Datasets:

MedRAG
/

textbooks

Dataset card Data Studio Files Files and versions Community

Split (1)

train · 126k rows

Subsets and Splits

No community queries yet

The top public SQL queries from the community will appear here once available.

id stringlengths 14 28	title stringclasses 18 values	content stringlengths 2 999	contents stringlengths 19 1.02k
Cell_Biology_Alberts_210	Cell_Biology_Alberts	In each water molecule (H2O) the two H atoms are linked to the O atom by covalent bonds. The two bonds are highly polar because the O is strongly attractive for electrons, whereas the H is only weakly attractive. Consequently, there is an unequal distribution of electrons in a water molecule, with a preponderance of positive charge on the two H atoms and of negative charge on the O. When a positively charged region of one water molecule (that is, one of its H atoms) approaches a negatively charged region (that is, the O) of a second water molecule, the electrical attraction between them can result in a hydrogen bond. These bonds are much weaker than covalent bonds and are easily broken by the random thermal motions that reflect the heat energy of the molecules. Thus, each bond lasts only a short time. But the combined effect of many weak bonds can be profound. For example, each water molecule can form hydrogen bonds through its two H atoms to two other water molecules, producing a	Cell_Biology_Alberts. In each water molecule (H2O) the two H atoms are linked to the O atom by covalent bonds. The two bonds are highly polar because the O is strongly attractive for electrons, whereas the H is only weakly attractive. Consequently, there is an unequal distribution of electrons in a water molecule, with a preponderance of positive charge on the two H atoms and of negative charge on the O. When a positively charged region of one water molecule (that is, one of its H atoms) approaches a negatively charged region (that is, the O) of a second water molecule, the electrical attraction between them can result in a hydrogen bond. These bonds are much weaker than covalent bonds and are easily broken by the random thermal motions that reflect the heat energy of the molecules. Thus, each bond lasts only a short time. But the combined effect of many weak bonds can be profound. For example, each water molecule can form hydrogen bonds through its two H atoms to two other water molecules, producing a
Cell_Biology_Alberts_211	Cell_Biology_Alberts	a short time. But the combined effect of many weak bonds can be profound. For example, each water molecule can form hydrogen bonds through its two H atoms to two other water molecules, producing a network in which hydrogen bonds are being continually broken and formed. It is only because of the hydrogen bonds that link water molecules together that water is a liquid at room temperature—with a high boiling point and high surface tension—rather than a gas.	Cell_Biology_Alberts. a short time. But the combined effect of many weak bonds can be profound. For example, each water molecule can form hydrogen bonds through its two H atoms to two other water molecules, producing a network in which hydrogen bonds are being continually broken and formed. It is only because of the hydrogen bonds that link water molecules together that water is a liquid at room temperature—with a high boiling point and high surface tension—rather than a gas.
Cell_Biology_Alberts_212	Cell_Biology_Alberts	Molecules, such as alcohols, that contain polar bonds and that can form hydrogen bonds with water dissolve readily in water. Molecules carrying charges (ions) likewise interact favorably with water. Such molecules are termed hydrophilic, meaning that they are water-loving. Many of the molecules in the aqueous environment of a cell necessarily fall into this category, including sugars, DNA, RNA, and most proteins. Hydrophobic (water-hating) molecules, by contrast, are uncharged and form few or no hydrogen bonds, and so do not dissolve in water. Hydrocarbons are an important example. In these molecules all of the H atoms are covalently linked to C atoms by a largely nonpolar bond; thus they cannot form effective hydrogen bonds to other molecules (see Panel 2–1, p. 90). This makes the hydrocarbon as a whole hydrophobic—a property that is exploited in cells, whose membranes are constructed from molecules that have long hydrocarbon tails, as we see in Chapter 10.	Cell_Biology_Alberts. Molecules, such as alcohols, that contain polar bonds and that can form hydrogen bonds with water dissolve readily in water. Molecules carrying charges (ions) likewise interact favorably with water. Such molecules are termed hydrophilic, meaning that they are water-loving. Many of the molecules in the aqueous environment of a cell necessarily fall into this category, including sugars, DNA, RNA, and most proteins. Hydrophobic (water-hating) molecules, by contrast, are uncharged and form few or no hydrogen bonds, and so do not dissolve in water. Hydrocarbons are an important example. In these molecules all of the H atoms are covalently linked to C atoms by a largely nonpolar bond; thus they cannot form effective hydrogen bonds to other molecules (see Panel 2–1, p. 90). This makes the hydrocarbon as a whole hydrophobic—a property that is exploited in cells, whose membranes are constructed from molecules that have long hydrocarbon tails, as we see in Chapter 10.
Cell_Biology_Alberts_213	Cell_Biology_Alberts	four Types of noncovalent attractions help Bring molecules Together in Cells Much of biology depends on the specific binding of different molecules caused by three types of noncovalent bonds: electrostatic attractions (ionic bonds), hydrogen bonds, and van der Waals attractions; and on a fourth factor that can push molecules together: the hydrophobic force. The properties of the four types of noncovalent attractions are presented in Panel 2–3 (pp. 94–95). Although each	Cell_Biology_Alberts. four Types of noncovalent attractions help Bring molecules Together in Cells Much of biology depends on the specific binding of different molecules caused by three types of noncovalent bonds: electrostatic attractions (ionic bonds), hydrogen bonds, and van der Waals attractions; and on a fourth factor that can push molecules together: the hydrophobic force. The properties of the four types of noncovalent attractions are presented in Panel 2–3 (pp. 94–95). Although each
Cell_Biology_Alberts_214	Cell_Biology_Alberts	Figure 2–2 Some energies important for cells. a crucial property of any bond— covalent or noncovalent—is its strength. Bond strength is measured by the amount of energy that must be supplied to break it, expressed in units of either kilojoules per mole (kJ/mole) or kilocalories per mole (kcal/mole). Thus if 100 kJ of energy must be supplied to break 6 × 1023 bonds of a specific type (that is, 1 mole of these bonds), then the strength of that bond is 100 kJ/mole. note that, in this diagram, energies are compared on a logarithmic scale. Typical strengths and lengths of the main classes of chemical bonds are given in Table 2–1.	Cell_Biology_Alberts. Figure 2–2 Some energies important for cells. a crucial property of any bond— covalent or noncovalent—is its strength. Bond strength is measured by the amount of energy that must be supplied to break it, expressed in units of either kilojoules per mole (kJ/mole) or kilocalories per mole (kcal/mole). Thus if 100 kJ of energy must be supplied to break 6 × 1023 bonds of a specific type (that is, 1 mole of these bonds), then the strength of that bond is 100 kJ/mole. note that, in this diagram, energies are compared on a logarithmic scale. Typical strengths and lengths of the main classes of chemical bonds are given in Table 2–1.
Cell_Biology_Alberts_215	Cell_Biology_Alberts	one joule (J) is the amount of energy required to move an object a distance of one meter against a force of one newton. This measure of energy is derived from the si units (système internationale d’Unités) universally employed by physical scientists. a second unit of energy, often used by cell biologists, is the kilocalorie (kcal); one calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1°C. one kJ is equal to 0.239 kcal (1 kcal = 4.18 kJ). Figure 2–3 Schematic indicating how two macromolecules with complementary surfaces can bind tightly to one another through noncovalent interactions. noncovalent chemical bonds have less than 1/20 the strength of a covalent bond. They are able to produce tight binding only when many of them are formed simultaneously. although only electrostatic attractions are illustrated here, in reality all four noncovalent forces often contribute to holding two macromolecules together (Movie 2.1).	Cell_Biology_Alberts. one joule (J) is the amount of energy required to move an object a distance of one meter against a force of one newton. This measure of energy is derived from the si units (système internationale d’Unités) universally employed by physical scientists. a second unit of energy, often used by cell biologists, is the kilocalorie (kcal); one calorie is the amount of energy needed to raise the temperature of 1 gram of water by 1°C. one kJ is equal to 0.239 kcal (1 kcal = 4.18 kJ). Figure 2–3 Schematic indicating how two macromolecules with complementary surfaces can bind tightly to one another through noncovalent interactions. noncovalent chemical bonds have less than 1/20 the strength of a covalent bond. They are able to produce tight binding only when many of them are formed simultaneously. although only electrostatic attractions are illustrated here, in reality all four noncovalent forces often contribute to holding two macromolecules together (Movie 2.1).
Cell_Biology_Alberts_216	Cell_Biology_Alberts	individual noncovalent attraction would be much too weak to be effective in the face of thermal motions, their energies can sum to create a strong force between two separate molecules. Thus sets of noncovalent attractions often allow the complementary surfaces of two macromolecules to hold those two macromolecules together (Figure 2–3). Table 2–1 compares noncovalent bond strengths to that of a typical covalent bond, both in the presence and in the absence of water. Note that, by forming competing interactions with the involved molecules, water greatly reduces the strength of both electrostatic attractions and hydrogen bonds.	Cell_Biology_Alberts. individual noncovalent attraction would be much too weak to be effective in the face of thermal motions, their energies can sum to create a strong force between two separate molecules. Thus sets of noncovalent attractions often allow the complementary surfaces of two macromolecules to hold those two macromolecules together (Figure 2–3). Table 2–1 compares noncovalent bond strengths to that of a typical covalent bond, both in the presence and in the absence of water. Note that, by forming competing interactions with the involved molecules, water greatly reduces the strength of both electrostatic attractions and hydrogen bonds.
Cell_Biology_Alberts_217	Cell_Biology_Alberts	The structure of a typical hydrogen bond is illustrated in Figure 2–4. This bond represents a special form of polar interaction in which an electropositive hydrogen atom is shared by two electronegative atoms. Its hydrogen can be viewed as a proton that has partially dissociated from a donor atom, allowing it to be shared by a second acceptor atom. Unlike a typical electrostatic interaction, this bond is highly directional—being strongest when a straight line can be drawn between all three of the involved atoms.	Cell_Biology_Alberts. The structure of a typical hydrogen bond is illustrated in Figure 2–4. This bond represents a special form of polar interaction in which an electropositive hydrogen atom is shared by two electronegative atoms. Its hydrogen can be viewed as a proton that has partially dissociated from a donor atom, allowing it to be shared by a second acceptor atom. Unlike a typical electrostatic interaction, this bond is highly directional—being strongest when a straight line can be drawn between all three of the involved atoms.
Cell_Biology_Alberts_218	Cell_Biology_Alberts	The fourth effect that often brings molecules together in water is not, strictly speaking, a bond at all. However, a very important hydrophobic force is caused by a pushing of nonpolar surfaces out of the hydrogen-bonded water network, where they would otherwise physically interfere with the highly favorable interactions between water molecules. Bringing any two nonpolar surfaces together reduces their contact with water; in this sense, the force is nonspecific. Nevertheless, we shall see in Chapter 3 that hydrophobic forces are central to the proper folding of protein molecules.	Cell_Biology_Alberts. The fourth effect that often brings molecules together in water is not, strictly speaking, a bond at all. However, a very important hydrophobic force is caused by a pushing of nonpolar surfaces out of the hydrogen-bonded water network, where they would otherwise physically interfere with the highly favorable interactions between water molecules. Bringing any two nonpolar surfaces together reduces their contact with water; in this sense, the force is nonspecific. Nevertheless, we shall see in Chapter 3 that hydrophobic forces are central to the proper folding of protein molecules.
Cell_Biology_Alberts_219	Cell_Biology_Alberts	One of the simplest kinds of chemical reaction, and one that has profound significance in cells, takes place when a molecule containing a highly polar covalent bond between a hydrogen and another atom dissolves in water. The hydrogen atom in such a molecule has given up its electron almost entirely to the companion atom, and so exists as an almost naked positively charged hydrogen nucleus—in an ionic bond is an electrostatic attraction between two fully charged atoms. *Values in parentheses are kcal/mole. 1 kJ = 0.239 kcal and 1 kcal = 4.18 kJ. (A) hydrogen bond ~0.3 nm long covalent bond ~0.1 nm long Figure 2–4 Hydrogen bonds. (a) Ball-andstick model of a typical hydrogen bond. The distance between the hydrogen and the oxygen atom here is less than the sum of their van der Waals radii, indicating a partial sharing of electrons. (B) The most common hydrogen bonds in cells.	Cell_Biology_Alberts. One of the simplest kinds of chemical reaction, and one that has profound significance in cells, takes place when a molecule containing a highly polar covalent bond between a hydrogen and another atom dissolves in water. The hydrogen atom in such a molecule has given up its electron almost entirely to the companion atom, and so exists as an almost naked positively charged hydrogen nucleus—in an ionic bond is an electrostatic attraction between two fully charged atoms. *Values in parentheses are kcal/mole. 1 kJ = 0.239 kcal and 1 kcal = 4.18 kJ. (A) hydrogen bond ~0.3 nm long covalent bond ~0.1 nm long Figure 2–4 Hydrogen bonds. (a) Ball-andstick model of a typical hydrogen bond. The distance between the hydrogen and the oxygen atom here is less than the sum of their van der Waals radii, indicating a partial sharing of electrons. (B) The most common hydrogen bonds in cells.
Cell_Biology_Alberts_220	Cell_Biology_Alberts	H2O H2O molecule to H3O+ OH– the other hydronium hydroxyl(B) ion ion other words, a proton (H+). When the polar molecule becomes surrounded by water molecules, the proton will be attracted to the partial negative charge on the O atom of an adjacent water molecule. This proton can easily dissociate from its original partner and associate instead with the oxygen atom of the water molecule, generating a hydronium ion (H3O+) (Figure 2–5A). The reverse reaction also takes place very readily, so in the aqueous solution protons are constantly flitting to and fro between one molecule and another.	Cell_Biology_Alberts. H2O H2O molecule to H3O+ OH– the other hydronium hydroxyl(B) ion ion other words, a proton (H+). When the polar molecule becomes surrounded by water molecules, the proton will be attracted to the partial negative charge on the O atom of an adjacent water molecule. This proton can easily dissociate from its original partner and associate instead with the oxygen atom of the water molecule, generating a hydronium ion (H3O+) (Figure 2–5A). The reverse reaction also takes place very readily, so in the aqueous solution protons are constantly flitting to and fro between one molecule and another.
Cell_Biology_Alberts_221	Cell_Biology_Alberts	Substances that release protons when they dissolve in water, thus forming H3O+, are termed acids. The higher the concentration of H3O+, the more acidic the solution. H3O+ is present even in pure water, at a concentration of 10–7 M, as a result of the movement of protons from one water molecule to another (Figure 2–5B). By convention, the H3O+ concentration is usually referred to as the H+ concentration, even though most protons in an aqueous solution are present as H3O+. To avoid the use of unwieldy numbers, the concentration of H3O+ is expressed using a logarithmic scale called the pH scale. Pure water has a pH of 7.0 and is said to be neutral—that is, neither acidic (pH <7) nor basic (pH >7).	Cell_Biology_Alberts. Substances that release protons when they dissolve in water, thus forming H3O+, are termed acids. The higher the concentration of H3O+, the more acidic the solution. H3O+ is present even in pure water, at a concentration of 10–7 M, as a result of the movement of protons from one water molecule to another (Figure 2–5B). By convention, the H3O+ concentration is usually referred to as the H+ concentration, even though most protons in an aqueous solution are present as H3O+. To avoid the use of unwieldy numbers, the concentration of H3O+ is expressed using a logarithmic scale called the pH scale. Pure water has a pH of 7.0 and is said to be neutral—that is, neither acidic (pH <7) nor basic (pH >7).
Cell_Biology_Alberts_222	Cell_Biology_Alberts	Acids are characterized as being strong or weak, depending on how readily they give up their protons to water. Strong acids, such as hydrochloric acid (HCl), lose their protons quickly. Acetic acid, on the other hand, is a weak acid because it holds on to its proton more tightly when dissolved in water. Many of the acids important in the cell—such as molecules containing a carboxyl (COOH) group— are weak acids (see Panel 2–2, pp. 92–93). Because the proton of a hydronium ion can be passed readily to many types of molecules in cells, altering their character, the concentration of H3O+ inside a cell (the acidity) must be closely regulated. Acids—especially weak acids—will give up their protons more readily if the concentration of H3O+ in solution is low and will tend to receive them back if the concentration in solution is high.	Cell_Biology_Alberts. Acids are characterized as being strong or weak, depending on how readily they give up their protons to water. Strong acids, such as hydrochloric acid (HCl), lose their protons quickly. Acetic acid, on the other hand, is a weak acid because it holds on to its proton more tightly when dissolved in water. Many of the acids important in the cell—such as molecules containing a carboxyl (COOH) group— are weak acids (see Panel 2–2, pp. 92–93). Because the proton of a hydronium ion can be passed readily to many types of molecules in cells, altering their character, the concentration of H3O+ inside a cell (the acidity) must be closely regulated. Acids—especially weak acids—will give up their protons more readily if the concentration of H3O+ in solution is low and will tend to receive them back if the concentration in solution is high.
Cell_Biology_Alberts_223	Cell_Biology_Alberts	The opposite of an acid is a base. Any molecule capable of accepting a proton from a water molecule is called a base. Sodium hydroxide (NaOH) is basic (the term alkaline is also used) because it dissociates readily in aqueous solution to form Na+ ions and OH– ions. Because of this property, NaOH is called a strong base. More important in living cells, however, are the weak bases—those that have a weak tendency to reversibly accept a proton from water. Many biologically important molecules contain an amino (NH2) group. This group is a weak base that can generate OH– by taking a proton from water: –NH2 + H2O →–NH3+ + OH– (see Panel 2–2, pp. 92–93).	Cell_Biology_Alberts. The opposite of an acid is a base. Any molecule capable of accepting a proton from a water molecule is called a base. Sodium hydroxide (NaOH) is basic (the term alkaline is also used) because it dissociates readily in aqueous solution to form Na+ ions and OH– ions. Because of this property, NaOH is called a strong base. More important in living cells, however, are the weak bases—those that have a weak tendency to reversibly accept a proton from water. Many biologically important molecules contain an amino (NH2) group. This group is a weak base that can generate OH– by taking a proton from water: –NH2 + H2O →–NH3+ + OH– (see Panel 2–2, pp. 92–93).
Cell_Biology_Alberts_224	Cell_Biology_Alberts	Because an OH– ion combines with a H3O+ ion to form two water molecules, an increase in the OH– concentration forces a decrease in the concentration of H3O+, and vice versa. A pure solution of water contains an equal concentration (10–7 M) of both ions, rendering it neutral. The interior of a cell is also kept close to neutrality by the presence of buffers: weak acids and bases that can release or take up protons near pH 7, keeping the environment of the cell relatively constant under a variety of conditions. Figure 2–5 Protons readily move in aqueous solutions. (a) The reaction that takes place when a molecule of acetic acid dissolves in water. at ph 7, nearly all of the acetic acid is present as acetate ion. (B) Water molecules are continuously exchanging protons with each other to form hydronium and hydroxyl ions. These ions in turn rapidly recombine to form water molecules. a Cell is formed from Carbon Compounds	Cell_Biology_Alberts. Because an OH– ion combines with a H3O+ ion to form two water molecules, an increase in the OH– concentration forces a decrease in the concentration of H3O+, and vice versa. A pure solution of water contains an equal concentration (10–7 M) of both ions, rendering it neutral. The interior of a cell is also kept close to neutrality by the presence of buffers: weak acids and bases that can release or take up protons near pH 7, keeping the environment of the cell relatively constant under a variety of conditions. Figure 2–5 Protons readily move in aqueous solutions. (a) The reaction that takes place when a molecule of acetic acid dissolves in water. at ph 7, nearly all of the acetic acid is present as acetate ion. (B) Water molecules are continuously exchanging protons with each other to form hydronium and hydroxyl ions. These ions in turn rapidly recombine to form water molecules. a Cell is formed from Carbon Compounds
Cell_Biology_Alberts_225	Cell_Biology_Alberts	a Cell is formed from Carbon Compounds Having reviewed the ways atoms combine into molecules and how these molecules behave in an aqueous environment, we now examine the main classes of small molecules found in cells. We shall see that a few categories of molecules, formed from a handful of different elements, give rise to all the extraordinary richness of form and behavior shown by living things.	Cell_Biology_Alberts. a Cell is formed from Carbon Compounds Having reviewed the ways atoms combine into molecules and how these molecules behave in an aqueous environment, we now examine the main classes of small molecules found in cells. We shall see that a few categories of molecules, formed from a handful of different elements, give rise to all the extraordinary richness of form and behavior shown by living things.
Cell_Biology_Alberts_226	Cell_Biology_Alberts	If we disregard water and inorganic ions such as potassium, nearly all the molecules in a cell are based on carbon. Carbon is outstanding among all the elements in its ability to form large molecules; silicon is a poor second. Because carbon is small and has four electrons and four vacancies in its outermost shell, a carbon atom can form four covalent bonds with other atoms. Most important, one carbon atom can join to other carbon atoms through highly stable covalent C–C bonds to form chains and rings and hence generate large and complex molecules with no obvious upper limit to their size. The carbon compounds made by cells are called organic molecules. In contrast, all other molecules, including water, are said to be inorganic.	Cell_Biology_Alberts. If we disregard water and inorganic ions such as potassium, nearly all the molecules in a cell are based on carbon. Carbon is outstanding among all the elements in its ability to form large molecules; silicon is a poor second. Because carbon is small and has four electrons and four vacancies in its outermost shell, a carbon atom can form four covalent bonds with other atoms. Most important, one carbon atom can join to other carbon atoms through highly stable covalent C–C bonds to form chains and rings and hence generate large and complex molecules with no obvious upper limit to their size. The carbon compounds made by cells are called organic molecules. In contrast, all other molecules, including water, are said to be inorganic.
Cell_Biology_Alberts_227	Cell_Biology_Alberts	Certain combinations of atoms, such as the methyl (–CH3), hydroxyl (–OH), carboxyl (–COOH), carbonyl (–C=O), phosphate (–PO32–), sulfhydryl (–SH), and amino (–NH2) groups, occur repeatedly in the molecules made by cells. Each such chemical group has distinct chemical and physical properties that influence the behavior of the molecule in which the group occurs. The most common chemical groups and some of their properties are summarized in Panel 2–1, pp. 90–91. Cells Contain four major families of small organic molecules	Cell_Biology_Alberts. Certain combinations of atoms, such as the methyl (–CH3), hydroxyl (–OH), carboxyl (–COOH), carbonyl (–C=O), phosphate (–PO32–), sulfhydryl (–SH), and amino (–NH2) groups, occur repeatedly in the molecules made by cells. Each such chemical group has distinct chemical and physical properties that influence the behavior of the molecule in which the group occurs. The most common chemical groups and some of their properties are summarized in Panel 2–1, pp. 90–91. Cells Contain four major families of small organic molecules
Cell_Biology_Alberts_228	Cell_Biology_Alberts	Cells Contain four major families of small organic molecules The small organic molecules of the cell are carbon-based compounds that have molecular weights in the range of 100–1000 and contain up to 30 or so carbon atoms. They are usually found free in solution and have many different fates. Some are used as monomer subunits to construct giant polymeric macromolecules— proteins, nucleic acids, and large polysaccharides. Others act as energy sources and are broken down and transformed into other small molecules in a maze of intracellular metabolic pathways. Many small molecules have more than one role in the cell—for example, acting both as a potential subunit for a macromolecule and as an energy source. Small organic molecules are much less abundant than the organic macromolecules, accounting for only about one-tenth of the total mass of organic matter in a cell. As a rough guess, there may be a thousand different kinds of these small molecules in a typical cell.	Cell_Biology_Alberts. Cells Contain four major families of small organic molecules The small organic molecules of the cell are carbon-based compounds that have molecular weights in the range of 100–1000 and contain up to 30 or so carbon atoms. They are usually found free in solution and have many different fates. Some are used as monomer subunits to construct giant polymeric macromolecules— proteins, nucleic acids, and large polysaccharides. Others act as energy sources and are broken down and transformed into other small molecules in a maze of intracellular metabolic pathways. Many small molecules have more than one role in the cell—for example, acting both as a potential subunit for a macromolecule and as an energy source. Small organic molecules are much less abundant than the organic macromolecules, accounting for only about one-tenth of the total mass of organic matter in a cell. As a rough guess, there may be a thousand different kinds of these small molecules in a typical cell.
Cell_Biology_Alberts_229	Cell_Biology_Alberts	All organic molecules are synthesized from and are broken down into the same set of simple compounds. As a consequence, the compounds in a cell are chemically related and most can be classified into a few distinct families. Broadly speaking, cells contain four major families of small organic molecules: the sugars, the fatty acids, the nucleotides, and the amino acids (Figure 2–6). Although many compounds present in cells do not fit into these categories, these four families of small organic molecules, together with the macromolecules made by linking them into long chains, account for a large fraction of the cell mass. Amino acids and the proteins that they form will be the subject of Chapter 3. A summary of the structures and properties of the remaining three families— sugars, fatty acids, and nucleotides—is presented in Panels 2–4, 2–5, and 2–6, respectively (see pages 96–101). The Chemistry of Cells is Dominated by macromolecules with Remarkable properties	Cell_Biology_Alberts. All organic molecules are synthesized from and are broken down into the same set of simple compounds. As a consequence, the compounds in a cell are chemically related and most can be classified into a few distinct families. Broadly speaking, cells contain four major families of small organic molecules: the sugars, the fatty acids, the nucleotides, and the amino acids (Figure 2–6). Although many compounds present in cells do not fit into these categories, these four families of small organic molecules, together with the macromolecules made by linking them into long chains, account for a large fraction of the cell mass. Amino acids and the proteins that they form will be the subject of Chapter 3. A summary of the structures and properties of the remaining three families— sugars, fatty acids, and nucleotides—is presented in Panels 2–4, 2–5, and 2–6, respectively (see pages 96–101). The Chemistry of Cells is Dominated by macromolecules with Remarkable properties
Cell_Biology_Alberts_230	Cell_Biology_Alberts	The Chemistry of Cells is Dominated by macromolecules with Remarkable properties By weight, macromolecules are the most abundant carbon-containing molecules in a living cell (Figure 2–7). They are the principal building blocks from which a cell is constructed and also the components that confer the most distinctive properties of living things. The macromolecules in cells are polymers that are constructed by covalently linking small organic molecules (called monomers) into of the cell of the cell SUGARS FATTY ACIDS POLYSACCHARIDES AMINO ACIDS NUCLEOTIDES PROTEINS FATS, LIPIDS, MEMBRANES NUCLEIC ACIDS	Cell_Biology_Alberts. The Chemistry of Cells is Dominated by macromolecules with Remarkable properties By weight, macromolecules are the most abundant carbon-containing molecules in a living cell (Figure 2–7). They are the principal building blocks from which a cell is constructed and also the components that confer the most distinctive properties of living things. The macromolecules in cells are polymers that are constructed by covalently linking small organic molecules (called monomers) into of the cell of the cell SUGARS FATTY ACIDS POLYSACCHARIDES AMINO ACIDS NUCLEOTIDES PROTEINS FATS, LIPIDS, MEMBRANES NUCLEIC ACIDS
Cell_Biology_Alberts_231	Cell_Biology_Alberts	SUGARS FATTY ACIDS POLYSACCHARIDES AMINO ACIDS NUCLEOTIDES PROTEINS FATS, LIPIDS, MEMBRANES NUCLEIC ACIDS Figure 2–6 The four main families of small organic molecules in cells. These small molecules form the monomeric building blocks, or subunits, for most of the macromolecules and other assemblies of the cell. some, such as the sugars and the fatty acids, are also energy sources. Their structures are outlined here and shown in more detail in the panels at the end of this chapter and in Chapter 3. long chains (Figure 2–8). They have remarkable properties that could not have been predicted from their simple constituents.	Cell_Biology_Alberts. SUGARS FATTY ACIDS POLYSACCHARIDES AMINO ACIDS NUCLEOTIDES PROTEINS FATS, LIPIDS, MEMBRANES NUCLEIC ACIDS Figure 2–6 The four main families of small organic molecules in cells. These small molecules form the monomeric building blocks, or subunits, for most of the macromolecules and other assemblies of the cell. some, such as the sugars and the fatty acids, are also energy sources. Their structures are outlined here and shown in more detail in the panels at the end of this chapter and in Chapter 3. long chains (Figure 2–8). They have remarkable properties that could not have been predicted from their simple constituents.
Cell_Biology_Alberts_232	Cell_Biology_Alberts	long chains (Figure 2–8). They have remarkable properties that could not have been predicted from their simple constituents. Proteins are abundant and spectacularly versatile, performing thousands of distinct functions in cells. Many proteins serve as enzymes, the catalysts that facilitate the many covalent bond-making and bond-breaking reactions that the cell needs. Enzymes catalyze all of the reactions whereby cells extract energy from food molecules, for example, and an enzyme called ribulose bisphosphate carboxylase helps to convert CO2 to sugars in photosynthetic organisms, producing most of the organic matter needed for life on Earth. Other proteins are used to build structural components, such as tubulin, a protein that self-assembles to make the cell’s long microtubules, or histones, proteins that compact the DNA in chromosomes. Yet other proteins act as molecular motors to produce force and	Cell_Biology_Alberts. long chains (Figure 2–8). They have remarkable properties that could not have been predicted from their simple constituents. Proteins are abundant and spectacularly versatile, performing thousands of distinct functions in cells. Many proteins serve as enzymes, the catalysts that facilitate the many covalent bond-making and bond-breaking reactions that the cell needs. Enzymes catalyze all of the reactions whereby cells extract energy from food molecules, for example, and an enzyme called ribulose bisphosphate carboxylase helps to convert CO2 to sugars in photosynthetic organisms, producing most of the organic matter needed for life on Earth. Other proteins are used to build structural components, such as tubulin, a protein that self-assembles to make the cell’s long microtubules, or histones, proteins that compact the DNA in chromosomes. Yet other proteins act as molecular motors to produce force and
Cell_Biology_Alberts_233	Cell_Biology_Alberts	Figure 2–7 The distribution of molecules in cells. The approximate composition of a bacterial cell is shown by weight. The composition of an animal cell is similar, even though its volume is roughly 1000 times greater. note that macromolecules dominate. The major inorganic ions include na+, K+, mg2+, Ca2+, and Cl–. movement, as for myosin in muscle. Proteins perform many other functions, and we shall examine the molecular basis for many of them later in this book.	Cell_Biology_Alberts. Figure 2–7 The distribution of molecules in cells. The approximate composition of a bacterial cell is shown by weight. The composition of an animal cell is similar, even though its volume is roughly 1000 times greater. note that macromolecules dominate. The major inorganic ions include na+, K+, mg2+, Ca2+, and Cl–. movement, as for myosin in muscle. Proteins perform many other functions, and we shall examine the molecular basis for many of them later in this book.
Cell_Biology_Alberts_234	Cell_Biology_Alberts	Although the chemical reactions for adding subunits to each polymer are different in detail for proteins, nucleic acids, and polysaccharides, they share important features. Each polymer grows by the addition of a monomer onto the end of a growing chain in a condensation reaction, in which one molecule of water is lost with each subunit added (Figure 2–9). The stepwise polymerization of monomers into a long chain is a simple way to manufacture a large, complex molecule, since the subunits are added by the same reaction performed over and over again by the same set of enzymes. Apart from some of the polysaccharides, most macromolecules are made from a limited set of monomers that are slightly different from one another—for example, the 20 different amino acids from which proteins are made. It is critical to life that the polymer chain is not assembled at random from these subunits; instead the subunits are added in a precise order, or sequence. The elaborate mechanisms that allow	Cell_Biology_Alberts. Although the chemical reactions for adding subunits to each polymer are different in detail for proteins, nucleic acids, and polysaccharides, they share important features. Each polymer grows by the addition of a monomer onto the end of a growing chain in a condensation reaction, in which one molecule of water is lost with each subunit added (Figure 2–9). The stepwise polymerization of monomers into a long chain is a simple way to manufacture a large, complex molecule, since the subunits are added by the same reaction performed over and over again by the same set of enzymes. Apart from some of the polysaccharides, most macromolecules are made from a limited set of monomers that are slightly different from one another—for example, the 20 different amino acids from which proteins are made. It is critical to life that the polymer chain is not assembled at random from these subunits; instead the subunits are added in a precise order, or sequence. The elaborate mechanisms that allow
Cell_Biology_Alberts_235	Cell_Biology_Alberts	made. It is critical to life that the polymer chain is not assembled at random from these subunits; instead the subunits are added in a precise order, or sequence. The elaborate mechanisms that allow enzymes to accomplish this task are described in detail in Chapters 5 and 6.	Cell_Biology_Alberts. made. It is critical to life that the polymer chain is not assembled at random from these subunits; instead the subunits are added in a precise order, or sequence. The elaborate mechanisms that allow enzymes to accomplish this task are described in detail in Chapters 5 and 6.
Cell_Biology_Alberts_236	Cell_Biology_Alberts	noncovalent Bonds specify Both the precise shape of a macromolecule and its Binding to other molecules Most of the covalent bonds in a macromolecule allow rotation of the atoms they join, giving the polymer chain great flexibility. In principle, this allows a macromolecule to adopt an almost unlimited number of shapes, or conformations, as random thermal energy causes the polymer chain to writhe and rotate. However, the shapes of most biological macromolecules are highly constrained because of the many weak noncovalent bonds that form between different parts of the same molecule. If these noncovalent bonds are formed in sufficient numbers, the polymer chain can strongly prefer one particular conformation, determined by the linear sequence of monomers in its chain. Most protein molecules and many of the small RNA molecules found in cells fold tightly into a highly preferred conformation in this way (Figure 2–10).	Cell_Biology_Alberts. noncovalent Bonds specify Both the precise shape of a macromolecule and its Binding to other molecules Most of the covalent bonds in a macromolecule allow rotation of the atoms they join, giving the polymer chain great flexibility. In principle, this allows a macromolecule to adopt an almost unlimited number of shapes, or conformations, as random thermal energy causes the polymer chain to writhe and rotate. However, the shapes of most biological macromolecules are highly constrained because of the many weak noncovalent bonds that form between different parts of the same molecule. If these noncovalent bonds are formed in sufficient numbers, the polymer chain can strongly prefer one particular conformation, determined by the linear sequence of monomers in its chain. Most protein molecules and many of the small RNA molecules found in cells fold tightly into a highly preferred conformation in this way (Figure 2–10).
Cell_Biology_Alberts_237	Cell_Biology_Alberts	The four types of noncovalent interactions important in biological molecules were presented earlier, and they are discussed further in Panel 2–3 (pp. 94–95). In addition to folding biological macromolecules into unique shapes, they can also add up to create a strong attraction between two different molecules (see Figure 2–3). This form of molecular interaction provides for great specificity, inasmuch as the close multipoint contacts required for strong binding make it possible for a macromolecule to select out—through binding—just one of the many thousands of other types of molecules present inside a cell. Moreover, because the strength of the binding depends on the number of noncovalent bonds that are formed, interactions of almost any affinity are possible—allowing rapid dissociation where appropriate.	Cell_Biology_Alberts. The four types of noncovalent interactions important in biological molecules were presented earlier, and they are discussed further in Panel 2–3 (pp. 94–95). In addition to folding biological macromolecules into unique shapes, they can also add up to create a strong attraction between two different molecules (see Figure 2–3). This form of molecular interaction provides for great specificity, inasmuch as the close multipoint contacts required for strong binding make it possible for a macromolecule to select out—through binding—just one of the many thousands of other types of molecules present inside a cell. Moreover, because the strength of the binding depends on the number of noncovalent bonds that are formed, interactions of almost any affinity are possible—allowing rapid dissociation where appropriate.
Cell_Biology_Alberts_238	Cell_Biology_Alberts	As we discuss next, binding of this type underlies all biological catalysis, making it possible for proteins to function as enzymes. In addition, noncovalent interactions allow macromolecules to be used as building blocks for the formation of Figure 2–8 Three families of macromolecules. each is a polymer formed from small molecules (called monomers) linked together by covalent bonds. Figure 2–9 Condensation and hydrolysis as opposite reactions. The macromolecules of the cell are polymers that are formed from subunits (or monomers) by a condensation reaction, and they are broken down by hydrolysis. The condensation reactions are all energetically unfavorable; thus polymer formation requires an energy input, as will be described in the text.	Cell_Biology_Alberts. As we discuss next, binding of this type underlies all biological catalysis, making it possible for proteins to function as enzymes. In addition, noncovalent interactions allow macromolecules to be used as building blocks for the formation of Figure 2–8 Three families of macromolecules. each is a polymer formed from small molecules (called monomers) linked together by covalent bonds. Figure 2–9 Condensation and hydrolysis as opposite reactions. The macromolecules of the cell are polymers that are formed from subunits (or monomers) by a condensation reaction, and they are broken down by hydrolysis. The condensation reactions are all energetically unfavorable; thus polymer formation requires an energy input, as will be described in the text.
Cell_Biology_Alberts_239	Cell_Biology_Alberts	Figure 2–10 The folding of proteins and RNa molecules into a particularly stable three-dimensional shape, or conformation. if the noncovalent bonds maintaining the stable conformation are disrupted, the molecule becomes a flexible chain that loses its biological activity. larger structures, thereby forming intricate machines with multiple moving parts that perform such complex tasks as DNA replication and protein synthesis (Figure 2–11).	Cell_Biology_Alberts. Figure 2–10 The folding of proteins and RNa molecules into a particularly stable three-dimensional shape, or conformation. if the noncovalent bonds maintaining the stable conformation are disrupted, the molecule becomes a flexible chain that loses its biological activity. larger structures, thereby forming intricate machines with multiple moving parts that perform such complex tasks as DNA replication and protein synthesis (Figure 2–11).
Cell_Biology_Alberts_240	Cell_Biology_Alberts	Living organisms are autonomous, self-propagating chemical systems. They are formed from a distinctive and restricted set of small carbon-based molecules that are essentially the same for every living species. Each of these small molecules is composed of a small set of atoms linked to each other in a precise configuration through covalent bonds. The main categories are sugars, fatty acids, amino acids, and nucleotides. Sugars are a primary source of chemical energy for cells and can be incorporated into polysaccharides for energy storage. Fatty acids are also important for energy storage, but their most critical function is in the formation of cell membranes. Long chains of amino acids form the remarkably diverse and versatile macromolecules known as proteins. Nucleotides play a central part in energy transfer, while also serving as the subunits for the informational macromolecules, RNA and DNA.	Cell_Biology_Alberts. Living organisms are autonomous, self-propagating chemical systems. They are formed from a distinctive and restricted set of small carbon-based molecules that are essentially the same for every living species. Each of these small molecules is composed of a small set of atoms linked to each other in a precise configuration through covalent bonds. The main categories are sugars, fatty acids, amino acids, and nucleotides. Sugars are a primary source of chemical energy for cells and can be incorporated into polysaccharides for energy storage. Fatty acids are also important for energy storage, but their most critical function is in the formation of cell membranes. Long chains of amino acids form the remarkably diverse and versatile macromolecules known as proteins. Nucleotides play a central part in energy transfer, while also serving as the subunits for the informational macromolecules, RNA and DNA.
Cell_Biology_Alberts_241	Cell_Biology_Alberts	Most of the dry mass of a cell consists of macromolecules that have been produced as linear polymers of amino acids (proteins) or nucleotides (DNA and RNA), covalently linked to each other in an exact order. Most of the protein molecules and many of the RNAs fold into a unique conformation that is determined by their sequence of subunits. This folding process creates unique surfaces, and it depends on a large set of weak attractions produced by noncovalent forces between atoms. e.g., sugars, amino acids, and nucleotides e.g., globular proteins and RNA e.g., ribosome Figure 2–11 Small molecules become covalently linked to form macromolecules, which in turn assemble through noncovalent interactions to form large complexes. small molecules, proteins, and a ribosome are drawn approximately to scale. Ribosomes are a central part of the machinery that the cell uses to make proteins: each ribosome is formed as a complex of about 90 macromolecules (protein and Rna molecules).	Cell_Biology_Alberts. Most of the dry mass of a cell consists of macromolecules that have been produced as linear polymers of amino acids (proteins) or nucleotides (DNA and RNA), covalently linked to each other in an exact order. Most of the protein molecules and many of the RNAs fold into a unique conformation that is determined by their sequence of subunits. This folding process creates unique surfaces, and it depends on a large set of weak attractions produced by noncovalent forces between atoms. e.g., sugars, amino acids, and nucleotides e.g., globular proteins and RNA e.g., ribosome Figure 2–11 Small molecules become covalently linked to form macromolecules, which in turn assemble through noncovalent interactions to form large complexes. small molecules, proteins, and a ribosome are drawn approximately to scale. Ribosomes are a central part of the machinery that the cell uses to make proteins: each ribosome is formed as a complex of about 90 macromolecules (protein and Rna molecules).
Cell_Biology_Alberts_242	Cell_Biology_Alberts	These forces are of four types: electrostatic attractions, hydrogen bonds, van der Waals attractions, and an interaction between nonpolar groups caused by their hydrophobic expulsion from water. The same set of weak forces governs the specific binding of other molecules to macromolecules, making possible the myriad associations between biological molecules that produce the structure and the chemistry of a cell. CaTalYsis anD The Use of eneRGY BY Cells	Cell_Biology_Alberts. These forces are of four types: electrostatic attractions, hydrogen bonds, van der Waals attractions, and an interaction between nonpolar groups caused by their hydrophobic expulsion from water. The same set of weak forces governs the specific binding of other molecules to macromolecules, making possible the myriad associations between biological molecules that produce the structure and the chemistry of a cell. CaTalYsis anD The Use of eneRGY BY Cells
Cell_Biology_Alberts_243	Cell_Biology_Alberts	CaTalYsis anD The Use of eneRGY BY Cells One property of living things above all makes them seem almost miraculously different from nonliving matter: they create and maintain order, in a universe that is tending always to greater disorder (Figure 2–12). To create this order, the cells in a living organism must perform a never-ending stream of chemical reactions. In some of these reactions, small organic molecules—amino acids, sugars, nucleotides, and lipids—are being taken apart or modified to supply the many other small molecules that the cell requires. In other reactions, small molecules are being used to construct an enormously diverse range of proteins, nucleic acids, and other macromolecules that endow living systems with all of their most distinctive properties. Each cell can be viewed as a tiny chemical factory, performing many millions of reactions every second. Cell metabolism is organized by enzymes	Cell_Biology_Alberts. CaTalYsis anD The Use of eneRGY BY Cells One property of living things above all makes them seem almost miraculously different from nonliving matter: they create and maintain order, in a universe that is tending always to greater disorder (Figure 2–12). To create this order, the cells in a living organism must perform a never-ending stream of chemical reactions. In some of these reactions, small organic molecules—amino acids, sugars, nucleotides, and lipids—are being taken apart or modified to supply the many other small molecules that the cell requires. In other reactions, small molecules are being used to construct an enormously diverse range of proteins, nucleic acids, and other macromolecules that endow living systems with all of their most distinctive properties. Each cell can be viewed as a tiny chemical factory, performing many millions of reactions every second. Cell metabolism is organized by enzymes
Cell_Biology_Alberts_244	Cell_Biology_Alberts	Cell metabolism is organized by enzymes The chemical reactions that a cell carries out would normally occur only at much higher temperatures than those existing inside cells. For this reason, each reaction requires a specific boost in chemical reactivity. This requirement is crucial, because it allows the cell to control its chemistry. The control is exerted through specialized biological catalysts. These are almost always proteins called enzymes, although RNA catalysts also exist, called ribozymes. Each enzyme accelerates, or catalyzes, just one of the many possible kinds of reactions that a particular molecule might undergo. Enzyme-catalyzed reactions are connected in series, so that the product of one reaction becomes the starting material, or substrate, for the next (Figure 2–13). Long linear reaction pathways are in turn linked to one another, forming a maze of interconnected reactions that enable the cell to survive, grow, and reproduce.	Cell_Biology_Alberts. Cell metabolism is organized by enzymes The chemical reactions that a cell carries out would normally occur only at much higher temperatures than those existing inside cells. For this reason, each reaction requires a specific boost in chemical reactivity. This requirement is crucial, because it allows the cell to control its chemistry. The control is exerted through specialized biological catalysts. These are almost always proteins called enzymes, although RNA catalysts also exist, called ribozymes. Each enzyme accelerates, or catalyzes, just one of the many possible kinds of reactions that a particular molecule might undergo. Enzyme-catalyzed reactions are connected in series, so that the product of one reaction becomes the starting material, or substrate, for the next (Figure 2–13). Long linear reaction pathways are in turn linked to one another, forming a maze of interconnected reactions that enable the cell to survive, grow, and reproduce.
Cell_Biology_Alberts_245	Cell_Biology_Alberts	Two opposing streams of chemical reactions occur in cells: (1) the catabolic pathways break down foodstuffs into smaller molecules, thereby generating both a useful form of energy for the cell and some of the small molecules that the cell needs as building blocks, and (2) the anabolic, or biosynthetic, pathways use the 20 nm 50 nm 10 µm 0.5 mm 20 mm	Cell_Biology_Alberts. Two opposing streams of chemical reactions occur in cells: (1) the catabolic pathways break down foodstuffs into smaller molecules, thereby generating both a useful form of energy for the cell and some of the small molecules that the cell needs as building blocks, and (2) the anabolic, or biosynthetic, pathways use the 20 nm 50 nm 10 µm 0.5 mm 20 mm
Cell_Biology_Alberts_246	Cell_Biology_Alberts	Figure 2–12 biological structures are highly ordered. Well-defined, ornate, and beautiful spatial patterns can be found at every level of organization in living organisms. in order of increasing size: (a) protein molecules in the coat of a virus (a parasite that, although not technically alive, contains the same types of molecules as those found in living cells); (B) the regular array of microtubules seen in a cross section of a sperm tail; (C) surface contours of a pollen grain (a single cell); (D) cross section of a fern stem, showing the patterned arrangement of cells; and (e) a spiral arrangement of leaves in a succulent plant. (a, courtesy of Robert Grant, stéphane Crainic, and James m. hogle; B, courtesy of lewis Tilney; C, courtesy of Colin macfarlane and Chris Jeffree; D, courtesy of Jim haseloff.)	Cell_Biology_Alberts. Figure 2–12 biological structures are highly ordered. Well-defined, ornate, and beautiful spatial patterns can be found at every level of organization in living organisms. in order of increasing size: (a) protein molecules in the coat of a virus (a parasite that, although not technically alive, contains the same types of molecules as those found in living cells); (B) the regular array of microtubules seen in a cross section of a sperm tail; (C) surface contours of a pollen grain (a single cell); (D) cross section of a fern stem, showing the patterned arrangement of cells; and (e) a spiral arrangement of leaves in a succulent plant. (a, courtesy of Robert Grant, stéphane Crainic, and James m. hogle; B, courtesy of lewis Tilney; C, courtesy of Colin macfarlane and Chris Jeffree; D, courtesy of Jim haseloff.)
Cell_Biology_Alberts_247	Cell_Biology_Alberts	Figure 2–13 How a set of enzyme-catalyzed reactions generates a metabolic pathway. each enzyme catalyzes a particular chemical reaction, leaving the enzyme unchanged. in this example, a set of enzymes acting in series converts molecule a to molecule f, forming a metabolic pathway. (for a diagram of many of the reactions in a human cell, abbreviated as shown, see figure 2–63.) small molecules and the energy harnessed by catabolism to drive the synthesis of the many other molecules that form the cell. Together these two sets of reactions constitute the metabolism of the cell (Figure 2–14). The details of cell metabolism form the traditional subject of biochemistry and most of them need not concern us here. But the general principles by which cells obtain energy from their environment and use it to create order are central to cell biology. We begin with a discussion of why a constant input of energy is needed to sustain all living things.	Cell_Biology_Alberts. Figure 2–13 How a set of enzyme-catalyzed reactions generates a metabolic pathway. each enzyme catalyzes a particular chemical reaction, leaving the enzyme unchanged. in this example, a set of enzymes acting in series converts molecule a to molecule f, forming a metabolic pathway. (for a diagram of many of the reactions in a human cell, abbreviated as shown, see figure 2–63.) small molecules and the energy harnessed by catabolism to drive the synthesis of the many other molecules that form the cell. Together these two sets of reactions constitute the metabolism of the cell (Figure 2–14). The details of cell metabolism form the traditional subject of biochemistry and most of them need not concern us here. But the general principles by which cells obtain energy from their environment and use it to create order are central to cell biology. We begin with a discussion of why a constant input of energy is needed to sustain all living things.
Cell_Biology_Alberts_248	Cell_Biology_Alberts	Biological order is made possible by the Release of heat energy from Cells The universal tendency of things to become disordered is a fundamental law of physics—the second law of thermodynamics—which states that in the universe, or in any isolated system (a collection of matter that is completely isolated from the rest of the universe), the degree of disorder always increases. This law has such profound implications for life that we will restate it in several ways.	Cell_Biology_Alberts. Biological order is made possible by the Release of heat energy from Cells The universal tendency of things to become disordered is a fundamental law of physics—the second law of thermodynamics—which states that in the universe, or in any isolated system (a collection of matter that is completely isolated from the rest of the universe), the degree of disorder always increases. This law has such profound implications for life that we will restate it in several ways.
Cell_Biology_Alberts_249	Cell_Biology_Alberts	For example, we can present the second law in terms of probability by stating that systems will change spontaneously toward those arrangements that have the greatest probability. If we consider a box of 100 coins all lying heads up, a series of accidents that disturbs the box will tend to move the arrangement toward a mixture of 50 heads and 50 tails. The reason is simple: there is a huge number of possible arrangements of the individual coins in the mixture that can achieve the 50–50 result, but only one possible arrangement that keeps all of the coins oriented heads up. Because the 50–50 mixture is therefore the most probable, we say that it is more “disordered.” For the same reason, it is a common experience that one’s living space will become increasingly disordered without intentional effort: the movement toward disorder is a spontaneous process, requiring a periodic effort to reverse it (Figure 2–15).	Cell_Biology_Alberts. For example, we can present the second law in terms of probability by stating that systems will change spontaneously toward those arrangements that have the greatest probability. If we consider a box of 100 coins all lying heads up, a series of accidents that disturbs the box will tend to move the arrangement toward a mixture of 50 heads and 50 tails. The reason is simple: there is a huge number of possible arrangements of the individual coins in the mixture that can achieve the 50–50 result, but only one possible arrangement that keeps all of the coins oriented heads up. Because the 50–50 mixture is therefore the most probable, we say that it is more “disordered.” For the same reason, it is a common experience that one’s living space will become increasingly disordered without intentional effort: the movement toward disorder is a spontaneous process, requiring a periodic effort to reverse it (Figure 2–15).