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What starts out very small and has the potential to grow considerably larger?
Trees, of course. But also populations. Give a population everything it needs to survive, and the growth of that population will be tremendous.
Patterns of Population Growth
Populations may show different patterns of growth. The growth pattern depends partly on the conditions under which a population lives.
Exponential Growth
Under ideal conditions, populations of most species can grow at exponential rates. Curve A inFigure below represents exponential growth. The population starts out growing slowly. As population size increases, the growth rate also increases. The larger the population becomes, the faster it grows.
Exponential and Logistic Growth. Curve A shows exponential growth. Curve B shows logistic growth.
Logistic Growth
Most populations do not live under ideal conditions. Therefore, most do not grow exponentially. Certainly, no population can keep growing exponentially for very long. Many factors may limit growth. Often, the factors are density dependent (known as density-dependent factors). These are factors that are influential when the population becomes too large and crowded. For example, the population may start to run out of food or be poisoned by its own wastes. As a result, population growth slows and population size levels off. Curve B in Figure above represents this pattern of growth, which is called logistic growth.
At what population size does growth start to slow in the logistic model of growth? That depends on the population’s carrying capacity (see Figure above). The carrying capacity (K) is the largest population size that can be supported in an area without harming the environment. Population growth hits a ceiling at that size in the logistic growth model.
K-Selected and r-Selected Species
Species can be divided into two basic types when it comes to how their populations grow.
• Species that live in stable environments are likely to be K-selected. Their population growth is controlled by density-dependent factors. Population size is generally at or near the carrying capacity. These species are represented by curve B in Figure above.
• Species that live in unstable environments are likely to be r-selected. Their potential population growth is rapid. For example, they have large numbers of offspring. However, individuals are likely to die young. Thus, population size is usually well below the carrying capacity. These species are represented by the lower part of curve A in Figure above. (r is the population growth rate. See the "Population Growth" concept.)
Summary
• Under ideal conditions, populations can grow exponentially.
• The growth rate increases as the population gets larger.
• Review
• Describe exponential population growth.
• Describe logistic growth.
• What are density-dependent factors?
• What does the carrying capacity refer to?
• What are K-selected and r-selected species?
• Most populations do not live under ideal conditions and grow logistically instead.
• Density-dependent factors slow population growth as population size nears the carrying capacity.
6.21: Human Population
How do humans adapt to their environment?
It could be said that the human population does not have to adapt to its environment, but forces the environment to change to suit us. We can live practically anywhere we want, eat all types of food, and build all types of housing. Because of all of these "adaptations," our population has grown, after a slow start, considerably fast.
The Human Population
Humans have been called the most successful "weed species" Earth has ever seen. Like weeds, human populations are fast growing. They also disperse rapidly. They have colonized habitats from pole to pole. Overall, the human population has had a pattern of exponential growth, as shown in Figure below. The population increased very slowly at first. As it increased in size, so did its rate of growth.
Growth of the Human Population. This graph gives an overview of human population growth since 10,000 BC. It took until about 1800 AD for the number of humans to reach 1 billion. It took only a little over 100 years for the number to reach 2 billion. The human population recently passed the 7 billion mark! Why do you think the human population began growing so fast?
Early Population Growth
Homo sapiens arose about 200,000 years ago in Africa. Early humans lived in small populations of nomadic hunters and gatherers. They first left Africa about 40,000 years ago. They soon moved throughout Europe, Asia, and Australia. By 10,000 years ago, they had reached the Americas. During this long period, birth and death rates were both fairly high. As a result, population growth was slow.
Humans invented agriculture about 10,000 years ago. This provided a bigger, more dependable food supply. It also let them settle down in villages and cities for the first time. The death rate increased because of diseases associated with domestic animals and crowded living conditions. The birth rate increased because there was more food and settled life offered other advantages. The combined effect was continued slow population growth.
Summary
• Early humans lived in small populations of nomadic hunters and gatherers. Both birth and death rates were fairly high. As a result, human population growth was very slow.
• The invention of agriculture increased both birth and death rates. The population continued to grow slowly.
Review
1. Describe human population growth rates.
2. How did the invention of agriculture affect human birth and death rates? How did it affect human population growth? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/06%3A_Ecology/6.20%3A_Population_Growth_Patterns.txt |
Do populations continuously grow?
Not necessarily. The growth of a population depends on a number of issues. Obviously, the average age of the individuals of that population is important. But other factors, such as the local economy, also play a role.
Demographic Transition
Major changes in the human population first began during the 1700s in Europe and North America. First death rates fell, followed somewhat later by birth rates.
Death Rates Fall
Several advances in science and technology led to lower death rates in 18th century Europe and North America:
• New scientific knowledge of the causes of disease led to improved water supplies, sewers, and personal hygiene.
• Better farming techniques and machines increased the food supply.
• The Industrial Revolution of the 1800s led to new sources of energy, such as coal and electricity. This increased the efficiency of the new agricultural machines. It also led to train transport, which improved the distribution of food.
For all these reasons, death rates fell, especially in children. This allowed many more children to survive to adulthood, so birth rates increased. As the gap between birth and death rates widened, the human population grew faster.
Birth Rates Fall
It wasn’t long before birth rates started to fall as well in Europe and North America. People started having fewer children because large families were no longer beneficial for several reasons.
• As child death rates fell and machines did more work, farming families no longer needed to have as many children to work in the fields.
• Laws were passed that required children to go to school. Therefore, they could no longer work and contribute to their own support. They became a drain on the family’s income.
Eventually, birth rates fell to match death rates. As a result, population growth slowed to nearly zero.
Stages of the Demographic Transition
These changes in population that occurred in Europe and North America have been called the demographic transition. The transition can be summarized in the following four stages, which are illustrated in Figure below:
• Stage 1—High birth and death rates lead to slow population growth.
• Stage 2—The death rate falls but the birth rate remains high, leading to faster population growth.
• Stage 3—The birth rate starts to fall, so population growth starts to slow.
• Stage 4—The birth rate reaches the same low level as the death rate, so population growth slows to zero.
Stages of the Demographic Transition. In the demographic transition, the death rate falls first. After a lag, the birth rate also falls. How do these changes affect the rate of population growth over time?
Summary
• Major changes in the human population first began during the 1700s. This occurred in Europe and North America.
• First, death rates fell while birth rates remained high. This led to rapid population growth.
• Later, birth rates also fell. As a result, population growth slowed.
Review
1. How did science and technology affect the human population?
2. List two important scientific changes that affected the human growth rate.
3. Outline the four stages of the demographic transition as it occurred in Europe and North America. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/06%3A_Ecology/6.22%3A_Demographic_Transition.txt |
Should the population of the planet by characterized by individual country, or as one general population?
How many people is too many? Is there a limit? Is there a carrying capacity for humans? These are important questions that do not have an easy answer. But as our population continues to grow, these questions and others should be discussed.
Recent Population Growth
At one time, scientists predicted that all human populations would pass through the samedemographic transition as Europe and North America. Now, they are not so sure. Death rates have fallen throughout the world. No country today remains in Stage 1 of the transition. However, birth rates are still high in many poor countries. These populations seem to be stuck in Stage 2. An example is the African country of Angola. Its population pyramid for 2005 is shown in Figure below. The wide base of the pyramid base reflects the high birth rate of this population.
Angola’s population pyramid is typical of Stage 2 of the demographic transition.
Many other countries have shifted to Stage 3 of the transition. Birth rates have started to fall. As a result, population growth is slowing. An example is Mexico. Its population pyramid for 1998 is shown in Figure below. It reflects a recent fall in the birth rate.
Mexico’s 1998 population pyramid is typical of Stage 3 population. How can you tell that the birth rate has started to fall?
Most developed nations have entered Stage 4. Sweden is an example (see Figure below). The birth rate has been low for many years in Sweden. Therefore, the rate of population growth is near zero.
Sweden’s 1998 population pyramid shows a population in Stage 4.
In some countries, birth rates have fallen even lower than death rates. As result, their population growth rates are negative. In other words, the populations are shrinking in size. These populations have top-heavy population pyramids, like the one for Italy shown in Figure below. This is a new stage of the demographic transition, referred to as Stage 5. You might think that a negative growth rate would be a good thing. In fact, it may cause problems. For example, growth-dependent industries decline. Supporting the large aging population is also a burden for the shrinking younger population of workers.
This 1998 population pyramid for Italy represents a Stage 5 population.
Future Population Growth
During the month of October 2011, the world's population surpassed 7 billion people. It took just 12 years for the population to increase by a billion people. At this rate, there may be well over 9 billion people by 2050, and easily 10 billion people by 2100. This raises many questions for both people and the planet.
The human population is now growing by more than 200,000 people a day. The human population may well be close to its carrying capacity. It has already harmed the environment. An even larger human population may cause severe environmental problems. This could lead to outbreaks of disease, starvation, and global conflict. There are three potential solutions:
1. Use technology to make better use of resources to support more people.
2. Change behaviors to reduce human numbers and how much humans consume.
3. Distribute resources more fairly among all the world’s people.
Which solution would you choose?
"If growth continued at this rate...by 2600 we would all be standing literally shoulder to shoulder" - Steven Hawking
Census Update: What the World Will Look like in 2050
On June 30, 2011, Time.com published Census Update: What the World Will Look Like in 2050 at www.time.com/time/nation/article/0,8599,2080404,00.html. According to the U.S. Census Bureau, in 2050, there will be 9.4 billion people:
• India will be the most populous nation, surpassing China sometime around 2025.
• The U.S. will remain the third most populous nation, with a population of 423 million (up from 308 million in 2010).
• Declining birth rates in Japan and Russia will cause them to fall from their current positions as the 9th and 10th most populous nations, respectively, to 16th and 17th.
• Nigeria will have a population of 402 million, up from 166 million people.
• Ethiopia's population will likely triple, from 91 million to 278 million, making the East African nation one of the top 10 most populous countries in the world.
So what does all this mean?
• The African continent is expected to experience significant population growth in the coming decades, which could compound existing food supply problems in some African nations.
• Immigration and differing birth rates among races will change the ethnic composition of the U.S.
• Population booms in Africa and India, the decline of Russia, and the expected plateau of China will all change the makeup of the estimated 9.4 billion people who will call Earth home in 2050.
Summary
• Other countries have completed similar demographic transitions. However, some countries seem stalled at early stages. They have high birth rates and rapidly growing populations.
• The total human population may have to stop growing eventually. Even if we reduce our use of resources and distribute them more fairly, at some point the carrying capacity will be reached.
• 4. What is the human population problem? What are some potential solutions? Which solution do you think is best?
Review
1. Why was a fifth stage added to the demographic transition model? Describe a population at this stage.
2. Which stage of the demographic transition is represented by the population pyramid in theFigure below? Why?
3. Evaluate how well the original demographic transition model represents human populations today. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/06%3A_Ecology/6.23%3A_Recent_and_Future_Population_Growth.txt |
What is biodiversity?
How many species exist? We don't really know for sure. But all those species together, from the smallest bacteria, the deadliest protist, the most bizarre fungi, the prettiest plant, and the biggest mammal, compile the diversity of life, or biodiversity.
What Is Biodiversity?
Biodiversity refers to the variety of life and its processes, including the variety of living organisms, the genetic differences among them, and the communities and ecosystems in which they occur. Scientists have identified about 1.9 million species alive today. They are divided into the six kingdoms of life shown in Figure below. Scientists are still discovering new species. Thus, they do not know for sure how many species really exist today. Most estimates range from 5 to 30 million species.
Known species represent only a fraction of all species that exist on Earth.
Cogs and Wheels
“The first rule of intelligent tinkering is to save all the pieces.” --attributed to Aldo Leopold, but probably a shortened version of: “To save every cog and wheel is the first precaution of intelligent tinkering.” - Aldo Leopold, Round River: from the Journals of Aldo Leopold, 1953
What are the “cogs” and “wheels” of life?
Although the concept of biodiversity did not become a vital component of biology and political science until nearly 40 years after Aldo Leopold’s death in 1948, Leopold – often considered the father of modern ecology - would have likely found the term an appropriate description of his “cogs and wheels.” Literally, biodiversity is the many different kinds (diversity) of life (bio-). Biologists, however, always alert to levels of organization, have identified three measures of life’s variation. Species diversity best fits the literal translation: the number of different species in a particular ecosystem or on Earth. A second measure recognizes variation within a species: differences among individuals or populations make upgenetic diversity. Finally, as Leopold clearly understood, the “cogs and wheels” include not only life but also the land, sea, and air that support life. Ecosystem diversity describes the many types of functional units formed by living communities interacting with their environments. Although all three levels of diversity are important, the term biodiversity usually refers to species diversity.
Summary
• Biodiversity refers to the number of species in an ecosystem or the biosphere as a whole.
Review
1. What is biodiversity?
2. What are the three measures of life’s variations?
3. What is meant by ecosystem diversity?
6.25: Importance of Biodiversity
Why is biodiversity important?
Think about how many species exist. Most likely well over 5 million. Now think about how much information about those species we do not yet understand. We do not know what we can learn from them.
Why Is Biodiversity Important?
Human beings benefit in many ways from biodiversity. Biodiversity has direct economic benefits. It also provides services to entire ecosystems.
Economic Benefits of Biodiversity
The diversity of species provides humans with a wide range of economic benefits:
• Wild plants and animals maintain a valuable pool of genetic variation. This is important because domestic species are genetically uniform. This puts them at great risk of dying out due to disease.
• Other organisms provide humans with many different products. Timber, fibers, adhesives, dyes, and rubber are just a few.
• Certain species may warn us of toxins in the environment. When the peregrine falcon nearly went extinct, for example, it warned us of the dangers of DDT.
• More than half of the most important prescription drugs come from wild species. Only a fraction of species have yet been studied for their medical potential.
• Other living things provide inspiration for engineering and technology. For example, the car design in Figure below was based on a fish.
From flowers to fish, biodiversity benefits humans in many ways.
Ecosystem Services of Biodiversity
Biodiversity generally increases the productivity and stability of ecosystems. It helps ensure that at least some species will survive environmental change. It also provides many other ecosystem services. For example:
• Plants and algae maintain the atmosphere. During photosynthesis, they add oxygen and remove carbon dioxide.
• Plants help prevent soil erosion. They also improve soil quality when they decompose.
• Microorganisms purify water in rivers and lakes. They also return nutrients to the soil.
• Bacteria fix nitrogen and make it available to plants. Other bacteria recycle the nitrogen from organic wastes and remains of dead organisms.
• Insects and birds pollinate flowering plants, including crop plants.
• Natural predators control insect pests. They reduce the need for expensive pesticides, which may harm people and other living things.
Summary
• Biodiversity has direct economic benefits. It also provides services to entire ecosystems.
• List three economic benefits of biodiversity.
• Identify three ecosystem services of biodiversity.
• Predict what would happen to other organisms in an ecosystem in which all the decomposers went extinct?
• Review
1. List three economic benefits of biodiversity.
2. Identify three ecosystem services of biodiversity.
3. Predict what would happen to other organisms in an ecosystem in which all the decomposers went extinct? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/06%3A_Ecology/6.24%3A_Biodiversity.txt |
This is one of the most powerful birds in the world. Could it go extinct?
The Philippine Eagle, also known as the Monkey-eating Eagle, is among the rarest, largest, and most powerful birds in the world. It is critically endangered, mainly due to massive loss of habitat due to deforestation in most of its range. Killing a Philippine Eagle is punishable under Philippine law by twelve years in jail and heavy fines.
Human Actions and the Sixth Mass Extinction
Over 99 percent of all species that ever lived on Earth have gone extinct. Five mass extinctions are recorded in the fossil record. They were caused by major geologic and climatic events. Evidence shows that a sixth mass extinction is occurring now. Unlike previous mass extinctions, the sixth extinction is due to human actions.
Some scientists consider the sixth extinction to have begun with early hominids during the Pleistocene. They are blamed for over-killing big mammals such as mammoths. Since then, human actions have had an ever greater impact on other species. The present rate of extinction is between 100 and 100,000 species per year. In 100 years, we could lose more than half of Earth’s remaining species.
Causes of Extinction
The single biggest cause of extinction today is habitat loss. Agriculture, forestry, mining, and urbanization have disturbed or destroyed more than half of Earth’s land area. In the U.S., for example, more than 99 percent of tall-grass prairies have been lost. Other causes of extinction today include:
• Exotic species introduced by humans into new habitats. They may carry disease, prey on native species, and disrupt food webs. Often, they can out-compete native species because they lack local predators. An example is described in Figure below.
• Over-harvesting of fish, trees, and other organisms. This threatens their survival and the survival of species that depend on them.
• Global climate change, largely due to the burning of fossil fuels. This is raising Earth’s air and ocean temperatures. It is also raising sea levels. These changes threaten many species.
• Pollution, which adds chemicals, heat, and noise to the environment beyond its capacity to absorb them. This causes widespread harm to organisms.
• Human overpopulation, which is crowding out other species. It also makes all the other causes of extinction worse.
The brown tree snake is an exotic species that has caused many extinctions on Pacific islands such as Guam.
Effects of Extinction
The results of a study released in the summer of 2011 have shown that the decline in the numbers of large predators like sharks, lions and wolves is disrupting Earth's ecosystem in all kinds of unusual ways. The study, conducted by scientists from 22 different institutions in six countries, confirmed the sixth mass extinction. The study states that this mass extinction differs from previous ones because it is entirely driven by human activity through changes in land use, climate, pollution, hunting, fishing and poaching. The effects of the loss of these large predators can be seen in the oceans and on land.
• Fewer cougars in the western US state of Utah led to an explosion of the deer population. The deer ate more vegetation, which altered the path of local streams and lowered overall biodiversity.
• In Africa, where lions and leopards are being lost to poachers, there is a surge in the number of olive baboons, who are transferring intestinal parasites to humans living nearby.
• In the oceans, industrial whaling led a change in the diets of killer whales, who eat more sea lions, seals, and otters and have dramatically lowered the population counts of those species.
The study concludes that the loss of big predators has likely driven many of the pandemics, population collapses and ecosystem shifts the Earth has seen in recent centuries.
Disappearing Frogs
Around the world, frogs are declining at an alarming rate due to threats like pollution, disease, and climate change. Frogs bridge the gap between water and land habitats, making them the first indicators of ecosystem changes.
Nonnative Species
Scoop a handful of critters out of the San Francisco Bay and you'll find many organisms from far away shores. Invasive kinds of mussels, fish, and more are choking out native species, challenging experts around the state to change the human behavior that brings them here.
How You Can Help Protect Biodiversity
There are many steps you can take to help protect biodiversity. For example:
• Consume wisely. Reduce your consumption wherever possible. Re-use or recycle rather than throw out and buy new. When you do buy new, choose products that are energy efficient and durable.
• Avoid plastics. Plastics are made from petroleum and produce toxic waste.
• Go organic. Organically grown food is better for your health. It also protects the environment from pesticides and excessive nutrients in fertilizers.
• Save energy. Unplug electronic equipment and turn off lights when not in use. Take mass transit instead of driving.
Lost Salmon
Why is the salmon population of Northern California so important? Salmon do not only provide food for humans, but also supply necessary nutrients for their ecosystems. Because of a sharp decline in their numbers, in part due to human interference, the entire salmon fishing season off California and Oregon was canceled in both 2008 and 2009. The species in the most danger of extinction is the California coho salmon.
Summary
• Evidence shows that a sixth mass extinction is occurring. The single biggest cause is habitat loss caused by human actions.
• There are many steps you can take to help protect biodiversity. For example, you can use less energy.
Review
1. How is human overpopulation related to the sixth mass extinction?
2. Why might the brown tree snake or the Philippine Eagle serve as “poster species” for causes of the sixth mass extinction?
3. Describe a hypothetical example showing how rising sea levels due to global warming might cause extinction.
4. Create a poster that conveys simple tips for protecting biodiversity. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/06%3A_Ecology/6.26%3A_Human_Actions_and_the_Sixth_Mass_Extinction.txt |
Renewable or nonrenewable, what's the difference?
That's like asking the difference between having an endless supply and having a limited supply. Will this planet eventually run out of oil? Probably. So oil is a nonrenewable resource.
Renewable and Nonrenewable Resources
A natural resource is something supplied by nature that helps support life. When you think ofnatural resources, you may think of minerals and fossil fuels. However, ecosystems and the services they provide are also natural resources. Biodiversity is a natural resource as well.
Renewable Resources
Renewable resources can be replenished by natural processes as quickly as humans use them. Examples include sunlight and wind. They are in no danger of being used up (seeFigure below). Metals and other minerals are renewable too. They are not destroyed when they are used and can be recycled.
Wind is a renewable resource. Wind turbines like this one harness just a tiny fraction of wind energy.
Living things are considered to be renewable. This is because they can reproduce to replace themselves. However, they can be over-used or misused to the point of extinction. To be truly renewable, they must be used sustainably. Sustainable use is the use of resources in a way that meets the needs of the present and also preserves the resources for future generations.
Nonrenewable Resources
Nonrenewable resources are natural resources that exist in fixed amounts and can be used up. Examples include fossil fuels such as petroleum, coal, and natural gas. These fuels formed from the remains of plants over hundreds of millions of years. We are using them up far faster than they could ever be replaced. At current rates of use, petroleum will be used up in just a few decades and coal in less than 300 years. Nuclear power is also considered to be a nonrenewable resource because it uses up uranium, which will sooner or later run out. It also produces harmful wastes that are difficult to dispose of safely.
Gasoline is made from crude oil. The crude oil pumped out of the ground is a black liquid called petroleum, which is a nonrenewable resource.
Coal is another nonrenewable resource.
Turning Trash Into Treasure
Scientists at the Massachusetts of Technology are turning trash into coal, which can readily be used to heat homes and cook food in developing countries. This coal burns cleaner than that from fossil fuels. It also save a tremendous amount of energy.
Summary
• Renewable resources can be replaced by natural processes as quickly as humans use them. Examples include sunlight and wind.
• Nonrenewable resources exist in fixed amounts. They can be used up. Examples include fossil fuels such as coal.
Review
1. Define natural resource. Give an example.
2. Distinguish between renewable and nonrenewable resources and give examples.
3. Infer factors that determine whether a natural resource is renewable or nonrenewable.
6.28: Soil and Water Resources
Could this land be used for agriculture?
Probably not. The quality of soil is very important in determining what can grow in a particular area. Good soil is not so easy to come by. Soil should be considered another resource that we, as a population, must strive to protect.
Soil and Water Resources
Theoretically, soil and water are renewable resources. However, they may be ruined by careless human actions.
Soil
Soil is a mixture of eroded rock, minerals, partly decomposed organic matter, and other materials. It is essential for plant growth, so it is the foundation of terrestrial ecosystems. Soil is important for other reasons as well. For example, it removes toxins from water and breaks down wastes.
Although renewable, soil takes a very long time to form—up to hundreds of millions of years. So, for human purposes, soil is a nonrenewable resource. It is also constantly depleted of nutrients through careless use, and eroded by wind and water. For example, misuse of soil caused a huge amount of it to simply blow away in the 1930s during the Dust Bowl (seeFigure below). Soil must be used wisely to preserve it for the future. Conservation practices include contour plowing and terracing. Both reduce soil erosion. Soil also must be protected from toxic wastes.
The Dust Bowl occurred between 1933 and 1939 in Oklahoma and other southwestern U.S. states. Plowing had exposed prairie soil. Drought turned the soil to dust. Intense dust storms blew away vast quantities of the soil. Much of the soil blew all the way to the Atlantic Ocean.
Water
Water is essential for all life on Earth. For human use, water must be fresh. Of all the water on Earth, only 1 percent is fresh, liquid water. Most of the rest is either salt water in the ocean or ice in glaciers and ice caps.
Although water is constantly recycled through the water cycle, it is in danger. Over-use and pollution of freshwater threaten the limited supply that people depend on. Already, more than 1 billion people worldwide do not have adequate freshwater. With the rapidly growing human population, the water shortage is likely to get worse.
Too Much of a Good Thing
Water pollution comes from many sources. One of the biggest sources is runoff. Runoff picks up chemicals such as fertilizer from agricultural fields, lawns, and golf courses. It carries the chemicals to bodies of water. The added nutrients from fertilizer often cause excessive growth of algae, creating algal blooms (see Figure below). The algae use up oxygen in the water so that other aquatic organisms cannot survive. This has occurred over large areas of the ocean, creating dead zones, where low oxygen levels have killed all ocean life. A very large dead zone exists in the Gulf of Mexico. Measures that can help prevent these problems include cutting down on fertilizer use. Preserving wetlands also helps because wetlands filter runoff water.
Algal Bloom. Nutrients from fertilizer in runoff caused this algal bloom.
Summary
• Soil and water are renewable resources but may be ruined by careless human actions. Soil can be depleted of nutrients. It can also be eroded by wind or water.
• Over-use and pollution of freshwater threaten the limited supply that people depend on.
Review
1. What is soil?
2. Why is soil considered a nonrenewable resource?
3. How much water is drinkable?
4. Why would you expect a dead zone to start near the mouth of a river, where the river flows into a body of water? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/06%3A_Ecology/6.27%3A_Renewable_and_Nonrenewable_Resources.txt |
Why is the atmosphere important?
Well, it contains all of the air that we breathe. The atmosphere also has other roles and functions, so when we interfere with the atmosphere, we interfere with some important biological processes. And this can have consequences.
The Atmosphere
The atmosphere plays an important part in maintaining Earth’s freshwater supply. It is part of the water cycle. It refills lakes and rivers with precipitation. The atmosphere also provides organisms with gases needed for life. It contains oxygen for cellular respiration and carbon dioxide for photosynthesis.
Air Pollution
Air pollution comes from many sources, including waste expelled into the atmosphere by numerous factories.
Earth’s atmosphere is vast. However, it has been seriously polluted by human activities. Air pollution consists of chemical substances and particles released into the atmosphere, mainly by human actions. The major cause of outdoor air pollution is the burning of fossil fuels. Power plants, motor vehicles, and home furnaces all burn fossil fuels and contribute to the problem (see Table below). Ranching and using chemicals such as fertilizers also cause air pollution. Erosion of soil in farm fields and construction sites adds dust particles to the air as well. Fumes from building materials, furniture, carpets, and paint add toxic chemicals to indoor air.
Pollutant Example/Major Source Problem
Sulfur oxides (SOx) Coal-fired power plants Acid Rain
Nitrogen oxides (NOx) Motor vehicle exhaust Acid Rain
Carbon monoxide (CO) Motor vehicle exhaust Poisoning
Carbon dioxide (CO2) All fossil fuel burning Global Warming
Particulate matter (smoke, dust) Wood and coal burning Respiratory disease, Global Dimming
Mercury Coal-fired power plants, medical waste Neurotoxicity
Smog Coal burning Respiratory problems; eye irritation
Ground-level ozone Motor vehicle exhaust Respiratory problems; eye irritation
In humans, air pollution causes respiratory and cardiovascular problems. In fact, more people die each year from air pollution than automobile accidents. Air pollution also affects ecosystems worldwide by causing acid rain, ozone depletion, and global warming. Ways to reduce air pollution from fossil fuels include switching to nonpolluting energy sources (such as solar energy) and using less energy. What are some ways you could use less energy?
Acid Rain
All life relies on a relatively narrow range of pH, or acidity. That’s because protein structure and function are very sensitive to pH. Air pollution can cause precipitation to become acidic. Nitrogen and sulfur oxides, mainly from motor vehicle exhaust and coal burning, create acids when they combine with water in the air. The acids lower the pH of precipitation, forming acid rain. If acid rain falls on the ground, it may damage soil and soil organisms. If it falls on plants, it may kill them (see Figure below). If it falls into lakes, it lowers the pH of the water and kills aquatic organisms.
Effects of Acid Rain. These trees in a European forest were killed by acid rain.
Ozone Depletion
There are two types of ozone. You can think of them as bad ozone and good ozone. Both are affected by air pollution.
• Bad ozone forms near the ground when sunlight reacts with pollutants in the air. Ground-level ozone is harmful to the respiratory systems of humans and other animals.
• Good ozone forms in a thin layer high up in the atmosphere, between 15 and 35 kilometers above Earth’s surface. This ozone layer shields Earth from most of the sun’s harmful UV radiation. It plays an important role in preventing mutations in the DNA of organisms.
Unfortunately, the layer of good ozone is being destroyed by air pollution. The chief culprits are chlorine and bromine gases. They are released in aerosol sprays, coolants, and other products. Loss of ozone has created an ozone hole over Antarctica. Ozone depletion results in higher levels of UV radiation reaching Earth. In humans, this increases skin cancers and eye cataracts. It also disturbs the nitrogen cycle, kills plankton, and disrupts ocean food webs. The total loss of the ozone layer would be devastating to most life. Its rate of loss has slowed with restrictions on pollutants, but it is still at risk.
Summary
• Air pollution consists of chemical substances and particles released into the air, mainly by human actions.
• The major cause of outdoor air pollution is the burning of fossil fuels.
• Indoor air can also be polluted.
• Air pollution causes acid rain, ozone depletion, and global warming.
Review
1. What two environmental effects are mainly associated with the burning of fossil fuels?
2. Explain how air pollution is related to acid rain and ozone depletion.
3. What benefits does the ozone layer provide? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/06%3A_Ecology/6.29%3A_Air_Pollution.txt |
Is the Earth really fragile?
Maybe not the planet, but how about the ecosystems? It may soon be hard to argue that global climate change does not exist. Climate change can definitely be seen in numerous ecosystems. So what will we do about it?
Global Climate Change
Another major problem caused by air pollution is global climate change. Gases such as carbon dioxide from the burning of fossil fuels increase the natural greenhouse effect. This raises the temperature of Earth’s surface.
What Is The Greenhouse Effect?
The greenhouse effect is a natural feature of Earth’s atmosphere. It occurs when gases in the atmosphere radiate the sun’s heat back down to Earth’s surface (see Figure below). Otherwise, the heat would escape into space. Without the greenhouse effect, Earth’s surfacetemperature would be far cooler than it is. In fact, it would be too cold to support life as we know it.
The Greenhouse Effect. Without greenhouse gases, most of the sun’s energy would be radiated from Earth’s surface back out to space.
Global Warming
Global warming refers to a recent increase in Earth’s average surface temperature (see Figure below). During the past century, the temperature has risen by almost 1°C (about 1.3°F). That may not seem like much. But consider that just 10°C is the difference between an ice-free and an ice-covered Earth.
The average annual temperature on Earth has been rising for the past 100 years.
Most scientists agree that global warming is caused by more carbon dioxide in the atmosphere (see Figure below). This increases the greenhouse effect. There is more carbon dioxide mainly because of the burning of fossil fuels. Destroying forests is another cause. With fewer forests, less carbon dioxide is removed from the atmosphere by photosynthesis.
This graph shows the recent trend in carbon dioxide in the atmosphere.
Effects of Climate Change
How has global warming affected Earth and its life? Some of its effects include:
• Decline in cold-adapted species such as polar bears.
• Melting of glaciers and rising sea levels.
• Coastal flooding and shoreline erosion.
• Heat-related human health problems.
• More droughts and water shortages.
• Changing patterns of precipitation.
• Increasing severity of storms.
• Major crop losses.
KQED: Climate Watch: California at the Tipping Point
The world's climate is changing and California is now being affected in both dramatic and subtle ways. In 2008, scientists determined that California’s temperatures increased by more than 2.1°F during the last century. What’s more, the data showed that human activity has played a significant role in that climate change. "What's just 2 degrees?" you may wonder. But, as the science shows, just 2 degrees is extremely significant.
What does all this temperature change mean? For starters, declining mountain snowpack and prolonged drought conditions could pose a threat to limited water supplies. Heat waves are projected to be longer, bringing increased danger from wildfires and heat-related deaths. Rising sea levels due to temperature shifts jeopardize life in coastal areas, both for human communities and the plants and animals that rely on intertidal and rich wetland ecosystems. Also, more precipitation is expected to fall as rain rather than snow, thereby increasing the risk of floods. And, as heat increases the formation of smog, poor air quality could get even worse.
Climate change may also profoundly affect the economy in California and elsewhere. Shorter ski seasons and damage to the marine ecosystem mean a reduction in tourism. Watershortages mean issues with the commercial and recreational fishing industry, and higher temperatures will affect crop growth and quality, weakening the agricultural industry, to name just a few of the economic issues associated with climate change.
KQED: Acidic Seas
Melting glaciers, rising temperatures and droughts are all impacts of global warming. But how does global warming actually affect the oceans? The sea, it turns out, absorbs carbon dioxide emissions. The ocean acts like a giant sponge, absorbing carbon dioxide emissions from the air. And as we add more and more carbon dioxide to air by burning fossil fuels, the ocean is absorbing it. On one level, it's done us a big favor. Scientists say that we would be experiencing much more extreme climate change were it not for the ocean's ability to remove the heat-trapping gas. However, these emissions are causing the oceans to become more acidic. Changing pH levels threaten entire marine food webs, from coral reefs to salmon.
As carbon dioxide levels increase in the atmosphere, the levels also increase in the oceans. What effects does this have? Can ocean acidification make it difficult for sea life to produce their hard exoskeletons?
What Can Be Done?
Efforts to reduce future global warming mainly involve energy use. We need to use less energy, for example, by driving more fuel-efficient cars. We also need to switch to energy sources that produce less carbon dioxide, such as solar and wind energy. At the same time, we can increase the amount of carbon dioxide that is removed from air. We can stop destroying forests and plant new ones.
Summary
• Gases such as carbon dioxide from the burning of fossil fuels increase the natural greenhouse effect. This is raising the temperature of Earth’s surface, and is called global warming.
Review
1. How does air pollution contribute to global warming?
2. What is the greenhouse effect?
3. What are three effects of global warming? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/06%3A_Ecology/6.30%3A_Global_Climate_Change.txt |
What are the most numerous organisms on the planet?
Bacteria. And all it takes is one to quickly grow, under just the right conditions, into millions and billions. Luckily, we know how to control bacteria when necessary. But bacteria do serve many important purposes. In fact, we could not survive without them.
Evolution of Prokaryotes
No doubt you’ve had a sore throat before, and you’ve probably eaten cheese or yogurt. If so, then you’ve encountered the fascinating world of prokaryotes. Prokaryotes are single-celled organisms that lack a nucleus. They also lack other membrane-bound organelles. Prokaryotes are tiny and sometimes bothersome, but they are the most numerous organisms on Earth. Without them, the world would be a very different place. Prokaryotes are the simplest organisms. The first cells and organisms to evolve would be classified as prokaryotic.
Prokaryotes are currently placed in two domains. A domain is the highest taxon, just above the kingdom. The prokaryote domains are Bacteria and Archaea (see Figure below). The third domain is Eukarya. It includes all eukaryotes. Unlike prokaryotes, eukaryotes have a nucleus in their cells.
The Three Domains of Life. All living things are grouped in three domains. The domains Bacteria and Archaea consist of prokaryotes. The Eukarya domain consists of eukaryotes.
It’s not clear how the three domains are related. Archaea were once thought to be offshoots of Bacteria that were adapted to extreme environments. For their part, Bacteria were considered to be ancestors of Eukarya. Scientists now know that Archaea share several traits with Eukarya that Bacteria do not share (see Table below). How can this be explained? One hypothesis is that Eukarya arose when an Archaean cell fused with a Bacterial cell. The two cells became the nucleus and cytoplasm of a new Eukaryan cell. How well does this hypothesis fit the evidence in Table below?
Characteristic Bacteria Archaea Eukarya
Flagella Unique to Bacteria Unique to Archaea Unique to Eukarya
Cell Membrane Unique to Bacteria Like Bacteria and Eukarya Unique to Eukarya
Protein Synthesis Unique to Bacteria Like Eukarya Like Archaea
Introns Absent in most Present Present
Peptidoglycan (in cell wall) Present Absent in most Absent
Summary
• Prokaryotes include Bacteria and Archaea. An individual prokaryote consists of a single cell without a nucleus.
• Bacteria live in virtually all environments on Earth.
• Archaea live everywhere on Earth, including extreme environments.
Review
1. What are prokaryotes?
2. What are two major differences between Bacteria and Archaea?
7.02: Prokaryote Classification
With so many different bacteria, how are they all classified?
By shape? By size? By some other criteria? As you can imagine, classifying bacteria is probably not an easy task. Bacteria are classified by their traits, some of which have to do with their shape, others with the cell wall, and even additional traits.
The Prokaryotic Domains
Domain Bacteria
Bacteria are the most diverse and abundant group of organisms on Earth. They live in almost all environments. They are found in the ocean, the soil, and the intestines of animals. They are even found in rocks deep below Earth’s surface. Any surface that has not been sterilized is likely to be covered with bacteria. The total number of bacteria in the world is amazing. It’s estimated to be 5 × 1030, or five million trillion. You have more bacteria in and on your body than you have body cells!
Bacteria called cyanobacteria are very important. They are bluish green in color (see Figure below) because they contain chlorophyll (but not chloroplasts, of course). They make food through photosynthesis and release oxygen into the air. These bacteria were probably responsible for adding oxygen to the air on early Earth. This changed the planet’s atmosphere. It also changed the direction of evolution. Ancient cyanobacteria also may have evolved into the chloroplasts of plant cells.
Cyanobacteria Bloom. The green streaks in this lake consist of trillions of cyanobacteria. Excessive nutrients in the water led to overgrowth of the bacteria.
Thousands of species of bacteria have been discovered, and many more are thought to exist. The known species can be classified on the basis of various traits. One classification is based on differences in their cell walls and outer membranes. It groups bacteria into Gram-positiveand Gram-negative bacteria, as described in Figure below.
Classification of Bacteria. Different types of bacteria stain a different color when stained with Gram stain. This makes them easy to identify.
Domain Archaea
Scientists still know relatively little about Archaea. This is partly because they are hard to grow in the lab. Many live inside the bodies of animals, including humans. However, none are known for certain to cause disease.
Archaea were first discovered in extreme environments. For example, some were found in hot springs. Others were found around deep sea vents. Such Archaea are called extremophiles, or “lovers of extremes.” Figure below describes three different types of Archaean extremophiles. The places where some of them live are thought to be similar to the environment on ancient Earth. This suggests that they may have evolved very early in Earth’s history.
Extremophile Archaea. Many Archaea are specialized to live in extreme environments. Just three types are described here.
Archaea are now known to live just about everywhere on Earth. They are particularly numerous in the ocean. Archaea in plankton may be one of the most abundant types of organisms on the planet. Archaea are also thought to play important roles in the carbon and nitrogen cycles. For these reasons, Archaea are now recognized as a major aspect of life on Earth.
Summary
• Bacteria live in virtually all environments on Earth.
• Archaea live everywhere on Earth, including extreme environments.
Review
1. Distinguish between Gram-positive and Gram-negative bacteria, and give an example of each.
2. Summarize the evolutionary significance of cyanobacteria.
3. What are extremophiles? Name three types.
4. Describe the habitat of extreme halophiles. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.01%3A_Prokaryote_Evolution.txt |
Does the shape matter?
It does if you're a bacterium. Prokaryotic cells are distinguished by their shape. And as you can imagine, shape may have something to do with mobility.
Prokaryote Structure
Most prokaryotic cells are much smaller than eukaryotic cells. Although they are tiny, prokaryotic cells can be distinguished by their shapes. The most common shapes are helices, spheres, and rods (see Figure below).
Prokaryotic Cell Shapes. The three most common prokaryotic cell shapes are shown here.
Plasma Membrane and Cell Wall
Like other cells, prokaryotic cells have a plasma membrane (see Figure below). It controls what enters and leaves the cell. It is also the site of many metabolic reactions. For example, cellular respiration and photosynthesis take place in the plasma membrane.
Most prokaryotes also have a cell wall. It lies just outside the plasma membrane. It gives strength and rigidity to the cell. Bacteria and Archaea differ in the makeup of their cell wall. The cell wall of Bacteria contains peptidoglycan, composed of sugars and amino acids. The cell wall of most Archaea lacks peptidoglycan.
Prokaryotic Cell. The main parts of a prokaryotic cell are shown in this diagram. The structure called a mesosome was once thought to be an organelle. More evidence has convinced most scientists that it is not a true cell structure at all. Instead, it seems to be an artifact of cell preparation. This is a good example of how scientific knowledge is revised as more evidence becomes available. Can you identify each of the labeled structures?
Cytoplasm and Cell Structures
Inside the plasma membrane of prokaryotic cells is the cytoplasm. It contains several structures, including ribosomes, a cytoskeleton, and genetic material. Ribosomes are sites where proteins are made. The cytoskeleton helps the cell keep its shape. The genetic material is usually a single loop of DNA. There may also be small, circular pieces of DNA, called plasmids. (see Figure below). The cytoplasm may contain microcompartments as well. These are tiny structures enclosed by proteins. They contain enzymes and are involved in metabolic processes.
Prokaryotic DNA. The DNA of a prokaryotic cell is in the cytoplasm because the cell lacks a nucleus.
Extracellular Structures
Many prokaryotes have an extra layer, called a capsule, outside the cell wall. The capsuleprotects the cell from chemicals and from drying out. It also allows the cell to stick to surfaces and to other cells. Because of this, many prokaryotes can form biofilms, like the one shown in Figure below. A biofilm is a colony of prokaryotes that is stuck to a surface such as a rock or a host’s tissues. The sticky plaque that collects on your teeth between brushings is a biofilm. It consists of millions of bacteria.
Most prokaryotes also have long, thin protein structures called flagella (singular, flagellum). They extend from the plasma membrane. Flagella help prokaryotes move. They spin around a fixed base, causing the cell to roll and tumble. As shown in Figure below, prokaryotes may have one or more flagella.
Bacterial Biofilm. The greatly magnified biofilm shown here was found on a medical catheter (tubing) removed from a patient’s body.
Variations in the Flagella of Bacteria. Flagella in prokaryotes may be located at one or both ends of the cell or all around it. They help prokaryotes move toward food or away from toxins.
Endospores
Many organisms form spores for reproduction. Some prokaryotes form spores for survival. Called endospores, they form inside prokaryotic cells when they are under stress. The stress could be UV radiation, high temperatures, or harsh chemicals. Endospores enclose the DNA and help it survive under conditions that may kill the cell. Endospores are commonly found in soil and water. They may survive for long periods of time.
Summary
• Most prokaryotic cells are much smaller than eukaryotic cells.
• Prokaryotic cells have a cell wall outside their plasma membrane.
• Prokaryotic DNA consists of a single loop. Some prokaryotes also have small, circular pieces of DNA called plasmids.
Review
1. Identify the three most common shapes of prokaryotic cells.
2. Describe a typical prokaryotic cell.
3. What are the roles of flagella and endospores in prokaryotes? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.03%3A_Prokaryote_Structure.txt |
What do bacteria need to grow?
Like most everything else, they need food. Given the right conditions, bacteria can grow from just a few cells to millions or billions overnight.
Prokaryote Metabolism
Like all living things, prokaryotes need energy and carbon. They meet these needs in a variety of ways. In fact, prokaryotes have just about every possible type of metabolism. They may get energy from light (photo) or chemical compounds (chemo). They may get carbon from carbon dioxide (autotroph) or other living things (heterotroph). Most prokaryotes are chemoheterotrophs. They depend on other organisms for both energy and carbon. Many break down organic wastes and the remains of dead organisms. They play vital roles as decomposers and help recycle carbon and nitrogen. Photoautotrophs are important producers. They are especially important in aquatic ecosystems.
Classification of Prokaryotes Based on Metabolism
Two major nutritional needs can be used to group prokaryotes. These are (1) carbon metabolism, their source of carbon for building organic molecules within the cells, and (2) energy metabolism, their source of energy used for growth.
In terms of carbon metabolism, prokaryotes are classified as either heterotrophic or autotrophic:
• Heterotrophic organisms use organic compounds, usually from other organisms, as carbon sources.
• Autotrophic organisms use carbon dioxide (CO2) as their only source or their main source of carbon. Many autotrophic bacteria are photosynthetic, and get their carbon from the carbon dioxide in the atmosphere.
Energy metabolism in prokaryotes is classified as one of the following:
• Phototrophic organisms capture light energy from the sun and convert it into chemical energy inside their cells.
• Chemotrophic organisms break down either organic or inorganic molecules to supply energy for the cell. Some chemotrophic organisms can also use their organic energy-supplying molecules as a carbon supply, which would make them chemoheterotrophs.
• Photoheterotrophs are organisms that capture light energy to convert to chemical energy in the cells, but they get carbon from organic sources (other organisms). Examples are purple non-sulfur bacteria, green non-sulfur bacteria and heliobacteria.
• Chemoheterotrophs are organisms that get their energy source and carbon source from organic sources. Chemoheterotrophs must consume organic building blocks that they are unable to make themselves. Most get their energy from organic molecules such as sugars. This nutritional mode is very common among eukaryotes, including humans.
• Photoautotrophs are cells that capture light energy, and use carbon dioxide as their carbon source. There are many photoautotrophic prokaryotes, which include cyanobacteria. Photoautotrophic prokaryotes use similar compounds to those of plants to trap light energy.
• Chemoautotrophs are cells that break down inorganic molecules to supply energy for the cell, and use carbon dioxide as a carbon source. Chemoautotrophs include prokaryotes that break down hydrogen sulfide (H2S the “rotten egg” smelling gas), and ammonia (NH4).Nitrosomonas, a species of soil bacterium, oxidizes NH4+ to nitrite (NO2-). This reaction releases energy that the bacteria use. Many chemoautotrophs also live in extreme environments such as deep sea vents.
This flowchart helps to determine if a species is an autotroph or a heterotroph, a phototroph or a chemotroph. For example, “Obtain carbon elsewhere?” asks if the source of carbon is another organism. If the answer is “yes”, the organism is heterotrophic. If the answer is “no,” the organisms is autotrophic.
Summary
• Prokaryotes fulfill their carbon and energy needs in various ways. They may be photoautotrophs, chemoautotrophs, photoheterotrophs, or chemoheterotrophs.
Review
1. Describe metabolism of most prokaryotes.
2. Define phototrophic and chemotrophic organisms.
3. What are photoautotrophs?
4. What are photoheterotrophs? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.04%3A_Prokaryote_Metabolism.txt |
Where do you find lots of bacteria?
Practically all surfaces. Bacteria can live and grow in practically any environment. It is this ability that has made bacteria the most numerous species on the planet.
Prokaryote Habitats
Prokaryotes have a wide range of metabolisms, and this determines where they live. They live in a particular habitat because they are able to “eat” whatever is around them. For example, there are bacteria and archaea that break down hydrogen sulfide to produce ATP. Hydrogen sulfide is the gas that gives rotten eggs and sewage their distinctive smell. It is poisonous to animals, but some prokaryotes depend on it for life.
Organisms that are obligate aerobes need oxygen to live. That is, they use oxygen as a terminal electron acceptor while making ATP (see the “Cellular Respiration” concept). Humans are obligate aerobes, and so are Mycobacterium tuberculosis bacteria. M. tuberculosis causes tuberculosis (TB). Obligate aerobes are found only in places with molecular oxygen.
An anaerobic organism is any organism that does not need oxygen for growth and even dies in its presence. Obligate anaerobes will die when exposed to atmospheric levels of oxygen.Clostridium perfringens bacteria, which are commonly found in soil around the world, are obligate anaerobes. Infection of a wound by C. perfringens bacteria causes the disease gas gangrene. Obligate anaerobes use molecules other than oxygen as terminal electron acceptors.
Facultative anaerobic organisms, which are usually prokaryotic, make ATP by aerobic respiration, if oxygen is present, but can also survive without oxygen. In the absence of oxygen they switch to the process of fermentation to make ATP. Fermentation is a type of heterotrophic metabolism that uses organic carbon instead of oxygen as a terminal electron acceptor. Examples of facultative anaerobic bacteria are the Staphylococci, Escherichia coli,Corynebacterium, and Listeria species. Many bacteria that cause human diseases are facultative anaerobic organisms.
Temperature
Like most organisms, prokaryotes live and grow best within certain temperature ranges. Prokaryotes can be classified by their temperature preferences, as shown in the Table below. Which type of prokaryote would you expect to find inside the human body?
Thermophiles live at relatively high temperatures, above 45°C (113°F). Thermophiles are found in geothermally heated regions of the Earth, such as hot springs like the Morning Glory pool in Yellowstone National Park (see Figure below), and deep sea hydrothermal vents. Some thermophiles live in decaying plant matter such as peat bogs and compost. Many thermophiles are archaea. Extreme thermophiles (or hyperthermophiles), live in temperatures hotter than 80°C (176°F).
Psychrophiles grow and reproduce in cold temperatures. The optimal growth temperature of some psychrophiles is 15°C or lower; they cannot grow in temperatures above 20°C. The environments that psychrophiles inhabit are found all over Earth. Psychrophiles live in such places as permafrost soils, deep-ocean waters, Arctic and Antarctic glaciers and snowfields.
Mesophiles grow best in moderate temperature, typically between 25°C and 40°C (77°F and 104°F). Mesophiles are often found living in or on the bodies of humans or other animals. The optimal growth temperature of many pathogenic mesophiles is 37°C (98°F), the normal human body temperature. Mesophilic organisms have important uses in food preparation, including cheese, yogurt, beer and wine.
Type of Prokaryote Preferred Temperature Where It Might Be Found
Thermophile above 45°C (113°F) in compost
Mesophile about 37°C (98°F) inside animals
Psychrophile below 20°C (68°F) in the deep ocean
The Morning Glory pool of Yellowstone National Park in the United States is a geothermal pool whose waters are heated to high temperatures by magma deep underground. Hyperthermophilic organisms, such as members of the archaeal genusSulfolobus can live at temperatures between 60°C-80°C and a pH of 3.
Summary
• Aerobic prokaryotes live in habitats with oxygen.
• Anaerobic prokaryotes live in habitats without oxygen.
• Prokaryotes may also be adapted to habitats that are hot, moderate, or cold in temperature.
Review
1. Why are many prokaryotes adapted for living at the normal internal temperature of the human body.
2. Compare psychrophiles to thermophiles.
3. Compare obligate aerobes to facultative anaerobes.
7.06: Prokaryote Reproduction
How do bacteria reproduce?
Essentially, they grow and divide. Show here is an example of Methicillin-resistantStaphylococcus aureus, or MRSA, bacteria. Notice how one bacterium is dividing into two.
Reproduction in Prokaryotes
Unlike multicellular organisms, increases in the size of prokaryotes (cell growth) and their reproduction by cell division are tightly linked. Prokaryotes grow to a fixed size and then reproduce through binary fission.
Binary Fission
Binary fission is a type of asexual reproduction. It occurs when a parent cell splits into two identical daughter cells. This can result in very rapid population growth. For example, under ideal conditions, bacterial populations can double every 20 minutes. Such rapid population growth is an adaptation to an unstable environment. Can you explain why?
Schematic diagram of cellular growth (elongation) and binary fission of bacilli. Blue and red lines indicate old and newly-synthesized bacterial cell wall, respectively. The DNA inside the bacterium is copied and the daughter cells receive an exact copy of the parent DNA. Fission involves a cytoskeletal protein FtsZ that forms a ring at the site of cell division.
Genetic Transfer
In asexual reproduction, all the offspring are exactly the same. This is the biggest drawback of this type of reproduction. Why? Lack of genetic variation increases the risk of extinction. Without variety, there may be no organisms that can survive a major change in the environment.
Prokaryotes have a different way to increase genetic variation. It’s called genetic transfer or bacterial conjugation. It can occur in two ways. One way is when cells “grab” stray pieces of DNA from their environment. The other way is when cells directly exchange DNA (usually plasmids) with other cells. For example, as shown in Figure below, the donor cell makes a structure called an F pilus, or sex pilus. The F pilus attaches one cell to another cell. The membranes of the two cells merge and genetic material, usually a plasmid, moves into the recipient cell. Genetic transfer makes bacteria very useful in biotechnology. It can be used to create bacterial cells that carry new genes.
A flowchart showing bacterial conjugation. The donor cell makes a structure called an F pilus, or sex pilus. The F pilus attaches one cell to another cell. The membranes of the two cells merge and genetic material, usually a plasmid, moves into the recipient cell.
Summary
• Prokaryotic cells grow to a certain size. Then they divide by binary fission. This is a type of asexual reproduction.
• Binary fission produces genetically identical offspring.
• Genetic transfer increases genetic variation in prokaryotes.
Review
1. Describe binary fission.
2. Why might genetic transfer be important for the survival of prokaryote species?
3. Under ideal conditions, in 2 hours how many bacteria can result from just a single bacterium? 5 hours? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.05%3A_Prokaryote_Habitats.txt |
This virus looks alive, but is it?
Notice the big virus. He (or she) looks very angry. But actually viruses cannot be a "he" or a "she" - or big either. In fact, viruses are the smallest things. Much smaller than most prokaryotes. We also cannot say that viruses are the smallest living things or organisms, as viruses do not meet the definition of living or of an organism.
Characteristics of Viruses
Which of the three domains of life do viruses belong to? None. Why? Viruses are usually considered to be nonliving. Viruses do not meet most of the criteria of life. They are not even made of cells.
A virus is a sub-microscopic particle that can infect living cells. Viruses are much smaller than prokaryotes, ranging in size from about 20–300 nanometers (nm), though some can be larger. Prokaryotes are typically 0.5–5.0 micrometers (µm) in length. For example, if a virus was about the size of three soccer balls lying side-by-side, then a prokaryote would be about the size of soccer field.
An individual virus is called a virion. It is a tiny particle much smaller than a prokaryotic cell. Because viruses do not consist of cells, they also lack cell membranes, cytoplasm, ribosomes, and other cell organelles. Without these structures, they are unable to make proteins or even reproduce on their own. Instead, they must depend on a host cell to synthesize their proteins and to make copies of themselves. Although viruses are not classified as living things, they share two important traits with living things. They have genetic material, and they can evolve. This is why the classification of viruses has been controversial. It calls into question just what it means to be alive. What do you think? How would you classify viruses?
The study of viruses is known as virology and people who study viruses are known asvirologists. Viruses infect and live inside the cells of living organisms. When viruses infect the cells of their host, they may cause disease. For example, viruses cause AIDS (Acquired immune deficiency syndrome), influenza (flu), chicken pox, and the common cold. Therapy is sometimes difficult for viral diseases. Antibiotics have no effect on viruses and only a few antiviral drugs are available for some diseases. One of the best ways to prevent viral diseases is with a vaccine, which produces immunity. But vaccines are available for only a few diseases.
Mimivirus, shown in the Figure below, is the largest known virus, with a diameter of 400 nm. Protein filaments measuring 100 nm stick out from the surface of the virus, which increases the diameter of the virus to about 600 nm. This is bigger than a small bacterial cell. The virus appears hexagonal under an electron microscope; the viral shape is icosahedral (having 20 faces or sides).
The largest known virus, called mimivirus, is so large that scientists first mistook it for a bacterium. It was first discovered in amoeba, in 1992, and was identified as a virus in 2003. Scientists believe that mimivirus may cause certain types of pneumonia in humans. The core contains DNA, with the majority of the DNA in genes, and only 10% DNA of unknown function ("junk" DNA).
Replication
Viruses can replicate only by infecting a host cell. They cannot reproduce on their own. Viruses are not cells; they are a strand of genetic material within a protective protein coat called a capsid. They infect a wide variety of organisms, including both eukaryotes and prokaryotes. Once inside the cell, they use the cell’s ATP, ribosomes, enzymes, and other cellular parts to replicate.
Habitats
Viruses can be found almost anywhere there is life, including living within prokaryotes. A phage is a virus that infects prokaryotes. Phages are estimated to be the most widely distributed and diverse entities in the biosphere, even more numerous than prokaryotic organisms. Phages can be found everywhere their hosts are found, such as in soil, in the intestine of animals, or seawater. Up to 109 virions have been found in a milliliter of seawater, and up to 70 percent of marine bacteria may be infected by phages. They are also found in drinking water and in some foods, including fermented vegetables and meats, where they control the growth of bacteria.
Summary
• Viruses are tiny particles, smaller than prokaryotic cells.
• Viruses are not cells and cannot replicate without help, but they have nucleic acids and can evolve.
Review
1. What is a virus?
2. What are the two main components of a virus?
3. How do viruses differ from living things? How are they similar to living things?
4. Briefly describe how viruses depend on host cells.
5. What two characteristics of life are evident in viruses?
6. What is a phage? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.07%3A_Virus_Characteristics.txt |
Is this a cell or a virus?
It is actually a representation of the HIV virus, the virus that causes AIDS. All the little “knobs” on the outside of the virus help to give the virus structure. And it is this structure that must be identified by a vaccine.
Structure of Viruses
Viruses vary in their structure. A virus particle consists of DNA or RNA within a protective protein coat called a capsid. The shape of the capsid may vary from one type of virus to another. The capsid is made from the proteins that are encoded by viral genes within their genome.
The shape of the capsid serves as one basis for classification of viruses. The capsid of the virus shown in Figure below is icosahedral. Virally coded proteins will self-assemble to form a capsid. Some viruses have an envelope of phospholipids and proteins. The envelope is made from portions of the host’s cell membrane. It surrounds the capsid and helps protect the virus from the host’s immune system. The envelope may also have receptor molecules that can bind with host cells. They make it easier for the virus to infect the cells.
Diagram of a Cytomegalovirus. The capsid encloses the genetic material of the virus. The envelope which surrounds the capsid is typically made from portions of the host cell membranes (phospholipids and proteins). Not all viruses have a viral envelope.
Helical Viruses
Helical capsids are made up of a single type of protein subunit stacked around a central axis to form a helical structure. The helix may have a hollow center, which makes it look like a hollow tube. This arrangement results in rod-shaped or filamentous virions. These virions can be anything from short and very rigid, to long and very flexible. The well-studied tobacco mosaic virus (TMV) is an example of a helical virus, as seen in the Figure below.
A helical virus, tobacco mosaic virus. Although their diameter may be very small, some helical viruses can be quite long, as shown here. 1. Nucleic acid; 2. Viral protein units, 3. Capsid. TMV causes tobacco mosaic disease in tobacco, cucumber, pepper, and tomato plants.
Icosahedral Viruses
Icosahedral capsid symmetry gives viruses a spherical appearance at low magnification, but the protein subunits are actually arranged in a regular geometrical pattern, similar to a soccer ball; they are not truly spherical. An icosahedral shape is the most efficient way of creating a hardy structure from multiple copies of a single protein. This shape is used because it can be built from a single basic unit protein which is used over and over again. This saves space in the viral genome.
Adenovirus, an icosahedral virus. An icosahedron is a three-dimensional shape made up of 20 equilateral triangles. Viral structures are built of repeated identical protein subunits, making the icosahedron the easiest shape to assemble using these subunits.
Complex Viruses
Complex viruses possess a capsid which is neither purely helical, nor purely icosahedral, and which may have extra structures such as protein tails or a complex outer wall. Viral protein subunits will self-assemble into a capsid, but the complex viruses DNA also codes for proteins which help in building the viral capsid. Many phage viruses are complex-shaped; they have an icosahedral head bound to a helical tail. The tail may have a base plate with protein tail fibers. Some complex viruses do not have tail fibers.
This “moon lander”-shaped complex virus infects Escherichia coli bacteria.
Enveloped Viruses
Some viruses are able to surround (envelop) themselves in a portion of the cell membrane of their host. The virus can use either the outer membrane of the host cell, or an internal membrane such as the nuclear membrane or endoplasmic reticulum. In this way the virus gains an outer lipid bilayer known as a viral envelope. This membrane is studded with proteins coded for by both the viral genome and the host genome. However, the lipid membrane itself and any carbohydrates present come entirely from the host cell. The influenza virus, HIV, and the varicella zoster virus (Figure below) are enveloped viruses.
An enveloped virus. Varicella zoster virus causes chicken pox and shingles.
The viral envelope can give a virus some advantages over other capsid-only viruses. For example, they have better protection from the host's immune system, enzymes and certain chemicals. The proteins in the envelope can include glycoproteins, which act as receptor molecules. These receptor molecules allow host cells to recognize and bind the virions, which may result in easier uptake of the virion into the cell. Most enveloped viruses depend on their envelopes to infect cells. However, because the envelope contains lipids, it makes the virus more susceptible to inactivation by environmental agents, such as detergents that disrupt lipids.
Summary
• Viruses have different shapes. They can be cylindrical, icosahedral, complex, or enveloped.
Review
1. Describe variation in capsid shape in viruses.
2. Compare the structures of a prokaryote and a virus. If you prefer, you may draw a diagram of each and label the different parts of each structure. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.08%3A_Virus_Structures.txt |
How are viruses classified?
In part by their shape. This picture represents a bacteriophage, a virus that infects bacteria. Notice the distinctive shape. This virus has a complex shape.
Classification of Viruses
Like the classification systems for cellular organisms, virus classification is the subject of ongoing debate. This is largely due to the nature of viruses, which are not living organisms by the classic definition, but neither are they necessarily non-living. Therefore, viruses do not fit neatly into the biological classification system of cellular organisms, as plants and animals do.
Virus classification is based mainly on characteristics of the viral particles, including the capsid shape, the type of nucleic acid (DNA or RNA, double stranded (ds) or single stranded (ss)) within the capsid, the process of replication, their host organisms, or the type of disease they cause. The Table below lists characteristics such as capsid shape, presence of an envelope, and the diseases the viruses can cause.
Viruses
Virus Family Virus Envelope Capsid shape Nucleic Acid Disease
Adenoviruses Adenovirus No Icosahedral dsDNA upper respiratory infections
Parvoviruses Parvovirus No Icosahedral ssDNA fifth disease, Canine parvovirus
Herpesviruses Herpes simplex virus, Varicella zoster virus, Epstein Barr virus Yes Icosahedral dsDNA Herpes, chicken pox, shingles, infectious mononucleosis
Hepadnaviruses Hepatitis B virus Yes Icosahedral dsDNA Hepatitis B
Reoviruses Rotavirus No Icosahedral dsRNA gastroenteritis
Retroviruses HIV, HTLV-I Yes Complex ssRNA HIV/AIDS, leukemia
Orthomyxoviruses Influenza viruses Yes Helical ssRNA Influenza (flu)
Rhabdoviruses Rabies virus Yes Helical ssRNA Rabies
Coronaviruses Corona virus Yes Complex ssRNA Common cold, Severe acute respiratory syndrome (SARS)
Cystoviruses Cystovirus Yes Icosahedral dsRNA InfectsPseudomonasbacteria
Summary
• Viruses can be classified on the basis of capsid shape, presence or absence of an envelope, and type of nucleic acid.
Review
1. Describe how viruses are classified.
2. What is the difference between viruses in the Herpesvirus family and the Retrovirus family?
3. What are the four types of nucleic acid that may be present in a virus?
4. Give an example of a disease caused by a member of the Orthomyxoviruses.
5. Give an example of a disease caused by a retrovirus.
7.10: Discovery and Origin of Viruses
Can you "discover" something without actually seeing it?
Well, yes you can, and that's precisely how viruses were discovered. Viruses are much smaller than bacteria, so special microscopes are needed to see them, but the existence of viruses was known prior to the development of these special microscopes.
Discovery and Origin of Viruses
Viruses are so small that they can be seen only with an electron microscope. Before electron microscopes were invented, scientists knew viruses must exist. How did they know? They had demonstrated that particles smaller than bacteria cause disease.
Discovery of Viruses
Researchers used special filters to remove bacteria from tissues that were infected. If bacteria were causing the infection, the filtered tissues should no longer be able to make other organisms sick. However, the filtered tissues remained infective. This meant that something even smaller than bacteria was causing the infection.
Scientists did not actually see viruses for the first time until the 1930s. That’s when the electron microscope was invented. In 1915, English bacteriologist Frederick Twort discovered bacteriophage, the viruses that attack bacteria. He noticed tiny clear spots within bacterial colonies, and hypothesized that something was killing the bacteria. The tobacco mosaic virus shown in Figure below was the first one to be seen.
Tobacco Mosaic Virus. This tobacco mosaic virus was the first virus to be discovered. It was first seen with an electron microscope in 1935.
Origin of Viruses
Where did viruses come from? How did the first viruses arise? The answers to these questions are not known for certain. Several hypotheses have been proposed. The two main hypotheses are stated below. Both may be valid and explain the origin of different viruses.
• Small viruses started as runaway pieces of nucleic acid that originally came from living cells such as bacteria.
• Large viruses were once parasitic cells inside bigger host cells. Over time, genes needed to survive and reproduce outside host cells were lost.
Summary
• Viruses were assumed to exist before they were first seen with an electron microscope in the 1930s.
• Multiple hypotheses for viral origins have been proposed.
Review
1. Why did scientists think viruses must exist even before they ever saw them with an electron microscope?
2. State two hypotheses for the origin of viruses.
3. What allowed scientists to see a virus for the first time? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.09%3A_Virus_Classification.txt |
Notice the viruses sitting on the bacteria?
Why is the virus sitting here? Remember, viruses are not living. So how do they replicate?
Replication of Viruses
Populations of viruses do not grow through cell division because they are not cells. Instead, they use the machinery and metabolism of a host cell to produce new copies of themselves. After infecting a host cell, a virion uses the cell’s ribosomes, enzymes, ATP, and other components to replicate. Viruses vary in how they do this. For example:
• Some RNA viruses are translated directly into viral proteins in ribosomes of the host cell. The host ribosomes treat the viral RNA as though it were the host’s own mRNA.
• Some DNA viruses are first transcribed in the host cell into viral mRNA. Then the viral mRNA is translated by host cell ribosomes into viral proteins.
In either case, the newly made viral proteins assemble to form new virions. The virions may then direct the production of an enzyme that breaks down the host cell wall. This allows the virions to burst out of the cell. The host cell is destroyed in the process. The newly released virus particles are free to infect other cells of the host.
Replication of RNA Viruses
An RNA virus is a virus that has RNA as its genetic material. Their nucleic acid is usually single-stranded RNA, but may be double-stranded RNA. Important human pathogenic RNA viruses include the Severe Acute Respiratory Syndrome (SARS) virus, Influenza virus, and Hepatitis C virus. Animal RNA viruses can be placed into different groups depending on their type of replication.
• Some RNA viruses have their genome used directly as if it were mRNA. The viral RNA is translated directly into new viral proteins after infection by the virus.
• Some RNA viruses carry enzymes which allow their RNA genome to act as a template for the host cell to a form viral mRNA.
• Retroviruses use DNA intermediates to replicate. Reverse transcriptase, a viral enzyme that comes from the virus itself, converts the viral RNA into a complementary strand of DNA, which is copied to produce a double stranded molecule of viral DNA. This viral DNA is then transcribed and translated by the host machinery, directing the formation of new virions. Normal transcription involves the synthesis of RNA from DNA; hence, reverse transcription is the reverse of this process. This is an exception to the central dogma of molecular biology.
Replication of DNA Viruses
A DNA virus is a virus that has DNA as its genetic material and replicates using a DNA-dependent DNA polymerase. The nucleic acid is usually double-stranded DNA but may also be single-stranded DNA. The DNA of DNA viruses is transcribed into mRNA by the host cell. The viral mRNA is then translated into viral proteins. These viral proteins then assemble to form new viral particles.
Reverse-Transcribing Viruses
A reverse-transcribing virus is any virus which replicates using reverse transcription, the formation of DNA from an RNA template. Some reverse-transcribing viruses have genomes made of single-stranded RNA and use a DNA intermediate to replicate. Others in this group have genomes that have double-stranded DNA and use an RNA intermediate during genome replication. The retroviruses, as mentioned above, are included in this group, of which HIV is a member. Some double-stranded DNA viruses replicate using reverse transcriptase. The hepatitis B virus is one of these viruses.
Bacteriophages
Bacteriophages are viruses that infect bacteria. They bind to surface receptor molecules of the bacterial cell and then their genome enters the cell. The protein coat does not enter the bacteria. Within a short amount of time, in some cases, just minutes, bacterial polymerase starts translating viral mRNA into protein. These proteins go on to become either new virions within the cell, helper proteins which help assembly of new virions, or proteins involved in cell lysis. Viral enzymes aid in the breakdown of the cell membrane. With some phages, just over twenty minutes after the phage infects the bacterium, over three hundred phages can be assembled and released from the host.
Summary
• After infecting a host cell, a virus uses the cell’s machinery and metabolism to produce new copies of itself.
Review
1. In general terms, describe viral replication.
2. Describe how DNA viruses replicate.
3. What are reverse-transcribing viruses? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.11%3A_Virus_Replication.txt |
Viral or bacterial?
Doesn't look like fun. The flu is caused by an influenza virus. And usually a slightly different virus every season.
Viruses and Human Disease
Viruses cause many human diseases. In addition to the flu and HIV, viruses cause rabies, measles, diarrheal diseases, hepatitis, polio, cold sores and other diseases (see Figure below). Viral diseases range from mild to fatal. One way viruses cause disease is by causing host cells to burst open and die. Viruses may also cause disease without killing host cells. They may cause illness by disrupting homeostasis in host cells.
Cold Sore. Cold sores are caused by a herpes virus.
Some viruses live in a dormant state inside the body. This is called latency. For example, the virus that causes chicken pox may infect a young child and cause the short-term disease chicken pox. Then the virus may remain latent in nerve cells within the body for decades. The virus may re-emerge later in life as the disease called shingles. In shingles, the virus causes painful skin rashes with blisters (see Figure below).
Shingles. Shingles is a disease caused by the same virus that causes chicken pox.
Some viruses can cause cancer. For example, human papillomavirus (HPV) causes cancer of the cervix in females. Hepatitis B virus causes cancer of the liver. A viral cancer is likely to develop only after a person has been infected with a virus for many years.
The Flu
Influenza, or flu, is a contagious respiratory illness caused by influenza viruses. Influenza spreads around the world in seasonal epidemics. An epidemic is an outbreak of a disease within a population of people during a specific time. Every year in the United States, about 200,000 people are hospitalized and 36,000 people die from the flu. Flu pandemics can kill millions of people. A pandemic is an epidemic that spreads through human populations across a large region (for example a continent), or even worldwide. Three influenza pandemics occurred in the 20th century and killed tens of millions of people, with each of these pandemics being caused by the appearance of a new strain of the virus. Most influenza strains can be inactivated easily by disinfectants and detergents.
Emerging Viral Diseases
Modern modes of transportation allow more people and products to travel around the world at a faster pace. They also open the airways to the transcontinental movement of infectious disease vectors. One example of this occurring is West Nile Virus, which scientists believe was introduced to the United States by an infected air traveler. With the use of air travel, people are able to go to foreign lands, contract a disease and not have any symptoms of illness until they get home, possibly exposing others to the disease along the way.
Often, new diseases result from the spread of an existing disease from animals to humans. A disease that can be spread from animals to humans is called a zoonosis. When a disease breaks out, scientists called epidemiologists investigate the outbreak, looking for its cause. Epidemiologists are like detectives trying to solve a crime. The information epidemiologists learn is important to understand the pathogen, and help prevent future outbreaks of disease.
A deadly strain of avian flu virus named H5N1 has posed the greatest risk for a new influenza pandemic since it first killed humans in Asia in the 1990s. The virus is passed from infected birds to humans. Fortunately, the virus has not mutated to a form that spreads easily between people.
Several lethal viruses that cause viral hemorrhagic fever have been discovered, two of which are shown in the Figure below. Ebola outbreaks have been limited mainly to remote areas of the world. However, they have gained extensive media attention because of the high mortality rate—23 percent to 90 percent—depending on the strain. The primary hosts of the viruses are thought to be apes in west central Africa, but the virus has also been isolated from bats in the same region.
The Ebola virus (left), and Marburg virus (right), are viruses that cause hemorrhagic fevers that can cause multiple organ failure and death.
People get exposed to new and rare zoonoses when they move into new areas and encounter wild animals. For example, severe acute respiratory syndrome (SARS) is a respiratory disease which is caused by the SARS coronavirus. An outbreak in China in 2003 was linked to the handling and consumption of wild civet cats sold as food in a market. In 2005, two studies identified a number of SARS-like coronaviruses in Chinese bats. It is likely that the virus spread from bats to civets, and then to humans.
Ebola is a rare and deadly disease caused by infection with a strain of Ebola virus. The 2014 Ebola epidemic is the largest in history, affecting multiple countries in West Africa, including Guinea, Sierra Leone and Liberia. Ebola is spread through direct contact with blood and body fluids of a person infected by and already showing symptoms of Ebola. Ebola is not spread through the air, water, food, or mosquitoes.
Summary
• Viruses cause many human diseases by killing host cells or disturbing their homeostasis.
• Viruses are not affected by antibiotics. Several viral diseases can be treated with antiviral drugs or prevented with vaccines.
Review
1. How do viruses cause human disease?
2. What is an epidemic? Why can the flu be considered an epidemic?
3. What is latency? Give an example of a virus that undergoes this process. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.12%3A_Viruses_and_Human_Disease.txt |
Why is the shape of the virus important?
The AIDS virus. One of the most devastating viruses known. Why? Notice the intricate anatomy of the virus, both inside and outside. It is the constant changing of those outside markers that make producing a vaccine against this virus difficult.
HIV
The Human Immunodeficiency Virus (HIV) is the virus that causes Acquired Immunodeficiency Syndrome (AIDS). Most researchers believe that the HIV originated in sub-Saharan Africa during the 20th century. HIV is transmitted by sexual contact and by contact with infected bodily fluids, such as blood, semen, breast milk, and vaginal secretions. It is also passed from mother to fetus. HIV is now a pandemic, with an estimated (as of 2008) 38.6 million people now living with the disease worldwide. It is estimated that AIDS has killed more than 25 million people since it was first recognized in 1981.
HIV is the retrovirus that destroys the immune system. HIV primarily infects helper T cells(specifically CD4+ T cells), macrophages, and dendritic cells. HIV infection leads to low levels of CD4+ T cells because the virus directly kills infected cells and the infected T cells are also attacked by the immune system. The infection of a CD4 cell is shown in the Figure below. When CD4+ T cell numbers decline below a certain critical level, cell-mediated immunity is lost, and the body becomes more prone to opportunistic infections. HIV infections will be discussed further in the immune system concepts.
If left untreated, most HIV-infected individuals will develop AIDS. AIDS is a collection of symptoms and infections resulting from the damage to the immune system by HIV. Because the immune systems of people with AIDS are so weak, bacteria and viruses that do not normally cause disease in healthy people can easily cause disease in an AIDS patient. Opportunistic infections associated with AIDS include:
• Pneumocystis pneumonia: a form of pneumonia caused by a fungus.
• Tuberculosis (TB), caused by the Mycobacterium tuberculosis bacteria.
• Lung infections caused by Mycobacterium other than tuberculosis (MOTT).
• Kaposi's sarcoma: a type of cancer that is caused by Kaposi's sarcoma-associated herpesvirus (KSHV).
Lifecycle of HIV. The image at bottom right is an SEM of HIV budding from cultured lymphocyte. The many round bumps on the cell surface are sites of assembly and budding of virions.
An HIV Infection
An HIV infection of a CD4 cell can be summarized as follows:
1. First, the viral particle attaches to the CD4 receptor and other associated receptors on the cell membrane. The viral envelope then fuses with the cell membrane, and the viral capsid moves into the cell.
2. Once the viral capsid enters the cell, reverse transcriptase frees the single-stranded RNA from the viral proteins and copies it into a complementary strand of DNA. This process of reverse transcription is error-prone and it is during this step that mutations may occur. Such mutations may cause drug resistance.
3. The reverse transcriptase then makes a complementary DNA strand to form a double-stranded viral DNA (vDNA).
4. The vDNA is then moved into the cell nucleus. The integration of the viral DNA into the host cell's genome is carried out by another viral enzyme called integrase. This integrated viral DNA may then lie dormant, during the latent stage of the HIV infection. Clinical latency for HIV can vary between two weeks and 20 years.
5. To actively produce viruses, certain cellular transcription factors need to be present. These transcription factors are plentiful in activated T cells. This means that those cells most likely to be killed by HIV are those currently fighting infection. The virus DNA is transcribed to mRNA which then leads to new virus protein and genome production.
6. Viral particles are assembled inside the cell and then exit the cell by budding. The virus gets its viral envelope from the cell’s plasma membrane. The cycle begins again when the new particles infect another cell.
HIV infection is treated with a cocktail of several antiretroviral drugs. The antiretroviral drugs prevent the virus from replicating and destroying more T cells, thus preventing the patients from developing AIDS. Treatment with antiretroviral drugs can dramatically increase the life expectancy of people with HIV.
Summary
• HIV is the virus that causes AIDS. HIV destroys the immune system.
• Opportunistic infections lead to AIDS.
Review
1. Describe the relationship between HIV and AIDS.
The graph below shows the relationship between the number of HIV particles and CD4 lymphocyte counts over the course of an untreated HIV infection. Use the graph to answer the following questions:
2. How does the increase of the number of HIV particles relate to the path of the disease?
3. Outline what happens to the number of viruses and numbers of lymphocytes between three and six weeks after initial infection.
4. Can the development of AIDS symptoms be delayed? Explain your answer. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.13%3A_HIV.txt |
Most people don't like having a shot at the doctor's office. But these are necessary. They protect you from some very dangerous viruses.
Control of Viruses
People have been able to control the spread of viruses even before they knew they existed. In 1717, Mary Montagu, the wife of an English ambassador to the Ottoman Empire, observed local women inoculating their children against smallpox, a contagious viral disease that was often deadly. Inoculation involves introducing a small amount of virus into a person’s body to allow their body to build up immunity to the virus. This early smallpox inoculation involved putting smallpox crusts into the nostril of a healthy person.
Vaccines
Because viruses use the machinery of a host cell to reproduce and stay within them, they are difficult to get rid of without killing the host cell. Vaccines were used to prevent viral infections long before the discovery of viruses. A vaccine is a mixture of antigenic material and other immune stimulants that will produce immunity to a certain pathogen or disease. The term "vaccine" comes from Edward Jenner's use of cowpox (vacca means cow in Latin), to immunize people against smallpox.
The material in the vaccine can either be weakened forms of a living pathogen or virus, dead pathogens (or inactivated viruses), purified material such as viral proteins, or genetically engineered pieces of a pathogen. The material in the vaccine will cause the body to mount an immune response, so the person will develop immunity to the disease. Smallpox was the first disease people tried to prevent by purposely inoculating themselves with other types of infections such as cowpox. Vaccination is an effective way of preventing viral infections. Vaccinations can be given in schools, shown in the Figure below, health clinics, and even at home. Their use has resulted in a dramatic decline in morbidity (illness) and mortality (death) associated with viral infections such as polio, measles, mumps, and rubella. Genetically engineered vaccines are produced through recombinant DNA technology. Most new vaccines are produced with this technology.
A young student receives a vaccine.
A worldwide vaccination campaign by the World Health Organization led to the eradication of smallpox in 1979. Smallpox is a contagious disease unique to humans and is caused by twoVariola viruses. The eradication of smallpox was possible because humans are the only carriers of the virus. To this day, smallpox is the only human infectious disease to have been completely eradicated from nature. Scientists are hoping to eradicate polio next.
Antiviral Drugs
While people have been able to prevent certain viral diseases by vaccinations for many hundreds of years, the development of antiviral drugs to treat viral diseases is a relatively recent development. Antiviral drugs are medications used specifically for treating the symptoms of viral infections. The first antiviral drug was interferon, a substance that is naturally produced by certain immune cells when an infection is detected. Over the past twenty years the development of antiretroviral drugs (also known as antiretroviral therapy, or ART) has increased rapidly. This has been driven by the AIDS epidemic.
Like antibiotics, specific antivirals are used for specific viruses. They are relatively harmless to the host, and therefore can be used to treat infections. Most of the antiviral drugs now available are designed to help deal with HIV and herpes viruses. Antivirals are also available for the influenza viruses and the Hepatitis B and C viruses, which can cause liver cancer.
Antiviral drugs are often imitation DNA building blocks which viruses incorporate into their genomes during replication. The life cycle of the virus is then halted because the newly synthesized DNA is inactive. Similar to antibiotics, antivirals are subject to drug resistance as the pathogens evolve to survive exposure to the treatment. HIV evades the immune system by constantly changing the amino acid sequence of the proteins on the surface of the virion. Researchers are now working to extend the range of antivirals to other families of pathogens.
Summary
• Several viral diseases can be treated with antiviral drugs or prevented with vaccines.
Review
1. What is a vaccine?
2. Describe the relationship between vaccination and immunity.
3. What diseases can be controlled with vaccinations? List five. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.14%3A_Control_of_Viruses.txt |
Can viruses be helpful, or just harmful?
Viruses are extensively used in research and medicine to both understand basic biology and to improve human health. Imagine having a tool that is able to inject its DNA into a host cell. The possibilities of using such a tool are endless. That tool would be a virus.
Viruses in Research
Viruses are an extremely important tool in the study of molecular and cellular biology. Since viruses infect cells by moving their genetic material into the host cell's nucleus, they are helpful in the investigation of the functions of cells. For example, the use of viruses in research has helped our understanding of the basics of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology.
Viruses and Medicine
Geneticists often use viruses as vectors to introduce genes into cells that they are studying. Aviral vector is a tool commonly used by molecular biologists to place genetic material into cells. To be a useful viral vector, the virus is modified so that it will not cause disease, and it will infect only certain types of cells. Phages are often used as vectors to genetically modify bacteria.
In a similar fashion, viral therapy uses viruses to genetically modify diseased cells and tissues. Viral therapy shows promise as a method of gene therapy and in the treatment of cancer. Gene therapy is the insertion of genes into a person’s cells and tissues to treat a disease. In the case of a genetic disease, the defective gene is replaced with a working gene. Although the technology is still new, it has been used with some success.
Scientists have focused on gene therapy for diseases caused by single-gene defects, such as cystic fibrosis, hemophilia, muscular dystrophy and sickle cell anemia. In gene therapy, the correct version of the gene is introduced to human cells by using a viral vector such as the adenovirus shown in the Figure below.
Phages have been used for over 60 years as an alternative to antibiotics in the former Soviet Union and Eastern Europe. They are seen as a hope against multi-drug-resistant strains of many bacteria because they can infect and kill these “superbugs”. However, in the case of MRSA (Methicillin-resistant Staphylococcus aureus), a phage infecting the bacterium produces a toxin that makes the bacterium more virulent and difficult to contain.
Viruses that infect cancer cells are being studied for their use in cancer treatments. Oncolytic viruses are viruses that lyse and kill cancer cells. Some researchers are hoping to treat some cancers with these viruses.
Gene therapy using an Adenovirus vector. A new gene is inserted into an adenovirus vector, which is used to introduce the modified DNA into a human cell. If the treatment is successful, the new gene will get into the nucleus and into the target cell DNA to make a functional protein.
Summary
• Viruses are useful tools in scientific research and medicine.
• Viruses help us understand molecular biology. They are also used in gene therapy.
Review
1. Why are viruses especially useful tools for understanding molecular biology?
2. What have scientists learned by studying the viruses and their invasion of host cells?
3. What is a viral vector? Are they harmful?
Resources
Can viruses be helpful, or just harmful?
Viruses are extensively used in research and medicine to both understand basic biology and to improve human health. Imagine having a tool that is able to inject its DNA into a host cell. The possibilities of using such a tool are endless. That tool would be a virus.
Viruses in Research
Viruses are an extremely important tool in the study of molecular and cellular biology. Since viruses infect cells by moving their genetic material into the host cell's nucleus, they are helpful in the investigation of the functions of cells. For example, the use of viruses in research has helped our understanding of the basics of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology.
Viruses and Medicine
Geneticists often use viruses as vectors to introduce genes into cells that they are studying. Aviral vector is a tool commonly used by molecular biologists to place genetic material into cells. To be a useful viral vector, the virus is modified so that it will not cause disease, and it will infect only certain types of cells. Phages are often used as vectors to genetically modify bacteria.
In a similar fashion, viral therapy uses viruses to genetically modify diseased cells and tissues. Viral therapy shows promise as a method of gene therapy and in the treatment of cancer. Gene therapy is the insertion of genes into a person’s cells and tissues to treat a disease. In the case of a genetic disease, the defective gene is replaced with a working gene. Although the technology is still new, it has been used with some success.
Scientists have focused on gene therapy for diseases caused by single-gene defects, such as cystic fibrosis, hemophilia, muscular dystrophy and sickle cell anemia. In gene therapy, the correct version of the gene is introduced to human cells by using a viral vector such as the adenovirus shown in the Figure below.
Phages have been used for over 60 years as an alternative to antibiotics in the former Soviet Union and Eastern Europe. They are seen as a hope against multi-drug-resistant strains of many bacteria because they can infect and kill these “superbugs”. However, in the case of MRSA (Methicillin-resistant Staphylococcus aureus), a phage infecting the bacterium produces a toxin that makes the bacterium more virulent and difficult to contain.
Viruses that infect cancer cells are being studied for their use in cancer treatments. Oncolytic viruses are viruses that lyse and kill cancer cells. Some researchers are hoping to treat some cancers with these viruses.
Gene therapy using an Adenovirus vector. A new gene is inserted into an adenovirus vector, which is used to introduce the modified DNA into a human cell. If the treatment is successful, the new gene will get into the nucleus and into the target cell DNA to make a functional protein.
Summary
• Viruses are useful tools in scientific research and medicine.
• Viruses help us understand molecular biology. They are also used in gene therapy.
Review
1. Why are viruses especially useful tools for understanding molecular biology?
2. What have scientists learned by studying the viruses and their invasion of host cells?
3. What is a viral vector? Are they harmful? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.15%3A_Viruses_in_Research_and_Medicine.txt |
Can you guess what organisms are pictured here?
Are they fat green worms on a red leaf? Here’s a clue: There are more organisms like these than any other on Earth. Here’s another clue: Each organism consists of a single cell without a nucleus.
The organisms are bacteria called Salmonella. If the word Salmonella rings a bell, that’s probably because Salmonella causes human diseases such as food poisoning. Many other types of bacteria also cause human diseases. But not all bacteria are harmful to people. In fact, we could not survive without many of the trillions of bacteria that live in or on the human body.
Bacteria and Humans
Bacteria and humans have many important relationships. Bacteria make our lives easier in a number of ways. In fact, we could not survive without them. On the other hand, bacteria can also make us sick.
Benefits of Bacteria
Bacteria provide vital ecosystem services. They are important decomposers. They are also needed for the carbon and nitrogen cycles. There are billions of bacteria inside the human intestines. They help digest food, make vitamins, and play other important roles. Humans also use bacteria in many other ways, including:
• Creating products, such as ethanol and enzymes.
• Making drugs, such as antibiotics and vaccines.
• Making biogas, such as methane.
• Cleaning up oil spills and toxic wastes.
• Killing plant pests.
• Transferring normal genes to human cells in gene therapy.
• Fermenting foods (see Figure below).
Fermented Foods. Fermentation is a type of respiration that doesn’t use oxygen. Fermentation by bacteria is used in brewing and baking. It is also used to make the foods pictured here.
Bacteria and Disease
You have ten times as many bacteria as human cells in your body. Most of these bacteria are harmless. However, bacteria can also cause disease. Examples of bacterial diseases include tetanus, syphilis, and food poisoning. Bacteria may spread directly from one person to another. For example, they can spread through touching, coughing, or sneezing. They may also spread via food, water, or objects.
Another way bacteria and other pathogens can spread is by vectors. A vector is an organism that spreads pathogens from host to host. Insects are the most common vectors of human diseases. Figure below shows two examples.
Bacterial Disease Vectors. Ticks spread bacteria that cause Lyme disease. Deerflies spread bacteria that cause tularemia.
Humans have literally walked into some new bacterial diseases. When people come into contact with wild populations, they may become part of natural cycles of disease transmission. Consider Lyme disease. It’s caused by bacteria that normally infect small, wild mammals, such as mice. A tick bites a mouse and picks up the bacteria. The tick may then bite a human who invades the natural habitat. Through the bite, the bacteria are transmitted to the human host.
Controlling Bacteria
Bacteria in food or water usually can be killed by heating it to a high temperature (generally, at least 74°C, or 165°F). Bacteria on many surfaces can be killed with chlorine bleach or other disinfectants. Bacterial infections in people can be treated with antibiotic drugs. For example, if you ever had “strep” throat, you were probably treated with an antibiotic.
Antibiotics have saved many lives. However, misuse and over-use of the drugs have led to antibiotic resistance in bacteria. Figure below shows how antibiotic resistance evolves. Some strains of bacteria are now resistant to most common antibiotics. These infections are very difficult to treat.
Evolution of Antibiotic Resistance in Bacteria. This diagram shows how antibiotic resistance evolves by natural selection.
Summary
• Bacteria and humans have many important relationships. Bacteria provide humans with a number of services. They also cause human diseases.
Review
1. How can bacteria cause disease?
2. List three benefits of bacteria.
3. How can bacteria be killed? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/07%3A_Prokaryotes_and_Viruses/7.16%3A_Bacteria_and_Humans.txt |
Prokaryote or eukaryote?
This organism consists of a single cell with several flagella. Is it a prokaryote, such as a bacterium? Actually, it’s larger than a prokaryotic cell, and it also has a nucleus. Therefore, this organism belongs to the domain Eukarya, the domain that includes humans. This particular eukaryote is one of the smallest, simplest organisms in the domain, called a protist. It’s scientific name is Giardia lamblia. As a human parasite, it can make us sick.
Figure 1: This scanning electron micrograph revealed some of the external ultrastructural details displayed by a flagellated Giardia lamblia protozoan parasite, which is the organism responsible for causing the diarrheal disease "giardiasis". (Public Domain; US Center for Disease Control).
Kingdom Protista
Protists are a group of all the eukaryotes that are not fungi, animals, or plants. As a result, it is a very diverse group of organisms. The eukaryotes that make up this kingdom, Kingdom Protista, do not have much in common besides a relatively simple organization. Protists can look very different from each other. Some are tiny and unicellular, like an amoeba, and some are large and multicellular, like seaweed. However, multicellular protists do not have highly specialized tissues or organs. This simple cellular-level organization distinguishes protists from other eukaryotes, such as fungi, animals, and plants. There are thought to be between 60,000 and 200,000 protist species, and many have yet to be identified. Protists live in almost any environment that contains liquid water. Many protists, such as the algae, are photosynthetic and are vital primary producers in ecosystems. Other protists are responsible for a range of serious human diseases, such as malaria and sleeping sickness.
The term protista was first used by Ernst Haeckel in 1866. Protists were traditionally placed into one of several groups based on similarities to a plant, animal, or fungus: the animal-likeprotozoa, the plant-like protophyta (mostly algae), and the fungus-like slime molds and water molds. These traditional subdivisions, which were largely based on non-scientific characteristics, have been replaced by classifications based on phylogenetics (evolutionary relatedness among organisms). However, the older terms are still used as informal names to describe the general characteristics of various protists.
Protists range from single-celled amoebas to multicellular seaweed. Protists may be similar to animals, plants, or fungi.
Summary
• Kingdom Protista includes all eukaryotes that are not animals, plants, or fungi.
• Kingdom Protista is very diverse. It consists of both single-celled and multicellular organisms.
Review
1. What are protists?
2. How are unicellular protists and multicellular protists similar?
3. How are protists classified? What are the main categories of protists?
8.02: Protist Evolution
What's the difference between a bacterium and a simple protist?
Were simple protists the first eukaryotic organisms to evolve? Probably. A protist is a eukaryote, so each cell has a nucleus. Otherwise, simple protists, like the Paramecium and amoeba, can be fairly similar to bacteria.
Evolution of Protists
Scientists think that protists are the oldest eukaryotes. If so, they must have evolved from prokaryotic cells. How did this happen? The endosymbiotic theory provides the most widely-accepted explanation. That’s because it is well supported by evidence.
The First Eukaryotic Cells
According to the endosymbiotic theory, the first eukaryotic cells evolved from a symbiotic relationship between two or more prokaryotic cells. Smaller prokaryotic cells were engulfed by (or invaded) larger prokaryotic cells. The small cells (now called endosymbionts) benefited from the relationship by getting a safe home and nutrients. The large cells (now called hosts) benefited by getting some of the organic molecules or energy released by the endosymbionts. Eventually, the endosymbionts evolved into organelles of the host cells. After that, neither could live without the other.
As shown in Figure below, some of the endosymbionts were aerobic bacteria. They were specialized to break down chemicals and release energy. They evolved into the mitochondriaof eukaryotic cells. Some of the small cells were cyanobacteria. They were specialized forphotosynthesis. They evolved into the chloroplasts of eukaryotic cells.
Endosymbiotic theory explains how eukaryotic cells arose.
Evidence for the Endosymbiotic Theory
Many pieces of evidence support the endosymbiotic theory. For example:
• Mitochondria and chloroplasts contain DNA that is different from the DNA found in the cell nucleus. Instead, it is similar to the circular DNA of bacteria.
• Mitochondria and chloroplasts are surrounded by their own plasma membranes, which are similar to bacterial membranes.
• New mitochondria and chloroplasts are produced through a process similar to binary fission. Bacteria also reproduce through binary fission.
• The internal structure and biochemistry of chloroplasts is very similar to that of cyanobacteria.
Summary
• Scientists think that protists are the oldest eukaryotes.
• Protists most likely evolved from prokaryotic cells, as explained by the endosymbiotic theory. This theory is well-supported by evidence.
Review
1. How did the first eukaryotic cells evolve, according to endosymbiotic theory?
2. Identify two pieces of evidence for endosymbiotic theory. Explain how this evidence supports the theory.
3. How are additional mitochondria produced? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.01%3A_Protist_Kingdom.txt |
Sexual or asexual reproduction for protists?
Notice how the Paramecium is dividing into two cells. This, obviously, is a form of asexual reproduction. But, remember that protists are an extremely diverse kingdom, and some protists can also reproduce sexually.
Characteristics of Protists
Like all other eukaryotes, protists have a nucleus containing their DNA. They also have other membrane-bound organelles, such as mitochondria and the endoplasmic reticulum. Most protists are single-celled. Some are multicellular. Because the protist kingdom is so diverse, their ways of getting food and reproducing vary widely.
Protist Habitats
Most protists are aquatic organisms. They need a moist environment to survive. They are found mainly in damp soil, marshes, puddles, lakes, and the ocean. Some protists are free-living organisms. Others are involved in symbiotic relationships. They live in or on other organisms, including humans.
Motility of Protists
Most protists have motility. This is the ability to move. Protists have three types of appendages for movement. As shown in Figure below, they may have flagella, cilia, or pseudopods (“false feet”). There may be one or more whip-like flagella. Cilia are similar to flagella, except they are shorter and there are more of them. They may completely cover the surface of the protist cell. Pseudopods are temporary, foot-like extensions of the cytoplasm.
Protists use cilia, pseudopods, or flagella to move.
Protist Reproduction
Protists have complex life cycles. Many have both asexual and sexual reproduction. An example is a protist called Spirogyra, a type of algae, shown Figure below. It usually exists as haploid cells that reproduce by binary fission. In a stressful environment, such as one that is very dry, Spirogyra may produce tough spores that can withstand harsh conditions. Spores are reproductive cells produced by protists and various other organisms. If two protist spores are close together, they can fuse to form a diploid zygote. This is a type of sexual reproduction. The zygote then undergoes meiosis, producing haploid cells that repeat the cycle.
Spirogyra is a genus of algae with a complex life cycle. Each organism consists of rectangular cells connected end-to-end in long filaments.
Protist Nutrition
Protists get food in one of three ways. They may ingest, absorb, or make their own organic molecules.
• Ingestive protists ingest, or engulf, bacteria and other small particles. They extend their cell wall and cell membrane around the food item, forming a food vacuole. Then enzymesdigest the food in the vacuole.
• Absorptive protists absorb food molecules across their cell membranes. This occurs bydiffusion. These protists are important decomposers.
• Photosynthetic protists use light energy to make food. They are major producers in aquaticecosystems.
Summary
• Protists have nuclear membranes around their DNA. They also have other membrane-bound organelles.
• Many protists live in aquatic habitats, and most are motile, or able to move.
• Protists have complex life cycles that may include both sexual and asexual reproduction.
• Protists get food through ingestion, absorption, or photosynthesis.
Review
1. Identify three structures that protists use to move.
2. Describe three ways that protists get food.
3. Describe asexual and sexual reproduction in protists.
8.04: Protozoa
What's like an animal, but not an animal?
An animal-like protist, or a protozoa. These protists have the ability to move, usually with some sort of cilia or flagella, and must obtain their energy from other sources. But obviously, they are much simpler than animals.
Animal-Like Protists: Protozoa
Animal-like protists are commonly called protozoa (singular, protozoan). Most protozoa consist of a single cell. They are animal-like because they are heterotrophs, and are capable of moving. Although protozoa are not animals, they are thought to be the ancestors of animals.
Ecology of Protozoa
Protozoa generally feed by engulfing and digesting other organisms. As consumers, they have various roles in food chains and webs. Some are predators. They prey upon other single-celled organisms, such as bacteria. In fact, protozoa predators keep many bacterial populations under control. Other protozoa are herbivores. They graze on algae. Still others are decomposers. They consume dead organic matter. There are also parasitic protozoa that live in or on living hosts. For example, the protozoan that causes malaria lives inside a human host. Protozoa are also important food sources for many larger organisms, including insectsand worms.
Classification of Protozoa
Protozoa can be classified on the basis of how they move. As shown in Table below, protozoa move in three different ways. Only sporozoa cannot move. Note that this classification is based only on differences in movement. It does not represent phylogenetic relationships.
Type of Protozoa How It Moves Example (Genus)
Amoeboid pseudopods
Amoeba
Ciliate cilia
Paramecium
Flagellate flagella
Giardia
Sporozoan does not move (as adult)
Plasmodium
Summary
• Animal-like protists are called protozoa. Most consist a single cell.
• Like animals, protozoa are heterotrophic and capable of moving.
• Examples of protozoa include amoebas and paramecia.
Review
1. How are protozoa similar to animals?
2. What roles do protozoa play in food chains and webs?
3. What type of protozoa is a Paramecium?
4. What type of protozoa is a Giardia? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.03%3A_Protist_Characteristics.txt |
Many people, if not most, believe seaweed to be a plant. Is it?
Seaweed is actually a plant-like protist, which are also known as algae. The green color is due to what pigment? Algae, like plants, obtain their energy through photosynthesis.
Plant-Like Protists: Algae
Plant-like protists are called algae (singular, alga). They are a large and diverse group. Some algae, the diatoms, are single-celled. Others, such as seaweed, are multicellular (see Figure below).
Diatoms are single-celled algae. Other forms of algae are multicellular.
Why are algae considered plant-like? The main reason is that they contain chloroplasts and produce food through photosynthesis. However, they lack many other structures of true plants. For example, algae do not have roots, stems, or leaves. Some algae also differ from plants in being motile. They may move with pseudopods or flagella. Although not plants themselves, algae were probably the ancestors of plants.
Ecology of Algae
Algae play significant roles as producers in aquatic ecosystems. Microscopic forms live suspended in the water column. They are the main component of phytoplankton. As such, they contribute to the food base of most marine ecosystems.
Multicellular seaweeds called kelp may grow as large as trees. They are the food base of ecosystems called kelp forests (see Figure below). Kelp forests are found throughout the ocean in temperate and arctic climates. They are highly productive ecosystems.
Kelp Forest. This kelp forest supports a large community of many other types of organisms.
Classification of Algae
Types of algae include red and green algae, and euglenids, and dinoflagellates (see Table below for examples). Scientists think that red and green algae evolved from endosymbiotic relationships with cyanobacteria. Their chloroplasts have two membranes because the cell membranes of the cyanobacteria became additional plasma membranes of the chloroplasts. Scientists think that euglenids and dinoflagellates evolved later, from endosymbiotic relationships with green and red algae. This is why their chloroplasts have three membranes. Differences in the types of chlorophyll in the four types of algae also support the hypothesized evolutionary relationships.
Type of Algae Origin of Chloroplast Type of Chloroplast
Red algae
cyanobacteria two membranes, chlorophyll like the majority of cyanobacteria
Green algae
cyanobacteria two membranes, chlorophyll like a minority of cyanobacteria
Euglenids
green algae three membranes, chlorophyll like green algae
Dinoflagellates
red algae three membranes, chlorophyll like red algae
Reproduction of Algae
Algae have varied life cycles. Two examples are shown in Figure below. Both cycles include phases of asexual reproduction (haploid, n) and sexual reproduction (diploid, 2n). Why go to so much trouble to reproduce? Asexual reproduction is fast, but it doesn’t create new genetic variation. Sexual reproduction is more complicated and risky, but it creates new gene combinations. Each strategy may work better under different conditions. Rapid population growth (asexual reproduction) is adaptive when conditions are favorable. Genetic variation (sexual reproduction) helps ensure that some organisms will survive if the environment changes.
Life Cycles of Algae: Zygotic Meiosis (A), Gametic Meiosis (B) and Sporic Meiosis (C). In life cycle A (left), diploid (2n) zygotes result from fertilization and then undergo meiosis to produce haploid (n) gametes. The gametes undergo mitosis and produce many additional copies of themselves. How are life cycles B and C different from life cycle A?
KQED: Algae Power
QUEST explores the potential of algae-–once considered nothing more than pond scum–-to become the fuel of the future. Entrepreneurs from throughout California are working to create the next generation of biofuels from algae. But will you ever be able to run your car off it?
Summary
• Plant-like protists are called algae. They include single-celled diatoms and multicellular seaweed.
• Like plants, algae contain chlorophyll and make food by photosynthesis.
• Types of algae include red and green algae, euglenids, and dinoflagellates.
Review
1. Compare and contrast algae and plants.
2. State pros and cons of asexual and sexual reproduction in algae.
3. Explain why dinoflagellates and euglenids have chloroplasts with three membranes instead of two. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.05%3A_Algae.txt |
What grows just about anywhere there is decaying material?
Mold. This slime mold, shown growing on dead wood, is a fungus-like protist. Though this mold does not have a mouth, essentially it is still “eating” this decaying material.
Fungus-Like Protists: Molds
Fungus-like protists are molds. They are absorptive feeders on decaying organic matter. They resemble fungi, and they reproduce with spores as fungi do. However, in other ways, they are quite different from fungi and more like other protists. For example, they have cell walls made of cellulose, whereas fungi have cell walls made of chitin. Like other protists, they have complicated life cycles with both asexual and sexual reproduction. They are motile during some stages of their life cycle. Two major types of fungus-like protists are slime molds and water molds.
Slime Molds
Slime molds are fungus-like protists commonly found on rotting logs and compost. They move very slowly in search of decaying matter to eat. When food is scarce, individual cells swarm together to form a blob-like mass, like the “dog vomit” slime mold in the Figure below. The mass glides along on its own secretions, engulfing decaying organic matter as it moves over it.
“Dog Vomit” Slime Mold. This slime mold looks like its name.
There are two types of slime molds when it comes to how they swarm: acellular and cellular.
• When acellular slime molds swarm, they fuse together to form a single cell with many nuclei.
• When cellular slime molds swarm, they remain as distinct cells.
Cellular slime molds are used as model organisms in molecular biology and genetics. They may be the key to how multicellular organisms evolved. Can you explain why?
Water Molds
Water molds are commonly found in moist soil and surface water. Many are plant pathogens that destroy crops. They infect plants such as grapes, lettuce, corn, and potatoes. Some water molds are parasites of fish and other aquatic organisms.
Summary
• Fungus-like protists are molds.
• Molds are absorptive feeders, found on decaying organic matter. They resemble fungi and reproduce with spores as fungi do.
• Examples of fungus-like protists include slime molds and water molds.
Review
1. How are fungus-like protists similar to fungi? What is one way they are different?
2. Why might cellular slime molds, but not acellular slime molds, be the key to how multicellular organisms evolved?
8.07: Protists and Human Disease
Can such little creatures make you sick?
They sure can. Not all of them, but some of them. And without proper medical treatment, the person may never recover.
Protists and Human Disease
Most protist diseases in humans are caused by animal-like protists, or protozoa. Protozoa make us sick when they become human parasites. Three examples of parasitic protozoa are described below.
Trypanosoma Protozoa
Members of the genus Trypanosoma are flagellate protozoa that cause sleeping sickness, which is common in Africa. They also cause Chagas disease, which is common in South America. The parasites are spread by insect vectors. The vector for Chagas disease is shown in Figure below. Trypanosoma parasites enter a person’s blood when the vector bites. Then they spread to other tissues and organs. The diseases may be fatal without medical treatment.
Vector for Chagas Disease. In Chagas disease, the Trypanosoma parasite is spread by an insect commonly called the “kissing bug.” A bite from this bug could be the kiss of death.
The discovery of Chagas disease is unique in the history of medicine. That’s because a single researcher—a Brazilian physician named Carlos Chagas—single-handedly identified and explained the new infectious disease. In the early 1900s, Chagas did careful lab and field studies. He determined the pathogen, vector, host, symptoms, and mode of transmission of the disease that is now named for him.
Giardia Protozoa
Giardia are flagellate protozoa that cause giardiasis. The parasites enter the body through food or water that has been contaminated by feces of infected people or animals. The protozoa attach to the lining of the host’s small intestine, where they prevent the host from fully absorbing nutrients. They may also cause diarrhea, abdominal pain, and fever. A picture of a Giardia protozoan opens this concept.
Plasmodium Protozoa
Plasmodium protozoa cause malaria. The parasites are spread by a mosquito vector. Parasites enter a host’s blood through the bite of an infected mosquito. The parasites infect the host’s red blood cells, causing symptoms such as fever, joint pain, anemia, and fatigue.
Malaria is common in tropical and subtropical climates throughout the world (see Figure below). In fact, malaria is one of the most common infectious diseases on the planet. Malaria is also a very serious disease. It kills several million people each year, most of them children. A vaccine to malaria is a possibility.
Worldwide Distribution of Malaria. This map shows where malaria is found. The area is determined by the mosquito vector. The mosquito can live year-round only in the red-shaded areas.
Summary
• Most protist diseases in humans are caused by protozoa. Protozoa make humans sick when they become human parasites.
• Trypanosoma protozoa cause Chagas disease and sleeping sickness.
• Giardia protozoa cause giardiasis, and Plasmodium protozoa cause malaria.
Review
1. Describe how the protozoa that cause Chagas disease are spread to human hosts.
2. State why malaria is commonly found only in tropical and subtropical regions of the world.
3. Terri lost her water bottle while hiking in Canada. It was a hot day, so she drank water from a stream to stay hydrated. A few days later, Terri became ill with abdominal pain, fever, and diarrhea. Her doctor thinks she has a protozoan infection. Which type of protozoa do you think is most likely responsible for Terri’s illness? How do you think Terri became infected? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.06%3A_Molds.txt |
What exactly is a fungus?
They're not animals or plants, and definitely not protists. So they cannot photosynthesize or eat. And they are much more than mushrooms.
Characteristics of Fungi
Do you see the organisms growing on the bread in the Figure below? They belong to the Kingdom Fungi. Molds growing on foods are some of the most common fungi in our everyday lives. These organisms may seem useless, gross, and costly. But fungi play very important roles in almost every terrestrial ecosystem on Earth.
The mold growing on this bread is a common fungus.
Fungi (singular, fungus) are a kingdom in the domain Eukarya. The fungi kingdom may contain more than a million species, but fewer than 100,000 have been identified. As shown in the Figure below, fungi include mushrooms and yeasts in addition to molds.
Several examples of fungi are pictured here.
Most fungi are multicellular, but some exist as single cells. Single-celled fungi are known as yeasts. Fungi spend most of their life cycle in the haploid state. They form diploid cells only during sexual reproduction. Like the cells of protists and plants, the cells of fungi have cell walls. But fungi are unique in having cell walls made of chitin instead of cellulose. Chitin is a tough carbohydrate that also makes up the exoskeleton (outer skeleton) of insects and related organisms.
Habitats of Fungi
Fungi are found all around the world, and grow in a wide range of habitats, including deserts. Most grow in terrestrial environments, but several species live only in aquatic habitats. Most fungi live in soil or dead matter, and in symbiotic relationships with plants, animals, or other fungi. Fungi, along with bacteria that are found in soil, are the primary decomposers of organic matter in terrestrial ecosystems. The decomposition of dead organisms returns nutrient to the soil, and the environment.
Summary
• Fungi are a kingdom in the domain Eukarya that includes molds, mushrooms, and yeasts.
• Most fungi are multicellular. They are unique in having cell walls made of chitin.
• Most fungi live on dead matter or soil. Some live in aquatic habitats. Many are involved in symbiotic relationships.
Review
1. What are fungi? Give two examples of fungi.
2. Explain the significance of the chitin cell wall of fungi.
3. List two habitats where fungi live.
8.09: Fungi Structure
Is the structure important?
Of course. Though mushrooms may be the most common type of fungus, fungi also include rusts, smuts, puffballs, truffles, morels, molds, and yeasts, as well as many less well-known organisms. And, except for yeast cells, they all have similar structures, which are usually hidden deep within their food source.
Structure of Fungi
Except for yeasts, which grow as single cells, most fungi grow as thread-like filaments, like those shown in Figure below. The filaments are called hyphae (singular, hypha). Each hypha consists of one or more cells surrounded by a tubular cell wall. A mass of hyphae make up the body of a fungus, which is called a mycelium (plural, mycelia).
The hyphae of most fungi are divided into cells by internal walls called septa (singular, septum). Septa usually have little pores that are large enough to allow ribosomes, mitochondria and sometimes nuclei to flow among cells. Hyphae that are divided into cells are called septate hyphae. However, the hyphae of some fungi are not separated by septa. Hyphae without septae are called coenocytic hyphae. Coenocytic hyphae are big, multinucleated cells.
These branches are hyphae, or filaments, of a mold called Penicillium.
A mycelium may range in size from microscopic to very large. In fact, one of the largest living organisms on Earth is the mycelium of a single fungus. A small part of a similar fungus is pictured in Figure below. The giant fungus covers 8.9 square kilometers (3.4 square miles) in an Oregon forest. That’s about the size of a small city. The fungus didn’t grow that large overnight. It’s estimated to be 2,400 years old, and it’s still growing!
The fungus shown here has been dubbed the “humongous fungus” because it covers such a large area.
Fruiting Bodies
Some fungi become noticeable only when producing spores (fruiting), either as mushrooms or molds. For example, you can see the fruiting bodies of the Armillaria fungus in the Figureabove, but the large “body” of the fungus, the mycelium, is hidden underground. This fruiting body, known as the sporocarp, is a multicellular structure on which spore-producing structures form. The fruiting body is part of the sexual phase of a fungal life cycle. The rest of the life cycle is characterized by the growth of mycelia.
Dimorphic Fungi
Some fungi take on different shapes, depending on their environmental conditions. These fungi are called dimorphic fungi, because they have “two forms.” For example, the fungusHistoplasma capsulatum, which causes the disease histoplasmosis, is thermally dimorphic; it has two forms that are dependent on temperature. In temperatures of about 25°C, it grows as a brownish mycelium, and looks like a mass of threads. At body temperature (37°C in humans), it grows as single, round yeast cells.
Summary
• Most fungi grow as thread-like filaments called hyphae.
• A mass of hyphae make up the body of a fungus, called a mycelium.
Review
1. Describe the general structure of multicellular fungi.
2. What is a fruiting body?
3. Relate the structures of hyphae, mycelia, and fruiting bodies to one another. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.08%3A_Fungi.txt |
So what do fungi "eat"?
Just about anything. From dead plants to rotting fruit. Shown here are fungi sprouting from dead material in the woods. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange.
Nutrition
Fungi get their nutrition by absorbing organic compounds from the environment. Fungi areheterotrophic: they rely solely on carbon obtained from other organisms for their metabolism and nutrition. Fungi have evolved in a way that allows many of them to use a large variety of organic substrates for growth, including simple compounds such as nitrate, ammonia, acetate, or ethanol. Their mode of nutrition defines the role of fungi in their environment.
Fungi obtain nutrients in three different ways:
1. They decompose dead organic matter. A saprotroph is an organism that obtains its nutrients from non-living organic matter, usually dead and decaying plant or animal matter, by absorbing soluble organic compounds. Saprotrophic fungi play very important roles as recyclers in ecosystem energy flow and biogeochemical cycles. Saprophytic fungi, such as shiitake (Lentinula edodes) and oyster mushrooms (Pleurotus ostreatus), decompose dead plant and animal tissue by releasing enzymes from hyphal tips. In this way they recycle organic materials back into the surrounding environment. Because of these abilities, fungi are the primary decomposers in forests (see Figure below).
2. They feed on living hosts. As parasites, fungi live in or on other organisms and get their nutrients from their host. Parasitic fungi use enzymes to break down living tissue, which may causes illness in the host. Disease-causing fungi are parasitic. Recall that parasitism is a type of symbiotic relationship between organisms of different species in which one, the parasite, benefits from a close association with the other, the host, which is harmed.
3. They live mutualistically with other organisms. Mutualistic fungi live harmlessly with other living organisms. Recall that mutualism is an interaction between individuals of two different species, in which both individuals benefit.
Both parasitism and mutualism are classified as symbiotic relationships, but they are discussed separately here because of the different effect on the host.
Forest Decomposers. These forest mushrooms may look fragile, but they do a powerful job. They decompose dead wood and other tough plant material.
Fungal hyphae are adapted to efficient absorption of nutrients from their environments, because hyphae have high surface area-to-volume ratios. These adaptations are also complemented by the release of hydrolytic enzymes that break down large organic molecules such as polysaccharides, proteins, and lipids into smaller molecules. These molecules are then absorbed as nutrients into the fungal cells. One enzyme that is secreted by fungi is cellulase, which breaks down the polysaccharide cellulose. Cellulose is a major component of plant cell walls. In some cases, fungi have developed specialized structures for nutrient uptake from living hosts, which penetrate into the host cells for nutrient uptake by the fungus.
Fungal mycelia. Fungi absorb nutrients from the environment through mycelia. The branching mycelia have a high surface-area-to-volume ratio which allows for efficient absorption of nutrients. Some fungi digest nutrients by releasing enzymes into the environment.
Mycorrhiza
A mycorrhiza (Greek for "fungus roots") is a symbiotic association between a fungus and the roots of a plant. In a mycorrhizal association, the fungus may colonize the roots of a host plant by either growing directly into the root cells, or by growing around the root cells. This association provides the fungus with relatively constant and direct access to glucose, which the plant produces by photosynthesis. The mycelia of the fungi increase the surface area of the plant’s root system. The larger surface area improves water and mineral nutrient absorption from the soil.
Summary
• Fungi are heterotrophic. They get their nutrition by absorbing organic compounds from the environment.
• Fungi, along with bacteria that are found in soil, are the primary decomposers of organic matter in terrestrial ecosystems.
Review
1. Describe how fungi obtain nutrients.
2. Explain the role of saprotrophic fungi? Give an example of this role.
3. What is a mycorrhiza? What are the advantages of a mycorrhiza? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.10%3A_How_Fungi_Eat.txt |
How do fungi reproduce? Sexually or asexually?
How about both? That would suggest that fungi can produce both diploid and haploid cells, which they can. Shown above are fungi mycelia and haploid spores. Spores allow fungi to reproduce through unfavorable conditions.
Reproduction of Fungi
The majority of fungi can reproduce both asexually and sexually. This allows them to adjust to conditions in the environment. They can spread quickly through asexual reproduction when conditions are stable. They can increase their genetic variation through sexual reproduction when conditions are changing and variation may help them survive.
Asexual Reproduction
Almost all fungi reproduce asexually by producing spores. A fungal spore is a haploid cell produced by mitosis from a haploid parent cell. It is genetically identical to the parent cell. Fungal spores can develop into new haploid individuals without being fertilized.
Spores may be dispersed by moving water, wind, or other organisms. Some fungi even have “cannons” that “shoot” the spores far from the parent organism. This helps to ensure that the offspring will not have to compete with the parent for space or other resources. You are probably familiar with puffballs, like the one in Figure below. They release a cloud of spores when knocked or stepped on. Wherever the spores happen to land, they do not germinate until conditions are favorable for growth. Then they develop into new hyphae.
Puffballs release spores when disturbed.
Yeasts do not produce spores. Instead, they reproduce asexually by budding. Budding is the pinching off of an offspring from the parent cell. The offspring cell is genetically identical to the parent. Budding in yeast is pictured in Figure below.
Yeast reproduce asexually by budding.
Sexual Reproduction
Sexual reproduction also occurs in virtually all fungi. This involves mating between two haploid hyphae. During mating, two haploid parent cells fuse, forming a diploid spore called a zygospore. The zygospore is genetically different from the parents. After the zygospore germinates, it can undergo meiosis, forming haploid cells that develop into new hyphae.
Summary
• The majority of fungi can reproduce both asexually and sexually. This allows them to adjust to conditions in the environment.
• Yeast reproduce asexually by budding. Other fungi reproduce asexually by producing spores.
• Sexual reproduction occurs when spores from two parents fuse and form a zygospore.
Review
1. Explain the advantages of fungal spores.
2. Identify ways that fungal spores may be dispersed.
3. Compare and contrast a fungal spore and zygospore.
8.12: Fungi Evolution
What were early fungi like?
Early fungi probably lived in water. And they were most likely single-celled organisms. Maybe they lived on dead and decaying material. Obviously, at least in overall size and structure, a mold is very different than a mushroom. Could early fungi have been similar to a mold?
Evolution of Fungi
DNA evidence suggests that almost all fungi have a single common ancestor. The earliest fungi may have evolved about 600 million years ago or even earlier. They were probablyaquatic organisms with a flagellum. Fungi first colonized the land at least 460 million years ago, around the same time as plants. Fossils of terrestrial fungi date back almost 400 million years (see Figure below). Starting about 250 million years ago, the fossil record shows fungi were abundant in many places. They may have been the dominant life forms on Earth at that time.
This rock contains fossilized fungi. The fungi lived 396 million years ago in what is now Scotland. They were preserved when they were covered with lava from a volcano. The lava cooled and hardened into rock.
Summary
• Almost all fungi have a single common ancestor.
• The earliest fungi may have evolved about 600 million years ago.
• Fungi colonized land at least 460 million years ago.
• By 250 million years ago, they may have been the dominant life forms on Earth.
Review
1. Summarize the evolution of fungi.
8.13: Fungi Classification
What type of fungus is this?
Obviously a mold. But what type of mold? There are thousands of known species of molds. How are they classified?
Classification of Fungi
For a long time, scientists considered fungi to be members of the plant kingdom because they have obvious similarities with plants. Both fungi and plants are immobile, have cell walls, and grow in soil. Some fungi, such as lichens, even look like plants (see Figure below).
Moss (Plant) and Lichen Growing on Tree Bark. Both fungi and moss are growing on this tree. Can you tell them apart?
The Kingdom Fungi
Today, fungi are no longer classified as plants. We now know that they have unique physical, chemical, and genetic traits that set them apart from plants and other eukaryotes. For example, the cell walls of fungi are made of chitin, not cellulose. Also, fungi absorb nutrients from other organisms, whereas plants make their own food. These are just a few of the reasons fungi are now placed in their own kingdom.
Fungal Phyla
Classification of fungi below the level of the kingdom is controversial. There is no single, widely-accepted system of fungal classification. Most classifications include several phyla (the next major taxon below the kingdom). Three of the most common phyla are compared inTable below.
Phylum Description Example
Zygomycota mainly terrestrial, live in soil and compost and on foods such as bread
black bread mold
Basidiomycota have many different shapes, considerable variation exists even within species
button mushrooms
Ascomycota found in all terrestrial ecosystems world-wide, even in Antarctica, often involved in symbiotic relationships
baker’s yeast
Summary
• Fungi used to be classified as plants. Now, they are known to have unique traits that set them apart from plants. For example, fungal cell walls contain chitin, not cellulose, and fungi absorb food rather than make their own.
• Below the level of the kingdom, classification of fungi is controversial.
Review
1. State why fungi were once classified as plants.
2. Explain the significance of the chitin cell wall of fungi.
3. Mushrooms belong to what phylum of fungi? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.11%3A_Fungi_Reproduction.txt |
Do all fungi feed only on dead organisms?
Not all. This fungus is a lichen, providing nutrients to the tree. The lichen gets sugars from the plant. Both benefit from this relationship.
Symbiotic Relationships of Fungi
Not all fungi feed on dead organisms. Many are involved in symbiotic relationships, including parasitism and mutualism.
Fungi as Parasites
In a parasitic relationship, the parasite benefits while the host is harmed. Parasitic fungi live in or on other organisms and get their nutrients from them. Fungi have special structures for penetrating a host. They also produce enzymes that break down the host’s tissues.
Parasitic fungi often cause illness and may eventually kill their host. They are the major cause of disease in agricultural plants. Fungi also parasitize animals, such as the insect pictured in Figure below. Fungi even parasitize humans. Did you ever have athelete’s foot? If so, you were the host of a parasitic fungus.
Parasitic Fungus and Insect Host. The white parasitic fungus named Cordyceps is shown here growing on its host—a dark brown moth.
Mutualism in Fungi
Fungi have several mutualistic relationships with other organisms. In mutualism, both organisms benefit from the relationship. Two common mutualistic relationships involving fungi are mycorrhiza and lichen.
• A mycorrhiza is a mutualistic relationship between a fungus and a plant. The fungus grows in or on the plant roots. The fungus benefits from the easy access to food made by the plant. The plant benefits because the fungus puts out mycelia that help absorb water and nutrients. Scientists think that a symbiotic relationship such as this may have allowed plants to first colonize the land.
• A lichen is an organism that results from a mutualistic relationship between a fungus and a photosynthetic organism. The other organism is usually a cyanobacterium or green alga. The fungus grows around the bacterial or algal cells. The fungus benefits from the constant supply of food produced by the photosynthesizer. The photosynthesizer benefits from the water and nutrients absorbed by the fungus. Figure below shows lichen growing on a rock.
Lichen Growing on Rock. Unlike plants, lichen can grow on bare rocks because they don’t have roots. That’s why lichens are often pioneer species in primary ecological succession. How do lichen get water and nutrients without roots?
Some fungi have mutualistic relationships with insects. For example:
• Leafcutter ants grow fungi on beds of leaves in their nests. The fungi get a protected place to live. The ants feed the fungi to their larvae.
• Ambrosia beetles bore holes in tree bark and “plant” fungal spores in the holes. The holes in the bark give the fungi an ideal place to grow. The beetles harvest fungi from their “garden.”
Summary
• Many fungi are involved in symbiotic relationships.
• Some fungi are parasites. They are specialized to penetrate a host and break down the host’s tissues. Parasitic fungi often cause illness and may eventually kill their host.
• Two common mutualistic relationships involving fungi are mycorrhiza (fungi and plant roots) and lichen (fungi and either cyanobacteria or green algae).
• Some fungi also have mutualistic relationships with insects.
Review
1. How significant are fungi as plant parasites?
2. Describe an example of a mutualistic relationship between fungi and insects.
3. Assume that you notice a fungus growing on a plant. What possible relationships might exist between the fungus and the plant? What type of evidence might help you identify which is the correct relationship?
4. Compare and contrast mycorrhiza and lichen. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.14%3A_Symbiotic_Relationships_of_Fungi.txt |
What's growing on this lemon?
Would you believe penicillin? Penicillin is a mold, which of course is a fungus, one that has helped millions, if not billions, of people.
Human Uses of Fungi
Whenever you eat pizza, you eat fungi, even if you don’t like your pizza with mushrooms. That’s because pizza dough contains yeast. Do you know other foods that are made with fungi?
Fungi for Food
Humans have collected and grown mushrooms for food for thousands of years. Figure belowshows some of the many types of mushrooms that people eat. Yeasts are used in bread baking and brewing alcoholic beverages. Other fungi are used in fermenting a wide variety of foods, including soy sauce, tempeh, and cheeses. Blue cheese has its distinctive appearance and flavor because of the fungus growing though it (see Figure below).
These are just a few of the many species of edible mushrooms consumed by humans.
Blue Cheese. The dark blue strands running through this cheese are a fungus. In fact, this cheese is moldy! The fungus isPenicillium roqueforti, a type of mold.
Fungi for Pest Control
Harmless fungi can be used to control pathogenic bacteria and insect pests on crops. Fungi compete with bacteria for nutrients and space, and they parasitize insects that eat plants. Fungi reduce the need for pesticides and other toxic chemicals.
Other Uses of Fungi
Fungi are useful for many other reasons.
• They are a major source of citric acid (vitamin C).
• They produce antibiotics such as penicillin, which has saved countless lives.
• They can be genetically engineered to produce insulin and other human hormones.
• They are model research organisms.
Summary
• Humans use fungi for many purposes, including as food or in the preparation of food.
• Humans also use fungi for pest control.
• In addition, fungi can be used to produce citric acid, antibiotics, and human hormones.
• Fungi are model research organisms as well.
Review
1. Describe two ways that humans use fungi.
2. How are fungi used to control pests?
8.16: Fungi and Human Disease
Would you eat these mushrooms?
I would not recommend it. But certain red mushrooms, Ganoderma Lucidum, have been found to be good for you. Red Mushrooms comprise a family of more than 200 mushroom species, which are good for our health. Of these, 6 species have a particularly high therapeutic effect.
Fungi and Human Disease
Fungi cause human illness in three different ways: poisonings, parasitic infections, and allergic reactions. Science on the SPOT: Fungus Fair explores some of these dangerous but also tasty and weirdly wonderful fungi.
Fungal Poisoning
Many fungi protect themselves from parasites and predators by producing toxic chemicals. If people eat toxic fungi, they may experience digestive problems, hallucinations, organ failure, and even death. Most cases of mushroom poisoning are due to mistaken identity. That’s because many toxic mushrooms look very similar to safe, edible mushrooms. An example is shown in Figure below.
Poisonous or Edible? The destroying angel mushroom on the left causes liver and kidney failure. The puffball mushroom on the right is tasty and harmless. Do you think you could tell these two species of mushrooms apart?
Fungal Parasites
Some fungi cause disease when they become human parasites. Two examples are fungi in the genera Candida and Trichophyton.
• Candida are yeast that cause candidiasis, commonly called a “yeast infection.” The yeast can infect the mouth or the vagina. If yeast enter the blood, they cause a potentially life threatening illness. However, this is rare, except in people with a depressed immune system.
• Trichophyton are fungi that cause ringworm. This is a skin infection characterized by a ring-shaped rash. The rash may occur on the arms, legs, head, neck, or trunk. The same fungi cause athlete’s foot when they infect the skin between the toes. Athlete’s foot is the second most common skin disease in the U.S.
Figure below shows signs of these two infections.
Ringworm produces a ring-shaped rash, but it isn’t caused by a worm. It’s caused by the same fungus that causes athlete’s foot.
Fungal Allergies
Mold allergies are very common. They are caused by airborne mold spores. When the spores enter the respiratory tract, the immune system responds to them as though they were harmful microbes. Symptoms may include sneezing, coughing, and difficulty breathing. The symptoms are likely to be more severe in people with asthma or other respiratory diseases. Long-term exposure to mold spores may also weaken the immune system.
Molds grow indoors as well as out. Indoors, they grow in showers, basements, and other damp places. Homes damaged in floods and hurricanes may have mold growing just about everywhere (see Figure below). Indoor mold may cause more health problems than outdoor mold because of the closed, confined space. Most people also spend more time indoors than out.
The mold growing on the walls and ceiling of this storm-damaged home may be harmful to human health.
Summary
• Fungi cause three different types of human illness: poisonings, parasitic infections, and allergies.
• Many poisonous mushrooms are eaten by mistake because they look like edible mushrooms.
• Parasitic yeasts cause candidiasis, ringworm, and athlete’s foot.
• Mold allergies are very common.
Review
1. Explain why you should never eat mushrooms you find in the woods unless you know for certain which type of mushrooms they are.
2. Compare and contrast ringworm and athlete’s foot.
3. How does mold cause allergies?
4. State why indoor mold may cause more health problems than outdoor mold. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/08%3A_Protists_and_Fungi/8.15%3A_Human_Uses_of_Fungi.txt |
What are plants?
Autumn. A time when leaves turn amazing colors. Of course, leaves are part of plants. But what are plants? What separates a plant from a fungus or protist? Or animal?
Plants
Imagine that human life cycles resembled those of the earliest plants. If you think about this analogy, you may begin to realize that many plants, which appear so inert to our roving eyes and active minds, actually lead secret lives of surprising variety.
You know that humans develop, or gradually change, from infants to quite different, sexually mature adults. You also know that meiosis in your own ovaries or testes produces haploid eggs or sperm, which must join in fertilization to become a new individual. Each of us, of course, began as that single cell made when a sperm united with an egg. Now, through mitosis and the miracle of development, we are made of trillions of cells organized into tissues, organs, and organ systems, which make us complex, amazing, active, individual beings. None of us would doubt that we have changed significantly since we began as single cells. Each of us has a unique identity that we keep throughout our entire lives, until death marks our end. We may give birth to other individuals by producing eggs or sperm, but only if they join with other sperm or eggs to produce new, separate lives.
If, however, we lived like some plants, your father would not have produced the sperm cell destined to provide half of your genes, although there would be such a sperm cell. Your mother would not have produced the egg cell destined to produce the other half. In fact, your parents, and you, would not be distinguishable as male or female. Instead, both parents (or maybe just one parent) would have released thousands of haploid spore cells, each of which would grow, by mitosis, into a new individual being, entirely different in form and habitat from its parents - and you. Small spores would become males, and large spores females, but as if sperm and egg had decided to postpone their “marriage” and grow up on their own, these beings would live very different, “non-human” lives.
Who are these beings? You are certainly not one of them, because you begin only when egg meets sperm. Their differences from you would be far greater than the differences between tadpole and frog, or caterpillar and butterfly, because every individual butterfly or frog could (theoretically) identify exactly which individual caterpillar or tadpole it used to be. Not so with these haploid creatures.
At some time during their relatively long lives, the male and female beings would produce sperm cells and egg cells by mitosis. Fertilization would not involve mating, of course. Depending on which kind of plant we chose as our model, sperm might swim on their own (with two or more flagella) from male to female being, or they might be blown by the wind, or carried by an animal. After sperm and egg join, you would begin your life as a single cell, and grow into an “adult,” eventually producing your own haploid spores. But you would never be able to identify your parents – if indeed you had two – nor would you know your children, because entire haploid lives would separate you. Why do plants lead such complex, multiple lives?
Most of the plants you are probably familiar with produce flowers. However, plants existed for hundreds of millions of years before they evolved flowers. In fact, the earliest plants were different from most modern plants in several important ways. They not only lacked flowers, but also lacked leaves, roots, and stems. You might not even recognize them as plants. So why are the earliest plants placed in the plant kingdom? What traits define a plant?
What are Plants?
Plants are multicellular eukaryotic organisms with cell walls made of cellulose. Plant cells also have chloroplasts. In addition, plants have specialized reproductive organs. These are structures that produce reproductive cells. Male reproductive organs produce sperm, and female reproductive organs produce eggs. Male and female reproductive organs may be on the same or different plants.
How Do Plants Obtain Food?
Almost all plants make food by photosynthesis. Only about 1 percent of the estimated 300,000 species of plants have lost the ability to photosynthesize. These other species are consumers, many of them predators. How do plants prey on other organisms? The Venus fly trap in Figure below shows one way this occurs.
Venus fly trap plants use their flowers to trap insects. The flowers secrete enzymes that digest the insects, and then they absorb the resulting nutrient molecules.
What Do Plants Need?
Plants need temperatures above freezing while they are actively growing and photosynthesizing. They also need sunlight, carbon dioxide, and water for photosynthesis. Like most other organisms, plants need oxygen for cellular respiration and minerals to buildproteins and other organic molecules. Most plants support themselves above the ground with stiff stems in order to get light, carbon dioxide, and oxygen. Most plants also grow roots down into the soil to absorb water and minerals. And, of course, we need the energy stored in plants through photosynthesis to survive. Life as we know it would not be possible without plants.
Summary
• Plants are multicellular eukaryotes. They have organelles called chloroplasts and cell walls made of cellulose.
• Plants also have specialized reproductive organs.
• Almost all plants make food by photosynthesis.
• Life as we know it would not be possible without plants.
Review
1. What traits do all plants share?
2. How do almost all plants obtain food?
3. What do plants need? Why do they need these items? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.01%3A_Plant_Characteristics.txt |
What is so special about this particular plant?
Look at this plant. You could say it has some interesting fruit. Some might say the fruit does not even look that tasty. However, this is a cacao tree, and its seeds are the source of chocolate. So, there are some people who would argue that this is one of the most important plants in the whole plant kingdom.
The Importance of Plants
The importance of plants to humans and just about all other life on Earth is staggering. Life as we know it would not be possible without plants. Why are plants so important?
• Plants supply food to nearly all terrestrial organisms, including humans. We eat either plants or other organisms that eat plants.
• Plants maintain the atmosphere. They produce oxygen and absorb carbon dioxide during photosynthesis. Oxygen is essential for cellular respiration for all aerobic organisms. It also maintains the ozone layer that helps protect Earth’s life from damaging UV radiation. Removal of carbon dioxide from the atmosphere reduces the greenhouse effect and global warming.
• Plants recycle matter in biogeochemical cycles. For example, through transpiration, plants move enormous amounts of water from the soil to the atmosphere. Plants such as peas host bacteria that fix nitrogen. This makes nitrogen available to all plants, which pass it on to consumers.
• Plants provide many products for human use, such as firewood, timber, fibers, medicines, dyes, pesticides, oils, and rubber.
• Plants create habitats for many organisms. A single tree may provide food and shelter to many species of insects, worms, small mammals, birds, and reptiles (see Figure below).
Red-eyed tree frogs like this one live in banana trees.
We obviously can’t live without plants, but sometimes they cause us problems. Many plants are weeds. Weeds are plants that grow where people don’t want them, such as gardens and lawns. They take up space and use resources, hindering the growth of more desirable plants. People often introduce plants to new habitats where they lack natural predators and parasites. The introduced plants may spread rapidly and drive out native plants. Many plants produce pollen, which can cause allergies. Plants may also produce toxins that harm human health (see Figure below).
Poison ivy causes allergic skin rashes. It’s easy to recognize the plant by its arrangement of leaves in groups of three. That’s the origin of the old saying, “leaves of three, leave it be.”
Why Study Plants?
Members of the plant kingdom play many crucial and sometimes surprising roles in the drama of life on Earth. You are probably familiar with some reasons plants are important. Why should you understand how plants live? Because plants play many roles, including but certainly not limited to:
1. Supplying Food and Energy
2. Maintaining Earth’s Atmosphere
3. Cycling Water and Nurturing Soils
4. Contributing to Nitrogen and Other Biogeochemical Cycles
5. Interdependence with Animals
6. Interdependence with Fungi
7. Interdependence Among Plants
8. Resources for Humans
9. Aesthetics for Humans
10. Scientific Use by Humans
11. Causing Problems
More than 100,000 natural compounds come from plants, and most of these have yet to be explored. Some of the most powerful and useful compounds come from plants. Who knew they could help us unlock some of the biology's mysteries - all using an approach of mapping biological pathways.
Summary
• Life as we know it would not be possible without plants.
Review
1. List three reasons that plants are important to life on Earth.
2. When is a plant considered a weed?
9.03: Plant Life Cycle Overview
How do plants reproduce?
Is it really due to the birds and the bees? Not always. Even though it is spotted, this plant is known as the kangaroo fern, not the cheetah fern. And all those spots are spores. So what's a spore? Each spore can grow into a new individual without the need for fertilization.
Life Cycle of Plants
All plants have a characteristic life cycle that includes alternation of generations. Plants alternate between haploid and diploid generations. Alternation of generations allows for both asexual and sexual reproduction. Beginning with the diploid sporophyte, spores form from meiosis. Asexual reproduction with spores produces haploid individuals called gametophytes, which produce haploid gametes by mitosis. Sexual reproduction with gametes and fertilization produces the diploid sporophyte. A typical plant’s life cycle is diagrammed in Figure below.
Life Cycle of Plants. This diagram shows the general life cycle of a plant.
Early plants reproduced mainly with spores and spent most of their life cycle as haploid gametophytes. Spores require little energy and matter to produce, and they grow into new individuals without the need for fertilization. In contrast, most modern plants reproduce with gametes using pollen and seeds, and they spend most of their life cycle as diploid sporophytes. Many modern plants can also reproduce asexually using roots, stems, or leaves. This is called vegetative reproduction. One way this can occur is shown in Figure below.
Strawberry plants have horizontal stems called stolons that run over the ground surface. If they take root, they form new plants.
Summary
• All plants have a characteristic life cycle that includes alternation of generations.
• Asexual reproduction with spores produces a haploid gametophyte generation.
• Sexual reproduction with gametes and fertilization produces a diploid sporophyte generation.
Review
1. Define alternation of generations.
2. What type of reproduction occurs in an alternation of generations life cycle?
3. Draw a diagram of a typical plant life cycle that illustrates the concept of alternation of generations. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.02%3A_Importance_of_Plants.txt |
Which moved onto land first, plants or animals?
This fossilized fern may be millions of years old. Over 200 million years ago, the first evidence of ferns related to several modern families appeared. The "great fern radiation" occurred in the late-Cretaceous, which ended 65 million years ago, when many modern families of ferns first appeared. And if animals were the first on land, would many have starved?
Evolution of Plants
As shown in Figure below, plants are thought to have evolved from an aquatic green alga protist. Later, they evolved important adaptations for land, including vascular tissues, seeds, and flowers. Each of these major adaptations made plants better suited for life on dry land and much more successful.
From a simple, green alga ancestor that lived in the water, plants eventually evolved several major adaptations for life on land.
The Earliest Plants
The earliest plants were probably similar to the stonewort, an aquatic algae pictured inFigure below. Unlike most modern plants, stoneworts have stalks rather than stiff stems, and they have hair-like structures called rhizoids instead of roots. On the other hand, stoneworts have distinct male and female reproductive structures, which is a plant characteristic. For fertilization to occur, sperm need at least a thin film of moisture to swim to eggs. In all these ways, the first plants may have resembled stoneworts.
Modern stoneworts may be similar to the earliest plants. Shown is a field of modern stoneworts (right), and an example from the Charophyta, a division of green algae that includes the closest relatives of the earliest plants (left).
Life on Land
By the time the earliest plants evolved, animals were already the dominant organisms in the ocean. Plants were also constrained to the upper layer of water that received enough sunlight for photosynthesis. Therefore, plants never became dominant marine organisms. But when plants moved onto land, everything was wide open. Why was the land devoid of other life? Without plants growing on land, there was nothing for other organisms to feed on. Land could not be colonized by other organisms until land plants became established.
Plants may have colonized the land as early as 700 million years ago. The oldest fossils of land plants date back about 470 million years. The first land plants probably resembled modern plants called liverworts, like the one shown in Figure below.
The first land plants may have been similar to liverworts like this one.
Colonization of the land was a huge step in plant evolution. Until then, virtually all life had evolved in the ocean. Dry land was a very different kind of place. The biggest problem was the dryness. Simply absorbing enough water to stay alive was a huge challenge. It kept early plants small and low to the ground. Water was also needed for sexual reproduction, so sperm could swim to eggs. In addition, temperatures on land were extreme and always changing. Sunlight was also strong and dangerous. It put land organisms at high risk of mutations.
Vascular Plants Evolve
Plants evolved a number of adaptations that helped them cope with these problems on dry land. One of the earliest and most important was the evolution of vascular tissues. Vascular tissues form a plant’s “plumbing system.” They carry water and minerals from soil to leaves for photosynthesis. They also carry food (sugar dissolved in water) from photosynthetic cellsto other cells in the plant for growth or storage. The evolution of vascular tissues revolutionized the plant kingdom. The tissues allowed plants to grow large and endure periods of drought in harsh land environments. Early vascular plants probably resembled the fern shown in Figure below.
Early vascular plants may have looked like this modern fern.
In addition to vascular tissues, these early plants evolved other adaptations to life on land, including lignin, leaves, roots, and a change in their life cycle.
• Lignin is a tough carbohydrate molecule that is hydrophobic (“water fearing”). It adds support to vascular tissues in stems. It also waterproofs the tissues so they don’t leak, which makes them more efficient at transporting fluids. Because most other organisms cannot break down lignin, it helps protect plants from herbivores and parasites.
• Leaves are rich in chloroplasts that function as solar collectors and food factories. The first leaves were very small, but leaves became larger over time.
• Roots are vascular organs that can penetrate soil and even rock. They absorb water andminerals from soil and carry them to leaves. They also anchor a plant in the soil. Roots evolved from rhizoids, which nonvascular plants had used for absorption.
• Land plants evolved a dominant diploid sporophyte generation. This was adaptive because diploid individuals are less likely to suffer harmful effects of mutations. They have two copies of each gene, so if a mutation occurs in one gene, they have a backup copy. This is extremely important on land, where there’s a lot of solar radiation.
With all these advantages, it’s easy to see why vascular plants spread quickly and widely on land. Many nonvascular plants went extinct as vascular plants became more numerous. Vascular plants are now the dominant land plants on Earth.
Summary
• The earliest plants are thought to have evolved in the ocean from a green alga ancestor.
• Plants were among the earliest organisms to leave the water and colonize land.
• The evolution of vascular tissues allowed plants to grow larger and thrive on land.
Review
1. What were the first plants to evolve?
2. What are vascular tissues of a plant? What is their function?
3. Explain why life on land was difficult for early plants.
4. Why did plants need to become established on land before animals could colonize the land? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.04%3A_Early_Evolution_of_Plants.txt |
Why are they called sunflowers?
When the plant is in the bud stage, the flower tends to track the movement of the sun across the horizon, hence the name sunflower. Flowering plants were the last group of plants to evolve. The flower contains both the male and female reproductive structures, and these plants have become tremendously successful. But these plants could not have evolved without the prior evolution of the seed. So what exactly is a seed?
Seed Plants Emerge
For reproduction, early vascular plants still needed moisture. Sperm had to swim from male to female reproductive organs for fertilization. Spores also needed some water to grow and often to disperse as well. Of course, dryness and other harsh conditions made it very difficult for tiny new offspring plants to survive. With the evolution of seeds in vascular plants, all that changed. Seed plants evolved a number of adaptations that made it possible to reproduce without water. As a result, seed plants were wildly successful. They exploded into virtually all of Earth’s habitats.
Why are seeds so adaptive on land? A seed contains an embryo and a food supply enclosed within a tough coating. An embryo is a zygote that has already started to develop and grow. Early growth and development of a plant embryo in a seed is called germination. The seed protects and nourishes the embryo and gives it a huge head start in the “race” of life. Many seeds can wait to germinate until conditions are favorable for growth. This increases the offspring’s chance of surviving even more.
Other reproductive adaptations that evolved in seed plants include ovules, pollen, pollen tubes, and pollination by animals.
• An ovule is a female reproductive structure in seed plants that contains a tiny female gametophyte. The gametophyte produces an egg cell. After the egg is fertilized by sperm, the ovule develops into a seed.
• A grain of pollen is a tiny male gametophyte enclosed in a tough capsule (see Figure below). It carries sperm to an ovule while preventing it from drying out. Pollen grains can’t swim, but they are very light, so the wind can carry them. Therefore, they can travel through air instead of water.
• Wind-blown pollen might land anywhere and be wasted. Another adaptation solved this problem. Plants evolved traits that attract specific animal pollinators. Like the bee in Figure below, a pollinator picks up pollen on its body and carries it directly to another plant of the same species. This greatly increases the chance that fertilization will occur.
• Pollen also evolved the ability to grow a tube, called a pollen tube, through which sperm could be transferred directly from the pollen grain to the egg. This allowed sperm to reach an egg without swimming through a film of water. It finally freed up plants from depending on moisture to reproduce.
Individual grains of pollen may have prickly surfaces that help them stick to pollinators such as bees. What other animals pollinate plants?
Seed Plants Diverge
The first seed plants formed seeds in cones. Cones are made up of overlapping scales, which are modified leaves (see Figure below). Male cones contain pollen, and female cones contain eggs. Seeds also develop in female cones. Modern seed plants that produce seeds in cones are called gymnosperms.
Gymnosperms produce seeds in cones (left). Each scale has a seed attached (right).
Later, seed plants called angiosperms evolved. They produce flowers, which consist of both male and female reproductive structures. The female reproductive structure in a flower includes an organ called an ovary. Eggs form in ovules inside ovaries, which also enclose and protect developing seeds after fertilization occurs. In many species of flowering plants, ovaries develop into fruits, which attract animals that disperse the seeds.
Summary
• The evolution of seeds and pollen allowed plants to reproduce on land without moisture.
• Flowering plants evolved flowers with ovaries that formed fruits. They have been the most successful plants of all.
Review
1. What is a seed?
2. Describe cones.
3. Compare and contrast gymnosperms and angiosperms, and give an example of each.
4. Which major plant adaptation—vascular tissues or seeds—do you think was more important in the evolution of plants? Choose one of the two adaptations, and write a logical argument to support your choice. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.05%3A_Evolution_of_Seed_Plants.txt |
How do you know which group one particular plant belongs to?
So many different types of plants. Does the plant have roots? Or flowers? Or just seeds? Or roots and stems but not seeds? These are all characteristics used to classify plants. How many different types of plants do you see in this Japanese garden?
Classification of Plants
The scientific classification of modern land plants is under constant revision. Informally, land plants can be classified into the groups listed in Table below. Major divisions and types of modern land plants are organized in this table. Why do the first five types of plants require a moist habitat?
The most basic division is between nonvascular plants and vascular plants. Vascular plants are further divided into those that reproduce without seeds and those that reproduce with seeds. Seed plants, in turn, are divided into those that produce seeds in cones and those that produce seeds in the ovaries of flowers. Seed plants are called gymnosperms. Seed plants called angiosperms produce seeds in the ovaries of flowers.
Major Division Types of Plants No. of Living Species Description
Nonvascular Plants
Liverworts 7,000
Hornworts 150
Mosses 10,000 They lack leaves and roots. They have no stems, so they grow low to the ground. They reproduce with spores. They need a moist habitat.
Vascular Plants
Clubmosses 1,200 They have roots and tiny leaves. They have no stems, so they grow low to the ground. They reproduce with spores. They need a moist habitat.
Ferns 11,000 They have large leaves in fronds. They have stiff stems, so they are tall growing; some are trees. They reproduce with spores. They need a moist habitat.
Ginkgoes 1
Cycads 160
Conifers 700
Gnetae 70 Most are trees with wood trunks. They have adaptations to dryness such as needle-like leaves. They reproduce with seeds and pollen. They produce seeds in cones.
Flowering Plants 258,650 They have tremendous diversity in size, shape, and other characteristics. They reproduce with seeds and pollen. They produce seeds in the ovaries of flowers. Ovaries may develop into fruits, which enhance seed dispersal.
Summary
The most basic division of living plants is between nonvascular and vascular plants.
• Vascular plants are further divided into seedless and seed plants.
• Seed plants called gymnosperms produce seeds in cones.
• Seed plants called angiosperms produce seeds in the ovaries of flowers.
Review
1. Compare the different types of plants in the Table above. Which type of plants would you say is most successful? Support your answer with data from the table.
9.07: Nonvascular Plants
Do all plants have roots?
The massive moss covering these branches seems to be dominating its habitat. And maybe it is. Mosses, being nonvascular plants, don't need roots to grow, so they can easily cover moist areas. Mosses commonly grow close together in clumps or mats in damp or shady locations. You may even have mats of moss growing in your backyard.
Nonvascular Plants
Nonvascular plants are bryophytes. Despite the dominance of vascular plants today, more than 17,000 species of bryophytes still survive. They include liverworts, hornworts, and mosses.
Characteristics of Nonvascular Plants
Most bryophytes are small. They not only lack vascular tissues; they also lack true leaves, seeds, and flowers. Instead of roots, they have hair-like rhizoids to anchor them to the ground and to absorb water and minerals (see Figure below). Bryophytes occupy niches in moist habitats, but, as they lack vascular tissue, they are not very efficient at absorbing water.
The rhizoids of a bryophyte may be so fine that they are just one cell thick.
Bryophytes also depend on moisture to reproduce. Sperm produced by a male gametophyte must swim through a layer of rainwater or dew to reach an egg produced by a female gametophyte. The tiny, diploid sporophyte generation then undergoes meiosis to produce haploid spores. The spores may also need moisture to disperse.
Evolution of Nonvascular Plants
Nonvascular plants were the first plants to evolve. Compared to other plants, their small size and lack of specialized structures, such as vascular tissue, stems, leaves, or flowers, explains why these plants evolved first. The first nonvascular plants to evolve were the liverworts. The hornworts evolved somewhat later, and mosses apparently evolved last. Of all the bryophytes, mosses are most similar to vascular plants. Presumably, they share the most recent common ancestor with vascular plants.
Diversity of Nonvascular Plants
The three types of modern nonvascular plants are pictured in Figure below.
• Liverworts are tiny nonvascular plants that have leaf-like, lobed, or ribbon-like photosynthetic tissues rather than leaves. Their rhizoids are very fine, they lack stems, and they are generally less than 10 centimeters (4 inches) tall. They often grow in colonies that carpet the ground.
• Hornworts are minute nonvascular plants, similar in size to liverworts. They also have very fine rhizoids and lack stems. Their sporophytes are long and pointed, like tiny horns. They rise several centimeters above the gametophytes of the plant.
• Mosses are larger nonvascular plants that have coarser, multicellular rhizoids that are more like roots. They also have tiny, photosynthetic structures similar to leaves that encircle a central stem-like structure. Mosses grow in dense clumps, which help them retain moisture.
Liverworts, hornworts, and mosses are modern bryophytes. Liverworts are named for the liver-shaped leaves of some species. Hornworts are named for their horn-like sporophytes.
Summary
• Nonvascular plants are called bryophytes.
• Nonvascular plants include liverworts, hornworts, and mosses. They lack roots, stems, and leaves.
• Nonvascular plants are low-growing, reproduce with spores, and need a moist habitat.
Review
1. Describe nonvascular plants.
2. What is a rhizoid?
3. Why were nonvascular plants the first plants to evolve? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.06%3A_Plant_Classification.txt |
How does water move from the roots to the top of these trees?
Redwood trees found in Yosemite National Park in California. Big? Of course. How do these trees grow so tall? It has a lot to do with a very efficient system to move water, sugars and other nutrients. But the first plants to have such a "vascular system" were not tall trees, but much smaller plants.
Vascular Plants
Vascular plants are known as tracheophytes, which literally means “tube plants.” The earliest vascular plants quickly came to dominate terrestrial ecosystems. Why were they so successful? It was mainly because of their tube-like vascular tissues.
Vascular Tissues
The vascular tissues for which these plants are named are specialized to transport fluid. They consist of long, narrow cells arranged end-to-end, forming tubes. There are two different types of vascular tissues, called xylem and phloem. Both are shown in Figure below.
• Xylem is vascular tissue that transports water and dissolved minerals from roots to stems and leaves. This type of tissue consists of dead cells that lack end walls between adjacent cells. The side walls are thick and reinforced with lignin, which makes them stiff and water proof.
• Phloem is vascular tissue that transports food (sugar dissolved in water) from photosynthetic cells to other parts of the plant for growth or storage. This type of tissue consists of living cells that are separated by end walls with tiny perforations, or holes.
Xylem and phloem are the two types of vascular tissues in vascular plants.
Evolution of Vascular Plants
The first vascular plants evolved about 420 million years ago. They probably evolved from moss-like bryophyte ancestors, but they had a life cycle dominated by the diploid sporophyte generation. As they continued to evolve, early vascular plants became more plant-like in other ways as well.
• Vascular plants evolved true roots made of vascular tissues. Compared with rhizoids, roots can absorb more water and minerals from the soil. They also anchor plants securely in the ground, so plants can grow larger without toppling over.
• Vascular plants evolved stems made of vascular tissues and lignin. Because of lignin, stems are stiff, so plants can grow high above the ground where they can get more light and air. Because of their vascular tissues, stems keep even tall plants supplied with water so they don’t dry out in the air.
• Vascular plants evolved leaves to collect sunlight. At first, leaves were tiny and needle-like, which helped reduce water loss. Later, leaves were much larger and broader, so plants could collect more light.
With their vascular tissues and other adaptations, early vascular plants had the edge over nonvascular plants. The could grow tall and take advantage of sunlight high up in the air. Bryophytes were the photosynthetic pioneers onto land, but early vascular plants were the photosynthetic pioneers into air.
Diversity of Seedless Vascular Plants
Surviving descendants of early vascular plants include clubmosses and ferns. There are 1,200 species of clubmoss and more than 20,000 species of fern. Both types of vascular plants are seedless and reproduce with spores. Two examples are pictured in Figures below and below.
• Clubmosses look like mosses and grow low to the ground. Unlike mosses, they have roots, stems, and leaves, although the leaves are very small.
• Ferns look more like “typical” plants. They have large leaves and may grow very tall. Some even develop into trees.
Clubmosses like these are often confused with mosses.
There’s no confusing ferns with mosses. Why do these ferns look more plant-like?
Summary
• Vascular plants are known as tracheophytes.
• Vascular tissues include xylem and phloem. They allow plants to grow tall in the air without drying out.
• Vascular plants also have roots, stems, and leaves.
Review
1. Compare xylem to phloem.
2. How did vascular tissues and lignin allow vascular plants to be “photosynthetic pioneers into air”?
3. Why are roots more advantageous to a plant than rhizoids?
4. What benefits do stems provide to a plant? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.08%3A_Vascular_Plants.txt |
How old can a plant be?
This is obviously a seed plant. It is a Gingko tree, which is an unique species in that there are no close living relatives. Gingkoes can live for a very long time. Some specimens of this species are thought to be over 2,500 years old. The Ginkgo is also known as a living fossil, with fossils related to modern Ginkgo from the Permian period, dating back 270 million years.
Seed Plants
Seed plants are called spermatophytes. The evolution of seeds by vascular plants was a very big deal. In fact, it was arguably as important as the evolution of vascular tissues. Seeds solved the problem of releasing offspring into a dry world. Once seeds evolved, vascular seed plants and their descendants diversified to fill terrestrial niches everywhere. Today, vascular seed plants dominate Earth.
Parts of a Seed
As shown in Figure below, a seed consists of at least three basic parts: the embryo, seed coat, and stored food.
• The embryo develops from a fertilized egg. While still inside the seed, the embryo forms its first leaf (cotyledon) and starts to develop a stem (hypocotyl) and root (radicle).
• The tough seed coat protects the embryo and keeps it from drying out until conditions are favorable for germination.
• The stored food in a seed is called endosperm. It nourishes the embryo until it can start making food on its own.
A typical plant seed, like this avocado seed, contains an embryo, seed coat, and endosperm. How does each part contribute to the successful development of the new plant?
Many seeds have additional structures that help them disperse. Some examples are shown in Figure below. Structures may help them travel in the wind or stick to animals. Dispersal of seeds away from parent plants helps reduce competition with the parents and increases the chance of offspring surviving.
Dandelion seeds have tiny “parachutes.” Maple seeds have “wings” that act like little gliders. Burdock seeds are covered with tiny hooks that cling to animal fur.
Classification of Seed Plants
The two major types of seed plants are the gymnosperms (seeds in cones) and angiosperms(seeds in ovaries of flowers). Figure below shows how the seeds of gymnosperms and angiosperms differ. Do you see the main difference between the two seeds? The angiosperm seed is surrounded by an ovary.
In gymnosperms, a seed develops on the scale of a cone. Only an angiosperm seed develops inside an ovary.
There are only about 1,000 living species of gymnosperms, whereas there are hundreds of thousands of living species of angiosperms. Living gymnosperms are typically classified in the divisions described in the Table below. Most modern gymnosperms are trees with woody trunks. The majority are conifers such as pine trees.
Division Description
Ginkgoes There is only one living species (Ginkgo biloba); some living trees are more than 2000 years old; they originated in Asia but now are cultivated all over the world; they have been used for medicines for thousands of years.
Conifers There are more than 700 living species; most are trees such as pines with needle-like leaves; they are often the dominant plants in their habitats; they are valuable to humans for paper and timber.
Cycads There are about 300 living species; they are typically trees with stout trunks and fern-like leaves; they live only in tropical and subtropical climates; they have large, brightly-colored seed cones to attract animal pollinators.
Gnetae There are fewer than 100 living species; most are woody vines with evergreen leaves; they live mainly in tropical climates; they are the least well known gymnosperms but the most similar to angiosperms.
Evolution of Seed Plants
The earliest seed plants probably evolved close to 300 million years ago. They were similar to modern ginkgoes and reproduced with pollen and seeds in cones. Early seed plants quickly came to dominate forests during the Mesozoic Era, or Age of the Dinosaurs, about 250 to 65 million years ago.
As seed plants continued to evolve, Earth’s overall climate became drier, so early seed plants evolved adaptations to help them live with low levels of water. Some also evolved adaptations to cold. They had woody trunks and needle-like, evergreen leaves covered with a thick coating of waxy cuticle to reduce water loss. Some of the trees were huge, like today’s giant sequoia, a modern conifer (see Figure below).
The person standing at the foot of this giant sequoia shows just how enormous the tree is. Some early seed plants also grew very large.
Eventually, some gymnosperms started to evolve angiosperm-like traits. For example, cycad ancestors were the first plants to use insects as pollinators. They also used birds and monkeys to disperse their brightly colored seeds. Of modern gymnosperms, Gnetae probably share the most recent common ancestor with angiosperms. Among other similarities, Gnetae produce nectar, a sweet, sugary liquid that attracts insect pollinators. Most modern flowering plants also produce nectar.
Summary
• Most vascular plants are seed plants, or spermatophytes. They reproduce with seeds and pollen.
• Some modern seed plants are gymnosperms that produce seeds in cones.
Review
1. Identify the parts of a seed and the role of each part.
2. Name and describe the divisions of gymnosperms. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.09%3A_Seed_Plants.txt |
So what exactly is a flower?
This closeup view of a lily flower shows the fine detail of this structure. Why are flowers so colorful? What is the purpose of all the parts? They were one of the last adaptations of the plant kingdom, suggesting immense evolutionary significance.
Flowering Plants
Angiosperms, or flowering seed plants, form seeds in ovaries. As the seeds develop, the ovaries may develop into fruits. Flowers attract pollinators, and fruits encourage animals to disperse the seeds.
Parts of a Flower
A flower consists of male and female reproductive structures. The main parts of a flower are shown in Figure below. They include the stamen, pistil, petals, and sepals.
• The stamen is the male reproductive structure of a flower. It consists of a stalk-like filament that ends in an anther. The anther contains pollen sacs, in which meiosis occurs and pollen grains form. The filament raises the anther up high so its pollen will be more likely to blow in the wind or be picked up by an animal pollinator.
• The pistil is the female reproductive structure of a flower. It consists of a stigma, style, andovary. The stigma is raised and sticky to help it catch pollen. The style supports the stigma and connects it to the ovary, which contains the egg. Petals attract pollinators to the flower. Petals are often brightly colored so pollinators will notice them.
• Sepals protect the developing flower while it is still a bud. Sepals are usually green, which camouflages the bud from possible consumers.
A flower includes both male and female reproductive structures.
Flowers and Pollinators
Many flowers have bright colors, strong scents, and sweet nectar to attract animal pollinators. They may attract insects, birds, mammals, and even reptiles. While visiting a flower, a pollinator picks up pollen from the anthers. When the pollinator visits the next flower, some of the pollen brushes off on the stigma. This allows cross-pollination, which increases genetic diversity.
Other Characteristics of Flowering Plants
Although flowers and their components are the major innovations of angiosperms, they are not the only ones. Angiosperms also have more efficient vascular tissues. Additionally, in many flowering plants the ovaries ripen into fruits. Fruits are often brightly colored, so animals are likely to see and eat them and disperse their seeds (see Figure below).
Brightly colored fruits attract animals that may disperse their seeds. It’s hard to miss the bright red apples on these trees.
Evolution of Flowering Plants
Flowering plants are thought to have evolved at least 200 million years ago from gymnosperms like Gnetae. The earliest known fossils of flowering plants are about 125 million years old. The fossil flowers have male and female reproductive organs but no petals or sepals.
Scientists think that the earliest flowers attracted insects and other animals, which spread pollen from flower to flower. This greatly increased the efficiency of fertilization over wind-spread pollen, which might or might not actually land on another flower. To take better advantage of this “animal labor,” plants evolved traits such as brightly colored petals to attract pollinators. In exchange for pollination, flowers gave the pollinators nectar.
Giving free nectar to any animal that happened to come along was not an efficient use of resources. Much of the pollen might be carried to flowers of different species and therefore wasted. As a result, many plants evolved ways to “hide” their nectar from all but very specific pollinators, which would be more likely to visit only flowers of the same species. For their part, animal pollinators co-evolved traits that allowed them to get to the hidden nectar. Two examples of this type of co-evolution are shown in Figure below.
The hummingbird has a long narrow bill to reach nectar at the bottom of the tube-shaped flowers. The bat is active at night, so bright white, night-blooming flowers attract it. In each case, the flowering plant and its pollinator co-evolved to become better suited for their roles in the symbiotic relationship.
Some of the most recent angiosperms to evolve are grasses. Humans started domesticating grasses such as wheat about 10,000 years ago. Why grasses? They have many large, edible seeds that contain a lot of nutritious stored food. They are also relatively easy to harvest. Since then, humans have helped shaped the evolution of grasses, as illustrated by the example in Figure below. Grasses supply most of the food consumed by people worldwide. What other grass seeds do you eat?
The plant on the left, called teosinte, is the ancestor of modern, domesticated corn, shown on the right. An intermediate stage is pictured in the middle. How were humans able to change the plant so dramatically?
Classification of Flowering Plants
There are more than a quarter million species of flowering plants, and they show tremendous diversity. Nonetheless, almost all flowering plants fall into one of three major groups: monocots, eudicots, or magnolids. The three groups differ in several ways. For example, monocot embryos form just one cotyledon, whereas eudicot and magnolid embryos form two cotyledons. The arrangement of their vascular tissues is also different. Examples of the three groups of flowering plants are given in Table below.
Group Sample Families Sample Families
Monocots
Grasses
Orchids
Eudicots
Daisies
Peas
Magnolids
Magnolias
Avocados
Summary
• Most modern seed plants are angiosperms that produce seeds in the ovaries of flowers.
• Ovaries may develop into fruits.
• Flowers attract pollinators and fruits are eaten by animals. Both traits aid the dispersal of seeds.
Review
1. Describe the male and female reproductive structures of flowers.
2. State how fruits help flowering plants reproduce.
3. Explain how flowering plants and their animal pollinators co-evolved.
4. Define monocot. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.10%3A_Flowering_Plants.txt |
Why do plant cells look like little rectangles?
A section of a pine embryo. Notice how all the cells seem to stack on each other, with no spaces in between. MIght this allow the cells to form structures that can grow upright?
Organs in Plants?
Your body includes organ systems, such as the digestive system, made of individual organs, such as the stomach, liver, and pancreas, which work together to carry out a certain function (in this case, breaking down and absorbing food). These organs, in turn, are made of different kinds of tissues, which are groups of cells which work together to perform a specific job. For example, your stomach is made of muscle tissue to facilitate movement and glandular tissue to secrete enzymes for chemical breakdown of food molecules. These tissues, in turn, are made of cells specialized in shape, size, and component organelles, such as mitochondria forenergy and microtubules for movement.
Plants, too, are made of organs, which in turn are made of tissues. Plant tissues, like ours, are constructed of specialized cells, which in turn contain specific organelles. It is these cells, tissues, and organs that carry out the dramatic lives of plants.
Plant Cells
Plant cells resemble other eukaryotic cells in many ways. For example, they are enclosed by a plasma membrane and have a nucleus and other membrane-bound organelles. A typical plant cell is represented by the diagram in Figure below.
Plant cells have all the same structures as animal cells, plus some additional structures. Can you identify the unique plant structures in the diagram?
Plant Cell Structures
Structures found in plant cells but not animal cells include a large central vacuole, cell wall, and plastids such as chloroplasts.
• The large central vacuole is surrounded by its own membrane and contains water and dissolved substances. Its primary role is to maintain pressure against the inside of the cell wall, giving the cell shape and helping to support the plant.
• The cell wall is located outside the cell membrane. It consists mainly of cellulose and may also contain lignin, which makes it more rigid. The cell wall shapes, supports, and protects the cell. It prevents the cell from absorbing too much water and bursting. It also keeps large, damaging molecules out of the cell.
• Plastids are membrane-bound organelles with their own DNA. Examples are chloroplastsand chromoplasts. Chloroplasts contain the green pigment chlorophyll and carry out photosynthesis. Chromoplasts make and store other pigments. They give flower petals their bright colors.
Types of Plant Cells
There are three basic types of cells in most plants. These cells make up ground tissue, which will be discussed in another concept. The three types of cells are described in Table below. The different types of plant cells have different structures and functions.
Type of Cell Structure Functions Example
Parenchymal
cube-shaped
loosely packed
thin-walled
relatively unspecialized
contain chloroplasts
photosynthesis
cellular respiration
storage
food storage tissues of potatoes
Collenchymal
elongated
irregularly thickened walls
support
wind resistance
strings running through a stalk of celery
Sclerenchymal very thick cell walls containing lignin
support
strength
tough fibers in jute (used to make rope)
Summary
• Plants have eukaryotic cells with large central vacuoles, cell walls containing cellulose, and plastids such as chloroplasts and chromoplasts.
• Different types of plant cells include parenchymal, collenchymal, and sclerenchymal cells. The three types differ in structure and function.
Review
1. Identify three structures found in plant cells but not animal cells. What is the function of each structure?
2. Describe parenchymal plant cells and state their functions. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.11%3A_Plant_Cells.txt |
What is this abstract pattern?
Is it just a random artistic piece? Is it a depiction of a pattern of bubbles? Would you believe it is part of a plant? It is actually the center portion of a carrot taproot. And these are all cells. Cells that have come together to form a tissue, with a specific function. What do you think is the main function of tissue in a plant's root?
Plant Tissues
As for all animals, your body is made of four types of tissue: epidermal, muscle, nerve, and connective tissues. Plants, too, are built of tissues, but not surprisingly, their very different lifestyles derive from different kinds of tissues. All three types of plant cells are found in most plant tissues. Three major types of plant tissues are dermal, ground, and vascular tissues.
Dermal Tissue
Dermal tissue covers the outside of a plant in a single layer of cells called the epidermis. You can think of the epidermis as the plant’s skin. It mediates most of the interactions between a plant and its environment. Epidermal cells secrete a waxy substance called cuticle, which coats, waterproofs, and protects the above-ground parts of plants. Cuticle helps prevent water loss, abrasions, infections, and damage from toxins.
This tissue includes several types of specialized cells. Pavement cells, large, irregularly shaped parenchymal cells which lack chloroplasts, make up the majority of the epidermis. Within the epidermis, thousands of pairs of bean-shaped schlerenchymal guard cells swell and shrink by osmosis to open and close stomata, tiny pores which control the exchange of oxygen and carbon dioxide gases and the release of water vapor. The lower surfaces of some leaves contain as many as 100,000 stomata per square centimeter.
The epidermis of Arabidopsis shows both pavement cells (A) and stomata made of sclerenchymal guard cells (B), which control water loss and gas exchange.
Ground Tissue
Ground tissue makes up much of the interior of a plant and carries out basic metabolic functions. Ground tissue in stems provides support and may store food or water. Ground tissues in roots may also store food.
Vascular Tissue
Vascular tissue runs through the ground tissue inside a plant. Your body was able to grow from a single cell to perhaps 100 trillion cells because, 21 days after fertilization, a tiny heart began to pump blood throughout your tiny self – and it hasn’t stopped since. The blood it pumps carries water, oxygen and nutrients to each one of your trillions of cells, and removes CO2 and other wastes. Of course plants don’t have hearts, but they do have vessels that transport water, minerals, and nutrients through the plant. These vessels are the vascular tissue, and consist of xylem and phloem. Xylem and phloem are packaged together in bundles, as shown in Figure below.
Bundles of xylem and phloem run through the ground tissue inside this stalk of celery. What function do these tissues serve?
Summary
• The three types of plant cells are found in each of the major types of plant tissues: dermal, ground, and vascular tissues.
• Dermal tissue covers the outside of a plant in a single layer of cells called the epidermis. It mediates most of the interactions between a plant and its environment.
• Ground tissue makes up most of the interior of a plant. It carries out basic metabolic functions and stores food and water.
• Vascular tissue runs through the ground tissue inside a plant. It consists of bundles of xylem and phloem, which transport fluids throughout the plant.
Review
1. Compare dermal, ground, and vascular tissues of plants.
2. What is cuticle? What is its role?
3. What are guard cells and stomata?
4. An important concept in biology is that form follows function. In other words, the structure of an organism, or part of an organism, depends on its function. Apply this concept to plants, and explain why plants have different types of cells and tissues. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.12%3A_Plant_Tissues.txt |
So how do plants grow?
There must be an area of growth, similar to how the bones in your fingers, arms, and legs grow longer. There is, and it is called the apical meristem, which is shown here.
Growth of Plants
Most plants continue to grow throughout their lives. Like other multicellular organisms, plants grow through a combination of cell growth and cell division. Cell growth increases cell size, while cell division (mitosis) increases the number of cells. As plant cells grow, they also become specialized into different cell types through cellular differentiation. Once cells differentiate, they can no longer divide. How do plants grow or replace damaged cells after that?
The key to continued growth and repair of plant cells is meristem. Meristem is a type of plant tissue consisting of undifferentiated cells that can continue to divide and differentiate.
Apical meristems are found at the apex, or tip, of roots and buds, allowing roots and stems to grow in length and leaves and flowers to differentiate. Roots and stems grow in length because the meristem adds tissue “behind” it, constantly propelling itself further into the ground (for roots) or air (for stems). Often, the apical meristem of a single branch will become dominant, suppressing the growth of meristems on other branches and leading to the development of a single trunk. In grasses, meristems at the base of the leaf blades allow for regrowth after grazing by herbivores – or mowing by lawnmowers.
Microphotograph of the root tip of a broad bean show rapidly dividing apical meristem tissue just behind the root cap. Numerous cells in various stages of mitosis can be observed.
Apical meristems differentiate into the three basic types of meristem tissue which correspond to the three types of tissue: protoderm produces new epidermis, ground meristem produces ground tissue, and procambium produces new xylem and phloem. These three types of meristem are considered primary meristem because they allow growth in length or height, which is known as primary growth.
Secondary meristems allow growth in diameter (secondary growth) in woody plants. Herbaceous plants do not have secondary growth. The two types of secondary meristem are both named cambium, meaning “exchange” or “change”. Vascular cambium produces secondary xylem (toward the center of the stem or root) and phloem (toward the outside of the stem or root), adding growth to the diameter of the plant. This process produces wood, and builds the sturdy trunks of trees. Cork cambium lies between the epidermis and the phloem, and replaces the epidermis of roots and stems with bark, one layer of which is cork.
Woody plants grow in two ways. Primary growth adds length or height, mediated by apical meristem tissue at the tips of roots and shoots – which is difficult to show clearly in cross-sectional diagrams. Secondary growth adds to the diameter of a stem or root; vascular cambium adds xylem (inward) and phloem (outward), and cork cambium replaces epidermis with bark.
Summary
• Most plants continue to grow as long as they live. They grow through a combination of cell growth and cell division (mitosis).
• The key to plant growth is meristem, a type of plant tissue consisting of undifferentiated cells that can continue to divide and differentiate.
• Meristem allows plant stems and roots to grow longer (primary growth) and wider (secondary growth).
Review
1. Define meristem and apical meristem.
2. What are the two types of secondary meristem?
3. Describe cork cambium.
4. What is primary growth and secondary growth? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.13%3A_Plant_Growth.txt |
Now those are some serious roots. But what exactly are roots?
There are taproots and fibrous roots, primary roots and secondary roots. And they always seem to know which way to grow. Roots are very special plant organs. How and why?
Roots
Plants have specialized organs that help them survive and reproduce in a great diversity of habitats. Major organs of most plants include roots, stems, and leaves.
Roots are important organs in all vascular plants. Most vascular plants have two types of roots: primary roots that grow downward and secondary roots that branch out to the side. Together, all the roots of a plant make up a root system.
Root Systems
There are two basic types of root systems in plants: taproot systems and fibrous rootsystems. Both are illustrated in Figure below.
• Taproot systems feature a single, thick primary root, called the taproot, with smaller secondary roots growing out from the sides. The taproot may penetrate as many as 60 meters (almost 200 feet) below the ground surface. It can plumb very deep water sources and store a lot of food to help the plant survive drought and other environmental extremes. The taproot also anchors the plant very securely in the ground.
• Fibrous root systems have many small branching roots, called fibrous roots, but no large primary root. The huge number of threadlike roots increases the surface area for absorption of water and minerals, but fibrous roots anchor the plant less securely.
Dandelions have taproot systems; grasses have fibrous root systems.
Root Structures and Functions
As shown in Figure below, the tip of a root is called the root cap. It consists of specialized cells that help regulate primary growth of the root at the tip. Above the root cap is primary meristem, where growth in length occurs.
A root is a complex organ consisting of several types of tissue. What is the function of each tissue type?
Above the meristem, the rest of the root is covered with a single layer of epidermal cells. These cells may have root hairs that increase the surface area for the absorption of water and minerals from the soil. Beneath the epidermis is ground tissue, which may be filled with stored starch. Bundles of vascular tissues form the center of the root. Waxy layers waterproof the vascular tissues so they don’t leak, making them more efficient at carrying fluids. Secondary meristem is located within and around the vascular tissues. This is where growth in thickness occurs.
The structure of roots helps them perform their primary functions. What do roots do? They have three major jobs: absorbing water and minerals, anchoring and supporting the plant, and storing food.
1. Absorbing water and minerals: Thin-walled epidermal cells and root hairs are well suited to absorb water and dissolved minerals from the soil. The roots of many plants also have a mycorrhizal relationship with fungi for greater absorption.
2. Anchoring and supporting the plant: Root systems help anchor plants to the ground, allowing plants to grow tall without toppling over. A tough covering may replace the epidermis in older roots, making them ropelike and even stronger. As shown in Figure below, some roots have unusual specializations for anchoring plants.
3. Storing food: In many plants, ground tissues in roots store food produced by the leaves during photosynthesis. The bloodroot shown in Figure below stores food in its roots over the winter.
Mangrove roots are like stilts, allowing mangrove trees to rise high above the water. The trunk and leaves are above water even at high tide. A bloodroot plant uses food stored over the winter to grow flowers in the early spring.
Root Growth
Roots have primary and secondary meristems for growth in length and width. As roots grow longer, they always grow down into the ground. Even if you turn a plant upside down, its roots will try to grow downward. How do roots “know” which way to grow? How can they tell down from up? Specialized cells in root caps are able to detect gravity. The cells direct meristem in the tips of roots to grow downward toward the center of Earth. This is generally adaptive for land plants. Can you explain why?
As roots grow thicker, they can’t absorb water and minerals as well. However, they may be even better at transporting fluids, anchoring the plant, and storing food (see Figure below).
Secondary growth of sweet potato roots provides more space to store food. Roots store sugar from photosynthesis as starch. What other starchy roots do people eat?
Summary
• Roots absorb water and minerals and transport them to stems. They also anchor and support a plant, and store food.
• A root system consists of primary and secondary roots.
• Each root is made of dermal, ground, and vascular tissues.
• Roots grow in length and width from primary and secondary meristem.
Review
1. What are root hairs? What is their role?
2. Identify three major functions of roots.
3. Contrast a taproot system with a fibrous root system.
4. Explain how roots “know” which way to grow. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.14%3A_Roots.txt |
How does water get up? How does sugar move down?
What structures hold the plant upright? Are tree trunks also stems? Of course they are. Trees don't start out big and tall - they grow from small plants to large trees. And it is these stems that allow them to grow upright. So, obviously the stem is very important and has numerous functions.
Stems
In vascular plants, stems are the organs that hold plants upright so they can get the sunlight and air they need. Stems also bear leaves, flowers, cones, and secondary stems. These structures grow at points called nodes (shown in Figure below). At each node, there is a bud of meristem tissue that can divide and specialize to form a particular structure.
The stem of a vascular plant has nodes where leaves and other structures may grow.
Another vital function of stems is transporting water and minerals from roots to leaves and carrying food from leaves to the rest of the plant. Without this connection between roots and leaves, plants could not survive high above ground in the air. In many plants, stems also store food or water during cold or dry seasons.
Stem Diversity
Stems show variation because many stems are specialized. Figure below shows examples of stem specialization. With specialized stems, plants can exploit a diversity of niches in virtually all terrestrial ecosystems.
Stem specializations such as these let plants grow in many different habitats.
Stem Tissues and Functions
Like roots, the stems of vascular plants are made of dermal, vascular, and ground tissues.
• A single-celled layer of epidermis protects and waterproofs the stem and controls gas exchange.
• In trees, some of the epidermal tissue is replaced by bark. Bark is a combination of tissues that provides a tough, woody external covering on the stems of trees. The inner part of bark is alive and growing; the outer part is dead and provides strength, support, and protection.
• Ground tissue forms the interior of the stem. The large central vacuoles of ground tissue cells fill with water to support the plant. The cells may also store food.
• Bundles of vascular tissue run through the ground tissue of a stem and transport fluids. Plants may vary in how these bundles are arranged.
Stem Growth
The stems of all vascular plants get longer through primary growth. This occurs in primary meristem at the tips and nodes of the stems. Most stems also grow in thickness through secondary growth. This occurs in secondary meristem, which is located in and around the vascular tissues. Secondary growth forms secondary vascular tissues and bark. In many trees, the yearly growth of new vascular tissues results in an annual growth ring like the one in Figure below. When a tree is cut down, the rings in the trunk can be counted to estimate the tree’s age.
The number of rings in this cross-section of tree trunk show how many years the tree lived. What does each ring represent?
Summary
• Stems hold plants upright, bear leaves and other structures, and transport fluids between roots and leaves.
• Like roots, stems contain dermal, ground, and vascular tissues.
• Trees have woody stems covered with bark.
Review
1. Describe two types of specialized stems. What is each type of stem specialized for?
2. What is bark? What purposes does it serve?
3. Apply lesson concepts to predict how the stem of a desert plant might be specialized for its environment. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.15%3A_Stems.txt |
Could life exist without the leaf?
A leaf looks so simple. But it is actually a very complex structure. And it may be one of the most important organs in all kingdoms. Life as we know it could not exist without leaves. Why? One word: photosynthesis.
Leaves
Plants have specialized organs that help them survive and reproduce in a great diversity of habitats. Major organs of most plants include roots, stems, and leaves. Leaves are the keys not only to plant life but to all terrestrial life. The primary role of leaves is to collect sunlight and make food by photosynthesis. Despite the fundamental importance of the work they do, there is great diversity in the leaves of plants. However, given the diversity of habitats in which plants live, it’s not surprising that there is no single best way to collect solar energy forphotosynthesis.
Leaf Variation
Leaves may vary in size, shape, and their arrangement on stems. Nonflowering vascular plants have three basic types of leaves: microphylls (“tiny leaves”), fronds, and needles.Figure below describes each type.
Leaf variation in nonflowering plants reflects their evolutionary origins. Can you explain how?
Flowering vascular plants also have diverse leaves. However, the leaves of all flowering plants have two basic parts in common: the blade and petiole. The blade of the leaf is the relatively wide, flat part of the leaf that gathers sunlight and undergoes photosynthesis. Thepetiole is the part that attaches the leaf to a stem of the plant. This occurs at a node.
Flowering plant leaves vary in how the leaves are arranged on the stem and how the blade is divided. This is illustrated in Figure below. Generally, the form and arrangement of leaves maximizes light exposure while conserving water, reducing wind resistance, or benefiting the plant in some other way in its particular habitat.
• Leaves arranged in whorls encircle upright stems at intervals. They collect sunlight from all directions.
• Leaves arranged in basal rosettes take advantage of warm temperatures near the ground.
• Leaves arranged in alternate or opposing pairs collect light from above. They are typically found on plants with a single, upright stem.
• The blades of simple leaves are not divided. This provides the maximum surface area for collecting sunlight.
• The blades of compound leaves are divided into many smaller leaflets. This reduces wind resistance and water loss.
Leaf variation in flowering plants may include variations in the arrangement of leaves and the divisions of the blade.
Seasonal Changes in Leaves
Even if you don’t live in a place where leaves turn color in the fall, no doubt you’ve seen photos of their “fall colors” (see Figure below). The leaves of many plants turn from green to other, glorious colors during autumn each year. The change is triggered by shorter days and cooler temperatures. Leaves respond to these environmental stimuli by producing less chlorophyll. This allows other leaf pigments—such as oranges and yellows—to be seen.
A deciduous tree goes through dramatic seasonal changes each year. Can you identify the seasons in the photo?
After leaves turn color in the fall, they may all fall off the plant for the winter. Plants that shed their leaves seasonally each year are called deciduous plants. Shedding leaves is a strategy for reducing water loss during seasons of extreme dryness. On the downside, the plant must grow new leaves in the spring, and that takes a lot of energy and matter. Some plants may “bank” energy over the winter by storing food. That way, they are ready to grow new leaves as soon as spring arrives.
Evergreen plants have a different strategy for adapting to seasonal dryness. They don’t waste energy and matter growing new leaves each year. Instead, they keep their leaves and stay green year-round. However, to reduce water loss, they have needle-like leaves with very thick cuticle. On the downside, needle-like leaves reduce the surface area for collecting sunlight. This is one reason that needles may be especially rich in chlorophyll, as you can see from the dark green pine needles in Figure below. This is also an important adaptation for low levels of sunlight, allowing evergreens to live far from the equator.
Compare the color of the evergreen needles and the deciduous leaf. Why is the darker color of the needles adaptive?
Summary
• The primary function of leaves is to collect sunlight and make food by photosynthesis.
• In a deciduous plant, leaves seasonally turn color and fall off the plant. They are replaced with new leaves later in the year.
• An evergreen plant keeps its green leaves year-round. It may have needle-like leaves to reduce water loss.
Review
1. Name the two main parts of an angiosperm leaf. What is the function of each part?
2. Identify strategies used by deciduous and evergreen plants to adapt to seasonal dryness.
3. Relate leaf variation to environmental variation. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.16%3A_Leaves.txt |
Oxygen — the oxygen that we breath — is just a waste product of what reaction?
Every split second that sunlight hits that leaf, photosynthesis is initiated, bringing energy into the ecosystem. It could be said that this is one of the most important - if not the absolutely most important - biochemical reactions. And it all starts with the leaf.
Factories for Photosynthesis
Photosynthesis is the process that uses energy from the sun, together with carbon dioxide and water, to make glucose and oxygen. The primary role of photosynthesis is to make the carbohydrate, suggesting that oxygen, which is released back into the atmosphere, is just a waste product.
You can think of a single leaf as a photosynthesis factory. A factory has specialized machines to produce a product. It’s also connected to a transportation system that supplies it with raw materials and carries away the finished product. In all these ways, a leaf resembles a factory. The cross section of a leaf in Figure below lets you look inside a leaf “factory.”
There’s more to a leaf than meets the eye. Can you identify the functions of each of the labeled structures in the diagram?
A leaf consists of several different kinds of specialized tissues that work together to make food by photosynthesis. The major tissues are mesophyll, veins, and epidermis.
• Mesophyll makes up most of the leaf’s interior. This is where photosynthesis occurs. Mesophyll consists mainly of parenchymal cells with chloroplasts.
• Veins are made primarily of xylem and phloem. They transport water and minerals to thecells of leaves and carry away dissolved sugar.
• The epidermis of the leaf consists of a single layer of tightly-packed dermal cells. They secrete waxy cuticle to prevent evaporation of water from the leaf. The epidermis has tiny pores called stomata (singular, stoma) that control transpiration and gas exchange with the air. For photosynthesis, stomata must control the transpiration of water vapor and the exchange of carbon dioxide and oxygen. Stomata are flanked by guard cells that swell or shrink by taking in or losing water through osmosis. When they do, they open or close the stomata (see Figure below).
For photosynthesis, stomata must control the transpiration of water vapor and the exchange of carbon dioxide and oxygen. Stomata are flanked by guard cells that swell or shrink by taking in or losing water through osmosis. When they do, they open or close the stomata.
Summary
• Specialized cells and tissues in leaves work together to perform photosynthesis.
Review
1. Explain how a leaf is like a factory.
2. Explain the role of stomata during photosynthesis.
3. What controls the opening of stomata?
9.18: Plant Life Cycles
Fertilization or pollination. How does this occur in the plant?
Pollination. A significant step in the life cycle of flowering plants. But fertilization must occur in the life cycles of all plants, not just those with flowers. Does it always use the birds or the bees?
General Plant Life Cycle
The life cycle of all plants is complex because it is characterized by alternation of generations. Plants alternate between diploid sporophyte and haploid gametophyte generations, and between sexual and asexual reproduction. The ability to reproduce both sexually and asexually gives plants the flexibility to adapt to changing environments. Their complex life cycle allows for great variation. A general plant life cycle is represented by the diagram in Figure below. From the figure, you can see that the diploid sporophyte has a structure called a sporangium (plural, sporangia) that undergoes meiosis to form haploid spores. A spore develops into a haploid gametophyte. The gametophyte has male or female reproductive organs that undergo mitosis to form haploid gametes (sperm or eggs). Fertilization of gametes produces a diploid zygote. The zygote grows and develops into a mature sporophyte, and the cycle repeats.
This diagram represents the life cycle that generally characterizes plants.
One of the two generations of a plant’s life cycle is typically dominant to the other generation. Whether it’s the sporophyte or gametophyte generation, individuals in the dominant generation live longer and grow larger. They are the green, photosynthetic structures that you would recognize as a fern, tree, or other plant (see Figure below). Individuals in the nondominant generation, in contrast, may be very small and rarely seen. They may live in or on the dominant plant.
The dominant generation in nonvascular plants is the gametophyte; in vascular plants, it’s the sporophyte. Why is a dominant sporophyte generation an advantage on land?
All of these photos show plants of the dominant generation in their life cycle.
Summary
• All plants have a life cycle with alternation of generations.
• Plants alternate between diploid sporophyte and haploid gametophyte generations, and between sexual reproduction with gametes and asexual reproduction with spores.
Review
1. Outline the general life cycle of plants.
2. What are sporangia? What do they do?
3. Describe the gametophyte.
4. Describe the sporophyte.
9.19: Life Cycle of Nonvascular Plants
Haploid or diploid. Which would you say is dominant?
That may depend on the plant. Start with moss. The typical nonvascular plant. But such a simple plant has a very interesting life cycle. Whereas most kinds of plants have two sets of chromosomes in their vegetative cells, mosses have only a single set of chromosomes. So, how does meiosis occur?
Life Cycle of Nonvascular Plants
Nonvascular plants include mosses, liverworts, and hornworts. They are the only plants with a life cycle in which the gametophyte generation is dominant. Figure below shows the life cycle of moss. The familiar, green, photosynthetic moss plants are gametophytes. The sporophytegeneration is very small and dependent on the gametophyte plant.
Like other bryophytes, moss plants spend most of their life cycle as gametophytes. Find the sporophyte in the diagram. Do you see how it is growing on the gametophyte plant?
The gametophytes of nonvascular plants have distinct male or female reproductive organs (see Figure below). Male reproductive organs, called antheridia (singular, antheridium), produce motile sperm with two flagella. Female reproductive organs, called archegonia(singular, archegonium), produce eggs.
The reproductive organs of bryophytes like this liverwort are male antheridia and female archegonia.
In order for fertilization to occur, sperm must swim in a drop of water from an antheridium to an egg in an archegonium. If fertilization takes place, it results in a zygote that develops into a tiny sporophyte on the parent gametophyte plant. The sporophyte produces haploid spores, and these develop into the next generation of gametophyte plants. Then the cycle repeats.
Summary
• In nonvascular plants, the gametophyte generation is dominant. The tiny sporophyte grows on the gametophyte plant.
Review
1. Describe antheridia and archegonia and their functions.
2. Create your own cycle diagram to represent the moss life cycle. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.17%3A_Photosynthesis.txt |
What's a fiddlehead?
This fern leaf structure is known as a fiddlehead. Can you understand why? Does it look tasty? Nearly all ferns have fiddleheads, and many recipes exist on how to prepare fiddleheads. But this part of the plant, the leaf or the frond, also plays a very important role in the fern's life cycle.
Life Cycle of Seedless Vascular Plants
Unlike nonvascular plants, all vascular plants—including seedless vascular plants—have a dominant sporophyte generation. Seedless vascular plants include clubmosses and ferns.Figure below shows a typical fern life cycle.
In the life cycle of a fern, the sporophyte generation is dominant.
A mature sporophyte fern has the familiar leafy fronds. The undersides of the leaves are dotted with clusters of sporangia. Sporangia produce spores that develop into tiny, heart-shaped gametophytes. Gametophytes have antheridia and archegonia. Antheridia produce sperm with many cilia; archegonia produce eggs. Fertilization occurs when sperm swim to an egg inside an archegonium. The resulting zygote develops into an embryo that becomes a new sporophyte plant. Then the cycle repeats.
Summary
• In vascular plants, the sporophyte generation is dominant.
• In seedless vascular plants such as ferns, the sporophyte releases spores from the undersides of leaves.
• The spores develop into tiny, separate gametophytes, from which the next generation of sporophyte plants grows.
Review
1. What role do leaves play in the reproduction of ferns?
2. Describe antheridia and archegonia and their functions.
3. Create your own cycle diagram to represent the life cycle of a fern.
9.21: Gymnosperm Life Cycle
Do pine trees produce flowers?
Of course not. But look closely at these pine cones. Notice the shape. Can you see the "petals?" They do seem oddly similar to flowers. These pine cones have a prominent role in the gymnosperm life cycle. So what is the function of a pine cone?
Life Cycle of Gymnosperms
Gymnosperms are vascular plants that produce seeds in cones. Examples include coniferssuch as pine and spruce trees. The gymnosperm life cycle has a dominant sporophyte generation. Both gametophytes and the next generation’s new sporophytes develop on the sporophyte parent plant. Figure below is a diagram of a gymnosperm life cycle.
The gymnosperm life cycle follows the general plant life cycle, but with some new adaptations. Can you identify them?
Cones form on a mature sporophyte plant. Inside male cones, male spores develop into male gametophytes. Each male gametophyte consists of several cells enclosed within a grain of pollen. Inside female cones, female spores develop into female gametophytes. Each female gametophyte produces an egg inside an ovule.
Pollination occurs when pollen is transferred from a male to female cone. If sperm then travel from the pollen to an egg so fertilization can occur, a diploid zygote results. The zygote develops into an embryo within a seed, which forms from the ovule inside the female cone. If the seed germinates, it may grow into a mature sporophyte tree, which repeats the cycle.
Summary
• In gymnosperms, the gametophyte generation takes place in a cone, which forms on the mature sporophyte plant.
• Each male gametophyte is just a few cells inside a grain of pollen. Each female gametophyte produces an egg inside an ovule.
• Pollination must occur for fertilization to take place. Zygotes develop into embryos inside seeds, from which the next sporophyte generation grows.
Review
1. Describe how gymnosperms use cones to reproduce.
2. Create your own cycle diagram to represent the life cycle of a gymnosperm.
9.22: Angiosperm Life Cycle
What's the most successful type of plant?
Flowering plants. Why? As you know, flowers come in many different styles and colors, and many are visually pleasing. This aids in pollination. Also notice the anatomy of this Hibiscus flower. Each part has evolved to play a role in the life cycle.
Life Cycle of Angiosperms
Angiosperms, or flowering plants, are the most abundant and diverse plants on Earth.Angiosperms evolved several reproductive adaptations that have contributed to their success. Like all vascular plants, their life cycle is dominated by the sporophyte generation. A typical angiosperm life cycle is shown in Figure below.
Life cycle of an angiosperm
The flower in Figure above is obviously an innovation in the angiosperm life cycle. Flowersform on the dominant sporophyte plant. They consist of highly specialized male and female reproductive organs. Flowers produce spores that develop into gametophytes. Male gametophytes consist of just a few cells within a pollen grain and produce sperm. Female gametophytes produce eggs inside the ovaries of flowers. Flowers also attract animalpollinators.
If pollination and fertilization occur, a diploid zygote forms within an ovule in the ovary. The zygote develops into an embryo inside a seed, which forms from the ovule and also contains food to nourish the embryo. The ovary surrounding the seed may develop into a fruit. Fruitsattract animals that may disperse the seeds they contain. If a seed germinates, it may grow into a mature sporophyte plant and repeat the cycle.
Summary
• In flowering plants, the gametophyte generation takes place in a flower, which forms on the mature sporophyte plant.
• Each male gametophyte is just a few cells inside a grain of pollen. Each female gametophyte produces an egg inside an ovule.
• Pollination must occur for fertilization to take place. Zygotes develop into embryos inside seeds, from which the next sporophyte generation grows.
Review
1. State the functions of flowers and fruits in angiosperm reproduction.
2. Create your own cycle diagram to represent the life cycle of a daisy. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.20%3A_Life_Cycle_of_Seedless_Vascular_Plants.txt |
Look closely at the petals of this flower. Do they look different?
This flower is from an aloe plant. Aloes are succulent plants, which have adaptations that allow them to store water in their enlarged fleshy leaves, stems, or roots. This allows them to survive in arid environments.
Plant Adaptations
Plants live just about everywhere on Earth. To live in so many different habitats, they have evolved adaptations that allow them to survive and reproduce under a diversity of conditions.
All plants are adapted to live on land. Or are they? All living plants today have terrestrial ancestors, but some plants now live in the water. They have had to evolve new adaptations for their watery habitat.
Adaptations to Water
Aquatic plants are plants that live in water. Living in water has certain advantages for plants. One advantage is, well, the water. There’s plenty of it and it’s all around. Therefore, most aquatic plants do not need adaptations for absorbing, transporting, and conserving water. They can save energy and matter by not growing extensive root systems, vascular tissues, or thick cuticles on leaves. Support is also less of a problem because of the buoyancy of water. As a result, adaptations such as strong woody stems and deep anchoring roots are not necessary for most aquatic plants.
Living in water does present challenges to plants, however. For one thing, pollination by wind or animals isn’t feasible under water, so aquatic plants may have adaptations that help them keep their flowers above water. For instance, water lilies have bowl-shaped flowers and broad, flat leaves that float. This allows the lilies to collect the maximum amount of sunlight, which does not penetrate very deeply below the water's surface. Plants that live in moving water, such as streams and rivers, may have different adaptations. For example, cattails have narrow, strap-like leaves that reduce their resistance to the moving water (see Figure below).
Water lilies and cattails have different adaptations for life in the water. Compare the leaves of the two kinds of plants. How do the leaves help the plants adapt to their watery habitats?
Adaptations to Extreme Dryness
Plants that live in extremely dry environments have the opposite problem: how to get and keep water. Plants that are adapted to very dry environments are called xerophytes. Their adaptations may help them increase water intake, decrease water loss, or store water when it is available.
The saguaro cactus pictured in Figure below has adapted in all three ways. When it was still a very small plant, just a few inches high, its shallow roots already reached out as much as 2 meters (7 feet) from the base of the stem. By now, its root system is much more widespread. It allows the cactus to gather as much moisture as possible from rare rainfalls. The saguaro doesn’t have any leaves to lose water by transpiration. It also has a large, barrel-shaped stem that can store a lot of water. Thorns protect the stem from thirsty animals that might try to get at the water inside.
The saguaro cactus has many adaptations for extreme dryness. How does it store water?
Adaptations to Air
Plants called epiphytes grow on other plants. They obtain moisture from the air and make food by photosynthesis. Most epiphytes are ferns or orchids that live in tropical or temperate rainforests (see Figure below). Host trees provide support, allowing epiphyte plants to obtain air and sunlight high above the forest floor. Being elevated above the ground lets epiphytes get out of the shadows on the forest floor so they can get enough sunlight for photosynthesis. Being elevated may also reduce the risk of being eaten by herbivores and increase the chance of pollination by wind.
These Elkhorn and Staghorn ferns are growing on a rainforest tree as epiphytes.
Epiphytes don’t grow in soil, so they may not have roots. However, they still need water for photosynthesis. Rainforests are humid, so the plants may be able to absorb the water they need from the air. However, many epiphytes have evolved modified leaves or other structures for collecting rainwater, fog, or dew. The leaves of the bromeliad shown in Figure below are rolled into funnel shapes to collect rainwater. The base of the leaves forms a tank that can hold more than 8 liters (2 gallons) of water. Some insects and amphibians may spend their whole life cycle in the pool of water in the tank, adding minerals to the water with their wastes. The tissues at the base of the leaf are absorbent, so they can take in both water and minerals from the tank.
The leaves of this bromeliad are specialized to collect, store, and absorb rainwater.
Summary
• Plants live just about everywhere on Earth, so they have evolved adaptations that allow them to survive and reproduce under a diversity of conditions.
• Various plants have evolved adaptations to live in the water, in very dry environments, or in the air as epiphytes.
Review
1. List special challenges that aquatic plants face.
2. What are xerophytes? Give an example.
3. Identify three general ways that plants can adapt to extreme dryness.
4. Describe how epiphytes can absorb moisture without growing roots in soil.
5. Why are epiphytes found mainly in rainforest ecosystems?
6. Apply the concept of symbiosis to epiphytes and their host plants. Do you think they have a symbiotic relationship? If so, which type of symbiotic relationship do you think they have? Explain your answer. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.23%3A_Plant_Adaptations.txt |
So what happens to a vineyard in the middle of winter?
The vines cannot die each year. Instead, the plants go into a state of dormancy, almost as if they are taking a long nap.
Plant Responses
Like all organisms, plants detect and respond to stimuli in their environment. Unlike animals, plants can’t run, fly, or swim toward food or away from danger. They are usually rooted to the soil. Instead, a plant’s primary means of response is to change how it is growing. Plants also don’t have a nervous system to control their responses. Instead, their responses are generally controlled by hormones, which are chemical messenger molecules.
Plant Tropisms
Plant roots always grow downward because specialized cells in root caps detect and respond to gravity. This is an example of a tropism. A tropism is a turning toward or away from a stimulus in the environment. Growing toward gravity is called geotropism. Plants also exhibit phototropism, or growing toward a light source. This response is controlled by a plant growth hormone called auxin. As shown in Figure below, auxin stimulates cells on the dark side of a plant to grow longer. This causes the plant to bend toward the light.
Phototropism is controlled by the growth hormone auxin.
Daily and Seasonal Responses
Plants also detect and respond to the daily cycle of light and darkness. For example, some plants open their leaves during the day to collect sunlight and then close their leaves at night to prevent water loss. Environmental stimuli that indicate changing seasons trigger other responses. Many plants respond to the days growing shorter in the fall by going dormant. They suspend growth and development in order to survive the extreme cold and dryness of winter. Dormancy ensures that seeds will germinate and plants will grow only when conditions are favorable.
Responses to Disease
Plants don’t have immune systems, but they do respond to disease. Typically, their first line of defense is the death of cells surrounding infected tissue. This prevents the infection from spreading. Many plants also produce hormones and toxins to fight pathogens. For example, willow trees produce salicylic acid to kill bacteria. The same compound is used in many acne products for the same reason. Exciting new research suggests that plants may even produce chemicals that warn other plants of threats to their health, allowing the plants to prepare for their own defense. As these and other responses show, plants may be rooted in place, but they are far from helpless.
Summary
• Like all organisms, plants detect and respond to stimuli in their environment. Their main response is to change how they grow.
• Plant responses are controlled by hormones. Some plant responses are tropisms.
• Plants also respond to daily and seasonal cycles and to disease.
Review
1. What is the primary way that plants respond to environmental stimuli? What controls their responses?
2. Define tropism. Name one example in plants.
3. State ways that plants respond to disease.
4. Why is it adaptive for plants to detect and respond to daily and seasonal changes? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/09%3A_Plants/9.24%3A_Plant_Responses.txt |
Animals are multicellular, eukaryotic organisms of the kingdom Animalia. All animals are motile (i.e., they can move spontaneously and independently at some point in their lives) and their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their lives. All animals are heterotrophs: they must ingest other organisms or their products for sustenance.
10: Animals
Is an insect an animal?
Of course it is. Is a snail an insect? No, snails are mollusks. Notice the large "foot" that allows movement, and the antennas are obvious. Actually, a snail's eyes are on the two long projections on its head, and the projections are called eyestalks. These are characteristics of this animal.
Characteristics of Animals
Animals are a kingdom of multicellular eukaryotes. They cannot make their own food. Instead, they get nutrients by eating other living things. Therefore, animals are heterotrophs.
Animal Cells
Like the cells of all eukaryotes, animal cells have a nucleus and other membrane-bound organelles (see Figure below). Unlike the cells of plants and fungi, animal cells lack a cell wall. This gives animal cells flexibility. It lets them take on different shapes so they can become specialized to do particular jobs. The human nerve cell shown in Figure below is a good example. Its shape suits its function of transmitting nerve impulses over long distances. A nerve cell would be unable to take this shape if it were surrounded by a rigid cell wall.
Animal Cell. The shape of an animal cell is not constrained by a rigid cell wall. A bacterial cell is shown above for comparison.
Human Nerve Cell. A human nerve cell is specialized to transmit nerve impulses. How do you think the cell’s shape helps it perform this function?
Animal Structure and Function
Animals not only have specialized cells. Most animals also have tissues and organs. In many animals, organs form organ systems, such as a nervous system. Higher levels of organization allow animals to perform many complex functions. What can animals do that most other living things cannot? Most animals share these characteristics: sensory organs, movement, and internal digestion. All of them are illustrated in Figure below.
• Animals can detect environmental stimuli, such as light, sound, and touch. Stimuli are detected by sensory nerve cells. The information is transmitted and processed by the nervous system. The nervous system, in turn, may direct the body to respond.
• All animals can move, at least during some stage of their life cycle. Muscles and nerves work together to allow movement. Being able to move lets animals actively search for food and mates. It also helps them escape from predators.
• Virtually all animals have internal digestion of food. Animals consume other organisms and may use special tissues and organs to digest them. (Many other organisms absorb nutrients directly from the environment.)
Most animals share these characteristics: sensory organs, movement, and internal digestion.
Animal Life Cycle and Reproduction
Many animals have a relatively simple life cycle. A general animal life cycle is shown in Figure below. Most animals spend the majority of their life as diploid organisms. Just about all animals reproduce sexually. Diploid adults undergo meiosis to produce sperm or eggs.Fertilization occurs when a sperm and an egg fuse. The zygote that forms develops into an embryo. The embryo eventually develops into an adult.
Animal Life Cycle. An animal life cycle that includes only sexual reproduction is shown here. Some animals also reproduce asexually. How does the animal life cycle compare with the life cycle of a plant?
Summary
• Animals are multicellular eukaryotes that lack cell walls.
• All animals are heterotrophs.
• Animals have sensory organs, the ability to move, and internal digestion. They also have sexual reproduction.
Review
1. Identify traits that characterize all animals.
2. State one way that animal cells differ from the cells of plants and fungi. What is the significance of this difference?
3. Describe animal digestion.
4. Describe a general animal life cycle.
10.02: Animal Classification
Plant or animal?
Animal. What type? Now that is a good question. This azure vase sponge is an animal, but how is it classified? It is estimated that there are easily over a million species of animals on Earth. How are all these species divided into their various classifications?
Classification of Animals
All animals share basic traits. But animals also show a lot of diversity. They range from simple sponges to complex humans.
Major Animal Phyla
Members of the animal kingdom are divided into more than 30 phyla. Table below lists the nine phyla with the greatest number of species. Each of the animal phyla listed in the table has at least 10,000 species.
Phylum Animals It Includes
Porifera sponges
Cnidaria jellyfish, corals
Platyhelminthes flatworms, tapeworms, flukes
Nematoda roundworms
Mollusca snails, clams, squids
Annelida earthworms, leeches, marine worms
Arthropoda insects, spiders, crustaceans, centipedes
Echinodermata sea stars, sea urchins, sand dollars, sea cucumbers
Chordata tunicates, lancelets, fish, amphibians, reptiles, birds, mammals
Invertebrate vs. Vertebrate
The first eight phyla listed in Table above include only invertebrate animals. Invertebrates are animals that lack a vertebral column, or backbone. The last phylum in the table, the Chordata, also includes many invertebrate species. Tunicates and lancelets are both invertebrates. Altogether, invertebrates make up at least 95 percent of all animal species. The remaining animals are vertebrates. Vertebrates are animals that have a backbone. All vertebrates belong to the phylum Chordata. They include fish, amphibians, reptiles, birds, and mammals.
Summary
• Vertebrates have a backbone, but invertebrates do not.
• Except for the chordates, all animal phyla consist only of invertebrates.
• Chordates include both vertebrates and invertebrates.
Review
1. State how the phylum Chordata differs from other animal phyla.
2. Compare and contrast invertebrates and vertebrates. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/10%3A_Animals/10.01%3A_Animal_Characteristics.txt |
What is this elephant doing?
Obviously he is spraying the zebras with water. Why? Is this elephant playing or is there another reason he is spraying the zebras? The elephant is actually trying to keep the zebras away from the waterhole. This is considered an animal behavior.
Animal Behavior
Did you ever see a dog sit on command? Have you ever watched a cat trying to catch a mouse? These are just two examples of the many behaviors of animals. Animal behavior includes all the ways that animals interact with each other and the environment. Examples of common animal behaviors are pictured in Figure below.
Examples of Animal Behavior. Can you think of other examples of animal behavior besides the three shown here?
The branch of biology that studies animal behavior is called ethology. Ethologists usually study how animals behave in their natural environment, rather than in a lab. They generally try to answer four basic questions about the behaviors they observe:
1. What causes the behavior? What is the stimulus, or trigger, for the behavior? What structures and functions of the animal are involved in the behavior?
2. How does the behavior develop? Is it present early in life? Or does it appear only as the animal matures? Are certain experiences needed for the behavior to develop?
3. Why did the behavior evolve? How does the behavior affect the fitness of the animal performing it? How does it affect the survival of the species?
4. How did the behavior evolve? How does it compare with similar behaviors in related species? In what ancestor did the behavior first appear?
Evolution of Animal Behavior
To the extent that behaviors are controlled by genes, they may evolve through natural selection. If behaviors increase fitness, they are likely to become more common over time. If they decrease fitness, they are likely to become less common.
Nature vs. Nurture
Some behaviors seem to be controlled solely by genes. Others appear to be due to experiences in a given environment. Whether behaviors are controlled mainly by genes or by the environment is often a matter of debate. This is called the nature-nurture debate. Nature refers to the genes an animal inherits. Nurture refers to the environment that the animal experiences.
In reality, most animal behaviors are not controlled by nature or nurture alone. Instead, they are influenced by both nature and nurture. In dogs, for example, the tendency to behave toward other dogs in a certain way is probably controlled by genes. However, the normal behaviors can’t develop in an environment that lacks other dogs. A puppy raised in isolation from other dogs may never develop the normal behaviors. It may always fear other dogs or act aggressively toward them.
How Behaviors Evolve
It’s easy to see how many common types of behavior evolve. That’s because they obviously increase the fitness of the animal performing them. For example, when wolves hunt together in a pack, they are more likely to catch prey (see Figure below). Therefore, hunting with others increases a wolf’s fitness. The wolf is more likely to survive and pass its genes to the next generation by behaving this way.
Wolves hunt together in packs. This is adaptive because it increases their chances of killing prey and obtaining food.
The evolution of certain other types of behavior is not as easy to explain. An example is a squirrel chattering loudly to warn other squirrels that a predator is near. This is likely to help the other squirrels avoid the predator. Therefore, it could increase their fitness. But what about the squirrel raises the alarm? This squirrel is more likely to be noticed by the predator. Therefore, the behavior may actually lower this squirrel’s fitness. How could such a behavior evolve through natural selection?
One possible answer is that helping others often means helping close relatives. Close relatives share many of the same genes that they inherited from their common ancestor. As a result, helping a close relative may actually increase the chances that copies of one’s own genes will be passed to the next generation. In this way, a behavior that puts oneself at risk could actually increase through natural selection. This form of natural selection is called kin selection.
Science Friday: When Eels Attack!
Electric eels have been known to jump out of the water and shock animals like horses. What would cause this extraordinary behavior? In this video by Science Friday, Kenneth Catania of Vanderbilt University discusses this behavior and the experiments that demonstrate this.
Science Friday: Run, Octopus, Run!
Octopus move around using a variety of motions. One unique motion they can perform is to “run” backwards on two arms. In this video by Science Friday, Chrissy Huffard at Stanford explains the possible use of this behavior
Summary
• Most animal behaviors are controlled by both genes and experiences in a given environment.
• To the extent that behaviors are controlled by genes, they may evolve.
• Behaviors that improve fitness increase through natural selection.
Review
1. Define animal behavior.
2. What is the nature-nurture debate?
3. What is kin selection?
4. How do behaviors become common? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/10%3A_Animals/10.03%3A_Evolution_of_Animal_Behavior.txt |
Is this Blue-footed Booby actually dancing?
The courtship of the Blue-footed Booby consists of the male flaunting his blue feet and dancing to impress the female. During the dance, the male will spread his wings and stamp his feet on the ground. Are these birds born with this skill, or do they learn it?
Innate Behavior
Behaviors that are closely controlled by genes with little or no environmental influence are called innate behaviors. These are behaviors that occur naturally in all members of a species whenever they are exposed to a certain stimulus. Innate behaviors do not have to be learned or practiced. They are also called instinctive behaviors. An instinct is the ability of an animal to perform a behavior the first time it is exposed to the proper stimulus. For example, a dog will drool the first time—and every time—it is exposed to food.
Significance of Innate Behavior
Innate behaviors are rigid and predictable. All members of the species perform the behaviors in the same way. Innate behaviors usually involve basic life functions, such as finding food or caring for offspring. Several examples are shown in Figure below. If an animal were to perform such important behaviors incorrectly, it would be less likely to survive or reproduce.
Examples of Innate Behavior. These innate behaviors are necessary for survival or reproduction. Can you explain why each behavior is important?
Intelligence and Innate Behavior
Innate behaviors occur in all animals. However, they are less common in species with higher levels of intelligence. Humans are the most intelligent species, and they have very few innate behaviors. The only innate behaviors in humans are reflexes. A reflex is a response that always occurs when a certain stimulus is present. For example, a human infant will grasp an object, such as a finger, that is placed in its palm. The infant has no control over this reaction because it is innate. Other than reflexes such as this, human behaviors are learned–or at least influenced by experience—rather than being innate.
Innate Behavior in Human Beings
All animals have innate behaviors, even human beings. Can you think of human behaviors that do not have to be learned? Chances are, you will have a hard time thinking of any. The only truly innate behaviors in humans are called reflex behaviors. They occur mainly in babies. Like innate behaviors in other animals, reflex behaviors in human babies may help them survive.
An example of a reflex behavior in babies is the sucking reflex. Newborns instinctively suck on a nipple that is placed in their mouth. It is easy to see how this behavior evolved. It increases the chances of a baby feeding and surviving. Another example of a reflex behavior in babies is the grasp reflex (Figure below). Babies instinctively grasp an object placed in the palm of their hand. Their grip may be surprisingly strong. How do you think this behavior might increase a baby’s chances of surviving?
One of the few innate behaviors in human beings is the grasp reflex. It occurs only in babies.
Summary
• Innate behaviors are instinctive. They are controlled by genes and always occur in the same way.
• Innate behaviors do not have to be learned or practiced.
• Innate behaviors generally involve basic life functions, so it’s important that they be performed correctly.
Review
1. What are innate behaviors? Give an example.
2. What would happen to an individual who poorly performs innate behaviors?
3. What is an instinct? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/10%3A_Animals/10.04%3A_Innate_Behavior_of_Animals.txt |
Fighting or playing?
You might think that these young tigers are fighting, but they’re really just playing. Like most other young mammals, tigers like to play. Why do mammals play? Is playing just for fun, or does it serve some other purpose as well? Playing is actually an important way of learning. By playing, these tigers are learning moves that will help them become successful predators as adults. Playing is just one of many ways that mammals and other animals learn how to behave.
Learned Behavior
Learning is a change in behavior that occurs as a result of experience. Compared with innate behaviors, learned behaviors are more flexible. They can be modified to suit changing conditions. This may make them more adaptive than innate behaviors. For example, drivers may have to modify how they drive (a learned behavior) when roads are wet or icy. Otherwise, they may lose control of their vehicle.
Animals may learn behaviors in a variety of ways. Some ways are quite simple. Others are more complex. Several types of learning are described in Figure below.
Types of Learning. Five different ways that animals may learn behaviors are shown here. What have you learned in each of these ways?
Insight Learning
Insight learning, which is based on past experience and reasoning, is a hallmark of the human animal. Humans have used insight learning to solve problems ranging from starting a fire to traveling to the moon. It usually involves coming up with new ways to solve problems. Insight learning generally happens quickly. An animal has a sudden flash of insight. Insight learning requires relatively great intelligence. Human beings use insight learning more than any other species. They have used their intelligence to solve problems ranging from inventing the wheel to flying rockets into space.
Think about problems you have solved. Maybe you figured out how to solve a new type of math problem or how to get to the next level of a video game. If you relied on your past experiences and reasoning to do it, then you were using insight learning.
One type of insight learning is making tools to solve problems. Scientists used to think that humans were the only animals intelligent enough to make tools. In fact, tool-making was believed to set humans apart from all other animals.
In 1960, primate expert Jane Goodall discovered that chimpanzees also make tools. She saw a chimpanzee strip leaves from a twig. Then he poked the twig into a hole in a termite mound. After termites climbed onto the twig, he pulled the twig out of the hole and ate the insects clinging to it. The chimpanzee had made a tool to “fish” for termites. He had used insight to solve a problem. Since then, chimpanzees have been seen making several different types of tools. For example, they sharpen sticks and use them as spears for hunting. They use stones as hammers to crack open nuts.
Scientists have also observed other species of animals making tools to solve problems. A crow was seen bending a piece of wire into a hook. Then the crow used the hook to pull food out of a tube.
An example of a gorilla using a walking stick is shown below (Figure below). Behaviors such as these show that other species of animals can use their experience and reasoning to solve problems. They can learn through insight.
This gorilla is using a branch as a tool. She is leaning on it to keep her balance while she reaches down into swampy water to catch a fish.
Summary
• Learning is a change in behavior that occurs as a result of experience.
• Learned behaviors are adaptive because they are flexible. They can change if the environment changes.
• Behaviors can be learned in several different ways, including through play.
Review
1. What is learning?
2. Name three types of learning in animals.
3. Compare and contrast instinct and learning. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/10%3A_Animals/10.05%3A_Learned_Behavior_of_Animals.txt |
Playing or fighting?
This display of aggression may be over a mate or land. But they definitely are not playing. This fight will continue until one is badly injured and flees, or the fight may continue to the death.
Social Behaviors
Different types of behavior evolved in animals because the behaviors helped them survive or reproduce. In many species, animals live together in a close-knit group with other members of their species. Such a group is referred to as a society. Animals that live in a society are known as social animals. They live and work together for the good of the group. This is called cooperation. Generally, each member of the group has a specific role that it plays in the society. Cooperation allows the group to do many things that a lone animal could never do. Look at the ants in Figure below. By working together, they are able to carry a large insect back to the nest to feed other members of their society.
Cooperation in a Social Insect. These ants are cooperating in a task that a single ant would be too small to do alone.
Communication
For individuals to cooperate, they need to communicate. Animals can communicate with sounds, chemicals, or visual cues. For example, to communicate with sounds, birds sing and frogs croak. Both may be communicating that they are good mates. Ants communicate with chemicals called pheromones. For example, they use the chemicals to mark trails to food sources so other ants can find them. Male dogs use pheromones in urine to mark their territory. They are “telling” other dogs to stay out of their yard. You can see several examples of visual communication in Figure below.
Visual Communication in Animals. Many animals use visual cues to communicate.
Aggression
Aggression is behavior that is intended to cause harm or pain. It may involve physical violence against other individuals. For example, two male gorillas may fight and use their canine teeth to inflict deep wounds. Expressing aggression this way may lead to serious injury and even death.
In many species display behaviors, rather than actual physical attacks, are used to show aggression. This helps prevent injury and death. Male gorillas, for example, are more likely to put on a display of aggression than to attack another male. In fact, gorillas have a whole series of display behaviors that they use to show aggression. They beat on their chest, dash back and forth, and pound the ground with their hands.
Competition
Aggressive behavior often occurs when individuals compete for the same resources. Animals may compete for territory, water, food, or mates. There are two basic types of competition: intraspecific and interspecific.
• Intraspecific competition occurs between members of the same species. For example, two male deer may compete for mates by clashing their antlers together.
• Interspecific competition occurs between members of different species. For example, one species of ant may attack and take over the colony of another ant species.
Summary
• Types of animal behavior include social behaviors such as cooperation and communication.
• Competition may lead to aggressive behaviors or displays of aggression.
• Review
• Explain why communication is necessary for social living.
• Describe communication in ants.
• What is aggression?
• What type of competition occurs between two dogs?
• Create a bulletin board or brief video to demonstrate the role of facial expressions in human communication. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/10%3A_Animals/10.06%3A_Social_Behavior_of_Animals.txt |
The seventh greatest wonder in the world. Really?
The Masai Mara Wildebeest Migration. The annual wildebeest migration in Kenya and Tanzania is arguably the most spectacular natural event in Africa. More than 3 million large mammals have made the vast Masai Mara and Serengeti plains their home, and each summer they migrate, looking for greener pastures. And yes, some call it "the seventh greatest wonder in the world."
Cyclic Behaviors
Many animal behaviors occur in a regular cycle. Two types of cyclic behaviors are circadian rhythms and migration.
Circadian rhythms are regular changes in biology or behavior that occur in a 24-hour cycle. In humans, for example, blood pressure and body temperature change in a regular way throughout each 24-hour day. Animals may eat and drink at certain times of day as well. Humans have daily cycles of behavior, too. Most people start to get sleepy after dark and have a hard time sleeping when it is light outside. In many species, including humans, circadian rhythms are controlled by a tiny structure called the biological clock. This structure is located in a gland at the base of the brain. The biological clock sends signals to the body. The signals cause regular changes in behavior and body processes. The amount of light entering the eyes helps control the biological clock. The clock causes changes that repeat every 24 hours.
Migration refers to seasonal movements of animals from one area to another. Migrants typically travel long distances, and travel the same paths each seasonal cycle. Usually, the migrants move to another area in order to find food or mates. Many birds, fish, and insectsmigrate. Mammals such as whales and caribou migrate as well. Figure below shows the migration route of a bird called a godwit. Another example of a behavior with a yearly cycle is hibernation. Hibernation is a state in which an animal’s body processes are slower than usual, and its body temperature falls. An animal uses less energy than usual during hibernation. This helps the animal survive during a time of year when food is scarce. Hibernation may last for weeks or months. Animals that hibernate include species of bats, squirrels, and snakes. Most people think that bears hibernate. In fact, bears do not go into true hibernation. In the winter, they go into a deep sleep. However, their body processes do not slow down very much. Their body temperature also remains about the same as usual. Bears can be awakened easily from their winter sleep.
Godwit Migration Route. Godwits make this incredibly long journey twice a year. In the fall, they migrate from the Arctic to Antarctica. They make the return flight in the spring.
KQED: Flyways: The Migratory Routes of Birds
For thousands of years and countless generations, migratory birds have flown the same long-distance paths between their breeding and feeding grounds. Understanding the routes these birds take, called flyways, helps conservation efforts and gives scientists better knowledge of global changes, both natural and man-made.
Summary
• Types of animal behavior include cyclic behaviors such as circadian rhythms and migration.
Review
1. What are circadian rhythms? Give an example.
2. Why is migration considered a cyclic behavior? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/10%3A_Animals/10.07%3A_Cyclic_Behavior_of_Animals.txt |
So when does the peacock extend his tail?
Peafowl are best known for the male's extravagant tail. Obviously though, a peacock cannot have his tail extended continuously. It would make it very difficult to move around. And it may be very tiring. So when does the peacock extend his tail? The peacock displays his tail as part of courtship.
Mating and Courtship
Mating refers to the union of a male and female of the same species for reproduction. The relationship between mates varies by species. Adults may have many mates, or they may mate with just one individual. Mates may stay together only while mating, for an entire breeding season, or even for life.
Females are likely to be more selective than males in choosing mates. In many species, males put on courtship displays to encourage females to choose them as mates. For example, to attract a mate, a male bowerbird builds an elaborate nest decorated with hundreds of small blue objects (see Figure below).
Bowerbird Decorating His Nest. A male bowerbird spends many hours collecting bits of blue glass and other small blue objects to decorate his nest. A female bowerbird inspects the nests of many males before choosing as a mate the male with the best nest.
Parental Care
In most species of fish, amphibians, and reptiles, parents provide no care to their offspring. In birds and mammals, on the other hand, parental care is common. Most often, the mother provides the care. However, in some species, both parents or just the father may be involved.
Parental care is generally longest and most involved in mammals. Besides feeding and protecting their offspring, parents may teach their offspring skills they will need to survive on their own. For example, meerkat adults teach their pups how to eat scorpions. They show the pups how to safely handle the poisonous insects and how to remove the stingers.
This mother killdeer is pretending she has a broken wing. She is trying to attract a predator’s attention in order to protect her chicks. This behavior puts her at risk of harm. How can it increase her fitness?
Defending Territory
Some species of animals are territorial. This means that they defend their area. The area they defend usually contains their nest and enough food for themselves and their offspring. A species is more likely to be territorial if there is not very much food in their area. Animals generally do not defend their territory by fighting. Instead, they are more likely to use display behavior. The behavior tells other animals to stay away. It gets the message across without the need for fighting. Display behavior is generally safer and uses less energy than fighting. Male gorillas use display behavior to defend their territory. They pound on their chests and thump the ground with their hands to warn other male gorillas to keep away from their area. The robin displays his red breast to warn other robins to stay away (Figure below).
The red breast of this male robin is easy to see. The robin displays his bright red chest to defend his territory. It warns other robins to keep out of his area.
Some animals deposit chemicals to mark the boundary of their territory. This is why dogs urinate on fire hydrants and other objects. Cats may also mark their territory by depositing chemicals. They have scent glands in their face. They deposit chemicals by rubbing their face against objects.
Summary
• Behaviors relating to reproduction include mating, courtship, and parenting behaviors.
Review
1. Describe an example of courtship behavior in animals.
2. In what vertebrates is parental care common?
3. Give two examples of parental care. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/10%3A_Animals/10.08%3A_Reproductive_Behavior_of_Animals.txt |
Thumbnail: Scanning Electron Microscope image of an ant. (Public Domain; ).
11: Invertebrates
What type of animal is a Sea Cucumber?
That is a Sea Cucumber, and it is an animal. The Sea Cucumber is a soft-bodied invertebratethat is related to the starfish and sea urchins. There are over 1,100 species that are found worldwide in intertidal zones as well as deep waters.
Characteristics of Invertebrates
The majority of animals today are invertebrates. They have a wide range of physical traits and ways of life. Modern invertebrates include animals as different as the sponge and tarantula. Why are both of these animals classified as invertebrates? What traits do they have common?
Examples of Invertebrates. Both a sponge (left) and tarantula (right) are invertebrates. Can you identify any traits they share?
One trait invertebrates like the sponge and tarantula share is lack of a backbone. In fact, they don’t have any bones at all. These are defining traits of all invertebrates. Some invertebrates have a skeleton, but it isn’t made of bone. Many other traits of invertebrates show considerable diversity.
Digestion
Invertebrates have one of two types of digestive system: an incomplete or complete digestive system. Both are shown in Figure below. An incomplete digestive system consists of a digestive cavity with one opening. The single opening serves as both mouth and anus. A complete digestive system consists of a digestive tract with two openings. One opening is the mouth. The other is the anus.
Two Types of Digestive Systems in Invertebrates. On the left is an incomplete digestive system, found in a jellyfish; on the right is the complete digestive system of a roundworm. Invertebrates may have either of these two types of digestive system. Find the parts of each digestive system in each drawing. How do the two systems differ?
Movement
All invertebrates can move on their own during at least some stage of their life cycle. However, they may differ in how they move. Several ways are described below.
• Some invertebrates are simply carried along by water currents. They cannot control their movement in a particular direction. An example is a jellyfish.
• Other invertebrates can contract muscles to move independently of water currents or on solid surfaces. They can also control the direction in which they move. An example is a roundworm. It can move forward and to the left or right.
• Still other invertebrates have specialized appendages for movement. For example, they may have jointed legs for walking or climbing or wings for flying. An example is an insect such as a fly.
Nervous System
Most invertebrates have a nervous system. The nervous system allows them to sense and respond to their environment. The simplest invertebrate nervous system is just a network of nerves that can sense touch, called a nerve net (see Figure below). Most invertebrates have a more complex nervous system. It may include a brain and several different sense organs.
The nervous system of invertebrates.
Reproduction
Most invertebrates reproduce sexually. Diploid adults produce haploid gametes (sperm and eggs). In some species, the same individuals produce both sperm and eggs. In other species, sperm and eggs are produced by separate male and female individuals. Fertilization occurs when a sperm and an egg fuse to form a diploid zygote. The zygote develops into an embryo and eventually into a new adult organism. On the way, it may pass through one or more larval stages. A larva (plural, larvae) is a juvenile, or immature, stage of an animal. It is generally quite different in form and function from the adult form of the species. For example, the larva may be able to swim freely, whereas the adult must remain permanently attached to a solid surface.
Some invertebrates can also reproduce asexually. This may occur by fission or budding.Fission takes place when an animal simply divides into two parts. Each part then regrows the missing part. The result is two whole organisms. Budding may take place when a parent forms a small bump, or bud. The bud remains attached to the parent while it develops into a new individual.
Summary
• The majority of living animals are invertebrates. Invertebrates lack a backbone.
• Invertebrates may have an incomplete or a complete digestive system.
• Invertebrates vary in how they move and in the complexity of their nervous system.
• Most invertebrates reproduce sexually. After hatching, many invertebrates pass through one or more larval stages that are different from the adult stage.
Review
1. Describe the range of variation in the nervous systems of invertebrates.
2. Compare and contrast incomplete and complete digestive systems.
3. Describe fission and budding.
4. Create a diagram to show the life cycle of an invertebrate with a larval stage. Include simple sketches of the adult and larval stages of the animal. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.01%3A_Invertebrate_Characteristics.txt |
How many different types of beetles are there?
There are about 350,000 species of beetles spread all over the world. But let's focus on this one. Look at the detail on this Rhinoceros beetle. The horns are used in fighting other males during mating season, and for digging. The body of an adult rhino beetle is covered by a thick exoskeleton. A pair of thick wings lay atop another set of wings underneath, allowing the rhinoceros beetle to fly. Compare those evolutionary adaptations to a simple sponge, and the evolutionary significance of invertebrates becomes obvious.
Invertebrate Evolution
Invertebrates evolved several important traits before vertebrates even appeared. These traits are now found in just about all animals.
Multicellularity
The first animal trait to evolve was multicellularity. This was highly adaptive. Multiple cells could do different jobs. They could evolve special adaptations that allowed them to do their job really well. However, the first invertebrates still lacked tissues. Sponges represent the first organism at the multicellular stage of invertebrate evolution.
Tissues
Living cnidarians, such as jellyfish, represent the next stage of invertebrate evolution. This was the evolution of tissues. It was the first step in the evolution of organs and organ systems. At first, invertebrates developed tissues from just two embryonic cell layers. There was an outer cell layer called ectoderm and an inner cell layer called endoderm. The two cell layers allowed different types of tissues to form.
Radial Symmetry
Another trait that evolved early on was symmetry. To understand symmetry, you need to see an animal that lacks symmetry. A sponge, like the one in Figure below, lacks symmetry. This means it cannot be divided into two identical halves. A symmetrical organism, in contrast, can be divided into two identical halves. Both the coral polyp and the beetle in Figure below have symmetry.
Symmetry in Invertebrates. Sponges lack symmetry. Radial symmetry evolved first. This was followed by bilateral symmetry. How do the two types of symmetry differ?
The coral polyp in Figure above has radial symmetry. This was the first type of symmetry to evolve. The coral has a distinct top and bottom but not distinct ends. It can be divided into identical halves like a pie, but not into right and left halves. Animals with radial symmetry have no sense of directions such as forward and backward or left and right. This makes controlled movement in these directions impossible.
Cephalization
Flatworms represent the next stage of invertebrate evolution. They evolved cephalization.This is the concentration of nerve tissue at one end of the body, forming a head region. This is highly adaptive. It allows central control of the entire organism. Cephalization was first step in the evolution of a brain.
Bilateral Symmetry
An outcome of cephalization was bilateral symmetry. This is demonstrated by the beetle in Figure above. With concentrated nerve tissue at the head but not at the tail end, the two ends of the body are distinct from each other. The animal can be divided down the middle to form identical right and left halves. It allows the animal to tell front from back and left from right. This is needed for controlled movements in these directions.
Mesoderm
Ancestors of flatworms also evolved mesoderm. This is a third layer of cells between the ectoderm and the endoderm (see Figure below). Evolution of this new cell layer allowed animals to develop new types of tissues, such as muscle.
Three Cell Layers in a Flatworm. A flatworm has three cell layers.
Complete Digestive System
Early invertebrates had an incomplete digestive system. There was just one opening for the mouth and anus. Ancestors of modern roundworms were the first animals to evolve acomplete digestive system. With a separate mouth and anus, food could move through the body in just one direction. This made digestion more efficient. An animal could keep eating while digesting food and getting rid of waste. Different parts of the digestive tract could also become specialized for different digestive functions. This led to the evolution of digestive organs.
Pseudocoelom and Coelom
Ancestors of roundworms also evolved a pseudocoelom. This is a partial body cavity that is filled with fluid. It allows room for internal organs to develop. The fluid also cushions the internal organs. The pressure of the fluid within the cavity provides stiffness. It gives the body internal support, forming a hydrostatic skeleton. It explains why roundworms are round and flatworms are flat. Later, a true coelom evolved. This is a fluid-filled body cavity, completely enclosed by mesoderm. It lies between the digestive cavity and body wall (see Figure below). Invertebrates with a true coelom include mollusks and annelids.
Cross Section of an Invertebrate with a Coelom. The coelom forms within the mesoderm.
Segmented Body
Segmentation evolved next. This is a division of the body into multiple segments. Both the earthworm and ant pictured in Figure below have segmented bodies. This trait increases flexibility. It permits a wider range of motion. All annelids and arthropods are segmented. Arthropods also evolved jointed appendages. For example, they evolved jointed legs for walking and “feelers” (antennae) for sensing.
Segmented Invertebrates. Earthworm (Annelid) and Black Ant (Arthropod). An earthworm consists of many small segments. An ant has three larger segments. Notice the ants jointed legs and “feelers.”
Notochord
Some invertebrates evolved a notochord. This is the stiff support rod in a chordate. The first chordates were probably similar to modern invertebrate chordates. The sea squirt in Figure below is an example. Later, some invertebrate chordates evolved into vertebrates.
Notochord. A sea squirt is an invertebrate with a notochord.
Summary
• Many important traits evolved in invertebrates. They include: multicellularity, tissues and organs, radial and bilateral symmetry, cephalization, mesoderm, complete digestive system, coelom, segmented body, and notochord.
Review
1. Distinguish among asymmetry, radial symmetry, and bilateral symmetry.
2. Define cephalization. What is its relationship to bilateral symmetry?
3. What is mesoderm? Name an invertebrate with mesoderm.
4. Define coelom. What invertebrates have a true coelom?
5. What is segmentation? Why is it advantageous?
6. Compare and contrast incomplete and complete digestive systems. Why is a complete digestive system more efficient? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.02%3A_Invertebrate_Evolution.txt |
So what exactly is a sponge?
Here we have a giant barrel sponge. How can something that looks like that be considered an animal? Where's the head? Where are the legs? Where's the mouth?
Sponges
Invertebrates are animals without a backbone. They are the most numerous animals on Earth. Most invertebrates are insects. However, simpler invertebrates evolved before insects. Some, like the sponges you will read about in this concept, have existed virtually unchanged for hundreds of millions of years. Their continued existence is evidence that they are well adapted for their habitats. They also evolved some of the most important traits that are found in almost all animals today. Without the traits that evolved in sponges and other simple invertebrates, you would not exist. Sponges are aquatic invertebrates that make up the phylum Porifera. The word "porifera" means pore-bearing. The phylum is aptly named. As you can see from Figure below, a sponge has a porous body. There are at least 5,000 living species of sponges. Almost all of them inhabit the ocean, living mainly on coral reefs or the ocean floor.
Sponge on a Coral Reef. This orange sponge is covered with pores. Can you predict the function of the pores?
Structure and Function of Sponges
Sponges come in a variety of shapes and sizes. For example, they may be shaped like tubes, fans, cones, or just blobs. They range in diameter from about a centimeter (0.4 inches) to over a meter (3.3 feet). Many species live in colonies that may be quite large. Adult sponges are sessile. This means they are unable to move from place to place. Root-like projections anchor them to solid surfaces such as rocks and reefs.
Sponges have an internal skeleton that gives them support and protection. An internal skeleton is called an endoskeleton. A sponge endoskeleton consists of short, sharp rods called spicules (see Figure below). Spicules are made of silica, calcium carbonate, or spongin, a tough protein. They grow from specialized cells in the body of the sponge.
Sponge Anatomy. A sponge lacks tissues and organs, but it has several types of specialized cells.
Sponges are filter feeders. They pump water into their body through their pores. The water flows through a large central cavity called the spongocoel (see Figure above). As the water flows by, specialized collar cells (which are also known as choanocytes) filter out food particles such as bacteria. Collar cells have tiny hairs that trap the particles. They also have a flagellum that whips the water and keeps it moving. Once the food is trapped, the collar cells digest it (see Figure below). Cells called amebocytes also help digest the food. They distribute the nutrients to the rest of the body as well. Finally, the water flows back out of the body through an opening called the osculum. As water flows through the sponge, oxygen diffuses from the water to the sponge’s cells. The cells also expel wastes into the water for removal through the osculum.
Collar Cell. The collar cells of sponges trap and digest food.
Sponge Reproduction
Sponges reproduce both asexually and sexually. Asexual reproduction occurs by budding.Figure below shows the sponge life cycle when sexual reproduction is involved. Adult sponges produce eggs and sperm. In many species, the same individuals produce both. However, they don’t produce eggs and sperm at the same time. As a result, self-fertilization is unlikely to occur. What is an advantage of avoiding self-fertilization?
The sponge life cycle includes sexual reproduction. Sponges may also reproduce asexually.
Sperm are released into the surrounding water through the osculum. If they enter a female sponge through a pore, they may be trapped by collar cells. Trapped sperm are delivered to eggs inside the female body, where fertilization takes place. The resulting zygote develops into a larva. Unlike the adult, the larva is motile. It is covered with cilia that propel it through the water. As the larva grows, it becomes more similar to an adult sponge and loses its ability to swim.
Ecology of Sponges
Sponges that live on coral reefs have symbiotic relationships with other reef species. They provide shelter for algae, shrimp, and crabs. In return, they get nutrients from the metabolism of the organisms they shelter. Sponges are a source of food for many species of fish. Because sponges are sessile, they cannot flee from predators. Their sharp spicules provide some defense. They also produce toxins that may poison predators that try to eat them.
Summary
• Sponges are aquatic invertebrates. They make up the phylum Porifera.
• Sponges have specialized cells and an endoskeleton.
• Sponges lack tissues and body symmetry.
• Adult sponges are sessile filter feeders.
• Sponge larvae have cilia for swimming.
Review
1. Define sessile. Name an invertebrate with a sessile adult stage.
2. Describe the skeleton of a sponge.
3. Sponges have specialized cells called collar cells. Describe how collar cells are specialized for the functions they serve.
4. Create a diagram of an adult sponge body plan that shows how sponges obtain food. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.04%3A_Sponges.txt |
The sea anemone. Plant or animal?
It may look like a plant, but it's not. Sea anemones are a group of water-dwelling, predatory animals in the phylum Cnidaria. A sea anemone is a polyp attached at the bottom to the surface beneath it. They can have anywhere from a few tens of tentacles to a few hundred tentacles. And they eat small fish and shrimp.
Cnidarians
Cnidarians are invertebrates such as jellyfish and corals. They belong to the phylum Cnidaria. All cnidarians are aquatic. Most of them live in the ocean. Cnidarians are a little more complex than sponges. They have radial symmetry and tissues. There are more than 10,000 cnidarianspecies. They are very diverse, as shown in Figure below.
Cnidarian Diversity. Cnidarians show a lot of variability.
Structure and Function of Cnidarians
All cnidarians have something in common. It’s a nematocyst, like the one shown in Figure below. A nematocyst is a long, thin, coiled stinger. It has a barb that may inject poison. These tiny poison "darts" are propelled out of special cells. They are used to attack prey or defend against predators.
Cnidarian Nematocyst. A cnidarian nematocyst is like a poison dart. It is ejected from a specialized cell.
There are two basic body plans in cnidarians. They are called the polyp and medusa. Both are shown in Figure below. The polyp has a tubular body and is usually sessile. The medusa(plural, medusae) has a bell-shaped body and is typically motile. Some cnidarian species alternate between polyp and medusa forms. Other species exist in just one form or the other.
Cnidarian Body Plans. Cnidarians may exist in the polyp (left) or medusa (right) form.
The body of a cnidarian consists of two cell layers, ectoderm and endoderm. The cellssurround a digestive cavity called the coelenteron (see Figure below). Cnidarians have a simple digestive system. The single opening is surrounded by tentacles, which are used to capture prey. The tentacles are covered with nematocyst cells. Digestion takes place in the coelenteron. Nutrients are absorbed and gases exchanged through the cells lining this cavity. Fluid in the coelenteron creates a hydrostatic skeleton.
Cnidarians have a simple nervous system consisting of a nerve net that can detect touch. They may also have other sensory structures. For example, jellyfish have light-sensing structures and gravity-sensing structures. These senses give them a sense of up versus down. It also helps them balance.
Cnidarian Reproduction
Figure below shows a general cnidarian life cycle. Polyps usually reproduce asexually. One type of asexual reproduction in polyps leads to the formation of new medusae. Medusae usually reproduce sexually. Sexual reproduction forms a zygote. The zygote develops into a larva called a planula. The planula, in turn, develops into a polyp. There are many variations on the general life cycle. Obviously, species that exist only as polyps or medusae have a life cycle without the other form.
General Cnidarian Life Cycle. Cnidarians may reproduce both asexually and sexually.
Ecology of Cnidarians
Cnidarians can be found in almost all ocean habitats. They may live in water that is shallow or deep, warm or cold. A few species live in freshwater. Some cnidarians live alone, while others live in colonies.
Corals form large colonies in shallow tropical water. They are confined to shallow water because they have a mutualistic relationship with algae that live inside them. The algae need sunlight for photosynthesis, so they must be relatively close to the surface of the water. Corals exist only as polyps. They catch plankton with their tentacles. Many secrete a calcium carbonate exoskeleton. Over time, this builds up to become a coral reef (see Figure below). Coral reefs provide food and shelter to many ocean organisms. They also help protect shorelines from erosion by absorbing some of the energy of waves. Coral reefs are at risk of destruction today.
Great Barrier Reef. The Great Barrier Reef is a coral reef off the coast of Australia.
Unlike corals, jellyfish spend most of their lives as medusae. They live virtually everywhere in the ocean. They are typically carnivores. They prey on zooplankton, other invertebrates, and the eggs and larvae of fish.
KQED: Amazing Jellies
Jellyfish. They are otherworldly creatures that glow in the dark, without brains or bones, some more than 100 feet long. And there are many different types. Jellyfish are free-swimming members of the phylum
Cnidaria. Jellyfish are found in every ocean, from the surface to the deep sea.
Summary
• Cnidarians include jellyfish and corals.
• Cnidarians are aquatic invertebrates. They have tissues and radial symmetry. They also have tentacles with stingers.
• There are two cnidarian body plans: the polyp and the medusa. They differ in several ways.
• Many corals secrete an exoskeleton that builds up to become a coral reef.
Review
1. What is a nematocyst? What is its function?
2. How do coral reefs form?
3. Compare and contrast cnidarian polyps and medusae. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.05%3A_Cnidarians.txt |
Would you believe that this gold-dotted creature is a flatworm?
No? Well it is. There are more than 25,000 different types of flatworms, so they can be very different in how they appear. And many don't even look like your typical worm.
Flatworms
Flatworms belong to the phylum Platyhelminthes. Examples of flatworms are shown in Figure below. There are more than 25,000 species in the flatworm phylum.
Platyhelminthes. Platyhelminthes include flatworms, tapeworms, and flukes.
Structure and Function of Flatworms
Flatworms range in length from about 1 millimeter (0.04 inches) to more than 20 meters (66 feet). They have a flat body because they do not have a coelom or even a pseudocoelom. They also lack a respiratory system. Instead, their cells exchange gases by diffusion directly with the environment. They have an incomplete digestive system.
Flatworms reflect several major evolutionary advances in invertebrates. They have three embryonic cell layers, including mesoderm. The mesoderm layer allows them to develop organ systems. For example, they have muscular and excretory systems. The muscular system allows them to move from place to place over solid surfaces. The excretory system lets them maintain a proper balance of water and salts. Flatworms also show cephalization and bilateral symmetry.
Flatworm Reproduction
Flatworms reproduce sexually. In most species, the same individuals produce both eggs and sperm. After fertilization occurs, the fertilized eggs pass out of the adult’s body and hatch into larvae. There may be several different larval stages. The final larval stage develops into the adult form, and the life cycle repeats.
Ecology of Flatworms
Both flukes and tapeworms are parasites with vertebrate hosts, including human hosts. Flukes live in the host’s circulatory system or liver. Tapeworms live in the host’s digestive system. Usually, more than one type of host is required to complete the parasite’s life cycle. Look at the life cycle of the liver fluke in Figure below. As an adult, the fluke has a vertebrate host. As a larva, it has an invertebrate host. If you follow the life cycle, you can see how each host becomes infected so the fluke can continue its life cycle.
Life Cycle of the Sheep Liver Fluke. The sheep liver fluke has a complicated life cycle with two hosts. How could such a complicated way of life evolve?
Tapeworms and flukes have suckers and other structures for feeding on a host. Tapeworms also have a scolex, a ring of hooks on their head to attach themselves to the host (see Figure below). Unlike other invertebrates, tapeworms lack a mouth and digestive system. Instead, they absorb nutrients directly from the host’s digestive system with their suckers.
Tapeworm Suckers and Hooks. The head of a tapeworm has several suckers. At the very top of the head is a “crown” of hooks called a scolex.
Not all flatworms are parasites. Some are free-living carnivores. They eat other small invertebrates and decaying animals. Most of the free-living species live in aquatic habitats, but some live in moist soil.
Summary
• Platyhelminthes are flatworms such as tapeworms and flukes.
• Flatworms have a mesoderm cell layer and simple organ systems. They also show cephalization and bilateral symmetry.
• Many flatworms are parasites with vertebrate hosts. Some are free-living carnivores that live mainly in aquatic habitats.
Review
1. Flatworms were the first to evolve the mesoderm. What advantage does the mesoderm provide?
2. Describe specialized feeding structures of parasitic platyhelminthes.
3. Some parasitic flatworms have a very complicated life cycle with more than one host. Infer why this might be adaptive. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.06%3A_Flatworms.txt |
When most people picture a worm, do they picture a roundworm?
Actually, they do not. Whereas flatworms are flat, roundworms obviously appear round. With over 80,000 species, there are plenty of different types of roundworms. But these are still not the types of worms most people picture when they think of worms.
Roundworms
Roundworms make up the phylum Nematoda. This is a very diverse animal phyla. It has more than 80,000 known species.
Structure and Function of Roundworms
Roundworms range in length from less than 1 millimeter to over 7 meters (23 feet) in length. As their name suggests, they have a round body. This is because they have a pseudocoelom. This is one way they differ from flatworms. Another way is their complete digestive system. It allows them to take in food, digest food, and eliminate wastes all at the same time.
Roundworms have a tough covering of cuticle on the surface of their body. It prevents their body from expanding. This allows the buildup of fluid pressure in the pseudocoelom. As a result, roundworms have a hydrostatic skeleton. This provides a counterforce for the contraction of muscles lining the pseudocoelom. This allows the worms to move efficiently along solid surfaces.
Roundworm Reproduction
Roundworms reproduce sexually. Sperm and eggs are produced by separate male and female adults. Fertilization takes place inside the female organism. Females lay huge numbers of eggs, sometimes as many as 100,000 per day! The eggs hatch into larvae, which develop into adults. Then the cycle repeats.
Ecology of Roundworms
Roundworms may be free-living or parasitic. Free-living worms are found mainly in freshwater habitats. Some live in soil. They generally feed on bacteria, fungi, protozoans, or decaying organic matter. By breaking down organic matter, they play an important role in the carbon cycle.
Parasitic roundworms may have plant, vertebrate, or invertebrate hosts. Several species have human hosts. For example, hookworms, like the one in Figure below, are human parasites. They infect the human intestine. They are named for the hooks they use to grab onto the host’s tissues. Hookworm larvae enter the host through the skin. They migrate to the intestine, where they mature into adults. Adults lay eggs, which pass out of the host in feces. Then the cycle repeats.
Hookworm Parasite. Hookworms like this one are common human parasites.
Tiny pinworms are the most common roundworm parasites of people in the U.S. In some areas, as many as one out of three children are infected. Humans become infected when they ingest the nearly microscopic pinworm eggs. The eggs hatch and develop into adults in the host’s digestive tract. Adults lay eggs that pass out of the host’s body to continue the cycle. Pinworms have a fairly simple life cycle with only one host.
Summary
• Roundworms make up the phylum Nematoda.
• Roundworms have a pseudocoelom and hydrostatic skeleton. Their body is covered with tough cuticle.
• Free-living roundworms are found mainly in freshwater habitats.
• Parasitic roundworms have a variety of hosts, including humans.
Review
1. How do free-living nematodes contribute to the carbon cycle?
2. Apply what you know about pinworms to develop one or more recommendations for preventing pinworm infections in humans.
3. Platyhelminthes and nematodes are both worms. Justify classifying them in different invertebrate phyla. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.07%3A_Roundworms.txt |
Fish or squid?
Neither. This is a mollusk, a cuttlefish to be specific. What is a mollusk? Well, to start, mollusks are aquatic species that are not fish. There are over 100,000 different mollusks, so there are bound to be some interesting looking organisms, like this one.
Mollusks
Have you ever been to the ocean or eaten seafood? If you have, then you probably have encountered members of the phylum Mollusca. Mollusks include snails, scallops, and squids, as shown in Figure below. There are more than 100,000 known species of mollusks. About 80 percent of mollusk species are gastropods.
This figure shows some of the more common and familiar mollusks.
Structure and Function of Mollusks
Mollusks are a very diverse phylum. Some mollusks are nearly microscopic. The largest mollusk, a colossal squid, may be as long as a school bus and weigh over half a ton! The basic body plan of a mollusk is shown in Figure below. The main distinguishing feature is a hard outer shell. It covers the top of the body and encloses the internal organs. Most mollusks have a distinct head region. The head may have tentacles for sensing the environment and grasping food. There is generally a muscular foot, which may be used for walking. However, the foot has evolved modifications in many species to be used for other purposes.
Basic Mollusk Body Plan. The basic body plan shown here varies among mollusk classes. For example, several mollusk species no longer have shells. Do you know which ones?
Two unique features of mollusks are the mantle and radula (see Figure above). The mantle is a layer of tissue that lies between the shell and the body. It secretes calcium carbonate to form the shell. It forms a cavity, called the mantle cavity, between the mantle and the body. The mantle cavity pumps water for filter feeding. The radula is a feeding organ with teeth made of chitin. It is located in front of the mouth in the head region. Herbivorous mollusks use the radula to scrape food such as algae off rocks. Predatory mollusks use the radula to drill holes in the shells of their prey.
Mollusks have a coelom and a complete digestive system. Their excretory system consists of tube-shaped organs called nephridia (see Figure above). The organs filter waste from body fluids and release the waste into the coelom. Terrestrial mollusks exchange gases with the surrounding air. This occurs across the lining of the mantle cavity. Aquatic mollusks “breathe” under water with gills. Gills are thin filaments that absorb gases and exchange them between the blood and surrounding water.
Mollusks have a circulatory system with one or two hearts that pump blood. The heart is a muscular organ that pumps blood through the circulatory system when its muscles contract. The circulatory system may be open or closed, depending on the species.
The major classes of mollusks vary in structure and function. You can read about some of their differences in Figure below.
Use this figure to compare and contrast gastropods, bivalves, and cephalopods
Mollusk Reproduction
Mollusks reproduce sexually. Most species have separate male and female sexes. Gametes are released into the mantle cavity. Fertilization may be internal or external, depending on the species. Fertilized eggs develop into larvae. There may be one or more larval stages. Each one is different from the adult stage. Mollusks have a unique larval form called a trochophore. It is a tiny organism with cilia for swimming.
Ecology of Mollusks
Mollusks live in most terrestrial, freshwater, and marine habitats. However, the majority of species live in the ocean. They can be found in both shallow and deep water and from tropical to polar latitudes. Mollusks are a major food source for other organisms, including humans. You may have eaten mollusks such as clams, oysters, scallops, or mussels.
The different classes of mollusks have different ways of obtaining food.
• Gastropods may be herbivores, predators, or internal parasites. They live in both aquatic and terrestrial habitats. Marine species live mainly in shallow coastal waters. Gastropods use their foot to crawl slowly over rocks, reefs, or soil, looking for food.
• Bivalves are generally sessile filter feeders. They live in both freshwater and marine habitats. They use their foot to attach themselves to rocks or reefs or to burrow into mud. Bivalves feed on plankton and nonliving organic matter. They filter the food out of the water as it flows through their mantle cavity.
• Cephalopods are carnivores that live only in marine habitats. They may be found in the open ocean or close to shore. They are either predators or scavengers. They generally eat other invertebrates and fish.
KQED: Cool Critters: Dwarf Cuttlefish
What's the coolest critter in the ocean under 4 inches long? The Dwarf Cuttlefish! Cuttlefish are marine animals that belong to the class Cephalopoda. Despite their name, cuttlefish are not fish but mollusks. Recent studies indicate that cuttlefish are among the most intelligent invertebrates, with one of the largest brain-to-body size ratios of all invertebrates. Cuttlefish have an internal shell called the cuttlebone and eight arms and two tentacles furnished with suckers, with which they secure their prey.
KQED: The Fierce Humboldt Squid
The Humboldt squid is a large, predatory invertebrate found in the waters of the Pacific Ocean. A mysterious sea creature up to 7 feet long, with 10 arms, a sharp beak and a ravenous appetite, packs of fierce Humboldt Squid attack nearly everything they see, from fish to scuba divers. Traveling in groups of 1,000 or more and swimming at speeds of more than 15 miles an hour, these animals hunt and feed together, and use jet propulsion to shoot out of the water to escape predators. Humboldt squid live at depths of between 600 and about 2,000 feet, coming to the surface at night to feed. They live for approximately two years and spend much of their short life in the ocean's oxygen-minimum zone, where very little other life exists. Because they live at such depths, little is known about these mysterious sea creatures. The Humboldt squid usually lives in the waters of the Humboldt Current, ranging from the southern tip of South America north to California, but in recent years, this squid has been found as far north as Alaska. Marine biologists are working to discover why they have headed north from their traditional homes off South America.
Where's the Octopus?
When marine biologist Roger Hanlon captured the first scene in this video, he started screaming. Hanlon, senior scientist at the Marine Biological Laboratory in Woods Hole, studies camouflage in cephalopods: squid, cuttlefish and octopuses. They are masters of optical illusion.
Summary
• Mollusks are invertebrates such as snails, scallops, and squids.
• Mollusks have a hard outer shell. There is a layer of tissue called the mantle between the shell and the body.
• Most mollusks have tentacles for feeding and sensing, and many have a muscular foot.
• Mollusks also have a coelom, a complete digestive system, and specialized organs for excretion.
• The majority of mollusks live in the ocean.
• Different classes of mollusks have different ways of obtaining food.
Review
1. List the three major classes of mollusks.
2. Describe the basic body plan of a mollusk.
3. Describe the mantle and radula.
4. What are gills? What is their function?
5. Create a Venn diagram to show important similarities and differences among the three major classes of mollusks. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.08%3A_Mollusks.txt |
How can this be an animal?
This is a worm. And even though it is blue, this is a Christmas tree worm. These "Christmas tree" structures are actually specialized mouth appendages. Each spiral is composed of feather-like tentacles which are heavily ciliated. These appendages trap prey and transport the food straight towards the worm's mouth. And these worms are annelids.
Annelids
The phylum Annelida is made up of segmented worms such as earthworms. Segmented worms are divided into many repeating segments. There are roughly 15,000 species of annelids. Most belong to one of three classes. A species in each class is pictured in Figure below.
Classes of Annelids. The majority of annelids are polychaetes. They live on the ocean floor, so you may not be familiar with them.
Structure and Function of Annelids
Annelids range in length from less than 1 millimeter to over 3 meters. They never attain the large size of some mollusks. Like mollusks, however, they have a coelom. In fact, the annelid coelom is even larger, allowing greater development of internal organs. Annelids have other similarities with mollusks, including:
• A closed circulatory system (like cephalopods).
• An excretory system consisting of tubular nephridia.
• A complete digestive system.
• A brain.
• Sensory organs for detecting light and other stimuli.
• Gills for gas exchange (but many exchange gas through their skin).
The segmentation of annelids is highly adaptive. For one thing, it allows more efficient movement. Each segment generally has its own nerve and muscle tissues. Thus, localized muscle contractions can move just those segments needed for a particular motion. Segmentation also allows an animal to have specialized segments to carry out particular functions. This allows the whole animal to be more efficient. Annelids have the amazing capacity to regrow segments that break off. This is called regeneration.
Annelids have a variety of structures on the surface of their body for movement and other functions. These vary, depending on the species. Several of the structures are described in Figure below.
Annelid External Structures. Many annelids have bristles and other types of external structures. Each structure is not present in all species.
Annelid Reproduction
Most species of annelids can reproduce both asexually and sexually. However, leeches can reproduce only sexually. Asexual reproduction may occur by budding or fission. Sexual reproduction varies by species.
• In some species, the same individual produces both sperm and eggs. But worms mate to exchange sperm, rather than self-fertilizing their own eggs. Fertilized eggs are deposited in a mucous cocoon. Offspring emerge from the cocoon looking like small adults. They grow to adult size without going through a larval stage.
• In polychaete species, there are separate sexes. Adult worms go through a major transformation to develop reproductive organs. This occurs in many adults at once. Then they all swim to the surface and release their gametes in the water, where fertilization takes place. Offspring go through a larval stage before developing into adults.
Ecology of Annelids
Annelids live in a diversity of freshwater, marine, and terrestrial habitats. They vary in what they feed on and how they obtain their food.
• Earthworms are deposit feeders. They burrow through the ground, eating soil and extracting organic matter from it. Earthworm feces, called worm casts, are very rich in plant nutrients. Earthworm burrows help aerate soil, which is also good for plants.
• Polychaetes live on the ocean floor. They may be sedentary filter feeders, active predators, or scavengers. Active species crawl along the ocean floor in search of food.
• Leeches are either predators or parasites. As predators, they capture and eat other invertebrates. As parasites, they feed off the blood of vertebrate hosts. They have a tubular organ, called a proboscis, for feeding.
Comparison of Worms
The following table compares the three worm phyla (Table below).
Phylum Common Name Body Cavity Segmented Digestive System Example
Platyhelminthes Flatworm No No Incomplete Tapeworm
Nematoda Roundworm Yes No Complete Heartworm
Annelida Segmented worm Yes Yes Complete Earthworm
Summary
• Annelids are segmented worms such as earthworms and leeches.
• Annelids have a coelom, closed circulatory system, excretory system, and complete digestive system. They also have a brain.
• Earthworms are important deposit feeders that help form and enrich soil.
• Leeches are either predators or parasites. Parasitic leeches feed off the blood of vertebrate hosts.
Review
1. Discuss the advantages of segmentation.
2. Define regeneration.
3. Polychaete worms have an interesting reproductive strategy. Describe this strategy and infer its adaptive significance.
4. What are deposit feeders? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.09%3A_Annelids.txt |
What has more species than any other animal phylum?
Arthropods are not only the largest phylum of invertebrates. They are by far the largest phylum of the animal kingdom. Roughly 80 percent of all animal species living on Earth today are arthropods. Obviously, arthropods have been extremely successful. What accounts for their success?
Arthropods
There are more than a million known species of arthropods. There may actually be ten times that many. Arthropods include insects, spiders, lobsters, and centipedes. The arthropods pictured in Figure below give just a hint of the phylum’s diversity.
Arthropod Diversity. Dust mites are among the smallest of arthropods. Japanese spider crabs are the largest. Besides size, what other differences among arthropods do you see in these photos?
Structure and Function of Arthropods
Arthropods range in length from about 1 millimeter to 4 meters (about 13 feet). They have a segmented body with a hard exoskeleton. They also have jointed appendages. The body segments are the head, thorax, and abdomen (see Figure below). In some arthropods, the head and thorax are joined together as a cephalothorax.
Arthropod Body Plan. Notice the three body segments of each organism.
The arthropod exoskeleton consists of several layers of cuticle. The exoskeleton prevents water loss and gives support and protection. It also acts as a counterforce for the contraction of muscles. The exoskeleton doesn’t grow as the animal grows. Therefore, it must be shed and replaced with a new one periodically through life. This is called molting.
The jointed appendages of arthropods may be used as legs for walking. Being jointed makes them more flexible. Try walking or climbing stairs without bending your knees, and you’ll see why joints are helpful. In most arthropods, the appendages on the head have been modified for other functions. Figure below shows some of head appendages found in arthropods. Sensory organs such as eyes are also found on the head.
Arthropod Head. Arthropods have evolved a variety of specialized appendages and other structures on their head.
Some arthropods have special excretory structures. They are called coxal glands and Malpighian tubules. Coxal glands collect and concentrate liquid waste from blood. They excrete the waste from the body through a pore. Malphigian tubules carry waste from the digestive tract to the anus. The waste is excreted through the anus.
Like mollusks and annelids, aquatic arthropods may have gills to exchange gases with thewater (discussed below). Terrestrial arthropods, on the other hand, have special respiratory structures to exchange gases with the air. These are described in Figure below.
How Terrestrial Arthropods Breathe Air. Terrestrial arthropods have respiratory structures that let them breathe air.
Underwater Spiders
In the ponds of northern Europe lives a tiny brown spider that spends its entire life underwater. But just like land spiders, it needs oxygen to breathe. So, how does this spider breath? Does it use book lungs? No. In fact, aquatic spiders, known as "diving bell spiders," have gills. Every so often, the spider leaves its underwater web to visit the surface and bring back a bubble of air that sticks to its hairy abdomen. It deposits the bubble into a little silk air tank. This "diving bell" is a gill that sucks oxygen from the water, allowing the spider to stay underwater for up to 24 hours.
Arthropod Reproduction
Arthropods have a life cycle with sexual reproduction. Most species go through larval stages after hatching. The larvae are very different from the adults. They change into the adult form in a process called metamorphosis. This may take place within a cocoon. A familiar example of metamorphosis is the transformation of a caterpillar (larva) into a butterfly (adult). Other arthropod species, in contrast, hatch young that look like small adults. These species lack both larval stages and metamorphosis.
Evolution of Arthropods
The oldest known arthropods are trilobites. A fossil trilobite is shown in Figure below. Trilobites were marine arthropods. They had many segments with paired appendages for walking. As arthropods continued to evolve, segments fused. Eventually, arthropods with three major segments evolved. Appendages were also lost or modified during the course of arthropod evolution.
Trilobite Fossil. This trilobite fossil represents the earliest arthropods. Trilobites first appeared more than 500 million years ago. They lived for at least 200 million years before going extinct. They left behind large numbers of fossils.
Arthropods were the first animals to live on land. The earliest terrestrial arthropods were probably millipedes. They moved to land about 430 million years ago. Early land arthropods evolved adaptations such as book lungs or trachea to breathe air. The exoskeleton was another important adaptation. It prevents an animal from drying out. It also provides support in the absence of buoyant water.
Classification of Arthropods
Living arthropods are divided into four subphyla. They are described in Table below. The Hexapoda subphylum includes mainly insects. There are so many insects and they are so important that they are described in greater detail below.
Subphylum (includes) Description Example
Myriapoda (centipedes, millipedes) terrestrial; herbivores or predators; 10–400 walking legs; poison claws for hunting
centipede
Chelicerata (spiders, scorpions, mites, ticks, horseshoe crabs, sea spiders) mainly terrestrial; predators or parasites; 8 walking legs; appendages called chelicerae for grasping prey; poison fangs for killing prey; no mandibles, maxillae, antennae; two body segments
spider
Crustacea (lobsters, crabs, shrimp, barnacles, krill) mainly aquatic, predators, scavengers, or filter feeders; two pairs of antennae and claws for hunting; unique larval stage (called “nauplius”) with head appendages for swimming
lobster
Hexapoda (ants, flies, grasshoppers, beetles, butterflies, moths, bees, springtails) mainly terrestrial or aerial; herbivores, predators, parasites, scavengers, or decomposers; 6 walking legs; many modified appendages, such as wings for flying
beetle
Summary
• Arthropods are the largest phylum in the animal kingdom.
• Most arthropods are insects. The phylum also includes spiders, centipedes, and crustaceans.
• The arthropod body consists of three segments with a hard exoskeleton and jointed appendages.
• Terrestrial arthropods have adaptations for life on land, such as trachea or book lungs for breathing air.
• The earliest arthropods were trilobites. The earliest land arthropods were millipedes.
Review
1. Identify the distinguishing trait of arthropods.
2. What is molting? Why does it occur?
3. What are the Malphigian tubules?
4. Describe two structures that allow arthropods to breathe air.
5. Assume you see a “bug” crawling over the ground. It has two body segments and lacks antennae. Which arthropod subphylum does the “bug” belong to? Explain your answer. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.10%3A_Arthropods.txt |
What dominates life on Earth?
Well, by numbers, it's not humans. This may look like a scary creature from your worst nightmare, but it wouldn’t hurt a fly. In fact, it is a fly! The picture shows the charming portrait of a horsefly, up close and personal. Those big, striped, colorful orbs are its eyes. Did you ever look through a kaleidoscope? If so, then you have an idea of what the world looks like to a horsefly.
What other organs do insects like this horsefly have? Besides sensing their environment, what other functions do their organs serve?
Insects
Most members of the subphylum Hexapoda are insects (class Insecta). In fact, more than half of all known organisms are insects. There may be more than 10 million insect species in the world, most of them yet to be identified. It’s clear that insects, and not humans, dominate life on Earth.
Hexapoda. A cricket on green leaf. Can you find the six legs attached to the thorax?
Structure and Function of Insects
Insects range in length from less than a millimeter to about the length of your arm. They can be found in most habitats, but they are mainly terrestrial. Many can fly, so they are also aerial. Like other arthropods, insects have a head, thorax, and abdomen. They have a wide variety of appendages, including six legs attached to the thorax.
Insects have a pair of antennae for “smelling” and “tasting” chemicals. Some insects can also use their antennae to detect sound. Other sensory organs on the head include several simple eyes and a pair of compound eyes. The compound eyes let insects see images. Butterflies and bees can even see in color. For feeding, the head contains one pair of mandibles and two pairs of maxillae. Insects consume a wide range of foods, and their mouthparts have become specialized. Several variations are shown in Figure below.
Mouthpart Specialization in Insects. The mouthparts of insects are adapted for different food sources. How do you think the different mouthparts evolved? (CC BY-NC 3.0; Christopher Auyeung - CK-12 Foundation).
An insect’s abdomen contains most of the internal organs. Like other arthropods, insects have a complete digestive system. They also have an open circulatory system and central nervous system. Like other terrestrial arthropods, they have trachea for breathing air and Malpighian tubules for excretion.
Insect Flight
The main reason that insects have been so successful is their ability to fly. Insects are the only invertebrates that can fly and were the first animals to evolve flight. Flight has important advantages. It’s a guaranteed means of escape from nonflying predators. It also aids in the search for food and mates.
Insects generally have two pairs of wings for flight. Wings are part of the exoskeleton and attached to the thorax. Insect wings show a lot of variation. As you can see in Figure below, butterfly wings are paper-thin, whereas beetle wings are like armor. Not all insect wings work the same way, either. They differ in how the muscles are attached and whether the two pairs of wings work independently or together. Besides flight, wings serve other functions. They may protect the body (beetles), communicate visually with other insects (butterflies), or produce sounds to attract mates (katydids).
Form and Function in Insect Wings. Beetles, butterflies, and katydids all have two pairs of wings that they use for flight. However, the wings are very different because they have other functions as well.
Insect Reproduction
Nearly all insects reproduce sexually. Some can also reproduce asexually. An example of an insect life cycle is shown in Figure below.
Insect Life Cycle. This diagram represents the life cycle of a mosquito. Most insects have a similar life cycle.
When an insect egg hatches, a larva emerges. The larva eats and grows and then enters the pupa stage. The pupa is immobile and may be encased in a cocoon. During the pupa stage, the insect goes through metamorphosis. Tissues and appendages of the larva break down and reorganize into the adult form. How did such an incredible transformation evolve? Metamorphosis is actually very advantageous. It allows functions to be divided between life stages. Each stage can evolve adaptations to suit it for its specific functions without affecting the adaptations of the other stage.
Insect Behavior
Insects are capable of a surprising range of behaviors. Most of their behaviors, such as flying and mating, are instinctive. These are behaviors that don’t need to be learned. They are largely controlled by genes. However, some insect behaviors are learned. For example, ants and bees can learn where food is located and keep going back for more.
Many species of insects have evolved complex social behaviors. They live together in large, organized colonies (see Figure below). This is true of ants, termites, bees, and wasps. Colonies may include millions of individual insects. Colony members divide up the labor of the colony. Different insects are specialized for different jobs. Some reproduce, while others care for the young. Still others get food or defend the nest.
Termite Nest. This cathedral-like structure is the nest of a huge colony of termites in Australia. In fact, it is the world’s largest known termite nest. It towers 7.5 meters (25 feet) above the ground and houses millions of termites.
Living in a large colony requires good communication. Ants communicate with chemicals called pheromones. For example, an ant deposits pheromones on the ground as it returns to the nest from a food source. It is marking the path so other ants can find the food. Honeybees communicate by doing a “waggle dance.”
KQED: Ants: The Invisible Majority
Most of us think ants are just pests. But not Brian Fisher. Known as “The Ant Guy,” he's on a mission to show the world just how important and amazing these little creatures are. In the process, he hopes to catalog all of the world's 30,000 ant species before they become casualties of habitat loss.
KQED: Ladybugs: A Population of Millions
Ladybugs, also known as ladybird beetles, have a life cycle of four to six weeks. In one year as many as six generations of ladybird beetles may hatch. In the spring, each adult female lays up to 300 eggs in small clusters on plants where aphids are present. After a week the wingless larvae hatch. Both the ladybird beetle larvae and adults are active predators, eating only aphids, scales, mites and other plant-eating insects. The ladybugs live on the vegetation where their prey is found, which includes roses, oleander, milkweed and broccoli. Adult ladybugs don’t taste very good. A bird careless enough to try to eat one will not swallow it.
By late May to early June, when the larvae have depleted the food supply, the adults migrate to the mountains. There, they eat mainly pollen. The ladybugs gain fat from eating the pollen and this tides them over during their nine-month hibernation. Thousands of adults hibernate overwinter in tight clusters, called aggregates, under fallen leaves and ground litter near streams. In the clear, warmer days of early spring, the ladybugs break up the aggregates and begin several days of mating.
Insects and Humans
Most humans interact with insects every day. Many of these interactions are harmless and often go unnoticed. However, insects cause humans a lot of harm. They spread human diseases. For example, the deadly bubonic plague of the middle ages was spread by fleas. Today, millions of people die each year from malaria, which is spread by mosquitoes. Insects also eat our crops. Sometimes they travel in huge swarms that completely strip the land of all plant material (see Figure below). On the other hand, we depend on insects for the very food we eat. Without insects to pollinate them, flowering plants—including many food crops—could not reproduce.
Locust Swarm. A swarm of locusts in the African country of Mauritania darkens the mid-day sky. The hungry insects will eat virtually all the plants in their path.
KQED: Better Bees: Super Bee and Wild Bee
Honeybees are one of the most well-known insects on the planet. Bees are naturalized on every continent except Antarctica. Honeybees have a highly developed social structure and depend on their community, or colony, for survival, with a colony containing up to 20,000 bees. When bees search plants for nectar, pollen sticks to the fuzzy hairs that cover their hind legs. At the next flower, some of the pollen rubs off and fertilizes that flower. In this way, bees help improve fruit production. Bees pollinate an estimated 130 different varieties of fruit, flowers, nuts and vegetables in the United States alone. Farmers obviously depend on bees to pollinate crops, such as fruit and nuts, but in recent years thousands of bee colonies have disappeared. This could be a devastating issue for farmers. Can anything be done? Meet two Northern California researchers looking for ways to make sure we always have bees to pollinate crops.
Summary
• Insects are arthropods in the class Hexapoda. They are the most numerous organisms in the world.
• Most insects are terrestrial, and many are aerial.
• Insects have six legs and a pair of antennae for sensing chemicals. They also have several eyes and specialized mouthparts for feeding.
• Insects are the only invertebrates than can fly. Flight is the main reason for their success.
• Insects may live in large colonies and have complex social behaviors.
• Insects spread disease and destroy crops. However, they are essential for pollinating flowering plants.
Review
1. List two traits that characterize insects.
2. State two important advantages of flight in insects.
3. Give examples of insect behavior.
4. Present facts and a logical argument to support the following statement: “Insects dominate life on Earth.”
5. Explain why distinctive life stages and metamorphosis are adaptive.
6. Diagram an insect life cycle. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.11%3A_Insects.txt |
Believe it or not, this is an animal. See the mouth and arms?
It is a sea lily, a crinoid echinoderm. Crinoids are essentially a mouth on the top surface that is surrounded by feeding arms. Although the basic echinoderm pattern of fivefold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a surface, but many become free-swimming as adults.
Echinoderms
Echinoderms are marine organisms that make up the phylum Echinodermata. They can be found in the ocean from the equator to the poles. There are roughly 6000 living species of echinoderms. They are among the most distinctive organisms within the animal kingdom. Members of the phylum include sea stars (starfish), sand dollars, and feather stars, shown in Figure below.
Examples of Echinoderms. You may have seen sea stars and sand dollars at the beach because they live in shallow water near the shore. Other echinoderms, such as feather stars, are less commonly seen because they live in the deep ocean.
Structure and Function of Echinoderms
Echinoderms are named for their “spiny skin.” However, the spines aren’t on their skin. They are part of the endoskeleton. The endoskeleton consists of calcium carbonate plates and spines, covered by a thin layer of skin. Adult echinoderms have radial symmetry. This is easy to see in the sea star and sand dollar in Figure above. However, echinoderms evolved from an ancestor with bilateral symmetry. Evidence for this is the bilateral symmetry of their larvae.
A unique feature of echinoderms is their water vascular system. This is a network of canals that extend along each body part. In most echinoderms, the canals have external projections called tube feet (see Figure below). The feet have suckers on the ends. Muscle contractions force water into the feet, causing them to extend outward. As the feet extend, they attach their suckers to new locations, farther away from their previous points of attachment. This results in a slow but powerful form of movement. The suckers are very strong. They can even be used to pry open the shells of prey.
Tube Feet of a Sea Star. The tube feet of a sea star (in white) are part of its water vascular system. There is a sucker on the end of each foot that allows the animal to “walk” slowly over a surface. The suckers are strong enough to pry open shells.
Echinoderms lack respiratory and excretory systems. Instead, the thin walls of their tube feet allow oxygen to diffuse in and wastes to diffuse out. Echinoderms also lack a centralized nervous system. They have an open circulatory system and lack a heart. On the other hand, echinoderms have a well-developed coelom and a complete digestive system. Echinoderms use pheromones to communicate with each other. They detect the chemicals with sensory cells on their body surface. Some echinoderms also have simple eyes (ocelli) that can sense light. Like annelids, echinoderms have the ability to regenerate a missing body part.
Echinoderm Reproduction
Some echinoderms can reproduce asexually by fission, but most echinoderms reproduce sexually. They generally have separate sexes and external fertilization. Eggs hatch into free-swimming larvae. The larvae undergo metamorphosis to change into the adult form. During metamorphosis, their bilateral symmetry changes to radial symmetry.
Echinoderm Classification
Living echinoderms are placed in five classes. These five classes show many similarities. Organisms in each class are described in Table below.
Class (includes) Description Example
Crinoidea
• feathers stars
• sea lilies
fewer than 100 species; many have more than five arms; earliest and most primitive echinoderms; live on the ocean floor, mainly in deep water; filter feeders
feather star
Asteroidea
• sea stars
almost 2000 species; most have five arms; many are brightly colored; live on the ocean floor, mainly in shallow water; predators or scavengers
sea star
Ophiuroidea
• brittle stars
about 2000 species; central disk distinct from arms; move by flapping their arms, which lack suckers; live on the ocean floor in shallow or deep water; predators, scavengers, deposit feeders, or filter feeders
brittle star
Echinoidea
• sea urchins
• sand dollars
• sea biscuits
• heart urchins
about 100 species; do not have arms but do have tube feet; have a specialized mouth part with teeth to scrape food from rocks; live on the ocean floor in shallow or deep water; predators, herbivores, or filter feeders
sea urchin
Holothuroidea
• sea cucumbers
about 1000 species; long body without arms; unlike other echinoderms, have a respiratory system; live on the ocean floor in shallow or deep water; deposit feeders, or filter feeders
sea cucumber
Summary
• Echinoderms are marine invertebrates. They include sea stars, sand dollars, and feather stars.
• Echinoderms have a spiny endoskeleton. They have radial symmetry as adults but bilateral symmetry as larvae.
• Echinoderms have a unique water vascular system with tube feet. This allows slow but powerful movement.
Review
1. Describe the echinoderm endoskeleton.
2. Give an example of an organism in each class of living echinoderms.
3. Adult sea stars and other echinoderms have obvious radial symmetry. What evidence supports the claim that echinoderms evolved from an ancestor with bilateral symmetry?
4. Explain the structure and function of the water vascular system.
11.13: Invertebrate Chordates
Would you believe this animal eats its own brain?
This is a sea squirt, which is a tunicate. Many physical changes occur to the tunicate's body during metamorphosis into an adult, with one of the most interesting being the digestion of the cerebral ganglion, which controls movement and is the equivalent of the human brain. From this comes the common saying that the sea squirt "eats its own brain."
Invertebrate Chordates
Living species of chordates are classified into three major subphyla: Vertebrata, Urochordata, and Cephalochordata. Vertebrates are all chordates that have a backbone. The other two subphyla are invertebrate chordates that lack a backbone. Members of the subphylum Urochordata are tunicates (also called sea squirts). Members of the subphylum Cephalochordata are lancelets. Both tunicates and lancelets are small and primitive. They are probably similar to the earliest chordates that evolved more than 500 million years ago.
Tunicates
There are about 3,000 living species of tunicates (see Figure below). They inhabit shallow marine waters. Larval tunicates are free-swimming. They have all four defining chordate traits (see the "Chordates" concept). Adult tunicates are sessile. They no longer have a notochord or post-anal tail.
Tunicates (Urochordata). Tunicates are one of two subphyla of invertebrate chordates.
Adult tunicates are barrel-shaped. They have two openings that siphon water into and out of the body. The flow of water provides food for filter feeding. Tunicates reproduce sexually. Each individual produces both male and female gametes. However, they avoid self-fertilization. Tunicates can also reproduce asexually by budding.
Lancelets
There are only about 25 living species of lancelets. They inhabit the ocean floor where the water is shallow. Lancelet larvae are free-swimming. The adults can swim but spend most of their time buried in the sand. Like tunicates, lancelets are filter feeders. They take in water through their mouth and expel it through an opening called the atriopore (see Figure below). Lancelets reproduce sexually and have separates sexes.
Lancelet (Cephalochordata). Unlike tunicates, lancelets retain all four defining chordate traits in the adult stage. Can you find them?
Summary
• Chordates include vertebrates and invertebrates that have a notochord.
• Invertebrate chordates do not have a backbone.
• Invertebrate chordates include tunicates and lancelets. Both are primitive marine organisms.
Review
1. Name and describe the two subphyla of invertebrate chordates. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/11%3A_Invertebrates/11.12%3A_Echinoderms.txt |
Thumbnail: Red-eyed tree frog (Agalychnis callidryas). (CC BY-SA 3.0; Charlesjsharp).
12: Vertebrates
What's a chordate?
Most chordates are animals with backbones. These range from small fish to giant whales, from playful dogs to ferocious cats. Not nearly as diverse as the invertebrates. But chordates do have many amazing species. The white tiger (Panthera tigris) is a chordate. The tiger is also from the class Mammalia, order Carnivora, and family Felidae, meaning it is a meat-eating cat.
Introduction to Chordates
The phylum Chordata consists of both invertebrate and vertebrate chordates. It is a large and diverse phylum. It includes some 60,000 species. Chordates range in length from about a centimeter to over 30 meters (100 feet). They live in marine, freshwater, terrestrial, and aerial habitats. They can be found from the equator to the poles. Several examples of chordates are pictured in Figure below.
Diversity of Chordates. These six species illustrate the diversity of the phylum Chordata.
Characteristics of Chordates
Chordates have three embryonic cell layers. They also have a segmented body with a coelom and bilateral symmetry. Chordates have a complete digestive system and a closed circulatory system. Their nervous system is centralized. There are four additional traits that are unique to chordates. These four traits, shown in Figure below, define the chordate phylum.
1. Post-anal tail: The tail is opposite the head and extends past the anus.
2. Dorsal hollow nerve cord: The nerve cord runs along the top, or dorsal, side of the animal. (In non chordate animals, the nerve cord is solid and runs along the bottom).
3. Notochord: The notochord lies between the dorsal nerve cord and the digestive tract. It provides stiffness to counterbalance the pull of muscles.
4. Pharyngeal slits: Pharyngeal slits are located in the pharynx. The pharynx is the tube that joins the mouth to the digestive and respiratory tracts.
Body Plan of a Typical Chordate. The body plan of a chordate includes a post-anal tail, notochord, dorsal hollow nerve cord, and pharyngeal slits.
In some chordates, all four traits persist throughout life and serve important functions. However, in many chordates, including humans, all four traits are present only during the embryonic stage. After that, some of the traits disappear or develop into other organs. For example, in humans, pharyngeal slits are present in embryos and later develop into the middle ear.
Classification of Chordates
Living species of chordates are classified into three major subphyla: Vertebrata, Urochordata, and Cephalochordata. Vertebrates are all chordates that have a backbone. The other two subphyla are invertebrate chordates that lack a backbone.
Summary
• Chordates include vertebrates and invertebrates that have a notochord.
• Chordates also have a post-anal tail, dorsal hollow nerve cord, and pharyngeal slits.
• Vertebrate chordates have a backbone.
Review
1. What is a vertebrate?
2. Identify the four defining traits of chordates.
3. Adult humans lack some of the defining traits of chordates. Why are humans still classified in the chordate phylum?
12.02: Placental Mammals
Is this kangaroo a placental mammal?
You know that female kangaroos have a pouch for the final development of their babies. So, no, kangaroos are not placental mammals. What is a placental mammal?
Therian Mammals
Like other female vertebrates, all female mammals have ovaries. These are the organs that produce eggs (see Figure below). Therian mammals also have two additional female reproductive structures that are not found in other vertebrates. They are the uterus and vagina.
• The uterus (plural, uteri) is a pouch-like, muscular organ. The embryo or fetus develops inside the uterus. Muscular contractions of the uterus push the offspring out during birth.
• The vagina is a tubular passageway through which the embryo or fetus leaves the mother’s body during birth. The vagina is also where the male deposits sperm during mating.
Female Reproductive System of a Therian Mammal (Human). Therian mammals are viviparous, giving birth to an embryo or infant rather than laying eggs. The female reproductive system of all therian mammals is similar to that of humans.
Therian mammals are divided into two groups: placental mammals and marsupial mammals. Each group has a somewhat different reproductive strategy.
Placental Mammals
Placental mammals are therian mammals in which a placenta develops during pregnancy. The placenta sustains the fetus while it grows inside the mother’s uterus. Placental mammals give birth to relatively large and mature infants. Most mammals are placental mammals.
The Placenta
The placenta is a spongy structure. It consists of membranes and blood vessels from both mother and embryo (see Figure below). The placenta passes oxygen, nutrients, and other useful substances from the mother to the fetus. It also passes carbon dioxide and other wastes from the fetus to the mother. The placenta lets blood from the fetus and mother exchange substances without actually mixing. Thus, it protects the fetus from being attacked by the mother’s immune system as a “foreign parasite.”
Placenta of a Placental Mammal (Human). The placenta allows the exchange of gases, nutrients, and other substances between the fetus and mother.
Pros and Cons of Placental Reproduction
The placenta permits a long period of fetal growth in the uterus. As a result, the fetus can become large and mature before birth. This increases its chances of surviving. On the other hand, supporting a growing fetus is very draining and risky for the mother. The mother has to eat more food to nourish the fetus. She also becomes heavier and less mobile as the fetus gets larger. As a result, she may be less able to escape from predators. Because the fetus is inside her, she can’t abandon it to save her own life if she is pursued or if food is scarce. Giving birth to a large infant is also risky. It may even result in the mother’s death.
Summary
• Therian mammals are viviparous. They give birth to an embryo or infant rather than laying eggs.
• The female reproductive system of a therian mammal includes a uterus and a vagina.
• There are two groups of therian mammals: placental mammals and marsupials.
• Placental mammals give birth to a relatively large and mature fetus. This is possible because they have a placenta to nourish the fetus and protect it from the mother’s immune system. This allows for a long period of growth and development before birth.
• Because the offspring of placental mammals is relatively large and mature at birth, it has a good chance of surviving. However, carrying and giving birth to a large fetus is risky for the mother. It also requires her to eat more food.
Review
1. What are therian mammals? What structures are found in these mammals that are absent in other vertebrates?
2. What are the functions of the uterus and vagina in therian mammals?
3. What is the placenta?
4. Describe how the placenta functions?
5. Placental mammals greatly outnumber the other two groups of mammals. Infer why placental mammals have been so successful. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.01%3A_Chordates.txt |
What traits set this animal apart from invertebrate chordates?
This small colorful fish is a vertebrate - it has a backbone. And vertebral columns first evolved in fish. Think about the invertebrate chordates. They live in the ocean, so it only makes sense that fish would be the first true vertebrates. Vertebrates are a subphylum of the phylum Chordata. Like all chordates, vertebrates have a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. What other characteristics do vertebrates have?
Characteristics of Vertebrates
The main distinguishing feature of vertebrates is their vertebral column, or backbone (see Figure below). The backbone runs from the head to the tail along the dorsal (top) side of the body. The vertebral column is the core of the endoskeleton. It allows a vertebrate to hold its shape. It also houses and protects the spinal (nerve) cord that passes through it. The vertebral column is made up of repeating units called vertebrae (singular, vertebra). In many species, there are shock-absorbing discs between the vertebrae to cushion them during movement.
Human Vertebral Column and Vertebrae. The human vertebral column consists of 33 vertebrae. Two vertebrae are shown here enlarged.
Vertebrate Endoskeleton
Another distinguishing feature of vertebrates is an endoskeleton made of bone or cartilage.Cartilage is a tough tissue that contains a protein called collagen. Bone is a hard tissue that consists of a collagen matrix, or framework, filled in with minerals such as calcium. Bone is less flexible than cartilage but stronger. An endoskeleton made of bone rather than cartilage allows animals to grow larger and heavier. Bone also provides more protection for soft tissues and internal organs.
As shown in Figure below, the vertebrate endoskeleton includes a cranium, or skull, to enclose and protect the brain. It also generally includes two pairs of limbs. Limb girdles (such as the human hips and shoulders) connect the limbs to the rest of the endoskeleton.
Vertebrate Endoskeletons. The vertebrate endoskeleton includes a vertebral column, cranium, limbs, and limb girdles. Can you find these parts in each endoskeleton shown here?
Other Vertebrate Traits
There are several additional traits found in virtually all vertebrates.
• Vertebrates have a system of muscles attached to the endoskeleton to enable movement. Muscles control movement by alternately contracting (shortening) and relaxing (lengthening). Generally, muscles work together in opposing pairs.
• Vertebrates have a closed circulatory system with a heart. Blood is completely contained within blood vessels that carry the blood throughout the body. The heart is divided into chambers that work together to pump blood. There are between two and four chambers in the vertebrate heart. With more chambers, there is more oxygen in the blood and more vigorous pumping action.
• Most vertebrates have skin covered with scales, feathers, fur, or hair. These features serve a variety of functions, such as waterproofing and insulating the body.
• Vertebrates have an excretory system that includes a pair of kidneys. Kidneys are organs that filter wastes from blood so they can be excreted from the body.
• Vertebrates have an endocrine system of glands that secrete hormones. Hormones are chemical messengers that control many body functions.
• Vertebrates have an adaptive immune system. The immune system is the organ system that defends the body from pathogens and other causes of disease. Being adaptive means that the immune system can "learn" to recognize specific pathogens. Then it can produce tailor-made proteins called antibodies to "attack" them. This allows the immune system to launch a rapid attack whenever the pathogens invade the body again.
• Vertebrates have a centralized nervous system. As shown in Figure below, the nervous system consists of a brain in the head region. It also includes a long spinal cord that runs from the brain to the tail end of the backbone. Long nerve fibers extend from the spinal cord to muscles and organs throughout the body.
Nervous System (Human). The vertebrate nervous system includes a brain and spinal cord. It also includes a body-wide network of nerves, called peripheral nerves. They connect the spinal cord with the rest of the body.
Summary
• Vertebrates are a subphylum of chordates that have a vertebral column and an endoskeleton made of cartilage or bone.
• Vertebrates also have complex organ systems, including a closed circulatory system with a heart, an excretory system with a pair of kidneys, and an adaptive immune system.
Review
1. Describe the vertebrate vertebral column, and list its functions.
2. Contrast cartilage and bone, and state the advantages of a bony endoskeleton relative to a cartilaginous endoskeleton.
3. Identify the components of the vertebrate nervous system.
4. What is an adaptive immune system? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.03%3A_Vertebrate_Characteristics.txt |
Why so many different types of large mammals?
Elephants, zebras, gazelles, giraffes, rhinoceros, lions, and hippopotamuses, to name just some of the animals of Africa. Each species must have its own niche, otherwise they could not coexist. So, there must be a role for each species within their ecosystem.
Evolution of Modern Mammals
The Cretaceous Period ended with another mass extinction. This occurred about 65 million years ago. All of the dinosaurs went extinct at that time. Did the extinction of the dinosaurs allow mammals to take over?
Traditional View
Scientists have long assumed that the extinction of the dinosaurs opened up many niches for mammals to exploit. Presumably, this led to an explosion of new species of mammals early in Cenozoic Era. Few mammalian fossils from the early Cenozoic have been found to support this theory. Even so, it was still widely accepted until recently.
View from the Mammalian Supertree
In 2007, an international team of scientists compared the DNA of almost all known species of living mammals. They used the data to create a supertree of mammalian evolution. The supertree shows that placental mammals started to diversify as early as 95 million years ago.
What explains the diversification of mammals long before the dinosaurs went extinct? What else was happening at that time? One change was a drop in Earth’s temperature. This may have favored endothermic mammals over ectothermic dinosaurs. Flowering plants were also spreading at that time. They may have provided new and plentiful foods for small mammals or their insect prey.
The supertree also shows that another major diversification of mammals occurred about 50 million years ago. Again, worldwide climate change may have been one reason. This time Earth’s temperature rose. The warmer temperature led to a greater diversity of plants. This would have meant more food for mammals or their prey.
Summary
• The mammalian supertree shows that placental mammals started to diversify as early as 95 million years ago.
Review
1. What is the mammalian supertree?
2. Explain why the extinction of most therapsids may have allowed mammals to evolve.
3. What explains the diversification of mammals long before the dinosaurs went extinct?
12.05: Vertebrate Reproduction
Does the embryo develop in the mother or in an egg?
Is the egg within the mother or not? Vertebrate reproduction occurs by one of three methods. These bird eggs demonstrate one of three types of vertebrate reproduction.
Vertebrate Reproduction
Vertebrates reproduce sexually, and almost all of them have separate male and female sexes. Recall that sexual reproduction is the joining of gametes during fertilization, producing genetically variable offspring. Generally, aquatic species have external fertilization, whereas terrestrial species have internal fertilization. Can you think of a reason why aquatic and terrestrial vertebrates differ in this way?
Vertebrates have one of the following three reproductive strategies: ovipary, ovovivipary, or vivipary.
• Ovipary refers to the development of an embryo within an egg outside the mother’s body. This occurs in most amphibians and reptiles and in all birds.
• Ovovivipary refers to the development of an embryo inside an egg within the mother’s body until it hatches. The mother provides no nourishment to the developing embryo inside the egg. This occurs in some species of fish and reptiles.
• Vivipary refers to the development and nourishment of an embryo within the mother’s body. Birth may be followed by a period of parental care of the offspring. This reproductive strategy occurs in almost all mammals.
What type of reproduction occurs in kangaroos?
Summary
• Vertebrates reproduce sexually, and almost all have separate male and female sexes.
• Aquatic species generally have external fertilization, whereas terrestrial species usually have internal fertilization.
• Vertebrates have one of three reproductive strategies, known as ovipary, ovovivipary, or vivipary.
Review
1. Define ovipary, ovovivipary, and vivipary.
2. Which vertebrates use each type of reproductive strategy?
12.06: Vertebrate Classification
The stingray. Fish?
Of course. But what type? Of the nine classes of vertebrates, five are fish. Each of the five classes has distinguishing characteristics that allow members to be classified appropriately. Stingray are cartilaginous fish, related to sharks.
Vertebrate Classification
There are about 50,000 vertebrate species, and they are placed in nine different classes. Five of the classes are fish. The other classes are amphibians, reptiles, birds, and mammals.Table below lists some of the distinguishing traits of each class. Classes are listed in order of evolution.
Class Distinguishing Traits Example
Hagfish They have a cranium but no backbone; they do not have jaws; their endoskeleton is made of cartilage; they are ectothermic.
hagfish
Lampreys They have a partial backbone; they do not have jaws; their endoskeleton is made of cartilage; they are ectothermic.
lamprey
Cartilaginous Fish They have a complete backbone; they have jaws; their endoskeleton is made of cartilage; they are ectothermic.
shark
Ray-Finned Fish They have a backbone and jaws; their endoskeleton is made of bones; they have thin, bony fins; they are ectothermic.
perch
Lobe-Finned Fish They have a backbone and jaws; their endoskeleton is made of bones; they have thick, fleshy fins; they are ectothermic.
coelacanth
Amphibians They have a bony endoskeleton with a backbone and jaws; they have gills as larvae and lungs as adults; they have four limbs; they are ectothermic
frog
Reptiles They have a bony endoskeleton with a backbone and jaws; they breathe only with lungs; they have four limbs; their skin is covered with scales; they have amniotic eggs; they are ectothermic.
alligator
Birds They have a bony endoskeleton with a backbone but no jaws; they breathe only with lungs; they have four limbs, with the two front limbs modified as wings; their skin is covered with feathers; they have amniotic eggs; they are endothermic.
bird
Mammals They have a bony endoskeleton with a backbone and jaws; they breathe only with lungs; they have four limbs; their skin is covered with hair or fur; they have amniotic eggs; they have mammary (milk-producing) glands; they are endothermic.
bear
Summary
• The 50,000 species of living vertebrates are placed in nine classes: hagfish, lampreys, cartilaginous fish, ray-finned fish, lobe-finned fish, amphibians, reptiles, birds, and mammals.
Review
1. Which was the first and last vertebrate classes to evolve?
2. What are the five fish vertebrate classes?
3. What are the defining characteristics of mammals?
4. What was the first class of vertebrates to live on land?
5. Sharks belong to what vertebrate class? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.04%3A_Evolution_of_Modern_Mammals.txt |
What type of fish is a simple perch?
Does it have a partial of complete vertebral column? What about a jaw? Do you think the endoskeleton is made of cartilage or bone? Why are these important evolutionary steps? A bony skeletal could support a larger body. Early bony fish evolved into modern ray-finned and lobe-finned fish, which then evolved into species that could move out of the water.
Vertebrate Evolution
The earliest vertebrates were jawless fish, similar to living hagfish. They lived between 500 and 600 million years ago. They had a cranium but no vertebral column. The phylogenetic tree in Figure below gives an overview of vertebrate evolution. As more data become available, new ideas about vertebrate evolution emerge.
Phylogenetic Tree of Vertebrate Evolution. The earliest vertebrates evolved almost 550 million years ago. Which class of vertebrates evolved last?
Evolution of Fish
Not too long after hagfish first appeared, fish similar to lampreys evolved a partial vertebral column. The first fish with a complete vertebral column evolved about 450 million years ago. These fish also had jaws and may have been similar to living sharks. Up to this point, all early vertebrates had an endoskeleton made of cartilage rather than bone. About 400 million years ago, the first bony fish appeared. A bony skeleton could support a larger body. Early bony fish evolved into modern ray-finned and lobe-finned fish.
Evolution of Other Vertebrate Classes
Amphibians, reptiles, mammals, and birds evolved after fish.
• The first amphibians evolved from a lobe-finned fish ancestor about 365 million years ago. They were the first vertebrates to live on land, but they had to return to water to reproduce. This meant they had to live near bodies of water.
• The first reptiles evolved from an amphibian ancestor at least 300 million years ago. They laid amniotic eggs and had internal fertilization. They were the first vertebrates that no longer had to return to water to reproduce. They could live just about anywhere.
• Mammals and birds both evolved from reptile-like ancestors. The first mammals appeared about 200 million years ago and the earliest birds about 150 million years ago.
Evolution of Endothermy
Until mammals and birds evolved, all vertebrates were ectothermic. Ectothermy means regulating body temperature from the outside through behavioral changes. For example, an ectotherm might stay under a rock in the shade in order to keep cool on a hot, sunny day. Almost all living fish, amphibians, and reptiles are ectothermic. Their metabolic rate and level of activity depend mainly on the outside temperature. They can raise or lower their own temperature only slightly through behavior alone.
Both mammals and birds evolved endothermy. Endothermy means regulating bodytemperature from the inside through metabolic or other physical changes. On a cold day, for example, an endotherm may produce more heat by raising its metabolic rate. On a hot day, it may give off more heat by increasing blood flow to the surface of the body. Keeping body temperature stable allows cells to function at peak efficiency at all times. The metabolic rate and activity level can also remain high regardless of the outside temperature. On the other hand, maintaining a stable body temperature requires more energy—and more food.
Summary
• The earliest vertebrates resembled hagfish and lived more than 500 million years ago.
• As other classes of fish appeared, they evolved traits such as a complete vertebral column, jaws, and a bony endoskeleton.
• Amphibians were the first tetrapod vertebrates as well as the first vertebrates to live on land.
• Reptiles were the first amniotic vertebrates.
• Mammals and birds, which both descended from reptile-like ancestors, evolved endothermy, or the ability to regulate body temperature from the inside.
Review
1. In what order did vertebrates evolve?
2. Birds evolved from what other type of vertebrate?
3. What were the first vertebrates to lay amniotic eggs?
4. Compare and contrast ectothermy and endothermy, including their pros and cons. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.07%3A_Vertebrate_Evolution.txt |
What are these geometric shapes?
These are the scales - the skin - of a blue Siamese fighting fish, and they almost look like a piece of art. They also look like they form a smooth, streamlined skin. Why is this important to a fish?
Structure and Function in Fish
Fish are aquatic vertebrates. They make up more than half of all vertebrate species. They are especially important in the study of vertebrate evolution because several important vertebrate traits evolved in fish. Fish show great diversity in body size. They range in length from about 8 millimeters (0.3 inches) to 16 meters (about 53 feet). Most are ectothermic and covered with scales. Scales protect fish from predators and parasites and reduce friction with the water. Multiple, overlapping scales provide a flexible covering that allows fish to move easily while swimming.
Adaptations for Water
Many structures in fish are adaptations for their aquatic lifestyle. Several are described below and shown in Figure below.
• Fish have gills that allow them to “breathe” oxygen in water. Water enters the mouth, passes over the gills, and exits the body through a special opening. Gills absorb oxygen from the water as it passes over them.
• Fish have a stream-lined body. They are typically long and narrow, which reduces water resistance when they swim.
• Most fish have several fins for swimming. They use some of their fins to propel themselves through the water and others to steer the body as they swim.
• Fish have a system of muscles for movement. Muscle contractions ripple through the body in waves from head to tail. The contractions whip the tail fin against the water to propel the fish through the water.
• Most fish have a swim bladder. This is a balloon-like internal organ that contains gas. By changing the amount of gas in the bladder, a fish can move up or down through the water column.
General Fish Body Plan. A fish has a stream-lined body with gills and fins.
Fish Organ Systems
Fish have a circulatory system with a two-chambered heart. Their digestive system is complete and includes several organs and glands. Jawed fish use their jaws and teeth to grind up food before passing it to the rest of the digestive tract. This allows them to consume larger prey.
Fish also have a centralized nervous system with a brain. Fish brains are small compared with the brains of other vertebrates, but they are large and complex compared with the brains of invertebrates. Fish also have highly developed sense organs that allow them to see, hear, feel, smell, and taste. Sharks and some other fish can even sense the very low levels of electricity emitted by other animals. This helps them locate prey.
Summary
• Fish are aquatic, ectothermic vertebrates.
• Many structures in fish are adaptations for their aquatic lifestyle. For example, fish have a stream-lined body that reduces water resistance while swimming.
• Fish have gills for “breathing” oxygen in water and fins for propelling and steering their body through water.
Review
1. What are gills? What purpose do they serve in fish?
2. Describe the function of fish scales.
3. Describe how fish use their muscles to swim.
4. What is a swim bladder? How is it used? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.08%3A_Fish_Structure_and_Function.txt |
How do fish reproduce?
Wild male and female Sockeye salmon before spawning. Sockeye salmon are blue tinged with silver in color while living in the ocean. Just prior to spawning, both sexes turn red with green heads. Sockeye spawn mostly in streams having lakes in their watershed. The young fish spend up to three years in the freshwater lake before migrating to the ocean. Migratory fish spend from one to four years in salt water, and thus are four to six years old when they return to spawn. Navigation to the home river is thought to be done using the characteristic smell of the stream, and possibly the sun.
Fish Reproduction and Development
Nearly all fish reproduce sexually, and most species have separate sexes. Those without separate sexes avoid self-fertilization by producing sperm and eggs at different times. Each fish typically produces a large number of gametes. In most fish species, fertilization takes place externally. These fish are oviparous. Eggs are laid and embryos develop outside the mother’s body. In a minority of fish, including sharks, eggs develop inside the mother’s body but without nourishment from the mother. These fish are ovoviviparous.
Spawning
In many species of fish, a large group of adults come together to release their gametes into the water at the same time. This is called spawning. It increases the chances that fertilization will take place. It also means that many embryos will form at once, which helps ensure that at least some of them will be able to escape predators.
With spawning, there is no way for fish parents to know which embryos are their own. Therefore, fish generally don’t provide any care to their eggs or offspring. There are some exceptions, however, including the fish described in Figure below, which is performing mouth brooding.
Mouth Brooding. Some species of fish carry their fertilized eggs in their mouth until they hatch. This is called mouth brooding. If you look closely, you can see the eggs inside the mouth of the cardinalfish pictured here.
Fish Larvae
Fish eggs hatch into larvae that are different from the adult form of the species (see Figure below). A larva swims attached to a large yolk sac, which provides the larva with food. The larva eventually goes through metamorphosis and changes into the adult form. However, it still needs to mature before it can reproduce.
Salmon Larva. This newly hatched salmon larva doesn’t look very fish-like. The structure hanging from the larva is the yolk sac.
Summary
• Nearly all fish reproduce sexually and have separate sexes.
• Fertilization is generally external, and most fish are oviparous. Many adults of the same species may come together in a group and release gametes into the water at the same time, which is called spawning.
• Fish hatch into larvae that are different from the adult form of the species.
Review
1. Explain why the practice of spawning is adaptive.
2. What is mouth brooding?
3. How are fish larvae different from the adult fish?
Explore More
Use this resource to answer the questions that follow.
1. Describe the stages in the life of a salmon.
2. How does a salmon remember where to return to spawn? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.09%3A_Fish_Reproduction_and_Development.txt |
What type of animal is a sea horse? Is it actually a fish?
It is, and there are about 50 species of seahorses. Although they are bony fish, they do not have scales, but rather a thin skin stretched over a series of bony plates arranged in rings throughout their body. Each species has a distinct number of rings. Seahorses have a coronet on their head, which is distinct to each individual, much like a human fingerprint. Seahorses also swim upright, a characteristic not shared by other fish. Seahorses are poor swimmers, so they are most likely to be found resting, with their prehensile tails wound around a stationary object. They have long snouts, which they use to suck up food, and eyes that can move independently of each other. Seahorses eat small shrimp, tiny fish, crustaceans, and plankton.
Classification of Fish
There are about 28,000 existing species of fish, and they are placed in five different classes. The classes are commonly referred to as hagfish, lampreys, cartilaginous fish, ray-finned fish, and lobe-finned fish (see the table in the previous lesson).
Hagfish
Hagfish are very primitive fish. They retain their notochord throughout life rather than developing a backbone, and they lack scales and fins. They are classified as vertebrates mainly because they have a cranium. Hagfish are noted for secreting large amounts of thick, slimy mucus. The mucus makes them slippery, so they can slip out of the jaws of predators.
Lampreys
Like hagfish, lampreys also lack scales, but they have fins and a partial backbone. The most striking feature of lampreys is a large round sucker, lined with teeth, that surrounds the mouth (see Figure below). Lampreys use their sucker to feed on the blood of other fish species.
Sucker Mouth of a Lamprey. The mouth of a lamprey is surrounded by a tooth-lined sucker.
Cartilaginous Fish
Cartilaginous fish include sharks, rays, and ratfish (see Figure below). In addition to an endoskeleton composed of cartilage, these fish have a complete backbone. They also have a relatively large brain. They can solve problems and interact with other members of their species. They are generally predators with keen senses. Cartilaginous fish lack a swim bladder. Instead, they stay afloat by using a pair of muscular fins to push down against the water and create lift.
Cartilaginous Fish. All of these fish belong to the class of cartilaginous fish with jaws. (a) Oceanic whitetip shark (b) Ray (c) Ratfish
One of the most important traits of cartilaginous fish is their jaws. Jaws allow them to bite food and break it into smaller pieces. This is a big adaptive advantage because it greatly expands the range of food sources they can consume. Jaws also make cartilaginous fish excellent predators. It you’ve ever seen the film Jaws, then you know that jaws make sharks very fierce predators (see also Figure below).
Jaws of a Shark. Sharks have powerful jaws with multiple rows of sharp, saw-like teeth. Most other fish are no match for these powerful predators.
Ray-Finned Fish
Ray-finned fish include the majority of living fish species, including goldfish, tuna, salmon, perch, and cod. They have a bony endoskeleton and a swim bladder. Their thin fins consist of webs of skin over flexible bony rays, or spines. The fins lack muscle, so their movements are controlled by muscles in the body wall. You can compare their ray fins with the fleshy fins of lobe-finned fish in Figure below.
Fins of Bony Fish. The fins of ray-finned and lobe-finned fish are quite different. How is the form of the fins related to their different functions in the two classes of fish? Ray Fin (left), Lobe Fin (right)
Lobe-Finned Fish
Lobe-finned fish are currently far fewer in number than ray-finned fish. Their fins, like the one shown in Figure above, contain a stump-like appendage of bone and muscle. There are two groups of lobe-finned fish still alive today: coelacanths and lungfish.
1. Coelacanths are ancient fish with just two living species. They are at risk of extinction because of their very small numbers.
2. Lungfish have a lung-like organ for breathing air. The organ is an adaptation of the swim bladder. It allows them to survive for long periods out of water.
Summary
• There are about 28,000 existing species of fish, and they are placed in five classes: hagfish, lampreys, cartilaginous fish, ray-finned bony fish, and lobe-finned bony fish.
Review
1. Assume that a new species of fish has been discovered deep in the ocean. It has a complete vertebral column made of cartilage. Which class should the new species be placed in? Name one other trait you would expect to find in the new species of fish. Explain your answers.
2. Fish with jaws may be very large. Infer how their jaws may be related to their large body size.
3. The majority of living fish belong to what class?
4. Describe the lungfish. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.10%3A_Fish_Classification.txt |
Are there actually ecosystems in the ocean?
There are. Different types of fish live in different types of ecosystems. Shown above are tropical fish in a coral reef ecosystem. Some fish are deep-ocean bottom dwellers, whereas others live in shallow waters. Other fish may not be able to survive in the ocean, as they need freshwater.
Evolution of Fish
The evolution of fish from hagfish to finned fish is a long and involved process. One step in this evolution involves the change in function of gills. Invertebrate chordates use their gills to filter food out of water, not to absorb oxygen. In the early evolution of fish, there was a switch to using gills to absorb oxygen instead of to filter food. Gills consist of many thin, folded tissues that provide a large surface area for oxygen uptake. With more oxygen absorbed by the gills, fish could become much larger and more active.
Fossilized fish shown in different sizes.
Timing of Fish Evolution
Ancestors of hagfish are thought to have been the earliest vertebrates. Their fossils date back to about 550 million years ago. Fossils of cartilaginous fish with jaws, resembling living sharks, first appeared in the fossil record about 450 million years ago. They were followed about 50 million years later by the bony fish.
The Bony Fish
At first, the lobe-finned bony fish were much more common than the ray-finned bony fishthat dominate today. Lobe-finned fish were also ancestral to amphibians. Their stump-like appendages and lung-like organs evolved into amphibian legs and lungs. Ray-finned bony fish may have been the first fish to evolve in freshwater. They eventually became the most diverse and dominant class of fish.
Ecology of Fish
The habitats and diets of fish are varied. They live throughout the ocean and also in freshwater lakes, ponds, rivers, and streams. However, there is one fish, the Mudskipper, that spends time on land.
Fish Food
Most fish are predators, but the nature of their prey and how they consume it differs from one class to another and even within classes.
• Hagfish are deep-ocean bottom dwellers. They feed on other fish, either living or dead. They enter the body of their prey through the mouth or anus. Then they literally eat their prey from the inside out.
• Lampreys generally live in shallow ocean water or freshwater. They either consume small invertebrates or suck blood from larger fish with their sucker mouth.
• Cartilaginous fish such as sharks may live on the bottom of the ocean. However, most live in the water column. They prey on other fish and aquatic mammals or else consume plankton.
• Bony fish may live in salt water or freshwater. They consume a wide range of foods. For example, they may eat algae, smaller fish, detritus, or dead organisms, depending on the species of fish.
Fish at Risk
Today, more than 1,000 species of fish are at risk of extinction. This is mainly because of human actions. Specific causes include over-fishing and habitat destruction caused by water pollution, dam building, and the introduction of non-native species.
Summary
• The evolution of fish included a shift from using the gills for filtering food to using them to absorb oxygen from water.
• The earliest fish, resembling living hagfish, evolved about 550 million years ago.
• Adaptations that eventually evolved in fish include a complete vertebral column, jaws, and an endoskeleton made of bones instead of cartilage.
• Fish live throughout the ocean and in freshwater lakes and streams.
• Most fish are predators, but the nature of their prey and how they consume it may vary.
• Many species of fish are threatened by human actions, such as water pollution and overfishing.
Review
1. Which fish were the ancestors of amphibians?
2. Describe the eating habits of hagfish and lampreys. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.11%3A_Fish_Evolution_and_Ecology.txt |
So how did vertebrates move from the water onto land?
There had to be some major modifications. Modifications in how the animal moves, how the animal breathes, and modifications in the animals skin.
Structure and Function in Amphibians
Amphibians are vertebrates that exist in two worlds: they divide their time between freshwater and terrestrial habitats. They share a number of features with air-breathing lungfish, but they also differ from lungfish in many ways. One way they differ is their appendages. Modern amphibians include frogs, salamanders, and caecilians, as shown Figure below.
Examples of Living Amphibians. In what ways do these three amphibians appear to be similar? In what ways do they appear to be different?
Amphibians are the first true tetrapods, or vertebrates with four limbs. Amphibians have less variation in size than fish, ranging in length from 1 centimeter (2.5 inches) to 1.5 meters (about 5 feet). They generally have moist skin without scales. Their skin contains keratin, a tough, fibrous protein found in the skin, scales, feathers, hair, and nails of tetrapod vertebrates, from amphibians to humans. Some forms of keratin are tougher than others. The form in amphibian skin is not very tough, and it allows gases and water to pass through their skin.
Amphibian Ectothermy
Amphibians are ectothermic, so their internal body temperature is generally about the same as the temperature of their environment. When it’s cold outside, their body temperature drops, and they become very sluggish. When the outside temperature rises, so does their body temperature, and they are much more active. What do you think might be some of the pros and cons of ectothermy in amphibians?
Amphibian Organ Systems
All amphibians have digestive, excretory, and reproductive systems. All three systems share a body cavity called the cloaca. Wastes enter the cloaca from the digestive and excretory systems, and gametes enter the cloaca from the reproductive system. An opening in the cloaca allows the wastes and gametes to leave the body.
Amphibians have a relatively complex circulatory system with a three-chambered heart. Their nervous system is also rather complex, allowing them to interact with each other and their environment. Amphibians have sense organs to smell and taste chemicals. Other sense organs include eyes and ears. Of all amphibians, frogs generally have the best vision and hearing. Frogs also have a larynx, or voice box, to make sounds.
Most amphibians breathe with gills as larvae and with lungs as adults. Additional oxygen is absorbed through the skin in most species. The skin is kept moist by mucus, which is secreted by mucous glands. In some species, mucous glands also produce toxins, which help protect the amphibians from predators. The golden frog shown in Figure below is an example of a toxic amphibian.
Toxic Frog. This golden frog is only about 5 centimeters (2 inches) long, but it’s the most poisonous vertebrate on Earth. One dose of its toxin can kill up to 20 humans!
Summary
• Amphibians are ectothermic vertebrates that divide their time between freshwater and terrestrial habitats.
• Amphibians are the first true tetrapods, or vertebrates with four limbs.
• Amphibians breathe with gills as larvae and with lungs as adults. They have a three-chambered heart and relatively complex nervous system.
Review
1. What is a tetrapod?
2. How does the temperature of the environment affect the level of activity of an amphibian?
3. What is the cloaca? What functions does it serve in amphibians?
4. Describe the different ways that amphibians may obtain oxygen. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.12%3A_Amphibian_Structure_and_Function.txt |
What changes must occur for these tadpoles to move onto land?
These are tadpoles of the Yellow-Bellied Toad. Of course these tadpoles are born in the water. You can see the beginning of the formation of the hind limbs.
Amphibian Reproduction and Development
Amphibians reproduce sexually with either external or internal fertilization. They attract mates in a variety of ways. For example, the loud croaking of frogs is their mating call. Each frog species has its own distinctive call that other members of the species recognize as their own. Most salamanders use their sense of smell to find a mate. The males produce a chemical odor that attracts females of the species.
Amphibian Eggs
Unlike other tetrapod vertebrates (reptiles, birds, and mammals), amphibians do not produce amniotic eggs. Therefore, they must lay their eggs in water so they won’t dry out. Their eggs are usually covered in a jelly-like substance, like the frog eggs shown in Figure below. The “jelly” helps keep the eggs moist and offers some protection from predators.
Frog Eggs. Frog eggs are surrounded by “jelly.” What is its function?
Amphibians generally lay large number of eggs. Often, many adults lay eggs in the same place at the same time. This helps to ensure that eggs will be fertilized and at least some of the embryos will survive. Once eggs have been laid, most amphibians are done with their parenting.
Amphibian Larvae
The majority of amphibian species go through a larval stage that is very different from the adult form, as you can see from the frog in Figure below. The early larval, or tadpole, stage resembles a fish. It lacks legs and has a long tail, which it uses to swim. The tadpole also has gills to absorb oxygen from water. As the larva undergoes metamorphosis, it grows legs, loses its tail, and develops lungs. These changes prepare it for life on land as an adult frog.
Frog Development: From Tadpole to Adult. A frog larva (tadpole) goes through many changes by adulthood. Notice the visible changes that occur at each stage. How do these changes prepare it for life as an adult frog?
Summary
• Amphibians reproduce sexually with either external or internal fertilization.
• Amphibians may attract mates with calls or scents.
• Amphibians do not produce amniotic eggs, so they must reproduce in water.
• Amphibian larvae go through metamorphosis to change into the adult form.
Review
1. Describe the life cycle of frogs.
2. Describe the parental involvement of most amphibians.
3. Define metamorphosis.
12.14: Amphibian Classification
Frog, toad, or salamander? What's the difference?
Look closely at the face of this salamander. It is strikingly similar to that of a frog or toad. As the first vertebrates to evolve from life in the sea to life on land, amphibians share a number of important evolutionary adaptations.
Classification of Amphibians
There are about 6,200 known species of living amphibians. They are placed in three different orders:
1. Frogs and toads
2. Salamanders and newts
3. Caecilians
Frogs and Toads
One feature that distinguishes frogs and toads from other amphibians is lack of a tail in adulthood. Frogs and toads also have much longer back legs than other amphibians. Their back legs are modified for jumping. Frogs can jump up to 20 times their own body length. That’s the same as you jumping at least 100 feet, or more than the length of a basketball court. Think how fast you could move if you could travel that far on one jump!
Frogs and toads are closely related, but they differ in several ways. Generally, frogs spend more time in water, and toads spend more time on land. As you can see from Figure below, frogs also have smoother, moister skin than toads, as well as longer hind legs.
Frog and Toad. Frogs (a) and toads (b) are placed in the same amphibian order. What traits do they share?
Salamanders and Newts
Unlike frogs and toads, salamanders and newts keep their tails as adults (see Figure below). They also have a long body with short legs, and all their legs are about the same length. This is because they are adapted for walking and swimming rather than jumping. An unusual characteristic of salamanders is their ability to regenerate, or regrow, legs that have been lost to predators.
Salamander and Newt. Salamanders and newts can walk or swim. Salamander on a leaf (left), newt swimming in the water (right).
Caecilians
Caecilians are most closely related to salamanders. As you can see from Figure below, they have a long, worm-like body without legs. Caecilians evolved from a tetrapod ancestor, but they lost their legs during the course of their evolution.
Swimming Caecilian. Caecilians are the only amphibians without legs.
Summary
• There are about 6,200 known species of living amphibians. They are classified into three orders: frogs and toads, salamanders and newts, and caecilians.
• Frogs and toads are adapted for jumping. Salamanders and newts may walk or swim. Caecilians live in the water or soil and are the only amphibians without legs.
Review
1. Distinguish frogs from toads.
2. What is an unusual characteristic of salamanders.
3. Compare and contrast the three orders of living amphibians. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.13%3A_Amphibian_Reproduction_and_Development.txt |
Why were amphibians so successful?
They might not have had many predators on land 365 million years ago. Given plenty of land and food, they had the resources to thrive. This is a frog fossil from the Eocene epoch. The vertebral column and four limbs are easily identifiable.
Evolution of Amphibians
Fossil evidence shows that amphibians evolved about 365 million years ago from a lobe-finned lungfish ancestor. As the earliest land vertebrates, they were highly successful. Some of them were much larger than today’s amphibians. For more than 100 million years, amphibians remained the dominant land vertebrates. Then some of them evolved into reptiles. Once reptiles appeared, with their amniotic eggs, they replaced amphibians as the dominant land vertebrates.
Ecology of Amphibians
Amphibians can be found in freshwater and moist terrestrial habitats throughout the world. The only continent without amphibians is Antarctica. Amphibians are especially numerous in temperate lakes and ponds and in tropical rainforests.
Amphibians as Prey and Predators
Amphibians are an important food source for animals such as birds, snakes, raccoons, and fish. Amphibians are also important predators. As larvae, they feed mainly on small aquatic animals such as water insects. They may also feed on algae. As adults, amphibians are completely carnivorous. They may catch and eat worms, snails, and insects, as the frog inFigure below is doing. Unlike other amphibians, caecilians are burrowers. They use their head to dig in the soil, and they feed on earthworms and other annelids. Caecilians can be found in moist soil near rivers and streams in tropical regions.
Frog Predator. A frog eating its insect prey.
The Threat of Amphibian Extinction
Currently, almost one third of all amphibian species face the threat of extinction. The reasons include habitat loss, pollution, climate change, and the introduction of non-native species. Most of these problems are the result of human actions.
Amphibians have permeable skin that easily absorbs substances from the environment. This may explain why they seem to be especially sensitive to pollution. Monitoring the health and survival of amphibians may help people detect pollution early, before other organisms are affected.
Summary
• Amphibians evolved about 365 million years ago from a lobe-finned fish ancestor.
• As the earliest land vertebrates, amphibians were highly successful for more than 100 million years until reptiles took over as the dominant land vertebrates.
• Amphibians are found throughout the world except in Antarctica and Greenland.
• Amphibians are important prey for animals such as birds, snakes, and raccoons. They are important predators of insects, worms, and other invertebrates.
• Up to one third of all amphibian species are at risk of extinction because of human actions, such as habitat destruction, climate change, and pollution.
Review
1. Explain why amphibians were able to become the dominant land vertebrates for millions of years.
2. What was the ancestor of amphibians?
3. Why were amphibians replaced by reptiles as the dominant land vertebrate?
12.16: Reptile Structure and Function
Why did amphibians evolve into reptiles?
It probably has to do with food and land. Having to live close to water limits the resources available to a species. Having the ability to live away from water allowed reptiles to search for additional food.
Structure and Function in Reptiles
Reptiles are a class of tetrapod vertebrates that produce amniotic eggs. They include crocodiles, alligators, lizards, snakes, and turtles. The reptile class is one of the largest classes of vertebrates. It consists of all amniotes except birds and mammals. Reptiles have several adaptations for living on dry land that amphibians lack. For example, as shown in Figure below, the skin of most reptiles is covered with scales. The scales, which are made of very tough keratin, protect reptiles from injury and prevent them from losing water.
Crocodile Scales. These crocodiles are covered with tough, waterproof scales.
Reptile Respiration
The scales of reptiles prevent them from absorbing oxygen through their skin, as amphibians can. Instead, reptiles breathe air only through their lungs. However, their lungs are more efficient than the lungs of amphibians, with more surface area for gas exchange. This is another important reptile adaptation for life on land.
Reptiles have various ways of moving air into and out of their lungs. Lizards and snakes use muscles of the chest wall for this purpose. These are the same muscles used for running, so lizards have to hold their breath when they run. Crocodiles and alligators have a large sheet of muscle below the lungs, called a diaphragm, that controls their breathing. This is a structure that is also found in mammals.
Ectothermy in Reptiles
Like amphibians, reptiles are ectotherms with a slow metabolic rate. Their metabolism doesn’t generate enough energy to keep their body temperature stable. Instead, reptiles regulate their body temperature through their behavior. For example, the crocodile in Figure below is soaking up heat from the environment by basking in the sun. Because of their ectothermy, reptiles can get by with as little as one tenth the food needed by endotherms such as mammals. Some species of reptiles can go several weeks between meals.
Heat Transfer to an Ectothermic Reptile. This crocodile is being warmed by the environment in three ways. Heat is radiating directly from the sun to the animal’s back. Heat is also being conducted to the animal from the rocks it rests on. In addition, convection currents are carrying warm air from surrounding rocks to the animal’s body.
Other Reptile Structures
Like amphibians, most reptiles have a heart with three chambers, although crocodiles and alligators have a four-chambered heart like birds and mammals. The reptile brain is also similar in size to the amphibian brain, taking into account overall body size. However, the parts of the reptile brain that control the senses and learned behavior are larger than in amphibians.
Most reptiles have good eyesight and a keen sense of smell. Snakes smell scents in the air using their forked tongue (see Figure below). This helps them locate prey. Some snakes have heat-sensing organs on their head that help them find endothermic prey, such as small mammals and birds.
Snake “Smelling” the Air. A snake flicks its tongue in and out to capture scent molecules in the air.
Summary
• Reptiles are a class of ectothermic, tetrapod vertebrates.
• Reptiles have several adaptations for living on dry land, such as tough keratin scales and efficient lungs for breathing air.
• Reptiles have a three-chambered heart and relatively well-developed brain.
Review
1. Describe reptile scales and the functions they serve.
2. What is a diaphragm? What does it do?
3. Describe two senses that snakes may use to locate prey.
4. Pretend you are a reptile such as a lizard. Explain how you might stay warm on a cold day. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.15%3A_Amphibian_Evolution_and_Ecology.txt |
What are the advantages of a water-tight egg?
Obviously, water-tight eggs can be laid anywhere. They do not have to be kept constantly moist. There is no danger of the developing fetus dehydrating. Shown above is a turtle hatching.
Reptile Reproduction
Most reptiles reproduce sexually and have internal fertilization. Males have one or two penises that pass sperm from their cloaca to the cloaca of a female. Fertilization occurs within the cloaca, and fertilized eggs leave the female’s body through the opening in the cloaca. In a minority of species, the eggs are retained inside the female’s body until they hatch. Then the offspring leave the mother’s body through the cloaca opening.
Amniotic Eggs
Unlike amphibians, reptiles produce amniotic eggs (see Figure below). The shell, membranes, and other structures of an amniotic egg protect and nourish the embryo. They keep the embryo moist and safe while it grows and develops. They also provide it with a rich, fatty food source (the yolk).
The amniotic egg is an important adaptation in fully terrestrial vertebrates. It first evolved in reptiles. The shells of reptile eggs are either hard or leathery.
Reptile Young
Unlike amphibians, reptiles do not have a larval stage. Instead, newly hatched reptiles look like smaller versions of the adults. They are able to move about on their own, but they are vulnerable to predators. Even so, most reptile parents provide no care to their hatchlings. In fact, most reptiles don’t even take care of their eggs. For example, female sea turtles lay their eggs on a sandy beach and then return to the ocean. The only exceptions are female crocodiles and alligators. They may defend their nest from predators and help the hatchlings reach the water. If the young remain in the area, the mother may continue to protect them for up to a year.
Summary
• Most reptiles reproduce sexually and have internal fertilization.
• Reptile eggs are amniotic, so they can be laid on land instead of in water.
• Reptiles do not have a larval stage, and their hatchlings are relatively mature.
• Reptile parents provide little if any care to their young.
Review
1. Outline the structure and function of an amniotic egg.
2. Describe young reptiles.
12.18: Reptile Classification
With so many possible colors, how would a chameleon be classified?
Chameleons are a distinctive and highly specialized type of lizard. They are distinguished partly by their parrot-like feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, crests or horns on their distinctively shaped heads, and the ability of some to change color. But there are approximately 160 species of chameleons. So how are they classified?
Classification of Reptiles
There are more than 8,200 living species of reptiles, with the majority being snakes or lizards. They are commonly placed in four different orders. The four orders are described in Table below.
Order Characteristics Example
Crocodilia: crocodiles, alligators, caimans, gharials They have four sprawling legs that can be used to gallop; they replace their teeth throughout life; they have strong jaws and a powerful bite; they have a more advanced brain and greater intelligence than other reptiles; they have a four-chambered heart.
caiman
Sphenodontia: tuataras They are the least specialized of all living reptiles; their brain is very similar to the amphibian brain; they have a three-chambered heart, but it is more primitive than the heart of other reptiles.
tuatara
Squamata: lizards, snakes
Lizards: most have four legs for running or climbing, and they can also swim; many change color when threatened; they have a three-chamberedheart.
Snakes: they do not have legs, although they evolved from a tetrapod ancestor; they have a very flexible jaw for swallowing large prey whole; some inject poison into their prey through fangs; they have a three-chambered heart.
lizard
Testudines:turtles, tortoises, terrapins They have four legs for walking; they have a hard shell covering most of their body; they have a three-chambered heart.
terrapin
Summary
• There are more than 8,200 living species of reptiles, and they are placed in four orders: Crocodilia, which includes crocodiles and alligators; Sphenodontia, or tuataras; Squamata, which includes lizards and snakes; and Testudines, such as turtles and tortoises.
Review
1. Compare and contrast crocodilians with other orders of reptiles. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.17%3A_Reptile_Reproduction.txt |
So what exactly is a dinosaur?
Dinosaurs were not lizards. Rather, they were a separate group of reptiles with a distinct upright posture not found in lizards. Dinosaurs can be described as large, powerful reptiles. And many were very big. But dinosaurs were more than that. They were a varied group of animals with over 1,000 non-avian species.
Evolution of Reptiles
The earliest amniotes evolved about 350 million years ago. They resembled small lizards, but they were not yet reptiles. Their amniotic eggs allowed them to move away from bodies of water and become larger. They soon became the most important land vertebrates.
Synapsids and Sauropsids
By about 320 million years ago, early amniotes had diverged into two groups, called synapsids and sauropsids. Synapsids were amniotes that eventually gave rise to mammals.Sauropsids were amniotes that evolved into reptiles, dinosaurs, and birds. The two groups of amniotes differed in their skulls. The earliest known reptile, pictured in Figure below, dates back about 315 million years.
Earliest Reptile: Hylonomus. The earliest known reptile is given the genus name Hylonomus. It was about 20 to 30 centimeters (8 to 12 inches) long, lived in swamps, and ate insects and other small invertebrates.
At first, synapsids were more successful than sauropsids. They became the most common vertebrates on land. However, during the Permian mass extinction 245 million years ago, most synapsids went extinct. Their niches were taken over by sauropsids, which had been relatively unimportant until then. This is called the Triassic takeover.
Rise and Fall of the Dinosaurs
By the middle of the Triassic about 225 million years ago, sauropsids had evolved into dinosaurs. Dinosaurs became increasingly important throughout the rest of the Mesozoic Era, as they radiated to fill most terrestrial niches. This is why the Mesozoic Era is called the Age of the Dinosaurs. During the next mass extinction, which occurred at the end of the Mesozoic Era, all of the dinosaurs went extinct. Many other reptiles survived, however, and they eventually gave rise to modern reptiles.
Evolution of Modern Reptile Orders
Figure below shows a traditional phylogenetic tree of living reptiles. Based on this tree, some of the earliest reptiles to diverge were ancestors of turtles. The first turtle-like reptiles are thought to have evolved about 250 million years ago. Ancestral crocodilians evolved at least 220 million years ago. Tuataras may have diverged from squamates (snakes and lizards) not long after that. Finally, lizards and snakes went their separate ways about 150 million years ago.
Traditional Reptile Phylogenetic Tree. This phylogenetic tree is based on physical traits of living and fossil reptiles. Trees based on DNA comparisons may differ from the traditional tree and from each other, depending on the DNA sequences used. Reptile evolution is currently an area of intense research and constant revision.
Summary
• The earliest amniotes appeared about 350 million years ago, and the earliest reptiles evolved from a sauropsida ancestor by about 315 million years ago.
• Dinosaurs evolved around 225 million years ago and dominated animal life on land until 65 million years ago, when they all went extinct.
• Other reptiles survived and evolved into the classes of reptiles that exist today.
Review
1. Identify amniotes called synapsids and sauropsids.
2. Give a brief overview of reptile evolution.
3. Explain why reptiles were able to replace amphibians as the dominant land vertebrates.
12.20: Reptile Ecology
Where do reptiles live?
Anywhere and everywhere. Shown here is a sea turtle. Obviously, this reptile lives in the ocean. But reptiles can also live in the desert, jungle, forest, or even your backyard. Reptiles live in practically every type of habitat.
Ecology of Reptiles
Today, reptiles live in a wide range of habitats. They can be found on every continent except Antarctica. Many turtles live in the ocean, while others live in freshwater or on land. Lizards are all terrestrial, but their habitats may range from deserts to rainforests, and from underground burrows to the tops of trees. Most snakes are terrestrial and live in a wide range of habitats, but some snakes are aquatic. Crocodilians live in and around swamps or bodies of freshwater or salt water.
Reptile Diets
What reptiles eat is also very diverse, but the majority of reptiles are carnivores. Large reptiles such as crocodilians are the top predators in their ecosystems, preying on birds, fish, deer, turtles, and sometimes domestic livestock. Their powerful jaws can crush bones and even turtle shells. Smaller reptiles—including tuataras, snakes, and many lizards—are also important predators, preying on insects, frogs, birds, and small mammals such as mice.
Most terrestrial turtles are herbivores. They graze on grasses, leaves, flowers, and fruits. Marine turtles and some species of lizards are omnivores, feeding on plants as well asinsects, worms, amphibians, and small fish.
Reptiles at Risk
Many species of reptiles, especially marine reptiles, are at risk of extinction. Some are threatened by habitat loss. For example, many beaches where turtles lay their eggs have been taken over and developed by people. Other marine reptiles have been over-hunted by humans. Marine turtles and their eggs are still eaten in some countries despite being protected species.
Some reptiles are preyed upon by non-native species introduced by humans. For example, marine iguanas on the Galápagos Islands are threatened by dogs and cats that people have brought to the islands. The iguanas are slow and tame and have no adaptations to these new predators.
Summary
• Reptiles can be found on every continent except Antarctica.
• Reptiles may live in terrestrial, freshwater, or marine habitats.
• Most reptiles are carnivores, and large reptiles are the top predators in their ecosystems.
• Many species of reptiles, especially marine reptiles, are at risk of extinction.
Review
1. Describe the habitats of most lizards.
2. What do reptiles eat? Describe the diet of a crocodilian.
3. Why are some reptiles at risk?
Explore More
Use this resource to answer the questions that follow.
1. What makes a good meal for giant anaconda?
2. How do anaconda sense their prey?
3. Describe how anaconda hunt and kill their prey.
4. Why is a good meal important for anaconda?
5. How long may a pregnant female anaconda go between meals?
6. How long might it take an anaconda to ingest and digest a large meal? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.19%3A_Reptile_Evolution.txt |
Why is flight so important to birds?
One of the defining traits of many birds is the ability to fly. Obviously, flight is a major evolutionary advantage. But together with the ability to fly must come a number of structural modifications. What do you think these might be?
Structure and Function in Birds
Birds are endothermic tetrapod vertebrates. They are bipedal, which means they walk on two legs. Birds also lay amniotic eggs with hard, calcium carbonate shells. Although birds are the most recent class of vertebrates to evolve, they are now the most numerous vertebrates on Earth. Why have birds been so successful? What traits allowed them to increase and diversify so rapidly? Birds can vary considerably in size, as you can see from the world’s smallest and largest birds, pictured in Figure below. The tiny bee hummingbird is just 5 centimeters (2 inches) long, whereas the ostrich towers over people at a height of 2.7 meters (9 feet). All modern birds have wings, feathers, and beaks. They have a number of other unique traits as well, most of which are adaptations for flight. Flight is used by birds as a means of locomotion in order to find food and mates and to avoid predators. Although not all modern birds can fly, they all evolved from ancestors that could.
Range of Body Size in Birds. The bee hummingbird is the smallest bird. The ostrich is the largest.
Wings and Feathers
Wings are an obvious adaptation for flight. They are actually modified front legs. Birds move their wings using muscles in the chest. These muscles are quite large, making up as much as 35 percent of a bird’s body weight.
Feathers help birds fly and also provide insulation and serve other purposes. Birds actually have two basic types of feathers: flight feathers and down feathers. Both are shown in Figure below. Flight feathers are long, stiff and waterproof. They provide lift and air resistance without adding weight. Down feathers are short and fluffy. They trap air next to a bird’s skin for insulation.
Types of Bird Feathers. These two types of bird feathers have different uses. How is each feather’s structure related to its function?
Organ Systems Adapted for Flight
Birds need a light-weight body in order to stay aloft. Even so, flying is hard work, and flight muscles need a constant supply of oxygen- and nutrient-rich blood. The organ systems of birds are adapted to meet these needs.
• Birds have light-weight bones that are filled with air. They also lack a jaw, which in many vertebrates is a dense, heavy bone with many teeth. Instead, birds have a light-weight keratin beak without teeth.
• Birds have air sacs that store inhaled air and push it into the lungs like bellows. This keeps the lungs constantly filled with oxygenated air. The lungs also contain millions of tiny passages that create a very large surface area for gas exchange with the blood (see Figure below).
• Birds have a relatively large, four-chambered heart. The heart beats rapidly to keep oxygenated blood flowing to muscles and other tissues. Hummingbirds have the fastest heart rate at up to 1,200 beats per minute. That’s almost 20 times faster than the human resting heart rate!
• Birds have a sac-like structure called a crop to store and moisten food that is waiting to be digested. They also have an organ called a gizzard that contains swallowed stones. The stones make up for the lack of teeth by grinding food, which can then be digested more quickly. Both structures make it easier for the digestive system to produce a steady supply of nutrients from food.
Organ System Adaptations for Flight. The intricate passageways in a bird’s lung are adapted for efficient gas exchange. Find the crop and gizzard in the digestive tract diagram. What are their functions? Bird Lung (left), Bird Digestive Tract (right)
Nervous System and Sense Organs
Birds have a large brain relative to the size of their body. Not surprisingly, the part of the brain that controls flight is the most developed part. The large brain size of birds is also reflected by their high level of intelligence and complex behavior. In fact, birds such as crows and ravens may be more intelligent than many mammals. They are smart enough to use objects such as twigs for tools. They also demonstrate planning and cooperation. Most birds have a poor sense of smell, but they make up for it with their excellent sense of sight. Predatory birds have especially good eyesight. Hawks, for example, have vision that is eight times sharper than human vision.
Summary
• Birds are endothermic tetrapod vertebrates. They are bipedal and have wings and feathers.
• Bird organ systems are adapted for flight. For example, they have light-weight air-filled bones and a large four-chambered heart.
• Birds also have relatively large brains and a high level of intelligence.
Review
1. Why do birds fly?
2. List two functions of feathers in birds.
3. Describe the bird crop and gizzard. What are their functions?
4. How do birds keep their lungs filled with oxygenated air?
5. Give an example of bird behavior that shows their relatively great intelligence. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.21%3A_Bird_Structure_and_Function.txt |
Is this pair of birds actually a “couple”?
Yes. Birds do actually pair up each mating season, if not for life. And the male better be prepared to treat his female properly. There is actually an elaborate process in which the female chooses her mate.
Bird Reproduction
Reproduction in birds may be quite complicated and lengthy. Birds reproduce sexually and have separate sexes and internal fertilization, so males and females must mate for fertilization to occur. Mating is generally preceded by courtship. In most species, parents also take care of their eggs and hatchlings.
Courtship and Mating
Courtship is behavior that is intended to attract a mate. It may involve singing specific courtship songs or putting on some type of visual display. For example, a bird may spread out and display its tail feathers or do a ritualized mating “dance.” Typically, males perform the courtship behavior, and females choose a mate from among competing males.
During mating, a male bird presses his cloaca against his mate’s cloaca and passes sperm from his cloaca to hers. After fertilization, eggs pass out of the female’s body, exiting through the opening in the cloaca.
Nesting and Incubation
Eggs are usually laid in a nest. The nest may be little more than a small depression in the ground, or it may be very elaborate, like the weaver bird nest in Figure below. Eggs that are laid on the ground may be camouflaged to look like their surroundings (also shown in Figure below). Otherwise, eggs are usually white or pastel colors such as pale blue or pink.
Variation in Bird Nests. A weaver bird uses grasses to weave an elaborate nest (left). The eggs of a ground-nesting gull are camouflaged to blend in with the nesting materials (right).
After birds lay their eggs, they generally keep the eggs warm with their body heat while the embryos inside continue to develop. This is called incubation, or brooding. In most species, parents stay together for at least the length of the breeding season. In some species, they stay together for life. By staying together, the males as well as females can incubate the eggs and later care for the hatchlings. Birds are the only nonhuman vertebrates with this level of male parental involvement.
Hatchlings
Nest of a marsh warbler (Acrocephalus palustris) with baby birds
Ground-nesting birds, such as ducks and chickens, have hatchlings that are able to run around and feed themselves almost as soon as they break through the eggshell. Being on the ground makes them vulnerable to predators, so they need to be relatively mature when they hatch in order to escape. In contrast, birds that nest off the ground—in trees, bushes, or buildings—have hatchlings that are naked and helpless. The parents must protect and feed the immature offspring for weeks or even months. However, this gives the offspring more time to learn from the parents before they leave the nest and go out on their own.
Parental Care
In birds, 90% to 95% of species are monogamous, meaning the male and female remain together for breeding for a few years or until one mate dies. Birds of all types, from parrots to eagles and falcons, are monogamous. Usually, the parents take turns incubating the eggs. Birds usually incubate their eggs after the last one has been laid. In polygamous species, where there is more than one mate, one parent does all of the incubating. The wild turkey is an example of a polygamous bird.
The length and type of parental care varies widely amongst different species of birds. At one extreme, in a group of birds called the magapodes (which are chicken-like birds), parental care ends at hatching. In this case, the newly-hatched chick digs itself out of the nest mound without parental help and can take care of itself right away. These birds are called precocial. Other precocial birds include the domestic chicken and many species of ducks and geese. At the other extreme, many seabirds care for their young for extended periods of time. For example, the chicks of the Great Frigatebird receive intensive parental care for six months, or until they are ready to fly, and then take an additional 14 months of being fed by the parents (Figure below). These birds are the opposite of precocial birds and are called altricial.
In most animals, male parental care is rare. But it is very common in birds. Often both parents share tasks such as defense of territory and nest site, incubation, and the feeding of chicks. Since birds often take great care of their young, some birds have evolved a behavior calledbrood parasitism. This happens when a bird leaves her eggs in another bird’s nest. The host bird often accepts and raises the parasite bird's eggs.
Great Frigatebird adults are known to care for their young for up to 20 months after hatching, the longest in a bird species. Here, a young bird is begging for food.
Summary
• Birds reproduce sexually and have internal fertilization.
• Mating is generally preceded by courtship.
• Birds' amniotic eggs have hard shells and are laid in a nest. The eggs are usually incubated until they hatch.
• Most species have a relatively long period of parental care.
Review
1. What is courtship? Give an example.
2. Contrast hatchling maturity in birds that are ground-nesting and those that nest off the ground. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.22%3A_Bird_Reproduction.txt |
Raptor vs. landfowl. Any obvious differences?
Of course there are. That is like comparing a turkey to an owl. And there are also flightless birds, birds that live near water, and parrots. With almost 10,000 species of birds, there are bound to be significant differences.
Classification of Birds
There are about 10,000 living species of birds. Almost all of them can fly, but there are several exceptions.
Flightless Birds
Some birds have lost the ability to fly during the course of their evolution. Several flightless birds are shown in Figure below. They include the ostrich, kiwi, rhea, cassowary, and moa. All of these birds have long legs and are adapted for running. The penguins shown in the figure are also flightless birds, but they have a very different body shape. That’s because they are adapted for swimming rather than running.
Flightless Birds. Flightless birds that are adapted for running include the ostrich, kiwi, rhea, cassowary, and moa. Penguins are flightless birds adapted for swimming.
Flying Birds
Birds that are able to fly are divided into 29 orders that differ in their physical traits and behaviors. Table below describes seven of the most common orders. As shown in the table, the majority of flying birds are perching birds, like the honeyeater described in the last row of the table. The order of perching birds has more species than all the other bird orders combined. In fact, this order of birds is the largest single order of land vertebrates.
Order Description Example
Landfowl: turkeys, chickens, pheasants They are large in size; they spend most of their time on the ground; they usually have a thick neck and short, rounded wings; their flight tends to be brief and close to the ground.
turkey
Waterfowl: ducks, geese, swans They are large in size; they spend most of their time on the water surface; they have webbed feet and are good swimmers; most are strong flyers.
ducks
Shorebirds: puffins, gulls, plovers They range from small to large; most live near the water, and some are sea birds; they have webbed feet and are good swimmers; most are strong flyers.
puffin
Diurnal Raptors: hawks, falcons, eagles They range from small to large; they are active during the day and sleep during the night; they have a sharp, hooked beak and strong legs with clawed feet; they hunt by sight and have excellent vision.
hawk
Nocturnal Raptors: burrowing owls, barn owls, horned owls They range from small to large; they are active during the night and sleep during the day; they have a sharp, hooked beak and strong legs with clawed feet; they have large, forward-facing eyes; they have excellent hearing and can hunt with their sense of hearing alone.
burrowing owl
Parrots: cockatoos, parrots, parakeets They range from small to large; they are found in tropical regions; they have a strong, curved bill; they stand upright on strong legs with clawed feet; many are brightly colored; they are very intelligent.
cockatoo
Perching Birds: honeyeaters, sparrows, crows They are small in size; they perch above the ground in trees and on buildings and wires; they have four toes for grasping a perch; many are songbirds.
honeyeater
Summary
• There are about 10,000 living species of birds, almost all of which can fly.
• Flying birds are divided into 29 orders. The most common orders include landfowl, waterfowl, shorebirds, diurnal and nocturnal raptors, parrots, and perching birds.
Review
1. Name and describe flightless birds.
2. Compare and contrast nocturnal and diurnal raptors.
3. Give examples of landfowl and waterfowl.
4. Describe parrots. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.23%3A_Bird_Classification.txt |
Did birds really evolve from dinosaurs?
One is an ostrich, the other is a mononykus dinosaur. The structural relationship is obvious. Mononykus moved about on two legs, was very nimble, and could run at high speeds, something that would have been useful in the open desert plains where they lived. It had a small skull, and its teeth were small and pointed, suggesting that it ate insects and small animals, such as lizards and mammals.
Evolution of Birds
Birds are thought to have evolved from a group of bipedal dinosaurs called theropods. The ancestor of birds was probably similar to the theropod called Deinonychus, which is represented by the sketch in Figure below. Fossils of Deinonychus were first identified in the 1960s. This was an extremely important discovery. It finally convinced most scientists that birds had descended from dinosaurs, which had been debated for almost a century.
Extinct Bird Relative: Deinonychus. Deinonychus shared many traits with birds. What similarities with birds to you see?
What was Deinonychus?
Deinonychus is the genus name of an extinct dinosaur that is considered to be one of the closest non-bird relatives of modern birds. It lived about 110 million years ago in what is now North America. Deinonychus was a predatory carnivore with many bird-like features. For example, it had feathers and wings. It also had strong legs with clawed feet, similar to modern raptors. Its respiratory, circulatory, and digestive systems were similar to those of birds as well. The location of fossilized eggs near Deinonychus fossils suggests that it may have brooded its eggs. This would mean that it was endothermic. (Can you explain why?) On the other hand, Deinonychus retained a number of reptile-like traits, such as jaws with teeth, and hands with claws at the tips of its wings.
Evolution of Flight
Scientists have long speculated about the evolution of flight in birds. They wonder how and why birds evolved wings from a pair of front limbs. Several hypotheses have been suggested. Here are just two:
1. Wings evolved in a bird ancestor that leapt into the air to avoid predators or to capture prey. Therefore, wings are modified arms that helped the animal leap higher.
2. Wings evolved in a bird ancestor that lived in trees. Thus, wings are modified arms that helped the animal glide from branch to branch.
Scientists still don’t know how or why wings and flight evolved, but they continue to search for answers. In addition to fossils, they are studying living vertebrates such as bats that also evolved adaptations for flight.
Summary
• Birds are thought to have evolved from theropod dinosaurs around 150 million years ago.
• Bird ancestors may have been similar to the extinct theropod Deinonychus, whose fossils convinced most scientists that birds evolved from dinosaurs.
• Scientists still don’t know how or why wings and flight evolved, but they continue to search for answers.
Review
1. What was the bird ancestor?
2. What was Deinonychus? What bird-like traits were evident in Deinonychus?
3. What are the two ideas associated with the evolution of flight? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.24%3A_Bird_Evolution.txt |
Where do birds live?
Practically anywhere they want. From some of the coldest regions on the planet to the warmest. Look at these penguins and where they live. Would you want to live there?
Ecology of Birds
Birds live and breed in most terrestrial habitats on all seven continents, from the Arctic to Antarctica. Because they are endothermic, birds can live in a wider range of climates than reptiles or amphibians, although the greatest diversity of birds occurs in tropical regions. Birds are important members of every ecosystem in which they live, occupying a wide range of ecological positions.
Bird Diets
Some birds are generalists. A generalist is an organism that can eat many different types of food. Other birds are highly specialized in their food needs and can eat just one type of food.
Raptors such as hawks and owls are carnivores. They hunt and eat mammals and other birds. Vultures are scavengers. They eat the remains of dead animals, such as roadkill. Aquatic birds generally eat fish or water plants. Perching birds may eat insects, fruit, honey, or nectar. Many fruit-eating birds play a key role in seed dispersal, and some nectar-feeding birds are important pollinators.
Bird beaks are generally adapted for the food they eat. For example, the sharp, hooked beak of a raptor is well suited for killing and tearing apart prey. The long beak of the hummingbird in Figure below co-evolved with the tube-shaped flowers from which it sips nectar.
Hummingbird Sipping Nectar. A hummingbird gets nectar from flowers and pollinates the flowers in return. What type of relationship exists between the bird and the flowering plant?
Birds at Risk
Hundreds of species of birds have gone extinct as a result of human actions. A well-known example is the passenger pigeon. It was once the most common bird in North America, but overhunting and habitat destruction led to its extinction in the 1800s. Habitat destruction and use of the pesticide DDT explain the recent extinction of the dusky seaside sparrow. This native Florida bird was declared extinct in 1990.
Today, some 1,200 species of birds are threatened with extinction by human actions. Humans need to take steps to protect this precious and important natural resource. What can you do to help?
The Golden Eagle
Although not as famous as its bald cousin, Golden Eagles are much easier to find in Northern California - one of the largest breeding populations for Golden Eagles. The largest of the raptors, Golden Eagles weigh typically between 8 and 12 pounds, and their wing span is around 6 to 7 feet. These eagles dive towards earth to catch prey, and can reach speeds of up to 200 mph!
The Great Horned Owl
Owls are amazing creatures. They have many adaptations that allow them to thrive in their environments. Their claws are enormous and powerful, they have excellent hearing, and fantastic vision in low light. And the Great Horned Owl can fly almost silently due to "fringes" on their feathers that help to break up the sound of air passing over their wings.
Summary
• Birds live and breed in most terrestrial habitats on all seven continents. They occupy a wide range of ecological positions.
• Raptors are carnivores; aquatic birds eat fish or water plants; and perching birds may eat insects, fruit, honey, or nectar.
• Some birds are pollinators that co-evolved with plants.
• Human actions have caused the extinction of hundreds of species of birds, and some 1,200 species are threatened with extinction today.
Review
1. What is a generalist?
2. Why did the hummingbird pictured sipping nectar above evolve such a long, pointed beak?
3. What bird would eat insects, fruit, honey, or nectar?
4. Draw a sketch of a hypothetical bird that preys on small mammals. The bird must exhibit traits that suit it for its predatory role.
12.26: Mammal Characteristics
One of these is not a mammal. Which one?
Mammals are a class of endothermic vertebrates. They have four limbs and produce amniotic eggs. Examples of mammals include bats, whales, mice, and humans. Clearly, mammals are a very diverse group. Nonetheless, they share many traits that set them apart from other vertebrates.
Characteristics of Mammals
Two characteristics are used to define the mammal class. They are mammary glands and body hair (or fur).
1. Female mammals have mammary glands. The glands produce milk after the birth of offspring. Milk is a nutritious fluid. It contains disease-fighting molecules as well as all the nutrients a baby mammal needs. Producing milk for offspring is called lactation.
2. Mammals have hair or fur. It insulates the body to help conserve body heat. It can also be used for sensing and communicating. For example, cats use their whiskers to sense their surroundings. They also raise their fur to look larger and more threatening (see Figure below).
Cat Communicating a Warning. By raising its fur, this cat is “saying” that it’s big and dangerous. This might discourage a predator from attacking.
Most mammals share several other traits. The traits in the following list are typical of, but not necessarily unique to, mammals.
• The skin of many mammals is covered with sweat glands. The glands produce sweat, the salty fluid that helps cool the body.
• Mammalian lungs have millions of tiny air sacs called alveoli. They provide a very large surface area for gas exchange.
• The heart of a mammal consists of four chambers. This makes it more efficient and powerful for delivering oxygenated blood to tissues.
• The brain of a mammal is relatively large and has a covering called the neocortex. This structure plays an important role in many complex brain functions.
• The mammalian middle ear has three tiny bones that carry sound vibrations from the outer to inner ear. The bones give mammals exceptionally good hearing. In other vertebrates, the three bones are part of the jaw and not involved in hearing.
• Mammals have four different types of teeth. The teeth of other vertebrates, in contrast, are all alike.
Dolphins are mammals that have adapted to swimming and reproducing in water.
Summary
• Mammals are a class of endothermic vertebrates.
• Mammals have four limbs and produce amniotic eggs.
• The mammal class is defined by the presence of mammary glands and hair (or fur).
• Other traits of mammals include sweat glands in their skin, alveoli in their lungs, a four-chambered heart, and a brain covering called the neocortex.
Review
1. List five traits that are shared by all mammals, including the two traits that are used to define the mammal class. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.25%3A_Bird_Ecology.txt |
Does this mammal breathe like all other mammals?
Essentially, yes it does. A whale is a mammal, so it has a pair of lungs, not gills like fish. As shown here, whales take oxygen out of the air, not out of water.
Eating and Digesting Food
Maintaining a high metabolic rate takes a lot of energy. The energy must come from food. Therefore, mammals need a nutritious and plentiful diet. The diets of mammals are diverse. Except for leaf litter and wood, almost any kind of organic matter may be eaten by mammals.
Some mammals are strictly herbivores or strictly carnivores. However, most mammals will eat other foods if necessary. Some mammals are omnivores. They routinely eat a variety of both plant and animal foods. Most mammals also feed on a variety of other species. The few exceptions include koalas, which feed only on eucalyptus plants, and giant pandas, which feed only on bamboo. Types of mammalian diets and examples of mammals that eat them are given in Table below. How would you classify your own diet?
Type of Diet Foods Eaten Examples of Mammals with this Type of Diet
herbivorous diet: plants leaves, grasses, shoots, stems, roots, tubers, seeds, nuts, fruits, bark, conifer needles, flowers
rabbit, mouse, sea cow, horse, goat, elephant, zebra, giraffe, deer, elk, hippopotamus, kangaroo, monkey
carnivorous diet: animals other mammals, birds, reptiles,amphibians, fish, mollusks, worms, insects
aardvark, anteater, whale, hyena, dog, jackal, dolphin, wolf, weasel, seal, walrus, cat, otter, mole
omnivorous diet: plants and animals any of the foods eaten in herbivorous and carnivorous diets
bear, badger, mongoose, fox, raccoon, human, rat, chimpanzee, pig
Different diets require different types of digestive systems. Mammals that eat a carnivorous diet generally have a relatively simple digestive system. Their food consists mainly of proteins and fats that are easily and quickly digested. Herbivorous mammals, on the other hand, tend to have a more complicated digestive system. Complex plant carbohydrates such as cellulose are more difficult to digest. Some herbivores have more than one stomach. The stomachs store and slowly digest plant foods.
Mammalian teeth are also important for digestion. Mammals have four different types of teeth. The teeth of other vertebrates, in contrast, are all alike. The four types of teeth are specialized for different feeding functions, as shown in Figure below. Together, the four types of teeth can cut, tear, and grind food. This makes food easier and quicker to digest.
Mammalian Teeth (Human). With their different types of teeth, mammals can eat a wide range of foods.
Lungs and Heart of Mammals
Keeping the rate of metabolism high takes a constant and plentiful supply of oxygen. That’s because cellular respiration, which produces energy, requires oxygen. The lungs and heart of mammals are adapted to meet their oxygen needs.
The lungs of mammals are unique in having alveoli. These are tiny, sac-like structures. Each alveolus is surrounded by a network of very small blood vessels (see Figure below). Because there are millions of alveoli in each lung, they greatly increase the surface area for gas exchange between the lungs and bloodstream. Human lungs, for example, contain about 300 million alveoli. They give the lungs a total surface area for gas exchange of up to 90 square meters (968 square feet). That’s about as much surface area as one side of a volleyball court!
Alveoli of Mammalian Lungs. Clusters of alveoli resemble tiny bunches of grapes. They are surrounded by many blood vessels for gas exchange.
Mammals breathe with the help of a diaphragm. This is the large muscle that extends across the bottom of the chest below the lungs. When the diaphragm contracts, it increases the volume of the chest. This decreases pressure on the lungs and allows air to flow in. When the diaphragm relaxes, it decreases the volume of the chest. This increases pressure on the lungs and forces air out.
The four-chambered mammalian heart can pump blood in two different directions. The right side of the heart pumps blood to the lungs to pick up oxygen. The left side of the heart pumps blood containing oxygen to the rest of the body. Because of the dual pumping action of the heart, all of the blood going to body cells is rich in oxygen.
The Mammalian Brain
Of all vertebrates, mammals have the biggest and most complex brain for their body size (see Figure below). The front part of the brain, called the cerebrum, is especially large in mammals. This part of the brain controls functions such as memory and learning.
Vertebrate Brains. Vertebrate brains come in a range of sizes. Even the brains of mammals show a lot of variation in size. The area of the neocortex is greatest in humans.
The brains of all mammals have a unique layer of nerve cells covering the cerebrum. This layer is called the neocortex (the pink region of the brains in Figure above). The neocortex plays an important role in many complex brain functions. In some mammals, such as rats, the neocortex is relatively smooth. In other mammals, especially humans, the neocortex has many folds. The folds increase the surface area of the neocortex. The larger this area is, the greater the mental abilities of an animal.
Intelligence of Mammals
Mammals are very intelligent. Of all vertebrates, they are the animals that are most capable of learning. Mammalian offspring are fed and taken care of by their parents for a relatively long time. This gives them plenty of time to learn from their parents. By learning, they can benefit from the experiences of their elders. The ability to learn is the main reason that the large mammalian brain evolved. It’s also the primary reason for the success of mammals.
Summary
• Mammals may be herbivores, carnivores, or omnivores. They have four types of teeth, so they can eat a wide range of foods.
• Traits of the heart and lungs keep the cells of mammals well supplied with oxygen and nutrients.
• Mammals have a relatively large brain and a high level of intelligence.
Review
1. Identify three mammals that are herbivores, three that are carnivores, and three that are omnivores.
2. What are alveoli? What is their function?
3. Explain how mammalian teeth differ from the teeth of other vertebrates.
4. Compare and contrast the mammalian brain with the brains of other vertebrates. How is the brain of mammals related to their ability to learn? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.27%3A_Mammal_Structure_and_Function.txt |
Is this bear cold?
Not really. Like all mammals, polar bears maintain a stable internal temperature. They do not need to stay warm by lying in the sun. This allows them to live in cold climates.
Endothermy in Mammals
Many structures and functions in mammals are related to endothermy. Mammals can generate and conserve heat when it’s cold outside. They can also lose heat when they become overheated. How do mammals control their body temperature in these ways?
How Mammals Stay Warm
Mammals generate heat mainly by keeping their metabolic rate high. The cells of mammals have many more mitochondria than the cells of other animals. The extra mitochondria generate enough energy to keep the rate of metabolism high. Mammals can also generate little bursts of heat by shivering. Shivering occurs when many muscles contract a little bit all at once. Each muscle that contracts produces a small amount of heat.
Conserving heat is also important, especially in small mammals. A small body has a relatively large surface area compared to its overall size. Because heat is lost from the surface of the body, small mammals lose a greater proportion of their body heat than large mammals. Mammals conserve body heat with their hair or fur. It traps a layer of warm air next to the skin. Most mammals can make their hair stand up from the skin, so it becomes an even better insulator. Even humans automatically contract these muscles when they are cold, causing goosebumps (see Figure below). Mammals also have a layer of fat under the skin to help insulate the body. This fatty layer is not found in other vertebrates.
Mammals raise their hair with tiny muscles in the skin. Even humans automatically contract these muscles when they are cold. They cause “goosebumps,” as shown here.
How Mammals Stay Cool
One way mammals lose excess heat is by increasing blood flow to the skin. This warms the skin so heat can be given off to the environment. That’s why you may get flushed, or red in the face, when you exercise on a hot day. You are likely to sweat as well. Sweating also reduces body heat. Sweat wets the skin, and when it evaporates, it cools the body. Evaporation uses energy, and the energy comes from body heat. Animals with fur, such as dogs, use panting instead of sweating to lose body heat (see Figure below). Evaporation of water from the tongue and other moist surfaces of the mouth and throat uses heat and helps cool the body.
Panting Dog. This dog is overheated. It is losing excess body heat by panting.
Summary
• Mammals have several ways of generating and conserving heat, such as a high metabolic rate and hair to trap heat.
• Mammals also have several ways to stay cool, including sweating or panting.
Review
1. Describe how mammals stay warm and conserve heat?
2. What is the function of sweating? Explain your answer.
3. How do animals with fur lose excess heat?
12.29: Mammal Living and Locomotion
What allows this cheetah to be so fast?
Do you think it has something to do with the placement of the limbs? Well, it does. Look closely at the cheetah's body. Can you describe why it is so fast?
Social Living in Mammals
Many mammals live in social groups. Social living evolved because it is adaptive. Consider these two examples:
1. Herbivores such as zebras and elephants live in herds. Adults in the herd surround and protect the young, who are most vulnerable to predators.
2. Lions live in social groups called prides. Adult females in the pride hunt cooperatively, which is more efficient than hunting alone. Then they share the food with the rest of the pride. For their part, adult males defend the pride’s territory from other predators.
Locomotion in Mammals
Mammals are noted for the many ways they can move about. Generally, their limbs are very mobile. Often, they can be rotated. Many mammals are also known for their speed. The fastest land animal is a predatory mammal. Can you guess what it is? Racing at speeds of up to 112 kilometers (70 miles) per hour, the cheetah wins hands down. In addition, the limbs of mammals let them hold their body up above the ground. That’s because the limbs are attached beneath the body, rather than at the sides as in reptiles (see Figure below).
Limb Positions in Reptiles and Mammals. The sprawling limbs of a reptile keep it low to the ground. A mammal has a more upright stance.
Mammals may have limbs that are specialized for a particular way of moving. They may be specialized for running, jumping, climbing, flying, or swimming. Mammals with these different modes of locomotion are pictured in Figure below.
Mammalian Locomotion. Mammals have many different modes of locomotion.
The deer in the Figure above is specialized for running. Why? It has long legs and hard hooves. Can you see why the other animals in the figure are specialized for their particular habitats? Notice how arboreal, or tree-living animals, have a variety of different specializations for moving in trees. For example, they may have:
• A prehensile, or grasping, tail. This is used for climbing and hanging from branches.
• Very long arms for swinging from branch to branch. This way of moving is called brachiation.
• Sticky pads on their fingers. The pads help them cling to tree trunks and branches.
Summary
• Mammals live in social groups, which are an adaptive beneficial trait.
• Mammals have many ways of moving about and may move very quickly.
Review
1. Name five types of movement found in mammals?
2. A certain mammal has very long forelimbs. What does that suggest about where the animal lives and how it moves?
3. What type of movement is found in the above question.
4. What is the purpose of a prehensile tail?
12.30: Marsupials
Marsupials have a different way of reproducing that reduces the mother’s risks. A marsupial is a therian mammal in which the embryo is born at an early, immature stage. The embryo completes its development outside the mother’s body in a pouch on her belly. Only a minority of therian mammals are marsupials. They live mainly in Australia. Examples of marsupials are pictured in Figure below.
Marsupials. Marsupials include the kangaroo, koala, and opossum. The koala, sometimes called the koala bear, is a marsupial native to Australia. And it is very different to any other bear.
The Marsupial Embryo
The marsupial embryo is nourished inside the uterus with food from a yolk sac instead of through a placenta. The yolk sac stores enough food for the short period of time the embryo remains in the uterus. After the embryo is born, it moves into the mother’s pouch, where it clings to a nipple. It remains inside the pouch for several months while it continues to grow and develop. Even after the offspring is big enough to leave the pouch, it may often return to the pouch for warmth and nourishment. Eventually, the offspring is mature enough to remain outside the pouch on its own.
Pros and Cons of Marsupial Development
In marsupials, the short period of development within the mother’s uterus reduces the risk of her immune system attacking the embryo. In addition, the marsupial mother doesn’t have to eat extra food or carry a large fetus inside her. The risks of giving birth to a large fetus are also avoided. Another pro is that the mother can expel the embryo from her pouch if she is pursued by a predator or if food is scarce. On the other hand, a newborn marsupial is tiny and fragile. Therefore, it may be less likely to survive than a newborn placental mammal.
The North American Marsupial: The Opossum
Most people think of opossums as scary creatures. Is this because they look kind of funny, walk kind of funny, have beady eyes and sharp teeth, and can emit a very foul odor? Maybe. But what is so different about opossums is that they are the only marsupial in North America. These opossums, however, can be beneficial to humans. They use their sharp teeth to crush bone – which means that they are good getting rid of unwanted rodents in your neighborhood. They have excellent immune systems and they emit that terrible odor for protection.
Summary
• Marsupials give birth to a tiny, immature embryo. The embryo then continues to grow and develop in a pouch on the mother’s belly.
• Marsupial development is less risky for the mother. However, the embryo is fragile, so it may be less likely to survive than the fetus of a placental mammal.
Review
1. What is a marsupial?
2. Where does a marsupial embryo develop? How is it nourished?
3. Name two advantages of marsupial development? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.28%3A_Mammal_Endothermy.txt |
Can a mammal lay an egg?
This is a mammal. But it is unlike placental or marsupial mammals. If a mammal does not develop within a placenta or a pouch, what type of development is left? Eggs!
Monotremes
Only five living species of mammals are not therian mammals. They are called monotremes.Monotremes are mammals that reproduce by laying eggs. The only living monotreme species are the platypus and echidnas (see Figure below and Figure below). They are found solely in Australia and New Guinea (an island not far from Australia).
This egg-laying, venomous, duck-billed, beaver-tailed, otter-footed mammal is a platypus, a monotreme mammal that reproduces by laying eggs.
Like the platypus, the echidna is a monotreme. The only living monotreme species inhabit Australia and New Guinea.
Eggs and Lactation in Monotremes
Female monotremes lack a uterus and vagina. Instead, they have a cloaca with one opening, like the cloacas of reptiles and birds. The opening is used to excrete wastes as well as lay eggs.
Monotreme eggs have a leathery shell, like the eggs of reptiles. The eggs are retained inside the mother’s body for at least a couple of weeks. During that time, the mother provides the eggs with nutrients. Platypus females lay their eggs in a burrow. Echidna females have a pouch in which they store their eggs. Female monotremes have mammary glands but lack nipples. Instead, they “sweat” milk from a patch on their belly.
Pros and Cons of Monotreme Reproduction
The mother’s risks are less in monotremes than in therian mammals. The mother doesn’t need to eat more or put herself at risk by carrying and delivering a fetus or an embryo. On the other hand, externally laid eggs are more difficult to protect than an embryo in a pouch or a fetus in a uterus. Therefore, monotreme offspring may be less likely to survive than the offspring of therian mammals.
Summary
• Monotremes reproduce by laying eggs.
• Monotremes have a cloaca instead of a uterus and vagina. The eggs pass through the opening of the cloaca.
• Monotreme reproduction is the least risky for the mother. However, eggs are harder to protect than is an embryo or a fetus in a pouch or uterus. Therefore, monotreme offspring may have a lower chance of surviving than the offspring of therian mammals.
Review
1. What are monotremes?
2. Describe eggs and egg laying in monotremes.
3. How does lactation differ in monotremes and therian mammals?
4. Create a chart that you could use to explain to a younger student the different ways that mammals reproduce.
12.32: Mammal Ancestors
Which mammalian trait evolved first? What was the first mammal like? When did the earliest mammal live?
Detailed answers to these questions are still in dispute, though it is probably safe to say the earliest mammals were not like this giraffe. Obviously, the giraffe has some specialized traits.
Major Events in Mammalian Evolution
Scientists do generally agree on the major events in the evolution of mammals. These are summarized in Table below. Refer back to the table as you read about the events in this concept. *mya = millions of years ago
Era Period Epoch Major Events Start (mya)*
Cenozoic Neogene Holocene Rise of human civilization; spread and dominance of modern humans 0.01
- - Pleistocene Spread and then extinction of many large mammals; appearance of modern humans 1.8
- - Pliocene Appearance of many existing genera of mammals, including the genus Homo 5.3
- - Miocene Appearance of remaining modern mammal families; diversification of horses and mastodons; first apes 23.0
- Paleogene Oligocene Rapid evolution and diversification of placental mammals 33.9
- - Eocene Appearance of several modern mammal families; diversification of primitive whales 55.8
- - Paleocene Appearance of the first large mammals 65.5
Mesozoic Cretaceous - Emergence of monotreme, marsupial, andplacental mammals; possible first appearance of four clades (superorders) of placental mammals (Afrotheria, Xenarthra, Laurasiatheria, Supraprimates) 145.5
- Jurassic - Spread of mammals, which remain small in size 199.6
- Triassic - Evolution of cynodonts to become smaller and more mammal-like; appearance of the first mammals 251.0
Paleozoic Permian - Evolution and spread of synapsids (pelycosaurs and therapsids) 299.0
- Carboniferous - Appearance of amniotes, the first fully terrestrial vertebrates 359.0
Mammalian Ancestors
Ancestors of mammals evolved close to 300 million years ago. They were amniotes called synapsids. Figure below shows how modern mammals evolved from synapsids. The stages of evolution from synapsids to mammals are described below.
Phylogeny of Mammalian Evolution. This diagram represents the evolution of mammals.
Pelycosaurs
Synapsids called pelycosaurs became the most common land vertebrates during the first half of the Permian Period. A pelycosaur genus called Dimetrodon is shown in Figure below. Dimetrodon had sprawling legs and walked like a lizard. It also had a fairly small brain. However, it had started to develop some of the traits of mammals. For example, it had teeth of different types.
Pelycosaur Synapsid: Dimetrodon. Dimetrodon was a pelycosaur. It lived about 275 million years ago.
Therapsids
Some pelycosaurs gave rise to a group of animals called therapsids. The earliest therapsids lived about 260 million years ago. At first, the therapsids looked a lot like Dimetrodon. But after a while, they could easily be mistaken for mammals. They evolved a number of mammalian traits, such as legs positioned under the body instead of along the sides. Therapsids became the most common and diverse land vertebrates during the second half of the Permian Period.
The Permian Period ended about 250 million years ago with a mass extinction. Most therapsids went extinct. Their niches were taken over by sauropsids. These were the amniotes that evolved into dinosaurs, reptiles, and birds. Not all therapsids went extinct, however. The few that remained no longer had to compete with many other therapsids. Some of them eventually evolved into mammals.
Cynodonts
The surviving therapsids were small animals. Some of the most successful were the cynodonts (see Figure below). They flourished worldwide during the first half of the Triassic Period. Some of them ate insects and were nocturnal, or active at night. Being nocturnal may have helped save them from extinction. Why? A nocturnal niche was one of the few niches that dinosaurs did not take over in the Triassic Period.
Cynodonts became more mammal-like as they continued to evolve. Some of their mammalian traits may have been adaptations to their nocturnal niche. For example:
• The ability to regulate body temperature might have been selected for because it would allow nocturnal animals to remain active in the cool of the night.
• A good sense of hearing might have been selected for because it would be more useful than good vision when hunting in the dark.
Probable Mammalian Ancestor: Cynodont. Cynodonts were mammal-like therapsids. They may have been ancestral to mammals. They were about the size of a rat.
By the end of the Triassic Period, cynodonts had become even smaller in size. They also had evolved many mammalian traits. For example, they had
• Four different types of teeth
• A relatively large brain
• Three tiny bones in the middle ear
• A diaphragm for breathing
• Endothermy
• Lactation
• Hair
Cynodonts probably gave rise to mammals about 200 million years ago. However, they are not considered to be mammals themselves. In fact, competition with early mammals may have led to their extinction. They went extinct sometime during the Jurassic or Cretaceous Period.
Summary
• Amniotes called synapsids were the ancestors of mammals.
• Synapsids named pelycosaurs had some of the traits of mammals by 275 million years ago.
• Some synapsids evolved into therapsids, which became widespread during the Permian Period.
• The few therapsids that survived the Triassic takeover were small, arboreal insect eaters. They were also nocturnal. Being active at night may explain why they survived and evolved still more mammalian traits.
Review
1. What were the synapsids? When were they most widespread?
2. Identify the therapsids. How were they related to mammals?
3. Describe cynodonts. What is their place in the evolution of mammals?
4. Describe mammalian traits found in cynodonts. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.31%3A_Monotremes.txt |
Could you argue that the duckbilled platypus has some characteristics of other species?
Like a bird or a fish? You could. This might suggest that an ancestor of this species may have been one of the early mammals to evolve.
Evolution of Early Mammals
The earliest mammals evolved from cynodonts. But the evolution of mammals didn’t end there. Mammals continued to evolve. Monotreme mammals probably split off from other mammals first. They were followed by marsupials. Placental mammals probably evolved last.
Evolution of Monotremes
The first monotremes may have evolved about 150 million years ago. Early monotreme fossils have been found in Australia. An example is a genus called Steropodon, shown in Figure below. It may have been the ancestor of the platypus. Early monotremes retained some of the traits of their therapsid ancestors. For example, they laid eggs and had a cloaca. These traits are still found in modern monotremes.
Probable Monotreme Ancestor: Steropodon. Like the platypus, Steropodon probably had a bill.
Evolution of Marsupials
The first marsupials may have evolved about 130 million years ago. One of the earliest was the extinct genus Sinodelphys. A fossil of this mammal is shown in. It is a remarkable fossil find. It represents a nearly complete animal. Even tufts of hair and imprints of soft tissues were preserved.
Sinodelphys was about 15 centimeters (6 inches) long. Its limb structure suggests that it was a climbing animal. It could escape from predators by climbing into trees. It probably lived on a diet of insects and worms.
Evolution of Placental Mammals
The earliest placental mammals may have evolved about 110 million years ago. The ancestor of placental mammals may be the extinct genus Eomaia. Fossils of Eomaia have been found in what is now China. It was only about 10 centimeters (4 inches) long. It was a tree climber and probably ate insects and worms. Eomaia had several traits of placental mammals. Figure below shows how Eomaia may have looked.
Probable Ancestor of Placental Mammals: Eomaia. Eomaia lived a little over 100 million years ago.
The placental mammal descendants of Eomaia were generally more successful than marsupials and monotremes. On most continents, placental mammals became the dominant mammals, while marsupials and monotremes died out. Marsupials remained the most common and diverse mammals in Australia. The reason for their success there is not yet resolved.
Summary
• Monotremes evolved about 150 million years ago. Like modern monotremes, they had a cloaca and laid eggs.
• Marsupials evolved about 130 million years ago. They were very small and ate insects and worms.
• Placental mammals evolved about 110 million years ago. They were also small and climbed trees.
• Placental mammals became the dominant land mammals. Most marsupials and monotremes died out, except in Australia.
Review
1. Outline the evolution of monotreme, marsupial, and placental mammals.
2. Describe Sinodelphys.
3. Describe Eomaia.
12.34: Mammal Classification
How would you classify this mammal?
Obviously it is a camel, but is there more to it than that? There are 17 orders of placental mammals. But then these mammals need to be further classified into families, genera, and finally species.
Classification of Placental Mammals
Traditional classifications of mammals are based on similarities in structure and function. Increasingly, mammals are being classified on the basis of molecular similarities. DNAanalyses has recently shown that the traditional orders include mammals that may not be closely related.
Traditional Classification
The most widely accepted traditional classification of mammals divides living placental mammals into 17 orders. These orders are shown in Table below. This classification of mammals was widely accepted for more than 50 years. Placental mammals are still commonly placed in these orders. However, this classification is not very useful for studies of mammalian evolution. That’s because it groups together some mammals that do not seem to be closely related by descent from a recent common ancestor.
Order Example Sample Trait
Insectivora
mole
small sharp teeth
Edentata
anteater
few or no teeth
Pholidota
pangolin
large plate-like scales
Chiroptera
bat
digits support membranous wings
Carnivora
coyote
long pointed canine teeth
Rodentia
mouse
incisor teeth grow continuously
Lagomorpha
rabbit
chisel-like incisor teeth
Perissodactyla
horse
odd-toed hooves
Artiodactyla
deer
even-toed hooves
Cetacea
whale
paddle-like forelimbs
Primates
monkey
five digits on hands and feet
Proboscidea
elephant
tusks
Hyracoidea
hyrax
rubbery pads on feet
Dermoptera
colugo
membrane of skin between legs for gliding
Pinnipedia
seal
feet with fins
Sirenia
manatee
paddle-like tail
Tubulidentata
aardvark
teeth without enamel
Phylogenetic Classification
The mammalian supertree classifies placental mammals phylogenetically. It uses the analysis of DNA sequences to group together mammals that are evolutionarily closely related, sharing a recent common ancestor. These groups are not necessarily the same as the traditional groups based on structure and function.
The supertree classification places placental mammals in four superorders. The four superorders and some of the mammals in them are:
• Afrotheria—aardvarks, elephants, manatees.
• Xenarthra—anteaters, sloths, armadillos.
• Laurasiatheria—bats, whales, hoofed mammals, carnivores.
• Supraprimates—primates, rabbits, rodents.
All four superorders appear to have become distinct from one another between 85 and 105 million years ago. The exact relationships among the superorders are still not clear. Revisions in this classification of mammals may occur as new data become available.
Summary
• Mammals used to be classified on the basis of similarities in structure and function into 17 different orders.
• Recently, DNA analyses have shown that the traditional orders include mammals that are not closely related.
• Phylogenetic classification, based on DNA data, groups placental mammals in four superorders. The superorders appear to have become distinct from each other 85–105 million years ago.
Review
1. Compare traditional and phylogenetic classifications of placental mammals. Explain which type of classification is more useful for understanding how mammals evolved.
2. Assume that a new species of placental mammal has been discovered. Scientists have examined it closely and studied its DNA. It has wings similar to a bat that it uses for gliding. Its DNA is most similar to the DNA of rodents such as mice. How would you classify the new mammal? Explain your answer. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.33%3A_Evolution_of_Early_Mammals.txt |
Thumbnail: Front view of man and woman. (Public Domain; Mikael Häggström).
13: Human Biology
What makes a muscle contract?
It starts with a signal from the nervous system. So it starts with a signal from your brain. The signal goes through your nervous system to your muscle. Your muscle contracts, and your bones move. And all this happens incredibly fast.
Muscle Contraction
Muscle contraction occurs when muscle fibers get shorter. Literally, the muscle fibers get smaller in size. To understand how this happens, you need to know more about the structure of muscle fibers.
Structure of Muscle Fibers
Each muscle fiber contains hundreds of organelles called myofibrils. Each myofibril is made up of two types of protein filaments: actin filaments, which are thinner, and myosin filaments, which are thicker. Actin filaments are anchored to structures called Z lines (Figure 13.13.2). The region between two Z lines is called a sarcomere. Within a sarcomere, myosin filaments overlap the actin filaments. The myosin filaments have tiny structures called cross bridges that can attach to actin filaments.
Figure 13.13.2: Sarcomere. A sarcomere contains actin and myosin filaments between two Z lines.
Sliding Filament Theory
The most widely accepted theory explaining how muscle fibers contract is called the sliding filament theory. According to this theory, myosin filaments use energy from ATP to “walk” along the actin filaments with their cross bridges. This pulls the actin filaments closer together. The movement of the actin filaments also pulls the Z lines closer together, thus shortening the sarcomere.
When all of the sarcomeres in a muscle fiber shorten, the fiber contracts. A muscle fiber either contracts fully or it doesn’t contract at all. The number of fibers that contract determines the strength of the muscular force. When more fibers contract at the same time, the force is greater.
Muscles and Nerves
Muscles cannot contract on their own. They need a stimulus from a nerve cell to “tell” them to contract. Let’s say you decide to raise your hand in class. Your brain sends electrical messages to nerve cells, called motor neurons, in your arm and shoulder. The motor neurons, in turn, stimulate muscle fibers in your arm and shoulder to contract, causing your arm to rise. Involuntary contractions of cardiac and smooth muscles are also controlled by nerves.
Summary
• According to the sliding filament theory, a muscle fiber contracts when myosin filaments pull actin filaments closer together and thus shorten sarcomeres within a fiber.
• When all the sarcomeres in a muscle fiber shorten, the fiber contracts.
Review
1. What is a sarcomere and Z-line?
2. What are the two protein filaments of a myofibril?
3. Explain how muscles contract according to the sliding filament theory.
4. A serious neck injury may leave a person paralyzed from the neck down. Explain why.
13.02: Organization of the Human Body
How is the human body similar to a well-tuned machine?
Many people have compared the human body to a machine. Think about some common machines, such as drills and washing machines. Each machine consists of many parts, and each part does a specific job, yet all the parts work together to perform an overall function. The human body is like a machine in all these ways. In fact, it may be the most fantastic machine on Earth.
The human machine is organized at different levels, starting with the cell and ending with the entire organism (see Figure below). At each higher level of organization, there is a greater degree of complexity.
The human organism has several levels of organization.
Cells
The most basic parts of the human machine are cells—an amazing 100 trillion of them by the time the average person reaches adulthood! Cells are the basic units of structure and function in the human body, as they are in all living things. Each cell carries out basic life processes that allow the body to survive. Many human cells are specialized in form and function, as shown in Figure below. Each type of cell in the figure plays a specific role. For example, nerve cells have long projections that help them carry electrical messages to other cells. Muscle cells have many mitochondria that provide the energy they need to move the body.
Different types of cells in the human body are specialized for specific jobs. Do you know the functions of any of the cell types shown here?
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: epithelial, muscle, nervous, and connective tissues. These four tissue types, which are shown in Figure below, make up all the organs of the human body.
The human body consists of these four tissue types.
• Connective tissue is made up of cells that form the body’s structure. Examples include bone and cartilage.
• Epithelial tissue is made up of cells that line inner and outer body surfaces, such as the skin and the lining of the digestive tract. Epithelial tissue protects the body and its internal organs, secretes substances such as hormones, and absorbs substances such as nutrients.
• Muscle tissue is made up of cells that have the unique ability to contract, or become shorter. Muscles attached to bones enable the body to move.
• Nervous tissue is made up of neurons, or nerve cells, that carry electrical messages. Nervous tissue makes up the brain and the nerves that connect the brain to all parts of the body.
Organs and Organ Systems
After tissues, organs are the next level of 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 brain, heart, lungs, skin, and kidneys. Human organs are organized into organ systems, many of which are shown in Figure below. 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.
Many of the organ systems that make up the human body are represented here. What is the overall function of each organ system?
Your body’s 12 organ systems are shown below (Table below). Your organ systems do not work alone in your body. They must all be able to work together. For example, one of the most important functions of organ systems is to provide cells with oxygen and nutrients and to remove toxic waste products such as carbon dioxide. A number of organ systems, including the cardiovascular and respiratory systems, all work together to do this.
Organ System Major Tissues and Organs Function
Cardiovascular Heart; blood vessels; blood Transports oxygen, hormones, and nutrients to the body cells. Moves wastes and carbon dioxide away from cells.
Lymphatic Lymph nodes; lymph vessels Defend against infection and disease, moves lymph between tissues and the blood stream.
Digestive Esophagus; stomach; small intestine; large intestine Digests foods and absorbs nutrients, minerals, vitamins, and water.
Endocrine Pituitary gland, hypothalamus; adrenalglands; ovaries; testes Produces hormones that communicate between cells.
Integumentary Skin, hair, nails Provides protection from injury and water loss, physical defense against infection by microorganisms, andtemperature control.
Muscular Cardiac (heart) muscle; skeletal muscle; smooth muscle; tendons Involved in movement and heat production.
Nervous Brain, spinal cord; nerves Collects, transfers, and processes information.
Reproductive
Female: uterus; vagina; fallopian tubes; ovaries
Male: penis; testes; seminal vesicles
Produces gametes (sex cells) and sex hormones.
Respiratory Trachea, larynx, pharynx, lungs Brings air to sites where gas exchange can occur between the blood and cells (around body) or blood and air (lungs).
Skeletal Bones, cartilage; ligaments Supports and protects soft tissues of body; produces blood cells; stores minerals.
Urinary Kidneys; urinary bladder Removes extra water, salts, and waste products from blood and body; controls pH; controls water and salt balance.
Immune Bone marrow; spleen; white blood cells Defends against diseases.
Summary
• The human body is organized at different levels, starting with the cell.
• Cells are organized into tissues, and tissues form organs.
• Organs are organized into organ systems such as the skeletal and muscular systems.
Review
1. What are the levels of organization of the human body?
2. Which type of tissue covers the surface of the body?
3. What are the functions of the skeletal system?
4. Which organ system supports the body and allows it to move?
5. Explain how form and function are related in human cells. Include examples.
6. Compare and contrast epithelial and muscle tissues. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.01%3A_Muscle_Contraction.txt |
What happens if stability is disrupted?
Remove one stone and the whole arch collapses. The same is true for the human body. All the systems work together to maintain stability or homeostasis. Disrupt one system, and the whole body may be affected.
Homeostasis
All of the organs and organ systems of the human body 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 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 processes.
Maintaining Homeostasis
The process in which organ systems work to maintain a stable internal environment is calledhomeostasis. Keeping a stable internal environment requires constant adjustments. Here are just three of the many ways that human organ systems help the body maintain homeostasis:
• Respiratory system: A high concentration of carbon dioxide in the blood triggers faster breathing. The lungs exhale more frequently, which removes carbon dioxide from the body more quickly.
• Excretory system: A low level of water in the blood triggers retention of water by the kidneys. The kidneys produce more concentrated urine, so less water is lost from the body.
• Endocrine system: A high concentration of sugar in the blood triggers secretion of insulin by an endocrine gland called the pancreas. Insulin is a hormone that helps cells absorb sugar from the blood.
So how does your body maintain homeostasis? The regulation of your internal environment is done primarily through negative feedback. Negative feedback is a response to a stimulus that keeps a variable close to a set value (Figure below). Essentially, it "shuts off" or "turns on" a system when it varies from a set value.
For example, your body has an internal thermostat. During a winter day, in your house a thermostat senses the temperature in a room and responds by turning on or off the heater. Your body acts in much the same way. When body temperature rises, receptors in the skin and the brain sense the temperature change. The temperature change triggers a command from the brain. This command can cause several responses. If you are too hot, the skin makes sweat and blood vessels near the skin surface dilate. This response helps decrease body temperature.
Another example of negative feedback has to do with blood glucose levels. When glucose (sugar) levels in the blood are too high, the pancreas secretes insulin to stimulate the absorption of glucose and the conversion of glucose into glycogen, which is stored in the liver. As blood glucose levels decrease, less insulin is produced. When glucose levels are too low, another hormone called glucagon is produced, which causes the liver to convert glycogen back to glucose.
Feedback Regulation. If a raise in body temperature (stimulus) is detected (receptor), a signal will cause the brain to maintain homeostasis (response). Once the body temperature returns to normal, negative feedback will cause the response to end. This sequence of stimulus-receptor-signal-response is used throughout the body to maintain homeostasis.
Positive Feedback
Some processes in the body are regulated by positive feedback. Positive feedback is when a response to an event increases the likelihood of the event to continue. An example of positive feedback is milk production in nursing mothers. As the baby drinks her mother's milk, the hormone prolactin, a chemical signal, is released. The more the baby suckles, the more prolactin is released, which causes more milk to be produced. Other examples of positive feedback include contractions during childbirth. When constrictions in the uterus push a baby into the birth canal, additional contractions occur.
Failure of Homeostasis
Many homeostatic mechanisms such as these work continuously to maintain stable conditions in the human body. Sometimes, however, the mechanisms fail. When they do, 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.
Summary
• All of the organ systems of the body work together to maintain homeostasis of the organism.
• If homeostasis fails, death or disease may result.
Review
1. What is homeostasis?
2. Describe how one of the human organ systems helps maintain homeostasis.
3. A house has several systems, such as the electrical system, plumbing system, and heating and cooling system. In what ways are the systems of a house similar to human body systems?
13.04: Small Intestine
Imagine the inside walls of the 23 feet of your small intestine covered with these finger-like projections. Why? What's their purpose, and why is the small intestine so long? These projections absorb. Absorb what? Minerals and nutrients from food. And the length of the small intestine allows as much of these important substances to be absorbed as possible.
The small intestine is a narrow tube about 7 meters (23 feet) long in adults. It is the site of most chemical digestion and virtually all absorption. The small intestine consists of three parts: the duodenum, jejunum and ileum (see the opening figure).
Digestion in the Small Intestine
The duodenum is the first and shortest part of the small intestine. Most chemical digestion takes place here, and many digestive enzymes are active in the duodenum (see Table below). Some are produced by the duodenum itself. Others are produced by the pancreas and secreted into the duodenum.
Enzyme What It Digests Where It Is Made
Amylase carbohydrates pancreas
Trypsin proteins pancreas
Lipase lipids pancreas, duodenum
Maltase carbohydrates duodenum
Peptidase proteins
duodenum
The liver is an organ of both digestion and excretion. It produces a fluid called bile, which is secreted into the duodenum. Some bile also goes to the gall bladder, a sac-like organ that stores and concentrates bile and then secretes it into the small intestine. In the duodenum, bile breaks up large globules of lipids into smaller globules that are easier for enzymes to break down. Bile also reduces the acidity of food entering from the highly acidic stomach. This is important because digestive enzymes that work in the duodenum need a neutral environment. The pancreas contributes to the neutral environment by secreting bicarbonate, a basic substance that neutralizes acid.
Absorption in the Small Intestine
The jejunum is the second part of the small intestine, where most nutrients are absorbed into the blood. As shown in Figure below, the mucous membrane lining the jejunum is covered with millions of microscopic, fingerlike projections called villi (singular, villus). Villi contain many capillaries, and nutrients pass from the villi into the bloodstream through the capillaries. Because there are so many villi, they greatly increase the surface area for absorption. In fact, they make the inner surface of the small intestine as large as a tennis court!
This image shows intestinal villi greatly magnified. They are actually microscopic.
The ileum is the third part of the small intestine. A few remaining nutrients are absorbed here. Like the jejunum, the inner surface of the ileum is covered with villi that increase the surface area for absorption.
Summary
• Virtually all absorption of nutrients takes place in the small intestine, which has a very large inner surface area because it is covered with millions of microscopic villi.
Review
1. Name the parts of the small intestine.
2. Where are most nutrients absorbed?
3. What is digested by trypsin, by lipase, and by maltase?
4. Describe the functions of the three parts of the small intestine.
5. What role do villi play in absorption? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.03%3A_Homeostasis.txt |
What's the worst thing you can do to hurt your health?
Besides pathogens, many other dangers in the environment may negatively affect human health. For example, air pollution can cause lung cancer. It can also make asthma and other diseases worse. Bioterrorism is another potential threat in the environment. It may poison large numbers of people or cause epidemics of deadly diseases. But the worst thing you can do to yourself is smoke cigarettes.
Carcinogens and Cancer
A carcinogen is anything that can cause cancer. Cancer is a disease in which cells divide out of control. Most carcinogens cause cancer by producing mutations in DNA.
Types of Carcinogens
There are several different types of carcinogens. They include pathogens, radiation, and chemicals. Some carcinogens occur naturally. Others are produced by human actions.
• Viruses cause about 15 percent of all human cancers. For example, the virus called hepatitis B causes liver cancer.
• UV radiation is the leading cause of skin cancer. The radioactive gas known as radon causes lung cancer.
• Tobacco smoke contains dozens of carcinogens, including nicotine and formaldehyde. Exposure to tobacco smoke is the leading cause of lung cancer.
• Some chemicals that were previously added to foods, such as certain dyes, are now known to cause cancer. Cooking foods at very high temperatures also causes carcinogens to form (see Figure below).
Barbecued foods are cooked at very high temperatures. This may cause carcinogens to form.
How Cancer Occurs
Mutations that lead to cancer usually occur in genes that control the cell cycle. These include tumor-suppressor genes and proto-oncogenes.
• Tumor-suppressor genes normally prevent cells with damaged DNA from dividing.Mutations in these genes prevent them from functioning normally. As a result, cells with damaged DNA are allowed to divide.
• Proto-oncogenes normally help control cell division. Mutations in these genes turn them into oncogenes. Oncogenes promote the division of cells with damaged DNA.
Cells that divide uncontrollably may form a tumor, or abnormal mass of cells. Tumors may be benign or malignant. Benign tumors remain localized and generally do not harm health.Malignant tumors are cancerous. There are no limits to their growth, so they can invade and damage neighboring tissues. Cells from malignant tumors may also break away from the tumor and enter the bloodstream. They are carried to other parts of the body, where new tumors may form. The most common and the most deadly cancers for U.S. adults are listed inTable below.
Gender Most Common Types of Cancer after Skin Cancer (% of all cancers) Most Common Causes of Cancer Deaths (% of all cancer deaths)
Males prostate cancer (33%), lung cancer (13%) lung cancer (31%), prostate cancer (10%)
Females breast cancer (32%), lung cancer (12%) lung cancer (27%), breast cancer (15%)
More cancer deaths in adult males and females are due to lung cancer than any other type of cancer. Lung cancer is most often caused by exposure to tobacco smoke. What might explain why lung cancer causes the most cancer deaths when it isn’t the most common type of cancer?
Cancer Treatment and Prevention
Most cancers can be treated, and some can be cured. The general goal of treatment is to remove the tumor without damaging other cells. A cancer patient is typically treated in more than one way. Possible treatments include surgery, drugs (chemotherapy), and radiation. Early diagnosis and treatment of cancer lead to the best chance for survival. That’s why it’s important to know the following warning signs of cancer:
• change in bowel or bladder habits
• sore that does not heal
• unusual bleeding or discharge
• lump in the breast or elsewhere
• chronic indigestion or difficulty swallowing
• obvious changes in a wart or mole
• persistent coughing or hoarseness
Having one or more warning signs does not mean you have cancer, but you should see a doctor to be sure. Getting routine tests for particular cancers can also help detect cancers early, when chances of a cure are greatest. For example, getting the skin checked regularly by a dermatologist is important for early detection of skin cancer (see Figure below).
Regular checkups with a dermatologist can detect skin cancers early. Why is early detection important?
You can take steps to reduce your own risk of cancer. For example, you can avoid exposure to carcinogens such as tobacco smoke and UV light. You can also follow a healthy lifestyle. Being active, eating a low-fat diet, and maintaining a normal weight can help reduce your risk of cancer.
Summary
• A carcinogen is anything that causes cancer.
• Most carcinogens produce mutations in genes that control the cell cycle.
Review
1. What is a carcinogen? What is cancer?
2. How do most carcinogens cause cancer? Give two examples of carcinogens.
3. Describe tumor-suppressor genes and describe how they cause cancer.
4. Identify three ways cancer can be treated.
5. List four warning signs of cancer.
6. What might explain why lung cancer causes the most cancer deaths when it isn’t the most common type of cancer? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.05%3A_Carcinogens_and_Cancer.txt |
Liquid to solid. What does this mean?
Well, that's exactly what the large intestine does. It takes the remains of digested food — that is, food in which all the nutrients and minerals have been removed, and prepares it for elimination.
The Large Intestine and Its Functions
From the small intestine, any remaining food wastes pass into the large intestine. The large intestine is a relatively wide tube that connects the small intestine with the anus. Like the small intestine, the large intestine also consists of three parts: the cecum (or caecum), colon, and rectum.
Absorption of Water and Elimination of Wastes
The cecum is the first part of the large intestine, where wastes enter from the small intestine. The wastes are in a liquid state. As they pass through the colon, which is the second part of the large intestine, excess water is absorbed. The remaining solid wastes are called feces. Feces accumulate in the rectum, which is the third part of the large intestine. As the rectum fills, the feces become compacted. After a certain amount of feces accumulate, they are eliminated from the body. A sphincter controls the anus and opens to let feces pass through.
Bacteria in the Large Intestine
Trillions of bacteria normally live in the large intestine. Most of them are helpful. In fact, we wouldn’t be able to survive without them. Some of the bacteria produce vitamins, which are absorbed by the large intestine. Other functions of intestinal bacteria include:
• controlling the growth of harmful bacteria.
• breaking down indigestible food components.
• producing substances that help prevent colon cancer.
• breaking down toxins before they can poison the body.
Summary
• The absorption of water from digestive wastes and the elimination of the remaining solid wastes occur in the large intestine.
• The large intestine also contains helpful bacteria.
Review
1. Describe the functions of the three parts of the large intestine.
2. How do bacteria in the large intestine help keep us healthy?
13.07: Balanced Eating
Why is the goodstuff in the smallest segment of this diagram?
If you're like most high school kids, one of the first things you do after school is search for something to eat. And you look for the chips or candy. As this diagram shows, you can eat those. Just not a lot.
Balanced Eating
Balanced eating is a way of eating that promotes good health. It means eating the right balance of different foods to provide the body with all the nutrients it needs. Fortunately, you don’t need to measure and record the amounts of different nutrients you each day in order to balance your eating. Instead, you can use MyPlate, MyPyramid and food labels.
MyPyramid and MyPlate
MyPyramid shows the relative amounts of foods in different food groups you should eat each day (see Figure below). You can visit the MyPyramid Web site at www.mypyramid.govto learn more about MyPyramid and customize it for your own gender, age, and activity level.
MyPyramid is a visual guideline for balanced eating.
Each food group represented by a colored band in MyPyramid is a good source of nutrients. The key in Figure above shows the food group each band represents. The wider the band, the more you should eat from that food group. The white tip of MyPyramid represents foods that should be eaten only once in a while, such as ice cream and potato chips. They contain few nutrients and may contribute excess Calories to the diet.
The figure “walking” up the side of MyPyramid represents the role of physical activity in balanced eating. Regular exercise helps you burn any extra energy that you consume in foods and provides many other health benefits. You should be active for about an hour a day most days of the week. The more active you are, the more energy you will use.
In June 2011, the United States Department of Agriculture replaced My Pyramid with MyPlate.MyPlate depicts the relative daily portions of various food groups. Seehttp://www.choosemyplate.gov/ for further information.
MyPlate is a visual guideline for balanced eating, replacing MyPyramid in 2011.
The following guidelines accompany MyPlate:
• Balancing Calories
• Enjoy your food, but eat less.
• Avoid oversized portions.
• Foods to Increase
• Make half your plate fruits and vegetables.
• Make at least half your grains whole grains.
• Switch to fat-free or low-fat (1%) milk.
• Foods to Reduce
• Compare sodium in foods like soup, bread, and frozen meals - and choose the foods with lower numbers.
• Drink water instead of sugary drinks.
Food Labels
Packaged foods are required by law to carry a nutrition facts label, like the one in Figure below. The labels show the nutrient content and ingredients of foods. Reading labels can help you choose foods that are high in nutrients you need more of (such as proteins) and low in nutrients you need less of (such as fats).
Nutrition facts labels like this one can help you make good food choices.
You should also look for ingredients such as whole grains, vegetables, and fruits. Avoid foods that contain processed ingredients, such as white flour or white rice. Processing removes nutrients. As a result, processed foods generally supply fewer nutrients than whole foods, even when they have been enriched or fortified with added nutrients.
Weight Gain and Obesity
Any unused energy in food, whether it comes from carbohydrates, proteins, or lipids, is stored in the body as fat. An extra 3,500 Calories of energy results in the storage of almost half a kilogram (1 pound) of stored body fat. People who consistently consume more food energy then they need may become obese. Obesity occurs when the body mass index is 30.0 kg/m2or greater. Body mass index (BMI) is an estimate of the fat content of the body. It is calculated by dividing a person’s weight (in kilograms) by the square of the person’s height (in meters). Obesity increases the risk of health problems such as type 2 diabetes and hypertension.
Eating Disorders
Some people who are obese have an eating disorder, called binge eating disorder, in which they compulsively overeat. An eating disorder is a mental illness in which people feel compelled to eat in a way that causes physical, mental, and emotional health problems. Other eating disorders include anorexia nervosa and bulimia nervosa. Treatments for eating disorders include counseling and medication.
Summary
• Balanced eating promotes good health.
• MyPlate, MyPyramid, and food labels are tools that can help you choose the right foods for balanced eating.
• Review
• Explain how to use MyPyramid and food labels to choose foods for balanced eating.
• What is an eating disorder? Give an example.
• Aleesha weighs 80 kg and is 1.6 m tall. What is her body mass index? Is she obese?
• Eating too much and exercising too little can lead to weight gain and obesity.
• Some people who are obese have an eating disorder. Eating disorders are mental illnesses that require treatment by health professionals. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.06%3A_Large_Intestine.txt |
Is some air actually bad for you?
This question shouldn't even need an answer. Yes, some air can be harmful.
Air Pollution and Illness
Almost 5 million people die each year because of air pollution. In fact, polluted air causes more deaths than traffic accidents. Air pollution harms the respiratory and circulatory systems. Both outdoor and indoor air can be polluted.
Outdoor Air Pollution
The Air Quality Index (AQI) is an assessment of the pollutants in the outdoor air based on their human health effects. The health risks associated with different values of AQI are shown in Figure below. When AQI is high, you should limit the time you spend outdoors. Avoiding exposure to air pollution can help limit its impact on your health. People with certain health problems, including asthma, are very sensitive to the effects of air pollution. They need to be especially careful to avoid it.
Air quality is especially important for sensitive people. They include people with asthma, other respiratory illnesses, and cardiovascular diseases.
AQI generally refers to the levels of ground-level ozone and particulates. Ozone is a gas that forms close to the ground when air pollutants are heated by sunlight. It is one of the main components of smog (see Figure below). Smog also contains particulates. Particulates are tiny particles of solids or liquids suspended in the air. They are produced mainly by the burning of fossil fuels. The particles settle in airways and the lungs, where they cause damage.
Smog clouds the city of Los Angeles, California. Visible air pollution in the form of smog is a sign that the air is unhealthy.
Indoor Air Pollution
Indoor air may be even more polluted than outdoor air. It may contain harmful substances such as mold, bacteria, and radon. It may also contain carbon monoxide. Carbon monoxide is a gas produced by furnaces and other devices that burn fuel. If it is inhaled, it replaces oxygen in the blood and quickly leads to death. Carbon monoxide is colorless and odorless, but it can be detected with a carbon monoxide detector like the one in Figure below.
A carbon monoxide detector warns you if the level of the gas is too high.
Summary
• Both outdoor and indoor air may contain pollutants that can cause human illness and death.
Review
1. How can you use the Air Quality Index to protect your health?
2. Explain why ground-level ozone is usually a worse problem in the summer than in the winter in North America.
3. Compare and contrast pollutants in outdoor and indoor air, including their effects on human health.
13.09: Bioterrorism
"The world has definitely changed." This statement is common at times. What might it refer to?
Bioterrorism is a threat against civilized people worldwide. To be prepared, all levels of government have developed and conducted terrorism drills. These include protecting responders from harmful biological substances.
Bioterrorism
Bioterrorism is the intentional release or spread of agents of disease. The agents may be viruses, bacteria, or toxins produced by bacteria. The agents may spread through the air, food, or water; or they may come into direct contact with the skin. Two of the best known bioterrorism incidents in the U.S. occurred early in this century:
1. In 2001, letters containing anthrax spores were mailed to several news offices and two U.S. Senate offices. A total of 22 people were infected, and 5 of them died of anthrax.
2. In 2003, a deadly toxin called ricin was detected in a letter addressed to the White House. The letter was intercepted at a mail-handling facility off White House grounds. Fortunately, the ricin toxin did not cause any illnesses or deaths.
Summary
• Bioterrorism is the intentional release or spread of agents of disease.
Review
1. Define bioterrorism.
2. Research additional recent acts of bioterrorism.
13.10: Human Skeletal System
The skeletal system consists of all the bones of the body. How important are your bones?
Try to imagine what you would look like without them. You would be a soft, wobbly pile of skin, muscles, and internal organs, so you might look something like a very large slug. Not that you would be able to see yourself—folds of skin would droop down over your eyes and block your vision because of your lack of skull bones. You could push the skin out of the way, if you could only move your arms, but you need bones for that as well!
The Skeleton
The human skeleton is an internal framework that, in adults, consists of 206 bones, most of which are shown in Figure below.
In addition to bones, the skeleton also consists of cartilage and ligaments:
• Cartilage is a type of dense connective tissue, made of tough protein fibers, that provides a smooth surface for the movement of bones at joints.
• A ligament is a band of fibrous connective tissue that holds bones together and keeps them in place.
The human skeleton consists of bones, cartilage, and ligaments.
The skeleton supports the body and gives it shape. It has several other functions as well, including:
1. protecting internal organs
2. providing attachment surfaces for muscles
3. producing blood cells
4. storing minerals
5. maintaining mineral homeostasis.
Maintaining mineral homeostasis is a very important function of the skeleton, because just the right levels of calcium and other minerals are needed in the blood for normal functioning of the body. When mineral levels in the blood are too high, bones absorb some of the minerals and store them as mineral salts, which is why bones are so hard. When blood levels of minerals are too low, bones release some of the minerals back into the blood, thus restoring homeostasis.
Summary
• The adult human skeleton includes 206 bones and other tissues.
• The skeleton supports the body, protects internal organs, produces blood cells, and maintains mineral homeostasis.
Review
1. What is cartilage? What is its role in the skeletal system?
2. List three functions of the human skeleton.
3. Explain how bones maintain mineral homeostasis in the body. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.08%3A_Air_Pollution_and_Illness.txt |
Are bones living?
It's common to think of bones as not living. But bones are very much living. In fact, you are constantly making new bone tissue. That means that you are also constantly getting rid of bone. Bone is full of blood and nerves and all sorts of cells and proteins, making it an extremely complex living tissue.
Structure of Bones
Many people think of bones as being dead, dry, and brittle. These adjectives correctly describe the bones of a preserved skeleton, but the bones in a living human being are very much alive. As shown in Figure below, the basic structure of bones is bone matrix, which makes up the underlying rigid framework of bones, composed of both compact bone and spongy bone. The bone matrix consists of tough protein fibers, mainly collagen, that become hard and rigid due to mineralization with calcium crystals. Bone matrix is crisscrossed by blood vessels and nerves and also contains specialized bone cells that are actively involved in metabolic processes.
Bone matrix provides bones with their basic structure. Notice the spongy bone in the middle, and the compact bone towards the outer region. The osteon is the functional unit of compact bone.
Bone Cells
There are three types of specialized cells in human bones: osteoblasts, osteocytes, and osteoclasts. These cells are responsible for bone growth and mineral homeostasis.
• Osteoblasts make new bone cells and secrete collagen that mineralizes to become bone matrix. They are responsible for bone growth and the uptake of minerals from the blood.
• Osteocytes regulate mineral homeostasis. They direct the uptake of minerals from the blood and the release of minerals back into the blood as needed.
• Osteoclasts dissolve minerals in bone matrix and release them back into the blood.
Bones are far from static, or unchanging. Instead, they are dynamic, living tissues that are constantly being reshaped. Under the direction of osteocytes, osteoblasts continuously build up bone, while osteoclasts continuously break it down.
Bone Tissues
Bones consist of different types of tissue, including compact bone, spongy bone, bone marrow, and periosteum. All of these tissue types are shown in Figure below.
• Compact bone makes up the dense outer layer of bone. Its functional unit is the osteon. Compact bone is very hard and strong.
• Spongy bone is found inside bones and is lighter and less dense than compact bone. This is because spongy bone is porous.
• Bone marrow is a soft connective tissue that produces blood cells. It is found inside the pores of spongy bone.
• Periosteum is a tough, fibrous membrane that covers and protects the outer surfaces of bone.
This bone contains different types of bone tissue. How does each type of tissue contribute to the functions of bone?
Summary
• Under the direction of osteocytes, osteoblasts continuously build up bone, while osteoclasts continuously break down bone. These processes help maintain mineralhomeostasis.
• Bone tissues include compact bone, spongy bone, bone marrow, and periosteum.
Review
1. Describe bone matrix.
2. Identify the three types of specialized bone cells and what they do.
3. Compare and contrast the structure and function of compact bone and spongy bone.
4. What is bone marrow? Where is it found?
13.12: Growth and Development of Bones
How do bones grow?
Bones are hard structures. So how do they grow? Well, bones are a living tissue. They have a blood supply. You are consistently making new bone. In fact, the human skeleton is replaced every 7-10 years. But how do bones grow? From their ends, where they have cartilage.
Growth and Development of Bones
Early in the development of a human fetus, the skeleton is made entirely of cartilage. The relatively soft cartilage gradually turns into hard bone through ossification. This is a process in which mineral deposits replace cartilage. As shown in Figure below, ossification of long bones, which are found in the arms and legs, begins at the center of the bones and continues toward the ends. By birth, several areas of cartilage remain in the skeleton, including growth plates at the ends of the long bones. This cartilage grows as the long bones grow, so the bones can keep increasing in length during childhood.
Long bones ossify and get longer as they grow and develop. These bones grow from their ends, known as the epiphysis, and the presence of a growth plate, or epiphyseal line, signifies that the bone is still growing.
In the late teens or early twenties, a person reaches skeletal maturity. By then, all of the cartilage has been replaced by bone, so no further growth in bone length is possible. However, bones can still increase in thickness. This may occur in response to increased muscle activity, such as weight training.
Summary
• Bones become increasingly ossified and grow larger during fetal development, childhood, and adolescence.
• When skeletal maturity is reached at about age 20, no additional growth in bone length can occur.
Review
1. Define ossification.
2. A newborn baby has a soft spot on the top of its head. Over the next few months, the soft spot gradually hardens. What explains this?
3. Jana is 17 years old and 172 cm tall. She plays basketball and hopes to grow at least 4 cm more before she turns 18 and goes to college. Jana recently injured her leg, and her doctor took an X-ray of it. Based on the X-ray, the doctor determined that Jana had reached skeletal maturity. How much taller is Jana likely to grow? Explain your answer. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.11%3A_Structure_of_Bones.txt |
What allows running?
Running. A means of terrestrial locomotion allowing humans and other animals to move rapidly on foot. The knees, which connect one part of the leg to the other, have to allow the legs to move. The knee is a joint, the part of the skeletal system that connects bones.
Joints
A joint is a place where two or more bones of the skeleton meet. With the help of muscles, joints work like mechanical levers, allowing the body to move with relatively little force. The surfaces of bones at joints are covered with a smooth layer of cartilage that reduces friction at the points of contact between the bones.
Types of Joints
There are three main types of joints: immovable, partly movable, and movable.
• Immovable joints allow no movement because the bones at these joints are held securely together by dense collagen. The bones of the skull are connected by immovable joints.
• Partly movable joints allow only very limited movement. Bones at these joints are held in place by cartilage. The ribs and sternum are connected by partly movable joints.
• Movable joints allow the most movement. Bones at these joints are connected by ligaments. Movable joints are the most common type of joints in the body, so they are described in more detail next.
Movable Joints
Movable joints are also known as synovial joints. This is because the space between the bones is filled with a thick fluid, called synovial fluid, that cushions the joint (see Figure below).
A movable, or synovial, joint is protected and cushioned by cartilage and synovial fluid.
There are a variety of types of movable joints, which are illustrated in Figure below. The joints are classified by how they move. For example, a ball-and-socket joint, such as the shoulder, has the greatest range of motion, allowing movement in several directions. Other movable joints, including hinge joints such as the knee, allow less movement.
Types of Movable Joints in the Human Skeleton. Movable joints can move in a variety of ways. Try moving each of the joints indicated in the diagram. Can you tell how their movements differ? Other joints in the human skeleton that are not depicted here include saddle, ellipsoid, and plane joints.
Summary
• Joints are places where two or more bones of the skeleton meet.
• With the help of muscles, joints allow the body to move with relatively little force.
• Some joints can move more than others.
Review
1. Define immovable joint, and give an example of bones that are connected by this type of joint.
2. Describe a synovial joint.
3. Describe the movement of a pivot joint, such as the elbow.
Explore More
Use this resource to answer the questions that follow.
1. Describe the motion of the following joints:
1. shoulder
2. knee
3. neck
4. wrist
13.14: Skeletal System Problems and Diseases
Do you think this would hurt? Why?
That would probably hurt. And hurt a lot. Broken bones, or fractures, may be one of the more common problems of the skeletal system. And this one would probably need surgery to fix. But, in addition to broken bones, there are other problems and diseases of the skeletal system.
Skeletal System Problems
Despite their hardness and strength, bones can suffer from injury and disease. Bone problems include fractures, osteoarthritis, and rickets.
• Fractures are breaks in bone, usually caused by excessive stress on bone. Fractures heal when osteoblasts form new bone. Soon after a fracture, the body begins to repair the break. The area becomes swollen and sore. Within a few days, bone cells travel to the break site and begin to rebuild the bone. It takes about two to three months before compact and spongy bone form at the break site. Sometimes the body needs extra help in repairing a broken bone. In such a case, a surgeon will piece a broken bone together with metal pins. Moving the broken pieces together will help keep the bone from moving and give the body a chance to repair the break.
• Osteoarthritis is a condition in which cartilage breaks down in joints due to wear and tear, causing joint stiffness and pain.
• Osteoporosis is a disease in which bones lose mass and become more fragile than they should be. Osteoporosis also makes bones more likely to break. Two of the easiest ways to prevent osteoporosis are eating a healthy diet that has the right amount of calcium and vitamin D and to do some sort of weight-bearing exercise every day. Foods that are a good source of calcium include milk, yogurt, and cheese. Non-dairy sources of calcium include Chinese cabbage, kale, and broccoli. Many fruit juices, fruit drinks, tofu, and cereals have calcium added to them. It is recommended that teenagers get 1300 mg of calcium every day. For example, one cup (8 fl. oz.) of milk provides about 300 mg of calcium, or about 30% of the daily requirement.
• Rickets is softening of the bones in children that occurs because bones do not have enough calcium. Rickets can lead to fractures and bowing of the leg bones, which is illustrated in the Figure below.
The bones of a child with rickets are so soft that the weight of the body causes them to bend.
Summary
• Skeletal system problems include fractures, osteoarthritis, and rickets.
Review
1. Osteoporosis is a disease in which osteoclasts are more active than osteoblasts. How is this likely to affect the bones? Why would a person with osteoporosis have a greater-than-normal risk of bone fractures? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.13%3A_Skeletal_System_Joints.txt |
What exactly are muscles?
Does the word "muscle" make you think of the biceps of a weightlifter, like the man in pictured above? Muscles such as biceps that move the body are easy to feel and see, but they aren’t the only muscles in the human body. Many muscles are deep within the body. They form the walls of internal organs such as the heart and stomach. You can flex your biceps like a body builder, but you cannot control the muscles inside you. It’s a good thing that they work on their own without any conscious effort on your part, because movement of these muscles is essential for survival.
What Are Muscles?
The muscular system consists of all the muscles of the body. Muscles are organs composed mainly of muscle cells, which are also called muscle fibers. Each muscle fiber is a very long, thin cell that can do something no other cell can do. It can contract, or shorten. Muscle contractions are responsible for virtually all the movements of the body, both inside and out. There are three types of muscle tissues in the human body: cardiac, smooth, and skeletal muscle tissues. They are shown in Figure below and described below.
Types of Muscle Tissue. Both skeletal and cardiac muscles appear striated, or striped, because their cells are arranged in bundles. Smooth muscles are not striated because their cells are arranged in sheets instead of bundles.
Smooth Muscle
Muscle tissue in the walls of internal organs such as the stomach and intestines is smooth muscle. When smooth muscle contracts, it helps the organs carry out their functions. For example, when smooth muscle in the stomach contracts, it squeezes the food inside the stomach, which helps break the food into smaller pieces. Contractions of smooth muscle are involuntary. This means they are not under conscious control.
Skeletal Muscle
Muscle tissue that is attached to bone is skeletal muscle. Whether you are blinking your eyes or running a marathon, you are using skeletal muscle. Contractions of skeletal muscle are voluntary, or under conscious control. When skeletal muscle contracts, bones move. Skeletal muscle is the most common type of muscle in the human body.
Cardiac Muscle
Cardiac muscle is found only in the walls of the heart. When cardiac muscle contracts, the heart beats and pumps blood. Cardiac muscle contains a great many mitochondria, which produce ATP for energy. This helps the heart resist fatigue. Contractions of cardiac muscle are involuntary, like those of smooth muscle. Cardiac muscle, like skeletal muscle, is arranged in bundles, so it appears striated, or striped.
Summary
• There are three types of human muscle tissue: smooth muscle (in internal organs), skeletal muscle, and cardiac muscle (only in the heart).
Review
1. Compare and contrast the three types of muscle tissue.
2. What can muscle cells do that other cells cannot?
3. Why are skeletal and cardiac muscles striated?
4. Where is smooth muscle tissue found?
5. What is the function of skeletal muscle? Give an example.
13.16: Skeletal Muscles
How do your bones move?
By the contraction and extension of your skeletal muscles. Notice how the muscles are attached to the bones. The muscles pull on the bones, causing movement.
Skeletal Muscles
There are well over 600 skeletal muscles in the human body, some of which are identified inFigure below. Skeletal muscles vary considerably in size, from tiny muscles inside the middle ear to very large muscles in the upper leg.
Skeletal Muscles. Skeletal muscles enable the body to move.
Structure of Skeletal Muscles
Each skeletal muscle consists of hundreds or even thousands of skeletal muscle fibers. The fibers are bundled together and wrapped in connective tissue, as shown Figure below. The connective tissue supports and protects the delicate muscle cells and allows them to withstand the forces of contraction. It also provides pathways for nerves and blood vessels to reach the muscles. Skeletal muscles work hard to move body parts. They need a rich blood supply to provide them with nutrients and oxygen and to carry away their wastes.
Skeletal Muscle Structure. A skeletal muscle contains bundles of muscle fibers inside a “coat” of connective tissue.
Skeletal Muscles and Bones
Skeletal muscles are attached to the skeleton by tough connective tissues called tendons(see Figure above). Many skeletal muscles are attached to the ends of bones that meet at a joint. The muscles span the joint and connect the bones. When the muscles contract, they pull on the bones, causing them to move.
Muscles can only contract. They cannot actively extend, or lengthen. Therefore, to move bones in opposite directions, pairs of muscles must work in opposition. For example, the biceps and triceps muscles of the upper arm work in opposition to bend and extend the arm at the elbow (see Figure below). What other body movements do you think require opposing muscle pairs?
Triceps and biceps muscles in the upper arm are opposing muscles.
Use It or Lose It
In exercises such as weight lifting, skeletal muscle contracts against a resisting force (see Figure below). Using skeletal muscle in this way increases its size and strength. In exercises such as running, the cardiac muscle contracts faster and the heart pumps more blood. Using cardiac muscle in this way increases its strength and efficiency. Continued exercise is necessary to maintain bigger, stronger muscles. If you don’t use a muscle, it will get smaller and weaker—so use it or lose it.
This exercise pits human muscles against a force. What force is it?
Summary
• Skeletal muscles are attached to the skeleton and cause bones to move when they contract.
Review
1. What is a muscle fiber?
2. What is the function of skeletal muscle?
3. How are skeletal muscles attached to bones?
4. Explain why many skeletal muscles must work in opposing pairs.
13.17: Nails and Hair
Would you believe this is a close-up of your hair and scalp?
Well maybe not yours. But some other person's. Hair is an integral part of the integumentary system. And although many people may lose some or all of the hair on top of their head, they still have hair on their arms and legs that perform important functions.
Nails and Hair
In addition to the skin, the integumentary system includes the nails and hair. Like the skin, these organs help the body maintain homeostasis.
Nails
Fingernails and toenails consist of specialized epidermal cells that are filled with keratin. The keratin makes them tough and hard, which is important for the functions they serve. Fingernails prevent injury by forming protective plates over the ends of the fingers. They also enhance sensation by acting as a counterforce to the sensitive fingertips when objects are handled. Nails are similar to claws in other animals. They cover the tips of fingers and toes. Fingernails and toenails both grow from nail beds. As the nail grows, more cells are added at the nail bed. Older cells get pushed away from the nail bed and the nail grows longer. There are no nerve endings in the nail. Otherwise cutting your nails would hurt a lot!
Hair
Hair is one of the defining characteristics of mammals. Its main component is keratin. A hair shaft consists of dead, keratin-filled cells that overlap each other like the shingles on a roof (see Figure below). Like roof shingles, the overlapping cells help shed water from the hair.
Shaft of Human Hair. This shaft of hair is magnified to show its overlapping cells.
Hair helps to insulate and protect the body. Head hair is especially important in preventing heat loss from the body. Eyelashes and eyebrows protect the eyes from water, dirt, and other irritants. Hairs in the nose trap dust particles and microorganisms in the air and prevent them from reaching the lungs. Hair also provides sensory input when objects brush against it or it sways in moving air.
Hair, hair follicle, and oil glands. The oil, called sebum, helps to prevent water loss from the skin. The sebaceous gland secretes sebum, which waterproofs the skin and hair.
Summary
• Nails and hair contain mostly keratin. They protect the body and enhance the sense of touch.
Review
1. A certain disease causes the loss of all body hair. How might homeostasis of the body be disturbed by the absence of hair? (Hint: What are the functions of hair?) | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.15%3A_Smooth_Skeletal_and_Cardiac_Muscles.txt |
A close-up view of a spider web? Some sort of exotic bacteria? What do you think this is?
This is actually a nerve cell, the cell of the nervous system. This cell sends electrical “sparks” that transmit signals throughout your body.
The Nervous System
A small child darts in front of your bike as you race down the street. You see the child and immediately react. You put on the brakes, steer away from the child, and yell out a warning, all in just a split second. How do you respond so quickly? Such rapid responses are controlled by your nervous system. The nervous system is a complex network of nervous tissue that carries electrical messages throughout the body. It includes the brain and spinal cord, the central nervous system, and nerves that run throughout the body, the peripheral nervous system (see Figure below). To understand how nervous messages can travel so quickly, you need to know more about nerve cells.
The human nervous system includes the brain and spinal cord (central nervous system) and nerves that run throughout the body (peripheral nervous system).
Nerve Cells
Although the nervous system is very complex, nervous tissue consists of just two basic types of nerve cells: neurons and glial cells. Neurons are the structural and functional units of the nervous system. They transmit electrical signals, called nerve impulses. Glial cells provide support for neurons. For example, they provide neurons with nutrients and other materials.
Neuron Structure
As shown in Figure below, a neuron consists of three basic parts: the cell body, dendrites, and axon.
• The cell body contains the nucleus and other cell organelles.
• Dendrites extend from the cell body and receive nerve impulses from other neurons.
• The axon is a long extension of the cell body that transmits nerve impulses to other cells. The axon branches at the end, forming axon terminals. These are the points where the neuron communicates with other cells.
The structure of a neuron allows it to rapidly transmit nerve impulses to other cells.
The axon of many neurons has an outer layer called a myelin sheath (see Figure above).Myelin is a lipid produced by a type of a glial cell known as a Schwann cell. The myelin sheath acts like a layer of insulation, similar to the plastic that encases an electrical cord. Regularly spaced nodes, or gaps, in the myelin sheath allow nerve impulses to skip along the axon very rapidly.
Types of Neurons
Neurons are classified based on the direction in which they carry nerve impulses.
• Sensory neurons carry nerve impulses from tissues and organs to the spinal cord and brain.
• Motor neurons carry nerve impulses from the brain and spinal cord to muscles and glands(see Figure below).
• Interneurons carry nerve impulses back and forth between sensory and motor neurons.
This axon is part of a motor neuron. It transmits nerve impulses to a skeletal muscle, causing the muscle to contract.
Summary
• Neurons are the structural and functional units of the nervous system. They consist of a cell body, dendrites, and axon.
• Neurons transmit nerve impulses to other cells.
• Types of neurons include sensory neurons, motor neurons, and interneurons.
Review
1. What are the two main parts of the nervous system?
2. List and describe the parts of a neuron.
3. What do motor neurons do?
4. What is myelin and the myelin sheath? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.18%3A_Nerve_Cells.txt |
How does a nervous system signal move from one cell to the next?
It literally jumps by way of a chemical transmitter. Notice the two cells are not connected, but separated by a small gap. The synapse. The space between a neuron and the next cell.
Nerve Impulses
Nerve impulses are electrical in nature. They result from 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 (see Figure below). It uses energy in ATP to pump positive sodium ions (Na+) out of the cell and potassium ions (K+) into the cell. As a result, the inside of the neuron is negatively charged compared to the extracellular fluid surrounding the neuron. This is due to many more positively charged ions outside the cell compared to inside the cell. This difference in electrical charge is called the resting potential.
The sodium-potassium pump maintains the resting potential of a neuron.
Action Potential
A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron. The reversal of charge is called an action potential. It begins when the neuron receives a chemical signal from another cell. The signal causes gates in sodium ion channels to open, allowing positive sodium ions to flow back into the cell. As a result, the inside of the cell becomes positively charged compared to the outside of the cell. This reversal of charge ripples down the axon very rapidly as an electric current (see Figure below).
An action potential speeds along an axon in milliseconds.
In neurons with myelin sheaths, ions flow across the membrane only at the nodes between sections of myelin. As a result, the action potential jumps along the axon membrane from node to node, rather than spreading smoothly along the entire membrane. This increases the speed at which it travels.
The place where an axon terminal meets another cell is called a synapse. The axon terminal and other cell are separated by a narrow space known as a synaptic cleft (see Figure below). When an action potential reaches the axon terminal, the axon terminal releases molecules of a chemical called a neurotransmitter. The neurotransmitter molecules travel across the synaptic cleft and bind to receptors on the membrane of the other cell. If the other cell is a neuron, this starts an action potential in the other cell.
At a synapse, neurotransmitters are released by the axon terminal. They bind with receptors on the other cell.
Summary
• A nerve impulse begins when a neuron receives a chemical stimulus.
• The nerve impulse travels down the axon membrane as an electrical action potential to the axon terminal.
• The axon terminal releases neurotransmitters that carry the nerve impulse to the next cell.
Review
1. Define resting potential and action potential.
2. Explain how resting potential is maintained
3. Describe how an action potential occurs.
4. What is a synapse? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.19%3A_Nerve_Impulses.txt |
The human brain. The "control center." What does it control?
Practically everything. From breathing and heartbeat to reasoning, memory, and language. And it is the main part of the central nervous system.
Central Nervous System
The nervous system has two main divisions: the central nervous system and the peripheral nervous system (see Figure below). The central nervous system (CNS) includes the brain and spinal cord (see Figure below). You can see an overview of the central nervous system at this link: vimeo.com/2024719.
The two main divisions of the human nervous system are the central nervous system and the peripheral nervous system. The peripheral nervous system has additional divisions.
This diagram shows the components of the central nervous system.
The Brain
The brain is the most complex organ of the human body and the control center of the nervous system. It contains an astonishing 100 billion neurons! The brain controls such mental processes as reasoning, imagination, memory, and language. It also interprets information from the senses. In addition, it controls basic physical processes such as breathing and heartbeat.
The brain has three major parts: the cerebrum, cerebellum, and brain stem. These parts are shown in Figure below and described in this section.
In this drawing, assume you are looking at the left side of the head. This is how the brain would appear if you could look underneath the skull.
• The cerebrum is the largest part of the brain. It controls conscious functions such as reasoning, language, sight, touch, and hearing. It is divided into two hemispheres, or halves. The hemispheres are very similar but not identical to one another. They are connected by a thick bundle of axons deep within the brain. Each hemisphere is further divided into the four lobes shown in Figure below.
• The cerebellum is just below the cerebrum. It coordinates body movements. Many nerve pathways link the cerebellum with motor neurons throughout the body.
• The brain stem is the lowest part of the brain. It connects the rest of the brain with the spinal cord and passes nerve impulses between the brain and spinal cord. It also controls unconscious functions such as heart rate and breathing.
Each hemisphere of the cerebrum consists of four parts, called lobes. Each lobe is associated with particular brain functions. Just one function of each lobe is listed here.
Spinal Cord
The spinal cord is a thin, tubular bundle of nervous tissue that extends from the brainstem and continues down the center of the back to the pelvis. It is protected by the vertebrae, which encase it. The spinal cord serves as an information superhighway, passing messages from the body to the brain and from the brain to the body.
Humanoid Robot Brains
The smartest people in the world have spent millions of dollars on developing high-tech robots. But even though technology has come a long way, these humanoid robots are nowhere close to having the "brain" and motor control of a human. Why is that? Learn about the motor control processes in the human brain, and how cutting-edge research is trying to implement it in robots below.
Summary
• The central nervous includes the brain and spinal cord.
• The brain is the control center of the nervous system. It controls virtually all mental and physical processes.
• The spinal cord is a long, thin bundle of nervous tissue that passes messages from the body to the brain and from the brain to the body.
Review
1. Name the organs of the central nervous system.
2. Which part of the brain controls conscious functions such as reasoning?
3. What are the roles of the brain stem?
4. Sam’s dad was in a car accident in which his neck was broken. He survived the injury but is now paralyzed from the neck down. Explain why. | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.20%3A_Central_Nervous_System.txt |
How does the signal get to your toes?
If the brain controls practically everything, how does the signal get to your toes? Or your legs? Or arms? By way of the peripheral nervous system, or all the nerves shown here other than the brain and spinal cord. Notice how they go everywhere.
Peripheral Nervous System
The peripheral nervous system (PNS) consists of all the nervous tissue that lies outside thecentral nervous system. It is shown in yellow in Figure below. It is connected to the central nervous system by nerves. A nerve is a cable-like bundle of axons. Some nerves are very long. The longest human nerve is the sciatic nerve. It runs from the spinal cord in the lower back down the left leg all the way to the toes of the left foot. Like the nervous system as a whole, the peripheral nervous system also has two divisions: the sensory division and the motor division.
• The sensory division of the PNS carries sensory information from the body to the central nervous system.
• The motor division of the PNS carries nerve impulses from the central nervous system to muscles and glands throughout the body. The nerve impulses stimulate muscles to contract and glands to secrete hormones. The motor division of the peripheral nervous system is further divided into the somatic and autonomic nervous systems.
The nerves of the peripheral nervous system are shown in blue in this image. Can you identify the sciatic nerve?
Somatic Nervous System
The somatic nervous system (SNS) controls mainly voluntary activities that are under conscious control. It is made up of nerves that are connected to skeletal muscles. Whenever you perform a conscious movement, from signing your name to riding your bike, your somatic nervous system is responsible.
The somatic nervous system also controls some unconscious movements, called reflexes. A reflex is a very rapid motor response that is not directed by the brain. In a reflex, nerve impulses travel to and from the spinal cord in a reflex arc, like the one in Figure below. In this example, the person jerks his hand away from the flame without any conscious thought. It happens unconsciously because the nerve impulses bypass the brain.
A reflex arc like this one enables involuntary actions. How might reflex responses be beneficial to the organism?
Autonomic Nervous System
All other involuntary activities not under conscious control are the responsibility of the autonomic nervous system (ANS). Nerves of the ANS are connected to glands and internal organs. They control basic physical functions such as heart rate, breathing, digestion, and sweat production. The autonomic nervous system also has two subdivisions: the sympathetic division and the parasympathetic division.
• The sympathetic division deals with emergency situations. It prepares the body for “fight or flight.” Do you get clammy palms or a racing heart when you have to play a solo or give a speech? Nerves of the sympathetic division control these responses.
• The parasympathetic division controls involuntary activities that are not emergencies. For example, it controls the organs of your digestive system so they can break down the food you eat.
Summary
• The peripheral nervous system consists of all the nervous tissue that lies outside the central nervous system. It is connected to the central nervous system by nerves.
• The peripheral nervous system has several divisions and subdivisions that transmit nerve impulses between the central nervous system and the rest of the body.
• Review
• Identify the two major divisions of the peripheral nervous system.
• What is the role of the sensory division?
• Compare and contrast the somatic and autonomic nervous systems.
• What are the two divisions of the autonomic nervous system?
• What is the role of the sympathetic division? | textbooks/bio/Introductory_and_General_Biology/Introductory_Biology_(CK-12)/13%3A_Human_Biology/13.21%3A_Peripheral_Nervous_System.txt |
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