Page 1
`
`NPS EX. 2030
`CFAD v. NPS
`IPR2015-01093
`
`

`
`WE
`
`THE COVER
`
`American bison (Bison bison) meander among the geysers
`in Yellowstone National Park in Wyoming.
`The American bison, or buffalo, is the largest mammal in
`North America. Before Europeans colonized "the continent,
`as many as 60 million bison ranged from Alberta to Mexico
`and from western New York to the Rocky Mountains. By
`1895, due to relentless overhunting for their meat and
`hides, fewer than 1,000 bison existed. At the turn of the
`century, however, conservationists in Canada and the
`United States began to work to forestall the extinction of
`the bison by creating wildlife preserves. Today virtually all
`of North America's 50,000 bison live in national parks and
`refuges. Chapter 49 discusses ways in which the discipline
`of conservation biology is attacking the pressures toward
`extinction that threaten thousands of species throughout
`the world today. Photograph © Erwin and Peggy Bauer.
`
`THE FRONTISPIECE
`
`The Yellowstone River in Paradise Valley, Montana, at the
`northern end of the Greater Yellowstone Ecosystem. Pho-
`tograph by Todd Wilkinson. From Conservation Biology 5,
`September 1991, page 334. Reprinted by permission of the
`Society for Conservation Biology and Blackwell Scientific
`Publications, Inc.
`
`LIFE: THE SCIENCE OF BIOLOGY, Third Edition
`Copyright © 1992 by Sinauer Associates, Inc.
`All rights reserved. This book may not be reproduced
`in whole or in part without permission.
`Address editorial correspondence to Sinauer Associates, Inc.,
`Sunderland, Mass. 01375.
`
`Address orders to W. H. Freeman and Co., Distribution Center,
`4419 West 1980 South, Salt Lake City, Utah 84104.
`
`Library of Congress Cataloging-in-Publication Data
`Purves, William K. (William Kirkwood), 1934-
`Life, the science of biology / William K. Purves, Gordon H.
`Orians, H. Craig Heller.
`p.
`cm.
`
`Includes bibliographical references and index.
`ISBN 0-7167-2276-3
`1. Biology.
`I. Orians, Gordon H.
`III. Title.
`QH308.2.P87 1992
`
`II. Heller, H. Craig.
`
`574——dc20
`
`91-19845
`CIP
`
`Book and cover design by Rodelinde Graphic Design
`
`Printed in U.S.A.
`
`4321
`
`
`
`Page 2
`
`Page 2
`
`

`
`
`
`i4:.’__$~.T,‘.,—A—£.=.-.-.‘._-—,._4.;-3.-.-ger_>.‘Vt~,...“p4—~—.-A
`
`
`
`
`
`
`
`Large Molecules
`
`_x’
`
`PREVIEW: _Living things are made up of many sub-
`stances, especially lipids, carbohydrates, proteins, and
`nucleic acids. Some lipids are energy-storing ”fue|s,”
`some form membranes, _an_d others serve as chemical
`messengers or as trappers of energy. Carbohydrates
`function as strengthening elements, as fuels, and in
`other ways. The diverse functions of proteins include
`accelerating chemical reactions, defending the animal
`
`body against microorganisms, and providing support
`and protection. Nucleic acids store, transmit, and inter-
`pret hereditary information.
`This chapter deals with the structures and functions
`of lipids (triglycerides, phospholipids, steroids, and car-
`Qtenoids), carbohydrates (monosaccharides, o|igosac-
`charides, and polysaccharides), amino acids, proteins,
`nucleotidea‘, DNA, and RNA.
`
`
`
`The macromolecules—giant molecules, or aggre-
`gates of molecules, with molecular weights in excess
`of 1,000 daltons—perform many essential functions
`in organisms. As we will see, these functions arise
`directly from the structures of the molecules. Some
`of the macromolecules fold into globular forms with
`surface features that enable them to recognize and
`interact with certain other molecules. Other macro-
`
`molecules form long, fibrous systems that provide
`strength and rigidity to parts of an organism; still
`others contract and allow the organism to move itself.
`Some macromolecules aggregate to form structures
`that determine what materials enter or leave the com-
`
`partments within an organism. The largest of the
`molecules are all polymers: molecules that are made
`by the combination of many smaller molecules. The
`small molecules that are a polymer’s subunits are
`called monomers. An oligomer contains only a few
`monomers.
`
`There is a flow of information among the various
`classes of macromolecules. The source of the infor-
`
`r"n’ation is DNA (deoxyribonucleic acid), the genetic
`material. Within the structure of DNA molecules lies
`
`the necessary information to dictate the structures of
`the many different proteins in an organism. Trans-
`mitting the information in DNA to proteins is the
`task of various types of RNAs (ribonucleic acids).
`Some of these proteins (the enzymes) act to accelerate
`chemical reactions in the cell. In this chapter, we take
`a brief look at the major classes of macromolecules
`in order to see how their structures relate to their
`
`functions; in later chapters we will return to the top-
`ics raised here and develop them in greater detail.
`
`‘
`
`FROM MONOMERS TO POLYMERS
`
`The largest molecules in living things—polysaccha-
`rides, proteins, and nucleic acids—are polymers built 0
`from simpler monomers. These polymerization re-
`actions belong to a class of reactions called conden-
`sations or dehydrations, which are of the general
`type
`
`A—H + B—OH ——> A—B + H20
`
`(A—H is a molecule consisting of a hydrogen atom
`attached to another part, A; Pr—OH is a molecule
`consisting of an —OH group attached to another
`part, B.) The product A—B is formed along with a
`molecule of water; the atoms of water are derived
`
`from the reactants, ‘with one hydrogen atom coming
`from one reactant, and an oxygen atom and the other
`hydrogen atom from the other reactant. Reactants
`are the molecules undergoing a chemical reaction.
`The actual polymerization reactions» that produce
`the different kinds of macromolecules differ in detail.
`
`In all cases, energy must be added to the system for
`polymers to form. Other kinds of specific molecules
`participate; their function is to activate the reactants
`—to provide the necessary energy for the reactions
`to be carried out. Large molecules are assembled
`through the repeated condensations of activated
`monomers.
`
`40
`
`Page 3
`
`Page 3
`
`

`
`LARGE MOLECULES 41
`
`species deposit fat (= lipid) droplets in their bodies
`as a means for storing energy——as you know, an
`excess of food results in fat deposition (Figure 3.2).
`Some plant species, such as olives, avocados, sesame
`seeds, and Castor beans, have substantial amounts of
`lipids in their seeds or fruits that serve as energy
`reserves for the next generation.
`
`Triglycerides
`
`One important group of lipids is the triglycerides,
`also known as simple lipids. Triglycerides that are solid
`at room temperature are called fats; those that are
`liquid at this temperature are called oils. The triglyc-
`erides are composed of two types of building blocks:
`fatty acids and glycerol. Fatty acids are carboxylic
`acids with long hydrocarbon tails. A typical fatty acid
`found in animal fats is palmitic acid, C15H31COOH
`(Figure 3.3).- Another example
`is
`stearic
`acid,
`C17H35COOH, which has two more carbon atoms and
`four more hydrogen atoms. These are both saturated
`fatty acids because their hydrocarbon tails contain no
`double bonds. Another common fatty acid, oleic acid
`(Figure 3.3), is unsaturated. Notice the double bond
`near the middle of the hydrocarbon chain in oleic
`acid, causing a kink in the molecule. Other fatty
`acids, such as linoleic acid, have more than one car-
`bon—carbon double bond and are thus polyunsatu-
`rated. These molecules have multiple kinks. Unsat-
`urated and polyunsaturated fatty acids can accept
`hydrogen atoms—that is,
`they can become hydro-
`genated. The addition of two hydrogen atoms across
`the double bond of oleic acid, for example, would
`produce stearic acid.
`Three fatty acid molecules combined with a mol-
`e_cule of glycerol give a molecule of a triglyceride
`(Figure 3.4). The three fatty acids in one triglyceride
`molecule are not always the same length, nor are
`they necessarily all either saturated or unsaturated.
`The kinks associated with double bonds are impor-
`
`
`
`These cells served as stores of energy for a mouse.
`
`Page 4
`
`
`
`
`Outside
`cell
`
`Cell
`membrane
`
`Organelle
`membrane
`
`
`
`
`Interior of
`organelle
`
`3,1 Lipid-Containing Membranes
`Lipid-"containing membranes separate the cell from its
`environment; they also separate the contents of some
`subcellular organelles from the rest of the cell. Materials
`that do not dissolve in lipids are generally unable to pass
`‘through the membranes from one region to another. Mol-
`ecules that are lipid-soluble (bright blue symbols in this
`representation) move through membranes with relative
`ease.
`
`LIPIDS
`
`The lipids are a diverseigroup of compounds that are
`insoluble in water but are readily soluble in organic
`(carbon—based) solvents such as ether. They release
`large amounts of energy when they break down.
`Each of these properties is significant in the biology
`of these compounds. Because lipids do not dissolve
`in water and water does not dissolve in lipids, a
`mixture of water and lipids forms two distinct layers.
`Also, many biological materials that are soluble in
`waterare much less soluble in lipids.
`Suppose that you must design compartments, sep-
`arated from each other and from their environment
`by barriers that limit the passage of materials. Given
`_the properties of lipids, an effective way to accom-
`plish this would be to use lipid—containing mem-
`branes to separate the compartments (Figure 3.1).
`This is, in fact, the system that has evolved in nature.
`Molecular traffic within an organism or into and out
`of its compartments is strictly limited by the solubility
`properties of the lipid portion of the surrounding
`membrane. Compounds that dissolve readily in lip-
`ids can move rapidly through biological membranes;
`but compounds that are insoluble in lipids are pre-
`vented from passing, or must be transported across
`the membrane by specific proteins, as will be de-
`scribed in Chapter 5.
`The role of li}2LC_l§._.i.t1 energy storage relates to the
`topics of oxidation and reduction, which will be de-
`scribed in Chapter 6 when we discuss the processing
`of energy. For now, suffice it to say that the lipids
`are marvelous storehouses for energy. Many animal
`
`Page 4
`
`

`
`42 CHAPTER THREE
`
`
`
`(:1) Palmitic acid
`
`0II
`CH3~CH2 —(CH2) 12 — CH2 — C — OH
`
`(b) Stearic acid
`
`0ll
`CH3~CH2 4(CH2)14 — CH2 — C — OH
`
`3.3 Fatty Acids
`(a) The absence of double bonds between carbon
`atoms in the chain means that palmitic acid is a sat-
`urated fatty acid; the straighti-_chain configu.r.é.!.ti,on in
`the model of the molecule is characteristic of satu-
`rated fatty acids. (b) Stearic acid has two more car-
`bons and four more hydrogens than palmitic acid
`and is also saturated. (c) Oleic acid has a double
`bond between two carbons in the chain and is
`therefore unsaturated. (d) With two double bonds
`in its chain, linoleic acid is polyunsaturated.
`
`(C) Oleic acid
`
`0
`||
`CH3 CH2 (CH2)5 CH2 CH—CH CH2 (CH2)5 CH2 C OH
`
`(d) Linoleic acid
`
`O
`||
`CH3 CH2 (CH2); CH2 CH_CH CH2 CH_CH CH2 (CH2)5 CH2 C OH
`
`tant in determining the fluidity and melting point of
`a lipid. Triglycerides with short or ansatuiated chains
`are usually oily liquids, whereas those with long and
`saturated chains are waxy solids.
`,f-rnimal fats such
`asfllar_cl_a_r;d tallow are usually solids with long—chain,
`saturated or singly unsaturated fatty acids. In these
`fats, hydrocarbon chain lengths range between 10
`and 20 carbon atoms. The triglycerides of plants tend
`to be more unsaturated, oily liquids. Natural peanut
`butter, for example, contains a great deal of oil. Pea-
`nut butter manufacturers often hydrogenate their
`product in order to reduce the double bonds and give
`a saturated, solid product.
`
`Phospholipids
`
`A triglyceride consists of glycerol with three fatty
`acids bound to it. Having certain phosphorus—con—
`taining compounds bound in the place of one of the
`fatty acids defines a class of substances known as
`phospholipids (Figure 3.5). Many phospholipids are
`important constituents of biological membranes.
`If
`you examine the structure of phospholipids closely,
`you will find it easy to understand how they are
`oriented in membranes. The phosphorus~containing
`portion of the phospholipid molecule carries one or
`more electric charges, so this portion is hydrophilic
`
`Fatty acids
`
`+
`
`Glycerol —-as Triglyceride
`
`Glycerol
`
`Tristearin
`O
`
`Stearic acid
`O
`ll
`CH3(CH2)]6 — c
`
`O l
`
`l
`CH3(CH2)16 T C
`
`0 I
`
`I
`CH3(CH2)16 ” C
`
`3.4 A Triglyceride and
`Its Components
`Tristearin is a triglyceride composed
`
`of glycerol and thvn - molecules of the
`fatty acid stearic acid. The synthesis
`
`w
`
`of a triglyceride from glycerol and
`three fatty acids is an example of a
`condensation. Condensations result
`in the release of water molecules. (In
`living things the reaction is more
`complex, but the end result is as
`shown here.)
`
`Page 5
`
`~ CH2
`
`H
`0
`
`I
`
`— CH2 CH3(CH2)16 — C — O — CH2
`
`H
`_CH _j_> CH3(CH2)16 _ C_ O_ CH
`
`}
`
`0H
`
`CH3(CH2)16 L C?‘ O "1 CH2
`
`Page 5
`
`

`
`LARGE MOLECULES 43
`
`0 l
`
`l
`R’-C -O~CH2
`o
`
`II
`N
`R —c —o—cH
`
`H2c—o
`
`
`
`§.§‘5;%‘3é%%
`
`(b) Phosphatidyl choline (a lecithin)
`
`O l
`
`cH3- CH2* (Cflzls ‘ CH2 _ C -0 _ CH2
`
`(I)
`.cfi3- CH2“ (CH2)1o" CH2_ C _O_CH
`
`H2c—o
`
`990099
`90939090909
`
`(a) Phosphatidate
`
`3.5 Some Phospholipids
`A phospholipid consists of glycerol combined with two
`molecules of fatty acid and a molecule containing phos-
`phorus. Examples of phosphorus-containing molecules
`are phosphoric acid, as in the yellow-shaded area of (11),
`and phosphocholine (yellow shaded area of b). Phospho-
`lipids that contain phosphocholine are called lecithins.
`(C) Cephalins are formed by the addition ofboth phos-
`phoric acid and ethanolamine, as included in the yellow
`region. In these diagrams, R’ and R” stand for ”residue”
`and represent any fatty-acid hydrocarbon chains. These
`chains, shown in red letters, are nonpolar, whereas the
`shaded phosphorus-containing portions are electrically
`charged. Other types of phospholipids exist.
`
`(water—l0ving; remember that water is a polar mole-
`cule). The two fatty acid regions, however, are hy-
`drophobic: (water-fearing). Thus in a biological mem-
`brane, phospholipids line up in such a way that the
`nonpolar, hydrophobic ”tails” pack tightly together
`to form the interior of the membrane, and the phos-
`phorus—containing ”heads” face outward (some to
`one side of the membrane and some to the other),
`where they interact with water, which is excluded
`from the interior of the membrane (Figure 3.6). The
`phospholipids form a bilayer,
`that is, a sheet two
`molecules thick. Biological membranes and their
`many important functions will be the subject of
`Chapter 5. For now, we emphasize that the dark lines
`of Figure 3.1 represent membranes that are composed
`Of phospholipid bilayers as depicted in Figure 3.6.
`
`(C) Phosphatidyl ethanolamine (a cephalin)
`
`Other lipids
`
`The lipids we have considered thus far (phospho-
`lipids and triglycerides) are chemically similar. The
`term lipid, however, defines compounds not on the
`basis of structural similarity, but in terms of their
`solubility. Remember that
`lipids are insoluble in
`water but readily Sultlblt‘
`in o1*g;.mi«., solvents such
`
`3.6 Phospholipids in Biological Membranes
`The nonpolar hydrocarbon (fatty-acid) chains gather
`together in the interior of the phospholipid bilayer by
`hydrophobic associations. The polar, phosphorus-
`containing hydrophilic heads of the molecules face out-
`wards toward either side of the membrane. This structure
`will be shown in more detail in Chapter 5.
`
`Phospholipid bilayer
`Of biological
`membrane
`
`Page 6
`
`Q g Hydrophilic ”head”
`i
`
`V
`Hydrophobic
`"
`fatty—acid tails
`
`?
`‘ It
`
`
`
`Hydrophilic ”head”
`
`
`
`
`
`9‘
`
`Page 6
`
`

`
`44 CHAPTER THREE
`
`sgwwwéz
`
`[3-Carotene
`
`OH
`
`Vitamin A
`
`3.7 Carotenoids
`Carotenoids are shown here in a shorthand chemical no-
`tation in which a carbon atom is present at each junction
`in the rings and at each bend of the chains. Each black
`dot corresponds to a methyl (——CH3) group. B-Carotene
`_ is symmetrical around the central (green) double bond;
`the ends of the molecule on either side of the double
`bond are the same, although the ends are rotated 180 de-
`grees with respect to one another. Two vitamin A mole-
`cules are produced by splitting B-carotene in the middle.
`
`_ as ether, chloroform, or benzene. Some other com-
`pounds with these properties (and hence classifiable
`as lipids) are the Carotenoids and the steroids.
`The Carotenoids are a family of light-absorbing pig-
`ments found in both plants and animals (Figure 3.7).
`Beta-carotene ([3-carotene) is one of the pigments
`used to trap light energy in leaves to power the pro-
`cess of photosynthesis (Chapter 8). It is B—carotene
`that causes plants to grow toward or away from light
`(a behavior called phototropism, discussed in Chap-
`ter 32). In humans, a molecule of B-carotene can be
`broken down into two vitamin A molecules, from
`
`which we make the pigment rhodopsin that is re-
`quired for vision (Chapter 37). Carotenoids are re-
`sponsible for the color of carrots, tomatoes, pump-
`kins, egg yolks, and butter.
`,_
`The steroids are a family:-of organic compounds‘
`based on a multiple ring structure in whichthe rings
`share carbons (Figure 3.8). Sg_;rne.,_s,te,roi,c_l_s _are_ impor-
`tant constituents of membranes. Others are among
`the hormones, chemical signals that carry messages
`from one part of the body to another (Chapter 34).
`Testosterone (Figure 3.8) is a steroid hormone that
`regul_ate_s_ sexual development
`in male vertebrates
`(animals with backbones), and the chemically similar
`estrogens play a similar role in females. Cortisone is
`one of a family of hormones that play a wide variety
`of regulatory roles in the digestion of carbohydrates
`and proteins, salt and water balance, and sexual de-
`velopment. Vitamin D is a steroid that regulates the
`absorption of calcium from the intestines. It is nec-
`essary for the proper deposition of calcium in bones;
`a deficiency of vitamin D leads to rickets, a bone-
`softening disease. Vitamin D is produced in human
`skin when certain other steroids are irradiated with
`
`sunlight or ultraviolet light.
`
`Cholesterol (also shown in Figure 3.8) is synthe-
`sized in the liver. In all cells except those of bacteria,
`cholesterol stiffens membranes. It is also the starting
`material for making -testosterone and several other
`"steroid hormones and for the bile salts that help to
`get fats into solution so they can be digested. Cho-
`lesterol is absorbed‘from foods such as milk, butter,
`and animal fats. When there is too much cholesterol
`
`in the blood, it is deposited in the arteries (along with
`other substances), a condition that may lead to arter-
`iosclerosis and heart attack.
`
`Chemically the lipids are quite varied, as you can
`see by glancing back at Figures 3.3 through 3.8. Their
`diversity matches the variety of their functions in
`living things: energy storage, digestion, membrane
`structure, bone formation, vision, and chemical sig-
`naling. Most lipids can be synthesized in the bodies
`of animals; the synthesis and storage of fats is an
`important means of locking energy away until it is
`needed. The few lipids that cannot be synthesized
`
`
`
`(:1) Testosterone
`
`
`
`(C) Vitamin D
`
`
`
`HO
`
`(d) Cholesterol
`
`Model of cholesterol
`
`3.8 Examples of Steroids
`Among the important steroids in vertebrates are (a) the
`male sex hormone testosterone, (b) the hormone corti- \
`sone, (c) vitamin D, and (cl) cholesterol. All of these ste-
`roids have a similar ring structure (orange).
`'
`"
`
`,
`
`*‘
`
`'
`
`Page 7
`
`'1
`
`Page 7
`
`

`
`
`
`ust be obtained in small amounts from the diet. For
`filumans
`the diet must include three particular un-
`saturated fatty acids and the fat—soluble vitamins: A,
`D, E’ and
`
`(:2) Straight-chain form
`
`0
`
`LARGE MOLECULES 45
`
`CARBOHYDRATES
`
`Carbohydrates are a diverse group of compounds
`with molecular weights ranging from less than 100
`to hundreds of thousands. They fall into three cate-
`gories: the monosaccharides, or simple sugars, which
`are monomers; the oligosaccharides, made up of a
`few monosaccharides linked together; and the poly-
`saccharides, polymeric carbohydrates that
`include
`starches, glycogen, cellulose, and many other impor-
`tant biological materials.
`(M0no- means "single/’
`gligo- means ”few,” and poly— means ”many”; sacchar-
`ide means ”sugar.”) There is no clear dividing line
`« between a large oligosaccharide and a smallpolysac-‘sl
`charide, for these are simply terms of convenience
`used to separate ”classes” within what is really a
`continuum of compounds of various sizes. All share
`a general formula of approximately C,,H2,,,Om; that is,
`there are twice as many hydrogen as oxygen atoms,
`and the number of carbon atoms is not always the
`same as the number of oxygen atoms.
`
`A
`
`Monosaccharides
`
`All living cells contain glucose, C6H12O6, a monosac-
`charide. It is produced in green plants by photosyn-
`thesis (Chapter 8), and it is also obtained by the
`digestion of certain polysaccharides. In cells it is me-
`tabolized to yield energy in the process of cellular
`respiration (Chapter 7). Glucose exists
`in both
`straight-chain and ring forms,
`in equilibrium with
`each other (Figure 3.9). There are two distinct ring
`forms of glucose (0L- and [3-glucose). These differ in
`the placement of the —H and ———OH groups attached
`to a particular carbon atom in the molecule (see the
`carbon atom identified as carbon 1 in Figure 3.9; the
`numbering convention shown there will be used
`throughout this book). OL- and B-glucose are chemi-
`cally and physically distinct substances, but they con-
`stantly interconvert in aqueous solution.
`A number of other simple sugars are illustrated in
`Figure 3.10. Many monosaccharides have the same
`formula as glucose, C6H12O.«,,
`including fructose
`("fruit sugar”), mannose, and galactose. These com-
`pounds are all isomers of each other—they are com-
`posed of the same kinds and numbers of atoms, but
`the atoms are combined differently and yield differ-
`ent arrangements such as those shown in Figure 3.10.
`The six-carbon sugars are referred to collectively as
`hexoses. There are also a number of five-carbon sug-
`firs, called pentoses. Some pentoses are found pri-
`marily in the cell walls of plants, as are several of the
`hexoses. Two pentoses are of particular importance:
`
`.4
`v——
`
`
`
`(c) Ring form
`(oL—Glucose)
`
`6 CHZOH
`
`
`
`(d)
`
`oc—Glucose
`
`(E)
`
`I3-Glucose
`
`I
`
`3.9 Forms of Glucose
`Glucose exists in several interconverting chemical forms /
`when dissolved in water. The straight-chain form (a) has
`an aldehyde group at carbon 1 (shaded in green). A reac-
`tion between the aldehyde group and the hydroxyl group
`at carbon 5 (b) gives rise to one of the ring forms (C). The
`ring form is usually represented as in (d), where the
`darker lines at the bottom imply that that edge of the
`molecule extends toward you and the upper edge ex-
`tends back into the page. Depending on the orientation
`of the aldehyde group at carbon 1 when the ring closes,
`either of two rapidly and spontaneously interconverting
`formsofzglucose, oi-glucose (d) or B-glucose (e) is formed.
`or-Glucose and B-glucose‘ differ only at carbon position 1.
`
`rib,ose.and deoxyribose (Figure 3.10), which form
`part- of the backbones of RNA and of DNA, respec-
`tively. Ribose and deoxyribose differ by one oxygen
`atom associated with one of
`the carbon atoms,
`carbon 2.
`
`Disaccharides
`
`Larger carbohydrates are made by the combination
`of two or more monosaccharide molecules. The mon-
`
`osaccharides may be covalently coupled to form spe-
`cific oligosaccharides and polysaccharides. The small-
`est oligosaccharides are the disaccharides and the
`trisaccharides, which are made up of two and three
`.-5_
`
`Page 8
`
`Page 8
`
`

`
`45 CHAPTHQTHREE
`
`Three-carbon sugar
`O
`
`3.10 Monosaccharides
`The three-carbon sugar (triose) glyceraldehyde has the formula C3H6O3; it is
`shown in the common straight~chain form. The pcntoscs, including ribose
`and deoxyribose, each have five carbons. The three hexoses (six-carbon sug-
`'ars) shown here all have the formula C6H1205, but they are chemically and
`biologically distinct from one another.
`
`Glyceraldehyde
`
`Five-carbon sugars
`
`OH
`
`SCHZOH
`O
`3/ \e:
`
`H I]
`4 ‘ H
`| H
`H |
`3$—2‘E
`OH
`OH
`Ribose
`
`5CH2OH
`
`4
`
`H
`
`
`
`OH
`
`1
`
`H
`
`H
`OH
`Deoxyribose
`
`Six-carbon sugars
`
`CHZOH
`
`H
`H
`on-Mannose
`
`
`
`OH
`H
`oc—Galactose
`
`6CH2OH
`I‘ V‘/Qi,(«b;)L
`OH
`O
`0
`HO 2
`5 H
`1CH2OH
`H
`3
`4
`H
`OH
`Fructose
`
`simple sugars} respectively. If one glucose molecule
`Combines with another, as shown in Figure 3.11, the
`disaccharide product must be one of two types: OL-
`linked or B—linked, depending on whether it is OL-
`glucose or B—glucose that reacts. An cx linkage with
`carbon 4 of a second glucose molecule gives us mal-
`tose, whereasxa B linkage gives cellobiose. Both mal-
`tose and cellobiose are disaccharides; both have the
`
`formula C12H22O11} both are composed of two glucose
`molecules (minus one molecule of water), but they
`are different compounds—they are recognized by
`different enzymes and undergo different chemical
`reactions. Two other commonly occurring disaccha-
`rides are sucrose and lactose (Figure 3.11). Sucrose
`(common table sugar; also C12H22O11) is made from
`one molecule of glucose and one of fructose. Lactose
`(milk sugar) consists of glucose and galactose.
`
`Polysaccharides
`
`As we saw in Figure 3.11, maltose consists of two ..
`glucose units connected by an a—linkage. Imagine a
`trisaccharide (three glucose units), a tetrasaccharide
`
`3.11 Disaccharides
`
`A disaccharide is composed of two monosaccharides. As shown in the reac-
`tion at the top left (a simplified version of the actual reaction in nature),
`maltose is produced when an or-1,4 linkage forms between two glucose mole-
`cules, while in cellobiose (bottom left) the two glucoses are linked B-1,4. Lac-
`tose (bottom right) is made by a B linkage between carbon 1 of galactose and
`carbon 4 of glucose. In sucrose (top right), carbon 1 of glucose is joined by an
`or-1,2 linkage to carbon 2 of fructose.
`
`
`
`CH OH
`2
`
`HO
`2
`
`H
`
`_ ’
`H Formation
`of cc-linkage
`
`H
`
`OH
`
`H
`
`OH
`
`H
`
`OH
`
`CH,OH
`
`
`
`H
`
`OH
`
`H
`
`OH
`
`CHZOH
`o
`
`H
`
`H
`
`HO
`
`OH
`
`H
`
`CH:
`\
`OH
`
`ct-Glucose
`
`B-Glucose
`
`on-Glucose
`
`B—Maltose
`
`B-Glucose
`
`oz-Glucose
`
`B-Fructose
`
`Sucrose
`
`
`
`cH2OH
`
`H
`OH
`
`H
`
`H10
`
`0 OH
`B
`.
`H
`1
`H Formation
`of B-linkage
`
`OH
`
`H~
`
`H‘
`
`OH
`
`
`
`CH,oH
`
`OH
`H
`Bclucose
`
`CI-I,0H
`
`H
`
`H
`OH
`
`O
`
`H
`
`OH
`'5
`H
`
`OH
`H
`B_G1uCOse
`
`
`
`H
`
`OH
`
`B-Glucose
`
`B-Glucose
`
`Cellobiose
`
`B-Galactose
`
`B-Lactose
`
`Page 9
`
`Page 9
`
`

`
`LARGE MOLECULES 47
`
`(n) Cellulose
`
`Glucose monomer
`
`cH2oH/ H
`
`0.
`
`H H
`oH H
`
`O
`
`o
`
`H
`
`H
`
`H
`
`on
`
`OH‘
`
`oH H
`H
`
`O
`CHZOH
`
`1-1
`
`H
`
`O
`
`H,oH
`o
`
`H
`
`H
`
`C
`
`H
`o
`
`H
`
`QB?
`
`o
`
`H
`
`H
`
`H
`
`O
`
`H
`
`EH1
`
`oH H
`H
`
`O
`CH20H
`
`
`
`,
`
`K
`
`Hydrogen bonding to other
`cellulose molecules can
`occur at these points
`
`(b) Starch
`
`H
`
`‘*0
`
`cH2oH
`
`CHZOH
`
`H
`
`“H20”
`O
`H
`OH
`H
`
`H
`OH 0
`1/
`CH2
`
`‘
`Branching occurs here
`
`cH2oH
`
`
`
`_ Unbranched
`starch
`molecule
`
`4
`
`Highly branclflz‘
`. glycogen
`molecule
`
`>
`
`3.12 Representative Carbohydrates
`(a) Cellulose is an unbranched polymer of glucose; hydro-
`gen bonding to other cellulose molecules can occur, as
`indicated here with dashed lines. Many adjacent cellulose
`molecules form the cellulose fibrils in plant cells. (b) In
`starch, branching may occur at the position indicated. In
`the micrograph, a red dye stains the starch grains in
`sweet potato cells. (c) Glycogen differs from starch in
`plants only in being more extensively branched.
`
`Page 10
`
`(four glucose units), and finally a giant polysaccha-
`ride consisting of hundreds or thousands of glucose
`units, each connected to the next by an or linkage
`from carbon 1 of one unit to carbon 4 of the next.
`This polymer is starch, an important storage com"-
`pound in the plant kingdom (Figure 3.1217).
`\
`Similarly, there is a giant polysaccharide made up
`solely of glucose but with the individual units con-
`nected by B linkages. This is cellulose, the predom-
`inant component of plant cell walls (Figure 3.12a).
`Both Starflh and cellulose are composed of nothing
`but g1uCOS€
`(when depolymerized, they yield only
`glucose), yet their biological functions and chemical
`and physical properties are entirely different. En-
`zymes that digest one will not affect the other at all.
`Aggregated starch forms a shapeless solid that crum-
`bles readily, whereas cellulose is largely crystalline
`and has an impressive ability to withstandWlongitu-
`dinal pulling without breaking. Starch is primarily a
`storage compound in plants, holding in reserve en-
`ergy and carbon that can be made available upon
`digestion. Cellulose is a key structural element in
`plant cell walls; for example, it provides much of the
`strength of wood. Humans have enzymes for the
`digestion of starch but not for the digestion of cel-
`lulose. The enzymes of many bacteria, fungi, and
`snails readily digest cellulose.
`Cotton is more than 90 percent cellulose and is a
`familiar example of this polysaccharide and its prop-
`erties. Cellulose is the standard building material for
`woody stalks, fibers, and all types of cell walls in
`plants. These rigid structures owe almost all their
`physical strength to cellulose, which is their toughest
`component and which usually makes up more than
`one—fourth of the plant cell wall. Cellulose is by far
`the most common organic compound on this planet,
`accounting for more than half the carbon present in
`plant life.
`Starch is not actually a single chemical substance;
`rather, the term denotes a large family of giant mol-
`ecules of broadly similar structure. All starches are
`polymers of glucose with or linkages. All are large,
`but some are enormous, containing tens of thousands
`of glucose units. An important variable is the degree
`of branching: Many starches have highly branched
`chains (Figure 3.12b). The starches that store glucose
`in plants are called amylose and are not highly
`branched. The highly branched polysaccharide that
`stores glucose in animals is glycogen (Figure 3.12c).
`Animals use glycogen to store energy in liver and
`muscle.
`
`What do we mean when we say that starch and
`glycogen are storage compounds for energy? Very
`simply, these compounds can readily be depolymer-
`ized to yield glucose monomers. Glucose, in turn,
`can be further digested, or metabolized—that is, it
`Can undergo chemical reactions—to yield energy for
`cellular work. Alternatively, glucose can be metabo-
`\
`
`Page 10
`
`

`
` ~
`
`48 CHAPTER THREE
`
`lized so that its carbon atoms are rearranged to form
`the skeletons of other compounds. Thus glycogen
`and starch are storage depots for carbon atoms as
`well as for energy. Each is chemically stable but read-
`ily mobilized by digestion and further metabolism.
`
`relatives such as crabs and lobsters, as well as in the
`cell walls of fungi. Fungi and insects (and their rel-
`atives) constitute more than 80 percent of the species
`ever described, and chitin is another of the most
`abundant substances on Earth.
`
`Derivative Carbohydrates
`
`PROTEINS
`
`Igixzative carbohydrates deviate from the general
`formula C,,H2,,,O,,, by containing other elements. Fig-
`ure 3.13 shows a sugar phosphate, amino sugars,
`and chitin as examples. A number of sugar phos-
`phates, such as fructose 1,6—bisphosphate, are im-
`portant intermediates in cellular respiration (Chapter
`7) and photosynthesis (Chapter 8). Sugar phosphates
`have phosphate groups attached to one or more
`—OH groups of the parent sugar. The two amino
`sugars shown in the figure, glucosamine and galac-
`_ tosamine, have an amino group in place of an —OH
`group. Galactosamine is a major component of car-
`tilage, the material that forms caps on the ends of
`bones and stiffens the protruding parts of the ears
`and nose. The polymer chitin is made from a deriv-
`a_ti_v_eof_ glucosamine. Chitin is the principal structural
`polysaccharide in the skeletons of insects and their
`/.
`
`
`
`CHZOH
`0
`H
`
`H
`OH
`
`H
`
`OH
`H
`
`O
`
`H
`
`H
`
`H
`
`O
`
`$=°
`N—H
`
`H
`
`O
`
`H
`
`O
`
`CHZOH
`O
`H
`
`H
`OH
`
`0‘
`
`‘
`
`H
`
`N—H
`I
`O=C
`I
`CH3
`
`CHZOH
`
`Chitin
`
`ITI-H
`
`H
`
`o=
`
`I
`CH3
`
`In Chapter 2 we considered the amino acids. These
`are the monomers from which a fascinating set of
`polymers are formed—the proteins. The proteins ac-
`count for many of the mechanical elements of living
`things, from parts of subcellular membranes to skin,
`bones, and tendons. In vertebrates, other proteins,
`the immunoglobulins
`(including the antibodies),
`form a major line of defense against foreign organ-
`isms. The specialized molecules needed to bring
`about all biochemical reactions are a major class of.
`proteins called enzymes. Our’ every movement re-
`sults from the contraction and relaxation of muscles,
`resulting in turn from the delicately regulated sliding
`of particular proteins in muscle cells past one an-
`other. Still other proteins act as adjustable channels
`through which sodiumjons (Na+), potassium ions
`(K+), and other ions are ‘passed from one side of a
`nerve-cell membrane to the other, resulting in phe-
`nomena such as the transmission of electric signals
`along a nerve. To understand this stunning variety
`of functions, we must first see and appreciate the
`structure of these molecules.
`
`Arnino Acids
`Twenty different amino acids ar