Chapter 24: The Chemistry of Life - ?· 774 Chapter 24 The Chemistry of Life CHAPTER 24 ... Section…

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<ul><li><p>774 Chapter 24</p><p>The Chemistry of Life</p><p>CHAPTER 24</p><p>What Youll LearnYou will learn the functionsof the four major classes ofbiological molecules: pro-teins, carbohydrates, lipids,and nucleic acids.</p><p>You will identify the build-ing blocks that form themajor biological molecules.</p><p>You will compare andcontrast the metabolicprocesses of cellular respira-tion, photosynthesis, andfermentation.</p><p>Why Its ImportantThe large biological moleculesin your body are essential tothe organization and opera-tion of its millions of cells.The structure of these mole-cules is directly related totheir function, and how theyfunction affects your healthand survival.</p><p>The silk that makes up thisspiders web is, gram for gram,stronger than steel, yet it islightweight and stretchable.Spider silk is made of protein,a biological molecule.</p><p>Visit the Chemistry Web site to find linksabout the chemistry of life.</p><p></p></li><li><p>24.1 Proteins 775</p><p>DISCOVERY LAB</p><p>Materials</p><p>400-mL beakerhot plate10-mL graduated cylinderboiling chip10% glucose solutiontest tubetongsBenedicts solutionstirring rodother food solutions such as10% starch or honey</p><p>Testing for Simple Sugars</p><p>Your body constantly uses energy. Many different food sources cansupply that energy, which is stored in the bonds of moleculescalled simple sugars. What foods contain simple sugars?</p><p>Safety Precautions</p><p>Procedure</p><p>1. Fill a 400-mL beaker one-third full of water. Place this water bathon a hot plate and begin to heat it to boiling.</p><p>2. Place 5.0 mL 10% glucose solution in a test tube.</p><p>3. Add 3.0 mL Benedicts solution to the test tube. Mix the two solu-tions using a stirring rod. Add a boiling chip to the test tube.</p><p>4. Using tongs, place the test tube in the boiling water bath and heatfor five minutes.</p><p>5. Record a color change from blue to yellow or orange as a positivetest for a simple sugar.</p><p>6. Repeat the procedure using food samples such as a 10% starchsolution, a 10% gelatin (protein) suspension, or a few drops ofhoney suspended in water.</p><p>Analysis</p><p>Was a color change observed? Which foods tested positive for thepresence of a simple sugar?</p><p>Objectives Describe the structures of</p><p>amino acids and proteins.</p><p> Explain the roles of proteinsin cells.</p><p>Vocabularyproteinamino acidpeptide bondpeptidedenaturationenzymesubstrateactive site</p><p>Section 24.1 Proteins</p><p>An amazing variety of chemical reactions take place in living organisms. Atthe forefront of coordinating the numerous and intricate reactions of life arethe large molecules called proteins, whose name comes from the Greek rootword protos, meaning first.</p><p>Protein StructureYou have learned that polymers are large molecules made of many repeatingbuilding blocks called monomers. Proteins are organic polymers made ofamino acids linked together in a specific way. But proteins are not just large,randomly arranged chains of amino acids. To function properly, each proteinmust be folded into a specific three-dimensional structure. The spider silkshown on the opposite page would not be the incredibly strong yet light-weight protein that it is if it were not constructed in its specific way. You willlearn in this section how proteins are made from their amino-acid buildingblocks and how different types of proteins function.</p></li><li><p>Amino acids As you saw in Chapter 23, many different functional groups arefound in organic compounds. Amino acids, as their name implies, are organicmolecules that have both an amino group and an acidic carboxyl group. Thegeneral structure of an amino acid is shown below.</p><p>Each amino acid has a central carbon atom around which are arranged fourgroups: an amino group ( NH2), a carboxyl group ( COOH), a hydrogenatom, and a variable side chain, R. The side chains range from a single hydro-gen atom to a complex double-ring structure. Examine the different side chainsof the amino acids shown in Figure 24-1. Identify the nonpolar alkanes, polarhydroxyl groups, acidic and basic groups such as carboxyl and amino groups,aromatic rings, and sulfur-containing groups. This wide range of side chainsgives the different amino acids a large variety of chemical and physical prop-erties and is an important reason why proteins can carry out so many differentfunctions.</p><p>Twenty different amino acids are commonly found in the proteins of liv-ing things. The name of each amino acid and its three-letter abbreviation arelisted in Table 24-1. What is the abbreviation for glycine?</p><p>The peptide bond The amino and carboxyl groups provide convenientbonding sites for linking amino acids together. Since an amino acid is bothan amine and a carboxylic acid, two amino acids can combine to form an</p><p>Figure 24-1</p><p>A large variety of side chains canbe found on amino acids. Someof them are shown on these rep-resentative amino acids and arehighlighted in green.</p><p>Amino acid plus yields</p><p>Peptide bond</p><p>plus</p><p>H R1</p><p>H OH</p><p>N C C OH</p><p>Amino acid</p><p>H R2</p><p>H OH</p><p>N C C OH</p><p>Dipeptide Water</p><p>H HR1</p><p>H OH</p><p>N C C N C C OH</p><p>OH</p><p>R2</p><p> 0 H2O</p><p>776 Chapter 24 The Chemistry of Life</p><p> R</p><p>Amino group Carboxyl group</p><p>Variable side chain</p><p>Hydrogen atom H O</p><p>H2N C C OH</p><p>Glycine Serine Cysteine Lysine</p><p>Glutamic acid Glutamine Valine Phenylalanine</p><p>H2N C C OH</p><p>H</p><p>H</p><p>O</p><p>H2N C C OH</p><p>CH2</p><p>OH</p><p>H</p><p>O</p><p>H2N C C OH</p><p>CH2</p><p>SH</p><p>H</p><p>O</p><p>H2N C C OH</p><p>CH2</p><p>CH2</p><p>CH2 NH2</p><p>CH2</p><p>H</p><p>O</p><p>H2N C C OH</p><p>CH3</p><p>H</p><p>O</p><p>H2N C C OH</p><p>CH2</p><p>H2N C C OH</p><p>H</p><p>O</p><p>O OH</p><p>CH2</p><p>CH2</p><p>C</p><p>H2N C C OH</p><p>H</p><p>O</p><p> CH3CH2</p><p>CH2</p><p>CH</p><p>O NH2C</p><p>H</p><p>O</p><p>Figure 24-2</p><p>The amino group of one aminoacid bonds to the carboxylgroup of another amino acid toform a dipeptide. The organicfunctional group formed is anamide linkage and is called apeptide bond.</p></li><li><p>amide, releasing water in the process. This reaction is a condensation reaction.As Figure 24-2 shows, the amino group of one amino acid reacts with thecarboxyl group of another amino acid to form an amide functional group.Where do the H and OH that form water come from?</p><p>The amide bond that joins two amino acids is referred to by biochemistsas a peptide bond.</p><p>A molecule that consists of two amino acids bound together by a peptidebond is called a dipeptide. Figure 24-3a shows the structure of a dipeptidethat is formed from the amino acids glycine (Gly) and phenylalanine (Phe).Figure 24-3b shows a different dipeptide, also formed by linking togetherglycine and phenylalanine. Is Gly-Phe the same compound as Phe-Gly? No,theyre different. Examine these two dipeptides to see that the order in whichamino acids are linked in a dipeptide is important.</p><p>Each end of the two-amino-acid unit in a dipeptide still has a free groupone end has a free amino group and the other end has a free carboxyl group.Each of those groups can be linked to the opposite end of yet another aminoacid, forming more peptide bonds. A chain of two or more amino acids linkedtogether by peptide bonds is called a peptide. Living cells always build pep-tides by adding amino acids to the carboxyl end of a growing chain.</p><p>Polypeptides As peptide chains increase in length, other ways of referringto them become necessary. A chain of ten or more amino acids joined by pep-tide bonds is referred to as a polypeptide. When a chain reaches a length ofabout 50 amino acids, its called a protein.</p><p>Because there are only 20 different amino acids that form proteins, itmight seem reasonable to think that only a limited number of different pro-tein structures are possible. But a protein can have from 50 to a thousand ormore amino acids, arranged in any possible sequence. To calculate the num-ber of possible sequences these amino acids can have, you need to considerthat each position on the chain can have any of 20 possible amino acids. Fora peptide that contains n amino acids, there are 20n possible sequences of theamino acids. So a dipeptide, with only two amino acids, can have 202, or400, different possible amino acid sequences. Even the smallest protein con-taining only 50 amino acids has 2050, or more than 1 1065, possiblearrangements of amino acids! It is estimated that human cells make between80 000 and 100 000 different proteins. You can see that this is only a verysmall fraction of the total number of proteins possible.</p><p>H</p><p>Peptide bond</p><p>O</p><p> C N</p><p>24.1 Proteins 777</p><p>The 20 Amino Acids</p><p>Amino acid Abbreviation</p><p>Alanine AlaArginine ArgAsparagine AsnAspartic acid AspCysteine CysGlutamic acid GluGlutamine GlnGlycine GlyHistidine HisIsoleucine IleLeucine LeuLysine LysMethionine MetPhenylalanine PheProline ProSerine SerThreonine ThrTryptophan TrpTyrosine TyrValine Val</p><p>Table 24-1</p><p>Glycylphenylalanine (Gly-Phe)Gly Phe</p><p>Phenylalanylglycine (Phe-Gly)Phe Gly</p><p>H HH</p><p>H OH</p><p>N C C N C C OH</p><p>OH</p><p>CH2</p><p>H H</p><p>H OH</p><p>N C C N C C OH</p><p>OH</p><p>H</p><p>CH2</p><p>Figure 24-3</p><p>Glycine and phenylalaninecan be combined in this configuration.</p><p>Glycine and phenylalaninecan also be combined in thisconfiguration. Why are thesetwo structures different substances?</p><p>b</p><p>a</p><p>a b</p></li><li><p>Figure 24-4</p><p>The folding of polypeptidechains into both helices (left)and sheets (right) involvesamino acids that are fairly closetogether in the chain being heldin position by hydrogen bonds.Other interactions among thevarious side chains are notshown here but play an impor-tant role in determining thethree-dimensional shape of apolypeptide.</p><p>Three-dimensional protein structure Long chains of amino acids start tofold into unique three-dimensional shapes even before they are fully synthe-sized, as intermolecular attractions form among the amino acids. Some areasof a polypeptide may twirl into helices, shaped like the coils on a telephonecord. Other areas may bend back and forth again and again into a sheet struc-ture, like the folds of an accordion. A polypeptide chain may also turn back onitself and change direction. A given protein may have several helices, sheets,and turns, or none at all. Figure 24-4 shows the folding patterns of a typicalhelix and a sheet. The overall three-dimensional shape of many proteins isglobular, or shaped like an irregular sphere. Other proteins have a long fibrousshape. The three-dimensional shape is determined by the interactions amongthe amino acids.</p><p>Changes in temperature, ionic strength, pH, and other factors can act todisrupt these interactions, resulting in the unfolding and uncoiling of a pro-tein. Denaturation is the process in which a proteins natural three-dimen-sional structure is disrupted. Cooking often denatures the proteins in foods.When an egg is hard-boiled, the protein-rich egg white solidifies due to thedenaturation of its protein. Because proteins function properly only whenfolded, denatured proteins generally are inactive.</p><p>The Many Functions of ProteinsProteins play many roles in living cells. They are involved in catalyzing reac-tions, transporting substances, regulating cellular processes, forming struc-tures, digesting foods, recycling wastes, and even serving as an energy sourcewhen other sources are scarce. See the Everyday Chemistry feature at theend of this chapter to learn how proteins play a role in the sense of smell.</p><p>Enzymes In most organisms, the largest number of different proteins func-tion as enzymes, catalyzing the many reactions that go on in living cells. Anenzyme is a biological catalyst. Recall that a catalyst speeds up a chemicalreaction without being consumed in the reaction. A catalyst usually lowersthe activation energy of a reaction by stabilizing the transition state.</p><p>How do enzymes function? The term substrate is used to refer to a reac-tant in an enzyme-catalyzed reaction. Substrates bind to specific sites onenzyme molecules, usually pockets or crevices. The pocket to which the sub-strates bind is called the active site of the enzyme. After the substrates bindto the active site, the active site changes shape slightly to fit more tightlyaround the substrates. This recognition process is called induced fit. In thediagram in Figure 24-5, youll see that the shapes of the substrates must fit</p><p>778 Chapter 24 The Chemistry of Life</p><p>Hydrogenbonds</p><p>Helix</p><p>Pleated sheet</p><p>N</p><p>NN</p><p>N</p><p>CC</p><p>CC</p><p>CC</p><p>C</p><p>H</p><p>H</p><p>H</p><p>O</p><p>O</p><p>O</p><p>H</p><p>O</p><p>C N</p><p>NN</p><p>N</p><p>CC</p><p>CC</p><p>CC</p><p>C</p><p>H</p><p>H</p><p>H</p><p>O</p><p>O</p><p>O</p><p>H</p><p>O</p><p>C C CN</p><p>NN</p><p>N</p><p>CC</p><p>CC</p><p>CC</p><p>C</p><p>H</p><p>H</p><p>H</p><p>O</p><p>O</p><p>O</p><p>H</p><p>O</p><p>N</p><p>NN</p><p>N</p><p>CC</p><p>CC</p><p>CC</p><p>C</p><p>H</p><p>H</p><p>H</p><p>O</p><p>O</p><p>O</p><p>H</p><p>O</p><p>HCC</p><p>CC</p><p>C</p><p>CC</p><p>N</p><p>N</p><p>O</p><p>HHN</p><p>HN O</p><p>O</p><p>CC NH</p><p>O</p><p>CO</p><p>HCC</p><p>CC</p><p>C</p><p>CC</p><p>N</p><p>N</p><p>O</p><p>HHN</p><p>HN O</p><p>O</p><p>CO</p><p>HCC</p><p>CC N OHN O</p></li><li><p>that of the active site, in the same way that puzzle pieces or a lock and key fittogether. A molecule only slightly different in shape from an enzymes nor-mal substrate doesnt bind as well to the active site or undergo the catalyzedreaction.</p><p>The structure that forms when substrates are bound to an enzyme is calledan enzyme-substrate complex. The large size of enzyme molecules allowsthem to form multiple bonds with their substrates, and the large variety ofamino acid side chains in the enzyme allows a number of different intermol-ecular forces to form. These intermolecular forces lower the activation ener-gy needed for the reaction in which bonds are broken and the substrates areconverted to product.</p><p>An example of an enzyme you may have used is papain, found in papayas,pineapple, and other plant sources. This enzyme catalyzes a reaction thatbreaks down protein molecules into free amino acids. Papain is the activeingredient in many meat tenderizers. When you sprinkle the dried form ofpapain on moist meat, you activate the papain so that it breaks down thetough protein fibers in the meat, making the meat more tender.</p><p>Transport proteins Some proteins are involved in transporting smallermolecules throughout the body. The protein hemoglobin, modeled inFigure 24-6, carries oxygen in the blood, from your lungs to the rest of yourbody. Other proteins combine with biological molecules called lipids to trans-port them from one part of your body to another through the bloodstream. Youwill learn about lipids later in this chapter.</p><p>24.1 Proteins 779</p><p>Figure 24-5</p><p>Substrates are brought closetogether as they fit into theuniquely shaped active site of anenzyme. The enzyme thenchanges shape as it moldsitself to the substrates, formingan enzyme-substrate complex.Bonds are broken and newbonds form to produce theproduct in the reaction.</p><p>Figure 24-6</p><p>Hemoglobin is a globularprotein with four polypeptidechains. The red structure in eachchain is heme, an organic groupcontaining an iron ion to whichoxygen binds. </p><p>The pink skin of this pig isdue to the hemoglobin in itsblood.</p><p>b</p><p>a</p><p>Enzyme</p><p>Substrates</p><p>Enzyme-substratecomplex</p><p>Enzyme unchanged</p><p>Product</p><p>Active site</p><p>Heme</p><p>a b</p></li><li><p>Structural proteins The sole function of some proteins is to form struc-tures vital to organisms. These molecules are known as structural proteins.The most abundant structural protein in most animals is collagen, which ispart of skin, ligaments, tendons, and bones. Figure 24-7 shows the structureof collagen. Other structural proteins make up hair, fur, wool, hooves, fin-gernails, cocoons, and feathers.</p><p>Hormones Hormones are messenger molecules that carry signals from onepart of the body to another. Some hormones are proteins. Insulin, a familiarexample, is a small (51 amino acids) protein hormone made by cells in thepancreas. When insulin is released into the bloodstream, it signals body cellsthat blood sugar is...</p></li></ul>