《生物学》课程参考书籍文献（英文版，Chapter 01 - Chapter 10）Part I The Origin of Living、Part II Biology of the Cell、Part III Energetics

2.3 Chemical bonds hold molecules together. Ionic Bonds Form Crystals The chlorine atom has 17 electrons: 2 in the inner energy evel. 8 in the next level and 7 in the outer level. Hence, one a group of atoms held together by energy in a stable associ- of the orbitals in the outer energy level has an unpaired ation is called a molecule. when a molecule contains atoms electron. The addition of another electron to the outer level f more than one element, it is called a compound. The fills that level and causes a negatively charged chloride ion, atoms in a molecule are joined by chemical bonds; these Ch, to form. bonds can result when atoms with opposite charges attract When placed together, meta allic sodium and gaseous (ionic bonds), when two atoms share one or more pairs of chlorine react swiftly and explosively, as the sodium atoms electrons (covalent bonds), or when atoms interact in other donate electrons to chlorine forming Nat and Cl ions. Be- rays.We will start by examining ionic bonds, which form cause opposite charges attract, the Na* and Ch remain asso- when atoms with opposite electrical charges (ions)attract. ciated in an ionic compound, Nacl, which is electrically neutral. However, the electrical attractive force holding a Closer look at table salt Nacl together directed specifically between particular Na+ and cl- ions. and no discrete sodium chloride mole- Common table salt, sodium chloride(Naci), is a lattice of cules form. Instead, the force exists between any one ion and ions in which the atoms are held together by ionic bonds all neighboring ions of the opposite charge, and the ions ag igure 2.9). Sodium has 11 electrons: 2 in the inner energy gregate in a cryst matrix with a precis se geometry. Such ag uter(valence)level. gregations are what we know as salt crystals. If a salt such as The valence electron is unpaired (free)and has a strong ten- Nacl is placed in water, the electrical attraction of the water can be achieved if the valence electron is lost to another .pIe for reasons we will point out later in this chapter, dency to join with another electron. A stable configuration nolecules forces holding the ions in their crystal matrix, atom that also has an unpaired electron. The loss of this ausing the salt to dissolve into a roughly equal mixture of electron results in the formation of a positively charged free Na* and Chions sodium ion. N An ionic bond is an attraction between ions of opposite charge in an ionic compound. Such bonds are not formed between particular ions in the ather, they exist between an ion and all of the oppositely charged ions in its immediate vicinity Sodium atom Chlorine atom 十 Sodium ion Chloride ion FIGURE 2.9 The formation of ionic bonds by sodium chloride. (a) When a sodium atom donates an electron to a chlorine atom, the sodium atom becomes a positively charged sodium ion, and the chlorine atom becomes a negatively charged chloride ion. (b) Sodium chloride forms a highly regular lattice of alternating sodium ions and chloride ions 26 Part I The Origin of Living things

26 Part I The Origin of Living Things Na Sodium atom Sodium ion Chlorine atom + – Chloride ion Na+ Cl Cl (a) FIGURE 2.9 The formation of ionic bonds by sodium chloride. (a) When a sodium atom donates an electron to a chlorine atom, the sodium atom becomes a positively charged sodium ion, and the chlorine atom becomes a negatively charged chloride ion. (b) Sodium chloride forms a highly regular lattice of alternating sodium ions and chloride ions. NaCl crystal Cl Cl Cl Cl Cl Na Na Na Na (b) Ionic Bonds Form Crystals A group of atoms held together by energy in a stable associ￾ation is called a molecule. When a molecule contains atoms of more than one element, it is called a compound. The atoms in a molecule are joined by chemical bonds; these bonds can result when atoms with opposite charges attract (ionic bonds), when two atoms share one or more pairs of electrons (covalent bonds), or when atoms interact in other ways. We will start by examining ionic bonds, which form when atoms with opposite electrical charges (ions) attract. A Closer Look at Table Salt Common table salt, sodium chloride (NaCl), is a lattice of ions in which the atoms are held together by ionic bonds (figure 2.9). Sodium has 11 electrons: 2 in the inner energy level, 8 in the next level, and 1 in the outer (valence) level. The valence electron is unpaired (free) and has a strong ten￾dency to join with another electron. A stable configuration can be achieved if the valence electron is lost to another atom that also has an unpaired electron. The loss of this electron results in the formation of a positively charged sodium ion, Na+. The chlorine atom has 17 electrons: 2 in the inner energy level, 8 in the next level, and 7 in the outer level. Hence, one of the orbitals in the outer energy level has an unpaired electron. The addition of another electron to the outer level fills that level and causes a negatively charged chloride ion, Cl–, to form. When placed together, metallic sodium and gaseous chlorine react swiftly and explosively, as the sodium atoms donate electrons to chlorine, forming Na+ and Cl– ions. Be￾cause opposite charges attract, the Na+ and Cl– remain asso￾ciated in an ionic compound, NaCl, which is electrically neutral. However, the electrical attractive force holding NaCl together is not directed specifically between particular Na+ and Cl– ions, and no discrete sodium chloride mole￾cules form. Instead, the force exists between any one ion and all neighboring ions of the opposite charge, and the ions ag￾gregate in a crystal matrix with a precise geometry. Such ag￾gregations are what we know as salt crystals. If a salt such as NaCl is placed in water, the electrical attraction of the water molecules, for reasons we will point out later in this chapter, disrupts the forces holding the ions in their crystal matrix, causing the salt to dissolve into a roughly equal mixture of free Na+ and Cl– ions. An ionic bond is an attraction between ions of opposite charge in an ionic compound. Such bonds are not formed between particular ions in the compound; rather, they exist between an ion and all of the oppositely charged ions in its immediate vicinity. 2.3 Chemical bonds hold molecules together.

Covalent Bonds Build Covalent bond Molecules with Several Covalent Stable molecules Molecules often consist of more than Covalent bonds form when two atoms two atoms. One reason that larger mole share one or more pairs of valence cules may be formed is that a given atom electrons. Consider hydrogen(H) as an is able to share electrons with example. Each hydrogen atom has an unpaired electron and an unfilled outer one other atom. An atom that requires energy level; for these reasons the hy H2(hydrogen gas two, three. or four additional electrons to fill its outer energy level completely drogen atom is unstable. When two nay acquire them by sharing its elec- hydrogen atoms are close to eacl trons with two or more other atoms other. however, each atom's electron For example, the carbon atom(C) can orbit both nuclei. In effect, the nu- contains six electrons. four of which are clei are able to share their electrons in its outer energy level. To satisfy the The result is a diatomic(two-atom) olecule of hydrogen gas(figure 2.10) octet rule, a carbon atom must gain ac- The molecule formed by the two ess to four additional electrons: that is drogen atoms is stable for three reasons: it must form four covalent bonds be- cause four covalent bonds may form in 1. It has no net charge. The di many ways, carbon atoms are found in atomic molecule formed as a result many different kinds of molecules of this sharing of electrons is not charged, because it still contains two protons and two electrons Chemical reactions 2. The octet rule is satisfied Each of the two hydrogen atoms FIGURE 2.10 The formation and breaking of chem Hydrogen gas. (a) Hydrogen gas is a cal bonds, the essence of chemistry, can be considered to have two or- diatomic molecule composed of two called a chemical reaction. All chemi biting electrons in its outer energy hydrogen atoms, each sharing its electron I reactions involve the shifting of at level. This satisfies the octet rule, with the other. (6) The flash of fire that oms from one molecule or ionic com because each shared electron orbits consumed the Hindenburg occurred when pound to another, without any change both nuclei and is included in the the hydrogen gas that was used to inflate the in the number or identity of the atoms outer energy level of botb atoms. dirigible combined explosively with oxygen For convenience, we refer to the origi 3. It has no free electrons. The gas in the air to form water nal molecules before the reaction starts bonds between the two atoms as reactants and the molecules result also pair the two free electrons. ing from the chemical reaction as prod Unlike ionic bonds. covalent bonds ucts. For exampl are formed between two specific atoms, giving rise to true, A-B+C-D→A—C+B+D discrete molecules. While ionic bonds can form regular crys- tals, the more specific associations made possible by covalent bonds allow the formation of complex molecular structures The extent to which chemical reactions occur is influ- enced by several important factors 1. Temperature. Heating up the reactants increases Covalent Bonds Can Be very Strong the rate of a reaction(as long as the temperature isnt The strength of a covalent bond depends on the number of so high as to destroy the molecules) shared electrons. Thus double bonds, which satisfy the oc- 2. Concentration of reactants and products. Reac tet rule by allowing two atoms to share two pairs of elec tions proceed more quickly when more reactants are trons, are stronger than single bonds, in which only one available. An accumulation of products typically lectron pair is shared. This means more chemical energy is peeds reactions in the reverse direction. quired to break a double bond than a single bond. The 3. Catalysts. A catalyst is a substance that increases the strongest covalent bonds are triple bonds, such as those rate of a reaction It doesn't alter the reaction's e that link the two nitrogen atoms of nitrogen gas molecules librium between reactants and products, but it does Covalent bonds are represented in chemical formulations as shorten the time needed to reach equilibrium, often lines connecting atomic symbols, where each line between dramatically. In organisms, proteins called enzymes two bonded atoms represents the sharing of one pair of catalyze almost every chemical reaction electrons. The structural formulas of hydrogen gas and oxygen gas are H-H and O=O, respectively, while their A covalent bond is a stable chemical bond formed when molecular formulas are h, and o two atoms share one or more pairs of electrons. Chapter 2 The Nature of Molecules 27

Covalent Bonds Build Stable Molecules Covalent bonds form when two atoms share one or more pairs of valence electrons. Consider hydrogen (H) as an example. Each hydrogen atom has an unpaired electron and an unfilled outer energy level; for these reasons the hy￾drogen atom is unstable. When two hydrogen atoms are close to each other, however, each atom’s electron can orbit both nuclei. In effect, the nu￾clei are able to share their electrons. The result is a diatomic (two-atom) molecule of hydrogen gas (figure 2.10). The molecule formed by the two hy￾drogen atoms is stable for three reasons: 1. It has no net charge. The di￾atomic molecule formed as a result of this sharing of electrons is not charged, because it still contains two protons and two electrons. 2. The octet rule is satisfied. Each of the two hydrogen atoms can be considered to have two or￾biting electrons in its outer energy level. This satisfies the octet rule, because each shared electron orbits both nuclei and is included in the outer energy level of both atoms. 3. It has no free electrons. The bonds between the two atoms also pair the two free electrons. Unlike ionic bonds, covalent bonds are formed between two specific atoms, giving rise to true, discrete molecules. While ionic bonds can form regular crys￾tals, the more specific associations made possible by covalent bonds allow the formation of complex molecular structures. Covalent Bonds Can Be Very Strong The strength of a covalent bond depends on the number of shared electrons. Thus double bonds, which satisfy the oc￾tet rule by allowing two atoms to share two pairs of elec￾trons, are stronger than single bonds, in which only one electron pair is shared. This means more chemical energy is required to break a double bond than a single bond. The strongest covalent bonds are triple bonds, such as those that link the two nitrogen atoms of nitrogen gas molecules. Covalent bonds are represented in chemical formulations as lines connecting atomic symbols, where each line between two bonded atoms represents the sharing of one pair of electrons. The structural formulas of hydrogen gas and oxygen gas are H—H and OO, respectively, while their molecular formulas are H2 and O2. Molecules with Several Covalent Bonds Molecules often consist of more than two atoms. One reason that larger mole￾cules may be formed is that a given atom is able to share electrons with more than one other atom. An atom that requires two, three, or four additional electrons to fill its outer energy level completely may acquire them by sharing its elec￾trons with two or more other atoms. For example, the carbon atom (C) contains six electrons, four of which are in its outer energy level. To satisfy the octet rule, a carbon atom must gain ac￾cess to four additional electrons; that is, it must form four covalent bonds. Be￾cause four covalent bonds may form in many ways, carbon atoms are found in many different kinds of molecules. Chemical Reactions The formation and breaking of chemi￾cal bonds, the essence of chemistry, is called a chemical reaction. All chemi￾cal reactions involve the shifting of at￾oms from one molecule or ionic com￾pound to another, without any change in the number or identity of the atoms. For convenience, we refer to the origi￾nal molecules before the reaction starts as reactants, and the molecules result￾ing from the chemical reaction as prod￾ucts. For example: A — B + C — D → A — C + B + D reactants products The extent to which chemical reactions occur is influ￾enced by several important factors: 1. Temperature. Heating up the reactants increases the rate of a reaction (as long as the temperature isn’t so high as to destroy the molecules). 2. Concentration of reactants and products. Reac￾tions proceed more quickly when more reactants are available. An accumulation of products typically speeds reactions in the reverse direction. 3. Catalysts. A catalyst is a substance that increases the rate of a reaction. It doesn’t alter the reaction’s equi￾librium between reactants and products, but it does shorten the time needed to reach equilibrium, often dramatically. In organisms, proteins called enzymes catalyze almost every chemical reaction. A covalent bond is a stable chemical bond formed when two atoms share one or more pairs of electrons. Chapter 2 The Nature of Molecules 27 FIGURE 2.10 Hydrogen gas. (a) Hydrogen gas is a diatomic molecule composed of two hydrogen atoms, each sharing its electron with the other. (b) The flash of fire that consumed the Hindenburg occurred when the hydrogen gas that was used to inflate the dirigible combined explosively with oxygen gas in the air to form water. H2 (hydrogen gas) Covalent bond + + – – (a) (b)

2.4 Water is the cradle of life (b) FIGURE 2.11 Water takes many forms. As a liquid, water fills our rivers and runs down over the land to the sea. (a)The iceberg on which the penguins are holding their meeting was formed in Antarctica from a huge block of ice that broke away into the ocean water. ( b)When water cool below 0oC, it forms beautiful crystals, familiar to us as snow and ice. However, water is not always plentiful. (e)At Badwater, in Death Valley, California, there is no hint of water except for the broken patterns of dried mud Chemistry of water Of all the molecules that are common on earth, only wa ter exists as a liquid at the relatively low temperatures that Hydrogen prevail on the earths surface, three-fourths of which is covered by liquid water(figure 2. 11). When life was origi nating, water provided a medium in which other molecules could move around and interact without being held place by strong covalent or ionic bonds. Life evolved as a result of these interactions, and it is still inextricably tied Bohr model Ball-and-stick model to water. Life began in water and evolved there for 3 bil-(a) lion years before spreading to land. About two-thirds of any organisms body is composed of water, and no organ- Water has a simple molecular structure.(@)Each molecule is composed of one ronment. It is no accident that tropical rain forests are hydrogen xygen atom shares one electron with each hydrogen atom. ( b) bursting with life, while dry deserts appear almost lifeless The greater electronegativity of the oxygen atom makes the water except when water becomes temporarily plentiful, such as molecule polar: water carries two partial negative charges(8-)near the oxygen atom and two partial positive charges( 8*), one on each hydrogen atom The Atomic Structure of Water Water has a simple atomic structure. It consists of an oxy- property, which derives directly from the structure of wa- gen atom bound to two hydrogen atoms by two single cova- ter, is responsible for much of the organization of living nt bonds(figure 2.12a). The resulting molecule is stable: it chemistry satisfies the octet rule, has no unpaired electrons, and carries The chemistry of life is water chemistry. The way in The single most outstanding chemical property of wa which life first evolved was determined in large part by ter is its ability to form weak chemical associations with the chemical properties of the liquid water in which only 5 to 10% of the strength of covalent bonds. This that evolution occurred 28 Part I The Origin of Living Things

Chemistry of Water Of all the molecules that are common on earth, only wa￾ter exists as a liquid at the relatively low temperatures that prevail on the earth’s surface, three-fourths of which is covered by liquid water (figure 2.11). When life was origi￾nating, water provided a medium in which other molecules could move around and interact without being held in place by strong covalent or ionic bonds. Life evolved as a result of these interactions, and it is still inextricably tied to water. Life began in water and evolved there for 3 bil￾lion years before spreading to land. About two-thirds of any organism’s body is composed of water, and no organ￾ism can grow or reproduce in any but a water-rich envi￾ronment. It is no accident that tropical rain forests are bursting with life, while dry deserts appear almost lifeless except when water becomes temporarily plentiful, such as after a rainstorm. The Atomic Structure of Water Water has a simple atomic structure. It consists of an oxy￾gen atom bound to two hydrogen atoms by two single cova￾lent bonds (figure 2.12a). The resulting molecule is stable: it satisfies the octet rule, has no unpaired electrons, and carries no net electrical charge. The single most outstanding chemical property of wa￾ter is its ability to form weak chemical associations with only 5 to 10% of the strength of covalent bonds. This property, which derives directly from the structure of wa￾ter, is responsible for much of the organization of living chemistry. The chemistry of life is water chemistry. The way in which life first evolved was determined in large part by the chemical properties of the liquid water in which that evolution occurred. 28 Part I The Origin of Living Things FIGURE 2.11 Water takes many forms. As a liquid, water fills our rivers and runs down over the land to the sea. (a) The iceberg on which the penguins are holding their meeting was formed in Antarctica from a huge block of ice that broke away into the ocean water. (b) When water cools below 0°C, it forms beautiful crystals, familiar to us as snow and ice. However, water is not always plentiful. (c) At Badwater, in Death Valley, California, there is no hint of water except for the broken patterns of dried mud. (a) (b) (c) FIGURE 2.12 Water has a simple molecular structure. (a) Each molecule is composed of one oxygen atom and two hydrogen atoms. The oxygen atom shares one electron with each hydrogen atom. (b) The greater electronegativity of the oxygen atom makes the water molecule polar: water carries two partial negative charges (δ–) near the oxygen atom and two partial positive charges (δ+), one on each hydrogen atom. δ– δ– δ– δ– δ+ δ+ δ+ δ+ 104.5° Oxygen Hydrogen Hydrogen Bohr model Ball-and-stick model H H 8+ 8n + + O (a) (b) 2.4 Water is the cradle of life

Water Atoms Act Like Tiny Magnets Both the oxygen and the hydrogen atoms attract the electrons they share in the covalent bonds of Oxygen atom a water molecule. this attraction is called elec tronegativity. However, the oxygen atom is more electronegative than the hydrogen atoms, so it attracts the electrons more strongly than do the hydrogen atoms. As a result, the shared electrons in a water molecule are far more likely to be found near the oxygen nucleus than An organic molecule near the hydrogen nuclei. This stronger attrac tion for electrons gives the oxygen atom two rtial negative charges(6-), as though the elec-FIGURE213 tron cloud were denser near the oxygen atom Structure of a hydrogen bond than around the hydrogen atoms. Because the water molecule as a whole is electrically neu- tral, each hydrogen atom carries a partial positive charge(o*). charges are much less than the unit charges of ions, how The greek letter delta(8)signifies a partial charg ever )Molecules that exhibit charge separation are called weaker than the full unit charge of an ion. polar molecules because of their magnet-like poles, and What would you expect the shape of a water molecule to water is one of the most polar molecules known. Tbe polarity be? Each of water's two covalent bonds has a partial charge of water underlies its chemistry and the chemistry of life at each end, 8- at the oxygen end and dt at the hydrogen end. Polar molecules interact with one another. as the 8- of The most stable arrangement of these charges is a tetrabe- one molecule is attracted to the 8* of another Because many The oxygen atom lies at the center of the tetrahedron, the is individually very weak and transient, lasting on aver drogen atoms occupy two of the apexes, and the partial only 100.000. 000.000 second(10-lI sec). However, the cum negative charges occupy the other two apexes. This results tive effects of large numbers of these bonds can be enor- in a bond angle of 104.5 between the two covalent oxygen- mous. Water forms an abundance of hydrogen bonds, hydrogen bonds. (In a regular tetrahedron, the bond angles which are responsible for many of its important physical would be 109.5 in water, the partial negative charges occu- properties(table 2. 2) py more space than the hydrogen atoms, and, therefore, they compress the oxygen-hydrogen bond angle slightly. partial positive ande is very polar, with ends that exhibit The water molecule The water molecule. thus. has distinct "ends "each with a negative charges. Opposite charges partial charge, like the two poles of a magnet. (These partial attract, forming weak linkages called hydrogen bonds. Table 2.2 The Properties of Water ation Example of Benefit to Life Cohesion Hydrogen bonds hold water molecules together Leaves pull water upward from the roots;seeds swell and germinate High specific heat Hydrogen bonds absorb heat when they break, and release Water stabilizes the temperature of heat when they form, minimizing temperature changes organisms and the environment igh heat of Many hydrogen bonds must be broken for water to evapo- Evaporation of water cools body surfaces Lower density Water molecules in an ice crystal are spaced relatively far Because ice is less dense than water of apart because of hydrogen bonding kes do not fre High polarity Polar water molecules are attracted to ions and polar com- kinds of molecules can move pounds, making them soluble in cells, permitting a diverse Chapter2 The Nature of Molecules 29

Water Atoms Act Like Tiny Magnets Both the oxygen and the hydrogen atoms attract the electrons they share in the covalent bonds of a water molecule; this attraction is called elec￾tronegativity. However, the oxygen atom is more electronegative than the hydrogen atoms, so it attracts the electrons more strongly than do the hydrogen atoms. As a result, the shared electrons in a water molecule are far more likely to be found near the oxygen nucleus than near the hydrogen nuclei. This stronger attrac￾tion for electrons gives the oxygen atom two partial negative charges (δ–), as though the elec￾tron cloud were denser near the oxygen atom than around the hydrogen atoms. Because the water molecule as a whole is electrically neu￾tral, each hydrogen atom carries a partial positive charge (δ+). The Greek letter delta (δ) signifies a partial charge, much weaker than the full unit charge of an ion. What would you expect the shape of a water molecule to be? Each of water’s two covalent bonds has a partial charge at each end, δ– at the oxygen end and δ+ at the hydrogen end. The most stable arrangement of these charges is a tetrahe￾dron, in which the two negative and two positive charges are approximately equidistant from one another (figure 2.12b). The oxygen atom lies at the center of the tetrahedron, the hydrogen atoms occupy two of the apexes, and the partial negative charges occupy the other two apexes. This results in a bond angle of 104.5° between the two covalent oxygen￾hydrogen bonds. (In a regular tetrahedron, the bond angles would be 109.5°; in water, the partial negative charges occu￾py more space than the hydrogen atoms, and, therefore, they compress the oxygen-hydrogen bond angle slightly.) The water molecule, thus, has distinct “ends,” each with a partial charge, like the two poles of a magnet. (These partial charges are much less than the unit charges of ions, how￾ever.) Molecules that exhibit charge separation are called polar molecules because of their magnet-like poles, and water is one of the most polar molecules known. The polarity of water underlies its chemistry and the chemistry of life. Polar molecules interact with one another, as the δ– of one molecule is attracted to the δ+ of another. Because many of these interactions involve hydrogen atoms, they are called hydrogen bonds (figure 2.13). Each hydrogen bond is individually very weak and transient, lasting on average only 100,000 1 ,000,000 second (10–11 sec). However, the cumula￾tive effects of large numbers of these bonds can be enor￾mous. Water forms an abundance of hydrogen bonds, which are responsible for many of its important physical properties (table 2.2). The water molecule is very polar, with ends that exhibit partial positive and negative charges. Opposite charges attract, forming weak linkages called hydrogen bonds. Chapter 2 The Nature of Molecules 29 Hydrogen atom Hydrogen bond An organic molecule Oxygen atom δ– δ+ FIGURE 2.13 Structure of a hydrogen bond. Table 2.2 The Properties of Water Property Explanation Example of Benefit to Life Cohesion High specific heat High heat of vaporization Lower density of ice High polarity Hydrogen bonds hold water molecules together Hydrogen bonds absorb heat when they break, and release heat when they form, minimizing temperature changes Many hydrogen bonds must be broken for water to evapo￾rate Water molecules in an ice crystal are spaced relatively far apart because of hydrogen bonding Polar water molecules are attracted to ions and polar com￾pounds, making them soluble Leaves pull water upward from the roots; seeds swell and germinate Water stabilizes the temperature of organisms and the environment Evaporation of water cools body surfaces Because ice is less dense than water, lakes do not freeze solid Many kinds of molecules can move freely in cells, permitting a diverse array of chemical reactions

Water Clings to Polar molecules The polarity of water causes it to be attracted to other polar molecules. When the other molecules are also water. the at- traction is referred to as cohesion. When the other mole cules are of a different substance. the attraction is called ad hesion. It is because water is cohesive that it is a liquid, and a gas, at moderat The cohesion of liquid water is also responsible for its surface tension Small insects can walk on water 2. 14)because at the air-water interface all of the hydrogen bonds in water face downward, causing the molecules of the water surface to cling together. Water is adhesive to any substance with which it can form hydrogen bonds. That why substances containing polar molecules get"wet"when FIGURE 2.1 they are immersed in water, while those that are composed Cohesion. Some insects, such as this water strider literally walk of nonpolar molecules(such as oils)do not. on water. In this photograph you can see the dimpling the insect's feet make on the water as its weight bears down on the surface. The attraction of water to substances like glass with sur face electrical charges is responsible for capillary action: if a that one foot brings to bear, the strider glides atop the surface of the water rather than sinking. of water water will rise in the tube above the level of the water in the beaker, because the adhesion of water to the glass surface, drawing it upward, is stronger than the force FIGURE 2.15 f gravity, drawing it down. The narrower the tube, the Capillary action. capillary action greater the electrostatic forces between the water and the causes the water within a narrow tube glass, and the higher the water rises(figure 2.15). to rise above the surrounding water he adhesion of the water to the glass Water Stores heat surface, which draws water upward, stronger than the force of gravity Water moderates temperature through two properties: its which tends to draw it down. The high specific heat and its high heat of vaporization. The narrower the tube, the greater the temperature of any substance is a measure of how rapidly surface area available for adhesion for a its individual molecules are moving. Because of the many given volume of water, and the higher hydrogen bonds that water molecules form with one anoth the water rises in the tube er,a large input of thermal energy is required to break these bonds before the individual water molecules can be- in moving about more freely and so have a higher temper- tent, to maintain a relatively constant internal temperature ature. Therefore, water is said to have a high specific heat, The heat generated by the chemical reactions inside cells which is defined as the amount of heat that must be ab- would destroy the cells, if it were not for the high specif orbed or lost by 1 gram of a substance to change its tem- heat of the water within them perature by 1 degree Celsius(C). Specific heat measures A considerable amount of heat energy (586 calories)is re- the extent to which a substance resists changing its temper- quired to change 1 gram of liquid water into a gas. Hence, ature when it absorbs or loses heat. Because polar substanc- water also has a high heat of vaporization. Because the es tend to form hydrogen bonds, and energy is needed to transition of water from a liquid to a gas requires the input break these bonds, the more polar a substance is, the higher of energy to break its many hydrogen bonds, the evapora- is its specific heat. The specific heat of water(1 calo- tion of water from a surface causes cooling rie/gram/C)is twice that of most carbon compounds and Many organisms dispose of excess body heat by evaporative nine times that of iron. Only ammonia, which is more cooling; for example, humans and many other vertebrates olar than water and forms very strong hydrogen bonds, sweat has a higher specific heat than water (1.23 At low temperatures, water molecules are locked into a calories/gram/C). Still, only 20% of the hydrogen bonds rystal-like lattice of hydrogen bonds, forming the solid we are broken as water heats from0°to100°C. call ice(figure 2. 16). Interestingly, ice is less dense than liquid Because of its high specific heat, water heats up more water because the hydrogen bonds in ice space the water slowly than almost any other compound and holds its tem- molecules relatively far apart. This unusual feature enables perature longer when heat is no longer applied. This char- icebergs to float. Were it otherwise, ice would cover nearly all acteristic enables organisms, which have a high water con- bodies of water, with only shallow surface melting annually Part I The Origin of Living things

Water Clings to Polar Molecules The polarity of water causes it to be attracted to other polar molecules. When the other molecules are also water, the at￾traction is referred to as cohesion. When the other mole￾cules are of a different substance, the attraction is called ad￾hesion. It is because water is cohesive that it is a liquid, and not a gas, at moderate temperatures. The cohesion of liquid water is also responsible for its surface tension. Small insects can walk on water (figure 2.14) because at the air-water interface all of the hydrogen bonds in water face downward, causing the molecules of the water surface to cling together. Water is adhesive to any substance with which it can form hydrogen bonds. That is why substances containing polar molecules get “wet” when they are immersed in water, while those that are composed of nonpolar molecules (such as oils) do not. The attraction of water to substances like glass with sur￾face electrical charges is responsible for capillary action: if a glass tube with a narrow diameter is lowered into a beaker of water, water will rise in the tube above the level of the water in the beaker, because the adhesion of water to the glass surface, drawing it upward, is stronger than the force of gravity, drawing it down. The narrower the tube, the greater the electrostatic forces between the water and the glass, and the higher the water rises (figure 2.15). Water Stores Heat Water moderates temperature through two properties: its high specific heat and its high heat of vaporization. The temperature of any substance is a measure of how rapidly its individual molecules are moving. Because of the many hydrogen bonds that water molecules form with one anoth￾er, a large input of thermal energy is required to break these bonds before the individual water molecules can be￾gin moving about more freely and so have a higher temper￾ature. Therefore, water is said to have a high specific heat, which is defined as the amount of heat that must be ab￾sorbed or lost by 1 gram of a substance to change its tem￾perature by 1 degree Celsius (°C). Specific heat measures the extent to which a substance resists changing its temper￾ature when it absorbs or loses heat. Because polar substanc￾es tend to form hydrogen bonds, and energy is needed to break these bonds, the more polar a substance is, the higher is its specific heat. The specific heat of water (1 calo￾rie/gram/°C) is twice that of most carbon compounds and nine times that of iron. Only ammonia, which is more polar than water and forms very strong hydrogen bonds, has a higher specific heat than water (1.23 calories/gram/°C). Still, only 20% of the hydrogen bonds are broken as water heats from 0° to 100°C. Because of its high specific heat, water heats up more slowly than almost any other compound and holds its tem￾perature longer when heat is no longer applied. This char￾acteristic enables organisms, which have a high water con￾tent, to maintain a relatively constant internal temperature. The heat generated by the chemical reactions inside cells would destroy the cells, if it were not for the high specific heat of the water within them. A considerable amount of heat energy (586 calories) is re￾quired to change 1 gram of liquid water into a gas. Hence, water also has a high heat of vaporization. Because the transition of water from a liquid to a gas requires the input of energy to break its many hydrogen bonds, the evapora￾tion of water from a surface causes cooling of that surface. Many organisms dispose of excess body heat by evaporative cooling; for example, humans and many other vertebrates sweat. At low temperatures, water molecules are locked into a crystal-like lattice of hydrogen bonds, forming the solid we call ice (figure 2.16). Interestingly, ice is less dense than liquid water because the hydrogen bonds in ice space the water molecules relatively far apart. This unusual feature enables icebergs to float. Were it otherwise, ice would cover nearly all bodies of water, with only shallow surface melting annually. 30 Part I The Origin of Living Things FIGURE 2.14 Cohesion. Some insects, such as this water strider, literally walk on water. In this photograph you can see the dimpling the insect’s feet make on the water as its weight bears down on the surface. Because the surface tension of the water is greater than the force that one foot brings to bear, the strider glides atop the surface of the water rather than sinking. FIGURE 2.15 Capillary action. Capillary action causes the water within a narrow tube to rise above the surrounding water; the adhesion of the water to the glass surface, which draws water upward, is stronger than the force of gravity, which tends to draw it down. The narrower the tube, the greater the surface area available for adhesion for a given volume of water, and the higher the water rises in the tube