starting with one diploid cell and generating four haploid cells. Each division, named meiosis I and meiosis II, has four stages: prophase, metaphase, anaphase, and telophase. These stages are similar to those of mitosis, but there are distinct and important differences.
Prior to meiosis, the cell’s DNA is replicated, generating chromosomes with two sister chromatids. A human cell prior to meiosis will have 46 chromosomes, 22 pairs of homologous autosomes, and 1 pair of sex chromosomes. Homologous chromosomes (Figure \(2\)), or homologs, are similar in size, shape, and genetic content; they contain the same genes, though they may have different alleles of those genes. The genes/alleles are at the same loci on homologous chromosomes. You inherit one chromosome of each pair of homologs from your mother and the other one from your father. Sexual reproduction is the primary method of reproduction for the vast majority of multicellular organisms, including almost all animals and plants. Fertilization joins two haploid gametes into a diploid zygote, the first cell of a new organism. The zygote enters G1 of the first cell cycle, and the organism begins to grow and develop through mitosis and cell division.
Meiosis
The process that produces haploid gametes is called meiosis. Meiosis is a type of cell division in which the number of chromosomes is reduced by half. It occurs only in certain special cells of an organism. In mammals, Meiosis occurs only in gamete producing cells within the gonads. During meiosis, homologous (paired) chromosomes separate, and haploid cells form that have only one chromosome from each pair. Figure \(3\) gives an overview of meiosis.
As you can see from the meiosis diagram, two cell divisions occur during the overall process, so a total of four haploid cells are produced. The two cell divisions are called meiosis I and meiosis II. Meiosis I begins after DNA replicates during interphase. Meiosis II follows meiosis I without DNA replicating again. Both meiosis I and meiosis II occur in four phases, called prophase, metaphase, anaphase, and telophase. You may recognize these four phases from mitosis, the division of the nucleus that takes place during routine cell division of eukaryotic cells.
Meiosis I
1. Prophase I: The nuclear envelope begins to break down, and the chromosomes condense. Centrioles start moving to opposite poles of the cell, and a spindle begins to form. Importantly, homologous chromosomes pair up, which is unique to prophase I. In prophase of mitosis and meiosis II, homologous chromosomes do not form pairs in this way. During prophase I, crossing-over occurs. The significance of crossing-over is discussed in the next section called variations.
2. Metaphase I: Spindle fibers attach to the paired homologous chromosomes. The paired chromosomes line up along the equator of the cell. This occurs only in metaphase I. In metaphase of mitosis and meiosis II, it is sister chromatids that line up along the equator of the cell.
3. Anaphase I: Spindle fibers shorten, and the chromosomes of each homologous pair start to separate from each other. One chromosome of each pair moves toward one pole of the cell, and the other chromosome moves toward the opposite pole.
4. Telophase I and Cytokinesis: The spindle breaks down, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells result. The daughter cells each have a random assortment of chromosomes, with one from each homologous pair. Both daughter cells go on to meiosis II.
Meiosis II
1. Prophase II: The nuclear envelope breaks down and the spindle begins to form in each haploid daughter cell from meiosis I. The centrioles also start to separate.
2. Metaphase II: Spindle fibers line up the sister chromatids of each chromosome along the equator of the cell.
3. Anaphase II: Sister chromatids separate and move to opposite poles.
4. Telophase II and Cytokinesis: The spindle breaks down, and new nuclear membranes form. The cytoplasm of each cell divides, and four haploid cells result. Each cell has a unique combination of chromosomes.
Gametogenesis
At the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to develop before they become mature gametes capable of fertilization. The development of diploid cells into gametes is called gametogenesis. It differs between males and females.
• A gamete produced by a male is called a sperm, and the process that produces a mature sperm is called spermatogenesis. During this process, a sperm cell grows a tail and gains the ability to “swim,” like the human sperm cell shown in the figure below.
• A gamete produced by a female is called an egg, and the process that produces a mature egg is called oogenesis. Just one egg is produced from the four haploid cells that result from meiosis. The single egg is a very large cell, as you can see from the human egg also shown in Figure \(5\).
Spermatogenesis
Spermatogenesis occurs in the wall of the seminiferous tubules, with stem cells at the periphery of the tube and the spermatozoa at the lumen of the tube. Immediately under the capsule of the tubule are diploid, undifferentiated cells. These stem cells, called spermatogonia (singular: spermatagonium), go through mitosis with one offspring going on to differentiate into a sperm cell, while the other gives rise to the next generation of sperm.
Meiosis begins with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell is produced called a secondary spermatocyte. This haploid cell must go through another meiotic cell division. The cell produced at the end of meiosis is called a spermatid. When it reaches the lumen of the tubule and grows a flagellum (or "tail"), it is called a sperm cell. Four sperm result from each primary spermatocyte that goes through meiosis.
Stem cells are deposited during gestation and are present at birth through the beginning of adolescence but in an inactive state. During adolescence, gonadotropic hormones from the anterior pituitary cause the activation of these cells and the production of viable sperm. This continues into old age.
Oogenesis
Oogenesis occurs in the outermost layers of the ovaries. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to increase in number, eventually resulting in up to one to two million cells in the embryo.
The cell starting meiosis is called a primary oocyte. This cell will begin the first meiotic division, but be arrested in its progress in the first prophase stage. At the time of birth, all future eggs are in the prophase stage. At adolescence, anterior pituitary hormones cause the development of a number of follicles in an ovary. This results in the primary oocyte finishing the first meiotic division. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.
Review
1. Explain how sexual reproduction occurs at the cellular level.
2. Summarize what happens during meiosis.
3. Compare and contrast gametogenesis in males and females.
4. Explain mechanisms that increase genetic variation in offspring produced by sexual reproduction.
5. Why do gametes need to be haploid? What would happen to the chromosome number after fertilization if they were diploid?
6. Describe one difference between prophase I of meiosis and prophase of mitosis.
7. Do all of the chromosomes that you got from your mother go into one of your gametes? Why or why not?
8. True or False. Crossing-over is the exchange of genetic material between sister chromatids.
9. True or False. Human sperms are haploid.
10. True or False. Sister chromatids separate from each other during meiosis I.
11. How many cells are produced after a single cell goes through meiosis?
12. Which stage of meiosis (prophase I or II; metaphase I or II; anaphase I or II; telophase I or II) best fits the descriptions below? Choose only one for each description.
1. Pairs of homologous chromosomes line up along the equator of the cell
2. Sister chromatids separate
3. Homologous chromosomes separate from each other
Explore More
A special type of cell division known as meiosis is responsible for your uniqueness. Learn more here:
Ever wonder why some babies have Down Syndrome? Check out this video:
Attributions
1. Family Photo by @donita, released into the public domain via Nappy
2. Chromosomal crossing over by Abbyprovenzano, CC BY-SA 3.0 via Wikimedia Commons
3. Major events in meiosos by NCBI, public domain via Wikimedia Commons
4. Meiosis by OpenStax, CC BY 4.0
5. Sperm egg, public domain via Wikimedia Commons
6. Spermatogenesis by OpenStax, CC BY 4.0
7. Oogenesis by OpenStax, CC BY 4.0
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/07%3A_Cell_Reproduction/7.5%3A_Sexual_Reproduction%3A_Meiosis_and_gametogenesis.txt |
What helps ensure the survival of a species?
Genetic variation. It is this variation that is the essence of evolution. Without genetic differences among individuals, "survival of the fittest" would not be likely. Either all survive, or all perish.
Genetic Variation
Sexual reproduction results in infinite possibilities of genetic variation. In other words, sexual reproduction results in offspring that are genetically unique. They differ from both parents and also from each other. This occurs for a number of reasons.
• When homologous chromosomes form pairs during prophase I of meiosis I, crossing-over can occur. Crossing-over is the exchange of genetic material between homologous chromosomes. It results in new combinations of genes on each chromosome.
• When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells, and different chromosomes segregate independently of each other. This called is called independent assortment. It results in gametes that have unique combinations of chromosomes.
• In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is likely to be a matter of chance. It is obviously another source of genetic variation in offspring. This is known as random fertilization.
All of these mechanisms working together result in an amazing amount of potential variation. Each human couple, for example, has the potential to produce more than 64 trillion genetically unique children. No wonder we are all different!
Crossing-Over
Crossing-over occurs during prophase I, and it is the exchange of genetic material between non-sister chromatids of homologous chromosomes. Recall during prophase I, homologous chromosomes line up in pairs, gene-for-gene down their entire length, forming a configuration with four chromatids, known as a tetrad. At this point, the chromatids are very close to each other and some material from two chromatids switch chromosomes, that is, the material breaks off and reattaches at the same position on the homologous chromosome (Figure \(2\)). This exchange of genetic material can happen many times within the same pair of homologous chromosomes, creating unique combinations of genes. This process is also known as recombination.
During prophase I, chromosomes condense and become visible inside the nucleus. As the nuclear envelope begins to break down, homologous chromosomes move closer together. The synaptonemal complex, a lattice of proteins between the homologous chromosomes, forms at specific locations, spreading to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. The synaptonemal complex also supports the exchange of chromosomal segments between non-sister homologous chromatids in a process called crossing over. The crossover events are the first source of genetic variation produced by meiosis. A single crossover event between homologous non-sister chromatids leads to an exchange of DNA between chromosomes. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.
Independent Assortment and Random Fertilization
During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where n is the number of chromosomes per set. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition.
In humans, there are over 8 million configurations in which the chromosomes can line up during metaphase I of meiosis. It is the specific process of meiosis, resulting in four unique haploid cells, that results in these many combinations. This independent assortment, in which the chromosome inherited from either the father or mother can sort into any gamete, produces the potential for tremendous genetic variation. Together with random fertilization, more possibilities for genetic variation exist between any two people than the number of individuals alive today. Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. A sperm cell, with over 8 million chromosome combinations, fertilizes an egg cell, which also has over 8 million chromosome combinations. That is over 64 trillion unique combinations, not counting the unique combinations produced by crossing-over.
Review
1. What is crossing-over and when does it occur?
2. Describe how crossing-over, independent assortment, and random fertilization lead to genetic variation.
3. How many combinations of chromosomes are possible from sexual reproduction in humans?
4. Create a diagram to show how crossing-over occurs and how it creates new gene combinations on each chromosome.
Attributions
1. Supplier Diversity Strategies by Profiles in Diversity Journal, CC BY 3.0 via Wikimedia Commons
2. Chromosomal Recombination by David Eccles (Gringer), licensed CC BY 2.5 via Wikimedia Commons
3. Crossing over by OpenStax, licensed CC BY 4.0
4. Independent assortment by OpenStax, licensed CC BY 4.0
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/07%3A_Cell_Reproduction/7.6%3A_Genetic_Variation.txt |
Mitosis or Meiosis?
Figure \(1\) shows a tiny embryo just beginning to form. Once an egg is fertilized, the resulting single cell must divide many times to develop a fetus. Both mitosis and meiosis involve cell division; is this type of cell division an example of mitosis or meiosis? The answer is mitosis. With each division, you are making a genetically exact copy of the parent cell, which only happens through mitosis.
Mitosis vs. Meiosis
Both mitosis and meiosis result in eukaryotic cell division. The primary difference between these divisions is the differing goals of each process. The goal of mitosis is to produce two daughter cells that are genetically identical to the parent cell. Mitosis happens when you grow. You want all your new cells to have the same DNA as the previous cells. The goal of meiosis is to produce sperm or eggs, also known as gametes. The resulting gametes are not genetically identical to the parent cell. Gametes are haploid cells, with only half the DNA present in the diploid parent cell. This is necessary so that when a sperm and an egg combine at fertilization, the resulting zygote has the correct amount of DNA—not twice as much as the parents. The zygote then begins to divide through mitosis.
Table \(1\): comparison of mitosis and meiosis
Mitosis Meiosis
Purpose To produce new cells To produce gametes
Number of Cells Produced 2 4
Rounds of Cell Division 1 2
Haploid or Diploid Diploid Haploid
Are daughter cells identical to parent cells? Yes No
Are daughter cells identical to each other? Yes No
Figure \(2\) shows a comparison of mitosis, meiosis, and binary fission.
• Binary fission occurs in bacterial. Note that bacterial cells have a single loop of DNA. The DNA of the cell is replicated. Each loop of DNA moves to the opposite side of the cell and the cell splits in half.
• Mitosis and Meiosis both occur in eukaryotic cells. In the example below the cell has 4 total chromosomes. These are replicated during the S phase.
• In mitosis, the chromosomes line up in the center of the cell. Then, sister chromatids separate and move to the opposite poles of the cell. The cell divides, producing two cells with 4 total chromosomes
• In meiosis, the homologous chromosomes line up in the center of the cell. Then each chromosome moves to opposite poles and the cell divides.
• Next, the chromosomes line up in the center of the cell. Then, sister chromatids separate and move to the opposite poles of the cell. This produces four cells with 2 chromosomes each. These are the gametes.
• Two gametes combine to form a zygote with 4 total chromosomes.
Chromosome Disorders
Changes in Chromosome Number
What would happen if an entire chromosome were missing or duplicated? What if a human had only 45 chromosomes? Or 47? This real possibility is usually due to mistakes during meiosis; the chromosomes do not fully separate from each other during sperm or egg formation. Specifically, nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number. Nondisjunction may occur during meiosis I or meiosis II Most human atypical chromosome numbers result in the death of the developing embryo, often before a woman even realizes she is pregnant. Occasionally, a zygote with an extra chromosome can become a viable embryo and develop.
Trisomy is a state where humans have an extra autosome. That is, they have three of a particular chromosome instead of two. For example, trisomy 18 results from an extra chromosome 18, resulting in 47 total chromosomes. To identify the chromosome number (including an abnormal number), a sample of cells is removed from an individual or developing fetus. Metaphase chromosomes are photographed and a karyotype is produced. A karyotype will display any abnormalities in chromosome number or large chromosomal rearrangements. Trisomy 8, 9, 12, 13, 16, 18, and 21 have been identified in humans. Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester. The most common trisomy in viable births is Trisomy 21.
Trisomy 21: Down Syndrome
One of the most common chromosome abnormalities is Down syndrome, due to nondisjunction of chromosome 21 resulting in an extra complete chromosome 21, or part of chromosome 21 (Figure \(5\)). Down syndrome is the only autosomal trisomy where an affected individual may survive to adulthood. Individuals with Down syndrome often have some degree of mental and physical impairments and a specific facial appearance. With proper assistance, individuals with Down syndrome can become successful, contributing members of society. The risk of having a child with Down syndrome is significantly higher among women age 35 and older.
Review
1. Define genetic disorder.
2. What is nondisjunction? Why may it cause genetic disorders?
3. Explain why genetic disorders caused by abnormal numbers of chromosomes most often involve the X chromosome.
4. How is Down syndrome detected in utero?
5. Compare and contrast genetic disorders and congenital disorders.
6. Explain why parents that do not have Down syndrome can have a child with Down syndrome.
7. What is the goal of mitosis? Or meiosis?
8. How many cells are created from cytokinesis following mitosis? Following meiosis?
9. Which process, mitosis or meiosis, creates genetically identical cells?
10. "Gametes are haploid cells." What does this sentence mean?
Explore More
https://bio.libretexts.org/link?17046#Explore_More
Explore More I
• Mitosis and Meiosis Simulation in the following video:
1. What are homologous chromosomes?
2. How does the location of specific genes compare between homologous chromosomes?
3. What is the outcome of mitosis?
4. What is a tetrad? Why are they an important feature of meiosis?
5. How does meiosis differ between females and males?
Explore More II
Explore How Cells Divide and answer the following questions:
1. How many daughter cells arise from mitosis? How many daughter cells are produced in meiosis?
2. How does the attachment of spindle fibers differ between mitosis and meiosis I?
3. Is anaphase I or anaphase II in meiosis more analogous to anaphase in mitosis? Explain your reasoning.
4. How many steps are there in mitosis? How many steps are there in meiosis?
5. How does interphase I of meiosis differ from interphase II of meiosis?
Attributions
1. 4 cell embryo by Nina Sesina, CC BY-SA 4.0 via Wikimedia Commons
2. Three growth types by domdomegg, CC BY-SA 4.0 via Wikimedia Commons
3. Nondisjunction by OpenStax, CC BY 4.0
4. Trisomy 21 by National Human Genome Research Institute, public domain via Wikimedia Commons
5. Brushfield eyes by Erin Ryan, released into the public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/07%3A_Cell_Reproduction/7.7%3A_Mitosis_vs._Meiosis_and_Disorders.txt |
Case Study Conclusion: Genetic Similarities and Differences
Humans are much more genetically similar to each other than they are different. Any two people on Earth are 99.9% genetically identical to each other. But the mere 0.1% that is different can be very important, as in the case of bone marrow donation to treat diseases such as leukemia. These variations are passed on in a family with sexual reproduction. A good match must exist between a bone marrow donor and recipient in genes that encode for human leukocyte antigen (HLA) proteins. If a patient receives a bone marrow transplant from a donor that has different types of HLAs than the patient does, antibodies in their immune system will identify the antigens as “nonself” and will launch an attack on the transplanted cells. Also, since bone marrow produces immune cells, antibodies in the transplanted tissue can actually attack the patient’s own cells through the same mechanism.
As you have also learned, a good HLA match is often difficult to find, even between full siblings. This is due to the genetic variations within gametes of a single person due to crossing over and random assortment. The variations are multiplied when two unique gametes fertilize. Kim has to find his match outside of his family. Every year, about 14,000 people in the United States must try to find a compatible donor from a bone marrow registry. These registries store information on HLA type from potential donors, like the woman shown above. She is swabbing her cheeks for a DNA match. It can take months to years to find a compatible match — if one is found at all.
In the meantime, Kim has to stop the production of abnormal WBCs with chemotherapy. Chemotherapy is the treatment of cancer with drugs ("anticancer drugs") that can destroy cancer cells. In current usage, the term "chemotherapy" usually refers to cytotoxic drugs which affect rapidly dividing cells in general, in contrast with targeted therapy (see below). Chemotherapy drugs interfere with cell division in various possible ways, e.g. with the duplication of DNA or the separation of newly formed chromosomes. Chemotherapy has the potential to harm healthy tissue, especially those tissues that have a high replacement rate (e.g. intestinal lining). Due to these side effects, patients may lose their hair follicles, Digestive system lining, and taste buds. These cells usually repair themselves after chemotherapy. Because some drugs work better together than alone, two or more drugs are often given at the same time. This is called "combination chemotherapy"; most chemotherapy regimens are given in a combination.
Chapter Summary
In this chapter, you learned about human sexual and asexual reproduction.
• The cell cycle is a repeating series of events that include growth, DNA synthesis, and cell division. It is more complicated in eukaryotic than prokaryotic cells.
• In a eukaryotic cell, the cell cycle has two major phases: interphase and mitotic phase. During interphase, the cell grows, performs routine life processes, and prepares to divide. During the mitotic phase, first, the nucleus divides (mitosis) and then the cytoplasm divides (cytokinesis), which produces two daughter cells.
• Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. After DNA replicates and the cell is about to divide, the DNA condenses and coils into the X-shaped form of a chromosome. Each replicated chromosome consists of two sister chromatids, which are joined together at a centromere.
• During mitosis, sister chromatids separate from each other and move to opposite poles of the cell. This happens in four phases, called prophase, metaphase, anaphase, and telophase.
• The cell cycle is controlled mainly by regulatory proteins that signal the cell to either start or delay the next phase of the cycle at key checkpoints.
• Cancer is a disease that occurs when the cell cycle is no longer regulated, for example, because the cell's DNA has become damaged. Cancerous cells grow out of control and may form a mass of abnormal cells called a tumor.
• In sexual reproduction, two parents produce gametes that unite in the process of fertilization to form a single-celled zygote. Gametes are haploid cells with only one of each pair of homologous chromosomes, and the zygote is a diploid cell with two of each pair of chromosomes.
• Meiosis is the type of cell division that produces four haploid daughter cells that may become gametes. Meiosis occurs in two stages, called meiosis I and meiosis II, each of which occurs in four phases (prophase, metaphase, anaphase, and telophase).
• Meiosis is followed by gametogenesis, the process in which the haploid daughter cells change into mature gametes. Males produce gametes called sperm in a process known as spermatogenesis, and females produce gametes called eggs in the process known as oogenesis.
• Sexual reproduction produces offspring that are genetically unique. Crossing-over, independent assortment, and the random union of gametes work together to result in an amazing amount of potential genetic variation.
• Sexual reproduction has the potential to produce tremendous genetic variation in offspring.
• During prophase I, the homologous chromosomes condense and become visible as the x shape we know, pair up to form a tetrad, and exchange genetic material by crossing over.
• In metaphase I, the tetrads line themselves up at the metaphase plate and homologous pairs orient themselves randomly.
• This variation is due to independent assortment and crossing-over during meiosis, and random union of gametes during fertilization.
• The goal of mitosis is to produce a new cell that is identical to the parent cell.
• The goal of meiosis is to produce gametes that have half the DNA of the parent cell.
• When chromosomes do not divide equally among gametes, the damaged gametes produce. This process is called nondisjunction.
• Trisomy is a state where humans have an extra autosome; they have three of a particular chromosome instead of two.
• The most common trisomy in viable births is Trisomy 21 (Down Syndrome) due to nondisjunction.
Chapter Summary Review
1. What are cyclin-dependent kinases? What is their role?
2. What are cell cycle checkpoints?
3. What is interphase?
4. Summarize each phase of the cell cycle.
5. Describe the structure of a chromosome in the prophase of mitosis.
6. What is cytokinesis and when does it occur?
7. What is centromere?
8. Describe the main steps of mitosis.
9. Cells go through a series of events that include growth, DNA synthesis, and cell division. Why are these events best represented by a cycle diagram?
10. Explain how the cell cycle is regulated.
11. Define and explain random assortment and random fertilization.
12. Why is DNA replication essential to the cell cycle?
13. True or False. When a eukaryotic cell divides, the nucleus divides first in the process of mitosis.
14. What happens during mitosis?
15. What is meiosis?
16. What is diploid? How many chromosomes are in a diploid human cell?
17. What is a zygote? How does the zygote form the organism?
18. What is the result of crossing-over?
19. How many cell divisions occur during meiosis?
20. Why are you genetically distinct?
21. Describe the steps of Meiosis I and Meiosis II.
22. Describe nondisjunction. List and explain some of the chromosome disorders.
23. Compare and contrast mitosis and meiosis.
Attributions
1. USARC officer by Timothy Hale, public domain via Wikimedia Commons
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/07%3A_Cell_Reproduction/7.8%3A_Case_Study_Conclusion%3A_Genes_and_Chapter_Summary.txt |
This chapter provides the molecular background for understanding heredity; explains Mendelian and non-Mendelian inheritance in humans; some genetic disorders and their treatment, and explores recent advances in genetics.
• 8.1: Case Study: Genes and Inheritance
People tend to look similar to their biological parents, but, you can also inherit traits from your parents that you can't see.
• 8.2: Laws of Inheritance
Mendel experimented with the inheritance of traits in pea plants at a time when the blending theory of inheritance was popular. This is the theory that offspring have a blend of the characteristics of their parents.
• 8.3: Genetics of Inheritance
Mendel did experiments with pea plants to show how traits such as seed shape and flower color are inherited. Based on his research, he developed his two well-known laws of inheritance: the law of segregation and the law of independent assortment.
• 8.4: Simple Inheritance
Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be completely dominant to the other. The pattern of inheritance of Mendelian traits depends on whether the traits are controlled by genes on autosomes or by genes on sex chromosomes.
• 8.5: Complex Inheritance
Many human traits have more complicated modes of inheritance than Mendelian traits. Such modes of inheritance are called non-Mendelian inheritance, and they include inheritance of multiple allele traits, traits with codominance or incomplete dominance, and polygenic traits, among others.
• 8.6: Genetic Disorders
Genetic disorders are diseases, syndromes, or other abnormal conditions that are caused by mutations in one or more genes or by chromosomal alterations. Genetic disorders are typically present at birth, but they should not be confused with congenital disorders, which are any disorders, regardless of cause, that are present at birth. Some congenital disorders are not caused by genetic mutations or chromosomal alterations and are caused by problems during embryonic or fetal development.
• 8.7: Case Study Conclusion: Cancer and Chapter Summary
Rebecca's family tree, as illustrated in the pedigree above, shows a high incidence of cancer among close relatives. But are genes the cause of cancer in this family? Only genetic testing, which is the sequencing of specific genes in an individual, can reveal whether a cancer-causing gene is being inherited in this family.
08: Inheritance
Case Study: Cancer in the Family
People tend to look similar to their biological parents, as illustrated by the family tree in Figure \(1\). But, you can also inherit traits from your parents that you can’t see. Rebecca becomes very aware of this fact when she visits her new doctor for a physical exam. Her doctor asks several questions about her family's medical history, including whether Rebecca has or had relatives with cancer. Rebecca tells her that her grandmother, aunt, and uncle, who have all passed away, all had cancer. They all had breast cancer, including her uncle, and her aunt additionally had ovarian cancer. Her doctor asks how old they were when they were diagnosed with cancer. Rebecca is not sure exactly, but she knows that her grandmother was fairly young at the time, probably in her forties.
Rebecca’s doctor explains that while the vast majority of cancers are not due to inherited factors, a cluster of cancers within a family may indicate that there are mutations in certain genes that increase the risk of getting certain types of cancer, particularly breast and ovarian cancer. Some signs that cancers may be due to these genetic factors are present in Rebecca’s family, such as cancer with an early age of onset (e.g. breast cancer before age 50), breast cancer in men, and breast cancer and ovarian cancer within the same person or family.
Based on her family medical history, Rebecca’s doctor recommends that she see a genetic counselor because these professionals can help determine whether the high incidence of cancers in her family could be due to inherited mutations in their genes. If so, they can test Rebecca to find out whether she has the particular variations of these genes that would increase her risk of getting cancer.
When Rebecca sees the genetic counselor, he asks how her grandmother, aunt, and uncle with cancer are related to her. She says that these relatives are all on her mother’s side — they are her mother’s mother and siblings. The genetic counselor records this information in the form of a specific type of family tree, called a pedigree, indicating which relatives had which type of cancer and how they are related to each other and to Rebecca. He also asks her ethnicity. Rebecca says that her family, on both sides, are Ashkenazi Jews, meaning Jews whose ancestors came from central and eastern Europe. “But what does that have to do with anything?” she asks. The counselor tells Rebecca that mutations in two tumor-suppressor genes called BRCA1 and BRCA2, located on chromosome 17 and 13, respectively, are particularly prevalent in people of Ashkenazi Jewish descent and greatly increase the risk of getting cancer. About 1 in 40 Ashkenazi Jewish people have one of these mutations, compared to about 1 in 800 in the general population. Her ethnicity, along with the types of cancer, age of onset, and the specific relationships between her family members who had cancer indicate to the counselor that she is a good candidate for genetic testing for the presence of these mutations.
Rebecca says that her 72-year-old mother never had cancer, and nor had many other relatives on that side of the family, so how could the cancers be genetic? The genetic counselor explains that the mutations in the BRCA1 and BRCA2 genes, although dominant, are not inherited by everyone in a family. Also, even people with mutations in these genes do not necessarily get cancer — the mutations simply increase their risk of getting cancer. For instance, 55 to 65% of women with a harmful mutation in the BRCA1 gene will get breast cancer before age 70, compared to 12% of women in the general population who will get breast cancer sometime over the course of their lives.
Rebecca is not sure she wants to know whether she has a higher risk of cancer. The genetic counselor understands her apprehension but explains that if she knows that she has harmful mutations in either of these genes, her doctor will screen her for cancer more often and at earlier ages. Therefore, any cancers she may develop are likely to be caught earlier when they are often much more treatable. Rebecca decides to go through with the testing, which involves taking a blood sample, and nervously waits for her results.
Chapter Overview: Genetics
At the end of this chapter, you will find out Rebecca ’s test results. By then, you will have learned how mutations in genes such as BRCA1 and BRCA2 can be passed down and cause disease. Especially, you will learn about:
• How Gregor Mendel discovered the laws of inheritance for certain types of traits.
• The science of heredity, known as genetics, and the relationship between genes and traits.
• Simple and more complex inheritance of some human traits.
• Genetic Disorders.
As you read this chapter, keep Rebecca’s situation in mind and think about the following questions:
1. What do the BRCA1 and BRCA2 genes normally do? How can they cause cancer?
2. Are BRCA1 and BRCA2 considered linked genes? And are they on autosomes or sex chromosomes?
3. After learning more about pedigrees, draw the pedigree for cancer in Rebecca’s family. Use the pedigree to help you think about why it is possible that her mother does not have one of the BRCA gene mutations, even if her grandmother, aunt, and uncle did have it.
4. Why do you think certain gene mutations are prevalent in certain ethnic groups?
Attributions
1. Caelius and Valerius family tree by Ann Martin, licensed CC BY 2.0 via Flickr
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/08%3A_Inheritance/8.1%3A_Case_Study%3A_Genes_and_Inheritance.txt |
Of Peas and People
These purplish-flowered plants are not just pretty to look at. Plants like these led to a huge leap forward in biology. The plants are common garden peas, and they were studied in the mid-1800s by an Austrian monk named Gregor Mendel. With his careful experiments, Mendel uncovered the secrets of heredity, or how parents pass characteristics to their offspring. You may not care much about heredity in pea plants, but you probably care about your own heredity. Mendel's discoveries apply to people as well as to peas — and to all other living things that reproduce sexually. In this concept, you will read about Mendel's experiments and the secrets of heredity that he discovered.
Mendel and His Pea Plants
Gregor Mendel, shown below, was born in 1822 and grew up on his parents’ farm in Austria. He did well in school and became a monk. He also went to the University of Vienna, where he studied science and math. His professors encouraged him to learn science through experimentation and to use math to make sense of his results. Mendel is best known for his experiments with pea plants like the one pictured above.
Blending Theory of Inheritance
During Mendel's time, the blending theory of inheritance was popular. This is the theory that offspring have a blend, or mix, of the characteristics of their parents. Mendel noticed plants in his own garden that weren’t a blend of the parents. For example, a tall plant and a short plant had offspring that were either tall or short but not medium in height. Observations such as these led Mendel to question the blending theory. He wondered if there was a different underlying principle that could explain how characteristics are inherited. He decided to experiment with pea plants to find out. In fact, Mendel experimented with almost 30,000 pea plants over the next several years!
Why Study Pea Plants?
Why did Mendel choose common, garden-variety pea plants for his experiments? Pea plants are a good choice because they are fast growing and easy to raise. They also have several visible characteristics that vary. These characteristics, some of which are illustrated in Figure \(3\). Each of these characteristics has two common traits ( values).
1. Seeds can be round or wrinkled
2. Seeds can have yellow or green cotyledons. Cotyledons refer to the tiny leaves inside the seeds.
3. Flowers can be white or violet
4. The seed pod can be full or constricted
5. The seed pod can be yellow or green
6. The flowers can occur along the stem (in axial pods) or at the end of a stem (in terminal pods)
7. Stems can be long (6-7 feet) or short (less than 1 foot).
Controlling Pollination
To research how characteristics are passed from parents to offspring, Mendel needed to control pollination. Pollination is the fertilization step in the sexual reproduction of plants. Pollen consists of tiny grains that are the male sex cells, or gametes, of plants. They are produced by a male flower part called the anther (Figure \(4\)). Pollination occurs when pollen is transferred from the anther to the stigma of the same or another flower. The stigma is a female part of a flower. It passes the pollen grains to female gametes in the ovary.
Pea plants are naturally self-pollinating. In self-pollination, pollen grains from anthers on one plant are transferred to stigmas of flowers on the same plant. Mendel was interested in the offspring of two different parent plants, so he had to prevent self-pollination. He removed the anthers from the flowers of some of the plants in his experiments. Then he pollinated them by hand with pollen from other parent plants of his choice. When pollen from one plant fertilizes another plant of the same species, it is called cross-pollination. The offspring that result from such a cross are called hybrids. When the term hybrid is used in this context, it refers to any offspring resulting from the breeding of two genetically distinct individuals.
Mendel's First Set of Experiments
At first, Mendel experimented with just one characteristic at a time. He began with flower color. As shown in Figure \(5\), Mendel cross-pollinated violet-flowered and white-flowered parent plants. The parent plants in the experiments are referred to as the P (for parent) generation.
F1 and F2 Generations
The offspring of the P generation are called the F1 (for filial, or “offspring”) generation. As shown in Figure \(5\), all of the plants in the F1 generation had violet flowers. None of them had white flowers. Mendel wondered what had happened to the white-flower characteristic. He assumed some type of inherited factor produces white flowers and some other inherited factor produces violet flowers. Did the white-flower factor just disappear in the F1 generation? If so, then the offspring of the F1 generation — called the F2 generation — should all have violet flowers like their parents.
To test this prediction, Mendel allowed the F1 generation plants to self-pollinate. He was surprised by the results. Some of the F2 generation plants had white flowers. He studied hundreds of F2 generation plants, and for every three violet-flowered plants, there was an average of one white-flowered plant.
Law of Segregation
Mendel did the same experiment for all seven characteristics. In each case, one value of the characteristic disappeared in the F1 plants and then showed up again in the F2 plants. And in each case, 75 percent of F2 plants had one value of the characteristic and 25 percent had the other value. Based on these observations, Mendel formulated his first law of inheritance. This law is called the law of segregation. It states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces.
Mendel's Second Set of Experiments
Mendel wondered whether different characteristics are inherited together. For example, are purple flowers and tall stems always inherited together? Or do these two characteristics show up in different combinations in offspring? To answer these questions, Mendel next investigated two characteristics at a time. For example, he crossed plants with yellow round seeds and plants with green wrinkled seeds. The results of this cross are shown in Figure \(5\).
F1 and F2 Generations
In this set of experiments, Mendel observed that plants in the F1 generation were all alike. All of them had yellow round seeds like one of the two parents. When the F1 generation plants were self-pollinated, however, their offspring—the F2 generation—showed all possible combinations of the two characteristics. Some had green round seeds, for example, and some had yellow wrinkled seeds. These combinations of characteristics were not present in the F1 or P generations.
Law of Independent Assortment
Mendel repeated this experiment with other combinations of characteristics, such as flower color and stem length. Each time, the results were the same as those shown in the figure above. The results of Mendel's second set of experiments led to his second law. This is the law of independent assortment. It states that factors controlling different characteristics are inherited independently of each other.
Mendel's Legacy
You might think that Mendel's discoveries would have made a big impact on science as soon as he made them, but you would be wrong. Why? Because Mendel's work was largely ignored. Mendel was far ahead of his time and working from a remote monastery. He had no reputation among the scientific community and limited previously published work. He also published his research in an obscure scientific journal. As a result, when Charles Darwin published his landmark book on evolution in 1869, although Mendel's work had been published just a few years earlier, Darwin was unaware of it. Consequently, Darwin knew nothing about Mendel's laws and didn’t understand heredity. This made Darwin's arguments about evolution less convincing to many people.
Then, in 1900, three different European scientists — named DeVries, Correns, and Tschermak — independently arrived at Mendel's laws. All three had done experiments similar to Mendel's and come to the same conclusions that he had drawn several decades earlier. Only then was Mendel's work rediscovered and Mendel himself given the credit he was due. Although Mendel knew nothing about genes, which were discovered after his death, he is now considered the father of genetics.
Review
1. What is the blending theory of inheritance? What observations led Mendel to question this theory?
2. Why were pea plants a good choice for Mendel's experiments?
3. Describe Mendel's first set of experiments, including the results.
4. State Mendel's two laws of inheritance.
5. How did the outcome of Mendel's second set of experiments lead to his second law?
6. Discuss Mendel's legacy.
7. In Mendel’s first set of experiments:
1. Why did he use pea plants with different characteristics for the parental generation?
2. Why do you think he only tested one characteristic at a time?
3. Why did he allow the plants in the F1 generation to self-pollinate?
4. If he observed 200 F2 plants, approximately how many would have purple flowers? Approximately how many would have white flowers? Explain your answers.
5. Which flower color seemed to dominate over the other? Explain your answer.
8. If Mendel’s law of independent assortment was not correct, and characteristics were always inherited together, what types of offspring do you think would have been produced by crossing plants with yellow round seeds and green wrinkled seeds? Explain your answer.
9. True or False. In Mendel’s experiments, the F1 generations are hybrids.
10. True or False. A single gamete of a pea plant contains factors that result in both a purple flower and a white flower.
Explore More
Every mother and father pass down traits to their children. Explore how Mendel's pea plant experiments helped us better understand the genetics of this process here:
Attributions
1. Sweet pea flower by Giligone licensed CC BY-SA 3.0 via Wikimedia Commons
2. Gregor Mendel by William Bateson, public domain via Wikimedia Commons
3. Mendel seven characteristics by Mariana Ruiz LadyofHats, released into the public domain via Wikimedia Commons
4. Flower structure by OpenStax, CC BY 4.0
5. Mendel's experiments by CNX, CC BY 4.0
6. Pea cross by Suzanne Wakim, licensed CC BY 4.0 adapted from on Dihybrid Cross by CNX OpenStax, licensed CC BY 4.0 via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/08%3A_Inheritance/8.2%3A_Laws_of_Inheritance.txt |
Like Father, Like Son
This father-son duo is serving in the army together. The shape of their faces and their facial features look very similar. If you saw them together, you might well guess that they are father and son. People have long known that the characteristics of living things are similar in parents and their offspring. However, it wasn’t until the experiments of Gregor Mendel that scientists understood how traits are inherited by offspring.
The Father of Genetics
Mendel did experiments with pea plants to show how traits such as seed shape and flower color are inherited. Based on his research, he developed his two well-known laws of inheritance: the law of segregation and the law of independent assortment. When Mendel died in 1884, his work was still virtually unknown. In 1900, three other researchers working independently came to the same conclusions that Mendel had drawn almost half a century earlier. Only then was Mendel's work rediscovered.
Mendel knew nothing about genes. They were discovered after his death. However, he did think that some type of "factors" controlled traits and were passed from parents to offspring. We now call these "factors" genes. Mendel's laws of inheritance, now expressed in terms of genes, form the basis of genetics, the science of heredity. For this reason, Mendel is often called the father of genetics.
The Language of Genetics
Today, we know that the traits of organisms are controlled by genes on chromosomes. To talk about inheritance in terms of genes and chromosomes, you need to know the language of genetics. Figure \(2\) shows the location of genes in a eukaryotic cell. The nucleus is a membrane-enclosed organelle found in most eukaryotic cells. The nucleus is the largest organelle in the cell and contains chromosomes which make up most of the cell's genetic information. Mitochondria also contain DNA, called mitochondrial DNA, but it makes up just a small percentage of the cell’s overall DNA content. The genetic information, which contains the information for the structure and function of the organism, is found encoded in DNA in the form of genes.
A gene is a short segment of DNA that contains information to encode an RNA molecule or a protein strand. DNA in the nucleus is organized in long linear strands that are attached to different proteins. These proteins help the DNA coil up for better storage in the nucleus. Think about how a string gets tightly coiled up if you twist one end while holding the other end. These long strands of coiled-up DNA and proteins are called chromosomes.
Each chromosome contains many genes. The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression. Gene expression is the process by which the information in a gene is "decoded" by various cell molecules to produce a functional gene product, such as a protein molecule or an RNA molecule. The human species is characterized by 23 pairs of chromosomes (Figure \(3\)).
Autosomes
Of the 23 pairs of human chromosomes, 22 pairs are autosomes (the lines numbered 1–22 in Figure \(3\)). Autosomes are chromosomes that contain genes for characteristics that are unrelated to sex. These chromosomes are the same in males and females. The great majority of human genes are located on autosomes. The genes located on these chromosomes are called autosomal genes.
Sex Chromosomes
The remaining pair of human chromosomes consists of the sex chromosomes, X and Y. Females have two X chromosomes, and males have one X and one Y chromosome. In females, one of the X chromosomes in each cell is inactivated and known as a Barr body. This ensures that females, like males, have only one functioning copy of the X chromosome in each cell.
As you can see from Figure \(3\), the X chromosome is much larger than the Y chromosome. The X chromosome has about 2,000 genes, whereas the Y chromosome has fewer than 100, none of which are essential to survival. (For comparison, the smallest autosome, chromosome 22, has over 500 genes.) Virtually all of the X chromosome genes are unrelated to sex. The genes located on the X chromosomes are called X-linked genes. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene, called SRY (which stands for sex-determining region Y gene), triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a female, so you can think of a female as the default sex of the human species. Can you think of a reason why the Y chromosome is so much smaller than the X chromosome?
The following terms are a good starting point. They are illustrated in Figure \(4\) that follows.
• A gene is the part of a chromosome that contains the genetic code for a given protein. For example, in pea plants, a given gene might code for flower color.
• The position of a given gene on a chromosome is called its locus (plural, loci). For example, a gene might be located near the center or at one end or the other of a chromosome.
• A given gene may have different normal versions called alleles. For example, in pea plants, there is a smooth seed allele (S) and a wrinkled seed allele (s) for the seed shape gene. Different alleles account for much of the variation in the traits of organisms including people.
• In sexually reproducing organisms, each individual has two copies of each type of chromosome. Paired chromosomes of the same type are called homologous chromosomes. They are about the same size and shape, and they have all the same genes at the same loci.
Genotype
When sexual reproduction occurs, sex cells called gametes unite during fertilization to form a single cell called a zygote. The zygote inherits two of each type of chromosome, with one chromosome of each type coming from the sperm donor and the other coming from the egg donor. Because homologous chromosomes have the same genes at the same loci, each individual also inherits two copies of each gene. The two copies may be the same allele or different alleles. The alleles an individual inherits for a given gene make up the individual’s genotype. As shown in the table below, an organism with two of the same allele (for example, BB or bb) is called a homozygote. An organism with two different alleles (in this example, Bb) is called a heterozygote.
Table \(1\): Alleles and genotypes
Alleles Genotypes Phenotypes
BB (homozygous dominant) purple flowers
B (purple) Bb (heterozygous) purple flowers
b (white) bb (homozygous recessive) white flowers
Phenotype
The expression of an organism’s genotype is referred to as its phenotype. The phenotype refers to the organism’s traits, such as purple or white flowers in pea plants. As you can see from Table \(1\), different genotypes may produce the same phenotype. In this example, both BB and Bb genotypes produce plants with the same phenotype, purple flowers. Why does this happen? In a Bb heterozygote, both alleles are expressed but only the B allele is seen in phenotype because it masks the expression of b, so the b allele doesn’t influence the phenotype. The allele B is called dominant, and the allele that doesn't show in the phenotype is called recessive.
The terms dominant and recessive may also be used to refer to phenotypic traits. For example, purple flower color in pea plants is a dominant trait. It shows up in the phenotype whenever a plant inherits even one dominant allele for the trait. Similarly, white flower color is a recessive trait. Like other recessive traits, it shows up in the phenotype only when a plant inherits two recessive alleles for the trait.
Review
1. Define genetics.
2. Why is Gregor Mendel sometimes called the father of genetics if genes were not discovered until after his death?
3. Correctly use the terms gene, allele, locus, and chromosome in one or more sentences.
4. Compare and contrast genotype and phenotype.
5. Imagine that there are two alleles, R and r, for a given gene. R is dominant to r. Answer the following questions about this gene.
1. What are the possible homozygous and heterozygous genotypes?
2. Which genotype or genotypes express the dominant R phenotype? Explain your answer.
3. Are R and r on different loci? Why or why not?
4. Can R and r be on the same exact chromosome? Why or why not? If not, where are they located?
6. If a child has the genotype Dd and inherited the D from their mother, where did the d likely come from?
1. Either their mother or their father
2. Their father
3. Their maternal grandmother
4. Their maternal grandfather
7. True or False. Each phenotype has only one genotype.
8. True or False. Recessive genes are never expressed in a phenotype.
9. True or False. An observable physical trait is a phenotype.
10. A gene for flower color and a gene for seed shape could be on the same:
1. chromosome
2. locus
3. allele
4. Both A and B
11. What does a gene usually codes for?
Attributions
1. Father and son by Sgt. Tracy Ellingsen, public domain
2. DNA terminology by Wa-Su Biology, dedicated CC0 via Wikimedia Commons
3. Chromosomes by Mariana Ruiz Villarreal (LadyofHats), CC BY-NC 3.0, for CK-12
4. Gene loci and allele by Keith Chan, CC BY-SA 4.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/08%3A_Inheritance/8.3%3A_Genetics_of_Inheritance.txt |
Dimples
This person is exhibiting a genetic trait — the dimples in their cheeks when they smiles. Genetic traits are characteristics that are encoded in DNA. Some genetic traits, like dimples, have a simple inheritance pattern like the traits that Gregor Mendel studied in pea plants. The way these traits are inherited by offspring from their parents is called simple inheritance.
What Is Simple Inheritance?
Simple (or Mendelian) inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be completely dominant to the other. The pattern of inheritance of simple traits depends on whether the traits are controlled by genes on autosomes or by genes on sex chromosomes.
• Autosomal traits are controlled by genes on one of the 22 pairs of human autosomes. Autosomes are all the chromosomes except the X or Y chromosome, and they do not differ between males and females, so autosomal traits are inherited in the same way regardless of the sex of the parent or offspring.
• Traits controlled by genes on the sex chromosomes are called sex-linked traits. Because of the small size of the Y chromosome, most sex-linked traits are controlled by genes on the X chromosome. These traits are called X-linked traits. Single-gene X-linked traits have a different pattern of inheritance than single-gene autosomal traits because males have just one X chromosome. Males always inherit their X chromosome from their mother, and they pass on their X chromosome to all of their daughters but none of their sons.
Studying Inheritance Patterns
There are two very useful tools for studying how traits are passed from one generation to the next. One tool is a pedigree, the other is a Punnett square.
Pedigree
The charts below are called pedigrees. A pedigree shows how a trait is passed from generation to generation within a family. A pedigree can show, for example, whether a trait is an autosomal dominant, autosomal recessive, or X-linked trait. Pedigrees show relationships and identify individuals with a given trait. In the pedigrees below:
• affected individuals are shown in red, unaffected individuals are shown in blue
• males are shown as squares, females are shown as circles
• The top row of a pedigree is the original couple. Two individuals who are connected by a horizontal line are breeding pairs. The children of the couple are connected to them by vertical lines.
• The next row of the pedigree shows the couple's children, as well as the partners of the children. And, the third row of the pedigree shows the next generation (the grandchildren of the couple at the top of the pedigree). Larger pedigrees can have more rows showing additional generations.
Figure \(2\) is an example of a pedigree of an autosomal dominant trait. This pedigree begins with an affected male and an unaffected female. The couple's first child (on the left) is an affected male whose partner is an unaffected female. They produce two children: an affected female and an unaffected male. The next child is an unaffected male partnered with an unaffected female. Their children are two unaffected females and an unaffected male. The next child is an unpartnered affected female. The fourth child is an unpartnered unaffected female. The youngest child is an affected female partnered with an unaffected male. Their children are an affected male, followed by an unaffected male, followed by an affected female, followed by an unaffected female.
In an autosomal dominant trait, a child that has the trait will always have at least one parent with the trait. In an autosomal recessive trait, two individuals without the trait can have a child with the trait.
Figure \(3\) is a pedigree of an autosomal recessive trait. In this pedigree, individuals that are half-shaded are heterozygous (and therefore do not show the trait). In this example, two heterozygous individuals have four children. Their youngest child is an affected male. This individual mates with an unaffected female to produce two heterozygous children and two unaffected children. The original couple's next youngest is a heterozygous female. The original couple's second child is an unaffected male, who partners with an unaffected female to produce two unaffected children. The original couple's oldest child is an unaffected female whose partner is a heterozygous male. Their children are an unaffected female, an unaffected male, and a heterozygous male.
Punnett Square
A Punnett square is a chart that allows you to easily determine the expected ratios of possible genotypes in the offspring of two parents. The mating between two parents is called a cross. The Punnett square is named after its developer, British geneticist Reginald C. Punnett. You can see a hypothetical example in Figure \(4\). In this case, the gene is autosomal, and both parents are heterozygotes (Aa) for the gene. Half the gametes produced by each parent will have the A allele and half will have the a allele. That's because the two alleles are on homologous chromosomes, which always separate and go to separate gametes during meiosis. According to Mendel's law of segregation, the alleles in the gametes from each parent are written down the side and across the top of the Punnett square. Filling in the cells of the Punnett square gives the possible genotypes of their children. It also shows the most likely ratios of the genotypes, which in this case is 25 percent AA, 50 percent Aa, and 25 percent aa.
Predicting Genotypes and phenotype with Punnett Squares
Mendel developed the law of segregation by following only a single characteristic, such as pod color, in his pea plants. In a monohybrid cross, such as the one in Figure \(5\), the Punnett square shows every possible combination when combining one maternal (biological mother) allele with one paternal (biological father) allele. In this example, both organisms are heterozygous for flower color Bb (purple). Both plants produce gametes that contain either the B and b alleles. If the gametes from both parents contain the dominant alleles, the resulting plant will be homozygous dominant and have purple flowers. If the gametes from both parents contain the recessive alleles, the resulting plant will be homozygous recessive and have white flowers. If the gamete from one parent contains the dominant allele and the gamete from the other parent contains the recessive allele, the resulting plant will be heterozygous and have purple flowers. The probability of any single offspring showing the dominant trait is 3:1, or 75%.
Dihybrid cross
For a monohybrid cross, we are only looking at a single gene. Therefore, the outside of the Punnett square will only have single letters (single alleles). For a dihybrid cross, pairs of alleles are used. This means the outside of the square will have pairs of letters. A Punnett square for a monohybrid cross is divided into four squares, whereas a Punnett square for a dihybrid cross is divided into 16 squares. How many boxes would a Punnett square need if three traits were examined? The squares are filled in with the possible combinations of alleles formed when gametes combine, such as in a zygote.
These types of crosses can be challenging to set up, and the square you create will be 4x4. This simple guide will walk you through the steps of solving a typical dihybrid cross common in genetics. The method can also work for any cross that involves two traits.
Consider this cross
This cross focuses on two traits in peas.
• The trait for yellow peas (Y) is dominant to the trait for green peas (y).
• The trait for round peas (R) is dominant to the trait for wrinkled peas (r).
Figure \(6\) outlines two generations of crosses. In the Parental (P) generation two homozygous plants are crossed: a plant that produces yellow round peas (YYRR) is crossed with a plant that produces green wrinkled peas (yyrr). The Punnett square for this cross is not shown, but all of the offspring would be heterozygous (have the YyRr genotype) and produce yellow round peas. To arrive at this:
• The parent with the YYRR genotype produces gametes that are all YR
• The parent with the yr genotype produces gametes that are all yr
• The YR and yr gametes produce YyRr offspring
Two heterozygous plants (YyRr) are crossed. What gametes do each of these plants produce? When gametes are produced, they can either have the dominant R or the recessive r. And, they can either have the dominant Y or the recessive y. Combine the R's and Ys of each parent to represent sperm and egg.
• A gamete that gets the Y allele:
• can either get the R allele and be YR
• or it can get the r allele and be Yr
• A gamete that gets the y allele:
• can either get the R allele and be yR
• or it can get the r allele and be yr
A plant that is YyRr will produce 4 different gametes: YR, Yr, yR, yr. These gametes are written on the outside of the Punnett square in Figure \(6\). Next the gametes are combined to form the offspring's genotypes (written in the center of the Punnet square). The resulting offspring will have the following ratios:
• 9/16 round, yellow peas; having the genotypes
• YYRR (1)
• YyRR (2)
• YYRr (2)
• YyRr (4)
• 3/16 round, green peas; having the genotypes yyRR (1) and yyRr (2)
• 3/16 wrinkled, yellow peas; having the genotypes YYrr (3)
• 1/16 wrinkled green peas; having the genotype yyrr
Autosomal single-gene Traits in Humans
Single-gene autosomal traits include widow's peak and freckles, both of which are illustrated below. Widow's peak refers to a point in the hairline at the center of the forehead. Assume that the dominant and recessive alleles for the widow's peak gene are represented by Wand w, respectively. Because this is a dominant trait, people with the genotype WW and the genotype Ww will have a widow's peak, and only people with the genotype ww will not have the trait.
Assume that the dominant and recessive alleles for freckles are represented by F and f, respectively. Because it is a dominant trait, people with the genotype FF and the genotype Ff will have freckles, and only people with the genotype ff will not have the trait.
Sex inheritance
What determines if a baby is a male or female? Recall that you have 23 pairs of chromosomes—and one of those pairs is the sex chromosomes. Everyone normally has two sex chromosomes. Later, you will learn that due to nondisjunction, males and females may have one less or one extra X chromosome. Your sex chromosomes can be X or Y. Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY). If a baby inherits an X chromosome from the father and an X chromosome from the mother, what will be the child’s sex? The baby will have two X chromosomes, so it will be female. If the father’s sperm carries the Y chromosome, the child will be male. Notice that a mother can only pass on an X chromosome, so the sex of the baby is determined by the father. The father has a 50 percent chance of passing on the Y or X chromosome, so there is a 50 percent chance that a child will be male, and there is a 50 percent chance a child will be female. This 50:50 chance occurs for each baby. A couple's first five children could all be boys. The sixth child still has a 50:50 chance of being a girl. A Punnett square can also be used to show how the X and Y chromosomes are passed from parents to their children. This is illustrated in the Punnett square below. It may help you understand the inheritance pattern of sex-linked traits.
X-Linked Mendelian Traits in Humans
One example of a sex-linked trait is red-green colorblindness. People with this type of colorblindness cannot tell the difference between red and green. They often see these colors as shades of brown (Figure \(11\)).
Males are much more likely to be colorblind than females because colorblindness is a sex-linked, recessive trait. Because males have just one X chromosome, they have only one allele for any X-linked trait. Therefore, a recessive X-linked allele is always expressed in males. Because females have two X chromosomes, they have two alleles for any X-linked trait. Females can have one X chromosome with the colorblind gene and one X chromosome with a normal gene for color vision. Since colorblindness is recessive, the dominant normal gene will mask the recessive colorblind gene. Females with one colorblindness allele and one normal allele are referred to as carriers. They carry the allele but do not express it. Females must inherit two copies of the recessive allele to express an X-linked recessive trait. This explains why X-linked recessive traits are less common in females than males and why they show a different pattern of inheritance than autosomal traits.
According to this Punnett square (Table \(1\)), the son of a woman who carries the colorblindness trait and a male with normal vision has a 50% chance of being colorblind. Figure \(10\) shows a simple pedigree for this trait.
Table \(1\): Punnet Square for color blindness
Xb XB
XB
XbX
(carrier female)
XBXB
(unaffected female)
Y
XbY
(colorblind male)
XBY
(unaffected male)
Another example of a recessive X-linked Mendelian trait is hemophilia. This is a disorder characterized by the inability of blood to clot normally. England's Queen Victoria was a carrier of the disorder. Two of Queen Victoria's five daughters inherited the hemophilia allele from their mother and were carriers. When they married royalty in other European countries, they spread the allele across Europe, including the royal families of Spain, Germany, and Russia. Victoria's son Prince Leopold also inherited the hemophilia allele from his mother and actually suffered from the disease. For these reasons, hemophilia was once popularly called "the royal disease."
Feature: My Human Body
Are you color blind or think you might be? If you inherited this X-linked recessive disorder, a world without clear differences between certain colors seems normal to you. It's all you have ever known. That's why some people who are color blind are not even aware of it. Simple tests have been devised to determine whether a person is color blind and the degree of this visual deficit. An example of such a test is pictured below. What do you see when you look at this circle? Can you clearly perceive the number 74? If so, you probably have normal red-green color vision. If you cannot see the number, you may have red-green color blindness.
Being color blind may cause a number of problems. These may range from minor frustrations to outright dangers. For example:
• If you are color blind, it may be difficult to color-coordinate clothing and furnishings. You may end up wearing color combinations that people with normal color vision think are odd or clashing.
• Many LED indicator lights are red or green. For example, power strips and electronic devices may have indicator lights to show whether they are on (green) or off (red).
• Test strips for pH, hard water, swimming pool chemicals, and other common tests are also often color coded. Litmus paper for testing pH, for example, turns red in the presence of an acid, but if you are color blind, you may not be able to read the test result.
• Do you like your steak well done? If you are color blind, you may not be able to tell if the meat is still undercooked (red) or grilled just right. You also may not be able to distinguish ripe (red) from unripe (green) fruits and vegetables such as tomatoes. And some foods, such as dark green spinach, may look more like mud than food and be totally unappetizing.
• Weather maps often are color coded. Is that rain (green) in your forecast or a wintry mix of sleet and freezing rain (pink or red)? If you can't tell the difference, you may go out on the roads when you shouldn't and put yourself in danger.
• Being able to distinguish red from green traffic lights may be a matter of life or death. This can be very difficult for someone with red-green color blindness. That's why in some countries, people with this vision defect are not allowed to drive.
Being aware of conditions such as colorblindness is also important for anyone creating content online. Developing webpages that are legible to all users is an important skill for a variety of jobs. You can use online tools (such as the Toptal Color Blind Filter) to ensure that the content you create is usable by all of your customers.
Review
1. Define genetic traits and simple inheritance.
2. Explain why autosomal and X-linked traits have different patterns of inheritance.
3. What is a pedigree, and why is it useful for studying how traits are passed from one generation to the next?
4. What is a Punnett square, and what does it show?
5. Identify examples of human autosomal and X-linked traits.
6. Imagine a hypothetical human gene that has two alleles, Q and q. Q is dominant to q and the inheritance of this gene is simple. Answer the following questions about this gene.
1. If a woman has the genotype Qq and her partner has the genotype QQ, list each of their possible gametes and what proportion of their gametes will have each allele.
2. What are the likely proportions of their offspring being QQ, Qq, or qq?
3. Is this an autosomal trait or an X-linked trait? How do you know?
4. What are the chances of their offspring exhibiting the dominant Q trait? Explain your answer.
7. Explain why fathers always pass their X chromosome down to their daughters.
8. True or False. Women are more likely to have X-linked diseases than men.
9. True or False. Most human autosomal traits are controlled by a single gene with two alleles, similar to Mendel’s pea plants.
10. For each of the scenarios below, choose whether you would use a Punnett square or a pedigree. Choose only the one that best fits the scenario.
1. A man and a woman have known genotypes and you want to predict the possible genotypes of their offspring.
2. You want to document which members of your family had or have breast cancer.
Attributions
1. Young woman with dimples by I'm so bored, CC BY-SA 3.0 via Wikimedia Commons
2. Autosomal dominant pedigree by Jerome Walker, CC BY-SA 3.0 via Wikimedia Commons
3. Autosomal recessive pedigree by Jerome Walker, CC BY-SA 3.0 via Wikimedia Commons
4. Punnett by miguelferig dedicated CC0 via Wikimedia Commons
5. Punnett square by Mariana Ruiz Villarreal (LadyofHats), CC BY-NC 3.0 via CK-12
6. Dihybrid Cross by CNX OpenStax, licensed CC BY 4.0 via Wikimedia Commons
7. Widow's peak created by Mandeep Grewal licensed CC BY-SA 4.0, from the pictures
1. Male Widows Peak by Jmblock2 licensed CC BY-SA 4.0 via Wikimedia Commons
2. Omer Mor by Omer Mor licensed CC BY-SA 4.0 via Wikimedia Commons
8. Freckles by Ayo Ogunseinde, via Unsplash license
9. X and Y chromosomes by Maria Jackson, Leah Marks, Gerhard May, and Joanna Wilson licensed CC BY-NC-ND 4.0 via Research Gate
10. Colorblindness pedigree by Jodi So, CC BY-NC 3.0 via CK-12
11. Ishara public domain via Wikimedia Commons
12. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/08%3A_Inheritance/8.4%3A_Simple_Inheritance.txt |
Family Portrait
This photo of a South African family shows some of the variations that exist in human skin color. The color of human skin can range from very light to very dark with every possible gradation in between. As you might expect, the skin color trait has a more complex genetic basis than just one gene with two alleles, which is the type of simple trait that Mendel studied in pea plants. Like skin color, many other human traits have more complicated modes of inheritance than Mendelian traits. Such modes of inheritance are called non-Mendelian inheritance, and they include inheritance of multiple allele traits, traits with codominance or incomplete dominance, and polygenic traits, among others, all of which are described below.
Multiple Allele Traits
The majority of human genes are thought to have more than two normal versions or alleles. Traits controlled by a single gene with more than two alleles are called multiple allele traits. An example is ABO blood type. Your blood type refers to which of certain proteins called antigens are found on your red blood cells. There are three common alleles for this trait, which are represented by the letters IA, IB, and i.
Table \(1\): ABO Blood Group
Genotype Phenotype (blood type)
IAIA A
IAi A
IBIB B
IBi B
ii O
IAIB AB
As shown in the table below, there are six possible ABO genotypes because the three alleles, taken two at a time, result in six possible combinations. The IA and IB alleles are dominant to the i allele. As a result, both IAIA and IAi genotypes have the same phenotype, with the A antigen in their blood (type A blood). Similarly, both IBIB and IBi genotypes have the same phenotype, with the B antigen in their blood (type B blood). No antigen is associated with the i allele, so people with the ii genotype have no antigens for ABO blood type in their blood (type O blood).
Codominance
Look at the genotype IAIB in the ABO blood group table. Alleles IA and IB for ABO blood type are neither dominant nor recessive to one another. Instead, they are codominant to each other. Codominance occurs when two alleles for a gene are expressed equally in the phenotype of heterozygotes. In the case of ABO blood type, IAIB heterozygotes have a unique phenotype, with both A and B antigens in their blood (type AB blood).
Incomplete Dominance
Another relationship that may occur between alleles for the same gene is incomplete dominance. This occurs when the dominant allele is not completely dominant, so an intermediate phenotype results in heterozygotes who inherit both alleles. Generally, this happens when the two alleles for a given gene both produce proteins but one protein is not functional. As a result, the heterozygote individual produces only half the amount of normal protein as is produced by an individual who is homozygous for the normal allele.
An example of incomplete dominance in humans is Tay Sachs disease. The normal allele for the gene, in this case, produces an enzyme that is responsible for breaking down lipids. A defective allele for the gene results in the production of a nonfunctional enzyme. Heterozygotes who have one normal and one defective allele produce half as much functional enzyme as the normal homozygote, and this is enough for normal development. However, homozygotes who have only defective alleles produce only the nonfunctional enzyme. This leads to the accumulation of lipids in the brain beginning in utero, which causes significant brain damage. Most individuals with Tay Sachs disease die at a young age, typically by the age of five years.
Polygenic Traits
Many human traits are controlled by more than one gene. These traits are called polygenic traits. The alleles of each gene have a minor additive effect on the phenotype. There are many possible combinations of alleles, especially if each gene has multiple alleles. Therefore, a whole continuum of phenotypes is possible.
An example of a human polygenic trait is adult height. Several genes, each with more than one allele, contribute to this trait, so there are many possible adult heights. For example, one adult’s height might be 1.655 m (5.430 feet), and another adult’s height might be 1.656 m (5.433 feet). Adult height ranges from less than 5 feet to more than 6 feet, with males being somewhat taller than females on average. The majority of people fall near the middle of the range of heights for their sex, as shown in the graph in Figure \(2\).
Environmental Effects on Phenotype
Many traits are affected by the environment as well as by genes. This may be especially true for polygenic traits. For example, adult height might be negatively impacted by poor diet or illness during childhood. Skin color is another polygenic trait. There is a wide range of skin colors in people worldwide. In addition to differences in skin color genes, differences in exposure to ultraviolet (UV) light cause some of the variations. As shown in Figure \(3\), exposure to UV light darkens the skin.
Pleiotropy
Some genes affect more than one phenotypic trait. This is called pleiotropy. There are numerous examples of pleiotropy in humans. They generally involve important proteins that are needed for the normal development or functioning of more than one organ system. An example of pleiotropy in humans occurs with the gene that codes for the main protein in collagen, a substance that helps form bones. This protein is also important in the ears and eyes. Mutations in the gene result in problems not only in bones but also in these sensory organs, which is how the gene's pleiotropic effects were discovered.
Another example of pleiotropy occurs with sickle cell anemia. This recessive genetic disorder occurs when there is a mutation in the gene that normally encodes the red blood cell protein called hemoglobin. People with the disorder have two alleles for sickle-cell hemoglobin, so named for the sickle shape (Figure \(4\)) that their red blood cells take on under certain conditions such as physical exertion. The sickle-shaped red blood cells clog small blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain bone deformities, kidney failure, and strokes.
Epistasis
Some genes affect the expression of other genes. This is called epistasis. Epistasis is similar to dominance, except that it occurs between different genes rather than between different alleles for the same gene.
Albinism is an example of epistasis. A person with albinism has virtually no pigment in the skin. The condition occurs due to an entirely different gene than the genes that encode skin color. Albinism occurs because a protein called tyrosinase, which is needed for the production of normal skin pigment, is not produced due to a gene mutation. If an individual has albinism mutation, he or she will not have any skin pigment, regardless of the skin color genes that were inherited.
Feature: My Human Body
Do you know your ABO blood type? In an emergency, knowing this valuable piece of information could possibly save your life. If you ever need a blood transfusion, it is vital that you receive blood that matches your own blood type. Why? If the blood transfused into your body contains an antigen that your own blood does not contain, antibodies in your blood plasma (the liquid part of your blood) will recognize the antigen as foreign to your body and cause a reaction called agglutination. In this reaction, the transfused red blood cells will clump together, as shown in the image below. The agglutination reaction is serious and potentially fatal.
Knowing the antigens and antibodies present in each of the ABO blood types will help you understand which type(s) of blood you can safely receive if you ever need a transfusion. This information is shown in the table below for all of the ABO blood types. For example, if you have blood type A, this means that your red blood cells have the A antigen and that your blood plasma contains anti-B antibodies. If you were to receive a transfusion of type B or type AB blood, both of which have the B antigen, your anti-B antibodies would attack the transfused red blood cells, causing agglutination.
Table \(2\): Antigens and antibodies in ABO blood types
Characteristics Type A Type B Type AB Type O
Red Blood Cell
Antibodies in Plasma
Anti-B
Anti-A
None
Anti-A and Anti-B
Antigens in Red Blood Cells
A antigen
B antigens
A and B antigens
None
You may have heard that people with blood type O are called universal donors and that people with blood type AB are called universal recipients. People with type O blood have neither A nor B antigens in their blood, so if their blood is transfused into someone with a different ABO blood type, it causes no immune reaction. In other words, they can donate blood to anyone. On the other hand, people with type AB blood have no anti-A or anti-B antibodies in their blood, so they can receive a transfusion of blood from anyone. Which blood type(s) can safely receive a transfusion of type AB blood, and which blood type(s) can be safely received by those with type O blood?
Review
1. What is non-Mendelian inheritance?
2. Explain why the human ABO blood group is an example of a multiple allele trait with codominance.
3. What is incomplete dominance? Give an example of this type of non-Mendelian inheritance in humans.
4. Explain the genetic basis of human skin color.
5. How may the human trait of adult height be influenced by the environment?
6. Define pleiotropy, and give a human example.
7. What is the difference between pleiotropy and epistasis?
8. Which of the following terms best matches each trait description? Choose only the one term that best fits each trait. (codominance; multiple allele trait; Mendelian trait; polygenic trait)
1. A trait controlled by four genes.
2. A trait where each allele of a heterozygote makes an equal contribution to the phenotype.
3. A trait controlled by a single gene that has three different versions.
4. A trait controlled by a single gene where one allele is fully dominant to the only other allele.
9. People with type AB blood have:
1. anti-O antibodies
2. anti-A and anti-B antibodies
3. A and B antigens
10. True or False. People with type O blood cannot receive a blood transfusion from anyone besides others with type O blood.
11. True or False. People with type O blood can be heterozygous for this trait.
Explore More
To learn more about non-Mendelian Inheritance, check out this video:
Attributions
1. Family by Henry M. Trotter, released into the public domain via Wikimedia Commons
2. Adult height graph by Mariana Ruiz Villarreal (LadyofHats), CC BY-NC 3.0 for CK-12 Foundation
3. Skin tanning by Onetwo1, licensed CC BY 3.0 via Wikimedia Commons
4. Sickle cells by OpenStax College, licensed CC BY 3.0 via Wikimedia Commons
5. Type A Blood, public domain via Wikimedia Commons
6. Blood type table based on image of ABO Blood type, public domain via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/08%3A_Inheritance/8.5%3A_Complex_Inheritance.txt |
Polly Who?
Each hand in Figure \(1\) has an extra pinky finger. This is a condition called polydactyly, which literally means "many digits." People with polydactyly may have extra fingers and/or toes, and the condition may affect just one hand or foot or both hands and feet. Polydactyly is often genetic in origin and may be part of a genetic disorder that is associated with other conditions.
What Are Genetic Disorders?
Genetic disorders are diseases, syndromes, or other conditions that are caused by mutations in one or more genes or by chromosomal alterations. Genetic disorders are typically present at birth, but they should not be confused with congenital disorders, which are any disorders, regardless of cause, that are present at birth. Some congenital disorders are not caused by genetic mutations or chromosomal alterations. Instead, they are caused by problems that arise during embryonic or fetal development or during the process of birth. An example of a nongenetic congenital disorder is fetal alcohol syndrome. This is a collection of birth defects, including facial anomalies and intellectual disability, caused by maternal alcohol consumption during pregnancy.
Genetic Disorders Caused by Mutations
Table \(1\): Autosomal and X-linked genetic disorders
Genetic Disorder Direct Effect of Mutation Signs and Symptoms of the Disorder Mode of Inheritance
Marfan syndrome defective protein in connective tissue heart and bone defects and unusually long, slender limbs and fingers autosomal dominant
Sickle cell anemia atypical hemoglobin protein in red blood cells sickle-shaped red blood cells that clog tiny blood vessels, causing pain and damaging organs and joints autosomal recessive
Vitamin D-resistant rickets lack of a substance needed for bones to absorb minerals soft bones that easily become deformed, leading to bowed legs and other skeletal deformities X-linked dominant
Hemophilia A reduced activity of a protein needed for blood clotting internal and external bleeding that occurs easily and is difficult to control X-linked recessive
Table \(1\) lists several genetic disorders caused by mutations in just one gene. Some of the disorders are caused by mutations in autosomal genes, others by mutations in X-linked genes. Which disorders would you expect to be more common in males than females?
Very few genetic disorders are controlled by dominant mutant alleles. A dominant allele is expressed in every individual who inherits even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce. Therefore, the mutant dominant allele is likely to die out of the population.
A recessive mutant allele, such as the allele that causes sickle cell anemia or cystic fibrosis, is not expressed in people who inherit just one copy of it. These people are called carriers. They do not have the disorder themselves, but they carry the mutant allele and their offspring can inherit it. Thus, the allele is likely to pass on to the next generation rather than die out.
Genetic Disorders Caused by Chromosomal Alterations
As we learned in the Cell Reproduction chapter, mistakes may occur during meiosis that results in nondisjunction. This is the failure of replicated chromosomes to separate properly during meiosis. Some of the resulting gametes will be missing all or part of a chromosome, while others will have an extra copy of all or part of the chromosome. If such gametes are fertilized and form zygotes, they usually do not survive. If they do survive, the individuals are likely to have serious genetic disorders.
Table \(2\) lists several genetic disorders that are caused by atypical numbers of chromosomes. Most chromosomal disorders involve the X chromosome. The X and Y chromosomes are the only chromosome pair in which the two chromosomes are very different in size. This explains why nondisjunction of the sex chromosomes tends to occur more frequently than nondisjunction of autosomes.
Table \(2\): Genetic Disorders Caused by Atypical Numbers of Chromosomes
Genetic Disorder Genotype Phenotypic Effects
Down syndrome extra copy (complete or partial) of chromosome 21 (see figure below) developmental delays, distinctive facial appearance, and other physical and developmental conditions (see figure below)
Turner’s syndrome one X chromosome but no other sex chromosome (XO) Chromosomally female with short height and infertility (inability to reproduce)
Triple X syndrome three X chromosomes (XXX) Chromosomally female with mild developmental delays and menstrual irregularities
Klinefelter’s syndrome one Y chromosome and two or more X chromosomes (XXY, XXXY) Chromosomally male with problems in sexual development and reduced levels of the male hormone testosterone
Diagnosing and Treating Genetic Disorders
A genetic disorder that is caused by a mutation can be inherited. Therefore, people with a genetic disorder in their family may be concerned about having children with the disorder. A genetic counselor can help them understand the risks of their children being affected. If they decide to have children, they may be advised to have prenatal (“before birth”) testing to see if the fetus has any genetic disorders. One method of prenatal testing is amniocentesis. In this procedure, a few fetal cells are extracted from the fluid surrounding the fetus in utero, and the fetal chromosomes are examined. Down syndrome and other chromosomal alterations can be detected in this way.
The symptoms of genetic disorders can sometimes be treated or prevented. For example, in the genetic disorder called phenylketonuria (PKU), the amino acid phenylalanine builds up in the body to harmful levels. PKU is caused by a mutation in a gene that normally codes for an enzyme needed to break down phenylalanine. The buildup of PKU can lead to serious health problems, such as intellectual disability and delayed development, among other serious problems. Babies in the United States and many other countries are screened for PKU soon after birth. If PKU is diagnosed, the infant can be fed a low-phenylalanine diet. This prevents the buildup of phenylalanine and the health problems associated with it. With a low phenylalanine diet, most symptoms of the disorder can be prevented.
Curing Genetic Disorders
Cures for genetic disorders are still in the early stages of development. One potential cure is gene therapy. Gene therapy is an experimental technique that uses genes to treat or prevent disease. In gene therapy, normal genes are introduced into cells to compensate for mutated genes. If a mutated gene causes a necessary protein to be nonfunctional or missing, gene therapy may be able to introduce a normal copy of the gene to produce the needed functional protein.
A gene that is inserted directly into a cell usually does not function, so a carrier called a vector is genetically engineered to deliver the gene (Figure \(3\)). Certain viruses, such as adenoviruses, are often used as vectors. They can deliver the new gene by infecting cells. The viruses are modified so they do not cause disease when used in people. If the treatment is successful, the new gene delivered by the vector will allow the synthesis of a functioning protein.
Feature: Human Biology in the News
Down syndrome is the most common genetic cause of intellectual disability. It occurs in about 1 in every 700 live births, and it currently affects nearly half a million Americans. Until recently, scientists thought that the changes leading to intellectual disability in people with Down syndrome all happen before birth.
Researchers recently discovered a genetic disorder that affects brain development in people with Down Syndrome throughout childhood and into adulthood. The newly discovered genetic disorder changes communication between nerve cells in the brain, resulting in the slower transmission of nerve impulses. This finding may eventually allow the development of strategies to promote brain functioning in Down syndrome patients and may also be applicable to other developmental disabilities such as autism.
Review
1. Define genetic disorder.
2. Identify three genetic disorders caused by mutations in a single gene.
3. Why are single-gene genetic disorders more commonly controlled by recessive than dominant mutant alleles?
4. What is nondisjunction? Why may it cause genetic disorders?
5. Explain why genetic disorders caused by a number of chromosomes most often involve the X chromosome.
6. How is Down syndrome detected in utero?
7. Use the example of PKU to illustrate how the symptoms of a genetic disorder can sometimes be prevented.
8. Explain how gene therapy works.
9. Compare and contrast genetic disorders and congenital disorders.
10. Explain why parents that do not have Down syndrome can have a child with Down syndrome.
11. Hemophilia A and Turner’s syndrome both involve problems with the X chromosome. What is the major difference between these two types of disorders in terms of how the X chromosome is affected?
12. Can you be a carrier of Marfan syndrome and not have the disorder? Explain your answer.
13. True or False. It is impossible for people to have more than three copies of one chromosome.
14. True or False. The gene for sickle cell anemia is on a sex chromosome.
Explore More
Scientists have promised that gene therapy will be the next big leap for medicine, but what is it exactly? Learn more here:
Attributions
1. Polydactyly by Baujat G, Le Merrer M. CC BY 2.0 via Wikimedia Commons
2. Down syndrome by CK-12, public domain based on
1. Down Syndrome Karyotype by National Human Genome Research Institute, public domain via Wikimedia Commons
2. Brushfield eyes by Erin Ryan, public domain via Wikimedia Commons
3. Virus by Darryl Leja at NHGRI public domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/08%3A_Inheritance/8.6%3A_Genetic_Disorders.txt |
Case Study Conclusion: Cancer in the Family
Rebecca’s family tree, as illustrated in Figure \(1\), shows a high incidence of cancer among close relatives. But are genes the cause of cancer in this family? Only genetic testing, which is the sequencing of specific genes in an individual, can reveal whether a cancer-causing gene is being inherited in this family.
Fortunately for Rebecca, the results of her genetic testing show that she does not have the mutations in the BRCA1 and BRCA2 genes that most commonly increase a person’s risk of getting cancer. However, it does not mean that she doesn’t have other mutations in these genes that could increase her risk of getting cancer. There are many other mutations in BRCA genes whose effect on cancer risk is not known, and there may be many more yet to be discovered. It is important to continue to study the variations in genes such as BRCA in different people to better assess their possible contribution to the development of the disease. As you now know from this chapter, many mutations are harmless, while others can cause significant health effects, depending on the specific mutation and the gene involved.
Mutations in BRCA genes are particularly likely to cause cancer because these genes encode for tumor-suppressor proteins that normally repair damaged DNA and control cell division. If these genes are mutated in a way that causes the proteins to not function properly, other mutations can accumulate and cell division can run out of control, which can cause cancer.
BRCA1 and BRCA2 are on chromosomes 17 and 13, respectively, which are autosomes. As Rebecca’s genetic counselor mentioned, mutations in these genes have a dominant inheritance pattern. Now that you know the pattern of inheritance of autosomal dominant genes if Rebecca’s grandmother did have one copy of a mutated BRCA gene, what are the chances that Rebecca’s mother also has this mutation? Because it is dominant, only one copy of the gene is needed to increase the risk of cancer, and because it is on autosomes instead of sex chromosomes, the sex of the parent or offspring does not matter in the inheritance pattern. In this situation, Rebecca’s grandmother’s eggs would have had a 50% chance of having a BRCA gene mutation, due to Mendel’s law of segregation. Therefore, Rebecca’s mother would have had a 50% chance of inheriting this gene. Even though Rebecca does not have the most common BRCA mutations that increase the risk of cancer, it does not mean that her also mother does not, because there would also only be a 50% chance that she would pass it on to Rebecca. Therefore, Rebecca’s mother should consider getting tested for mutations in the BRCA genes as well. Ideally, the individuals with cancer in a family should be tested first when a genetic cause is suspected so that if there is a specific mutation being inherited, it can be identified and the other family members can be tested for that same mutation.
Mutations in both BRCA1 and BRCA2 are often found in Ashkenazi Jewish families. However, these genes are not linked in the chromosomal sense, because they are on different chromosomes and are therefore inherited independently, in accordance with Mendel’s law of independent assortment. Why would certain gene mutations be prevalent in particular ethnic groups? If people within an ethnic group tend to produce offspring with each other, their genes will remain prevalent within the group. These may be genes for harmless variations such as skin, hair, or eye color, or harmful variations such as the mutations in the BRCA genes. Other genetically based diseases and disorders are sometimes more commonly found in particular ethnic groups, such as cystic fibrosis in people of European descent and sickle-cell anemia in people of African descent. You will learn more about the prevalence of certain genes and traits in particular ethnic groups and populations in the chapter on Human Variation.
As you learned in this chapter, genetics is not the sole determinant of phenotype. The environment can also influence many traits, such as adult height and skin color. The environment also plays a major role in the development of cancer. 90 to 95% of all cancers do not have an identified genetic cause and are often caused by mutagens in the environment such as UV radiation from the sun or toxic chemicals in cigarette smoke. But for families like Rebecca’s, knowing their family health history and genetic makeup may help them better prevent or treat diseases that are caused by their genetic inheritance. If a person knows they have a gene that can increase their risk of cancer, they can make lifestyle changes, have early and more frequent cancer screenings, and may even choose to have preventative surgeries that may help reduce their risk of getting cancer and increase their odds of long-term survival if cancer does occur. The next time you go to the doctor and they ask whether any members of your family have had cancer, you will have a deeper understanding of why this information is so important to your health.
Chapter Summary
In this chapter, you learned about genetics — the science of heredity. Specifically, you learned that:
• Chromosomes are structures made of DNA and proteins that are encoded with genetic instructions for making proteins. The instructions are organized into units called genes, most of which contain instructions for a single protein.
• Humans normally have 23 pairs of chromosomes. Of these, 22 pairs are autosomes, which contain genes for characteristics unrelated to sex. The other pair consists of sex chromosomes (XX in females, XY in males). Only the Y chromosome contains genes that determine sex.
• Humans have an estimated 20,000 to 22,000 genes. The majority of human genes have two or more possible versions, called alleles.
• Mendel experimented with the inheritance of traits in pea plants, which have two different forms of several visible characteristics. Mendel crossed pea plants with different forms of traits.
• In Mendel's first set of experiments, he crossed plants that only differed in one characteristic. The results led to Mendel's first law of inheritance, called the law of segregation. This law states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces.
• In Mendel's second set of experiments, he experimented with two characteristics at a time. The results led to Mendel's second law of inheritance, called the law of independent assortment. This law states that the factors controlling different characteristics are inherited independently of each other.
• Mendel's laws of inheritance, now expressed in terms of genes, form the basis of genetics, the science of heredity. Mendel is often called the father of genetics.
• The position of a gene on a chromosome is its locus. A given gene may have different versions called alleles. Paired chromosomes of the same type are called homologous chromosomes and they have the same genes at the same loci.
• The alleles an individual inherits for a given gene make up the individual's genotype. An organism with two of the same allele is called a homozygote, and an individual with two different alleles is called a heterozygote.
• The expression of an organism's genotype is referred to as its phenotype. A dominant allele is always expressed in the phenotype, even when just one dominant allele has been inherited. A recessive allele is expressed in the phenotype only when two recessive alleles have been inherited.
• In sexual reproduction, two parents produce gametes that unite in the process of fertilization to form a single-celled zygote. Gametes are haploid cells with only one of each pair of homologous chromosomes, and the zygote is a diploid cell with two of each pair of chromosomes.
• Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be completely dominant to the other. The pattern of inheritance of Mendelian traits depends on whether the traits are controlled by genes on autosomes or by genes on sex chromosomes.
• Examples of human autosomal Mendelian traits include dimples and earlobe attachment. Examples of human X-linked traits include red-green color blindness and hemophilia.
• Two tools for studying inheritance are pedigrees and Punnett squares. A pedigree is a chart that shows how a trait is passed from generation to generation. A Punnett square is a chart that shows the expected ratios of possible genotypes in the offspring of two parents.
• Non-Mendelian inheritance refers to the inheritance of traits that have a more complex genetic basis than one gene with two alleles and complete dominance.
• Multiple allele traits are controlled by a single gene with more than two alleles. An example of a human multiple allele trait is ABO blood type.
• Codominance occurs when two alleles for a gene are expressed equally in the phenotype of heterozygotes. A human example of codominance occurs in the AB blood type, in which the IA and IB alleles are codominant.
• Incomplete dominance is the case in which the dominant allele for a gene is not completely dominant to a recessive allele, so an intermediate phenotype occurs in heterozygotes who inherit both alleles. A human example of incomplete dominance is Tay Sachs disease, in which heterozygotes produce half as much functional enzyme as normal homozygotes.
• Polygenic traits are controlled by more than one gene, each of which has a minor additive effect on the phenotype. This results in a continuum of phenotypes. Examples of human polygenic traits include skin color and adult height. Many of these types of traits, as well as others, are affected by the environment as well as by genes.
• Pleiotropy refers to the situation in which a gene affects more than one phenotypic trait. A human example of pleiotropy occurs with sickle cell anemia, which has multiple effects on the body.
• Epistasis is when one gene affects the expression of other genes. An example of epistasis is albinism, in which the albinism mutation negates the expression of skin color genes.
• Genetic disorders are diseases, syndromes, or other abnormal conditions that are caused by mutations in one or more genes or by chromosomal alterations.
• Examples of genetic disorders caused by single-gene mutations include Marfan syndrome (autosomal dominant), sickle cell anemia (autosomal recessive), vitamin D-resistant rickets (X-linked dominant), and hemophilia A (X-linked recessive). Very few genetic disorders are caused by dominant mutations because these alleles are less likely to be passed on to successive generations.
• Nondisjunction is the failure of replicated chromosomes to separate properly during meiosis. This may result in genetic disorders caused by abnormal numbers of chromosomes. An example is Down syndrome, in which the individual inherits an extra copy of chromosome 21. Most chromosomal disorders involve the X chromosome. An example is Klinefelter's syndrome (XXY, XXXY).
• Prenatal genetic testing, for example, by amniocentesis, can detect chromosomal alterations in utero. The symptoms of some genetic disorders can be treated or prevented. For example, symptoms of phenylketonuria (PKU) can be prevented by following a low-phenylalanine diet throughout life.
• Cures for genetic disorders are still in the early stages of development. One potential cure is gene therapy, in which normal genes are introduced into cells by a vector such as a virus to compensate for mutated genes.
Chapter Summary Review
1. Which sentence is correct?
1. Different alleles of the same gene are located at the same locus on homologous chromosomes.
2. Different alleles of the same gene are located at different loci on homologous chromosomes.
3. Different genes of the same alleles are located at the same locus on homologous chromosomes.
4. Different alleles of the same gene are located at different loci on the same chromosome.
2. A person has a hypothetical Aa genotype. Answer the following questions about this genotype.
1. What do A and a represent?
2. If the person expresses only the phenotype associated with A, is this an example of complete dominance, codominance, or incomplete dominance? Explain your answer. Also, describe what the observed phenotypes would be if it were either of the two incorrect answers.
3. Explain how a mutation that occurs in a parent can result in a genetic disorder in their child. Be sure to include which type of cell or cells in the parent must be affected in order for this to happen.
4. What is an allele that is not expressed in a heterozygote called?
5. True or False. Sex is determined by a gene on an autosome.
6. True or False. In sexual reproduction, parents and offspring are never identical.
7. True or False. In humans, a gamete will have 23 chromosomes.
8. True or False. The expression of an organism’s phenotype produces its genotype.
9. True or False. It is entirely likely for a gene to have more than two alleles.
10. Mendel’s law of independent assortment states that
1. two factors of the same characteristic separate into different gametes.
2. there are dominant and recessive factors.
3. factors controlling different characteristics are inherited independently of each other.
4. there are two factors that control inheritance.
11. Linked genes:
1. are on homologous chromosomes.
2. are on the same chromosome.
3. are on adjacent chromosomes.
4. are on non-homologous chromosomes.
12. A woman has red-green color blindness, which is an X-linked recessive trait. Her husband does not have red-green color blindness. Which of the following is correct?
1. Half of their daughters will have red-green color blindness.
2. All of their daughters will have red-green color blindness.
3. All of their sons will have red-green color blindness.
4. All of their children will have red-green color blindness.
13. Which of the following is an example of Mendelian inheritance?
1. A trait that has three alleles
2. A trait that is controlled by two genes
3. A trait that is controlled by a single gene with one dominant and one recessive allele
4. A trait that has two alleles, both of which are expressed equally in the phenotype
Attributions
1. Pedigree by Rachel Henderson by CK-12 licensed CC BY-NC 3.0
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/08%3A_Inheritance/8.7%3A_Case_Study_Conclusion%3A_Cancer_and_Chapter_Summary.txt |
This chapter outlines how Darwin developed his theory of evolution by natural selection, Wallace's contribution to the theory, and evidence for evolution. The chapter also describes tools for studying evolution, processes of microevolution and macroevolution, and how Earth formed and life first evolved.
• 9.1: Case Study: Everyday Evolution
One night in April 2009, Mateo woke up soaked in sweat. He had a fever of 102.4 degrees F, chills, an intense headache, and body aches. He soon developed a sore throat and a bad cough.
• 9.2: Darwin, Wallace, and the Theory of Evolution by Natural Selection
Eighteenth-century Englishman Charles Darwin is one of the most famous scientists who ever lived. His place in the history of science is well deserved. Darwin's theory of evolution by natural selection represents a giant leap in human understanding. Darwin's theory contains two major ideas: (1) that evolution occurs. and (2) that evolution occurs by natural selection. Natural selection is the process in which living things with beneficial traits produce more offspring than others do.
• 9.3: Evidence for Evolution
Fossils are a window into the past. They provide clear evidence that evolution has occurred. Scientists who find and study fossils are called paleontologists. How do they use fossils to understand the past? The oldest horse fossils show what the earliest horses were like. They were only 0.4 m tall, or about the size of a fox, and they had four long toes. Other evidence shows they lived in wooded marshlands, where they probably ate soft leaves.
• 9.4: Microevolution
Individuals do not evolve because their genes do not change over time. Instead, evolution occurs at the level of the population. A population consists of organisms of the same species that live in the same area. In terms of evolution, the population is assumed to be a relatively closed group. This means that most mating takes place within the population. Evolutionary change that occurs over relatively short periods of time within populations is called microevolution.
• 9.5: Macroevolution
This garter snake preys on a variety of small animals, including small amphibians called rough-skinned newts. The newts produce a powerful toxin that is concentrated in their skin. Garter snakes have evolved resistance to this toxin through a series of lucky genetic mutations, allowing them to safely prey upon the newts. The predator-prey relationship between these animals has created an evolutionary "arms" race.
• 9.6: Tools for Studying Evolution
This interesting image is a 19th century representation of Earth that is based on an ancient Hindu myth. According to the myth, Earth rests on the backs of elephants, which in turn stand on the back of a giant turtle.
• 9.7: Adaptation in Humans
Milk naturally contains not only proteins and lipids; it also contains carbohydrates. Specifically, milk contains the sugar lactose. Lactose is a disaccharide (two-sugar) compound that consists of one molecule each of galactose and glucose, as shown in the structural formula below. Lactose makes up between 2 and 8 percent of milk by weight. The exact amount varies both within and between species.
• 9.8: Case Study Conclusion: Flu and Chapter Summary
In April 2009, the world was hit with a swine flu pandemic. The Centers for Disease Control estimates that within that first year, 43 to 89 million people worldwide contracted the swine flu, and that it contributed to 8,870 to 18,300 deaths. Some people with swine flu were spared serious complications, such as Mateo, who you read about in the beginning of this chapter.
Thumbnail: A silhouette of human evolution. (CC BY SA 3.0 Unported; Tkgd2007).
09: Biological Evolution
Case Study: Flu, from Pigs to You
One night in April 2009, Mateo woke up soaked in sweat. He had a fever of 102.4 °F, chills, an intense headache, and body aches. He soon develops a sore throat and a bad cough. The next day he felt so sick and exhausted that he could hardly get out of bed, and his fever and other symptoms lasted for days. Clearly, this was not just a mild cold virus — Mateo most likely had influenza, commonly known as the flu.
While watching TV as he recovered in bed, Mateo saw a news report about a new “swine flu” strain of the influenza virus that was spreading in people throughout North America, particularly in Mexico. It was called the swine flu because scientists thought it most likely originated in pigs, based on similarities in its genetic sequence with viruses that infect pigs. However, contact with pigs was not necessary for people to catch swine flu. This version seemed to spread directly between people, similar to the typical seasonal flu virus.
Mateo’s symptoms were similar to those described in the news report on swine flu. Although he was beginning to recover, he saw that others were not so lucky. Many people with swine flu developed severe pneumonia, and some even died. Because this was a new strain of flu virus that was significantly different than the previous seasonal flu viruses, the existing flu vaccine was largely ineffective against swine flu. Therefore, the only way to try to prevent infection by the swine flu virus was to limit exposure to it, including avoiding contact with people with the flu and using good hand washing practices. The news report showed people in Mexico wearing masks as they went about their daily lives, to try to prevent exposure to the virus.
By June 2009, Mateo was back to normal, but many other people worldwide were not. Within just a few months, the swine flu had spread from North America to over 70 countries and territories throughout the world. The World Health Organization declared the spread of swine flu to be a pandemic, meaning that a significant portion of the world’s population was infected. In September 2009, over 99% of the influenza viruses circulating in the U.S. were the swine flu strain, which is also known as the 2009 H1N1 virus. If you had the flu in the U.S. during this time period, chances are high that it was the swine flu.
How could a new viral strain like this emerge so suddenly? And how could it change from infecting pigs to infecting humans? This is an example of evolution in action. You may think of evolution as something that occurred in the distant past, for instance, how humans evolved from earlier primates. But evolution is occurring all the time. As you will learn in this chapter, evolution is the process by which characteristics of biological entities, such as living organisms or viruses, change over time. Evolution can occur very slowly or more quickly, but it is particularly rapid in viruses and bacteria. In the Case Study Conclusion for this chapter, you will learn specifically about how the 2009 H1N1 virus evolved from a virus that infects pigs to one that infects humans.
Chapter Overview: Biological Evolution
In this chapter, you will learn about the theory of evolution, evidence for evolution, how evolution works, and the evolution of living organisms on Earth. Specifically, you will learn about:
• Darwin’s theory of evolution by natural selection and how he developed this theory.
• Evidence for the theory of evolution from fossils, DNA, and observations of living organisms.
• Microevolution, which is an evolution that occurs over a relatively short period of time within a population.
• How allele frequencies in a population change due to the forces of evolution, which include mutation, gene flow, genetic drift, and natural selection.
• Macroevolution, which is an evolution that occurs at or above the species level. This includes the generation of new species and coevolution between species.
• Influences on the timing of macroevolution.
• The tools used by scientists to study evolution including the fossil record, methods of establishing the age of fossils, and molecular clocks based on DNA or amino acid sequences.
As you read this chapter and learn more about evolution, think about the following questions about the swine flu virus.
1. Viruses can replicate quickly. Why does this contribute to their rapid rate of evolution?
2. Mutation plays an important role in the evolution of viruses. How does mutation relate to evolution?
3. One of the reasons why the 2009 H1N1 swine flu virus evolved is that different types of influenza viruses can exchange genetic material with each other if they infect the same host, in a process called reassortment. Why might this lead to a new strain of influenza virus with different characteristics? How is this similar to the genetic variation produced by sexual reproduction?
4. It is thought that contact between North American and Eurasian pigs, possibly through international trade, may have contributed to the evolution of the swine flu virus. What are some other examples in which the movement of organisms or contact between organisms has contributed to evolutionary changes?
Attributions
1. Sow and five piglets by Scott Bauer, U.S. Department of Agriculture, public domain via Wikimedia Commons
2. Masked Train Passengers by Eneas De Troya from Mexico City, México, CC BY 2.0 via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/09%3A_Biological_Evolution/9.1%3A_Case_Study%3A_Everyday_Evolution.txt |
Stepping Back in Time
The Grand Canyon, shown in Figure \(1\), is an American icon and one of the wonders of the natural world. It is also a record of the past. Look at the rock layers in the picture. If you were to walk down a trail to the bottom of the canyon, with each step-down, you would be taking a step back in time. That’s because lower layers of rock represent the more distant past. The rock layers and the fossils they contain show the prehistory of the region and its organisms over a 2-billion-year time span. Although Charles Darwin never visited the Grand Canyon, he saw rock layers and fossils in other parts of the world. They were one inspiration for his theory of evolution. Darwin’s theory rocked the scientific world. In this concept, you will read why.
What is the Theory of Evolution by Natural Selection?
Eighteenth-century Englishman Charles Darwin is one of the most famous scientists who ever lived. His place in the history of science is well deserved. Darwin’s theory of evolution by natural selection represents a giant leap in human understanding. It explains and unifies all of biology. Darwin’s theory actually contains two major ideas:
1. One idea is that evolution occurs. In other words, organisms change over time. Life on Earth has changed as descendants diverged from common ancestors in the past.
2. The other idea is that evolution occurs by natural selection. Natural selection is the process in which living things with beneficial traits produce more offspring than others do. This results in changes in the traits of living things over time.
In Darwin’s day, most people believed that all species were created at the same time and remained unchanged thereafter. They also believed that Earth was only 6,000 years old. Therefore, Darwin’s ideas revolutionized biology. How did Darwin come up with these important ideas? It all started when he went on a voyage.
Voyage of the Beagle
In 1831, when Darwin was just 22 years old, he set sail on a scientific expedition on a ship called the HMS Beagle. Darwin was the naturalist on the voyage. As a naturalist, it was his job to observe and collect specimens of plants, animals, rocks, and fossils wherever the expedition went ashore. The route the ship took and the stops they made are shown on the map below. Darwin was fascinated by nature, so he loved his job on the Beagle. He spent more than three years of the five-year trip exploring nature on distant continents and islands. While he was away, a former teacher published Darwin’s accounts of his observations. By the time Darwin finally returned to England, he had become famous as a naturalist.
Darwin’s Observations
During the long voyage, Darwin made many observations that helped him form his theory of evolution. For example:
• He visited tropical rainforests and other new habitats where he saw many plants and animals he had never seen before, such as the giant iguana and booby bird pictured below. These observations impressed him with the great diversity of life.
• He experienced an earthquake that lifted the ocean floor 2.7 meters (9 feet) above sea level. He also found rocks containing fossil seashells in mountains high above sea level. These observations suggested that continents and oceans had changed dramatically over time and continue to change in dramatic ways.
• He visited rock ledges that had clearly once were beaches that had gradually built up over time. This suggested that slow, steady processes also change Earth’s surface.
• He dug up fossils of gigantic extinct mammals, such as the ground sloth, fossils of which are also pictured below. This was hard evidence that organisms looked very different in the past. It suggested that living things — like the Earth’s surface — change over time.
The Galápagos Islands
Darwin’s most important observations were made on the Galápagos Islands (shown on the map above of the Beagle voyage). The Galápagos Islands are a group of 16 small volcanic islands that are 966 kilometers (600 miles) off the west coast of South America. Individual Galápagos islands differ from one another in important ways. Some are rocky and dry; others have better soil and more rainfall. Darwin noticed that the plants and animals on the different islands also differed. For example, the giant tortoises on one island had saddle-shaped shells, whereas those on another island had dome-shaped shells, as you can see in the photos below. People who lived on the islands could even tell which island a tortoise came from by its shell. This started Darwin thinking about the origin of species. He wondered how each island came to have its own type of tortoise.
Other Influences on Darwin
Science, like evolution, always builds on the past. Darwin didn’t develop his theory completely on his own. He was influenced by the ideas of earlier thinkers.
Writings of Earlier Scientists
Three scientists whose writings influenced Darwin were Lamarck, Lyell, and Malthus.
1. Jean Baptiste Lamarck (1744–1829) was an important French naturalist. He was one of the first scientists to propose that species change over time. However, Lamarck was wrong about how species change. His idea of the inheritance of acquired characteristics is incorrect. Traits an organism develops during its own life cannot be passed on to offspring, as Lamarck believed.
2. Charles Lyell (1797–1875) was a well-known English geologist. Darwin took his book, Principles of Geology, with him on the Beagle. In the book, Lyell argued that gradual geological processes have slowly shaped Earth’s surface over very long periods of time. From this, Lyell inferred that Earth must be far older than most people believed.
3. Thomas Malthus (1766–1834) was an English economist. He wrote an essay titled On Population. In the essay, Malthus argued that human populations grow faster than the resources they depend on. When populations become too large, famine and disease break out. In the end, this keeps populations in check by killing off the weakest members.
Knowledge of Artificial Selection
These weren’t the only influences on Darwin. He was also aware that humans could breed plants and animals to have useful traits. By selecting which plants or animals were allowed to reproduce, they could change an organism’s traits over time. The pigeons in the figure below are good examples. Darwin called this type of change in organisms artificial selection. He used the word artificial to distinguish it from natural selection.
Darwin Develops His Theory
Darwin spent many years thinking about the work of Lamarck, Lyell, and Malthus; what he had seen on his voyage; and what he knew about artificial selection. What did all this mean? How did it all fit together? Eventually, it all came together in his theory of evolution by natural selection. It’s easy to see how these influences helped shape Darwin’s ideas, although it actually took Darwin years to formulate his theory. His reasoning went like this:
1. Like Lamarck, Darwin assumed that species can change over time. The fossils he found helped convince him of that.
2. From Lyell, Darwin saw that Earth and its life were very old. Thus, there had been enough time for evolution to produce the great diversity of life that Darwin had observed.
3. From Malthus, Darwin knew that populations could grow faster than their resources. This “overproduction of offspring” led to a “struggle for existence,” in Darwin’s words.
4. From artificial selection, Darwin knew that some offspring have chance variations that can be inherited. In nature, offspring with certain variations might be more likely to survive the “struggle for existence” and reproduce. If so, they would pass their favorable variations to their offspring.
5. Darwin coined the term fitness to refer to an organism’s relative ability to survive and produce fertile offspring. Nature selects the variations that are most useful. Therefore, he called this type of selection natural selection.
6. Darwin knew artificial selection could change domestic species over time. He inferred that natural selection could also change wild species over time. In fact, he thought that if a species changed enough, it might evolve into a new species.
It's Wallace’s Theory Too
Did you ever hear the saying that “great minds think alike?” It certainly applies to Charles Darwin and another English naturalist named Alfred Russel Wallace. Wallace lived at about the same time as Darwin and also traveled to distant places to study nature. Wallace wasn’t as famous as Darwin, but he developed basically the same theory of evolution. While working in what is now Malaysia, Wallace sent Darwin a paper he had written explaining his evolutionary theory. Wallace's ideas served to confirm what Darwin already thought. It also pushed Darwin to finish and publish his book, On the Origin of Species. Published in 1859, the book changed science forever. It clearly spelled out Darwin’s theory of evolution by natural selection and provided convincing arguments and evidence to support it.
Applying Darwin's and Wallace’s Theory
The following example applies Darwin’s and Wallace's theory of evolution by natural selection. It explains how giraffes came to have such long necks, like those shown in the photo below.
• In the past, giraffes had short necks. But there was a chance variation in neck length. Some giraffes had necks a little longer than the average.
• Then, as now, giraffes fed on tree leaves. Perhaps the climate became drier, and leaves became scarcer. There would be more giraffes than the trees could support. Thus, there would be a “struggle for existence.”
• Giraffes with longer necks had an advantage. They could reach leaves other giraffes could not. Therefore, long-necked giraffes were more likely to survive and reproduce. In other words, they had greater fitness.
• These giraffes passed the long-neck trait to their offspring. With each successive generation, the population contained giraffes with longer necks. Eventually, all the giraffes had very long necks.
Feature: Reliable Sources
In the first chapter of his book On the Origin of Species, Charles Darwin discussed how artificial selection, also called selective breeding, had been successful in changing the traits of animals, including pigeons, cats, cattle, and dogs. He used this discussion as a springboard to introduce his idea of natural selection as well as to provide support for it.
The use of selective breeding to change the traits of other species has a very long history. In fact, archaeological evidence indicates that selective breeding of both plants and animals began as early as 10,000 years ago in the Middle East when previous hunter-gatherers began to domesticate animals and cultivate cereal plants. Around this time, changes in climate led to increasing drought, which forced people to concentrate around permanent water sources. These population concentrations could not be supported by wild animals and plants in the vicinity, providing a stimulus for the invention of agriculture and the use of selective breeding to increase the amount of available food. For thousands of years, species of plants such as wheat and rice and of animals such as goats and sheep were selectively bred and changed from their wild ancestors.
In the New World, the wild grain called teosinte, pictured on the left in Figure \(7\), was selectively bred by Native Americans to produce larger and more numerous edible kernels. The result was modern maize (commonly called corn), shown on the right in the same picture. Teosinte was very small with fewer grains on it. The modern corn is bulky and with a lot more grain on it. After maize was created, it spread across the Americas and was introduced to Europe by European explorers and traders. Today, maize is still a dietary staple and the most widely grown grain crop in the Americas.
The wild ancestors of domesticated wheat and rice were easy to identify because the modern species resemble their wild counterparts. However, that wasn't the case with maize, which looks very different from teosinte. Maize also appeared quite suddenly in the archaeological record, so its origin has been of special interest.
Go online to learn more about the selective breeding of teosinte to maize. Use only reliable sources such as university websites to find answers to the following questions:
1. Where and when was teosinte selectively bred to produce maize?
2. How did the change from wild teosinte to modern maize occur so rapidly?
3. What is the genetic basis of this change?
Review
1. State Darwin’s theory of evolution by natural selection.
2. Describe two observations Darwin made on his voyage on the Beagle that helped him develop his theory of evolution.
3. What is the inheritance of acquired characteristics? Which scientist developed this mistaken idea?
4. What is artificial selection? How does it work?
5. How did Alfred Russel Wallace contribute to the theory of evolution by natural selection?
6. Apply Darwin’s theory of evolution by natural selection to a specific case. For example, explain how Galápagos tortoises could have evolved saddle-shaped shells.
7. Why did Darwin’s observations of Galápagos tortoises cause him to wonder how species originate?
8. Explain how the writings of Charles Lyell and Thomas Malthus helped Darwin develop his theory of evolution by natural selection.
9. If a person builds big muscles due to a special diet and a lot of weightlifting, are big muscles a trait that will be automatically passed down to their children? Why or why not?
10. If a hypothetical ecosystem had unlimited resources available for all the organisms living in it, how do you think this would affect evolution?
11. What is the best definition of “fitness” in terms of evolution?
1. The amount of lean muscle mass in an organism
2. The ability of an organism to exercise for a long period of time
3. An organism’s ability to survive to an old age
4. An organism’s ability to survive and produce fertile offspring
12. In natural selection, organisms are selected by ___________ ; in artificial selection, organisms are selected by __________ .
13. Explain why naturally occurring variations between individuals are important for evolution.
14. True or False. Modern maize evolved from teosinte through natural selection.
15. True or False. The theory of evolution states that living organisms on earth all evolved at once and then stopped changing.
16. True or False. Fossils of extinct animals are one type of evidence that supports Darwin’s theory of evolution.
Attributions
1. Rim of the Grand Canyon by presumed Ratte~commonswiki, CC BY 2.5 via Wikimedia Commons
2. Voyage of the Beagle by Sémhur, licensed CC BY-SA 4.0 via Wikimedia Commons
3. Compilation by CK-12 based on
1. Galapagos Marine Iguana by A.Davey, licensed CC BY 2.0 via Flickr.com
2. Blue-footed Booby by Nicolas de Camaret, licensed CC BY 2.0 via Flickr.com
3. Megatherium americanum by LadyofHats, released into the public domain via Wikimedia Commons
4. Compilation by CK-12 based on
1. Galapagos Giant Tortoise by Nicolas de Camaret, licensed CC BY 2.0 via Flickr.com
2. Geochelone nigra by Catriona MacCallum, CC BY 2.5 via Wikimedia Commons
5. Compilation of pigeons by Suzanne Wakim licensed CC BY-SA 4.0 based on
1. Paloma bravía by Diego Delso, licensed CC BY-SA 4.0 via Wikimedia Commons
2. Silesian cropper by jim gifford, licensed CC BY-SA 2.0 via Wikimedia Commons
3. Fantail by jim gifford, licensed CC BY-SA 2.0 via Wikimedia Commons
6. Samburu reticulated giraffe by Dan Lundberg, licensed CC BY-SA 2.0 via Wikimedia Commons
7. Corn selection by John Doebley, CC BY 2.5 via Wikimedia Commons
8. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/09%3A_Biological_Evolution/9.2%3A_Darwin_Wallace_and_the_Theory_of_Evolution_by_Natural_Selection.txt |
A Horse Is a Horse, of Course, of Course
This drawing was created in 1848, but it's likely that you recognize the animal it depicts as a horse. Although horses haven't changed that much since this drawing was made, they have a long evolutionary history during which they changed significantly. How do we know? The answer lies in the fossil record.
Evidence from Fossils
Fossils are a window into the past. They provide clear evidence that evolution has occurred. Scientists who find and study fossils are called paleontologists. How do they use fossils to understand the past? Consider the example of the horse, outlined in figure \(2\). Fossils spanning a period of more than 50 million years show how the horse evolved.
The oldest horse fossils show what the earliest horses were like. They were only 0.4 m tall, or about the size of a fox, and they had four long toes. Other evidence shows they lived in wooded marshlands, where they probably ate soft leaves. Over time, the climate became drier, and grasslands slowly replaced the marshes. Later fossils show that horses changed as well.
• They became taller, which would help them see predators while they fed in tall grasses. Eventually, they reached a height of about 1.6 m.
• They evolved a single large toe that eventually became a hoof. This would help them run swiftly and escape predators.
• Their molars (back teeth) became longer and covered with hard cement. This would allow them to grind tough grasses and grass seeds without wearing out their teeth.
Evidence from Living Species
Scientists can learn a great deal about evolution by studying living species. They can compare the anatomy, embryos, and DNA of modern organisms to help understand how they evolved.
Comparative Anatomy
Comparative anatomy is the study of the similarities and differences in the structures of different species. Similar body parts may be homologous structures or analogous structures. Both provide evidence for evolution.
Homologous structures are structures that are similar in related organisms because they were inherited from a common ancestor. These structures may or may not have the same function in the descendants. Figure \(3\) shows the upper appendages of several different mammals. They all have the same basic pattern of bones, although they now have different functions. All of these mammals inherited this basic bone pattern from a common ancestor.
Analogous structures are structures that are similar in unrelated organisms. The structures are similar because they evolved to do the same job, not because they were inherited from a common ancestor. For example, the wings of bats and birds, shown in the figure that follows, look similar on the outside and have the same function. However, wings evolved independently in the two groups of animals. This is apparent when you compare the pattern of bones inside the wings.
Comparative Embryology
Comparative embryology is the study of the similarities and differences in the embryos of different species. Similarities in embryos are likely to be evidence of common ancestry. All vertebrate embryos, for example, have gill slits and tails. All of the embryos in Figure \(4\), except for fish, lose their gill slits by adulthood, and some of them also lose their tail. In humans, the tail is reduced to the tail bone. Thus, similarities organisms share as embryos may no longer be present by adulthood. This is why it is valuable to compare organisms in the embryonic stage.
Vestigial Structures
Structures like the human tail bone are called vestigial structures. Evolution has reduced their size because the structures are no longer used. The human appendix is another example of a vestigial structure. It is a tiny remnant of a once-larger organ. In a distant ancestor, it was needed to digest food, but it serves no purpose in the human body today. Why do you think structures that are no longer used shrink in size? Why might a full-sized, unused structure reduce an organism’s fitness?
Comparing DNA
Darwin could compare only the anatomy and embryos of living things. Today, scientists can compare their DNA. Similar DNA sequences are the strongest evidence for evolution from a common ancestor. Look at the diagram in Figure \(5\). The diagram is a cladogram, a branching diagram showing related organisms. Each branch represents the emergence of new traits that separate one group of organisms from the rest. The cladogram in the figure shows how humans and apes are related based on their DNA sequences.
Evidence from Biogeography
Biogeography is the study of how and why organisms live where they do. It provides more evidence for evolution. Let’s consider the camel family as an example.
Biogeography of Camels: An Example
Today, the camel family includes different types of camels (Figure \(6\)). All of today’s camels are descended from the same camel ancestors. These ancestors lived in North America about a million years ago.
Early North American camels migrated to other places. Some went to East Asia via a land bridge during the last ice age. A few of them made it all the way to Africa. Others went to South America by crossing the Isthmus of Panama. Once camels reached these different places, they evolved independently. They evolved adaptations that suited them for the particular environment where they lived. Through natural selection, descendants of the original camel ancestors evolved the diversity they have today.
Island Biogeography
The biogeography of islands yields some of the best evidence for evolution. Consider the birds called finches that Darwin studied on the Galápagos Islands (Figure \(7\))). All of the finches probably descended from one bird that arrived on the islands from South America. Until the first bird arrived, there had never been birds on the islands. The first bird was a seed eater. It evolved into many finch species, each adapted for a different type of food. This is an example of adaptive radiation. This is the process by which a single species evolves into many new species to fill available ecological niches.
Eyewitnesses to Evolution
In the 1970s, biologists Peter and Rosemary Grant went to the Galápagos Islands to re-study Darwin’s finches. They spent more than 30 years on the project, but their efforts paid off. They were able to observe evolution by natural selection actually taking place.
While the Grants were on the Galápagos, a drought occurred, so fewer seeds were available for finches to eat. Birds with smaller beaks could crack open and eat only the smaller seeds. Birds with bigger beaks could crack open and eat seeds of all sizes. As a result, many of the smaller-beaked birds died in the drought, whereas birds with bigger beaks survived and reproduced. As shown in Figure \(8\), within 2 years, the average beak size in the finch population increased. In other words, evolution by natural selection had occurred.
Review
1. How do paleontologists learn about evolution?
2. Describe what fossils reveal about the evolution of the horse.
3. What are vestigial structures? Give an example.
4. Define biogeography.
5. Describe an example of island biogeography that provides evidence of evolution.
6. Humans and apes have five fingers they can use to grasp objects. Are these analogous or homologous structures? Explain.
7. Compare and contrast homologous and analogous structures. What do they reveal about evolution?
8. Why does comparative embryology show similarities between organisms that do not appear to be similar as adults?
9. What does a cladogram show?
10. Explain how DNA is useful in the study of evolution.
11. A bat wing is more similar in anatomical structure to a cat forelimb than to a bird wing. Answer the following questions about these structures.
1. Which pairs are homologous structures?
2. Which pairs are analogous structures?
3. Based on this, do you think a bat is more closely related to a cat or to a bird? Explain your answer.
4. If you wanted to test the answer you gave to part c, what is a different type of evidence you could obtain that might help answer the question?
12. True or False. Fossils are the only type of evidence that supports the theory of evolution.
13. True or False. Adaptive radiation is a type of evolution that produces new species.
Explore More
The Galapagos finches remain one of our world's greatest examples of adaptive radiation. Watch as these evolutionary biologists detail their 40-year project to document the evolution of these famous finches:
Attributions
1. Cheval de Dongolah by F Joseph Cardini, released into the public domain via Wikimedia Commons
2. Horse evolution by Mcy jerry licensed CC BY-SA 3.0 via Wikimedia Commons
3. Analogous & Homologous Structures by Vanessablakegraham, CC BY-SA 3.0 via Wikimedia Commons
4. Haeckel drawings by Romanes, G. J, released into the public domain via Wikimedia Commons
5. The great apes by Merrilydancingape, CC BY-SA 3.0 via Wikimedia Commons
6. Map by CK-12 foundation licensed CC BY-NC 3.0
7. Finch Beaks by Christopher Auyeung vua CK-12 foundation licensed CC BY-NC 3.0
8. Evolution of finch beaks by Lumen Learning, CC BY-SA 3.0
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/09%3A_Biological_Evolution/9.3%3A_Evidence_for_Evolution.txt |
School Days
Except for their plastic lunch coolers, you might think that this picture of children on their way to school came from the 1800s. In fact, the picture is a photograph that was taken in 2006. The children are part of a religious community called the Amish, whose founders first came to the U.S. in the 1700s. Amish people shun modern conveniences such as electricity and automobiles. Their lives are more similar to the lives of their founders than to those of most other people in the U.S. today. As you will learn when you read this concept, the Amish are an example of one way in which populations may evolve.
Genes in Populations
Individuals do not evolve because their genes do not change over time. Instead, evolution occurs at the level of the population. A population consists of organisms of the same species that live in the same area. In terms of evolution, the population is assumed to be a relatively closed group. This means that most mating takes place within the population. Evolutionary change that occurs over relatively short periods of time within populations is called microevolution. The science that focuses on evolution within populations is population genetics. It is a combination of evolutionary theory and Mendelian genetics.
The Gene Pool
The genetic makeup of an individual is the individual’s genotype. A population consists of many individuals and therefore many genotypes. All the genotypes together make up the population’s gene pool. The gene pool consists of all the genes of all the members of the population. For each gene, the gene pool includes all the different alleles of the gene that exist in the population. An allele is referred to as a version of a gene. For a given gene, the population is characterized by the frequency of the different alleles in the gene pool. Allele frequency is how often an allele occurs in a gene pool relative to the other alleles for the same gene.
Forces of Evolution
The factors that cause allele frequencies to change are called the forces of evolution. There are four such forces: mutation, gene flow, genetic drift, and natural selection.
Genetic Drift
Genetic drift is a random change in allele frequencies that occurs in a small population. When a small number of parents produce just a few offspring, allele frequencies in the offspring may differ, just by chance, from allele frequencies in the parents. This is like tossing a coin. If you toss a coin just a few times, you may, by chance, get more or less than the expected 50 percent heads and 50 percent tails. Due to such chance variations in small populations, allele frequencies drift over time.
There are two special conditions under which genetic drift occurs. They are called the bottleneck effect and founder effect.
1. The bottleneck effect occurs when a population suddenly gets much smaller. This might happen because of a natural disaster such as a forest fire or disease epidemic. By chance, allele frequencies of the survivors may be different from those of the original population.
2. The founder effect occurs when a few individuals start or found a new population. By chance, allele frequencies of the founders may be different from allele frequencies of the population they left. An example of the founder effect occurred in the Amish, as described in figure \(2\).
Mutation
Mutation creates new genetic variation in a gene pool. It is how all new alleles first arise. In sexually reproducing species, the mutations that matter for evolution are those that occur in gametes. Only these mutations can be passed to offspring. For any given gene, the chance of a mutation occurring in a given gamete is very low. Thus, mutations alone do not have much effect on allele frequencies. However, mutations provide the genetic variation needed for other forces of evolution to act.
Gene Flow
Gene flow occurs when individuals move into or out of a population. If the rate of migration is high, this can have a significant effect on allele frequencies. Allele frequencies may change in the population the migrants leave as well as in the population the migrants enter. An example of gene flow occurred during the Vietnam War in the 1960s and 1970s. Many young American servicemen had children with Vietnamese women. Most of the servicemen returned to the United States after the war. However, they left copies of their genes behind in their offspring. In this way, they changed the allele frequencies in the Vietnamese gene pool. Do you think the gene pool of the U.S. was also affected? Why or why not?
Natural Selection
Natural selection occurs when there are differences in fitness among members of a population. As a result, some individuals pass more genes to the next generation than do other members of the population. This causes allele frequencies to change over time. The example of sickle cell anemia, which is shown in the following table and described below, shows how natural selection can keep even a harmful allele in a gene pool.
Table \(1\): Sickle Cell Anemia and Natural Selection
Genotype Phenotype Fitness
AA 100% normal hemoglobin Somewhat reduced fitness because of no resistance to malaria
AS Enough normal hemoglobin to prevent sickle-cell anemia Highest fitness because of resistance to malaria
SS 100% abnormal hemoglobin, causing sickle-cell anemia Greatly reduced fitness because of sickle-cell anemia
The allele (S) for sickle cell anemia is a harmful, autosomal recessive allele. It is caused by a mutation in the normal allele (A) for hemoglobin (the oxygen-carrying protein on red blood cells). Malaria is a deadly tropical disease that is common in many African populations. Heterozygotes (AS) with the sickle cell allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool.
The sickle cell example shows that fitness depends on phenotypes and also on the environment. What do you think might happen if malaria were to be eliminated in an African population with a relatively high frequency of the S allele? How might the fitness of the different genotypes change? How might this affect the frequency of the S allele?
The sickle cell trait is controlled by a single gene. Natural selection for polygenic traits, which are controlled by multiple genes, is more complex, although it is less complicated if you consider just phenotypes for polygenic traits rather than genotypes. There are three major ways that natural selection can affect the distribution of phenotypes for a polygenic trait. The three ways are shown in the graphs in Figure \(3\).
1. Disruptive selection occurs when phenotypes in the middle of the range are selected against. This results in two overlapping phenotypes, one at each end of the distribution. An example is a sexual dimorphism. This refers to differences between the phenotypes of males and females of the same species. In humans, for example, males and females have different average heights and body shapes.
2. Stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against. This narrows the range of variation. An example is human birth weight. Babies that are very large or very small at birth are less likely to survive, and this keeps birth weight within a relatively narrow range.
3. Directional selection occurs when one of two extreme phenotypes is selected for. This shifts the distribution toward that extreme. This is the type of natural selection that the Grants observed in the beak size of Galápagos finches. Larger beaks were selected for during drought, so beak size increased over time.
Feature: Human Biology in the News
Recently reported research may help solve one of the most important and long-lasting mysteries of human biology. The mystery is why people with the AS genotype for sickle cell hemoglobin are protected from malaria. As you read above, their sickle cell hemoglobin gives them higher fitness in malaria areas than normal homozygotes (AA) who have only normal hemoglobin.
The malaria parasite and its mosquito vector were discovered in the late 1800s. The genetic basis of sickle cell hemoglobin anemia and the resistance to malaria it confers were discovered around 1950. Since then, scientists have assumed, and some evidence has suggested, that the few sickle-shaped red blood cells of heterozygotes make them less hospitable hosts for the malaria parasite than the completely normal red blood cells of AA homozygotes. This seems like a reasonable hypothesis, but is it the correct one? The new research suggests a different hypothesis.
Working with genetically engineered mice as model organisms, researchers in Portugal discovered that an enzyme that produces the gas carbon monoxide is expressed at much higher levels in the presence of sickle cell hemoglobin than normal hemoglobin. Furthermore, the gas seems to protect the infected host from developing the lesions and symptoms of malaria, even though it does not seem to interfere with the life cycle of the malaria parasite in red blood cells. These findings may lead to new therapies for treating malaria, which is still one of the most serious public health problems in the world. The findings may also shed light on other abnormal hemoglobin variants that are known to protect against malaria.
Review
1. Why are populations, rather than individuals, the units of evolution?
2. What is a gene pool?
3. List and define the four forces of evolution.
4. Why is mutation needed for evolution to occur, even though it usually has little effect on allele frequencies?
5. What is the founder effect? Give an example.
6. Identify three types of natural selection for polygenic traits.
7. Explain why genetic drift is most likely to occur in a very small population.
8. In some species, females prefer to mate with males that have certain genetically determined characteristics, such as bright coloration or a large, showy tail. How will this alter allele frequency in a population?
9. Which of the following may cause genetic drift?
1. A natural disaster
2. A large population where members mate with each other and also with new migrants that come into the population.
3. An island with no birds that becomes populated by a small number of a species of bird.
4. Both A and C
10. True or False. Allele frequencies can change within an organism.
11. True or False. Most populations on Earth are in Hardy-Weinberg equilibrium.
12. True or False. Genotype frequency can change if there is migration into or out of the population.
Attributions
1. Amish on their way to school by Gladjoboy, licensed CC BY 2.0 via Wikimedia Commons
2. Polydactyly by Baujat G, Le Merrer M. CC BY 2.0 via Wikimedia Commons
3. Selection type chart, by Azcolvin429, CC BY-SA 3.0; via Wikimedia.org
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/09%3A_Biological_Evolution/9.4%3A_Microevolution.txt |
An Evolutionary "Arms" Race
The garter snake in Figure \(1\) preys on a variety of small animals, including small amphibians called rough-skinned newts. The newts produce a powerful toxin that is concentrated in their skin. Garter snakes have evolved resistance to this toxin through a series of lucky genetic mutations, allowing them to safely prey upon the newts. The predator-prey relationship between these animals has created an evolutionary "arms" race. It has resulted in extremely high toxin levels in the newts and ever greater resistance to the toxin in the snakes. This is an example of the coevolution of two species. Coevolution is a type of macroevolution.
What Is Macroevolution?
Macroevolution is an evolution that occurs at or above the level of the species. It is the result of microevolution taking place over many generations. Macroevolution may involve evolutionary changes in two interacting species, as in coevolution, or it may involve the emergence of one or more brand new species.
Origin of Species
One of the main topics in macroevolution is how new species arise. The process by which a new species evolves is called speciation. How does speciation occur? How does one species evolve into two or more new species? To understand how a new species forms, it is important to review what a species is. A species is a group of organisms that can breed and produce fertile offspring together in nature. For a new species to arise, some members of a species must become reproductively isolated from the rest of the species. This means they can no longer interbreed with other members of the species. How does this happen? Usually, they become geographically isolated first.
Allopatric Speciation
Assume that some members of a species become geographically separated from the rest of the species. If they remain separated long enough, they may evolve genetic differences. If the differences prevent them from interbreeding with members of the original species, they have evolved into a new species. Speciation that occurs in this way is called allopatric speciation. An example of allopatric speciation involves Kaibab squirrels that live on the rim of the Grand Canyon, as shown in figure \(2\).
Notes about these two groups of squirrels:
• Kaibab squirrels are found only on the north rim of the Grand Canyon, on the Kaibab Plateau. Abert's squirrels occupy a larger area on the south rim of the Grand Canyon.
• Kaibab squirrels became geographically isolated from Abert's squirrels, which are found on the south rim of the canyon.
• In isolation, Kaibab squirrel« evolved distinct characteristics, such as a complete whitetail.
• Abert's squirrels are the original species from which Kaibab squirrels diverged. Kaibab squirrels are currently classified as a subspecies of Abert's squirrels.
• Kaibab squirrels may eventually become different enough to be classified as a separate species.
Sympatric Speciation
Less often, a new species arises without geographic separation. This is called sympatric speciation.
1. Hawthorn flies lay eggs in hawthorn trees. The eggs hatch into larvae that feed on hawthorn fruits. Both the flies and trees are native to the U.S.
2. Apple trees were introduced to the U.S. by European settlers in the 1600s. Now, apple trees often grow near hawthorn trees. Some hawthorn flies started to lay eggs in nearby apple trees. When the eggs hatched, the larvae fed on apples.
3. Over time, the two fly populations — those that feed on hawthorn fruits and those that feed on apples — evolved reproductive isolation because they breed at different times. Their breeding season matches the season when apples or hawthorn fruits mature.
4. Because they rarely interbreed, the two populations of flies are evolving other genetic differences. They appear to be in the process of becoming separate species. As this example shows, behaviors, as well as physical traits, may evolve and lead to speciation.
Coevolution
Evolution generally occurs in response to changes in the environment. Environmental change often involves other species of organisms. In fact, many species evolve along with other species with which they interact. This is called coevolution. As one species changes, the other species must also change in order to adapt. The coevolution of rough-skinned newts and garter snakes is described above. Many other cases of coevolution occur in flowering plants and the species that pollinate them. The flowering plant and hummingbird in Figure \(4\) are an example. They have evolved matching structures. The tubular flowers of the plant are matched by the long, narrow beak of the hummingbird.
Timing of Macroevolution
Is evolution slow and steady? Or does it occur in fits and starts? It may depend on what else is going on, such as changes in climate or geologic conditions.
• When climate and geologic conditions are stable, evolution may occur steadily and gradually. This is how Darwin thought evolution occurred. This model of the timing of evolution is called gradualism.
• When climate or geologic conditions are changing, evolution may occur more quickly. Long periods of little change may be interrupted by bursts of relatively rapid change. This model of the timing of evolution is called punctuated equilibrium. It is generally better supported by the fossil record than is gradualism.
Review
1. Define speciation.
2. Describe how allopatric speciation occurs.
3. What is gradualism? When is it most likely to apply?
4. Describe the timing of evolutionary change according to the punctuated equilibrium model.
5. Why is sympatric speciation less likely to occur than allopatric speciation?
6. Why would macroevolution occur more quickly when there are major changes in the environment, such as changing the climate or geologic conditions, than when the environment is stable?
7. What is reproductive isolation? Why is it necessary for speciation to occur?
8. Kaibab squirrels are an example of what kind of speciation?
9. Imagine there is a large lake that dries up in certain regions, creating several smaller, separate lakes. The original lake had a particular species of fish and some fish got trapped in each of the smaller lakes as the large lake dried up.
1. Is there a greater chance of speciation in the fish in the smaller, separate lakes or in the original large lake? Explain your answer.
2. If new fish species evolve from the original species in the small, separate lakes, would this be sympatric or allopatric speciation? Explain your answer.
3. If speciation occurred in the small lakes as described in part b, and then flooding occurred and the small lakes joined to become one large lake again, do you think the fish are likely to become one species again? Why or why not?
10. True or False. Speciation due to two populations breeding at different times in the same area is an example of sympatric speciation.
11. True or False. Coevolution always occurs between individuals of the same species.
12. Explain what the “punctuated” and “equilibrium” periods are in “punctuated equilibrium”.
Explore More
There is a dizzying diversity of species on our planet. From genetic evidence, we know that all of those species evolved from a single ancient ancestor. But how does one species split into many? Through the evolutionary process of speciation. This video illustrates the speciation process in birds to help you understand the basis of the earth's biodiversity:
Attributions
1. Coast garter snake by Steve Jurvetson, licensed CC BY 2.0 via Wikimedia Commons
2. Allopatric speciation by CK-12 released into the public domain
3. Sympatric speciation composite by Mandeep Grewal, licensed CC BY-SA 2.0
1. Apple maggot by Joseph Berger CC BY 3.0 via forestry images
2. Hawthorn berries by Andrew Smith, licensed CC BY-SA 2.0 via Wikimedia Commons
3. Apples on a tree by CSIRO, CC BY 3.0 via Wikimedia Commons
4. Purple-throated carib hummingbird by Charlesjsharp CC BY 3.0 via Kiddle
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/09%3A_Biological_Evolution/9.5%3A_Macroevolution.txt |
Myths About Earth
This interesting image is a 19th-century representation of Earth that is based on an ancient Hindu myth. According to the myth, Earth rests on the backs of elephants, which in turn stand on the back of a giant turtle. Virtually all human cultures and religions have developed myths about Earth and its origins. For example, until fairly recently, many Westerners thought that Earth was created in a day and that this occurred just a few thousand years ago. However, a diversity of evidence has since convinced the scientific community that Earth actually formed by natural processes from stardust a mind-boggling 4.5 to 4.6 billion years ago. Evidence also suggests that life first appeared on Earth up to 4 billion years ago and has been evolving ever since.
Earth in a Day
It can be difficult to wrap your mind around such vast amounts of time as the age of Earth and its early life forms. A useful way for envisioning the relative amounts of time that passed between Earth's origin and important events in biological evolution is to condense the total period of time to a 24-hour day, as shown in Figure \(2\). On this scale, Earth would have formed at midnight, and the first life would have appeared at about 3:00 a.m. Humans would have appeared only during the last minute of the day. If we are such newcomers on planet Earth, how do we know about the vast period of time that went before us? How have we learned about the distant past?
The Fossil Record
Much of what we know about the history of life on Earth is based on the fossil record, so this is an extremely important tool in the study of evolution. The fossil record is the record of life that unfolded over four billion years on Earth as reconstructed from the discovery and analysis of fossils. Fossils are the preserved remains or traces of organisms that lived in the past. The soft parts of organisms almost always decompose quickly after death. On occasion, the hard parts — mainly bones, teeth, or shells — remain long enough to mineralize and form fossils. An example of a complete fossil skeleton is pictured inFigure \(3\).
To be preserved as fossils, remains must be covered quickly by sediments or preserved in some other way. For example, they may be frozen in glaciers or trapped in tree resin or rock, like the frog shown in Figure \(4\). Sometimes traces of organisms — such as footprints or burrows — are preserved. The conditions required for fossils to form rarely occur. Therefore, the chance of any given organism being preserved as a fossil is extremely low.
In order for fossils to “tell” us the story of life, their chronology must be established. This means that fossils must be dated. Only then can they help scientists reconstruct how life changed over time. Fossils can be dated in two different ways, called relative dating and absolute dating.
• Relative dating determines which of two fossils is older or younger than the other, but not their age in years. Relative dating is based on the positions of fossils in rock layers. Lower layers were laid down earlier, so they are assumed to contain older fossils. This is illustrated in the figure below.
• Absolute dating determines how long ago a fossil organism lived, giving the fossil age in years. Absolute dating may be based on the amount of carbon-14 or other radioactive elements that remains in a fossil.
Molecular Clocks
Molecular clocks are also valuable tools for studying evolution. A molecular clock uses DNA sequences (or the amino acid sequence of proteins that DNA encodes) to estimate how long it has been since related species diverged from a common ancestor. Molecular clocks are based on the assumption that mutations accumulate through time at a steady average rate for a given region of DNA. Species that have accumulated greater differences in their DNA sequences are assumed to have diverged from their common ancestor in the more distant past. Molecular clocks based on different regions of DNA may be used together for more accuracy. Look at the comparisons of DNA in the table below. Based on these data, which organism do you think shared the most recent common ancestor with humans?
Table \(1\): DNA similarities of chimpanzee, mouse, chicken, and fruit fly species is to human DNA.
Organism Similarity with Human DNA (percent)
Chimpanzee 98
Mouse 85
Chicken 60
Fruit Fly 44
Feature: Myth vs. Reality
Myth: Gaps in the fossil record disprove evolution.
Reality: Gaps in the fossil record, where transitional fossils between ancestral and descendant groups have not been found, are to be expected. The chances of organisms being fossilized are low. Some organisms do not preserve well, and conditions needed for fossilization are only rarely present. If evolution is occurring rapidly, the chances of transitional fossils forming are even lower. Even if fossils of transitional organisms do form, they must be discovered by researchers to be added to the fossil record. The vast majority of fossils have not been found. Researchers are studying the fossilization process to shed light on how much of the fossil record has not yet been discovered.
Fortunately, like fingerprints at a murder scene, the fossil record is just one type of evidence for evolution. In addition to fossils, molecular sequences and other types of evidence are all used together to reveal how life on Earth evolved.
Review
1. Based on a 24-hour day, at what time did mammals evolve? How much of Earth's past had already taken place by that time? When did the first living things evolve?
2. What is the fossil record?
3. Why is the fossil record incomplete?
4. Compare and contrast relative and absolute dating of fossils.
5. Explain what molecular clocks can reveal about the evolution of life.
6. Why is it important for the study of evolution to know a fossil’s relative age compared to another fossil?
7. If fossil A is located above fossil B and fossil B is located above fossil C in different rock layers, arrange the three fossils in order of their likely age, from oldest to youngest.
8. Which tool could you use to study the evolutionary relationships between species that are still alive?
1. Carbon-14 dating
2. Molecular clocks
3. Relative position in the fossil record
4. None of the above
9. Use the History of Earth in a Day model above to answer the following questions.
1. Which came first, free oxygen on Earth or the evolution of animals?
2. During which geologic period did multicellular life evolve?
3. About how much of Earth’s history had elapsed before eukaryotes evolved?
4. What is the name of our current era?
10. True or False. Fossils are always composed of actual tissue from extinct organisms.
11. True or False. Absolute dating of fossils is usually done using a molecular clock.
Explore More
What would it look like if we took Earth's 4.5 billion year history and stuffed it into a normal day's 24-hour time frame? Check it out here:
Carbon dating allows us to estimate the ages of
material. Learn more here:
Attributions
1. The Hindu Earth public domain via Wikimedia Commons
2. Geologic clock by Woudloper, released into the public domain via Wikimedia Commons
3. Thylacoleo skeleton by Karora, released into the public domain via Wikimedia Commons
4. Fossilized frog by Kevin Walsh from Oxford, England, licensed CC BY 2.0 via Wikimedia Commons
5. Relative dating of fossils by Jillcurie, CC BY-SA 3.0 via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/09%3A_Biological_Evolution/9.6%3A_Tools_for_Studying_Evolution.txt |
Got Lactase?
Do you remember this “got milk?” slogan from the 1990s? It was used in ads for milk in which celebrities were pictured wearing milk “mustaches.” While the purpose of the “got milk?” ads was to sell more milk, there is no denying that drinking milk can be good for one’s health. Milk is naturally high in protein and minerals. It can also be low in fat or even fat-free if treated to remove the lipids that naturally occur in milk. However, before you reach for a tall, cold glass of milk, you might want to ask yourself another question: “got lactase?”
Adaptation to Lactose
Do you drink milk? Or do you avoid drinking milk and consuming milk products because they cause you discomfort? If the latter is the case, then you may have trouble digesting milk.
Milk, Lactose, and Lactase
Milk naturally contains not only proteins and lipids; it also contains carbohydrates. Specifically, milk contains the sugar lactose. Lactose is a disaccharide (two-sugar) compound that consists of one molecule each of galactose and glucose, as shown in the structural formula below. Lactose makes up between 2 and 8 percent of milk by weight. The exact amount varies both within and between species.
Lactose in milk must be broken down into its two component sugars to be absorbed by the small intestine. The enzyme lactase is needed for this process, as shown in the diagram below. Human infants are almost always born with the ability to synthesize lactase. This allows them to readily digest the lactose in their mother’s milk (or in infant formula). However, in the majority of infants, lactase synthesis begins to decline at about 2 years of age, and less and less lactase is produced throughout childhood.
Lactose intolerance is the inability of older children and adults to digest lactose in milk. People who are lactose intolerant may be able to drink small quantities of milk without any problems, but if they try to consume larger amounts, they are likely to suffer adverse effects. For example, they may have abdominal bloating and cramping, flatulence (gas), diarrhea, nausea, and vomiting. The symptoms may occur from one-half to two hours after milk is consumed and are generally worse when the quantity of milk consumed is greater. The symptoms result from the inability of the small intestine to digest and absorb lactose, so the lactose is passed on to the large intestine, where normal intestinal bacteria start breaking it down through the process of fermentation. This process releases gas and causes other symptoms of lactose intolerance.
Lactose intolerance is actually the original and normal condition of the human species, as it is of all other mammalian species. Early humans were hunter-gatherers that subsisted on wild plant and animal foods. The animal foods may have included meat and eggs but did not include milk because animals had not been domesticated. Therefore, beyond the weaning period, milk was not available for people to drink in early human populations. It makes good biological sense to stop synthesizing an enzyme that the body does not need. After a young child is weaned, it is a waste of materials and energy to keep producing lactase when milk is no longer likely to be consumed.
Overall, an estimated 60 percent of the world’s adult human population is thought to be lactose intolerant today. You can see the geographic distribution of modern human lactose intolerance on the map in Figure \(4\). Lactose intolerance (dark blue) approaches 100 percent in populations throughout southern South America, southern Africa, and East and Southeast Asia.
Lactose intolerance is not considered to be a medical problem because its symptoms can be avoided by not consuming milk or milk products. Dietary control of lactose intolerance may be a matter of trial and error, however, because different people may be able to consume different quantities of milk before symptoms occur. If you are lactose intolerant, be aware that low-fat and fat-free milk may contain somewhat more lactose than full-fat milk because the former often have added milk solids that are relatively high in lactose.
Lactase Persistence
Lactase persistence is the opposite of lactose intolerance. People who are lactase persistent continue to produce the enzyme lactase beyond infancy and generally throughout life. As a consequence, they are able to digest lactose and drink milk at older ages without adverse effects. The map above can also be read to show where lactase persistence occurs today. Populations with a low percentage of lactose intolerance (including North Americans of European descent) have high percentages of lactase-persistent people.
Lactase persistence is a uniquely human trait that is not found in any other mammalian species. Why did lactase persistence evolve in humans? When some human populations began domesticating and keeping herds of animals, animal milk became a potential source of food. Animals such as cows, sheep, goats, camels, and even reindeer (Figure \(5\)) can be kept for their milk. These kinds of animal milk also contain lactose, so natural selection would be strong for any individuals who kept producing lactase beyond infancy and could make use of this nutritious food. Eventually, the trait of lactase persistence would increase in frequency and come to be the predominant trait in dairying populations.
It is likely that lactase persistence occurs as a result of both genes and the environment. Some people inherit genes that help them keep producing lactase after infancy. Geneticists think that several different mutations for lactase persistence arose independently in different populations within the last 10,000 years. Part of lactase persistence may be due to continued exposure to milk in the diet in childhood and adulthood. In other words, a person may be genetically predisposed to synthesize lactase at older ages because of a mutation but may need the continued stimulation of milk drinking to keep producing lactase.
Thrifty Gene or Drifty Gene?
Besides variation in lactase persistence, human populations may vary in how efficiently they use calories in the foods they consume. People in some populations seem to be able to get by on quantities of food that would be inadequate for others, so they tend to gain weight easily. What explains these differences in people?
Thrifty Gene Hypothesis
In 1962, human geneticist James Neel proposed the thrifty gene hypothesis. According to this hypothesis, so-called “thrifty genes” evolved in some human populations because they allowed people to get by on fewer calories and store the rest as body fat when food was plentiful. According to Neel’s hypothesis, thrifty genes would have increased in frequency through natural selection because they would help people survive during times of famine. People with the genes would be fatter and able to rely on their stored body fat for calories when food was scarce.
Such thrifty genes would have been advantageous in early human populations of hunter-gathers if food scarcity was recurrent stress. However, in modern times, when most people have access to enough food year-round, thrifty genes would no longer be advantageous. In fact, under conditions of plentiful food, having thrifty genes would predispose people to gain weight and develop obesity. They would also tend to develop a chronic disease associated with obesity, particularly type II diabetes. Diabetes mellitus is a disease that occurs when there are problems with the pancreatic hormone insulin, which normally helps cells take up glucose from the blood and controls blood glucose levels. In type II diabetes, body cells become relatively resistant to insulin, leading to high blood glucose. This causes symptoms including excessive thirst and urination. Without treatment, diabetes can lead to serious consequences, such as blindness and kidney failure.
Neel proposed his thrifty gene hypothesis not on the basis of genetic evidence for thrifty genes but as a possible answer to the mystery of why genes that seem to promote diabetes have not been naturally selected out of some populations. The mystery arose from observations that certain populations — such as South Pacific Islanders, sub-Saharan Africans, and southwestern Native Americans — developed high levels of obesity and diabetes after they abandoned traditional diets and adopted Western diets. This is reflected in figure \(6\) which shows the 2017-2018 rates of diabetes in the US. Prevalence of diagnosed diabetes was highest among American Indians/Alaska Natives (14.7%), people of Hispanic origin (12.5%), and non-Hispanic blacks (11.7%), followed by non-Hispanic Asians (9.2%) and non-Hispanic whites (7.5%). An important note for studies based on race: any two humans have 99.9% similar DNA. The 0.1% difference causes variation in physical traits that humans have used to construct races. Biologically, all humans belong to just one race. Different traits are selected in different environments due to natural selection and genetic drift.
Assessing the Thrifty Gene Hypothesis
One of the assumptions underlying the thrifty gene hypothesis is that human populations that recently developed high rates of obesity and diabetes after Western contact had a long history of recurrent famine. Anthropological evidence contradicts this assumption for at least some of the populations in question. For example, South Pacific Islanders have long lived in a “land of plenty,” with lush tropical forests year-round on islands surrounded by warm waters full of fish. Another assumption underlying the thrifty gene hypothesis is that hunter-gatherer people became significantly fatter during periods of plenty. Again, there is little or no evidence that hunter-gatherers traditionally deposited large fat stores when food was readily available.
Some geneticists have searched directly for so-called thrifty genes. Studies have revealed many genes with small effects associated with obesity or diabetes. However, these genes can explain only a few percentage points of the total population variation in obesity or diabetes.
The Drifty Gene and Other Hypotheses
Given the lack of evidence for the thrifty gene hypothesis, several researchers have suggested alternative hypotheses to explain population variation in obesity and diabetes. One hypothesis posits that susceptibility to obesity and diabetes may be a side effect of heat adaptation. According to this idea, some populations evolved lower metabolic rates as an adaptation to heat stress, because lower metabolic rates reduced the amount of heat that the body produced. The lower metabolic rates also predisposed people to gain excess weight and develop obesity and diabetes.
A thrifty phenotype hypothesis has also been proposed. This hypothesis suggests that individuals who have inadequate nutrition during fetal development might develop an insulin-resistant phenotype. The insulin-resistant phenotype would supposedly prepare these individuals for a life of famine, based on the environment within the womb. In a famine-free environment, however, the thrifty phenotype would lead to the development of diabetes.
The most recent alternative to the thrifty gene hypothesis is the drifty gene hypothesis proposed by biologist John Speakman. He argues that genes protecting humans from obesity were under strong natural selection pressure for a very long period of time while human ancestors were subject to the risk of predation. According to this view, being able to outrun predators would have been an important factor in selecting against fatness. When the risk of predation was lessened, perhaps as early as 2 million years ago, genes keeping fatness in check would no longer be selected for. Without selective pressure for these genes, their frequencies could change randomly due to genetic drift. In some populations, by chance, frequencies of the genes could decrease to relatively low levels, whereas in other populations the frequencies could be much higher.
Feature: Myth vs. Reality
Myth: Lactose intolerance is an allergy to milk.
Reality: Lactose intolerance is not an allergy because it is not an immune system response. Rather, it is a sensitivity to milk that is caused by lactase deficiency so the sugar in milk cannot be digested. Milk allergy does exist, but it is a different condition that occurs in only about 4 percent of people. It results when milk proteins (not milk sugar) trigger an immune reaction. How can you determine whether you have lactose intolerance or a milk allergy? If you can drink lactose-free milk without symptoms, it is likely that you are lactose intolerant and not allergic to milk. However, if lactose-free milk also produces symptoms, it is likely that you have a milk allergy. Note that it is possible to have both conditions.
Myth: If you are lactose intolerant, you will never be able to drink milk or consume other dairy products without suffering adverse physical symptoms.
Reality: Lactose intolerance does not mean that consuming milk and other dairy products is out of the question. Besides lactose-free milk, which is widely available, many dairy products have relatively low levels of lactose, so you may be able to consume at least small amounts of them without discomfort. For example, you may be able to consume milk in the form of yogurt without any problems because the bacteria in yogurt produce lactase that breaks down the lactose. Greek yogurt may be your best bet because it is lower in lactose, to begin with. Aged cheeses also tend to have relatively low levels of lactose because of the cheese-making process. Finally, by gradually adding milk or milk products to your diet, you may be able to increase your tolerance to lactose.
Review
1. Distinguish between the terms lactose and lactase.
2. What is lactose intolerance, and what percentage of all people have it?
3. Where and why did lactase persistence evolve?
4. What is the thrifty gene hypothesis?
5. How well is the thrifty gene hypothesis supported by evidence?
6. Describe an alternative hypothesis to the thrifty gene hypothesis.
7. Do you think that a lack of exposure to dairy products might affect a person’s lactase level? Why or why not?
8. Describe an experiment you would want to do or data you would want to analyze that would help to test the thrifty phenotype hypothesis. Remember, you are studying people, so be sure it is ethical! Discuss possible confounding factors that you should control for in this study, or that might affect the interpretation of your results.
9. Explain the relationship between insulin, blood glucose, and type II diabetes.
10. True or False. Lactose persistence evolved more recently than lactose intolerance.
11. True or False. The drifty gene hypothesis is dependent on the assumption that fatter people cannot run as effectively as thinner people.
12. What two ethnic groups in the U.S. have a particularly high rate of death from diabetes? What other types of data would you want to observe to determine whether certain ethnic groups are more susceptible to diabetes? Explain why this additional data would be helpful.
Explore More
Does obesity lead to diabetes, or could it be the other way around? Hear a provocative answer to this question in this moving TED talk:
Attributions
1. Got milk released into the public domain via Wikimedia Commons
2. Lactose Haworth by NEUROtiker released into the public domain via Wikimedia Commons
3. Adapted by Mandeep Grewal from Hydrolysis of lactose by Yikrazuul released into the public domain via Wikimedia Commons
4. Laktoseintoleranz by Rainer Zenz released into the public domain via Wikimedia Commons
5. Lapper og Reinsdyr by Nasjonalbiblioteket from Norway uploaded by Anne-Sophie Ofrim, licensed CC BY 2.0 via Wikimedia Commons
6. Diabetes graph and data by CDC, public domain
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/09%3A_Biological_Evolution/9.7%3A_Adaptation_in_Humans.txt |
Case Study Conclusion: Flu, from Pigs to You
In April 2009, the world was hit with a swine flu pandemic. The Centers for Disease Control estimates that within that first year, 43 to 89 million people worldwide contracted the swine flu and that it contributed to 8,870 to 18,300 deaths. Some people with swine flu were spared serious complications, such as Mateo, who you read about it at the beginning of this chapter. At the time, the swine flu spread rapidly because as a newly evolved viral strain, most people had no natural immunity against it, and the existing flu vaccine could not prevent it. But by November 2009, a swine flu vaccine was developed, and now it is included in the annual flu vaccine in the U.S. By August 2010, the World Health Organization declared the H1N1 swine flu pandemic to be over. The virus is still around, but because of the vaccine and the natural immunity of those who had the virus previously, its infection rate is no longer of pandemic proportions.
The swine flu virus appears to have originated in pigs and later evolved the ability to infect humans. How could this happen? Scientists think that a process called reassortment played a critical role. In reassortment, influenza viruses can exchange genetic material with each other if they have infected the same cells. This creates new combinations of genes, somewhat similar to the genetic mixing that occurs in sexual reproduction when two parents with different genes reproduce with each other. As you know, genes help dictate the characteristics of an organism, or in this case, a virus. Therefore, the production of novel combinations of genes due to viral reassortment can lead to the evolution of new viral characteristics.
In addition to reassortment, influenza viruses have other characteristics that cause them to evolve quickly. In contrast to sexual reproduction, the replication of viruses to produce new “offspring” particles is much more rapid. As you have learned in this chapter, evolution is typically a slow process that takes place over many generations. But if these generations are produced rapidly, as in the case of viruses and bacteria, it speeds the rate of evolution. Additionally, RNA viruses have a very high rate of genetic mutation. The rapid evolution of the influenza virus is one of the reasons why the annual seasonal flu vaccine is not always effective against every strain.
But why did this flu pandemic come from pigs? Pigs are actually an ideal “mixing bowl” for the evolution of influenza viruses because pigs can become infected with influenza viruses from other species, including birds and humans. Therefore, genetic reassortment can occur in pigs between viral strains that normally infect different species. This is what scientists think occurred to produce the 2009 H1N1 swine flu virus. The 2009 H1N1 has gene segments from the birds, humans, and two different pig influenza viruses, and is therefore called a “quadruple reassortant” virus. In the case of the 2009 H1N1, this resulted in a new influenza strain that could infect humans, and be passed directly from person to person.
Scientists do not know exactly when and where the 2009 H1N1 evolved, but they think that the reassortment event may have occurred several years prior to the 2009 pandemic. This is based on evidence gathered from “molecular evolution” techniques, which are similar to the molecular clock technique described in this chapter. Influenza viruses are known to mutate at a relatively steady rate. The genetic sequences of the new 2009 H1N1 strain were compared to the sequences in related, older influenza viruses to count the number of new mutations, in order to give an estimate of when the new viral strain evolved.
Probably one of the final events that resulted in the generation of the 2009 H1N1 virus was contact between North American and Eurasian pigs. This is because prior to 2009, there were “triple reassortant” variants of H1N1 with gene segments from a bird, human, and North American pig influenza already in existence. The 2009 H1N1 strain additionally contained gene segments from influenza from Eurasian pigs, resulting in the “quadruple reassortant” virus. Scientists think that contact between pigs from these different regions, through international trade or other methods of contact, could have created this new strain. As you have learned in this chapter, the migration of organisms to new locations as well as contact between different organisms can influence evolution in many ways. Some examples are the migration of ancestral camels throughout the world, the coevolution of flowers and their pollinators, and the “founder effect” of small populations that move to new locations, such as the Amish.
Along with fossils, comparative anatomy and embryology, DNA analysis, and biogeography, evidence for evolution includes direct observation of it occurring. Peter and Rosemary Grant observed evolution occurring in the change in beak size of Galápagos finches. The evolution of the swine flu virus is another example of evolution in action. Evolution is not just a thing of the past — it is an ongoing and important process that affects our ecosystem, species, and even our health. Like viruses, bacteria also evolve rapidly, and the evolution of antibiotic resistance in bacteria is a growing public health concern. You can see that evolution is very relevant to our lives today.
Chapter Summary
In this chapter, you learned about the theory of evolution, evidence for evolution, how evolution works, and the evolution of living organisms on Earth. Specifically, you learned:
• Darwin’s theory of evolution by natural selection states that living things with beneficial traits produce more offspring than others do. This leads to changes in the traits of living things over time.
• During his voyage on the Beagle, Darwin made many observations that helped him develop his theory of evolution, particularly on the Galápagos Islands.
• Darwin was influenced by other early thinkers, including Lamarck, Lyell, and Malthus. He was also influenced by his knowledge of artificial selection.
• Wallace’s paper on evolution confirmed Darwin’s ideas. It also pushed him to publish his book, On the Origin of Species. The book clearly spells out his theory and provides extensive evidence and well-reasoned arguments to support it.
• Fossils provide a window into the past and are evidence for evolution. Scientists who find and study fossils are called paleontologists.
• Scientists compare the anatomy, embryos, and DNA of living things to understand how they evolved. Evidence for evolution is provided by homologous and analogous structures.
• Biogeography is the study of how and why plants and animals live where they do, which provides additional evidence for evolution. On island chains, such as the Galápagos, one species may evolve into many new species to fill available niches. This is called adaptive radiation.
• Peter and Rosemary Grant re-studied Galápagos finches. During a drought in the 1970s, they were able to directly observe evolution occurring.
• Microevolution refers to evolution that occurs over a relatively short period of time within a population. Macroevolution refers to evolution that occurs at or above the level of species as the result of microevolution taking place over many generations.
• The population is the unit of evolution, and population genetics is the science that studies evolution at the population level. A population’s gene pool consists of all the genes of all the members of the population. For a given gene, the population is characterized by the frequency of different alleles in the gene pool.
• There are four forces of evolution: mutation, which creates new alleles; gene flow, in which migration changes allele frequencies; genetic drift, which is a random change in allele frequencies that may occur in a small population; and natural selection, in which allele frequencies change because of differences in fitness among individuals.
• New species arise in the process of speciation. Allopatric speciation occurs when some members of a species become geographically isolated and evolve genetic differences. If the differences prevent them from interbreeding with the original species, a new species has evolved. Sympatric speciation occurs without geographic isolation first occurring.
• Coevolution occurs when interacting species evolve together. An example is flowering plants and their pollinators.
• Darwin thought that evolution occurs steadily and gradually. This model of evolution is called gradualism. The fossil record better supports the model of punctuated equilibrium. In this model, long periods of little change are interrupted by bursts of relatively rapid change.
• The fossil record is the record of life on Earth as reconstructed from the discovery and analysis of fossils. It is one of the most important tools in the study of evolution, but it is incomplete because fossilization is rare. To be added to the fossil record, fossils must be dated using relative or absolute dating methods.
• Molecular clocks are additional tools for reconstructing how life on Earth evolved. Molecular clocks use DNA or protein sequences to estimate how much time has passed since related species diverged from a common ancestor.
• The geologic time scale is a timeline of Earth's history. It divides Earth's chronology into smaller units of time such as eons and eras that are based on major changes in geology, climate, and living things.
• Milk contains the sugar lactose, a disaccharide. Lactose must be broken down into its two component sugars to be absorbed by the small intestine, and the enzyme lactase is needed for this process.
• In about 60 percent of people worldwide, the ability to synthesize lactase and digest lactose declines after the first two years of life. These people become lactose intolerant and cannot consume much milk without suffering symptoms such as bloating, cramps, and diarrhea.
• In populations that herded milking animals for thousands of years, lactase persistence evolved. People who were able to synthesize lactase and digest lactose throughout life were strongly favored by natural selection. People who descended from these early herders generally still have lactase persistence. That includes many Europeans and European-Americans.
• Human populations may vary in how efficiently they use calories in food. Some people (especially South Pacific Islanders, Native Americans, and sub-Saharan Africans) seem to be able to get by on fewer calories than would be adequate for others, so they tend to easily gain weight, become obese, and develop diseases such as diabetes.
• The thrifty gene hypothesis answers the question of how genes for this ability could have evolved. It proposes that “thrifty genes” were selected for because they allowed people to use calories efficiently and store body fat when food was plentiful so they had a reserve to use when food was scarce. Thrifty genes become detrimental and lead to obesity and diabetes when food is plentiful all of the time.
• Several assumptions underlying the thrifty gene hypothesis have been called into question, and genetic research has been unable to actually identify thrifty genes. Alternate hypotheses to the thrifty gene hypothesis have been proposed, including the drifty gene hypothesis. The latter hypothesis explains variation in the tendency to become obese by genetic drift on neutral genes.
Chapter Summary Review
1. Data from Peter and Rosemary Grant’s study on the evolution of beak size in Galápagos finches is shown above. The top graph (1976) shows the distribution of beak size in the population before a drought, and the bottom graph (1978) shows beak size after the drought. The drought reduced seed availability. Finches with big beaks can crack open and eat seeds of all sizes, while finches with small beaks can only crack open and eat small seeds. Answer the following questions about this data.
1. How was the average beak size affected by the drought? Although scientists would calculate this mathematically, you may answer just based on your observation of the graphs.
2. Explain how natural selection and the “struggle for existence” likely changed the beak size in this population.
3. Is this an example of microevolution or macroevolution? Explain your answer.
4. Explain why variation is important for evolution by natural selection, using the data above as a specific example.
5. What do you notice about the distribution of beak sizes in the 1978 graph — are all the beaks one size? If not, why not?
6. Is the change in beak size shown here an example of stabilizing selection, disruptive selection, or directional selection?
2. Which of the following is an example of macroevolution?
1. Speciation
2. Coevolution
3. Structures that become larger in a population
4. A and B
3. Speciation is:
1. The movement of a species to a new niche
2. An evolution that occurs within a species
3. The evolution of a new species from an existing species
4. The development of analogous structures
4. True or False. An individual’s genotype is known as their gene pool.
5. True or False. New species can evolve without geographic separation.
6. True or False. In punctuated equilibrium, the periods of relatively little evolutionary change are shorter than the periods of dramatic change.
7. Describe one example of a major environmental change that influenced the evolution of life on Earth. This change could include climate change, geologic change, change in existing species, change in the atmosphere, etc.
8. Explain why mass extinction events often cause rapid evolutionary changes afterward.
9. Choose one. Species with homologous structures are (more/less) likely to be closely related than species with analogous structures.
10. Explain why the fossils of extinct animals provide evidence for evolution.
11. Which of the following is an example of evolution by natural selection?
1. Humans breeding dogs for certain characteristics
2. Bats developing wings as an adaptation for flight
3. A and B
4. None of the above
12. Compare and contrast Darwin’s theory of evolution by natural selection and Lamarck’s idea of inheritance of acquired characteristics.
13. Explain how microevolution and macroevolution relate to each other.
14. The fact that embryonic humans have gill slits is evidence for:
1. Coevolution
2. Evolution of analogous structures
3. Common ancestry of vertebrates
4. Gene flow
15. The study of allele frequencies in a group of the same species in the same time and place is known as _________ genetics.
16. Explain how biogeography can be used to study adaptive radiation.
Attributions
1. Antigenic shift by Mouagip derived from NIAID, public domain via Wikimedia Commons
2. Evolution of beak size by Jodi So via CK-12 licensed CC BY-NC 3.0
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/09%3A_Biological_Evolution/9.8%3A_Case_Study_Conclusion%3A_Flu_and_Chapter_Summary.txt |
This chapter outlines the organization of the human body and describes human cells, tissues, organs, organ systems, and body cavities. It also explains how organ systems interact and how feedback mechanisms maintain homeostasis in the body.
• 10.1: Case Study: Getting to Know Your Body
Looking at the photo of a football game above, you can see why it is so important that the players wear helmets. Football often involves forceful impact to the head as players tackle each other. This can cause damage to the brain - either temporarily as in the case of a concussion, or long-term and more severe types of damage. Helmets are critical to reduce the incidence of traumatic brain injuries (TBIs), but they do not fully prevent them.
• 10.2: Organization of the Body
This six-legged robot was created for research, but it looks like it might be fun to play with. It's obviously a complex machine. Think about some other, more familiar machines, such as power drills, washing machines, and lawn mowers. Each machine consists of many parts, and each part does a specific job, yet all the parts work together to perform certain functions.
• 10.3: Human Cells and Tissues
This photo looks like a close-up of an old-fashioned dust mop, and the object it shows has a somewhat similar function. However, the object is greatly enlarged in the photo. Can you guess what it is? The answer may surprise you.
• 10.4: Human Organs and Organ Systems
An organ is a collection of tissues joined in a structural unit to serve a common function. Organs exist in most multicellular organisms, including not only humans and other animals but also plants. In single-celled organisms such as bacteria, the functional equivalent of an organ is an organelle.
• 10.5: Human Body Cavities
The human body, like that of many other multicellular organisms, is divided into a number of body cavities. A body cavity is a fluid-filled space inside the body that holds and protects internal organs. Human body cavities are separated by membranes and other structures. The two largest human body cavities are the ventral cavity and dorsal cavity. These two body cavities are subdivided into smaller body cavities.
• 10.6: Interaction of Organ Systems
Communication among organ systems is vital if they are to work together as a team. They must be able to respond to each other and change their responses as needed to keep the body in balance. Communication among organ systems is controlled mainly by the autonomic nervous system and the endocrine system.
• 10.7: Homeostasis and Feedback
Homeostasis is the condition in which a system such as the human body is maintained in a more-or-less steady state. It is the job of cells, tissues, organs, and organ systems throughout the body to maintain many different variables within narrow ranges that are compatible with life. Keeping a stable internal environment requires continually monitoring the internal environment and constantly making adjustments to keep things in balance.
• 10.8: Case Study Conclusion: Pressure and Chapter Summary
As you learned in this chapter, the human body consists of many complex systems that normally work together efficiently like a well-oiled machine to carry out life's functions.
10: Introduction to the Human Body
Case Study: Under Pressure
Looking at the photo of a football game in Figure \(1\), you can see why it is so important that the players wear helmets because players may fall on their heads or on top of each other's heads. Football often involves forceful impact to the head as players tackle each other. This can cause damage to the brain — either temporarily as in the case of a concussion, or long-term and more severe types of damage. Helmets are critical to reducing the incidence of traumatic brain injuries (TBIs), but they do not fully prevent them.
Take the example of 43-year-old Dayo. As a former professional football player who also played in college and high school, Dayo sustained many high-impact head injuries over the course of their football playing years. Dayo uses they/ them pronouns. A few years ago, Dayo began experiencing a variety of troubling symptoms, including the loss of bladder control (i.e. the involuntary leakage of urine), memory loss, and difficulty in walking. Symptoms such as these are often signs of damage to the nervous system, which includes the brain, spinal cord, and nerves, but they can result from many different types of injuries or diseases that affect the nervous system. In order to treat Dayo properly, their doctors needed to do several tests to determine the exact cause of their symptoms. These included a spinal tap to see if they had an infection, and an MRI (magnetic resonance imaging) to see if there were any problems with their brain structure.
The MRI revealed the cause of Dayo’s symptoms. There are fluid-filled spaces within the brain called ventricles, and Dayo’s ventricles were enlarged compared to normal ventricles. Based on this observation combined with the results of other tests, Dayo’s doctor diagnosed them with hydrocephalus, a term that literally means “water head.” Hydrocephalus occurs when the fluid that fills the ventricles, called cerebrospinal fluid, builds up excessively. This causes the ventricles to become enlarged and puts pressure on the brain, which can cause a variety of neurological symptoms including the ones Dayo was experiencing. You can see the difference between normal ventricles and ventricles that are enlarged due to hydrocephalus in the illustration below. Notice how the brain becomes “squeezed” due to hydrocephalus in the image on the right.
Hydrocephalus often occurs at birth, due to genetic factors or events that occurred during fetal development. Because babies are born with skull bones that are not fully fused, the skull of a baby born with hydrocephalus can expand and relieve some of the pressure on the brain, as reflected in the enlarged head size shown above. But adults have fully fused, inflexible skulls, so when hydrocephalus occurs in an adult, the brain experiences all of the increased pressure.
Why did Dayo develop hydrocephalus? There are many possible causes of hydrocephalus in adults, including tumors, infections, hemorrhages, and TBIs. Given their repeated and long history of TBIs due to football, and the absence of any evidence of infection, tumor, or other cause, Dayo’s doctor thinks their head injuries were most likely responsible for their hydrocephalus.
Although hydrocephalus is serious, there are treatments. Read the rest of this chapter to learn about the cells, tissues, organs, cavities, and systems of the body, how they are interconnected, and the importance of keeping the body in a state of homeostasis, or balance. The amount of cerebrospinal fluid in the ventricles is normally kept at a relatively steady level, and the potentially devastating symptoms of hydrocephalus are an example of what can happen when a system in the body becomes unbalanced. At the end of the chapter, you will learn about Dayo’s treatment and prognosis.
Chapter Overview: Introduction to the Human Body
In this chapter, you will learn about the general organization and functions of the human body. Specifically, you will learn about:
• The organization of the body from atoms and molecules up through cells, tissues, organs, and organ systems.
• How organ systems work together to carry out the functions of life.
• The variety of different specialized cell types in humans, the four major types of human tissues, and some of their functions.
• What organs are and the 11 major organ systems of the human body.
• Spaces in the body called body cavities, and the organs they hold and protect.
• The tissues and fluid that protect the brain and spinal cord.
• How organ systems communicate and interact in body processes such as cellular respiration, digestion, the fight-or-flight response to stressors, and physical activities such as sports.
• How homeostasis is maintained to keep the body in a relatively steady state, and the problems that can be caused by loss of homeostasis, such as diabetes.
As you read the chapter, think about the following questions:
1. What is the normal function of cerebrospinal fluid?
2. What is a spinal tap and how does it test for infection?
3. In Dayo’s case, what organs and organ systems are probably affected by their hydrocephalus? What are some ways in which these organ systems interact?
4. The level of cerebrospinal fluid is normally kept in a state of homeostasis. What are other examples of types of homeostasis that keep your body functioning properly?
Attributions
1. Army vs. Navy Football game by U.S. Navy photo by Photographer’s Mate 2nd Class Jayme Pastoric, public domain via Wikimedia Commons
2. Hydrocephalus by CDC released into the public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/10%3A_Introduction_to_the_Human_Body/10.1%3A_Case_Study%3A_Getting_to_Know_Your_Body.txt |
A Fantastic Machine
This six-legged robot in Figure \(1\) was created for research, but it looks like it might be fun to play with. It’s obviously a complex machine. Think about some other, more familiar machines, such as power drills, washing machines, and lawnmowers. Each machine consists of many parts, and each part does a specific job, yet all the parts work together to perform certain functions. Many people have compared the human body to a machine, albeit an extremely complex one. Like real machines, the human body also consists of many parts that work together to perform certain functions, which in the case of the human body include keeping the organism alive. The human body may be the most fantastic machine on Earth, as you will discover when you learn more about it in this concept.
What the Human Machine Can Do
Imagine a machine that has all of the following attributes. It can generate a “wind” of 166 km/hr (100 mi/hr), and it can relay messages faster than 400 km/hr (249 mi/hr). It contains a pump that moves about a million barrels of fluid over its lifetime, and it has a control center that contains billions of individual components. The machine in question can even repair itself if necessary and not wear out for up to a century or more. It has all these abilities, and yet it consists mainly of water. What is it? It is the human body.
Organization of the Human Body
The human body is a complicated, highly organized structure that consists of trillions of parts that function together to achieve all the functions needed to maintain life. The biology of the human body incorporates the body’s structure, the study of which is called anatomy, and the body’s functioning, the study of which is called physiology.
The organization of the human body can be seen as a hierarchy of increasing size and complexity, starting at the level of atoms and molecules and ending at the level of the entire organism, which is an individual living thing. You can see the intervening levels of organization in Figure \(2\) and read about them in the figure and the sections that follow.
To study the smallest level of organization, scientists consider the simplest building blocks of matter: atoms and molecules. The chemical level of organization considers these two building blocks as atoms bond to form molecules with three-dimensional structures. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles such as the proton, electron, and neutron. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.
The cellular level is considered when a variety of molecules combine to form the fluid and organelles of a body cell. A cell is the smallest independently functioning unit of a living organism. Even bacteria, which are extremely small, independently living organisms, have a cellular structure. Each bacterium is a single cell. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells. A human cell, such as a smooth muscle cell, typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid together with a variety of tiny functioning units called organelles.
The tissue level can be studied when a community of similar cells forms a body tissue. A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function. For example, when many smooth muscle cells come together both structurally and functionally, these cells collectively form a layer of smooth muscle tissue.
An organ is an anatomically distinct body structure composed of two or more tissue types. Each organ performs one or more specific physiological functions. The human bladder, which is composed of smooth muscle tissue, transitional epithelial tissue, and several types of connective tissue serves the function of storing urine produced by the kidneys.
An organ system level is a group of organs that work together to perform major functions or meet the physiological needs of the body. In the organ example above, both the kidneys and the bladder are organs of the urinary system. The kidneys produce urine, which is moved to the bladder by the ureters. Urine can then leave the bladder, and the body, through the urethra. These four organs work together to rid the body of liquid waste.
Cells
The basic units of structure and function of the human body, as in all living things, are cells — an amazing 37 trillion of them by the time the average person reaches adulthood! Each cell carries out basic life processes that allow the body to survive. In addition, most human cells are specialized in structure and function to carry out other specific roles. In fact, the human body may consist of as many as 200 different types of cells, each of which has a special job to do. Just a few of these different human cell types are pictured in Figure \(3\). The cells in the figure have obvious differences in structure that reflect their different functions. For example, nerve cells have long projections sticking out from the body of the cell. These projections help them carry electrical messages to other cells.
Tissues
After the cell, the tissue is the next level of organization in the human body. A tissue is a group of connected cells that have a similar function. There are four basic types of human tissues: connective, epithelial, muscle, and nervous tissues. These four tissue types, which are shown in Figure \(4\), make up all the organs of the human body. Connective tissue is composed of cells that are suspended in a matrix. Epithelial tissue is mostly composed of cells that are tightly packed together in sheets. Muscle tissue is also composed of tightly packed cells and some types of muscle, such as the skeletal muscle shown in Figure \(4\) contains striation due to the organization of muscle fibers. Nervous tissue is composed of cells with long extensions.
Organs and Organ Systems
After tissues, organs are the next level of the organization of the human body. An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples of human organs include the heart, brain, lungs, skin, and kidneys. Human organs are organized into organ systems; the digestive system is shown in figure Figure \(5\). An organ system is a group of organs that work together to carry out a complex overall function. Each organ of the system does part of the larger job.
A Well-Oiled Machine
All of the organs and organ systems of the human body normally work together like a well-oiled machine. This is because they are closely regulated by the nervous and endocrine systems. The nervous system controls virtually all body activities, and the endocrine system secretes hormones that help to regulate these activities. Functioning together, the organ systems supply body cells with all the substances they need and eliminate their wastes. They also keep temperature, pH, and other conditions at just the right levels to support life.
Review
1. How is the human body like a complex machine?
2. Compare and contrast human anatomy and human physiology.
3. Summarize the hierarchical organization of the human body.
4. Relate cell structure to cell function, and give examples of specific cell types in the human body.
5. Define tissue, and identify the four types of tissues that make up the human body.
6. What is an organ? Give three examples of organs in the human body.
7. Define organ system, and name five organ systems in the human body.
8. True or False. How cells use oxygen is an example of physiology.
9. The organ system that secretes hormones is called the _______________ system.
10. A neuron is a:
1. specialized cell
2. unspecialized cell
3. an organ
4. an organ system
11. Which organ system’s function is to provide structure to the body and protect internal organs?
12. How is the human body regulated so all of its organs and organ systems work together
13. True or False. Organs consist of one or more types of tissue.
14. Give one example of how the respiratory and circulatory systems work together.
Explore More
The human body consists of more than just human cells, tissues, organs, and organ systems. It also includes a huge number of single-celled organisms that live inside the body and have a great and largely unexplored role in our health. You can learn more about the three pounds of microbes that you carry around with you by watching this eye-opening TED talk.
Ever wish you were as smart as a computer? Well, engineers wish they could build computers as smart as you. Learn more here:
Attributions
1. Six-legged walking robot LAURON IV by FZI Forschungszentrum Informatik Karlsruhe - Abteilung IDS, released into the public domain via Wikimedia Commons
2. Levels of organization by OpenStax, CC BY 4.0
3. Animals variety animal cells by Sunshineconnelly CC BY 3.0 via Wikimedia Commons
4. Four types of tissue by NIH, public domain via Wikimedia Commons
5. Digestive System by National Institute of Diabetes and Digestive and Kidney Disease, released into the public domain
6. Text adapted from
1. Human Biology by CK-12 licensed CC BY-NC 3.0
2. Anatomy by OERI CC BY 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/10%3A_Introduction_to_the_Human_Body/10.2%3A_Organization_of_the_Body.txt |
Dust Mop
This photo in Figure \(1\) looks like a close-up of an old-fashioned dust mop, and the object it shows has a somewhat similar function. However, the object is greatly enlarged in the photo. Can you guess what it is? The answer may surprise you. It is a scanning electron micrograph of human epithelial cells that line the bronchial passages. The floppy, dust-mop-like extensions are actually microscopic structures called cilia projecting from the outer surface of the epithelial cells. The function of the cilia is to trap dust, pathogens, and other particles in the air before it enters the lungs. The cilia also sway back and forth to sweep the trapped particles upward toward the throat, from which they can be expelled from the body.
Human Cells
Like the ciliated bronchial cells in the micrograph above, many other cells in the human body are very distinctive and well suited for special functions. To perform their special functions, cells may vary in a number of ways.
Variation in Human Cells
Some cells act as individual cells and are not attached to one another. Red blood cells are a good example. Their main function is to transport oxygen to other cells throughout the body, so they must be able to move freely through the circulatory system. Many other cells, in contrast, act together with other similar cells as part of the same tissue, so they are attached to one another and cannot move freely. For example, epithelial cells lining the respiratory tract are attached to each other to form a continuous surface that protects the respiratory system from particles and other hazards in the air.
Many cells can divide readily and form new cells. Skin cells are constantly dying and being shed from the body and replaced by new skin cells, and bone cells can divide to form new bone for growth or repair. Some other cells, in contrast, such as certain nerve cells, can divide and form new cells only under exceptional circumstances. That’s why nervous system injuries such as a severed spinal cord generally cannot heal by the production of new cells, resulting in a permanent loss of function.
Many human cells have the primary job of producing and secreting a particular substance, such as a hormone or an enzyme. For example, special cells in the pancreas produce and secrete the hormone insulin, which regulates the level of glucose in the blood. Some of the epithelial cells that line the bronchial passages produce mucus, a sticky substance that helps trap particles in the air before it passes into the lungs.
Different but Identical
All the different cell types within an individual human organism are genetically identical, so no matter how different the cells are, they all have the same genes. How can such different types of cells arise? The answer is the differential regulation of genes. Cells with the same genes can be very different because different genes are expressed depending on the cell type.
Examples of Human Cell Types
Many common types of human cells — such as bone cells and white blood cells — actually consist of several subtypes of cells. Each subtype, in turn, has a special structure and function. A closer look at these cell types will give you a better appreciation for the diversity of structures and functions of human cells.
Bone Cells
There are four main subtypes of bone cells, as shown in Figure \(2\). Each type has a different form and function:
1. Osteocytes are star-shaped bone cells that make up the majority of bone tissue. They are the most common cells in mature bone and can live as long as the organism itself. They also control the function of bone cells called osteoblasts and osteoclasts.
2. Osteoblasts are cells with single nuclei that synthesize new bone. They function in organized groups of connected cells called osteons to form the organic and mineral matrix of bone.
3. Osteogenic cells are undifferentiated stem cells that differentiate to form osteoblasts in the tissue that covers the outside of the bone.
4. Osteoclasts are very large, multinucleated cells that are responsible for the breakdown of bones through resorption. The breakdown of bone is very important in bone health because it allows for bone remodeling.
White Blood Cells
White blood cells (also called leukocytes) are even more variable than bone cells. Five subtypes of white blood cells are shown in Figure \(3\). All of them are immune system cells involved in defending the body, but each subtype has a different function. They also differ in the normal proportion of all leukocytes they make up.
1. Monocytes make up about 5 percent of leukocytes. They are the biggest cells with extensions and a kidney-shaped nucleus. They engulf and destroy (phagocytize) pathogens in tissues.
2. Eosinophils make up about 2 percent of leukocytes. They have and a bilobed nucleus. They attack larger parasites and set off allergic responses.
3. Basophils make up less than 1 percent of leukocytes. Like eosinophils, these cells also have granules and a bilobed nucleus. They release proteins called histamines that are involved in inflammation.
4. Lymphocytes make up about 30 percent of leukocytes. These are small cells with a large circular nucleus. They include B cells and T cells. B cells produce antibodies against non-self antigens, and T cells destroy virus-infected cells and cancer cells.
5. Neutrophils are the most numerous white blood cells, making up about 62 percent of leukocytes. They have granules and a multilobed nucleus. They phagocytize single-celled bacteria and fungi in the blood.
Tissues
Groups of connected cells form tissues. The cells in a tissue may all be the same type or they may be of multiple types. In either case, the cells in the tissue work together to carry out a specific function. There are four main types of human tissues: connective, epithelial, muscle, and nervous tissues.
Connective Tissue
The most diverse and abundant of all tissues, connective tissue holds cells together and supports the body. Connective tissue is made up of cells suspended in a non-cellular matrix. The matrix (also known as ground substance) is secreted by the connective tissue cells and determines the characteristics of the connective tissue. It is the consistency of the matrix that determines the function of the connective tissue. The matrix can be liquid, gel-like or solid, all depending on the type of connective tissue. For example, the extracellular matrix of bone is a rigid mineral framework. The extracellular matrix of blood is liquid plasma. Connective tissues such as bone and cartilage generally form the body's structure. There are many sub-types of the four major types of tissues in a human body, see the flow chart in Figure \(5\).
Connective Tissue Proper
Fibroblast cells are responsible for synthesizing protein fibers for the matrix. Collagen fibers are strong, elastic fibers are flexible and reticular fibers form a supportive framework for organs and basement membranes. There are two subcategories of connective tissue proper.
Loose connective tissue
Thin and soft, this tissue contains many collagen and elastic fibers in a jell-like matrix. The cells in loose connective tissue are not close together. This tissue functions in binding the skin to underlying structures. There are three types of loose connective tissue.
1. Areolar connective tissue is a common form of loose connective tissue. It is found in the skin and mucous membranes, where it binds the skin or membrane to underlying tissues such as muscles. It is also found around blood vessels and internal organs where it links and supports them.
2. Adipose connective tissue is commonly known as fat. This tissue contains fat cells that are specialized for lipid storage. In addition to storing energy, this tissue also cushions and protects the organs.
3. Reticular connective tissue is mostly composed of reticular protein fibers which make a skeleton, known as stroma, for the lymphatic and white blood cells. This type of tissue is found in the spleen and other lymphatic system structures.
Dense connective tissue proper
This tissue consists of three categories, dense regular connective tissue, dense irregular connective tissue, and elastic connective tissue. These tissues differ on the arrangement and composition of the fibrous elements of the extracellular matrix.
1. Dense regular connective tissue has extracellular fibers that all run in the same direction and plane. Muscle tendons are a type of dense regular connective tissue.
2. Dense irregular connective tissue contains collagen and elastic fibers which are found running in all different directions and planes. The dermis of the skin is composed of dense irregular connective tissue.
3. Elastic connective tissue: Made up of freely branching elastic fibers with fibroblasts in the spaces between the fibers, this tissue allows the kind of stretch that is found in the walls of arteries.
Cartilage
This connective tissue is relatively solid and is a non-vascularized tissue (does not have a blood supply). The matrix is produced by cells called chondroblasts. When these cells slow down, they reside is small spaces called lacunae. These mature cells in the lacunae are called chondrocytes. There are three types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage.
1. Hyaline cartilage is the most common type of cartilage, contains many collagen fibers and is found in many places including the nose, between the ribs and the sternum and in the rings of the trachea.
2. Elastic cartilage has many elastic fibers in the matrix and supports the shape of the ears and forms part of the larynx.
3. Fibrocartilage is tough and contains many collagen fibers and is responsible for cushioning the knee joint and for forming the disks between the vertebrae.
Bone
Bone is a hard, mineralized tissue found in the skeleton. The bone matrix contains many collagen fibers as well as inorganic mineral salts, calcium carbonate, and calcium phosphate, all features that make it a very rigid structure. Bone cells, called osteoblasts, secrete the osteoid substance that eventually hardens around the cells to form an ossified matrix. The osteon forms the basic unit of compact bone. Within the osteon, the osteocytes (mature bone cells) are located in lacunae. Because the bone matrix is very dense, the osteocytes get their nutrition from the central canal via tiny canals called canaliculi.
Blood
Blood is considered a type of fluid connective tissue because the matrix of blood is not solid. The fluid matrix is called plasma, and formed elements of this tissue include white blood cells, red blood cells, and platelets. Read more about the composition and function of blood in the cardiovascular system chapter.
Epithelial Tissue
Epithelial tissue is made up of cells that line inner and outer body surfaces, such as the skin and the inner surface of the digestive tract. Epithelial tissue that lines inner body surfaces and body openings is called mucous membrane. This type of epithelial tissue produces mucus, a slimy substance that coats mucous membranes and traps pathogens, particles, and debris. Epithelial tissue protects the body and its internal organs, secretes substances such as hormones in addition to mucus, and absorbs substances such as nutrients.
Epithelial Cell Classification
Most epithelial tissue is described with two names. The first name describes the number of cell layers present and the second describes the shape of the cells. One layer of epithelial cells is called simple and more than one layer of epithelial cells is called stratified. There are three basic shapes of epithelial cells, squamous, cuboidal, and columnar. Squamous cells are thin and flat; cuboidal cells have a shape of a cube; columnar cells have a shape of a pillar. For example, simple squamous epithelial tissue describes a single layer of cells that are flat and scale-like in shape.
Locations and Functions of Epithelial Tissues
These tissues are found at various locations in our body and they have many functions. Some locations and functions are listed below:
• Simple squamous epithelium: This tissue is located in the sacs of the lungs and kidney where the exchange of nutrients and gas is essential.
• Simple cuboidal epithelium: This tissue is located in the glands and their ducts and kidneys. The main function of this tissue is secretion.
• Simple Columnar epithelium: This tissue lines the Gastrointestinal tract. The main function of this tissue is absorption and secretion.
• Pseudostratified epithelium: This is a simple tissue with the appearance of stratification. This tissue is located in the respiratory tract. This tissue may contain cilia to move mucus.
• Stratified squamous epithelium: This tissue is located where protection is needed such as skin.
• Stratified cuboidal epithelium: This tissue is located in the sweat glands for protection.
• Stratified columnar epithelium: This tissue is located in some sweat glands. The main function is to protect and secrete sweat components.
• Transitional epithelium: This tissue lines bladder, urethra, and ureters. The tissue allows the urinary organs to expand and stretch.
Muscle Tissue
Muscle tissue is made up of cells that have the unique ability to contract or become shorter. There are three major types of muscle tissue, as pictured in Figure \(14\): skeletal, smooth, and cardiac muscle tissues.
1. Skeletal muscles are striated, or striped in appearance, because of their internal structure. Skeletal muscles are attached to bones, and when they pull on the bones, they enable the body to move. Skeletal muscles are under voluntary control.
2. Smooth muscles are nonstriated muscles. They are found in the walls of blood vessels and in the reproductive, gastrointestinal, and respiratory tracts. Smooth muscles are not under voluntary control.
3. Cardiac muscles are striated and found only in the heart. Their contractions cause the heart to beat. Cardiac muscles are not under voluntary control.
Nervous Tissue
Nervous tissue is made up of neurons and other types of cells generally called glial cells (Figure \(15\)). Neurons are composed of cell body and extensions. The cell body contains the nucleus and the extensions make connections with the other tissues and neurons. Neurons transmit electrical messages and the glial cells play supporting roles. Nervous tissue makes up the central nervous system (mainly the brain and spinal cord) and peripheral nervous system (the network of nerves that runs throughout the rest of the body).
Feature: My Human Body
If you are a blood donor, then you have donated tissue. Blood is a tissue that you can donate when you are alive. You may have indicated on your driver’s license application that you wish to be a tissue donor in the event of your death. Deceased people can donate many different tissues, including skin, bone, heart valves, and the corneas of the eyes. If you are not already registered as a tissue donor, the information below may help you decide if you would like to register.
Each year, approximately 30,000 people donate tissues, which supply tissues for up to 1 million tissue transplants. One tissue donor can enhance or save the life of more than 50 people! Unlike organs, which generally must be transplanted immediately after the donor dies, donated tissues can be processed and stored for a long time for later use. Donated tissues can be used to replace burned skin and damaged bone and to repair ligaments. Corneal tissues can be used for corneal transplants that restore sight in blind people. In fact, each year 48,000 patients have their sight restored with corneal transplants. Unfortunately, there are not enough tissues to go around, and the need for donated tissues keeps rising.
Review
1. Give an example of cells that function individually and move freely, and give an example of cells that act together and are attached to other cells of the same type.
2. What are examples of cells that can readily divide and cells that can divide only under rare circumstances?
3. Identify a type of cell that secretes an important substance and name the substance it secretes.
4. Explain how different cell types come about when all the cells in an individual human being are genetically identical.
5. Compare and contrast four subtypes of human bone cells.
6. Identify three types of human white blood cells, and state their functions.
7. Why are bone and blood both classified as connective tissues?
8. Name another type of connective tissue, and describe its role in the human body.
9. Based on the information in the table above of types of epithelial tissues, list four general functions of this type of tissue in the human body.
10. Compare and contrast the three types of muscle tissues.
11. Identify the four types of nervous tissues, where each type is found, and its basic function.
12. Of the main types of human tissue, name two that can secrete hormones.
13. Cells in a particular tissue:
1. Are all of the same type
2. Have different genes from cells in other tissues
3. Work together to carry out a function
4. Are always connected physically to each other
14. Why are mucous membranes often located in regions that interface between the body and the outside world?
15. Skin is a type of _____________ tissue.
16. Body fat is a type of ____________ tissue.
Explore More
Each person’s body is completely unique, which means that everyone reacts differently to drugs and other medical treatments. In the TED talk below, tissue engineer Nina Tandon talks about a possible solution to this problem: making artificial tissues that are engineered to be the same as the patient’s and then using the tissues to test the effectiveness of specific drugs or other treatments.
Attributions
1. Bronchiolar epithelium by Louisa Howard, released into the public domain via Wikimedia Commons
2. Bone cells by OpenStax College, licensed CC BY 3.0 via Wikimedia Commons
3. White blood cells by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. licensed CC BY 3.0 via Wikimedia Commons
4. Types of tissues by OpenStax College, licensed CC BY 3.0 via Wikimedia Commons
5. Connective Tissue by Mandeep Grewal CC BY-NC 3.0
6. Loose Connective Tissue by Adrignola original uploader was Sunshineconnelly, licensed CC BY 2.5 via Wikimedia Commons
7. Adipose tissue by OpenStax College CC BY 3.0 via Wikimedia Commons
8. Reticular tissue by OpenStax College CC BY 3.0 via Wikimedia Commons
9. Dense regular and irregular by OpenStax College CC BY 3.0 via Wikimedia Commons
10. Types of cartilage by OpenStax College CC BY 3.0 via Wikimedia Commons
11. Bone Connective Tissue by Darshani Kansara licensed CC BY-SA 4.0
1. Transverse Section Of Bone by BDB licensed CC BY-SA 2.5 via Wikimedia Commons
12. Components of the Blood by OpenStax College CC BY 3.0 via Lumen Learning
13. Classification of epithelial tissues by the US Government Public domain via Wikimedia Commons.
14. Three types of muscle cells by OpenStax College CC BY 3.0 via Wikimedia Commons.
15. Cells of Nervous tissue by OpenStax College CC BY 3.0 via Wikimedia Commons.
16. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/10%3A_Introduction_to_the_Human_Body/10.3%3A_Human_Cells_and_Tissues.txt |
“Achy Breaky Heart”
Billy Ray Cyrus's song "Achy Breaky Heart" has lyrics suc as: Heartache, heartbreak...it all has to do with love. Did you ever wonder why the heart is associated with love? The heart was once thought to be the center of all thought processes, as well as the site of all emotions. This notion may have stemmed from very early anatomical dissections that found many nerves can be traced to the region of the heart. The fact that the heart may start racing when one is excited or otherwise emotionally aroused may have contributed to this idea as well. In fact, the heart is not the organ that controls thoughts or emotions. The organ that controls those functions is the brain. In this concept, you’ll be introduced to the heart, brain, and other major organs of the human body.
Human Organs
An organ is a collection of tissues joined in a structural unit to serve a common function. Organs exist in most multicellular organisms, including not only humans and other animals but also plants. In single-celled organisms such as bacteria, the functional equivalent of an organ is an organelle.
Tissues in Organs
Although organs consist of multiple tissue types, many organs are composed of the main tissue that is associated with the organ’s major function and other tissues that play supporting roles. The main tissue may be unique to that specific organ. For example, the main tissue of the heart is the cardiac muscle, which performs the heart’s major function of pumping blood and is found only in the heart. The heart also includes nervous and connective tissues that are required for it to perform its major function. For example, nervous tissues control the beating of the heart, and connective tissues make up heart valves that keep blood flowing in just one direction through the heart.
Vital Organs
The human body contains five organs that are considered vital for survival. They are the heart, brain, kidneys, liver, and lungs. The locations of these five organs and several other internal organs are shown in Figure \(2\). If any of the five vital organs stops functioning, the death of the organism is imminent without medical intervention.
1. The heart is located in the center of the chest, and its function is to keep blood flowing through the body. Blood carries substances to cells that they need and also carries away wastes from cells.
2. The brain is located in the head and functions as the body’s control center. It is the seat of all thoughts, memories, perceptions, and feelings.
3. The two kidneys are located in the back of the abdomen on either side of the body. Their function is to filter blood and form urine, which is excreted from the body.
4. The liver is located on the right side of the abdomen. It has many functions, including filtering blood, secreting bile that is needed for digestion, and producing proteins necessary for blood clotting.
5. The two lungs are located on either side of the upper chest. Their main function is exchanging oxygen and carbon dioxide with the blood.
Human Organ Systems
Functionally related organs often cooperate to form whole organ systems. Figure \(3\) and Figure \(4\) show 11 human organ systems, including separate diagrams for the male and female reproductive systems. Some of the organs and functions of the organ systems are identified in the figure. Each system is also described in more detail in the text that follows. Most of these human organ systems are also the subject of separate chapters in this book.
Integumentary System
Organs of the integumentary system include the skin, hair, and nails. The skin is the largest organ in the body. It encloses and protects the body and is the site of many sensory receptors. The skin is the body’s first defense against pathogens, and it also helps regulate body temperature and eliminate wastes in sweat.
Skeletal System
The skeletal system consists of bones, joints, teeth. The bones of the skeletal system are connected by tendons, ligaments, and cartilage. Functions of the skeletal system include supporting the body and giving it shape. Along with the muscular system, the skeletal system enables the body to move. The bones of the skeletal system also protect internal organs, store calcium, and produce red and white blood cells.
Muscular System
The muscular system consists of three different types of muscles, including skeletal muscles, which are attached to bones by tendons and allow for voluntary movements of the body. Smooth muscle tissues control the involuntary movements of internal organs, such as the organs of the digestive system, allowing food to move through the system. Smooth muscles in blood vessels allow vasoconstriction and vasodilation and thereby help regulate body temperature. Cardiac muscle tissues control the involuntary beating of the heart, allowing it to pump blood through the blood vessels of the cardiovascular system.
Nervous System
The nervous system includes the brain and spinal cord, which make up the central nervous system, and nerves that run throughout the rest of the body, which make up the peripheral nervous system. The nervous system controls both voluntary and involuntary responses of the human organism and also detects and processes sensory information.
Endocrine System
The endocrine system is made up of glands that secrete hormones into the blood, which carries the hormones throughout the body. Endocrine hormones are chemical messengers that control many body functions, including metabolism, growth, and sexual development. The master gland of the endocrine system is the pituitary gland, which produces hormones that control other endocrine glands. Some of the other endocrine glands include the pancreas, thyroid gland, and adrenal glands.
Cardiovascular System
The cardiovascular system (also called the circulatory system) includes the heart, blood, and three types of blood vessels: arteries, veins, and capillaries. The heart pumps blood, which travels through the blood vessels. The main function of the cardiovascular system is transport. Oxygen from the lungs and nutrients from the digestive system are transported to cells throughout the body. Carbon dioxide and other waste materials are picked up from the cells and transported to organs such as the lungs and kidneys for elimination from the body. The cardiovascular system also equalizes body temperature and transports endocrine hormones to cells in the body where they are needed.
Urinary System
The urinary system includes the pair of kidneys, which filter excess water and a waste product called urea from the blood and form urine. Two tubes called ureters carry the urine from the kidneys to the urinary bladder, which stores the urine until it is excreted from the body through another tube named the urethra. The kidneys also produce an enzyme called renin and a variety of hormones. These substances help regulate blood pressure, the production of red blood cells, and the balance of calcium and phosphorus in the body.
Respiratory System
Organs and other structures of the respiratory system include the nasal passages, lungs, and a long tube called the trachea, which carries air between the nasal passages and lungs. The main function of the respiratory system is to deliver oxygen to the blood and remove carbon dioxide from the body. Gases are exchanged between the lungs and blood across the walls of capillaries lining tiny air sacs (alveoli) in the lungs.
Lymphatic System
The lymphatic system is sometimes considered to be part of the immune system. It consists of a network of lymph vessels and ducts that collect excess fluid (called lymph) from extracellular spaces in tissues and transport the fluid to the bloodstream. The lymphatic system also includes many small collections of tissue, called lymph nodes, and an organ called the spleen, both of which remove pathogens and cellular debris from the lymph or blood. In addition, the thymus gland in the lymphatic system produces some types of white blood cells (lymphocytes) that fight infections.
Digestive System
The digestive system consists of several main organs — including the mouth, esophagus, stomach, and small and large intestines — that form a long tube called the gastrointestinal (GI) tract. Food moves through this tract where it is digested, its nutrients absorbed, and its waste products excreted. The digestive system also includes accessory organs (such as the pancreas and liver) that produce enzymes and other substances needed for digestion but through which food does not actually pass.
Male and Female Reproductive Systems
The reproductive system is the only body system that differs substantially between individuals. There is a range of Biological sex, but most books divide them into male and female. We will discuss the Biology of sex in detail in the reproductive and development chapters.
Feature: Human Biology in the News
Organ transplantation has been performed by surgeons for more than six decades, and you’ve no doubt heard of people receiving heart, lung, and kidney transplants. However, you may have never heard of a penis transplant. The first U.S. penis transplant was performed in May of 2016 at Massachusetts General Hospital in Boston. The 15-hour procedure involved a team of more than 50 physicians, surgeons, and nurses. The patient was a 64-year-old man who had lost his penis to cancer in 2012. The surgical milestone involved grafting microscopic blood vessels and nerves of the donor organ to those of the recipient. As with most transplant patients, this patient will have to take immunosuppressing drugs for the rest of his life so his immune system will not reject the organ. The transplant team said that their success with this transplant “holds promise for patients with devastating genitourinary injuries and disease.” They also hope their experiences will be helpful for gender reassignment surgery.
Review
1. What is the main tissue in the heart, and what is its role?
2. What non-muscle tissues are found in the heart? What are their functions?
3. Identify two vital organs in the human body. Identify their locations and functions.
4. List three human organ systems. For each organ system, identify some of its organs and functions.
5. Compare and contrast the male and female reproductive systems.
6. For each of the following pairs of organ systems, describe one way in which they work together and/or overlap.
1. Skeletal system and muscular system
2. Muscular system and digestive system
3. Endocrine system and reproductive system
4. Cardiovascular system and urinary system
7. What is the largest organ of the human body?
8. What are the three organ systems involved in regulating human body temperature?
9. Teeth are part of which system?
1. Integumentary
2. Skeletal
3. Nervous
4. A and B
10. Hair is part of which organ system?
11. True or False. Organs only exist in animals.
12. True or False. The respiratory system helps to remove wastes from the body.
Explore More
Professor Anthony Atala is working to answer an important question: Can we grow new replacement organs rather than transplanting organs from other people? In his state-of-the-art lab, he and his associates are actually growing human organs, including blood vessels, bladders, and kidneys. Watch the fascinating TED talk below to see how they are doing it.
While organ transplant saves countless lives, they oftentimes fail. Learn more here:
Attributions
1. Twemoji by Twitter, licensed CC BY 4.0 via Wikimedia Commons
2. Internal organs by Mikael Häggström released into the public domain via Wikimedia Commons
3. Organ Systems by Lindsay M. Biga, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Devon Quick & Jon Runyeon CC BY-SA 4.0 via Open Oregon Education.
4. Organ Systems by Lindsay M. Biga, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Devon Quick & Jon Runyeon CC BY-SA 4.0 via Open Oregon Education.
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/10%3A_Introduction_to_the_Human_Body/10.4%3A_Human_Organs_and_Organ_Systems.txt |
Contain the Brain
The brain is a delicate organ that regulates the physiology of the whole body. Figure \(1\) shows the brain with its superficial structures. Brain coverings and the skull provide protection to the brain. The space where the brain is located in the skull is called the cranial cavity.
What Are Body Cavities?
The human body, like that of many other multicellular organisms, is divided into a number of body cavities. A body cavity is a fluid-filled space inside the body that holds and protects internal organs. Human body cavities are separated by membranes and other structures. The two largest human body cavities are the ventral cavity and the dorsal cavity. These two body cavities are subdivided into smaller body cavities. Both the dorsal and ventral cavities and their subdivisions are shown in Figure \(2\).
Ventral Cavity
The ventral cavity is at the anterior, or front, of the trunk. Organs contained within this body cavity include the lungs, heart, stomach, intestines, and reproductive organs. You can see some of the organs in the ventral cavity in Figure \(3\). The ventral cavity allows for considerable changes in the size and shape of the organs within it as they perform their functions. For example, organs such as the lungs, stomach, or uterus can expand or contract without distorting other tissues or disrupting the activities of nearby organs.
The ventral cavity is subdivided into the thoracic and abdominopelvic cavities.
• The thoracic cavity fills the chest and is subdivided into two pleural cavities and the pericardial cavity. The pleural cavities hold the lungs, and the pericardial cavity holds the heart.
• The abdominopelvic cavity fills the lower half of the trunk and is subdivided into the abdominal cavity and the pelvic cavity. The abdominal cavity holds digestive organs and the kidneys, and the pelvic cavity holds reproductive organs and organs of excretion.
Dorsal Cavity
The dorsal cavity is at the posterior, or back, of the body, including both the head and the back of the trunk. The dorsal cavity is subdivided into the cranial and spinal cavities.
• The cranial cavity fills most of the upper part of the skull and contains the brain.
• The spinal cavity is a very long, narrow cavity inside the vertebral column. It runs the length of the trunk and contains the spinal cord.
The brain and spinal cord are protected by the bones of the skull and the vertebrae of the spine. They are further protected by the meninges, a three-layer membrane that encloses the brain and spinal cord. A thin layer of cerebrospinal fluid is maintained between two of the meningeal layers. This clear fluid is produced by the brain, and it provides extra protection and cushioning for the brain and spinal cord.
Feature: My Human Body
The meninges membranes that protect the brain and spinal cord inside their cavities may become inflamed, generally due to a bacterial or viral infection. This condition is called meningitis. Meningitis can lead to serious long-term consequences such as deafness, epilepsy, or cognitive deficits, especially if not treated quickly. Meningitis can also rapidly become life-threatening, so it is classified as a medical emergency.
Learning the symptoms of meningitis may help you or a loved one get prompt medical attention if you ever develop the disease. Common symptoms include fever, headache, and neck stiffness. Other symptoms may include confusion or altered consciousness, vomiting, and an inability to tolerate light or loud noises. Young children often exhibit less specific symptoms, such as irritability, drowsiness, or poor feeding.
Meningitis is diagnosed with a lumbar puncture (commonly known as a "spinal tap"), in which a needle is inserted into the spinal canal to collect a sample of cerebrospinal fluid. The fluid is analyzed for the presence of pathogens in a medical lab. If meningitis is diagnosed, treatment consists of antibiotics and sometimes antiviral drugs. Corticosteroids may also be administered to reduce inflammation and the risk of complications such as brain damage. Supportive measures such as IV fluids may also be provided.
Some types of meningitis can be prevented with a vaccine. Ask your health care professional whether you have had the vaccine or should get it. Giving antibiotics to people who have had significant exposure to certain types of meningitis may reduce their risk of developing the disease. If someone you know is diagnosed with meningitis, see your doctor for advice if you are concerned about contracting the disease.
Review
1. What is a body cavity?
2. Compare and contrast ventral and dorsal body cavities.
3. Identify the subdivisions of the ventral cavity and the organs each contains.
4. Describe the subdivisions of the dorsal cavity and its contents.
5. Identify and describe all the tissues that protect the brain and spinal cord.
6. What do you think might happen if fluid were to build up excessively in one of the body cavities?
7. Explain why a woman’s body can accommodate a full-term fetus during pregnancy, without damage to her internal organs.
8. Which body cavity does the needle enter in a lumbar puncture?
9. What are the names given to the three body cavity divisions where the heart is located?
10. What are the names given to the three body cavity divisions where the kidneys are located?
11. True or False. The stomach is located in the dorsal cavity.
12. True or False. A body cavity must open to the outside world.
13. True or False. The vertebral column surrounds the spinal cavity.
14. The _________ cavity is directly below the thoracic cavity.
15. What is the name of the fluid that protects the brain and spinal cord?
1. meningeal
2. cerebrospinal
3. lumbar
4. cranial
Explore More
Learn about this man's personal account of having meningitis here:
Attributions
1. Brain anatomy released into the public domain via Wikimedia Commons
2. Scheme body cavities by NCI (original) / Mysid (SVG), released into the public domain via Wikimedia Commons
3. Abdominal Organs Anatomy by BruceBlaus, licensed CC BY-SA 4.0 via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/10%3A_Introduction_to_the_Human_Body/10.5%3A_Human_Body_Cavities.txt |
Teamwork
Every player on a softball team has a special job to perform. Each of the orange team’s players in Figure \(1\) has his part of the infield or outfield covered if the ball comes his way. Other players on the orange team cover other parts of the field or pitch or catch the ball. Playing softball clearly requires teamwork. The human body is like a softball team in that regard. All the organ systems of the human body must work together as a team to keep the body alive and well. Teamwork within the body begins with communication.
Communication among Organ Systems
Communication among organ systems is vital if they are to work together as a team. They must be able to respond to each other and change their responses as needed to keep the body in balance. Communication among organ systems is controlled mainly by the autonomic nervous system and the endocrine system.
The autonomic nervous system is the part of the nervous system that controls involuntary functions. For example, the autonomic nervous system controls heart rate, blood flow, and digestion. You don’t have to tell your heart to beat faster or to consciously squeeze muscles to push food through the digestive system. In fact, you don’t have to even think about these functions at all. The autonomic nervous system orchestrates all the signals needed to control them. It sends messages between parts of the nervous system and between the nervous system and other organ systems via chemical messengers called neurotransmitters.
The endocrine system is the system of glands that secrete hormones directly into the bloodstream. Once in the blood, endocrine hormones circulate to cells everywhere in the body. The endocrine system is under the control of the hypothalamus, a part of the brain. The hypothalamus secretes hormones that travel directly to cells of the pituitary gland, which is located beneath it. The pituitary gland is the master gland of the endocrine system. Most of its hormones either turn on or turn off other endocrine glands. For example, if the pituitary gland secretes thyroid stimulating hormone, the hormone travels through the circulation to the thyroid gland, which is stimulated to secrete thyroid hormone. Thyroid hormone then travels to cells throughout the body, where it increases their metabolism.
Examples of Organ System Interactions
An increase in cellular metabolism requires more cellular respiration. Cellular respiration is a good example of organ system interactions because it is a basic life process that occurs in all living cells.
Cellular Respiration
Cellular respiration is the intracellular process that breaks down glucose with oxygen to produce carbon dioxide and energy in the form of ATP molecules. It is the process by which cells obtain usable energy to power other cellular processes. Which organ systems are involved in cellular respiration? The glucose needed for cellular respiration comes from the digestive system via the cardiovascular system. The oxygen needed for cellular respiration comes from the respiratory system also via the cardiovascular system. The carbon dioxide produced in cellular respiration leaves the body by the opposite route. In short, cellular respiration requires at a minimum the digestive, cardiovascular, and respiratory systems.
Fight-or-Flight Response
The well-known fight-or-flight response is a good example of how the nervous and endocrine systems control other organ system responses. The fight-or-flight response begins when the nervous system perceives sudden danger, as shown in Figure \(2\). The brain sends a message to the endocrine system (via the pituitary gland) for the adrenal glands to secrete their hormones cortisol and adrenaline. These hormones flood the circulation and affect other organ systems throughout the body, including the cardiovascular, urinary, sensory, and digestive systems. Specific responses include increased heart rate, bladder relaxation, tunnel vision, and a shunting of blood away from the digestive system and toward the muscles, brain, and other vital organs needed to fight or flee.
Digesting Food
Digesting food requires teamwork between the digestive system and several other organ systems, including the nervous, cardiovascular, and muscular systems. When you eat a meal, the organs of the digestive system need more blood to perform their digestive functions. Food entering the digestive systems causes nerve impulses to be sent to the brain; in response, the brain sends messages to the cardiovascular system to increase heart rate and dilate blood vessels in the digestive organs. Food passes through the organs of the digestive tract by rhythmic contractions of smooth muscles in the walls of the organs, so the muscular system is also needed for digestion. After food is digested, nutrients from the food are absorbed into the blood of the vessels lining the small intestine. Any remaining food waste is excreted through the large intestine.
Playing Softball
The men playing softball in Figure \(1\) are using multiple organ systems in this voluntary activity. Their nervous systems are focused on observing and preparing to respond to the next play. Their other systems are being controlled by the autonomic nervous system. Organ systems they are using include the muscular, skeletal, respiratory, and cardiovascular systems. Can you explain how each of these organ systems is involved in playing softball?
Feature: Reliable Sources
Teamwork among organ systems allows the human organism to work like a finely tuned machine. Or at least it does until one of the organ systems fails. When that happens, other organ systems interacting in the same overall process will also be affected. This is especially likely if the system affected plays a controlling role in the process. An example is type 1 diabetes. This disorder occurs when the pancreas does not secrete the endocrine hormone insulin. Insulin normally is secreted in response to an increasing level of glucose in the blood, and it brings the level of glucose back to normal by stimulating body cells to take up insulin from the blood.
Learn more about type 1 diabetes. Use several reliable Internet sources to answer the following questions:
1. What causes the endocrine system to fail to produce insulin in type 1 diabetes?
2. Which organ systems are affected by high blood glucose levels if type 1 diabetes is not controlled? What are some of the specific effects?
3. How can blood glucose levels be controlled in patients with type 1 diabetes?
Review
1. What is the autonomic nervous system?
2. How do the autonomic nervous system and endocrine system communicate with other organ systems so the systems can interact?
3. Explain how the brain communicates with the endocrine system.
4. What is the role of the pituitary gland in the endocrine system?
5. Identify organ systems that play a role in cellular respiration.
6. How does the hormone adrenaline prepare the body to fight or flee? What specific physiological changes does it bring about?
7. Explain the role of the muscular system in the digestion of food.
8. Describe how three different organ systems are involved when a player makes a particular play in softball, such as catching a fly ball.
9. True or False. The autonomic nervous system controls conscious movements.
10. True or False. Hormones travel throughout the body.
11. True or False. The pituitary gland directly secretes thyroid hormone.
12. What are two types of molecules that the body uses to communicate between organ systems?
13. Explain why hormones can have such a wide variety of effects on the body.
14. Heart rate can be affected by:
1. Hormones
2. Neurotransmitters
3. The fight-or-flight response
4. All of the above
15. Which gland secretes the hormone cortisol?
Explore More
Without the muscles lining the GI tract, you would be unable to digest food. Watch this short animation of food moving through the GI tract. It illustrates very clearly the necessary interaction of the muscular and digestive systems in the digestive process.
Attributions
1. Marines play softball, public domain
2. Brain by National Cancer Institute, released into the public domain via Wikimedia Commons
3. Fight or Flight Response by Jvnkfood (original), converted to PNG and reduced to 8-bit by Pokéfan95, licensed CC BY 4.0 via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/10%3A_Introduction_to_the_Human_Body/10.6%3A_Interaction_of_Organ_Systems.txt |
Steady as She Goes
This device looks simple, but it controls a complex system that keeps a home at a steady temperature. The device is an old-fashioned thermostat. The dial shows the current temperature in the room and also allows the occupant to set the thermostat to the desired temperature. A thermostat is a commonly cited model of how living systems, including the human body, maintain a steady state called homeostasis.
What is Homeostasis?
Homeostasis is the condition in which a system such as the human body is maintained in a more-or-less steady state. It is the job of cells, tissues, organs, and organ systems throughout the body to maintain many different variables within narrow ranges that are compatible with life. Keeping a stable internal environment requires continuous monitoring of the internal environment and constantly making adjustments to keep things in balance.
Setpoint and Normal Range
For any given variable, such as body temperature or blood glucose level, there is a particular setpoint that is the physiological optimum value. For example, the setpoint for human body temperature is about 37 ºC (98.6 ºF). As the body works to maintain homeostasis for temperature or any other internal variable, the value typically fluctuates around the set point. Such fluctuations are normal as long as they do not become too extreme. The spread of values within which such fluctuations are considered insignificant is called the normal range. In the case of body temperature, for example, the normal range for an adult is about 36.5 to 37.5 ºC (97.7 to 99.5 ºF).
Maintaining Homeostasis
Homeostasis is normally maintained in the human body by an extremely complex balancing act. Regardless of the variable being kept within its normal range, maintaining homeostasis requires at least four interacting components: stimulus, sensor, control center, and effector.
1. The stimulus is provided by the variable that is being regulated. Generally, the stimulus indicates that the value of the variable has moved away from the set point or has left the normal range.
2. The sensor monitors the values of the variable and sends data on it to the control center.
3. The control center matches the data with normal values. If the value is not at the set point or is outside the normal range, the control center sends a signal to the effector.
4. The effector is an organ, gland, muscle, or other structure that acts on the signal from the control center to move the variable back toward the set point.
Each of these components is illustrated in Figure \(2\). The diagram on the left is a general model showing how the components interact to maintain homeostasis. The stimulus activates the sensor. The sensor activates the control system that regulates the effector. The diagram on the right shows the example of body temperature. From the diagrams, you can see that maintaining homeostasis involves feedback, which is data that feeds back to control a response. High body temperature may stimulate the temperature regulatory center of the brain to activate the sweat glands to bring the body temperature down. When body temperature reaches normal range, it acts as negative feedback to stop the process. Feedback may be negative or positive. All the feedback mechanisms that maintain homeostasis use negative feedback. Biological examples of positive feedback are much less common.
Negative Feedback
In a negative feedback loop, feedback serves to reduce an excessive response and keep a variable within the normal range. Examples of processes controlled by negative feedback include body temperature regulation and control of blood glucose.
Body Temperature
Body temperature regulation involves negative feedback whether it lowers the temperature or raises it (Figure \(3\)).
Cooling Down
The human body’s temperature regulatory center is the hypothalamus in the brain. When the hypothalamus receives data from sensors in the skin and brain that body temperature is higher than the setpoint, it sets into motion the following responses:
• Blood vessels in the skin dilate (vasodilation) to allow more blood from the warm body core to flow close to the surface of the body, so heat can be radiated into the environment.
• As blood flow to the skin increases, sweat glands in the skin are activated to increase their output of sweat (diaphoresis). When the sweat evaporates from the skin surface into the surrounding air, it takes the heat with it.
• Breathing becomes deeper, and the person may breathe through the mouth instead of the nasal passages. This increases heat loss from the lungs.
Heating Up
When the brain’s temperature regulatory center receives data that body temperature is lower than the setpoint, it sets into motion the following responses:
• Blood vessels in the skin contract (vasoconstriction) to prevent blood from flowing close to the surface of the body. This reduces heat loss from the surface.
• As the temperature falls lower, random signals to skeletal muscles are triggered, causing them to contract. This causes shivering, which generates a small amount of heat.
• The thyroid gland may be stimulated by the brain (via the pituitary gland) to secrete more thyroid hormones. This hormone increases metabolic activity and heat production in cells throughout the body.
• The adrenal glands may also be stimulated to secrete the hormone adrenaline. This hormone causes the breakdown of glycogen (the carbohydrate used for energy storage in animals) to glucose, which can be used as an energy source. This catabolic chemical process is exothermic, or heat producing.
Blood Glucose
In the control of the blood glucose level, certain endocrine cells in the pancreas called alpha and beta cells, detect the level of glucose in the blood. Then they respond appropriately to keep the level of blood glucose within the normal range.
• If the blood glucose level rises above the normal range, pancreatic beta cells release the hormone insulin into the bloodstream. Insulin signals cells to take up the excess glucose from the blood until the level of blood glucose decreases to the normal range.
• If the blood glucose level falls below the normal range, pancreatic alpha cells release the hormone glucagon into the bloodstream. Glucagon signals cells to break down stored glycogen to glucose and release the glucose into the blood until the level of blood glucose increases to the normal range.
Positive Feedback
In a positive feedback loop, feedback serves to intensify a response until an endpoint is reached. Examples of processes controlled by positive feedback in the human body include blood clotting and childbirth.
Blood Clotting
When a wound causes bleeding, the body responds with a positive feedback loop to clot the blood and stop blood loss. Substances released by the injured blood vessel wall begin the process of blood clotting. Platelets in the blood start to cling to the injured site and release chemicals that attract additional platelets. As the platelets continue to amass, more of the chemicals are released and more platelets are attracted to the site of the clot. The positive feedback accelerates the process of clotting until the clot is large enough to stop the bleeding.
Childbirth
Figure \(4\) shows the positive feedback loop that controls childbirth. The process normally begins when the head of the infant pushes against the cervix. This stimulates nerve impulses, which travel from the cervix to the hypothalamus in the brain. In response, the hypothalamus sends the hormone oxytocin to the pituitary gland, which secretes it into the bloodstream so it can be carried to the uterus. Oxytocin stimulates uterine contractions, which push the baby harder against the cervix. In response, the cervix starts to dilate in preparation for the passage of the baby. This cycle of positive feedback continues, with increasing levels of oxytocin, stronger uterine contractions, and wider dilation of the cervix until the baby is pushed through the birth canal and out of the body. At that point, the cervix is no longer stimulated to send nerve impulses to the brain, and the entire process stops.
When Homeostasis Fails
Homeostatic mechanisms work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do, homeostatic imbalance may result, in which cells may not get everything they need or toxic wastes may accumulate in the body. If homeostasis is not restored, the imbalance may lead to disease or even death. Diabetes is an example of a disease caused by homeostatic imbalance. In the case of diabetes, blood glucose levels are no longer regulated and may be dangerously high. Medical intervention can help restore homeostasis and possibly prevent permanent damage to the organism.
Feature: My Human Body
Diabetes is diagnosed in people who have abnormally high levels of blood glucose after fasting for at least 12 hours. A fasting level of blood glucose below 100 is normal. A level between 100 and 125 places you in the pre-diabetes category, and a level higher than 125 results in a diagnosis of diabetes.
Of the two types of diabetes, type 2 diabetes is the most common, accounting for about 90 percent of all cases of diabetes in the United States. Type 2 diabetes typically starts after the age of 40. However, because of the dramatic increase in recent decades in obesity in younger people, the age at which type 2 diabetes is diagnosed has fallen. Even children are now being diagnosed with type 2 diabetes. Today, about 30 million Americans have type 2 diabetes, and another 90 million have pre-diabetes.
You are likely to have your blood glucose level tested during a routine medical exam. If your blood glucose level indicates that you have diabetes, it may come as a shock to you because you may not have any symptoms of the disease. You are not alone, because as many as one in four diabetics does not know they have the disease. Once the diagnosis of diabetes sinks in, you may be devastated by the news. Diabetes can lead to heart attacks, strokes, blindness, kidney failure, and loss of toes or feet. The risk of death in adults with diabetes is 50 percent greater than it is in adults without diabetes, and diabetes is the seventh leading cause of death in adults. In addition, controlling diabetes usually requires frequent blood glucose testing, watching what and when you eat and taking medications or even insulin injections. All of this may seem overwhelming.
The good news is that changing your lifestyle may stop the progression of type 2 diabetes or even reverse it. Here’s how:
• Lose weight. Any weight loss is beneficial. Losing as little as seven percent of your weight may be all that is needed to stop diabetes in its tracks. It is especially important to eliminate excess weight around your waist.
• Exercise regularly. You should try to exercise five days a week for at least 30 minutes. This will not only lower your blood sugar and help your insulin work better; it will also lower your blood pressure and improve your heart health. Another bonus of exercise is that it will help you lose weight by increasing your basal metabolic rate.
• Adopt a healthy diet. Decrease your consumption of refined carbohydrates such as sweets and sugary drinks. Increase your intake of fiber-rich foods such as fruits, vegetables, and whole grains. About a quarter of each meal should consist of high-protein foods, such as fish, chicken, dairy products, legumes, or nuts.
• Control stress. Stress can increase your blood glucose and also raise your blood pressure and risk of heart disease. When you feel stressed out, do breathing exercises or take a brisk walk or jog. Also, try to replace stressful thoughts with more calming ones.
• Establish a support system. Enlist the help and support of loved ones as well as medical professionals such as a nutritionist and diabetes educator. Having a support system will help ensure that you are on the path to wellness and that you can stick to your plan.
Review
1. What is homeostasis?
2. Define the setpoint and normal range for physiological measures.
3. Identify and define the four interacting components that maintain homeostasis in feedback loops.
4. Compare and contrast negative and positive feedback loops.
5. Explain how negative feedback controls body temperature.
6. Give two examples of physiological processes that are controlled by positive feedback loops.
7. A negative feedback loop:
1. brings a variable’s level back to a normal range
2. can lower, but not raise, body temperature
3. is the type of feedback involved in blood clotting
4. A and B
8. During breastfeeding, the stimulus of the baby sucking on the nipple increases the amount of milk produced by the mother. The more sucking, the more milk is usually produced.
1. Is this an example of negative or positive feedback? Explain your answer.
2. What do you think might be the evolutionary benefit of the milk production regulation mechanism described in part a?
9. Explain why homeostasis is regulated by negative feedback loops, rather than positive feedback loops.
10. A setpoint is usually:
1. the top of a normal range
2. the bottom of a normal range
3. in the middle of a normal range
4. the point at which changes can no longer occur
11. The level of a sex hormone, testosterone (T), is controlled by negative feedback. Another hormone, gonadotropin-releasing hormone (GnRH), is released by the hypothalamus of the brain, which triggers the pituitary gland to release luteinizing hormone (LH). LH stimulates the gonads to produce T. When there is too much T in the bloodstream, it feeds back on the hypothalamus, causing it to produce less GnRH. While this does not describe all the feedback loops involved in regulating T, answer the following questions about this particular feedback loop.
1. What is the stimulus in this system? Explain your answer.
2. What is the control center in this system? Explain your answer.
3. What is the pituitary considered in this system: stimulus, sensor, control center, or effector? Explain your answer.
Attributions
1. Honeywell thermostat by Vincent de Groot, licensed CC BY 4.0 via Wikimedia Commons
2. Negative feedback loop by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
3. Temperature Regulation dedicated CC0 via Wikimedia Commons
4. Pregnancy-Positive Feedback by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/10%3A_Introduction_to_the_Human_Body/10.7%3A_Homeostasis_and_Feedback.txt |
Case Study Conclusion: Under Pressure
As you learned in this chapter, the human body consists of many complex systems that normally work together efficiently like a well-oiled machine to carry out life’s functions. For example, Figure \(1\) illustrates how the brain and spinal cord are protected by layers of membrane called meninges and fluid that flows between the meninges and in spaces called ventricles inside the brain. This fluid is called cerebrospinal fluid (CSF) and as you have learned, one of its important functions is to cushion and protect the brain and spinal cord, which make up most of the central nervous system (CNS). CSF additionally circulates nutrients and removes waste products from the CNS. CSF is produced continually in the ventricles, circulates throughout the CNS, and then is reabsorbed by the bloodstream. If too much CSF is produced, its flow blocked, or if not enough is reabsorbed, the system becomes out of balance, and CSF can build up in the ventricles. This causes an enlargement of the ventricles called hydrocephalus that can put pressure on the brain, resulting in the types of neurological problems that former professional football player, Dayo, described at the beginning of this chapter, is suffering from.
Recall that Dayo’s symptoms included loss of bladder control, memory loss, and difficulty walking. The cause of their symptoms was not immediately clear, although their doctors suspected that it related to the nervous system since the nervous system acts as the control center of the body, controlling and regulating many other organ systems. Dayo’s memory loss directly implicated the involvement of the brain, since that is the site of thoughts and memory. The urinary system is also controlled in part by the nervous system, and the inability to hold urine appropriately can be a sign of a neurological issue. Dayo’s trouble walking involved the muscular system, which works alongside the skeletal system to enable movement of the limbs. In turn, the contraction of muscles is regulated by the nervous system. You can see why a problem in the nervous system can cause a variety of different symptoms by affecting multiple organ systems in the human body.
To try to find the exact cause of Dayo’s symptoms, their doctors performed a lumbar puncture, or spinal tap, which is the removal of some CSF through a needle inserted into the lower part of the spinal canal. Doctors then analyzed Dayo’s CSF for the presence of pathogens such as bacteria to determine whether an infection was the cause of their neurological symptoms. When no evidence of infection was found, Doctors used an MRI to observe the structures of Dayo's brain. This is when Doctors discovered Dayo's enlarged ventricles, which are a hallmark of hydrocephalus.
To treat Dayo’s hydrocephalus, a surgeon implanted a device called a shunt in Dayo's brain to remove the excess fluid (Figure \(2\)). One side of the shunt consists of a small tube, called a catheter, which was inserted into Dayo’s ventricles. Excess CSF is then drained through a one-way valve to the other end of the shunt, which was threaded under their skin to their abdominal cavity, where the CSF is released and can be reabsorbed by the bloodstream.
Implantation of a shunt is the most common way to treat hydrocephalus, and for some people, it can allow them to recover almost completely. However, there can be complications associated with a brain shunt. The shunt can have mechanical problems or cause an infection. Also, the rate of draining must be carefully monitored and adjusted to balance the rate of removal of CSF with the rate of its production. If it is drained too fast, it is called overdraining, and if it is drained too slowly, it is called underdraining. In the case of underdraining, the pressure on the brain and associated neurological symptoms will persist. In the case of overdraining, the ventricles can collapse, which can cause serious problems such as the tearing of blood vessels and hemorrhaging. To avoid these problems, some shunts have an adjustable pressure valve where the rate of draining can be adjusted by placing a special magnet over the scalp. You can see how the proper balance between CSF production and removal is so critical – both in the causes of hydrocephalus and in its treatment.
In what other ways does your body regulate balance, or maintain a state of homeostasis? In this chapter, you learned about the feedback loops that keep body temperature and blood glucose within normal ranges. Other important examples of homeostasis in the human body are the regulation of the pH in the blood and the balance of water in the body. You will learn more about homeostasis in different body systems in the coming chapters.
Thanks to Dayo’s shunt, their symptoms are starting to improve, but they have not fully recovered. Time may tell whether the removal of the excess CSF from their ventricles will eventually allow them to recover normal functioning or whether permanent damage to their nervous system has already been done. The flow of CSF might seem simple but when it gets out of balance, it can easily wreak havoc on multiple organ systems because of the intricate interconnectedness of the systems within the human “machine."
Chapter Summary
This chapter provided an overview of the organization and functioning of the human body. You learned that:
• The human body consists of multiple parts that function together to maintain life. The biology of the human body incorporates the body’s structure, or anatomy, and the body’s functioning, or physiology.
• The organization of the human body is a hierarchy of increasing size and complexity, starting at the level of atoms and molecules and ending at the level of the entire organism.
• Cells are the level of organization above atoms and molecules, and they are the basic units of structure and function of the human body. Each cell carries out basic life functions as well as other specific roles. Cells of the human body show a lot of variation.
• Variations in cell function are generally reflected in variations in cell structure.
• Some cells are unattached to other cells and can move freely; others are attached to each other and cannot move freely. Some cells can divide readily and form new cells; others can divide only under exceptional circumstances. Many cells are specialized to produce and secrete particular substances.
• All the different cell types within an individual have the same genes. Cells can vary because different genes are expressed depending on the cell type.
• Many common types of human cells consist of several subtypes of cells, each of which has a special structure and function. For example, subtypes of bone cells include osteocytes, osteoblasts, osteogenic cells, and osteoclasts.
• A tissue is a group of connected cells that have a similar function. There are four basic types of human tissues that make up all the organs of the human body: epithelial, muscle, nervous, and connective tissues.
• Connective tissues, such as bone and blood, are made up of cells that are separated by non-living material, called the extracellular matrix.
• Epithelial tissues, such as skin and mucous membranes, protect the body and its internal organs and secrete or absorb substances.
• Muscle tissues are made up of cells that have the unique ability to contract. They include skeletal, smooth, and cardiac muscle tissues.
• Nervous tissues are made up of neurons, which transmit electrical messages, and glial cells of various types, which play supporting roles. Types of nervous tissues include gray matter, white matter, nerves, and ganglia.
• An organ is a structure that consists of two or more types of tissues that work together to do the same job. Examples include the brain and heart.
• Many organs are composed of a major tissue that performs the organ’s main function, as well as other tissues that play supporting roles.
• The human body contains five organs that are considered vital for survival. They are the heart, brain, kidneys, liver, and lungs. If any of these five organs stops functioning, the death of the organism is imminent without medical intervention.
• An organ system is a group of organs that work together to carry out a complex overall function. For example, the skeletal system provides structure to the body and protects internal organs.
• There are 11 major organ systems in the human organism. They are the integumentary, skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, and reproductive systems. Only the reproductive system varies significantly between males and females.
• The human body is divided into a number of body cavities. A body cavity is a fluid-filled space in the body that holds and protects internal organs. The two largest human body cavities are the ventral cavity and the dorsal cavity.
• The ventral cavity is at the anterior, or front, of the trunk. It is subdivided into the thoracic cavity and abdominopelvic cavity.
• The dorsal cavity is at the posterior, or back, of the body, and includes the head and the back of the trunk. It is subdivided into the cranial cavity and spinal cavity.
• Organ systems of the human body must work together to keep the body alive and functioning normally. This requires communication among organ systems. This is controlled by the autonomic nervous system and endocrine system. The autonomic nervous controls involuntary body functions, such as heart rate and digestion. The endocrine system secretes hormones into the blood that travel to body cells and influence their activities.
• Cellular respiration is a good example of organ system interactions because it is a basic life process that occurs in all living cells. It is the intracellular process that breaks down glucose with oxygen to produce carbon dioxide and energy. Cellular respiration requires the interaction of the digestive, cardiovascular, and respiratory systems.
• The fight-or-flight response is a good example of how the nervous and endocrine systems control other organ system responses. It is triggered by a message from the brain to the endocrine system and prepares the body for flight or a fight. Many organ systems are stimulated to respond, including the cardiovascular, respiratory, and digestive systems.
• Digesting food requires teamwork between the digestive system and several other organ systems, including the nervous, cardiovascular, and muscular systems.
• Playing softball or doing other voluntary physical activities may involve the interaction of nervous, muscular, skeletal, respiratory, and cardiovascular systems.
• Homeostasis is the condition in which a system such as the human body is maintained in a more-or-less steady state. It is the job of cells, tissues, organs, and organ systems throughout the body to maintain homeostasis.
• For any given variable, such as body temperature, there is a particular set point that is the physiological optimum value. The spread of values around the setpoint that is considered insignificant is called the normal range.
• Homeostasis is generally maintained by a negative feedback loop that includes a stimulus, sensor, control center, and effector. Negative feedback serves to reduce an excessive response and to keep a variable within the normal range. Negative feedback loops control body temperature and blood glucose level.
• Sometimes homeostatic mechanisms fail, resulting in homeostatic imbalance. Diabetes is an example of a disease caused by homeostatic imbalance. Aging can bring about a reduction in the efficiency of the body’s control system, making the elderly more susceptible to disease.
• Positive feedback loops are not common in biological systems. Positive feedback serves to intensify a response until an endpoint is reached. Positive feedback loops control blood clotting and childbirth.
The severe and broad impact of hydrocephalus on the body’s systems highlights the importance of the nervous system and its role as the master control system of the body. In the next chapter, you will learn much more about the structures and functioning of this fascinating and important system.
Chapter Summary Review
1. Compare and contrast tissues and organs.
2. Osteocyte cells are part of which type of tissue and organ system?
3. Adipose tissue, or body fat, is the same general type of tissue as:
1. mucous membranes
2. gray matter
3. skin
4. blood
4. Which type of tissue lines the inner and outer surfaces of the body?
5. True or False. The extracellular matrix that surrounds cells is always solid.
6. True or False. Skin is an organ.
7. What is a vital organ? What happens if a vital organ stops working?
8. Name three organ systems that transport or remove wastes from the body.
9. Name two types of tissue in the digestive system.
10. For each of the following body functions, choose the organ system that is most associated with the function. Organ systems: integumentary; skeletal; muscular; nervous; endocrine; cardiovascular; lymphatic; respiratory; digestive; urinary; reproductive
1. Processes sensory information
2. Secretes hormones
3. Releases carbon dioxide from the body to the outside world
4. Produces gametes
5. Controls water balance in the body
11. The spleen is part of which organ system?
1. Digestive
2. Lymphatic
3. Integumentary
4. Urinary
12. Describe one way in which the integumentary and cardiovascular systems work together to regulate homeostasis in the human body.
13. Name the two largest body cavities in humans and describe their general locations.
14. What are the names given to the three body cavity divisions where the reproductive organs are located?
15. True or False. There are two pleural cavities.
16. True or False. Body cavities are filled with air.
17. The pituitary gland is in which organ system? Describe how the pituitary gland increases metabolism.
18. When the level of thyroid hormone in the body gets too high, it acts on other cells to reduce the production of more thyroid hormone. What type of feedback loop does this represent?
19. Hypothetical organ A is the control center in a feedback loop that helps maintain homeostasis. It secretes molecule A1 which reaches organ B, causing organ B to secrete molecule B1. B1 negatively feeds back onto organ A, reducing the production of A1 when the level of B1 gets too high.
1. What is the stimulus in this feedback loop?
2. If the level of B1 falls significantly below the setpoint, what do you think happens to the production of A1? Why?
3. What is the effector in this feedback loop?
4. If organs A and B are part of the endocrine system, what type of molecules do you think A1 and B1 are likely to be?
20. What are the two main systems that allow various organ systems to communicate with each other?
21. The hypothalamus is part of the:
1. spinal cord
2. thoracic cavity
3. kidneys
4. brain
22. What are two functions of the hypothalamus that you learned about in this chapter?
Attributions
1. Brain and Nearby Structures by NIH Image Gallery, public domain via Flickr
2. Diagram showing a brain shunt by Cancer Research UK, CC BY 4.0 via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/10%3A_Introduction_to_the_Human_Body/10.8%3A_Case_Study_Conclusion%3A__Pressure__and_Chapter_Summary.txt |
This chapter describes neurons and other cells of the nervous system and compares and contrasts divisions of the nervous system, including central, peripheral, somatic, and autonomic divisions. The chapter explains how nerve impulses occur and how we sense stimuli. It also describes disorders of the nervous system and the effects of psychoactive drugs on the nervous system.
• 11.1: Case Study: The Control Center of Your Body
Each of these brightly-colored sticky notes represents a piece of information that someone doesn't want to forget. Although we are all forgetful sometimes, most people do not have trouble remembering things that are important or routine to us, such as our friend's name or how to get to class.
• 11.2: Introduction to the Nervous System
The nervous system is the human organ system that coordinates all of the body's voluntary and involuntary actions by transmitting electrical signals to and from different parts of the body. Specifically, the nervous system extracts information from the internal and external environments using sensory receptors. It then usually sends signals encoding this information to the brain, which processes the information to determine an appropriate response.
• 11.3: Neurons
Neurons, also called nerve cells, are electrically excitable cells that are the main functional units of the nervous system. Their function is to transmit nerve impulses. They are the only type of human cells that can carry out this function.
• 11.4: Nerve Impulses
This amazing cloud-to-surface lightning occurred when a difference in electrical charge built up in a cloud relative to the ground.
• 11.5: Central Nervous System
This very odd-looking drawing is called a homunculus that represents a cross-sectional wedge of the human brain.
• 11.6: Peripheral Nervous System
Did you ever see two people play the same piano? How do they coordinate all the movements of their own fingers, let alone synchronize them with those of their partner? The peripheral nervous system plays an important part in this challenge.
• 11.7: Human Senses
This figure appears at first glance to be just a pattern of colored leaves, but hidden within it is the three-dimensional shape of an ant.
• 11.8: Psychoactive Drugs
Who knew that a cup of coffee could also be a work of art? A talented barista can make coffee look as good as it tastes. If you are a coffee drinker, you probably know that coffee can also affect your mental state.
• 11.9: Case Study Conclusion: Memory and Chapter Summary
The nervous system coordinates all of the body's voluntary and involuntary activities. It interprets information from the outside world through sensory systems and makes appropriate responses through the motor system, through communication between the PNS and CNS. The brain directs the rest of the nervous system and controls everything from basic vital functions such as heart rate and breathing to high-level functions such as problem-solving and abstract thought.
Thumbnail: Cerebral lobes. (CC BY SA 3.0 Unported; Gutenberg Encyclopedia).
11: Nervous System
Case Study: Fading Memory
Each of these brightly colored sticky notes represents a piece of information that someone doesn’t want to forget. Although we are all forgetful sometimes, most people do not have trouble remembering things that are important or routine to us, such as our friend’s name or how to get to class. Our brain, the control center of the nervous system and the rest of the body, normally allows us to retain and recall information. But if the brain becomes damaged, a person may need to rely excessively on external reminders — like this wall of sticky notes — rather than being able to trust their own memory. That is if they are able to remember to write things down in the first place.
One person having trouble with their memory is Rosa, who is 68 years old. Rosa has been having difficulty remembering where she has set down objects in her house and forgot about a few doctor’s appointments and lunches she planned with friends. Her family began to notice that she would sometimes not recall recent conversations, requiring them to repeat things to her. Rosa would also sometimes struggle to find the right word in a conversation and would put objects in unusual places, such as the milk in a cabinet instead of the refrigerator. While most people do things like this occasionally, it seemed to Rosa and her family that it was happening to her more often recently.
She also had some other symptoms that were impacting her life, such as having trouble paying her bills on time and managing her budget, which she had previously done well. Rosa ascribed these lapses in memory and mental functioning to the normal effects of aging, but her family was concerned. They noticed that she was also more irritable than usual and would sometimes verbally lash out at them, which was not like her. When she became disoriented on a walk around her neighborhood and a neighbor had to escort her home, her family convinced her to see a doctor.
Besides a complete physical exam and lab tests, Rosa’s doctor interviewed Rosa and her family about her memory, ability to carry out daily tasks, and mood changes. He also administered a variety of tests to assess her memory and cognitive functioning, such as her ability to solve problems and use numbers and language correctly. Finally, he ordered a scan of her brain to investigate whether a tumor or some other observable cause was causing changes in the functioning of her brain.
Based on the results of these tests, Rosa’s doctor came to the conclusion that she most likely has mild Alzheimer’s disease (AD). AD results from abnormal changes in the molecules and cells of the brain, characterized by clumps of proteins called amyloid plaques between brain cells and tangled bundles of protein fibers called neurofibrillary tangles within certain brain cells. The affected brain cells stop functioning properly, lose their connections to other brain cells, and eventually will die. The picture below shows part of a cross-section of a brain from a patient who had severe AD compared to a similar section of a healthy brain. You can see how severely shrunken the brain with AD is, due to the death of many brain cells.
AD is a progressive disease, which means the damage and associated symptoms get worse over time. Clinicians have categorized the progression into three main stages — mild, moderate, and severe AD. Typically, AD cannot be definitively diagnosed until after death when the brain tissue can be directly examined for plaques and tangles. However, based on Rosa’s symptoms and the results of her tests, her doctor thinks she most likely has mild AD, when the brain changes and resulting symptoms are not yet severe.
Although there is currently no cure for AD, and Rosa will eventually get worse, her doctor says that medications and behavioral therapies may improve and prolong her functioning and quality of life over the next few years. He prescribes a medication that improves communication between brain cells, which has been shown to help some people with AD.
As you read this chapter, you will learn much more about how the brain and the rest of the nervous system work, and the multitude of functions they control in the body. By the end of the chapter, you will have enough knowledge about the nervous system to learn more about why AD causes the symptoms that it does, how Rosa’s medication works, and some promising new approaches that may help physicians diagnose and treat AD patients at earlier stages.
Chapter Overview: Nervous System
In this chapter, you will learn about the human nervous system, which includes the brain, spinal cord, and nerves. Specifically, you will learn about:
• The organization of the nervous system, including the central and peripheral nervous systems and their organs and subdivisions.
• The cells of the nervous system — neurons and glia — their parts, and their functions.
• How messages are sent by neurons through the nervous system and to and from the rest of the body.
• How these messages, or nerve impulses, are transmitted by electrical changes within neurons, and through chemical molecules to other cells.
• The structure and functions of different parts of the central nervous system, which includes the brain and spinal cord, and some of the things that can go wrong when they are damaged.
• The structure and functions of the peripheral nervous system, which includes the nerves that carry motor and sensory information to and from the body to control voluntary and involuntary activities.
• The human senses and how visual information, sounds, smells, tastes, touch, and balance are detected by sensory receptor cells and then sent to the brain for interpretation.
• How legal and illegal drugs can have psychoactive effects on the brain-altering mood, perceptions, thinking, and behavior — which can sometimes lead to addiction.
As you read the chapter, think about the following questions:
1. Based on Rosa’s symptoms, which parts of her brain may have been affected by Alzheimer’s disease?
2. How are messages sent between cells in the nervous system? What molecules are involved in this process? What are the ways in which drugs can alter this process?
3. Why can’t Rosa’s brain just grow new cells to replace the ones that have died?
Attributions
1. Stickies by woodleywonderworks, licensed CC BY 3.0 via Flickr
2. Healthy and AD brain by NIH Image Gallery, public domain via Flickr
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/11%3A_Nervous_System/11.1%3A_Case_Study%3A__The_Control_Center_of_Your_Body.txt |
In the Blink of an Eye
As you drive into a parking lot, a skateboarder suddenly flies in front of your car across your field of vision. You see the skateboarder in the nick of time and react immediately. You slam on the brakes and steer sharply to the right — all in the blink of an eye. You avoid a collision, but just barely. You’re shaken up but thankful that no one was hurt. How did you respond so quickly? Such rapid responses are controlled by your nervous system.
Overview of the Nervous System
The nervous system, illustrated in Figure \(2\), is the human organ system that coordinates all of the body’s voluntary and involuntary actions by transmitting electrical signals to and from different parts of the body. Specifically, the nervous system extracts information from the internal and external environments using sensory receptors. It then usually sends signals encoding this information to the brain, which processes the information to determine an appropriate response. Finally, the brain sends signals to muscles, organs, or glands to bring about the response. In the example above, your eyes detected the skateboarder, the information traveled to your brain, and your brain instructed your body to act so as to avoid a collision.
Signals of the Nervous System
The signals sent by the nervous system are electrical signals called nerve impulses, and they are transmitted by special nervous system cells named neurons, or nerve cells, like the one in Figure \(3\) (all the parts of a neuron are explained in the next section). Dendrites of a neuron receive nerve impulses from other cells. Long projection (called axons) from neurons carries nerve impulses directly to specific target cells. Schwann cells wrapped around the axon are called glial cells. They create a myelin sheath which allows the nerve impulse to travel very rapidly through the axons. A cell that receives nerve impulses from a neuron may be excited to perform a function, inhibited from carrying out an action, or otherwise controlled. In this way, the information transmitted by the nervous system is specific to particular cells and is transmitted very rapidly.
In fact, the fastest nerve impulses travel at speeds greater than 100 meters per second! Compare this to the chemical messages carried by the hormones that are secreted into the blood by endocrine glands. These hormonal messages are “broadcast” to all the cells of the body, and they can travel only as quickly as the blood flows through the cardiovascular system.
Organization of the Nervous System
As you might predict, the human nervous system is very complex. It has multiple divisions, beginning with its two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS), as shown in Figure \(4\). The CNS includes the brain and spinal cord, and the PNS consists mainly of nerves, which are bundles of axons from neurons. The nerves of the PNS connect the CNS to the rest of the body. You can learn much more about the CNS by reading the concept Central Nervous System.
The PNS is divided into two major parts, called the autonomic and somatic nervous systems. The somatic nervous system controls activities that are under voluntary control, such as turning a steering wheel. The autonomic nervous system controls activities that are not under voluntary control, such as digesting a meal. The autonomic nervous system has two divisions: the sympathetic division, which controls the fight-or-flight response during emergencies, and the parasympathetic division, which controls the routine “housekeeping” functions of the body at other times. You can learn more about the PNS and its subdivisions by reading the concept Peripheral Nervous System.
Review
1. List the general steps by which the nervous system generates an appropriate response to information from the internal and external environments.
2. What are neurons?
3. Compare and contrast the central and peripheral nervous systems.
4. Which major division of the peripheral nervous system allows you to walk to class? Which major division of the peripheral nervous system controls your heart rate?
5. Identify the functions of the three divisions of the autonomic nervous system.
6. What is an axon and what is its function?
7. True or False. A nerve impulse always causes the target cell to perform an action.
8. True or False. The spinal cord is not considered part of the peripheral nervous system.
9. Define nerve impulses.
10. Explain why signals in the nervous system are generally more targeted and specific than signals in the endocrine system.
11. Explain generally how the brain and spinal cord can interact with and control the rest of the body.
12. ___________ actions are performed without the person thinking about them.
13. The fight-or-flight response is controlled by the:
1. autonomic nervous system
2. somatic nervous system
3. central nervous system
4. parasympathetic nervous system
14. How are nerves and neurons related?
15. What type of information from the outside environment do you think is detected by sensory receptors in your ears?
Attributions
1. Skateboarder by JESHOOTS-com via Pixabay license
2. Nervous System diagram by the Emirr, CC BY 3.0 via Wikimedia Commons
3. Neuron by NickGorton, licensed CC BY-SA 3.0 via Wikimedia Commons
4. Nervous System Flowchart by Suzanne Wakim dedicated CC0
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/11%3A_Nervous_System/11.2%3A_Introduction_to_the_Nervous_System.txt |
Life as Art
This colorful picture could be an abstract work of modern art. You might imagine it hanging in an art museum or art gallery. In fact, the picture illustrates real life rather than an artistic creation. It is a micrograph of human nervous tissue. The neon green structures in the picture are neurons. The neuron is one of two basic types of cells in the nervous system, the other type being the glial cell.
Neurons, also called nerve cells, are electrically excitable cells that are the main functional units of the nervous system. Their function is to transmit nerve impulses. They are the only type of human cells that can carry out this function.
Neuron Structure
Figure \(2\) shows the structure of a typical neuron. The main parts of a neuron are labeled in the figure and described below.
• The cell body is the part of a neuron that contains the cell nucleus and other cell organelles. It is usually quite compact, and may not be much wider than the nucleus.
• Dendrites are thin structures that are extensions of the cell body. Their function is to receive nerve impulses from other cells and carry them to the cell body. A neuron may have many dendrites, and each dendrite may branch repeatedly to form a dendrite “tree” with more than 1,000 “branches.” The end of each branch can receive nerve impulses from another cell, allowing a given neuron to communicate with tens of thousands of other cells.
• The axon is a long, thin extension of the cell body. It transmits nerve impulses away from the cell body and toward other cells. The axon branches at the end, forming multiple axon terminals. These are the points where nerve impulses are transmitted to other cells, often to the dendrites of other neurons. An area called a synapse occurs at each axon terminal. Synapses are complex membrane junctions that transmit signals to other cells. An axon may branch hundreds of times, but there is never more than one axon per neuron.
• Spread out along axons, especially the long axons of nerves, are many sections of the myelin sheath. These are lipid layers that cover sections of the axon. The myelin sheath is a very good electrical insulator, similar to the plastic or rubber that encases an electrical cord.
• Regularly spaced gaps between sections of myelin sheath occur along the axon. These gaps are called nodes of Ranvier, and they allow the transmission of nerve impulses along the axon. Nerve impulses skip from node to node, allowing nerve impulses to travel along the axon very rapidly.
• A Schwann cell (also on an axon) is a type of glial cell. Its function is to produce the myelin sheath that insulates axons in the peripheral nervous system. In the central nervous system, a different type of glial cell, called an oligodendrocyte, produces the myelin sheath.
Neurogenesis
Fully differentiated neurons, with all their special structures, cannot divide and form new daughter neurons. Until recently, scientists thought that new neurons could no longer be formed after the brain developed prenatally. In other words, they thought that people were born with all the brain neurons they would ever have, and as neurons died, they would not be replaced. However, new evidence shows that additional neurons can form in the brain, even in adults, from the division of undifferentiated neural stem cells that are found throughout the brain. The production of new neurons is called neurogenesis. The extent to which it can occur is not known, but it is not likely to be very great in humans.
Neurons in Nervous Tissues
The nervous tissue in the brain and spinal cord consists of gray matter and white matter. Gray matter contains mainly the cell bodies of neurons. It is gray only in cadavers; living gray matter is actually more pink than gray (see image below). White matter consists mainly of axons covered with myelin sheath, which gives them their white color. White matter also makes up nerves of the peripheral nervous system. Nerves consist of long bundles of myelinated axons that extend to muscles, organs, or glands throughout the body. The axons in each nerve are bundled together like wires in a cable. Axons in nerves may be more than a meter long in an adult. The longest nerve runs from the base of the spine to the toes.
Types of Neurons
There are hundreds of different types of neurons in the human nervous system. These types exhibit a variety of structures and functions. Nonetheless, many neurons can be classified functionally based on the direction in which they carry nerve impulses.
• Sensory (also called afferent) neurons carry nerve impulses from sensory receptors in tissues and organs to the central nervous system. They change physical stimuli such as touch, light, and sound into nerve impulses.
• Motor (also called efferent) neurons, like the one in figure \(2\), carry nerve impulses from the central nervous system to muscles and glands. They change nerve signals into the activation of these structures.
• Interneurons carry nerve impulses back and forth often between sensory and motor neurons within the spinal cord or brain.
Glial Cells
Besides neurons, nervous tissues also consist of glial cells (also called neuroglia). They are now known to play many vital roles in the nervous system. There are several different types of glial cells, each with a different function. Schwann cells and Oligodendrocytes are glial cells that produce myelin sheath.
Feature: My Human Body
Would you like your brain to make new neurons that could help you become a better learner? What college student wouldn’t want a little more brainpower when it comes to learning new things? If research on rats applies to humans, then sustained aerobic exercise such as running can increase neurogenesis in the adult brain, and specifically in the hippocampus, a brain structure important for learning temporally and/or spatially complex tasks as well as memory. Although the research is still at the beginning stages, it suggests that exercise may actually lead to a “smarter” brain. However, even if the research results are not confirmed in the future for humans, it can’t hurt to get more aerobic exercise, because it is certainly beneficial for your body if not for your brain.
Review
1. Identify the three main parts of a neuron and their functions.
2. Describe the myelin sheath and nodes of Ranvier. How does their arrangement allow nerve impulses to travel very rapidly along axons?
3. What is a synapse?
4. Define neurogenesis. What is the potential for neurogenesis in the human brain?
5. Relate neurons to different types of nervous tissues.
6. Compare and contrast sensory and motor neurons.
7. Identify the role of interneurons.
8. For each type of neuron below, identify whether it is a sensory neuron, motor neuron, or interneuron.
1. A neuron in the spinal cord receives touch information and then transmits that information to another spinal cord neuron that controls the movement of an arm muscle.
2. A neuron that takes taste information from your tongue and sends it to your brain.
3. A spinal cord neuron stimulates a muscle to contract.
9. The myelin sheath is made by:
1. Sensory neurons
2. White neurons
3. Peripheral nervous system neurons
4. Glial cells
10. True or False. Synapses often exist where a dendrite and an axon terminal meet.
11. True or False. There is only one axon terminal per neuron.
Explore More
Multiple sclerosis (MS) is a progressive degenerative disease that is caused by the demyelination of axons in the central nervous system. When myelin degrades, the conduction of nerve impulses along the nerve can be impaired or lost, and the nerve eventually withers. Watch this inspirational TED talk in which the speaker shares how being diagnosed with MS changed her life and led her to become an MS nurse.
After his death in 1955, Albert Einstein's brain was studied by scientists worldwide—all wanting to gain insight into the anatomy of a genius. But it wasn't until the 1980s when Marian Diamond noticed that Einstein had more glial cells than average. Glia, stemming from Greek for "glue", was previously thought to have performed a strictly support role for the neurons. Now it is clear that glia may play a more active, non-electrical role in brain activity.
Attributions
1. Interneurons of Adult Visual Cortex by Wei-Chung Allen Lee, Hayden Huang, Guoping Feng, Joshua R. Sanes, Emery N. Brown, Peter T. So, Elly Nedivi, licensed CC BY 2.5 via Wikimedia Commons
2. Neuron by Chiara Mazzasette adapted from OpenStax, licensed CC BY 4.0 via Wikimedia Commons
3. White and gray matter by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
4. Sensory Neuron Test Water by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/11%3A_Nervous_System/11.3%3A_Neurons.txt |
When Lightning Strikes
This amazing cloud-to-surface lightning occurred when a difference in electrical charge built up in a cloud relative to the ground. When the buildup of charge was great enough, a sudden discharge of electricity occurred. A nerve impulse is similar to a lightning strike. Both a nerve impulse and a lightning strike occur because of differences in electrical charge, and both result in an electric current.
Generating Nerve Impulses
A nerve impulse, like a lightning strike, is an electrical phenomenon. A nerve impulse occurs because of a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions, which are electrically charged atoms or molecules.
Resting Potential
When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane of the neuron. The sodium-potassium pump is a mechanism of active transport that moves sodium ions out of cells and potassium ions into cells. The sodium-potassium pump moves both ions from areas of lower to higher concentration, using energy in ATP and carrier proteins in the cell membrane. Figure \(3\)shows in greater detail how the sodium-potassium pump works. Sodium is the principal ion in the fluid outside of cells, and potassium is the principal ion in the fluid inside of cells. These differences in concentration create an electrical gradient across the cell membrane, called resting potential. Tightly controlling membrane resting potential is critical for the transmission of nerve impulses.
Action Potential
An action potential, also called a nerve impulse, is an electrical charge that travels along the membrane of a neuron. It can be generated when a neuron’s membrane potential is changed by chemical signals from a nearby cell. In an action potential, the cell membrane potential changes quickly from negative to positive as sodium ions flow into the cell through ion channels, while potassium ions flow out of the cell, as shown in Figure \(3\).
The change in membrane potential results in the cell becoming depolarized. An action potential works on an all-or-nothing basis. That is, the membrane potential has to reach a certain level of depolarization, called the threshold, otherwise, an action potential will not start. This threshold potential varies but is generally about 15 millivolts (mV) more positive than the cell's resting membrane potential. If a membrane depolarization does not reach the threshold level, an action potential will not happen. You can see in Figure \(4\) that two depolarizations did not reach the threshold level of -55mV.
The first channels to open are the sodium ion channels, which allow sodium ions to enter the cell. The resulting increase in positive charge inside the cell (up to about +40 mV) starts the action potential. This is called the depolarization of the membrane. Potassium ion channels then open, allowing potassium ions to flow out of the cell, which ends the action potential. The inside of the membrane becomes negative again. This is called repolarization of the membrane. Both of the ion channels then close, and the sodium-potassium pump restores the resting potential of -70 mV. The action potential will move down the axon toward the synapse like a wave would move along the surface of the water. Figure \(4\)shows the change in potential of the axon membrane during an action potential. The nerve goes through a brief refractory period before racing resting potential. During the refractory period, another action potential cannot be generated
In myelinated neurons, ion flows occur only at the nodes of Ranvier. As a result, the action potential signal "jumps" along the axon membrane from node to node rather than spreading smoothly along the membrane, as they do in axons that do not have a myelin sheath. This is due to a clustering of Na+ and K+ ion channels at the Nodes of Ranvier. Unmyelinated axons do not have nodes of Ranvier, and ion channels in these axons are spread over the entire membrane surface.
Transmitting Nerve Impulses
The place where an axon terminal meets another cell is called a synapse. This is where the transmission of a nerve impulse to another cell occurs. The cell that sends the nerve impulse is called the presynaptic cell, and the cell that receives the nerve impulse is called the postsynaptic cell.
Some synapses are purely electrical and make direct electrical connections between neurons. However, most synapses are chemical synapses. The transmission of nerve impulses across chemical synapses is more complex.
Chemical Synapses
At a chemical synapse, both the presynaptic and postsynaptic areas of the cells are full of the molecular machinery that is involved in the transmission of nerve impulses. As shown in Figure \(5\), the presynaptic area contains many tiny spherical vessels called synaptic vesicles that are packed with chemicals called neurotransmitters. When an action potential reaches the axon terminal of the presynaptic cell, it opens channels that allow calcium to enter the terminal. Calcium causes synaptic vesicles to fuse with the membrane, releasing their contents into the narrow space between the presynaptic and postsynaptic membranes. This area is called the synaptic cleft. The neurotransmitter molecules travel across the synaptic cleft and bind to receptors, which are proteins that are embedded in the membrane of the postsynaptic cell.
The effect of a neurotransmitter on a postsynaptic cell depends mainly on the type of receptors that it activates, making it possible for a particular neurotransmitter to have different effects on various target cells. A neurotransmitter might excite one set of target cells, inhibit others, and have complex modulatory effects on still others, depending on the type of receptors. However, some neurotransmitters have relatively consistent effects on other cells.
Review
1. Define nerve impulse.
2. What is the resting potential of a neuron, and how is it maintained?
3. Explain how and why an action potential occurs.
4. Outline how a signal is transmitted from a presynaptic cell to a postsynaptic cell at a chemical synapse.
5. What generally determines the effects of a neurotransmitter on a postsynaptic cell?
6. Identify three general types of effects neurotransmitters may have on postsynaptic cells.
7. Explain how an electrical signal in a presynaptic neuron causes the transmission of a chemical signal at the synapse.
8. The flow of which type of ion into the neuron results in an action potential?
1. How do these ions get into the cell?
2. What does this flow of ions do to the relative charge inside the neuron compared to the outside?
9. The sodium-potassium pump:
1. is activated by an action potential
2. requires energy
3. does not require energy
4. pumps potassium ions out of cells
10. True or False. Some action potentials are larger than others, depending on the amount of stimulation.
11. True or False. Synaptic vesicles from the presynaptic cell enter the postsynaptic cell.
12. True or False. An action potential in a presynaptic cell can ultimately cause the postsynaptic cell to become inhibited.
13. Name three neurotransmitters.
Attributions
1. Adapted by Mandeep Grewal from Lincoln Lightning by U.S. Navy photo by Photographers Mate 2nd Class Aaron Ansarov; public domain via Wikimedia Commons
2. Scheme sodium-potassium pump by LadyofHats Mariana Ruiz Villarreal, released into the public domain via Wikimedia Commons
3. Action potential licensed CC BY 3.0 by OpenStax
4. Action potential by Chris 73, licensed CC BY 3.0 via Wikimedia Commons
5. Chemical synapse schema cropped by Looie496 created file, US National Institutes of Health, National Institute on Aging created original, released into the public domain via Wikimedia Commons
6. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/11%3A_Nervous_System/11.4%3A_Nerve_Impulses.txt |
Homunculus
Figure \(1\) is a very odd-looking drawing and is called a homunculus. The mass represents a cross-sectional wedge of the human brain. The drawing shows some areas of the brain associated with different parts of the body. As you can see, larger areas of the brain in this region are associated with the hands, face, and tongue than the legs and feet. Given the importance of speech, manual dexterity, and face-to-face social interactions in human beings, it is not surprising that relatively large areas of the brain are needed to control these body parts. The brain is the most complex organ in the human body and part of the central nervous system.
What Is the Central Nervous System?
The central nervous system (CNS) is the part of the nervous system that includes the brain and spinal cord. Figure \(2\) shows the central nervous system as one of the two main divisions of the total nervous system. The other main division is the peripheral nervous system (PNS). The CNS and PNS work together to control virtually all body functions. You can read much more about the PNS in the concept Peripheral Nervous System.
The delicate nervous tissues of the central nervous system are protected by major physical and chemical barriers. Physically, the brain and spinal cord are surrounded by tough meninges, a three-layer protective sheath that also contains cushioning cerebrospinal fluid. The bones of the skull and spinal vertebrae also contribute to physically protecting the brain and spinal cord. Chemically, the brain and spinal cord are isolated from the circulation — and most toxins or pathogens in the blood — by the blood-brain barrier. The blood-brain barrier is a highly selective membrane formed of endothelial cells (a type of glial cells) that separates the circulating blood from the extracellular fluid in the CNS. The barrier allows water, certain gases, glucose, and some other molecules needed by the brain and spinal cord to cross from the blood into the CNS while keeping out potentially harmful substances. These physical and chemical barriers make the CNS less susceptible to injury. However, damage to the CNS is likely to have more serious consequences.
The Brain
The brain is the control center not only of the rest of the nervous system but of the entire organism. The adult brain makes up only about 2 percent of the body’s weight, but it uses about 20 percent of the body’s total energy. The brain contains an estimated one hundred billion neurons, and each neuron has thousands of synaptic connections to other neurons. The brain also has about the same number of glial cells as neurons. No wonder the brain uses so much energy! In addition, the brain uses mostly glucose for energy. As a result, if the brain is deprived of glucose, it can lead to unconsciousness. The brain is able to store some glucose in the form of glycogen, but in much smaller amounts than are found in the liver and skeletal muscles.
The brain controls such mental processes as reasoning, imagination, memory, and language. It also interprets information from the senses and commands the body how to respond. It controls basic physical processes such as breathing and heartbeat as well as voluntary activities such as walking and writing. The brain has three major parts: the cerebrum, cerebellum, and brain stem (Figure \(3\)). The figure shows the brain from the left side of the head. It shows how the brain would appear if the skull and meninges were removed. The brain stem via its medulla links to the spinal cord. The cerebellum is a small section at the back of the brain. The largest part of the brain is the cerebrum.
Cerebrum
The cerebrum is the largest part of the brain. It controls conscious, intellectual functions. For example, it controls reasoning, language, memory, sight, touch, and hearing. When you read a book, play a video game, or recognize a classmate, you are using your cerebrum.
Hemispheres and Lateralization of the Cerebrum
The cerebrum is divided from front to back into two halves called the left and right hemispheres. The two hemispheres are connected by a thick bundle of axons, known as the corpus callosum, which lies deep within the brain. The corpus callosum is the main avenue of communication between the two hemispheres. It connects each point in the cerebrum to the mirror-image point in the opposite hemisphere.
The right and left hemispheres of the cerebrum are similar in shape, and most areas of the cerebrum are found in both hemispheres. Some areas, however, show lateralization, or a concentration in one hemisphere or the other. For example, in most people, language functions are more concentrated in the left hemisphere, whereas abstract reasoning and visual-spatial abilities are more concentrated in the right hemisphere.
For reasons that are not yet clear, each hemisphere of the brain interacts primarily with the opposite side of the body. The left side of the brain receives messages from and sends commands to the right side of the body, and the right side of the brain receives messages from and sends commands to the left side of the body. Sensory nerves from the spinal cord to the brain and motor nerves from the brain to the spinal cord both cross the midline of the body at the level of the brain stem.
Cerebral Cortex
Most of the information processing in the brain actually takes place in the cerebral cortex. This is a rind of gray matter and other tissues just a few millimeters thick that makes up the outer surface of the cerebrum in both hemispheres of the brain. The cerebral cortex has many folds in it that greatly increase the amount of surface area of the brain that can fit within the skull. Because of all the folds in the human cerebral cortex, it has a surface area of about 2,500 cm2(2.5 ft2). The size and importance of the cerebral cortex are far greater in the human brain than the brains of any other vertebrates including nonhuman primates.
Lobes of the Cerebral Cortex
Each hemisphere of the cerebrum is further divided into the four lobes shown in Figure \(4\) and described below.
1. The frontal lobes are located at the front of the brain behind the forehead. The frontal lobes are associated with executive functions such as attention, self-control, planning, problem-solving, reasoning, abstract thought, language, and personality.
2. The parietal lobes are located behind the frontal lobes at the top of the head. The parietal lobes are involved in sensation, including temperature, touch, and taste. Reading and arithmetic are also functions of the parietal lobes.
3. The temporal lobes are located at the sides of the head below the frontal and parietal lobes. The temporal lobes enable hearing, the formation and retrieval of memories, and the integration of memories and sensations.
4. The occipital lobes are located at the back of the head below the parietal lobes. The occipital lobes are the smallest of the four pairs of lobes. They are dedicated almost solely to vision.
Inner Structures of the Brain
Several structures are located deep within the brain and are important for communication between the brain and spinal cord or the rest of the body. These structures include the hypothalamus and thalamus. Figure \(5\) shows where these structures are located in the brain. The cerebrum, hypothalamus, and thalamus exist in two halves, one in each hemisphere.
Hypothalamus
The hypothalamus is located just above the brain stem and is about the size of an almond. The hypothalamus is responsible for certain metabolic processes and other activities of the autonomic nervous system, including body temperature, heart rate, hunger, thirst, fatigue, sleep, wakefulness, and circadian (24-hour) rhythms. The hypothalamus is also an important emotional center of the brain. The hypothalamus can regulate so many body functions because it responds to many different internal and external signals, including messages from the brain, light, steroid hormones, stress, and invading pathogens, among others.
One way the hypothalamus influences body functions is by synthesizing hormones that directly influence body processes. For example, it synthesizes the hormone oxytocin, which stimulates uterine contractions during childbirth and the letdown of milk during lactation. It also synthesizes the hormone vasopressin (also called antidiuretic hormone), which stimulates the kidneys to reabsorb more water and excrete more concentrated urine. These two hormones are sent from the hypothalamus via a stalk-like structure called the infundibulum (see diagram above) directly to the posterior (back) portion of the pituitary gland, which secretes them into the blood.
The main way the hypothalamus influences body functions is by controlling the pituitary gland, known as the master gland of the endocrine system. The hypothalamus synthesizes neurohormones called releasing factors that travel through the infundibulum directly to the anterior (front) part of the pituitary gland. The releasing factors generally either stimulate or inhibit the secretion of anterior pituitary hormones, most of which control other glands of the endocrine system.
Thalamus
The thalamus, which is located near the hypothalamus (Figure \(5\)), is a major hub for information traveling back and forth between the spinal cord and cerebrum. It filters sensory information traveling to the cerebrum. It relays sensory signals to the cerebral cortex and motor signals to the spinal cord. It is also involved in the regulation of consciousness, sleep, and alertness.
Cerebellum
The cerebellum is just below the cerebrum and at the back of the brain behind the brain stem (Figure \(3\)). It coordinates body movements and is involved in movements that are learned with repeated practice. For example, when you hit a softball with a bat or touch type on a keyboard you are using the cerebellum. Many nerve pathways link the cerebellum with motor neurons throughout the body.
Brain Stem
Sometimes called the “lower brain,” the brain stem is the lower part of the brain that is joined to the spinal cord. There are three parts to the brainstem: the midbrain, the pons, and the medulla oblongata, which are shown in Figure \(6\) below. The brain stem is primarily involved in the unconscious autonomic functions as well as several types of sensory information. It also helps coordinate large body movements such as walking and running. The midbrain deals with sight and sound information and translates these inputs before sending them to the forebrain. The pons relays messages to other parts of the brain (primarily the cerebrum and cerebellum) and helps regulate breathing. Some researchers have hypothesized that the pons plays a role in dreaming. Some of the functions of the Pons are shared by the medulla oblongata, also called the medulla. The medulla controls several subconscious homeostatic functions such as breathing, heart and blood vessel activity, swallowing, and digestion.
One of the brain stem’s most important roles is that of an “information highway.” That is, all of the information coming from the body to the brain and the information from the cerebrum to the body go through the brain stem. Sensory pathways for such things as pain, temperature, touch, and pressure sensation go upward to the cerebrum, and motor pathways for movement and other body processes go downward to the spinal cord. Most of the axons in the motor pathways cross from one side of the CNS to the other as they pass through the medulla oblongata. As a result, the right side of the brain controls much of the movement on the left side of the body, and the left side of the brain controls much of the movement on the right side of the body.
Spinal Cord
The spinal cord is a long, thin, tubular bundle of nervous tissues that extends from the brain stem and continues down the center of the back to the pelvis. It is highlighted in yellow in Figure \(7\). The spinal cord is enclosed within but is shorter than, the vertebral column.
Structure of the Spinal Cord
The center of the spinal cord consists of gray matter, which is made up mainly of cell bodies of neurons, including interneurons and motor neurons. The gray matter is surrounded by white matter that consists mainly of myelinated axons of motor and sensory neurons. Spinal nerves, which connect the spinal cord to the PNS, exit from the spinal cord between vertebrae (Figure \(8\)).
Functions of the Spinal Cord
The spinal cord serves as an information superhighway. It passes messages from the body to the brain and from the brain to the body. Sensory (afferent) nerves carry nerve impulses to the brain from sensory receptor cells everywhere in and on the body. Motor (efferent) nerves carry nerve impulses away from the brain to glands, organs, or muscles throughout the body.
The spinal cord also independently controls certain rapid responses called reflexes without any input from the brain. You can see how this may happen in Figure \(9\). A sensory receptor responds to a sensation and sends a nerve impulse along a sensory nerve to the spinal cord. In the spinal cord, the message passes to an interneuron and from the interneuron to a motor nerve, which carries the impulse to a muscle. The muscle contracts in response. These neuron connections form a reflex arc, which requires no input from the brain. No doubt you have experienced such reflex actions yourself. For example, you may have reached out to touch a pot on the stove, not realizing that it was very hot. Virtually at the same moment that you feel the burning heat, you jerk your arm back and remove your hand from the pot.
Injuries to the Spinal Cord
Physical damage to the spinal cord may result in paralysis, which is a loss of sensation and movement in part of the body. Paralysis generally affects all the areas of the body below the level of the injury because nerve impulses are interrupted and can no longer travel back and forth between the brain and body beyond that point. If an injury to the spinal cord produces nothing more than swelling, the symptoms may be transient. However, if nerve fibers (axons) in the spinal cord are badly damaged, the loss of function may be permanent. Experimental studies have shown that spinal nerve fibers attempt to regrow, but tissue destruction usually produces scar tissue that cannot be penetrated by the regrowing nerves, as well as other factors that inhibit nerve fiber regrowth in the central nervous system.
Feature: My Human Body
Each year, many millions of people have a stroke, and stroke is the second leading cause of death in adults. Stroke, also known as cerebrovascular accident, occurs when poor blood flow to the brain results in the death of brain cells. There are two main types of strokes:
• Ischemic strokes occur due to a lack of blood flow because of a blood clot in an artery going to the brain.
• Hemorrhagic strokes occur due to bleeding from a broken blood vessel in the brain.
Either type of stroke may result in paralysis, loss of the ability to speak or comprehend speech, loss of bladder control, personality changes, and many other potential effects, depending on the part of the brain that is injured. The effects of a stroke may be mild and transient or more severe and permanent. A stroke may even be fatal. It generally depends on the type of stroke and how extensive it is.
Are you at risk of stroke? The main risk factor for stroke is age: about two-thirds of strokes occur in people over the age of 65. There is nothing you can do about your age, but most other stroke risk factors can be reduced with lifestyle changes or medications. The risk factors include high blood pressure, tobacco smoking, obesity, high blood cholesterol, diabetes mellitus, and atrial fibrillation.
Chances are good that you or someone you know is at risk of a stroke, so it is important to recognize a stroke if one occurs. Stoke is a medical emergency, and the more quickly treatment is given, the better the outcome is likely to be. In the case of ischemic strokes, the use of clot-busting drugs may prevent permanent brain damage if administered within 3 or 4 hours of the stroke. Remembering the signs of a stroke is easy.
They are summed up by the acronym FAST, as explained in the chart below.
Review
1. What is the central nervous system?
2. How is the central nervous system protected?
3. What is the overall function of the brain?
4. Identify the three main parts of the brain and one function of each part.
5. Describe the hemispheres of the brain.
6. Explain and give examples of lateralization of the brain.
7. Identify one function of each of the four lobes of the cerebrum.
8. Summarize the structure and function of the cerebral cortex.
9. Explain how the hypothalamus controls the endocrine system.
10. Describe the spinal cord.
11. What is the main function of the spinal cord?
12. Explain how reflex actions occur.
13. Why do severe spinal cord injuries usually cause paralysis?
14. What do you think are some possible consequences of severe damage to the brain stem? How might this compare to the consequences of severe damage to the frontal lobe? Explain your answer.
15. Information travels very quickly in the nervous system, but generally, the longer the path between areas, the longer it takes. Based on this, explain why you think reflexes often occur at the spinal cord level and do not require input from the brain.
Explore More
More than 40 million people worldwide suffer from Alzheimer’s disease, a brain disorder, and the number is expected to grow dramatically in the coming decades. The disease was discovered more than a century ago, but little progress has been made in finding a cure. Watch this exciting TED talk in which scientist Samuel Cohen shares a new breakthrough in Alzheimer's research as well as a message of hope that a cure for Alzheimer’s will be found.
Attributions
1. Sensory Homunculus by Popadius adapted from OpenStax, licensed CC BY 3.0 via Wikimedia Commons
2. Overview of nervous system by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
3. Brain by Laura Guerin, CC BY-NC 3.0 via CK-12
4. Brain lobes by Laura Guerin, CC BY-NC 3.0 via CK-12
5. Hypothalamus-Pituitary Complex by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
6. Brain stem by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
7. Spinal cord by BruceBlaus licensed CC BY 3.0 via Wikimedia Commons
8. Spinal readjustment by Tomwsulcer dedicated CC0 via Wikimedia Commons
9. Short and long reflexes by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
10. Stroke Communications Kit by CDC, public domain
11. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/11%3A_Nervous_System/11.5%3A_Central_Nervous_System.txt |
One Piano, Four Hands
Did you ever see two people play the same piano? How do they coordinate all the movements of their own fingers, let alone synchronize them with those of their partner? The peripheral nervous system plays an important part in this challenge.
What Is the Peripheral Nervous System?
The peripheral nervous system (PNS) consists of all the nervous tissue that lies outside of the central nervous system (CNS). The main function of the PNS is to connect the CNS to the rest of the organism. It serves as a communication relay, going back and forth between the CNS and muscles, organs, and glands throughout the body.
Tissues of the Peripheral Nervous System
The tissues that make up the PNS are nerves and ganglia. Ganglia are nervous tissues that act as relay points for messages transmitted through nerves of the PNS. Nerves are cable-like bundles of axons that make up the majority of PNS tissues. Nerves are generally classified on the basis of the direction in which they carry nerve impulses as sensory, motor, or mixed nerves. See examples of sensory and motor never in Figure \(3\).
• Sensory nerves transmit information from sensory receptors in the body to the CNS. Sensory nerves are also called afferent nerves.
• Motor nerves transmit information from the CNS to muscles, organs, and glands. Motor nerves are also called efferent nerves.
• Mixed nerves contain both sensory and motor neurons, so they can transmit information in both directions. They have both afferent and efferent functions.
Divisions of the Peripheral Nervous System
The PNS is divided into two major systems, called the autonomic nervous system and the somatic (or sensory-somatic) nervous system. Both systems of the PNS interact with the CNS and include sensory and motor neurons, but they use different circuits of nerves and ganglia.
Somatic Nervous System
The somatic nervous system primarily senses the external environment and controls voluntary activities in which decisions and commands come from the cerebral cortex of the brain. For example, when you feel too warm, decide to turn on the air conditioner, and walk across the room to the thermostat, you are using your somatic nervous system. In general, the somatic nervous system is responsible for all of your conscious perceptions of the outside world and all of the voluntary motor activities you perform in response. Whether it’s playing piano, driving a car, or playing basketball, you can thank your somatic nervous system for making it possible.
Structurally, the somatic nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves (Figure \(2\)). Cranial nerves are in the head and neck and connect directly to the brain. Sensory cranial nerves sense smells, tastes, light, sounds, and body position. Motor cranial nerves control muscles of the face, tongue, eyeballs, throat, head, and shoulders. The motor nerves also control the salivary glands and swallowing. Four of the 12 cranial nerves participate in both sensory and motor functions as mixed nerves, having both sensory and motor neurons.
Spinal nerves of the somatic nervous system emanate from the spinal column between vertebrae. All of the spinal nerves are mixed nerves, containing both sensory and motor neurons. Spinal nerves also include motor nerves that stimulate skeletal muscle contraction, allowing for voluntary body movements.
Autonomic Nervous System
The autonomic nervous system primarily senses the internal environment and controls involuntary activities. It is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. In general, the autonomic nervous system is responsible for all the activities that go on inside your body without your conscious awareness or voluntary participation.
Structurally, the autonomic nervous system consists of sensory and motor nerves that run between the CNS (especially the hypothalamus in the brain) and internal organs (such as the heart, lungs, and digestive organs) and glands (such as the pancreas and sweat glands). Sensory neurons in the autonomic system detect internal body conditions and send messages to the brain. Motor nerves in the autonomic system function by controlling the contractions of smooth or cardiac muscle or glandular tissue. For example, when sensory nerves of the autonomic system detect a rise in body temperature, motor nerves signal smooth muscles in blood vessels near the body surface to undergo vasodilation, and the sweat glands in the skin secrete more sweat to cool the body.
The autonomic nervous system, in turn, has two subdivisions: the sympathetic division and parasympathetic division. The two subdivisions of the autonomic system are summarized in Figure \(4\). Both affect the same organs and glands, but they generally do so in opposite ways.
• The sympathetic division controls the fight-or-flight response. Changes occur in organs and glands throughout the body that prepare the body to fight or flee in response to a perceived danger. For example, the heart rate speeds up, air passages in the lungs become wider, more blood flows to the skeletal muscles, and the digestive system temporarily shuts down.
• The parasympathetic division returns the body to normal after the fight-or-flight response has occurred. For example, it slows down the heart rate, narrows air passages in the lungs, reduces blood flow to the skeletal muscles, and stimulates the digestive system to start working again. The parasympathetic division also maintains the internal homeostasis of the body at other times.
Disorders of the Peripheral Nervous System
Unlike the CNS, which is protected by bones, meninges, and cerebrospinal fluid, the PNS has no such protections. The PNS also has no blood-brain barrier to protect it from toxins and pathogens in the blood. Therefore, the PNS is more subject to injury and disease than is the CNS. Causes of nerve injury include diabetes, infectious diseases such as shingles, and poisoning by toxins such as heavy metals. Disorders of the PNS often have symptoms such as loss of feeling, tingling, burning sensations, or muscle weakness. If a traumatic injury results in a nerve being transacted (cut all the way through), it may regenerate, but this is a very slow process and may take many months.
Review
1. Describe the general structure of the peripheral nervous system, and state its primary function.
2. What are ganglia?
3. Identify three types of nerves based on the direction in which they carry nerve impulses.
4. Outline all of the divisions of the peripheral nervous system.
5. Compare and contrast the somatic and autonomic nervous systems.
6. When and how does the sympathetic division of the autonomic nervous system affect the body?
7. What is the function of the parasympathetic division of the autonomic nervous system? What specific effects does it have on the body?
8. Name and describe two disorders of the peripheral nervous system.
9. Give one example of how the CNS interacts with the PNS to control a function in the body.
10. For each of the following types of information, identify whether the neuron carrying it is sensory or motor and whether it is most likely in the somatic or autonomic nervous system.
1. Visual information
2. Blood pressure information
3. Information that causes muscle contraction in digestive organs after eating
4. Information that causes muscle contraction in skeletal muscles based on the person’s decision to make a movement
11. The cranial nerves:
1. Carry sensory information
2. Carry motor information
3. Are part of the somatic nervous system
4. All of the above
12. True or False. All of the spinal nerves carry both sensory and motor information.
13. True or False. The sympathetic nervous system enhances digestion to provide more energy for the body.
Explore More
Mindfulness techniques have been shown to reduce symptoms of depression as well as those of anxiety and stress. They have also been shown to be useful for pain management and performance enhancement. Specific mindfulness programs include Mindfulness-Based Stress Reduction (MBSR) and Mindfulness Mind-Fitness Training (MMFT). You can learn more about MBSR by watching the video below.
Ever wonder why "hot" peppers are perceived as hot? Check out this link:
Attributions
1. Ashton playing the piano by Dominic Smith, licensed CC BY-NC-SA 2.0 via Flickr
2. The nervous system licensed CC BY-SA 4.0 via Lumen Learning
3. Afferent nerve by Pearson Scott Foresman, Public domain via Wikimedia Commons
4. Autonomic nervous system by Geo-Science-International, dedicated CC0 via Wikimedia Commons
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/11%3A_Nervous_System/11.6%3A_Peripheral_Nervous_System.txt |
Seeing is Believing
Figure \(1\) appears at first glance to be just a pattern of colored leaves, but hidden within it is the three-dimensional shape of an ant. Can you see the ant among the leaves? This figure is an example of a stereogram, which is a two-dimensional picture that reveals a three-dimensional object when viewed correctly. If you can’t see the hidden image, it doesn’t mean that there is anything wrong with your eyes. It’s all in how your brain interprets what your eyes are sensing. The eyes are special sensory organs, and vision is one of our special senses.
Special and General Senses
The human body has two basic types of senses, called special senses and general senses. Special senses have specialized sense organs that gather sensory information and change it into nerve impulses. Special senses include the vision for which the eyes are the specialized sense organs, hearing (ears), balance (ears), taste (tongue), and smell (nasal passages). General senses, in contrast, are all associated with the sense of touch and lack special sense organs. Instead, sensory information about touch is gathered by the skin and other body tissues, all of which have important functions besides gathering sensory information. Whether the senses are special or general, however, all of them depend on cells called sensory receptors.
Sensory Receptors
A sensory receptor is a specialized nerve cell that responds to a stimulus in the internal or external environment by generating a nerve impulse. The nerve impulse then travels along with the sensory (afferent) nerve to the central nervous system for processing and to form a response.
There are several different types of sensory receptors that respond to different kinds of stimuli:
• Mechanoreceptors respond to mechanical forces such as pressure, roughness, vibration, and stretching. Most mechanoreceptors are found in the skin and are needed for the sense of touch. Mechanoreceptors are also found in the inner ear where they are needed for the senses of hearing and balance.
• Thermoreceptors respond to variations in temperature. They are found mostly in the skin and detect temperatures that are above or below body temperature.
• Nociceptors respond to potentially damaging stimuli, which are generally perceived as pain. They are found in internal organs as well as on the surface of the body. Different nociceptors are activated depending on the particular stimulus. For example, some detect damaging heat or cold, others detect excessive pressure, and still, others detect painful chemicals such as very hot spices in food.
• Photoreceptors detect and respond to light. Most photoreceptors are found in the eyes and are needed for the sense of vision.
• Chemoreceptors respond to certain chemicals. They are found mainly in taste buds on the tongue, where they are needed for the sense of taste; and in nasal passages, where they are needed for the sense of smell.
Touch
Touch is the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. These types of stimuli are detected by mechanoreceptors, thermoreceptors, and nociceptors all over the body, but most noticeably in the skin. These receptors are especially concentrated on the tongue, lips, face, palms of the hands, and soles of the feet. Various types of tactile receptors in the skin are shown in Figure \(2\).
Vision
Vision, or sight, is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light and forms images. However, the eye is not sufficient for us to see. The brain also plays a necessary role in vision.
How the Eye Works
Figure \(3\) shows the anatomy of the human eye in cross-section. The eye gathers and focuses light to form an image and then changes the image to nerve impulses that travel to the brain. How the eye performs these functions is summarized in the following steps.
1. Light passes first through the cornea, which is a clear outer layer that protects the eye and helps to focus the light by refracting, or bending, it.
2. Light next enters the interior of the eye through an opening called the pupil. The size of this opening is controlled by the colored part of the eye, called the iris, which adjusts the size based on the brightness of the light. The iris causes the pupil to narrow in bright light and widen in dim light.
3. The light then passes through the lens, which refracts the light even more and focuses it on the retina at the back of the eye as an inverted image.
4. The retina contains photoreceptor cells of two types, called rods and cones. Rods, which are found mainly in all areas of the retina other than the very center, are particularly sensitive to low levels of light. Cones, which are found mainly in the center of the retina, are sensitive to light of different colors and allow color vision. The rods and cones convert the light that strikes them to nerve impulses.
5. The nerve impulses from the rods and cones travel to the optic nerve via the optic disc, which is a circular area at the back of the eye where the optic nerve connects to the retina.
Role of the Brain in Vision
The optic nerves from both eyes meet and cross just below the bottom of the hypothalamus in the brain. The information from both eyes is sent to the visual cortex in the occipital lobe of the cerebrum, which is part of the cerebral cortex. The visual cortex is the largest system in the human brain and is responsible for processing visual images. It interprets messages from both eyes and “tells” us what we are seeing.
Vision Problems
Vision problems are very common. Two of the most common are myopia and hyperopia, and they often start in childhood or adolescence. Another common problem, called presbyopia, occurs in most people beginning in middle adulthood. All three problems result in blurred vision due to the failure of the eyes to focus images correctly on the retina.
Myopia
Myopia, or nearsightedness, occurs when the light that comes into the eye does not directly focus on the retina but in front of it, as shown in Figure \(4\). This causes the image of distant objects to be out of focus but does not affect the focus of close objects. Myopia may occur because the eyeball is elongated from front to back or because the cornea is too curved. Myopia can be corrected through the use of corrective lenses, either eyeglasses or contact lenses. Myopia can also be corrected by refractive surgery performed with a laser.
Hyperopia
Hyperopia, or farsightedness, occurs when the light that comes into the eye does not directly focus on the retina but behind it, as shown in Figure \(5\). This causes the image of close objects to be out of focus but does not affect the focus of distant objects. Hyperopia may occur because the eyeball is too short from front to back or because the lens is not curved enough. Hyperopia can be corrected through the use of corrective lenses or laser surgery.
Presbyopia
Presbyopia is a vision problem associated with aging in which the eye gradually loses its ability to focus on close objects. The precise cause of presbyopia is not known for certain, but evidence suggests that the lens may become less elastic with age, and the muscles that control the lens may lose power as people grow older. The first signs of presbyopia – eyestrain, difficulty seeing in dim light, problems focusing on small objects, and fine print – are usually first noticed between the ages of 40 and 50. Most older people with this problem use corrective lenses to focus on close objects because surgical procedures to correct presbyopia have not been as successful as those for myopia and hyperopia.
Hearing
Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. Sound waves enter the ear through the ear canal and travel to the eardrum (see the diagram of the ear in Figure \(6\)). The sound waves strike the eardrum and make it vibrate. The vibrations then travel through the three tiny bones (hammer, anvil, and stirrup) of the middle ear, which amplify the vibrations. From the middle ear, the vibrations pass to the cochlea in the inner ear. The cochlea is a coiled tube filled with liquid. The liquid moves in response to the vibrations, causing tiny hair cells (which are mechanoreceptors) lining the cochlea to bend. In response, the hair cells send nerve impulses to the auditory nerve, which carries the impulses to the brain. The brain interprets the impulses and “tells” us what we are hearing.
Taste and Smell
Taste and smell are both abilities to sense chemicals, so taste and olfactory (odor) receptors are chemoreceptors. Both types of chemoreceptors send nerve impulses to the brain along sensory nerves, and the brain “tells” us what we are tasting or smelling.
Taste receptors are found in tiny bumps on the tongue called taste buds. You can see a diagram of a taste receptor cell and related structures in Figure \(7\). Taste receptor cells make contact with chemicals in food through tiny openings called taste pores. When certain chemicals bind with taste receptor cells, it generates nerve impulses that travel through afferent nerves to the CNS. There are separate taste receptors for sweet, salty, sour, bitter, and meaty tastes. The meaty or savory taste is called umami.
Olfactory receptors line the passages inside the nasal passages (Figure \(8\)). There are millions of olfactory receptors, which sense chemicals in the air. Unlike taste receptors, which can sense only five different tastes, olfactory receptors can sense hundreds of different odors and send signals to the olfactory bulb of the brain. Did you ever notice that food seems to have less taste when you have a stuffy nose? This occurs because the sense of smell contributes to the sense of taste, and a stuffy nose interferes with the ability to smell.
Feature: Human Biology in the News
The most common cause of blindness in the Western hemisphere is age-related macular degeneration (AMD). About 15 million people in the United States have this type of blindness, and 30 million people are affected worldwide. At present, there is no cure for AMD. The disease occurs with the death of a layer of cells called retinal pigment epithelium, which normally provides nutrients and other support to the macula of the eye. The macula is an oval-shaped pigmented area near the center of the retina that is specialized for high visual acuity and has the retina’s greatest concentration of cones. When the epithelial cells die and the macula is no longer supported or nourished, the macula also starts to die. Patients experience a black spot in the center of their vision, and as the disease progresses, the black spot grows outward. Patients eventually lose the ability to read and even to recognize familiar faces before developing total blindness.
In 2016, a landmark surgery was performed as a trial on a patient with severe AMD. In the first-ever operation of its kind, Dr. Pete Coffey of the University of London implanted a tiny patch of cells behind the retina in each of the patient’s eyes. The cells were retinal pigmented epithelial cells that had been grown in a lab from stem cells, which are undifferentiated cells that have the ability to develop into other cell types. By six months out from the operation, the new cells were still surviving, and the doctor was hopeful that the patient’s vision loss would stop and even be reversed. At that point, several other operations had already been planned to test the new procedure. If these cases are a success, Dr. Coffey predicts that the surgery will become as routine as cataract surgery and prevent millions of patients from losing their vision.
Review
1. Compare and contrast special senses and general senses.
2. What are sensory receptors?
3. List five types of sensory receptors and the type of stimulus each detects.
4. Describe the range of tactile stimuli that are detected in the sense of touch.
5. Explain how the eye collects and focuses light to form an image and converts it to nerve impulses.
6. Identify two common vision problems, including both their causes and their effects on vision.
7. Explain how the structures of the ear collect and amplify sound waves and transform them into nerve impulses.
8. What role does the ear play in balance? Which structures of the ear are involved in balance?
9. Describe two ways that the body senses chemicals and the special sense organs that are involved in these senses.
10. Explain why your skin can detect different types of stimuli, such as pressure and temperature.
11. Choose one. Sensory information is sent to the central nervous system via (efferent/afferent) nerves.
12. Identify a mechanoreceptor used in two different human senses, and describe the type of mechanical stimuli that each one detects.
13. If a person is blind but their retina is functioning properly, where do you think the damage might be? Explain your answer.
14. When you see colors, what receptor cells are activated? Where are these receptors located? What lobe of the brain is primarily used to process visual information?
15. The auditory nerve carries:
1. Smell information
2. Taste information
3. Balance information
4. Sound information
Explore More
Some people “see” sounds, “hear” colors, or “taste” words. This rare ability is called synesthesia, and it is thought to be caused by cross-wiring of the senses in the brain. To learn more about this intriguing phenomenon, watch this fascinating TED animation:
Many people experience the dizzying effects of vertigo at some point in their lives. Learn more here:
Attributions
1. Bigant by GifTagger assumed CC BY 3.0 via Wikimedia Commons
2. Skin tactile receptors by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
3. Eye anatomy by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. CC BY 3.0 via Wikimedia Commons
4. Myopia by National Eye Institute, public domain via Wikimedia Commons
5. Hyperopia by National Eye Institute, public domain via Wikimedia Commons
6. Human ear public domain via Wikimedia Commons
7. Taste buds by Jonas Töle dedicated CC0 via Wikimedia Commons
8. Head olfactory nerve by Patrick J. Lynch, medical illustrator, CC BY 2.5 via Wikimedia Commons
9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/11%3A_Nervous_System/11.7%3A_Human_Senses.txt |
Art in a Cup
Who knew that a cup of coffee could also be a work of art? A talented barista can make coffee look as good as it tastes. If you are a coffee drinker, you probably know that coffee can also affect your mental state. It can make you more alert and may improve your concentration. That’s because the caffeine in coffee is a psychoactive drug. In fact, caffeine is the most widely consumed psychoactive substance in the world. In North America, for example, 90 percent of adults consume caffeine daily.
Psychoactive drugs are substances that change the function of the brain and result in alterations of mood, thinking, perception, and/or behavior. Psychoactive drugs may be used for many purposes, including therapeutic, ritual, or recreational purposes. Besides caffeine, other examples of psychoactive drugs include cocaine, LSD, alcohol, tobacco, codeine, and morphine. Psychoactive drugs may be legal prescription medications (e.g., codeine and morphine), legal nonprescription drugs (e.g., alcohol and tobacco), or illegal drugs (cocaine and LSD).
Cannabis (or marijuana) is also a psychoactive drug, but its status is in flux, at least in the United States. Depending on the jurisdiction, cannabis may be used recreationally and/or medically, and it may be either legal or illegal. Legal prescription medications such as opioids are also used illegally by increasingly large numbers of people. Some legal drugs, such as alcohol and nicotine, are readily available almost everywhere, as illustrated by the sign pictured in Figure \(2\).
Classes of Psychoactive Drugs
Psychoactive drugs are divided into different classes according to their pharmacological effects. Several classes are listed below, along with examples of commonly used drugs in each class.
• Stimulants are drugs that stimulate the brain and increase alertness and wakefulness. Examples of stimulants include caffeine, nicotine, cocaine, and amphetamines such as Adderall.
• Depressants are drugs that calm the brain, reduce anxious feelings, and induce sleepiness. Examples of depressants include ethanol (in alcoholic beverages) and opioids such as codeine and heroin.
• Anxiolytics are drugs that have a tranquilizing effect and inhibit anxiety. Examples of anxiolytic drugs include benzodiazepines such as diazepam (Valium), barbiturates such as phenobarbital, opioids, and antidepressant drugs such as sertraline (Zoloft).
• Euphoriants are drugs that bring about a state of euphoria, or intense feelings of well-being and happiness. Examples of euphoriants include the so-called club drug MDMA (ecstasy), amphetamines, ethanol, and opioids such as morphine.
• Hallucinogens are drugs that can cause hallucinations and other perceptual anomalies. They also cause subjective changes in thoughts, emotions, and consciousness. Examples of hallucinogens include LSD, mescaline, nitrous oxide, and psilocybin.
• Empathogens are drugs that produce feelings of empathy, or sympathy with other people. Examples of empathogens include amphetamines and MDMA.
Many psychoactive drugs have multiple effects so they may be placed in more than one class. An example is MDMA, pictured below, which may act both as a euphoriant and as an empathogen. In some people, MDMA may also have stimulant or hallucinogenic effects. As of 2016, MDMA had no accepted medical uses, but it was undergoing testing for use in the treatment of post-traumatic stress disorder and certain other types of anxiety disorders.
Mechanisms of Action
Psychoactive drugs generally produce their effects by affecting brain chemistry, which in turn may cause changes in a person’s mood, thinking, perception, and/or behavior. Each drug tends to have a specific action on one or more neurotransmitters or neurotransmitter receptors in the brain. Generally, they act as either agonists or antagonists.
• Agonists are drugs that increase the activity of particular neurotransmitters. They might act by promoting the synthesis of the neurotransmitters, reducing their reuptake from synapses, or mimicking their action by binding to receptors for the neurotransmitters.
• Antagonists are drugs that decrease the activity of particular neurotransmitters. They might act by interfering with the synthesis of the neurotransmitters or by blocking their receptors so the neurotransmitters cannot bind to them.
Consider the example of the neurotransmitter GABA. This is one of the most common neurotransmitters in the brain, and it normally has an inhibitory effect on cells. GABA agonists, which increase its activity, include ethanol, barbiturates, and benzodiazepines, among other psychoactive drugs. All of these drugs work by promoting the activity of GABA receptors in the brain.
Uses of Psychoactive Drugs
You may have been prescribed psychoactive drugs by your doctor. For example, you may have been prescribed an opioid drug such as codeine for pain (most likely in the form of Tylenol with added codeine). Chances are you also use nonprescription psychoactive drugs, such as caffeine for mental alertness. These are just two of the many possible uses of psychoactive drugs.
Medical Uses
Medical uses of psychoactive drugs include general anesthesia, in which pain is blocked and unconsciousness is induced. General anesthetics are most often used during surgical procedures and may be administered in gaseous form, as in the photo below. General anesthetics include the drugs halothane and ketamine. Other psychoactive drugs are used to manage pain without affecting consciousness. They may be prescribed either for acute pain in cases of trauma such as broken bones; or for chronic pain such as pain caused by arthritis, cancer, or fibromyalgia. Most often, the drugs used for pain control are opioids, such as morphine and codeine.
Many psychiatric disorders are also managed with psychoactive drugs. For example, antidepressants such as sertraline are used to treat depression, anxiety, and eating disorders. Anxiety disorders may also be treated with anxiolytics, such as buspirone and diazepam. Stimulants such as amphetamines are used to treat attention deficit disorder. Antipsychotics such as clozapine and risperidone, as well as mood stabilizers such as lithium, are used to treat schizophrenia and bipolar disorder.
Ritual Uses
Certain psychoactive drugs, particularly hallucinogens, have been used for ritual purposes since prehistoric times. For example, Native Americans have used the mescaline-containing peyote cactus (pictured below) for religious ceremonies for as long as 5,700 years. In prehistoric Europe, the mushroom Amanita muscaria, which contains a hallucinogenic drug called muscimol, was used for similar purposes. Various other psychoactive drugs — including jimsonweed, psilocybin mushrooms, and cannabis — have also been used by various peoples for ritual purposes for millennia.
Recreational Uses
The recreational use of psychoactive drugs generally has the purpose of altering one’s consciousness and creating a feeling of euphoria commonly called a “high.” Some of the drugs used most commonly for recreational purposes include cannabis, ethanol, opioids, and stimulants such as nicotine. Hallucinogens are also used recreationally, primarily for the alterations in thinking and perception that they cause.
Some investigators have suggested that the urge to alter one’s state of consciousness is a universal human drive, similar to the drive to satiate thirst, hunger, or sexual desire. They think that the drive to alter one’s state of mind is even present in children, who may attain an altered state by repetitive motions such as spinning or swinging. Some nonhuman animals also exhibit a drive to experience altered states. For example, they may consume fermented berries or fruit and become intoxicated. The way cats respond to catnip (Figure \(6\)) is another example.
Addiction, Dependence, and Rehabilitation
Psychoactive substances often bring about subjective changes that the user may find pleasant (for example, euphoria) or advantageous (for example, increased alertness). These changes are rewarding and positively reinforcing, so they have the potential for misuse, addiction, and dependence. Addiction refers to the compulsive use of a drug despite the negative consequences that such use may entail. Sustained use of an addictive drug may produce dependence on the drug. Dependence may be physical and/or psychological. It occurs when cessation of drug use produces withdrawal symptoms. Physical dependence produces physical withdrawal symptoms, which may include tremors, pain, seizures, or insomnia. Psychological dependence produces psychological withdrawal symptoms, such as anxiety, depression, paranoia, or hallucinations.
Rehabilitation for drug dependence and addiction typically involves psychotherapy, which may include both individual and group therapy. Organizations such as Alcoholics Anonymous (AA) and Narcotics Anonymous (NA) may also be helpful for people trying to recover from addiction. These groups are self-described as international mutual aid fellowships with the primary purpose of helping addicts achieve and maintain sobriety. In some cases, rehabilitation is aided by the temporary use of psychoactive substances that reduce cravings and withdrawal symptoms without creating addiction themselves. For example, the drug methadone is commonly used in the treatment of heroin addiction.
Feature: Human Biology in the News
Currently, in the United States, a lot of media attention is being given to a rising tide of opioid addiction and overdose deaths. Opioids are drugs derived from the opium poppy or synthetic versions of such drugs. They include illegal drug heroin and prescription painkillers such as codeine, morphine, hydrocodone, oxycodone, and fentanyl. In 2016, fentanyl received wide media attention when it was announced that an accidental fentanyl overdose was responsible for the death of rock-music icon Prince. Fentanyl is an extremely strong and dangerous drug, said to be 50 to 100 times stronger than morphine, making the risk of overdose death from fentanyl very high.
The dramatic increase in opioid addiction and overdose deaths has been called an opioid epidemic. It is considered to be the worst drug crisis in American history. Consider the following facts:
• In 1999, there were more than twice as many accidental deaths from motor vehicle crashes than from drug overdoses. By 2014, these causes of accidental death were reversed, with close to 40 percent more accidental deaths from drug overdoses than car crashes. The majority of these drug overdose deaths were from heroin and opioid painkillers.
• In 1999, the stimulant drug cocaine killed about twice as many people as did heroin. By 2014, deaths from heroin were up by 439 percent. During the same interval, deaths from cocaine also rose slightly but were a much smaller proportion of all drug deaths than those caused by heroin.
The opioid epidemic in the United States has occurred in all demographic groups, including every ethnic, age, gender, and socioeconomic category. What has caused this epidemic? The answer appears to be an equally dramatic increase in the medical use of prescription painkillers. In 1991, about 76 million prescriptions were written for painkillers. In 2011, the number of prescriptions for these drugs had risen to 219 million, an almost three-fold increase. During these same two decades, Mexican drug cartels began shipping huge amounts of heroin to the United States. Heroin became cheaper and easier to buy than prescription painkillers. Many people who became addicted to prescription opioids switched to heroin. About 80 percent of new heroin users in 2014 reported started out misusing prescription painkillers.
Doctors, public health professionals, and politicians have all called for new policies, funding, programs, and laws to address the opioid epidemic. Changes that have already been made include a shift from criminalizing to medicalizing the problem, an increase in treatment programs, and more widespread distribution and use of the opioid-overdose antidote naloxone (Narcan). Opioids can slow or stop a person's breathing, which is what usually causes overdose deaths. Naloxone helps the person wake up and keeps them breathing until emergency medical treatment can be provided.
Review
1. What are psychoactive drugs?
2. Identify six classes of psychoactive drugs and an example of a drug in each class.
3. Compare and contrast psychoactive drugs that are agonists and psychoactive drugs that are antagonists.
4. Describe two medical uses of psychoactive drugs.
5. Give an example of a ritual use of a psychoactive drug.
6. Why do people generally use psychoactive drugs recreationally?
7. Define addiction.
8. Identify possible withdrawal symptoms associated with physical dependence on a psychoactive drug.
9. Why might a person with a heroin addiction be prescribed the psychoactive drug methadone?
10. The prescription drug Prozac inhibits the reuptake of the neurotransmitter serotonin, causing more serotonin to be present in the synapse. Prozac can elevate mood, which is why it is sometimes used to treat depression. Answer the following questions about Prozac.
1. Is Prozac an agonist or an antagonist for serotonin? Explain your answer.
2. Is Prozac a psychoactive drug? Explain your answer.
11. Name 3 classes of psychoactive drugs that include opioids.
12. True or False. All psychoactive drugs are either illegal or available by prescription only.
13. True or False. Anxiolytics might be prescribed by a physician.
14. Name two drugs that activate receptors for the neurotransmitter GABA. Why do you think these drugs generally have a depressant effect?
Explore More
Learn more about psychiatric drugs that are being researched to treat mental health disorders. In this inspiring TED talk, neurobiologist David Anderson explains how modern psychiatric drugs treat the chemistry of the whole brain and why a more nuanced view of how the brain functions could lead to targeted psychiatric drugs that work better and avoid side effects.
Attributions
1. Murano Coffee by C. Michael Neely CC BY-SA 3.0 via Wikimedia Commons
2. Liquor Specials by Steve Snodgrass
3. CC BY 2.0 via Wikimedia Commons
4. Ecstasy monogram by DEA,
5. public domain via Wikimedia Commons
6. Preoxygenation before anesthetic induction by ISAF Headquarters Public Affairs Office, licensed
7. CC BY 2.0 via Wikimedia Commons
8. Lophophora williamsii pm by Peter A. Mansfeld,
9. CC BY 3.0 via Wikimedia Commons
10. Cat on the back by Pbtflakes Free Art License; via Wikicommons
11. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/11%3A_Nervous_System/11.8%3A_Psychoactive_Drugs.txt |
Case Study Conclusion: Fading Memory
Figure \(1\) illustrates some of the molecular and cellular changes that occur in Alzheimer’s disease (AD), which Rosa was diagnosed with at the beginning of this chapter, after experiencing memory problems and other changes in her cognitive functioning, mood, and personality. These abnormal changes in the brain include the development of amyloid plaques between brain cells and neurofibrillary tangles inside of neurons. These hallmark characteristics of AD are associated with the loss of synapses between neurons, and ultimately the death of neurons.
After reading this chapter, you should have a good appreciation for the importance of keeping neurons alive and communicating with each other at synapses. The nervous system coordinates all of the body’s voluntary and involuntary activities. It interprets information from the outside world through sensory systems and makes appropriate responses through the motor system, through communication between the PNS and CNS. The brain directs the rest of the nervous system and controls everything from basic vital functions such as heart rate and breathing to high-level functions such as problem-solving and abstract thought. The nervous system is able to perform these important functions by generating action potentials in neurons in response to stimulation and sending messages between cells at synapses, typically using chemical neurotransmitter molecules. When neurons are not functioning properly, lose their synapses, or die, they cannot carry out the signaling that is essential for the proper functioning of the nervous system.
AD is a progressive neurodegenerative disease, meaning that the damage to the brain becomes more extensive as time goes on. Figure \(2\) illustrates how the damage progresses from before AD is diagnosed (preclinical AD), to mild and moderate AD, and finally to severe AD.
You can see that the damage starts in a relatively small location towards the bottom of the brain. One of the earliest brain areas to be affected by AD is the hippocampus. The hippocampus is important for learning and memory. This explains why many of Rosa’s symptoms of mild AD involve deficits in memory, such as trouble remembering where she placed objects, recent conversations, and appointments.
As AD progresses, more of the brain is affected, including areas involved in emotional regulation, social behavior, planning, language, spatial navigation, and higher-level thought. Rosa is beginning to show signs of problems in these areas, including irritability, lashing out at family members, getting lost in her neighborhood, problems finding the right words, putting objects in unusual locations, and difficulty in managing her finances. You can see that as AD progresses, damage spreads further across the cerebrum, which you now know controls conscious functions such as reasoning, language, and interpretation of sensory stimuli. You can also see how the frontal lobe, which controls executive functions such as planning, self-control, and abstract thought, becomes increasingly damaged.
Increasing damage to the brain causes corresponding deficits in functioning. In moderate AD, patients have increased memory, language, and cognitive deficits compared to mild AD. They may not recognize their own family members, and may wander and get lost, engage in inappropriate behaviors, become easily agitated, and have trouble carrying out daily activities such as dressing. In severe AD, much of the brain is affected. Patients usually cannot recognize family members or communicate and are fully dependent on others for their care. They begin to lose the ability to control their basic functions, such as bladder and bowel control and proper swallowing. Eventually, AD causes death, usually as a result of this loss of basic functions.
For now, Rosa only has mild AD is still able to function relatively well with care from her family. The medication her doctor gave her has helped improve some of her symptoms. It is a cholinesterase inhibitor, which blocks an enzyme that normally degrades the neurotransmitter acetylcholine. With more of the neurotransmitter available, more of it can bind to neurotransmitter receptors on postsynaptic cells. Therefore, this drug acts as an agonist for acetylcholine, which enhances communication between neurons in Rosa’s brain. This increase in neuronal communication can help restore some of the functions lost in early Alzheimer’s disease and may slow the progression of symptoms.
But medication such as this is only a short-term measure and does not halt the progression of the underlying disease. Ideally, the damaged or dead neurons would be replaced by new, functioning neurons. Why does this not happen automatically in the body? As you have learned, neurogenesis is very limited in adult humans, so once neurons in the brain die, they are not normally replaced to any significant extent. However, scientists are studying the ways in which neurogenesis might be able to be increased in cases of disease or injury to the brain. Also, they are investigating the possibility of using stem cell transplants to replace damaged or dead neurons with new neurons. But this research is in very early stages and is not currently a treatment for AD.
One promising area of research is in the development of methods to allow earlier detection and treatment of AD, given that the changes in the brain may actually start 10 to 20 years before the diagnosis of AD. For example, a radiolabeled chemical called Pittsburgh Compound B (PiB) binds to amyloid plaques in the brain and in the future may be used in conjunction with brain imaging techniques to detect early signs of AD. Scientists are also looking for biomarkers in bodily fluids such as blood and cerebrospinal fluid that might indicate the presence of AD before symptoms appear. Finally, researchers are also investigating possible early and subtle symptoms, such as changes in how people move or a loss of smell, to see whether they can be used to identify people who will go on to develop AD. This research is in the early stages, but the hope is that patients can be identified earlier to provide earlier and possibly more effective treatment and to allow families more time to plan.
Scientists are also still trying to fully understand the causes of AD, which affects more than 5 million Americans. Some genetic mutations have been identified that play a role, but environmental factors also appear to be important. With more research into the causes and mechanisms of AD, hopefully, a cure can be found, and people like Rosa can live a longer and better life.
Chapter Summary
In this chapter, you learned about the human nervous system. Specifically, you learned that:
• The nervous system is the organ system that coordinates all of the body’s voluntary and involuntary actions by transmitting signals to and from different parts of the body. It has two major divisions, the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord.
• The PNS consists mainly of nerves that connect the CNS with the rest of the body. It has two major divisions: the somatic nervous system and the autonomic nervous system. The somatic system controls activities that are under voluntary control. The autonomic system controls activities that are involuntary.
• The autonomic nervous system is further divided into the sympathetic division, which controls the fight-or-flight response; the parasympathetic division, which controls most routine involuntary responses; and the enteric division, which provides local control for digestive processes.
• Signals sent by the nervous system are electrical signals called nerve impulses. They are transmitted by special, electrically excitable cells called neurons, which are one of two major types of cells in the nervous system.
• Glial cells are the other major type of nervous system cells. There are many types of glial cells, and they have many specific functions. In general, glial cells function to support, protect, and nourish neurons.
• The main parts of a neuron include the cell body, dendrites, and axon. The cell body contains the nucleus. Dendrites receive nerve impulses from other cells, and the axon transmits nerve impulses to other cells at axon terminals. A synapse is a complex membrane junction at the end of an axon terminal that transmits signals to another cell.
• Axons are often wrapped in an electrically-insulating myelin sheath, which is produced by glial cells. Electrical impulses called action potentials occur at gaps in the myelin sheath, called nodes of Ranvier, which speeds the conduction of nerve impulses down the axon.
• Neurogenesis, or the formation of new neurons by cell division, may occur in a mature human brain but only to a limited extent.
• The nervous tissue in the brain and spinal cord consists of gray matter, which contains mainly the cell bodies of neurons; and white matter, which contains mainly myelinated axons of neurons. Nerves of the peripheral nervous system consist of long bundles of myelinated axons that extend throughout the body.
• There are hundreds of types of neurons in the human nervous system, but many can be classified on the basis of the direction in which they carry nerve impulses. Sensory neurons carry nerve impulses away from the body and toward the central nervous system, motor neurons carry them away from the central nervous system and toward the body, and interneurons often carry them between sensory and motor neurons.
• A nerve impulse is an electrical phenomenon that occurs because of a difference in electrical charge across the plasma membrane of a neuron.
• The sodium-potassium pump maintains an electrical gradient across the plasma membrane of a neuron when it is not actively transmitting a nerve impulse. This gradient is called the resting potential of the neuron.
• An action potential is a sudden reversal of the electrical gradient across the plasma membrane of a resting neuron. It begins when the neuron receives a chemical signal from another cell or some other type of stimulus. The action potential travels rapidly down the neuron’s axon as an electric current.
• A nerve impulse is transmitted to another cell at either an electrical or a chemical synapse. At a chemical synapse, neurotransmitter chemicals are released from the presynaptic cell into the synaptic cleft between cells. The chemicals travel across the cleft to the postsynaptic cell and bind to receptors embedded in its membrane.
• There are many different types of neurotransmitters. Their effects on the postsynaptic cell generally depend on the type of receptor they bind to. The effects may be excitatory, inhibitory, or modulatory in more complex ways. Both physical and mental disorders may occur if there are problems with neurotransmitters or their receptors.
• The CNS includes the brain and spinal cord. It is physically protected by bones, meninges, and cerebrospinal fluid. It is chemically protected by the blood-brain barrier.
• The brain is the control center of the nervous system and of the entire organism. The brain uses a relatively large proportion of the body’s energy, primarily in the form of glucose.
• The brain is divided into three major parts, each with different functions: brain stem, cerebellum, and cerebrum. The cerebrum is further divided into left and right hemispheres. Each hemisphere has four lobes: frontal, parietal, temporal, and occipital. Each lobe is associated with specific senses or other functions.
• The cerebrum has a thin outer layer called the cerebral cortex. Its many folds give it a large surface area. This is where most information processing takes place.
• Inner structures of the brain include the hypothalamus, which controls the endocrine system via the pituitary gland; and the thalamus, which has several involuntary functions.
• The spinal cord is a tubular bundle of nervous tissues that extends from the head down the middle of the back to the pelvis. It functions mainly to connect the brain with the PNS. It also controls certain rapid responses called reflexes without input from the brain.
• A spinal cord injury may lead to paralysis (loss of sensation and movement) of the body below the level of the injury because nerve impulses can no longer travel up and down the spinal cord beyond that point.
• The PNS consists of all the nervous tissue that lies outside of the CNS. Its main function is to connect the CNS to the rest of the organism.
• The tissues that make up the PNS are nerves and ganglia. Ganglia act as relay points for messages that are transmitted through nerves. Nerves are classified as sensory, motor, or a mix of the two.
• The PNS is not as well protected physically or chemically as the CNS, so it is more prone to injury and disease. PNS problems include injury from diabetes, shingles, and heavy metal poisoning. Two disorders of the PNS are Guillain-Barre syndrome and Charcot-Marie-Tooth disease.
• The human body has two major types of senses, special senses, and general senses. Special senses have specialized sense organs and include vision (eyes), hearing (ears), balance (ears), taste (tongue), and smell (nasal passages). General senses are all associated with touch and lack special sense organs. Touch receptors are found throughout the body but particularly in the skin.
• All senses depend on sensory receptor cells to detect sensory stimuli and transform them into nerve impulses. Types of sensory receptors include mechanoreceptors (mechanical forces), thermoreceptors (temperature), nociceptors (pain), photoreceptors (light), and chemoreceptors (chemicals).
• Touch includes the ability to sense pressure, vibration, temperature, pain, and other tactile stimuli. The skin includes several different types of touch receptor cells.
• Vision is the ability to sense light and see. The eye is the special sensory organ that collects and focuses light, forms images, and changes them to nerve impulses. Optic nerves send information from the eyes to the brain, which processes the visual information and “tells” us what we are seeing.
• Common vision problems include myopia (nearsightedness), hyperopia (farsightedness), and presbyopia (age-related decline in close vision).
• Hearing is the ability to sense sound waves, and the ear is the organ that senses sound. It changes sound waves to vibrations that trigger nerve impulses, which travel to the brain through the auditory nerve. The brain processes the information and “tells” us what we are hearing.
• The ear is also the organ that is responsible for the sense of balance, which is the ability to sense and maintain an appropriate body position. The ears send impulses on head position to the brain, which sends messages to skeletal muscle via the peripheral nervous system. The muscles respond by contracting to maintain balance.
• Taste and smell are both abilities to sense chemicals. Taste receptors in taste buds on the tongue sense chemicals in food and olfactory receptors in the nasal passages sense chemicals in the air. The sense of smell contributes significantly to the sense of taste.
• Psychoactive drugs are substances that change the function of the brain and result in alterations of mood, thinking, perception, and/or behavior. They include prescription medications such as opioid painkillers, legal substances such as nicotine and alcohol, and illegal drugs such as LSD and heroin.
• Psychoactive drugs are divided into different classes according to their pharmacological effects. They include stimulants, depressants, anxiolytics, euphoriants, hallucinogens, and empathogens. Many psychoactive drugs have multiple effects so they may be placed in more than one class.
• Psychoactive drugs generally produce their effects by affecting brain chemistry. Generally, they act either as agonists, which enhance the activity of particular neurotransmitters; or as antagonists, which decrease the activity of particular neurotransmitters.
• Psychoactive drugs are used for various purposes, including medical, ritual, and recreational purposes.
• Misuse of psychoactive drugs may lead to addiction, which is the compulsive use of a drug despite negative consequences. Sustained use of an addictive drug may produce physical or psychological dependence on the drug. Rehabilitation typically involves psychotherapy and sometimes the temporary use of other psychoactive drugs.
In addition to the nervous system, there is another system of the body that is important for coordinating and regulating many different functions – the endocrine system. You will learn about the endocrine system in the next chapter.
Chapter Summary Review
1. Imagine that you decide to make a movement. To carry out this decision, a neuron in the cerebral cortex of your brain (neuron A) fires a nerve impulse that is sent to a neuron in your spinal cord (neuron B). Neuron B then sends the signal to a muscle cell, causing it to contract, resulting in movement. Answer the following questions about this pathway.
1. Which part of the brain is neuron A located in — the cerebellum, cerebrum, or brain stem? Explain how you know.
2. The cell body of neuron A is located in a lobe of the brain that is involved in abstract thought, problem-solving and planning. Which lobe is this?
3. Part of neuron A travels all the way down to the spinal cord to meet neuron B. Which part of neuron A travels to the spinal cord?
4. Neuron A forms a chemical synapse with neuron B in the spinal cord. How is the signal from neuron A transmitted to neuron B?
5. Is neuron A in the central nervous system (CNS) or peripheral nervous system (PNS)?
6. The axon of neuron B travels in a nerve to a skeletal muscle cell. Is the nerve part of the CNS or PNS? Is this an afferent nerve or an efferent nerve?
7. What part of the PNS is involved in this pathway — the autonomic nervous system or the somatic nervous system? Explain your answer.
2. What are the differences between a neurotransmitter receptor and a sensory receptor?
3. Which part of a postsynaptic neuron typically receives the signals from a presynaptic neuron?
1. The axon terminal
2. The nodes of Ranvier
3. The dendrites
4. The cell body
4. True or False. Glial cells produce action potentials.
5. True or False. The spinal cord consists of white matter only.
6. True or False. Axons may be more than a meter long in adult humans.
7. If a person has a stroke and as a result has trouble using language correctly, which hemisphere of their brain was most likely damaged? Explain your answer.
8. The right side of the brain generally controls the which side of the body?
1. right side
2. left side
3. head region
4. trunk and leg regions
9. Electrical gradients are responsible for the resting potential and action potential in neurons. Answer the following questions about the electrical characteristics of neurons.
1. Define what an electrical gradient is, in the context of a cell.
2. What is responsible for maintaining the electrical gradient that results in the resting potential?
3. Compare and contrast the resting potential and the action potential.
4. Where along a myelinated axon does the action potential occur? Why does it happen here?
10. What does it mean that the action potential is “all-or-none?”
11. What determines whether a neurotransmitter has an excitatory or inhibitory effect? Choose the best answer.
1. The neurotransmitter itself
2. The specific receptor for the neurotransmitter on the postsynaptic cell
3. The number of synaptic vesicles in the axon terminal
4. Whether it is in a sensory neuron or a motor neuron
12. Compare and contrast Schwann cells and oligodendrocytes.
13. True or False. The cerebellum makes up most of the brain and is divided into four lobes.
14. True or False. The hypothalamus is part of the brain.
15. Which lobe of the brain processes touch information?
1. Parietal
2. Occipital
3. Cochlea
4. Temporal
16. Information about sounds is mainly sent to which lobe of the brain?
1. Parietal
2. Occipital
3. Cochlea
4. Temporal
17. Rods and cones in the retina are:
1. Mechanoreceptors
2. Nociceptors
3. Photoreceptors
4. Chemoreceptors
18. For the senses of smell and hearing, name their respective sensory receptor cells, what type of receptor cells they are, and what stimuli they detect.
19. True or False. Sensory information such as smell, taste, and sound, are carried to the CNS by cranial nerves.
20. True or False. The parasympathetic nervous system is a division of the central nervous system.
Attributions
1. Characteristics of AD by National Institute on Aging, National Institutes of Health; public domain via Wikimedia Commons
2. Alzheimer’s Disease, Spreads through the Brain by National Institute on Aging, National Institutes of Health; public domain via Flickr.com
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/11%3A_Nervous_System/11.9%3A_Case_Study_Conclusion%3A__Memory_and_Chapter_Summary.txt |
This chapter describes the endocrine system and its vital roles in communication, control, and homeostasis within the human body. The focus is on the pituitary gland, as the master gland of the endocrine system, and three other endocrine glands: the thyroid gland, adrenal glands, and pancreas. The chapter also explains the differing mechanisms of steroid and non-steroid endocrine hormones.
• 12.1: Case Study: Hormones and Health
18 year-old Gabrielle checks her calendar. It has been 42 days since her last menstrual period, two weeks later than the length of the average woman's menstrual cycle. Although many women would suspect pregnancy if their period was late, Gabrielle has not been sexually active.
• 12.2: Introduction to the Endocrine System
The patient in this photo has the characteristic moon face of a disorder named Cushing's syndrome.
• 12.3: Endocrine Hormones
The medication pictured above with the brand name Progynon was a drug used to control the effects of menopause in women.
• 12.4: Pituitary Gland
This adorable nursing infant is part of a positive feedback loop. When he suckles on the nipple, it sends nerve impulses to his mother’s hypothalamus, which "tell" her pituitary gland to release the hormone prolactin into her bloodstream.
• 12.5: Thyroid Gland
A goiter is an abnormal enlargement of the thyroid gland, which is located in the neck. The formation of a goiter may occur in a number of different thyroid disorders. You'll learn why in this concept.
• 12.6: Adrenal Glands
The adrenal glands are endocrine glands that produce a variety of hormones. Adrenal hormones include the fight-or-flight hormone adrenaline and the steroid hormone cortisol. The two adrenal glands are located on both sides of the body, just above the kidneys. The right adrenal gland is smaller and has a pyramidal shape. The left adrenal gland is larger and has a half-moon shape.
• 12.7: Pancreas
Giving yourself an injection can be difficult, but for someone with diabetes, it may be a matter of life or death. The person in the photo has diabetes and is injecting himself with insulin, the hormone that helps control the level of glucose in the blood. Insulin is produced by the pancreas.
• 12.8: Case Study Conclusion: Hormonal and Chapter Summary
Gabrielle, who you read about in the beginning of this chapter, has polycystic ovary syndrome (PCOS). PCOS is named for the multiple fluid-filled sacs, or cysts, that are present in the ovaries of women with this syndrome. You can see these cysts in the illustration above, which compares a normal ovary with a polycystic ovary. The cysts result from follicles in the ovary that did not properly produce and release an egg. Mature eggs are normally released from follicles monthly during the process
Thumbnail: Thyroid and parathyroid glands. (Public Domain; NIH).
12: Endocrine System
Case Study: Hormonal Havoc
18-year-old Gabrielle checks her calendar. It has been 42 days since her last menstrual period, two weeks later than the length of the average woman’s menstrual cycle. Although many women would suspect pregnancy if their period was late, Gabrielle has not been sexually active. She is not even sure she is “late” because her period has never been regular. Ever since her first period at 13 years of age, her cycle lengths have varied greatly, and there are months where she does not get a period at all. Her mother told her that a girl’s period is often irregular when it first starts, but Gabrielle’s still has not become regular five years later. She decides to go to the student health center on her college campus to get it checked out.
The doctor asks her about the timing of her menstrual periods and performs a pelvic exam. She also notices that Gabrielle is overweight, has acne, and excess facial hair. As she explains to Gabrielle, while these physical characteristics can be perfectly normal, in combination with an irregular period they can be signs of a disorder of the endocrine, or hormonal, system called polycystic ovary syndrome (PCOS).
In order to check for PCOS, the doctor refers Gabrielle for a pelvic ultrasound and sends her to the lab to get blood work done. When her lab results come back, Gabrielle learns that her levels of androgens (a group of hormones) are high, and so is her blood glucose (sugar). The ultrasound showed that she has multiple fluid-filled sacs known as cysts in her ovaries. Based on Gabrielle’s symptoms and test results, the doctor tells her that she does indeed have PCOS.
PCOS is common in young women. It is estimated that between 1 in 10 to 20 women of childbearing age have PCOS — as many as five million women in the United States. You may know someone with PCOS or may have it yourself.
Read the rest of this chapter to learn about the glands and hormones of the endocrine system, their functions, how they are regulated, and the disorders — such as PCOS — that can arise when hormones are not regulated properly. At the end of the chapter, you will learn more about PCOS, its possible long-term consequences including fertility problems and diabetes, and how these negative outcomes can sometimes be prevented with lifestyle changes and medications.
Chapter Overview: Endocrine System
In this chapter, you will learn about the endocrine system, a system of glands that secrete hormones that regulate many of the body’s functions. Specifically, you will learn about:
• The glands that make up the endocrine system and how hormones act as chemical messengers in the body.
• The general types of endocrine system disorders.
• The types of endocrine hormones, including steroid hormones such as sex hormones, and non-steroid hormones such as insulin; and how they affect the functions of their target cells by binding to different types of receptor proteins.
• How the levels of hormones are regulated mostly through negative, but sometimes through positive, feedback loops.
• The master gland of the endocrine system, the pituitary gland, controls other parts of the endocrine system through the hormones that it secretes; and how the pituitary itself is regulated by hormones secreted from the hypothalamus of the brain.
• The thyroid gland and its hormones, which regulate processes such as metabolism and calcium homeostasis; how the thyroid is regulated; and the disorders that can occur when there are problems in thyroid hormone regulation, such as hyperthyroidism and hypothyroidism.
• The adrenal glands, which secrete hormones that regulate processes such as metabolism, electrolyte balance, responses to stress, and reproductive functions; and the disorders that can occur when there are problems in adrenal hormone regulation, such as Cushing’s syndrome and Addison’s disease.
• The pancreas, which secretes hormones that regulate blood glucose levels such as insulin; and disorders of the pancreas and its hormones including diabetes.
As you read this chapter, think about the following questions:
1. Why can hormones have such a broad-range effect on the body, such as is seen in PCOS?
2. Which hormones normally regulate blood glucose and how is this related to diabetes?
3. What are androgens? How do you think their functions relate to some of the symptoms that Gabrielle is experiencing?
Attributions
1. Calendar by Andreanna Moya CC BY 2.0 via flickr.com
2. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/12%3A_Endocrine_System/12.1%3A_Case_Study%3A_Hormones_and_Health.txt |
Moon Face
The patient in Figure \(1\) has the characteristic moon face of a disorder named Cushing’s syndrome. Other signs and symptoms of this disorder include abnormal weight gain, acne, and excessive hairiness, among many other abnormalities. What can cause so many different problems in one patient? The answer is the overproduction of the endocrine system hormone cortisol.
Overview of the Endocrine System
The endocrine system is a system of glands called endocrine glands that release chemical messenger molecules called hormones into the bloodstream. Other glands of the body, including sweat glands and salivary glands, also secrete substances but not into the bloodstream. Instead, they secrete them through ducts that carry them to nearby body surfaces. These other glands are called exocrine glands.
Endocrine hormones must travel through the bloodstream to the cells they affect, and this takes time. Because endocrine hormones are released into the bloodstream, they travel throughout the body wherever blood flows. As a result, endocrine hormones may affect many cells and have body-wide effects. Endocrine hormones may cause effects that last for days, weeks, or even months.
Glands of the Endocrine System
The major glands of the endocrine system are shown in Figure \(2\). The glands in the figure are described briefly in the rest of this section. Refer to the figure as you read about the glands in the following text.
Pituitary Gland
The pituitary gland is located at the base of the brain. It is controlled by the nervous system via the brain structure called the hypothalamus, to which it is connected by a thin stalk. The pituitary gland consists of two lobes, called the anterior (front) lobe and posterior (back) lobe. The posterior lobe stores and secretes hormones synthesized by the hypothalamus. The anterior lobe synthesizes and secretes its own endocrine hormones, also under the influence of the hypothalamus. One endocrine hormone secreted by the pituitary gland is growth hormone, which stimulates cells throughout the body to synthesize proteins and divide. Most of the other endocrine hormones secreted by the pituitary gland control other endocrine glands. Generally, these hormones direct the other glands to secrete either more or less of their hormones. This is why the pituitary gland is often referred to as the “master gland” of the endocrine system.
Remaining Glands of the Endocrine System
Each of the other glands of the endocrine system is summarized below. Several of these endocrine glands are also discussed in greater detail in other concepts in the present chapter.
• The thyroid gland is a large gland in the neck. Thyroid hormones such as thyroxine increase the rate of metabolism in cells throughout the body. They control how quickly cells use energy and make proteins.
• The four parathyroid glands are located in the neck behind the thyroid gland. The parathyroid hormone helps keep the level of calcium in the blood within a narrow range. It stimulates bone cells to dissolve calcium and release it into the blood.
• The pineal gland is a tiny gland located near the center of the brain. It secretes the hormone melatonin, which controls the sleep-wake cycle and several other processes. The production of melatonin is stimulated by darkness and inhibited by light. Cells in the retina of the eye detect light and send signals to a structure in the brain named the suprachiasmatic nucleus (SCN). Nerve fibers carry the signals from the SCN to the pineal gland via the autonomic nervous system.
• The pancreas is located near the stomach. Its endocrine hormones include insulin and glucagon, which work together to control the level of glucose in the blood. The pancreas also secretes digestive enzymes into the small intestine.
• The two adrenal glands are located above the kidneys. Adrenal glands secrete several different endocrine hormones, including the hormone adrenaline, which is involved in the fight-or-flight response. Other endocrine hormones secreted by the adrenal glands have a variety of functions. For example, the hormone aldosterone helps to regulate the balance of minerals in the body. The hormone cortisol, which causes Cushing's syndrome when it is produced in excess, is also an adrenal gland hormone.
• The gonads include the ovaries in females and testes in males. They secrete sex hormones, such as testosterone (in males) and estrogen (in females). These hormones control sexual maturation during puberty and the production of gametes (sperm or egg cells) by the gonads after sexual maturation.
• The thymus gland is located in front of the heart. It is the site where immune system cells called T cells mature. T cells are critical to the adaptive immune system, in which the body adapts to specific pathogens.
Endocrine System Disorders
Diseases of the endocrine system are relatively common. An endocrine system disease usually involves the secretion of too much or not enough of a hormone. When too much hormone is secreted, the condition is called hypersecretion. When not enough hormone is secreted, the condition is called hyposecretion.
Hypersecretion
Hypersecretion by an endocrine gland is often caused by a tumor. For example, a tumor of the pituitary gland can cause hypersecretion of growth hormone. If this occurs in childhood and goes untreated, it results in very long arms and legs and abnormally tall stature by adulthood (see ). This condition is commonly known as gigantism. Martin Van Buren Bates is depicted in Figure \(3\) standing next to a man of average size. Bates was a Civil War-era American famed for his incredibly large size. He was at least 7 feet 9 inches tall and weighed close to 500 pounds. He was normal in size at birth but started to grow very rapidly by about age 6 years, presumably because of the hypersecretion of growth hormone.
Hyposecretion
Hyposecretion by an endocrine gland is often caused by the destruction of the hormone-secreting cells of the gland. As a result, not enough of the hormone is secreted. An example of this is type 1 diabetes, in which the body’s own immune system attacks and destroys cells of the pancreas that secrete insulin. This type of diabetes is generally treated with frequent injections of insulin.
Hormone Insensitivity
In some cases, an endocrine gland secretes a normal amount of hormone, but target cells do not respond normally to it. This may occur because target cells have become resistant to the hormone. An example of this type of endocrine disorder is Androgen Insensitivity Disorder. Individuals with this disorder are born with an X and Y chromosome but develop and raised as females. This is due to a mutation in the Androgen Receptor (AR) gene which is located on the X chromosome. Testosterone is an androgen hormone that causes testes to descend and typical male characteristics to develop. People with this form of the condition have the external sex characteristics of females but do not have a uterus and therefore do not menstruate and are unable to conceive a child (infertile). They are typically raised as females and have a female gender identity. Affected individuals have male internal sex organs (testes) that are undescended, which means they are located in the pelvis or abdomen.
Review
1. What is the endocrine system? What is its general function?
2. Compare and contrast endocrine and exocrine glands.
3. How do endocrine system messages differ from those of the nervous system?
4. Describe the role of the pituitary gland in the endocrine system.
5. List three endocrine glands other than the pituitary gland, and identify their functions.
6. Which endocrine gland has an important function in the immune system? What is that function?
7. Define hypersecretion and hyposecretion.
8. Name an endocrine disorder in which too much of a hormone is produced.
9. What are two reasons people with diabetes might have signs and symptoms of inadequate insulin?
10. Choose one. Cushing’s syndrome is an example of (hyposecretion/hypersecretion).
11. True or False. The hypothalamus is the master gland of the endocrine system.
12. True or False. Mammary glands that produce milk for offspring are part of the endocrine system.
13. Melatonin is produced by the:
1. A. Pituitary gland
2. B. Hypothalamus
3. C. Pineal gland
4. D. Pancreas
14. Besides location, what is the main difference between the anterior lobe of the pituitary and the posterior lobe of the pituitary?
15. Which endocrine glands differ between males and females? Which hormones do they produce?
Explore More
Most people want to live a long, healthy life. Geneticist Cynthia Kenyon’s research suggests that endocrine hormones may be a key to human longevity. Watch this fascinating TED talk to learn how.
Emily Quinn is an artist and activist. In this video, she talks about the hardship that she experienced while growing up as an individual with Androgen Insensitivity Syndrome.
Attributions
1. Cushing's face by Ozlem Celik, Mutlu Niyazoglu, Hikmet Soylu and Pinar Kadioglu CC BY 2.5 via Wikimedia Commons
2. Endocrine glands by Mariana Ruiz Villarreal CC BY-NC 3.0 via CK-12 Foundation
3. Martin Van Buren Bates by Magnus Manske; public domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/12%3A_Endocrine_System/12.2%3A_Introduction_to_the_Endocrine_System.txt |
Pills from Pee
The medication pictured above with the brand name Progynon was a drug used to control the effects of menopause in women. The pills first appeared in 1928 and contained the human sex hormone estrogen. Estrogen secretion declines in women around the time of menopause and may cause symptoms such as mood swings and hot flashes. The pills were supposed to ease the symptoms by supplementing estrogen in the body. The manufacturer of Progynon obtained estrogen for the pills from the urine of pregnant women because it was a cheap source of the hormone. Progynon is still used today to treat menopausal symptoms. Although the drug has been improved over the years, it still contains estrogen. Estrogen is an example of an endocrine hormone.
How Do Endocrine Hormones Work?
Endocrine hormones like estrogen are messenger molecules that are secreted by endocrine glands into the bloodstream. They travel throughout the body in the circulation. Although they reach virtually every cell in the body in this way, each hormone affects only certain cells, called target cells. A target cell is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins — either on the cell surface or within the cell — that are specific to that hormone. An endocrine hormone travels through the bloodstream until it finds a target cell with a matching receptor to which it can bind. When the hormone binds to the receptor, it causes changes within the cell. The manner in which it changes the cell depends on whether the hormone is a steroid hormone or a non-steroid hormone.
Steroid Hormones
A steroid hormone such as estrogen is made of lipids. It is fat soluble, so it can diffuse across a target cell’s plasma membrane, which is also made of lipids. Once inside the cell, a steroid hormone binds with receptor proteins in the cytoplasm. As you can see in the diagram below, the steroid hormone and its receptor form a complex, called a steroid complex, which moves into the nucleus where it influences the expression of genes. Examples of steroid hormones include cortisol, which is secreted by the adrenal glands, and sex hormones, which are secreted by the gonads.
Non-steroid Hormones
A non-steroid hormone is made of amino acids. It is not fat soluble, so it cannot diffuse across the plasma membrane of a target cell. Instead, it binds to a receptor protein on the cell membrane. In the following diagram, you can see that the binding of the hormone with the receptor activates an enzyme in the cell membrane. The enzyme then stimulates another molecule, called the second messenger, which influences processes inside the cell. Most endocrine hormones are non-steroid hormones. Examples include glucagon and insulin, both produced by the pancreas.
Regulation of Endocrine Hormones
Endocrine hormones regulate many body processes, but what regulates the secretion of endocrine hormones? Most endocrine hormones are controlled by feedback mechanisms. A feedback mechanism is a loop in which a product feeds back to control its own production. Feedback loops may be either negative or positive.
• Most endocrine hormones are regulated by negative feedback loops. Negative feedback keeps the concentration of a hormone within a relatively narrow range and maintains homeostasis.
• Very few endocrine hormones are regulated by positive feedback loops. Positive feedback causes the concentration of a hormone to become increasingly higher.
Regulation by Negative Feedback
A negative feedback loop controls the synthesis and secretion of hormones by the thyroid gland. This loop includes the hypothalamus and pituitary gland in addition to the thyroid, as shown in Figure \(4\). When the levels of thyroid hormones circulating in the blood fall too low, the hypothalamus secretes thyrotropin releasing hormone (TRH). This hormone travels directly to the pituitary gland through the thin stalk connecting the two structures. In the pituitary gland, TRH stimulates the pituitary to secrete thyroid stimulating hormone (TSH). TSH, in turn, travels through the bloodstream to the thyroid gland and stimulates it to secrete thyroid hormones. This continues until the blood levels of thyroid hormones are high enough. At that point, the thyroid hormones feedback to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Eventually, the levels of thyroid hormones in the blood start to fall too low again. When that happens, the hypothalamus releases TRH, and the loop repeats.
Regulation by Positive Feedback
Prolactin is a non-steroid endocrine hormone secreted by the pituitary gland. One of the functions of prolactin is to stimulate a nursing mother’s mammary glands to produce milk. The regulation of prolactin in the mother is controlled by a positive feedback loop that involves the nipples, hypothalamus, pituitary gland, and mammary glands. Positive feedback begins when a baby suckles on the mother’s nipple. Nerve impulses from the nipple reach the hypothalamus, which stimulates the pituitary gland to secrete prolactin. Prolactin travels in the blood to the mammary glands and stimulates them to produce milk. The release of milk causes the baby to continue suckling, which causes more prolactin to be secreted and more milk to be produced. The positive feedback loop continues until the baby stops suckling at the breast.
Feature: Myth vs. Reality
Anabolic steroids are synthetic versions of the naturally occurring male sex hormone testosterone. Male hormones have androgenic, or masculinizing, effects, but they also have anabolic, or muscle-building effects. The anabolic effects are the reason that synthetic steroids are used by athletes. In addition to building muscles, they also accelerate the development of bones and red blood cells, increase endurance so athletes can train harder and longer, and speed up muscle recovery. Unfortunately, these benefits of steroid use come with costs. If you ever consider taking anabolic steroids to build muscles and improve athletic performance, consider the following myths and corresponding realities.
Myth: Steroids are safe.
Reality: Steroid use may cause several serious side effects. Prolonged use may increase the risk of liver cancer, heart disease, and high blood pressure.
Myth: Steroids will not stunt your growth.
Reality: Teens who take steroids before they have finished growing in height may have their growth stunted so they remain shorter throughout life than they would otherwise have been. Such stunting occurs because steroids increase the rate at which skeletal maturity is reached. Once skeletal maturity occurs, additional growth in height is impossible.
Myth: Steroids do not cause drug dependency.
Reality: Steroid use may cause dependency as evidenced by the negative effects of stopping steroid use. These negative effects may include insomnia, fatigue, and depressed mood, among others.
Myth: There is no such thing as “roid rage.”
Reality: Steroid use has been shown to increase aggressiveness in some people. It has also been implicated in a number of violent acts committed by people who had not demonstrated violent tendencies until they started using steroids.
Myth: Only males use steroids.
Reality: Although steroid use is more common in males than females, some females also use steroids. They use them to build muscle and improve physical performance, generally either for athletic competition or for self-defense.
Review
1. What are endocrine hormones?
2. Define the target cell in the context of endocrine hormones.
3. Explain how steroid hormones influence target cells.
4. How do non-steroid hormones affect target cells?
5. Compare and contrast negative and positive feedback loops.
6. Outline the way feedback controls the production of thyroid hormones.
7. Describe the feedback mechanism that controls milk production by the mammary glands.
8. Why do endocrine hormones only affect some of the cells in the body? Choose the best answer.
1. They only reach certain cells.
2. Many hormones cannot cross the plasma membrane of cells.
3. Some cells feedback negatively in response to a hormone.
4. Only some cells have receptor proteins that can bind to a given hormone.
9. People with a condition called hyperthyroidism produce too much thyroid hormone. What do you think this does to the level of TSH? Explain your answer.
10. Which is more likely to maintain homeostasis — negative feedback or positive feedback? Explain your answer.
11. Does testosterone bind to receptors on the plasma membrane of target cells or in the cytoplasm of target cells? Explain your answer.
12. True or False. Endocrine hormones can affect the expression of genes.
13. True or False. Non-steroid hormones cannot affect intracellular processes.
14. True or False. Insulin binds to receptors on the plasma membrane of cells.
15. Which hormone is secreted by the pituitary gland?
1. Prolactin
2. Insulin
3. Cortisol
4. Thyrotropin releasing hormone
Explore More
For a funny and fast-paced lesson that covers endocrine hormones in greater detail, watch this CrashCourse video:
Attributions
1. Glass bottle for 'Progynon' pills, United Kingdom, 1928-1948 by Schering, CC BY-SA 4.0 via Science Museum Group Collection
2. Steroid hormone by LadyofHats CC BY-NC 3.0 via CK-12 Foundation
3. Non-Steroid hormone by LadyofHats CC BY-NC 3.0 via CK-12 Foundation
4. Thyroid feedback by Rupali Raju CC BY-NC 3.0 via CK-12 Foundation
5. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/12%3A_Endocrine_System/12.3%3A_Endocrine_Hormones.txt |
Milk on Demand
This adorable nursing infant is part of a positive feedback loop. When he suckles on the nipple, it sends nerve impulses to his mother’s hypothalamus, which “tell” her pituitary gland to release the hormone prolactin into her bloodstream. Prolactin travels to the mammary glands in the breasts and stimulates milk production, which motivates the infant to keep suckling.
What Is the Pituitary Gland?
The pituitary gland is the master gland of the endocrine system, the system of glands that secrete hormones into the bloodstream. Endocrine hormones control virtually all physiological processes. For example, they control growth, sexual maturation, reproduction, body temperature, blood pressure, and metabolism. The pituitary gland is considered the master gland of the endocrine system because it controls the rest of the endocrine system. Many pituitary hormones either promote or inhibit hormone secretion by other endocrine glands.
Structure and Function of the Pituitary Gland
The pituitary gland is about the size of a pea. It protrudes from the bottom of the hypothalamus at the base of the inner brain (Figure \(2\)). The pituitary is connected to the hypothalamus by a thin stalk (called the infundibulum). Blood vessels and nerves in the stalk allow direct connections between the hypothalamus and the pituitary gland. The pituitary gland consists of two bulb-like lobes: an anterior lobe and a posterior lobe (Figure \(3\)).
Anterior Lobe
The anterior pituitary is at the front of the pituitary gland. It synthesizes and releases hormones into the blood. Table \(1\) shows some of the endocrine hormones released by the anterior pituitary, including their targets and effects.
Table \(1\): Anterior Pituitary Hormones
Hormone Target Effect(s)
Adrenocorticotropic hormone (ACTH) Adrenal glands Stimulates the cortex of each adrenal gland to secrete its hormones
Thyroid-stimulating hormone (TSH) Thyroid gland Stimulates the thyroid gland to secrete thyroid hormone
Growth hormone (GH) Body cells Stimulates body cells to synthesize proteins and grow
Follicle-stimulating hormone (FSH) Ovaries, testes Stimulates the ovaries to develop mature eggs; stimulates the testes to produce sperm
Luteinizing hormone (LH) Ovaries, testes Stimulates the ovaries and testes to secrete sex hormones; stimulates the ovaries to release eggs
Prolactin (PRL) Mammary glands Stimulates the mammary glands to produce milk
The anterior pituitary gland is regulated mainly by hormones from the hypothalamus. The hypothalamus secretes hormones called releasing hormones and inhibiting hormones that travel through capillaries directly to the anterior lobe of the pituitary gland. The hormones stimulate the anterior pituitary to either release or stop releasing particular pituitary hormones. Several of these hypothalamic hormones and their effects on the anterior pituitary are shown in Table \(2\).
Table \(2\): Hypothalamic Hormones and Their Effects on the Anterior Pituitary
Hypothalamic Hormone Effect on Anterior Pituitary
Thyrotropin releasing hormone (TRH) Release of thyroid stimulating hormone (TSH)
Corticotropin releasing hormone (CRH) Release of adrenocorticotropic hormone (ACTH)
Gonadotropin releasing hormone (GnRH) Release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
Growth hormone releasing hormone (GHRH) Release of growth hormone (GH)
Growth hormone inhibiting hormone (GHIH) (Somatostatin) Stopping of growth hormone release
Prolactin releasing hormone (PRH) Release of prolactin
Prolactin inhibiting hormone (PIH) (Dopamine) Stopping of prolactin release
Posterior Lobe
The posterior pituitary is at the back of the pituitary gland. This lobe does not synthesize any hormones. Instead, the posterior lobe stores hormones that come from the hypothalamus along the axons of nerves connecting the two structures (Figure \(3\)). The posterior pituitary then secretes the hormones into the bloodstream as needed. Hypothalamic hormones secreted by the posterior pituitary include vasopressin and oxytocin.
• Vasopressin (also called antidiuretic hormone, or ADH) helps to maintain homeostasis in body water. It stimulates the kidneys to conserve water by producing more concentrated urine. Specifically, vasopressin targets ducts in the kidneys and makes them more permeable to water. This allows more water to be resorbed by the body rather than excreted in the urine.
• Oxytocin (OXY) targets cells in the uterus to stimulate uterine contractions, for example, during childbirth. It also targets cells in the breasts of a nursing mother to stimulate the letdown of milk.
Review
1. Explain why the pituitary gland is called the master gland of the endocrine system.
2. Compare and contrast the two lobes of the pituitary gland and their general functions.
3. Identify two hormones released by the anterior pituitary, their targets, and their effects.
4. Explain how the hypothalamus influences the output of hormones by the anterior lobe of the pituitary gland.
5. Name and give the function of two hypothalamic hormones released by the posterior pituitary gland.
6. True or False. The pituitary gland only secretes hormones that are involved in reproduction.
7. True or False. The brain does not produce hormones, only glands produce hormones.
8. If a releasing hormone is secreted from the hypothalamus to the pituitary gland, which part of the pituitary receives it? Explain your answer.
9. Answer the following questions about prolactin releasing hormone (PRH) and prolactin inhibiting hormone (PIH).
1. Where are these hormones produced?
2. Where are their target cells located?
3. What are their effects on their target cells?
4. What are their ultimate effects on milk production? Explain your answer.
10. e. When a baby nurses, which of these hormones is most likely released in the mother? Explain your answer.
11. For each of the following hormones, state whether it is synthesized in the pituitary or the hypothalamus.
1. Gonadotropin releasing hormone (GnRH)
2. Growth hormone (GH)
3. Oxytocin
12. d. Adrenocorticotropic hormone (ACTH)
Attributions
1. Nursing by honey-bee, CC BY 2.0 via Wikimedia Commons
2. Pituitary Gland by Laura Guerin, CC BY-NC 3.0 via CK-12 Foundation
3. Pituitary gland representation by Diberri licensed CC BY-SA 3.0 via en.Wikipedia
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/12%3A_Endocrine_System/12.4%3A_Pituitary_Gland.txt |
Too Much of a Good Thing
The individual in Figure \(1\) has a goiter. A goiter is an abnormal enlargement of the thyroid gland, which is located in the neck. The formation of a goiter may occur in a number of different thyroid disorders. You’ll learn why in this concept.
Thyroid Structure
The thyroid gland is one of the largest endocrine glands in the body. It is located in the front of the neck below Adam’s apple (see Figure \(2\)). The gland is butterfly-shaped and composed of two lobes. The lobes are connected by a narrow band of thyroid tissue called an isthmus.
Internally, the thyroid gland is composed mainly of follicles. A follicle is a small cluster of cells surrounding a central cavity, which stores hormones and other molecules made by the follicular cells. Thyroid follicular cells are unique in being highly specialized to absorb and use iodine. They absorb iodine as iodide ions (I-) from the blood and use the iodide to produce thyroid hormones. The cells also use some of the iodide they absorb to form a protein called thyroglobulin, which serves to store iodide for later hormone synthesis. The outer layer of cells of each follicle secretes thyroid hormones as needed. Scattered among the follicles are another type of thyroid cells, called parafollicular cells (or C cells). These cells synthesize and secrete the hormone calcitonin.
Functions of the Thyroid
Like all endocrine glands, the function of the thyroid is to synthesize hormones and secrete them into the bloodstream. Once in the blood, they can travel to cells throughout the body and influence their functions.
Thyroid Hormones: T4 and T3
There are two main thyroid hormones produced by the follicles: thyroxine (T4), which contains four iodide ions and is represented by the structural diagram below; and triiodothyronine (T3), which contains three iodide ions. T3 is much more powerful than T4, but T4 makes up about 90 percent of circulating thyroid hormone, and T3 makes up only about 10 percent. However, most of the T4 is converted to T3 by target tissues.
Like steroid hormones, T3 and T4 cross cell membranes everywhere in the body and bind to intracellular receptors to regulate gene expression. However, unlike steroid hormones, thyroid hormones can cross cell membranes only with the help of special transporter proteins. Once inside the nucleus of cells, T3 and T4 turn on genes that control protein synthesis. Thyroid hormones increase the rate of metabolism in cells, so cells absorb more carbohydrates, use more energy, and produce more heat. Thyroid hormones also increase the rate and force of the heartbeat. In addition, they increase the sensitivity of cells to fight-or-flight hormones (that is, catecholamine hormones such as adrenaline).
The production of both T4 and T3 is regulated primarily by thyroid stimulating hormone (TSH), which is secreted by the anterior pituitary gland (see the diagram below). TSH production, in turn, is regulated by thyrotropin releasing hormone (TRH), which is produced by the hypothalamus. The thyroid gland, pituitary gland, and hypothalamus form a negative feedback loop to keep thyroid hormone secretion within a normal range. TRH and TSH production is suppressed when T4 levels start to become too high. The opposite occurs when T4 levels start to become too low.
Calcitonin
The calcitonin produced by the parafollicular cells of the thyroid gland has the role of helping to regulate blood calcium levels by stimulating the movement of calcium into bone. Calcitonin is secreted in response to rising blood calcium levels. It decreases blood calcium levels by enhancing calcium absorption and deposition in bone. Calcitonin works hand-in-hand with parathyroid hormone, which is secreted by the parathyroid glands and has the opposite effects as calcitonin. Together, these two hormones maintain calcium homeostasis.
Thyroid Disorders
As with other endocrine disorders, thyroid disorders are generally associated with either over or under-secretion of hormones. Abnormal secretion of thyroid hormones may occur for a variety of reasons.
Hyperthyroidism
Hyperthyroidism occurs when the thyroid gland produces excessive amounts of thyroid hormones. The most common cause of hyperthyroidism is Graves’ disease. Graves’ disease is an autoimmune disorder in which abnormal antibodies produced by the immune system stimulate the thyroid to secrete excessive quantities of its hormones. This stimulation overrides the usual negative feedback mechanism that normally controls thyroid hormone output. Graves’ disease often results in the formation of an enlarged thyroid (goiter) because of the continued stimulation to produce more hormones.
Besides a goiter, other signs and symptoms of hyperthyroidism may include protruding eyes (see photo below), heart palpitations, excessive sweating, diarrhea, weight loss despite increased appetite, muscle weakness, and unusual sensitivity to heat. Medications can be prescribed to mitigate the symptoms of the disease. Anti-thyroid drugs can also be given to decrease the production of thyroid hormones. If the drugs are ineffective, the gland can be partially or entirely removed. This can be done surgically or with the administration of radioactive iodine. Removal of the thyroid produces hypothyroidism.
Hypothyroidism
Hypothyroidism occurs when the thyroid gland produces insufficient amounts of thyroid hormones. It can result from surgical removal of the thyroid. However, worldwide, the most common cause of hypothyroidism is dietary iodine deficiency. In cases of iodine deficiency, the negative feedback loop controlling the release of thyroid hormone causes repeated stimulation of the thyroid. This results in the thyroid gland growing in size and producing a goiter. Although the gland gets larger, it cannot increase hormone output because of the lack of iodine in the diet.
Iodine deficiency is uncommon in the Western world because iodine is added to salt. Where iodine deficiency is not a problem, the most common cause of hypothyroidism is Hashimoto’s thyroiditis. This is another autoimmune disease, but in this case, the immune system destroys the thyroid gland, producing hypothyroidism. Hashimoto’s thyroiditis tends to run in families so it is likely to have a genetic component. It usually appears after the age of 30 and is more common in females than males.
Hypothyroidism produces many signs and symptoms, as shown in Figure \(6\). These may include abnormal weight gain, tiredness, baldness, cold intolerance, and slow heart rate. Hypothyroidism is generally treated with thyroid hormone replacement therapy. This may be needed for the rest of a person’s life. Hypothyroidism in a pregnant woman can have serious adverse consequences for the fetus. During the fetal period, cells of the developing brain are a major target for thyroid hormones, which play a crucial role in brain maturation. When levels of thyroid hormones are too low, the fetus may suffer permanent deficits in cognitive abilities. Deafness is also a potential outcome of hypothyroidism in utero.
Feature: Myth vs. Reality
Thyroid disorders are relatively common, affecting as many as 20 million people in the United States. Because the disorders are common, there are also many common myths about them.
Myth: If you have a thyroid problem, you will know something is wrong because you will have obvious symptoms.
Reality: The majority of people with a thyroid disorder are not aware they have it because the symptoms are often mild, nonspecific, and easy to ignore. Generally, blood tests of thyroid hormone levels are needed to make a conclusive diagnosis.
Myth: If you are diagnosed with a thyroid disorder, you will have to take medication for the rest of your life.
Reality: Whether you need to continue thyroid medication for life depends on the cause of the disorder. For example, some women develop hypothyroidism during pregnancy but no longer need medication after the pregnancy is over and hormone levels return to normal.
Myth: As soon as you start taking thyroid medication, your symptoms will resolve.
Reality: It often takes weeks or even months for thyroid hormone levels to return to normal and symptoms to disappear.
Myth: You can take an over-the-counter iodine supplement to correct hypothyroidism.
Reality: In the United States, where dietary iodine is almost always adequate, iodine deficiency is unlikely to be the cause of hypothyroidism. Therefore, taking supplemental iodine is not likely to correct the problem.
Myth: If thyroid symptoms are mild, you don’t need to take medication.
Reality: Because thyroid hormones are responsible for so many vital body functions, failing to treat even a mild thyroid disorder may lead to a range of other problems, such as osteoporosis or infertility.
Myth: Goiter may be caused by eating “goitrogenic” vegetables, such as broccoli, Brussels sprouts, and spinach.
Reality: Although these foods can interfere with the thyroid’s ability to process iodide, you would have to eat huge amounts of them to cause goiter.
Myth: Thyroid disorders occur only after middle age and only in women.
Reality: Thyroid disorders may occur at any age and in any sex. Hypothyroidism occurs more commonly in older adults, but hyperthyroidism occurs more commonly in younger adults. Although women are more likely to develop thyroid disorders, about 20 percent of cases occur in men.
Review
1. Describe the structure and location of the thyroid gland.
2. Identify the types of cells within the thyroid gland that produce hormones.
3. Compare and contrast T4 and T3.
4. How do T4 and T3 affect body cells?
5. Explain how T4 and T3 production is regulated.
6. What is the function of calcitonin?
7. Identify the chief cause and effects of hyperthyroidism.
8. What are two possible causes of hypothyroidism?
9. List signs and symptoms of hypothyroidism.
10. Why may both hyperthyroidism and hypothyroidism cause goiters?
11. Choose one symptom each for hyperthyroidism and hypothyroidism and explain why they occur based on the functions of thyroid hormones.
12. Which hormone is produced by the thyroid gland?
1. T3
2. Calcitonin
3. Parathyroid hormone
4. TSH
5. A and B
13. In cases of hypothyroidism due to Hashimoto’s thyroiditis or removal of the thyroid gland to treat hyperthyroidism, patients are often given medication to replace the missing thyroid hormone. Explain why the level of replacement thyroid hormone must be carefully monitored and adjusted if needed.
14. True or False. T3 and T4 bind to receptors on the plasma membrane of target cells.
15. Which disease causes too much thyroid hormone to be produced?
1. Hashimoto’s thyroiditis
2. Graves’ disease
3. Goiter
4. Iodine deficiency
Attributions
1. Goiter by Almazi, public domain via Wikimedia Commons
2. Thyroid by NIH, public domain via Arnavaz at French Wikipedia
3. Triiodothyronine by Ayacop Public Domain via Wikimedia Commons
4. Thyroid system
5. Proptosis and lid retraction from Graves' Disease by Jonathan Trobe, M.D. - University of Michigan Kellogg Eye Center (The Eyes Have It), CC BY 3.0 via Wikimedia Commons
6. Signs and symptoms of hypothyroidism by Mikael Häggström public domain via Wikimedia Commons
7. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0 | textbooks/bio/Human_Biology/Human_Biology_(Wakim_and_Grewal)/12%3A_Endocrine_System/12.5%3A_Thyroid_Gland.txt |
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