774 Chapter 24
The Chemistry of Life
CHAPTER 24
What Yoursquoll LearnYou will learn the functionsof the four major classes ofbiological molecules pro-teins carbohydrates lipidsand nucleic acids
You will identify the build-ing blocks that form themajor biological molecules
You will compare andcontrast the metabolicprocesses of cellular respira-tion photosynthesis andfermentation
Why Itrsquos ImportantThe large biological moleculesin your body are essential tothe organization and opera-tion of its millions of cellsThe structure of these mole-cules is directly related totheir function and how theyfunction affects your healthand survival
The silk that makes up thisspiderrsquos web is gram for gramstronger than steel yet it islightweight and stretchableSpider silk is made of proteina biological molecule
Visit the Chemistry Web site atchemistrymccom to find linksabout the chemistry of life
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51461464
241 Proteins 775
DISCOVERY LAB
Materials
400-mL beakerhot plate10-mL graduated cylinderboiling chip10 glucose solutiontest tubetongsBenedictrsquos solutionstirring rodother food solutions such as10 starch or honey
Testing for Simple Sugars
Your body constantly uses energy Many different food sources cansupply that energy which is stored in the bonds of molecules
called simple sugars What foods contain simple sugars
Safety Precautions
Procedure
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
2 Place 50 mL 10 glucose solution in a test tube
3 Add 30 mL Benedictrsquos solution to the test tube Mix the two solu-tions using a stirring rod Add a boiling chip to the test tube
4 Using tongs place the test tube in the boiling water bath and heatfor five minutes
5 Record a color change from blue to yellow or orange as a positivetest for a simple sugar
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
Analysis
Was a color change observed Which foods tested positive for thepresence of a simple sugar
Objectivesbull Describe the structures of
amino acids and proteins
bull Explain the roles of proteinsin cells
Vocabularyproteinamino acidpeptide bondpeptidedenaturationenzymesubstrateactive site
Section 241 Proteins
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
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 largerandomly 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
null
63896034
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
Each amino acid has a central carbon atom around which are arranged fourgroups an amino group (mdash NH2) a carboxyl group (mdash 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 groupsaromatic 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
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
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
Figure 24-1
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
Amino acid plus yields
Peptide bond
plus
H R1
H OH
mdash mdash
N mdash C mdash C mdash OH
mdashmdash
Amino acid
H R2
H OH
mdash mdash
N mdash C mdash C mdash OH
mdashmdash
Dipeptide Water
H HR1
H OH
mdash mdash
N mdash C mdash C mdash N mdash C mdash C mdash OH
mdash
OH
mdash mdashmdash
mdash
R2
mdashmdash
0 H2O
776 Chapter 24 The Chemistry of Life
mdash mdashR
Amino group Carboxyl group
Variable side chain
Hydrogen atom H O
H2N mdash C mdash C mdash OH
mdashmdash
Glycine Serine Cysteine Lysine
Glutamic acid Glutamine Valine Phenylalanine
H2N mdash C mdash C mdash OH
Hmdash
H
mdash
Omdashmdash
H2N mdash C mdash C mdash OH
CH2
mdash
OH
mdash
H
mdash
O
mdashmdash
H2N mdash C mdash C mdash OH
CH2
mdash
SH
mdash
H
mdash
O
mdashmdash
H2N mdash C mdash C mdash OH
CH2
CH2
CH2 mdash NH2
CH2
mdashmdash
mdashmdash
H
mdash
O
mdashmdash
H2N mdash C mdash C mdash OH
CH3
mdash
H
mdash
O
mdashmdash
H2N mdash C mdash C mdash OH
CH2
mdashmdash
H2N mdash C mdash C mdash OH
H
mdash
O
mdashmdash
mdashO OHmdashmdash
CH2
CH2
C
mdashmdash
mdash
H2N mdash C mdash C mdash OH
H
mdash
O
mdashmdash
mdashmdash CH3
CH2
CH2
CH
O mdashNH2mdashmdashC
mdashmdash
mdash
H
mdash
O
mdashmdash
Figure 24-2
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
null
100204475
amide releasing water in the process This reaction is a condensation reactionAs Figure 24-2 shows the amino group of one amino acid reacts with thecarboxyl group of another amino acid to form an amide functional groupWhere do the H and OH that form water come from
The amide bond that joins two amino acids is referred to by biochemistsas a peptide bond
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 Notheyrsquore different Examine these two dipeptides to see that the order in whichamino acids are linked in a dipeptide is important
Each end of the two-amino-acid unit in a dipeptide still has a free groupmdashone end has a free amino group and the other end has a free carboxyl groupEach 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
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 itrsquos called a protein
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
mdash mdash
H
Peptide bond
O
mdash C mdash Nmdash
mdash
241 Proteins 777
The 20 Amino Acids
Amino acid Abbreviation
Alanine AlaArginine ArgAsparagine AsnAspartic acid AspCysteine CysGlutamic acid GluGlutamine GlnGlycine GlyHistidine HisIsoleucine IleLeucine LeuLysine LysMethionine MetPhenylalanine PheProline ProSerine SerThreonine ThrTryptophan TrpTyrosine TyrValine Val
Table 24-1
Glycylphenylalanine (Gly-Phe)Gly Phe
Phenylalanylglycine (Phe-Gly)Phe Gly
H HH
H OH
mdash mdash
N mdash C mdash C mdash N mdash C mdash C mdash OH
mdash
OH
mdash mdashmdash
mdash
CH2
mdashmdash
H H
H OH
mdash mdash
N mdash C mdash C mdash N mdash C mdash C mdash OH
mdash
OH
mdash mdashmdash
H
mdashmdash
mdash
CH2
mdashmdash
Figure 24-3
Glycine and phenylalaninecan be combined in this configuration
Glycine and phenylalaninecan also be combined in thisconfiguration Why are thesetwo structures different substances
b
a
a b
null
18766637
Figure 24-4
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 bondsOther interactions among thevarious side chains are notshown here but play an impor-tant role in determining thethree-dimensional shape of apolypeptide
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 sheetsand 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
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 proteinrsquos natural three-dimen-sional structure is disrupted Cooking often denatures the proteins in foodsWhen 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
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
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
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 yoursquoll see that the shapes of the substrates must fit
778 Chapter 24 The Chemistry of Life
Hydrogenbonds
Helix
Pleated sheet
N
NN
N
CC
CC
CC
C
H
H
H
O
O
O
H
O
C N
NN
N
CC
CC
CC
C
H
H
H
O
O
O
H
O
C C CN
NN
N
CC
CC
CC
C
H
H
H
O
O
O
H
O
N
NN
N
CC
CC
CC
C
H
H
H
O
O
O
H
O
HCC
CC
C
CC
N
N
O
HHN
HN O
O
CC NH
O
CO
HCC
CC
C
CC
N
N
O
HHN
HN O
O
CO
HCC
CC N OHN O
null
18019461
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 enzymersquos nor-mal substrate doesnrsquot bind as well to the active site or undergo the catalyzedreaction
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
An example of an enzyme you may have used is papain found in papayaspineapple 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
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
241 Proteins 779
Figure 24-5
Substrates are brought closetogether as they fit into theuniquely shaped active site of anenzyme The enzyme thenchanges shape as it ldquomoldsitselfrdquo to the substrates formingan enzyme-substrate complexBonds are broken and newbonds form to produce theproduct in the reaction
Figure 24-6
Hemoglobin is a globularprotein with four polypeptidechains The red structure in eachchain is heme an organic groupcontaining an iron ion to whichoxygen binds
The pink skin of this pig isdue to the hemoglobin in itsblood
b
a
Enzyme
Substrates
Enzyme-substratecomplex
Enzyme unchanged
Product
Active site
Heme
a b
null
10239865
Structural proteins The sole function of some proteins is to form struc-tures vital to organisms These molecules are known as structural proteinsThe 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
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 abundant and should be stored A lack of insulin resultsin diabetes a disease in which there is too much sugar in the bloodstreamAnother protein hormone chorionic gonadotropin is synthesized by a devel-oping embryo Release of this hormone causes the development of a placen-ta that nourishes the embryo
Because modern technology has made possible the laboratory synthesis ofproteins some protein hormones are being synthetically produced for use asmedicines Insulin thyroid hormones and growth hormones are some exam-ples Both natural and synthetic proteins are used in a variety of productsmdashfrom meat tenderizer to cleaning solutions to health and beauty aids
780 Chapter 24 The Chemistry of Life
Figure 24-7
Collagen is a fibrous protein thatconsists of three helical polypep-tides (right) that are coiledaround one another to form thestrong fibers of connective tissue(left) The large ropelike struc-tures in the photo are bundles ofcollagen fibers held together bycross-linkage Collagen is anextremely large protein Each ofthe three strands is about 1000amino acids long
Section 241 Assessment
1 Compare the structures of amino acids dipeptidespolypeptides and proteins Which has the largestmolecular mass The smallest
2 Draw the structure of the dipeptide Gly-Ser cir-cling the peptide bond
3 List four functions that different proteins have inliving organisms
4 Thinking Critically How do the properties of pro-teins make them such useful catalysts How dothey differ from other catalysts you have studied
5 Applying Concepts Identify an amino acid fromFigure 24-1 that can be classified into each of thecategories in the following pairs
a nonpolar versus polarb aromatic versus aliphaticc acidic versus basic
chemistrymccomself_check_quiz
null
9545044
242 Carbohydrates 781
Objectivesbull Describe the structures of
monosaccharides disaccha-rides and polysaccharides
bull Explain the functions of car-bohydrates in living things
Vocabularycarbohydratemonosaccharidedisaccharidepolysaccharide
Section Carbohydrates
Analyzing the term carbohydrate offers a hint about the structure of thisgroup of molecules Early observations that these compounds have the gen-eral chemical formula Cn(H2O)n and appear to be hydrates of carbon led totheir being called carbohydrates Although it is now known that there are nofull water molecules attached to carbohydrates the name has stayed
Kinds of CarbohydratesWhy do marathon runners eat large quantities of pasta before a big racePasta as well as milk fruit bread and potatoes is rich in carbohydrates Themain function of carbohydrates in living organisms is as a source of energyboth immediate and stored Carbohydrates are compounds that contain mul-tiple hydroxyl groups (mdash OH) as well as a carbonyl functional group(CO) These molecules range in size from single monomers to polymersmade of hundreds or thousands of monomer units
Monosaccharides The simplest carbohydrates often called simple sugarsare monosaccharides The most common monosaccharides have either fiveor six carbon atoms They have a carbonyl group on one carbon and hydrox-yl groups on most of the other carbons The presence of a carbonyl groupmakes these compounds either aldehydes or ketones depending upon thelocation of the carbonyl group Multiple polar groups make monosaccharideswater soluble and give them high melting points
Glucose is a six-carbon sugar that has an aldehyde structure Glucose ispresent in high concentration in blood because it serves as the major source ofimmediate energy in the body For this reason glucose is often called bloodsugar Closely related to glucose is galactose which differs only in how ahydrogen and a hydroxyl group are oriented in space around one of the six car-bon atoms As you recall from Chapter 22 this relationship makes glucose andgalactose stereoisomers that is their atoms are arranged differently in spaceFructose also known as fruit sugar because it is the major carbohydrate inmost fruits is a six-carbon monosaccharide that has a ketone structureFructose is a structural isomer of glucose
When monosaccharides are in aqueous solution they exist in both open-chain and cyclic structures The cyclic structures are more stable and are thepredominant form of monosaccharides at equilibrium Note in Figure 24-8that the carbonyl groups are present only in the open-chain structures In thecyclic structures they are converted to hydroxyl groups
242
Cyclic form Open-chain form
H OH
H
HO H
OH
OH
CH2OH
H CC
C O
C C
Galactose
Cyclic form Open-chain form OH H
HOH HO
O
CC
C C
CH2OH
CH2OH
H
Fructose
H
Hmdash Cmdash OH
Cmdash Omdashmdash
HmdashCmdash OH
H
3
H
C
HmdashCmdash OH
Omdashmdash
HmdashCmdashOH
HmdashCmdashOH
H
3
Cyclic form Open-chain form
H
H
OH
HO
H H
OHOH
CH2OH
H CC
C O
C C
Glucose
H
C
HmdashCmdash OH
Omdashmdash
HmdashCmdashOH
HOmdashCmdash H HOmdashCmdash H HOmdashCmdash H
HmdashCmdash OH
HmdashCmdash OH
HOmdashCmdash H
HmdashCmdashOH
HmdashCmdashOH
H
3
Figure 24-8
Glucose galactose and fructoseare monosaccharides In aqueoussolutions they exist in an equi-librium between their open-chain and cyclic forms which areinterconverted rapidly
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1666096
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41952675
Disaccharides Like amino acids monosaccharides can be linked togetherby a condensation reaction in which water is released When two monosac-charides bond together a disaccharide is formed See Figure 24-9 The newbond formed is an ether functional group (C mdash O mdash C) Where does thewater that is produced in this reaction come from
One common disaccharide is sucrose which is also known as table sugarbecause sucrose is used mainly as a sweetener Sucrose is formed by linkingglucose and fructose Another common disaccharide is lactose the mostimportant carbohydrate in milk It often is called milk sugar Lactose isformed when glucose and galactose bond together Some foods that containcommon disaccharides are shown in Figure 24-10a
Disaccharides are too large to be absorbed into the bloodstream from thehuman digestive system so they must first be broken down into monosac-charides This is accomplished by a number of enzymes in the digestive sys-tem one of which is sucrase in the small intestine Sucrase breaks downsucrose into glucose and fructose Another enzyme called lactase breaksdown lactose into glucose and galactose Some people do not have an activeform of the enzyme lactase Unless they ingest lactase enzyme along withfoods that contain milk they experience gas bloating and much discomfortas lactose accumulates in their digestive systems These people are said to belactose intolerant
Polysaccharides You may have seen large carbohydrate polymers referredto as complex carbohydrates in nutritional references Another name for acomplex carbohydrate is polysaccharide which is a polymer of simple sugarsthat contains 12 or more monomer units The same type of bond that joins twomonosaccharides in a disaccharide links them together in a polysaccharidePasta is a good source of polysaccharides The pasta shown in Figure 24-10bcontains large amounts of starch a polysaccharide from plants
782 Chapter 24 The Chemistry of Life
Figure 24-9
When glucose and fructose bondtogether the disaccharidesucrose is formed Note thatwater also is a product of thiscondensation reactionRemember that each ringstructure is made of carbonatoms which are not shown forsimplicity
OH
CH2OH CH2OH
HO
Glucose
OHO
HO 0
OH
HO
CH2OH
Fructose
O
OHO
CH2OH CH2OH
HO
OHO
OHCH2OH
H2O
Sucrose Water
O
HO
O H
Figure 24-10
When you have a snack ofmilk and cookies you are ingest-ing the disaccharides lactose andsucrose
Pasta is rich in the polysac-charide starchb
a
a b
LAB
See page 963 in Appendix E forModeling Sugars
null
13076616
Three important polysaccharides are starch cellulose and glycogen Allare composed solely of glucose monomers as Figure 24-11 illustratesHowever thatrsquos their only similarity as all three have different propertiesand functions Plants make both starch and cellulose Starch is a soft water-soluble molecule used to store energy whereas cellulose is a water-insolublepolymer that forms rigid plant cell walls such as those found in woodGlycogen is the animal counterpart of starch It is made by animals to storeenergy mostly in the liver and muscles
How can these three polymers all be made solely of glucose monomersyet have such different properties The answer lies in the way the bonds thatlink the monomers together are oriented in space Because of this differencein bond shape humans can digest starch but not cellulose Digestiveenzymes canrsquot fit cellulose into their active sites due to the specific lock-and-key fit needed for enzyme action As a result the cellulose in the fruits veg-etables and grains that we eat is labeled ldquodietary fiberrdquo because it passesthrough the digestive system largely unchanged
Glycogen
Glucosesubunit
Glucosesubunit
Starch
Cellulose
Crosslink bond
Glucose subunit
242 Carbohydrates 783
Figure 24-11
The starch in potatoes the cellu-lose in lettuce leaves and theglycogen in meat are all poly-mers of glucose Cellulose has alinear unbranched structurethat resembles a chain-linkfence Starch molecules can beeither branched or unbranchedand glycogen is highly branched
Section 242 Assessment
6 Compare the structures of monosaccharides disac-charides and polysaccharides Which has thelargest molecular mass The smallest
7 What is the main function of carbohydrates in liv-ing organisms
8 Compare and contrast the structures of starch andcellulose How do the structural differences affectour ability to digest these two polysaccharides
9 Thinking Critically If a carbohydrate has 2n
possible isomers where n is equal to the number ofcarbon atoms in the carbohydrate structure that are
chiral (meaning a carbon atom with four differentgroups bonded to it) calculate the number ofpossible isomers for each of the following mono-saccharides Use Figure 24-8 to help you
a galactoseb glucosec fructose
10 Interpreting Scientific Illustrations Drawthe structure of the disaccharide sucrose circlingthe ether functional group that bonds the monomersugars together
chemistrymccomself_check_quiz
null
7664325
Section 243 Lipids
Objectivesbull Describe the structures of
fatty acids triglyceridesphospholipids and steroids
bull Explain the functions oflipids in living organisms
bull Identify some reactions thatfatty acids undergo
bull Relate the structure andfunction of cell membranes
Vocabularylipidfatty acidtriglyceridesaponificationphospholipidwaxsteroid
The wax that you use to polish your car the fat that drips out of your burg-er and the vitamin D that fortifies the milk you drank for lunchmdashwhat dothese diverse compounds have in common They are all lipids
What is a lipidA lipid is a large nonpolar biological molecule Because lipids are nonpo-lar they are insoluble in water Lipids have two major functions in livingorganisms They store energy efficiently and they make up most of the struc-ture of cell membranes Unlike proteins and carbohydrates lipids are notpolymers with repeated monomer subunits
Fatty acids Although lipids are not polymers many lipids have a majorbuilding block in common This building block is the fatty acid a long-chain carboxylic acid Most naturally occurring fatty acids contain between12 and 24 carbon atoms Their structure can be represented by the formula
Most fatty acids have an even number of carbon atoms which is a result oftheir being constructed two carbons at a time in enzymatic reactions
Fatty acids can be grouped into two main categories depending on the pres-ence or absence of double bonds between carbon atoms Fatty acids that con-tain no double bonds are referred to as saturated Those that have one or moredouble bonds are called unsaturated Figure 24-12 shows the structures of twocommon fatty acids Stearic acid is an 18-carbon saturated fatty acid oleic acidis an 18-carbon unsaturated fatty acid What makes oleic acid unsaturated
CH3(CH2)nCOOH
Stearic acid Oleic acidHO O
CH2
mdashmdash
C
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
HO O
CH2
mdashmdashmdash mdash
C
CH2
CH2
CH2
CH2
CH2
CH2
CH
CH
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
Figure 24-12
Two fatty acids abundant in ourdiets are the 18-carbon saturatedstearic acid and the 18-carbonunsaturated oleic acid How isthe structure of the moleculeaffected by the presence of adouble bond Refer to Table C-1in Appendix C for a key to atomcolor conventions
784 Chapter 24 The Chemistry of Life
null
11002601
null
5057328
An unsaturated fatty acid can become saturated if it reacts with hydrogenAs you learned in Chapter 23 hydrogenation is an addition reaction in whichhydrogen gas reacts with carbon atoms that are linked by multiple bondsEach unsaturated carbon atom can pick up one hydrogen atom to become sat-urated For example oleic acid can be hydrogenated to form stearic acid
The double bonds in naturally occurring fatty acids are almost all in the cisgeometric isomer form Recall from Chapter 22 that the cis isomer has identi-cal groups oriented on the same side of the molecule around a double bondBecause of the cis orientation unsaturated fatty acids have a kink or bend intheir structure that prevents them from packing together They donrsquot form asmany intermolecular attractions as saturated fatty acid molecules As a resultunsaturated fatty acids have lower melting points
Triglycerides Although fatty acids are abundant in living organisms theyare rarely found alone They most often are found bonded to glycerol a mol-ecule with three carbons each containing a hydroxyl group When three fattyacids are bonded to a glycerol backbone through ester bonds a triglycerideis formed The formation of a triglyceride is shown in Figure 24-13
Triglycerides can be either solids or liquids at room temperature If liquidthey are usually called oils If solid at room temperature theyrsquore called fatsMost mixtures of triglycerides from plant sources such as corn olive andpeanut oils are liquids because the triglycerides contain unsaturated fattyacids that have fairly low melting points Animal fats such as butter containa larger proportion of saturated fatty acids They have higher melting pointsand usually are solids at room temperature
Fatty acids are stored in fat cells of your body as triglycerides Whenenergy is abundant fat cells store the excess energy in the fatty acids oftriglycerides When energy is scarce the cells break down the triglyceridesforming free fatty acids and glycerol Further breakdown of the fatty acidsreleases the energy used to form them Although enzymes break downtriglycerides in living cells the reaction can be duplicated outside of cellsby using a strong base such as sodium hydroxide This reactionmdashthehydrolysis of a triglyceride using an aqueous solution of a strong base toform carboxylate salts and glycerolmdashis saponification shown below
243 Lipids 785
Figure 24-13
Ester bonds in a triglyceride areformed when the hydroxylgroups of glycerol combine withthe carboxyl groups of the fattyacids
CH2OH
CHOH
CH2OH
HO C(CH2)14CH3
HOC(CH2)16CH3
HOC(CH2)18CH3
Glycerol 3 Fatty acids A triglyceride Watermdash mdash
O
0
mdash mdash
O
mdash mdash
O
mdash mdash
O
CH mdash O mdash C mdash (CH2)16 mdash CH3 3H2O
mdash mdash
O
CH2 mdash O mdash C mdash (CH2)14 mdash CH3
mdash mdash
O
CH2 mdash O mdash C mdash (CH2)18 mdash CH3
Triglyceride Glycerol A soapA base
mdash mdash
O
mdash mdash
O
CH mdash O mdash C mdash (CH2)14CH3 3NaOH 0 CHOH 3CH3(CH2)14 mdash C mdash ONa
mdash mdash
O
mdash mdash
O
CH2 mdash O mdash C mdash (CH2)14CH3
CH2 mdash O mdash C mdash (CH2)14CH3 CH2OH
CH2OH
BiologyCONNECTION
The venom of poisonous snakescontains a class of enzymes
known as phospholipases Theseenzymes catalyze the breakdownof phospholipids triglycerides inwhich one fatty acid has beenreplaced by a phosphate groupThe venom of the eastern dia-mondback rattlesnake contains aphospholipase that hydrolyzesthe ester bond at the middle carbon of phospholipids If thelarger of the two breakdownproducts of this reaction gets intothe bloodstream it dissolves themembranes of red blood cellscausing them to rupture A bitefrom the eastern diamondbackcan lead to death if not treatedimmediately
null
15542812
A Saponification ReactionApplying Concepts The reaction between atriglyceride and a strong base such as sodiumhydroxide is called saponification In this reac-tion the ester bonds in the triglyceride arehydrolyzed by the base The sodium salts of thefatty acids called soaps precipitate out and glyc-erol is left in solution
Materials solid vegetable shortening 250-mLbeaker 600-mL beaker 60M NaOH ethanolsaturated NaCl solution stirring rod hot platetongs 25-mL graduated cylinder evaporatingdish cheesecloth (20 cm 20 cm) funnel
Procedure
1 Place a 250-mL beaker on the hot plate Add25 g solid vegetable shortening to the beakerTurn the hot plate on at a medium setting
2 As the vegetable shortening melts slowly add12 mL ethanol and then 5 mL 60M NaOH tothe beaker CAUTION Ethanol is flammableNaOH causes skin burns Wear gloves
3 Heat the mixture stirring occasionally forabout 15 minutes but do not allow it to boil
4 When the mixture begins to thicken use tongsto remove the beaker from the heat Allowthe beaker to cool for five minutes then placeit in a cold water bath in the 600-mL beaker
5 Add 25 mL saturated NaCl solution to the mix-ture in the beaker The soap is not very solubleand will appear as small clumps
6 Collect the solid soap clumps by filtering themthrough a cheesecloth-lined funnel
7 Using gloved hands press the soap into anevaporating dish Allow the soap to air dry forone or two days
8 Remove your gloves and wash your hands
Analysis1 What type of bonds present in the triglycerides
are broken during the saponification reaction
2 What is the common name for the sodium saltof a fatty acid
3 How does soap remove dirt from a surface
4 Write a word equation for the saponificationreaction in this lab
miniLAB
Saponification is used to make soaps which usually are the sodium salts offatty acids A soap has both a polar end and a nonpolar end Soaps can beused to clean nonpolar dirt and oil with water because the nonpolar dirt andoil bond to the nonpolar end of the soap molecules and the polar end of thesoap molecules is soluble in water Thus the dirt-laden soap molecules canbe rinsed away with the water You can make soap by doing the miniLABon this page
Phospholipids Another important type of triglyceride phospholipids arefound in greatest abundance in cellular membranes A phospholipid is atriglyceride in which one of the fatty acids is replaced by a polar phosphategroup As you can see in Figure 24-14a the polar part of the molecule forms
786 Chapter 24 The Chemistry of Life
Figure 24-14
A phospholipid has a polarhead and two nonpolar tails
The membranes of livingcells are formed by a doublelayer of lipids called a bilayerThe polar heads face out ofboth sides of the bilayer wherethey are in contact with thewatery environment inside andoutside the cell The nonpolartails point into the center of thebilayer
b
a
Polar head Nonpolar tails Polar heads Nonpolar tails
a b
null
53081093
a ldquoheadrdquo and the nonpolar fatty acids look like tails How are the polar andnonpolar parts of a phospholipid arranged in the membranes of cells A typ-ical cell membrane has two layers of phospholipids which are arranged withtheir nonpolar tails pointing inward and their polar heads pointing outwardSee Figure 24-14b This arrangement is called a lipid bilayer Because thelipid bilayer structure acts as a barrier the cell is able to regulate the materi-als that enter and leave through the membrane
Waxes Another type of lipid waxes also contain fatty acids A wax is alipid that is formed by combining a fatty acid with a long-chain alcohol Thegeneral structure of these soft solid fats with low melting points is shownbelow with x and y representing variable numbers of CH2 groups
Both plants and animals make waxes Plant leaves are often coated withwax which prevents water loss Notice in Figure 24-15 how raindrops ldquobeaduprdquo on the leaves of a plant indicating the presence of the waxy layer Thehoneycomb of bees also is made of a wax commonly called beeswaxCombining the 16-carbon fatty acid palmitic acid and a 30-carbon alcoholchain makes a common form of beeswax Candles are sometimes made ofbeeswax because it burns slowly and evenly
Steroids Not all lipids contain fatty acid chains Steroids are lipids thathave multiple cyclic rings in their structures All steroids are built from thebasic four-ring steroid structure shown below
Some hormones such as many sex hormones are steroids that function toregulate metabolic processes Cholesterol another steroid is an importantstructural component of cell membranes Vitamin D also contains the four-ring steroid structure and plays a role in the formation of bones
243 Lipids 787
Figure 24-15
Plants produce wax that coatstheir leaves (top) The wax pro-tects the leaves from drying outThe honeycomb of a beehive isconstructed from beeswax(bottom)
CH3(CH2)x mdash C mdash O mdash (CH2)yCH3
O
mdashmdash
Section 243 Assessment
11 Write the equation for the complete hydrogenationof the polyunsaturated fatty acid linoleic acid
CH3(CH2)4CHCHCH2CHCH(CH2)7COOH
12 List an important function of each of these typesof lipids
a triglycerides c waxesb phospholipids d steroids
13 Compare and contrast the structures of a steroida phospholipid a wax and a triglyceride
14 Thinking Critically What possible solvent canbe used to extract lipids from cell membranes
15 Interpreting Scientific Illustrations Draw thegeneral structure of a phospholipid Label thepolar and nonpolar portions of the structure
chemistrymccomself_check_quiz
null
124444855
Figure 24-16
Nucleotides are the monomersfrom which nucleic acid polymersare formed
788 Chapter 24 The Chemistry of Life
Section 244 Nucleic Acids
Objectivesbull Identify the structural com-
ponents of nucleic acids
bull Relate the function of DNAto its structure
bull Describe the structure andfunction of RNA
Vocabularynucleic acidnucleotide
Nucleic acids are the fourth class of biological molecules that you will studyThey are the information-storage molecules of the cell This group of nitro-gen-containing molecules got its name from the cellular location in whichthe molecules are primarily foundmdashthe nucleus It is from this control cen-ter of cells that nucleic acids carry out their major functions
Structure of Nucleic AcidsA nucleic acid is a nitrogen-containing biological polymer that is involved inthe storage and transmission of genetic information The monomer thatmakes up a nucleic acid is called a nucleotide Each nucleotide has threeparts an inorganic phosphate group a five-carbon monosaccharide sugarand a nitrogen-containing structure called a nitrogen base Examine each partof Figure 24-16a Although the phosphate group is the same in allnucleotides the sugar and the nitrogen base vary
In a nucleic acid the sugar of one nucleotide is bonded to the phosphate ofanother nucleotide as shown in Figure 24-16b Thus the nucleotides arestrung together in a chain or strand containing alternating sugar and phos-phate groups Each sugar is also bonded to a nitrogen base that sticks out fromthe chain The nitrogen bases on adjoining nucleotide units are stacked oneabove the other in a slightly askew position much like the steps in a staircaseYou can see this orientation in Figure 24-16c Intermolecular forces hold eachnitrogen base close to the nitrogen bases above and below it
DNA The Double HelixYoursquove probably heard of DNA (deoxyribonucleic acid) one of the twokinds of nucleic acids found in living cells DNA contains the master plansfor building all the proteins in an organismrsquos body
Each nucleotide contains a nitro-gen-containing base a five-carbonsugar and a phosphate group
a
Sugar
Nucleotide
mdash mdash
OH OH
HHH H
O
HO
HO mdash P mdash O mdashmdash CH2
mdash O
CC
C C
C
H
N
C N NH2
C
C
H
mdashmdash
mdashmdash
Phosphate group
Nitrogen-containingbase
Phosphate
Sugar Base
Base
Base
Base
Phosphate
Sugar
Phosphate
Sugar
Phosphate
Sugar
mdashmdashO
Nucleic acids are linearchains of alternating sugarsand phosphates Attached toevery sugar is a nitrogen basewhich is oriented roughly per-pendicular to the linear chain
b Nucleotides areoffset and thusresemble a staircase
c
null
123295525
null
5992533
244 Nucleic Acids 789
Figure 24-17
The structure of DNA is a doublehelix that resembles a twistedzipper
Hmdash
Hmdash
Deoxyribose
Deoxyribose
DeoxyriboseO
O O
mdashmdash
mdashmdash mdashmdashCH3
H
H
Nmdash
H
N
N
H
NNHmdash Hmdash
Hmdash
Hydrogenbond
Omdashmdash
H
H
N
H mdash
NN
N
N
H
N
Thymine Adenine Cytosine Guanine
N
N
Deoxyribose
HN
N
N
The structure of DNA The structure of DNA consists of two long chainsof nucleotides wound together to form a spiral structure Each nucleotide inDNA contains a phosphate group the five-carbon sugar deoxyribose and anitrogen base The alternating sugar and phosphate groups in each chainmake up the outside or backbone of the spiral structure The nitrogen basesare on the inside Because the spiral structure is composed of two chains itis known as a double helix You can see from Figure 24-17 that the DNAmolecule is similar to a zipper in which the two ends have been twisted inopposite directions The two sugar-phosphate backbones form the outsidesof the zipper What forms the zipperrsquos teeth
DNA contains four different nitrogen bases adenine (A) thymine (T)cytosine (C) and guanine (G) As Figure 24-18 shows both adenine and gua-nine contain a double ring Thymine and cytosine are single-ring structuresLooking back at Figure 24-17 you can see that each nitrogen base on onestrand of the helix is oriented next to a nitrogen base on the opposite strandin the same way that the teeth of a zipper are oriented The side-by-side basepairs are close enough so that hydrogen bonds form between them Becauseeach nitrogen base has a unique arrangement of organic functional groups thatcan form hydrogen bonds the nitrogen bases always pair in a specific way sothat the optimum number of hydrogen bonds form As Figure 24-18 showsguanine always binds to cytosine and adenine always binds to thymine TheGmdashC and AmdashT pairs are called complementary base pairs
It is because of complementary base pairing that the amount of adenine ina molecule of DNA always equals the amount of thymine and the amount ofcytosine always equals the amount of guanine In 1953 James Watson andFrancis Crick used this observation to make one of the greatest scientific dis-coveries of the twentieth century when they determined the double-helixstructure of DNA They accomplished this feat without actually carrying outmany laboratory experiments themselves Instead they analyzed and syn-thesized the work of numerous scientists who had carefully carried out stud-ies on DNA The X-ray diffraction patterns of DNA fibers taken by MauriceWilkins and Rosalind Franklin were of special importance because theyclearly showed the dimensions of the DNA molecule and the moleculersquos hel-ical structure Watson and Crick used these results and molecular modelingtechniques to build their DNA structure with balls and sticks This discoveryillustrates the importance of being able to visualize molecules in order touncover patterns in bonding arrangements How might such molecular mod-eling be done today
The function of DNA Watson and Crick used their model to predict howDNArsquos chemical structure enables it to carry out its function DNA storesthe genetic information of a cell in the cellrsquos nucleus Before the celldivides the DNA is copied so that the new generation of cells gets the same
Figure 24-18
In DNA base pairing existsbetween a double-ringed baseand a single-ringed baseAdenine and thymine alwayspair forming two hydrogenbonds between them Guanineand cytosine always form threehydrogen bonds when they pair
null
1982731
genetic information Having determined that the two chains of the DNAhelix are complementary Watson and Crick realized that complementarybase pairing provides a mechanism by which the genetic material of a cellis copied The problem-solving LAB shows how DNA copies or repli-cates itself
The four nitrogen bases of DNA serve as the letters of the alphabet in theinformation-storage language of living cells The specific sequence of theseletters represents an organismrsquos master instructions just as the sequence ofletters in the words of this sentence convey special meaning The sequenceof bases is different in every species of organism allowing for an enormousdiversity of life formsmdashall from a language that uses only four letters It isestimated that the DNA in a human cell has about three billion complemen-tary base pairs arranged in a sequence unique to humans
790 Chapter 24 The Chemistry of Life
problem-solving LAB
How does DNA replicateFormulating Models DNA must be copied orreplicated before a cell can divide so that each ofthe two new cells that are formed by cell divisionhas a complete set of genetic instructions It is veryimportant that the replicating process be accuratethe new DNA molecules must be identical to theoriginal Watson and Crick noticed that theirmodel for the three-dimensional structure of DNAprovided a mechanism for accurate replication
When DNA begins to replicate the two nucle-otide strands start to unzip An enzyme breaksthe hydrogen bonds between the nitrogen basesand the strands separate to expose the nitrogenbases Other enzymes deliver free nucleotidesfrom the surrounding medium to the exposednitrogen bases adenine hydrogen-bonding withthymine and cytosine bonding with guanine
Thus each strand builds a complementary strandby base pairing with free nucleotides Thisprocess is shown in Diagram a When the freenucleotides have been hydrogen-bonded intoplace their sugars and phosphates bond cova-lently to those on adjacent nucleotides to formthe new backbone Each strand of the originalDNA molecule is now bonded to a new strand
AnalysisDiagram b shows a small segment of a DNA mol-ecule Copy the base sequence onto a clean sheetof paper being careful not to make copyingerrors Show the steps of replication to producetwo segments of the DNA
Thinking Critically1 How does the base sequence of a newly syn-
thesized strand compare with the originalstrand to which it is bonded
2 If the original DNA segment is colored red andthe free nucleotides are colored blue whatpattern of colors will the newly replicated DNAsegments have Will all new segments havethe same color pattern What does this pat-tern mean
3 What would happen if you had made an errorwhen copying the base sequence onto yoursheet of paper How might an organism beaffected if this error had occurred during repli-cation of its DNA
A T C G G T T A A A C GT A G C C A A T T T G C
C
C
C
CC
C
TT
T
T
TT
T
T
T
AA
A
A
A
ATA
TA TA
TAT A
A
A
A
G
GG C G
CG
G
GG
a b
null
6060453
RNARNA (ribonucleic acid) also is a nucleic acid Its general structure differsfrom that of DNA in three important ways As you know DNA contains thenitrogen bases adenine cytosine guanine and thymine RNA contains ade-nine cytosine guanine and uracil thymine is never found in RNASecondly RNA contains the sugar ribose DNA contains the sugar deoxyri-bose which has a hydrogen atom in place of a hydroxyl group at one posi-tion Compare these different structures shown in Figure 24-19
The third difference between DNA and RNA arises as a result of thesestructural differences DNA is normally arranged in a double helix in whichhydrogen bonding links the two chains together through their bases RNA isusually single stranded with no such hydrogen bonds forming
Whereas DNA functions to store genetic information RNA allows cells touse the information found in DNA You have learned that the genetic infor-mation of a cell is contained in the sequence of nitrogen bases in the DNAmolecule Cells use this base sequence to make RNA with a correspondingsequence The RNA is then used to make proteins each with an amino acidsequence that is determined by the order of nitrogen bases in RNA A set ofthree of these nitrogen bases in RNA is called a codon The sequences ofbases in these codons are referred to as the genetic code Because proteinsare the molecular tools that actually carry out most activities in a cell theDNA double helix is ultimately responsible for controlling the thousands ofchemical reactions that take place
244 Nucleic Acids 791
mdash mdash
H O
Thymine
O
mdashmdash
Uracil
H
N
mdash mdash H HN
mdash mdash
O
O
mdashmdash
H
N
mdash mdash
N
H
H3C H
Deoxyribose
OH
H H
O
H
OH
H
HOCH2
OH
H H
O
H
OH
H
HOCH2
Ribose
OHH
Section 244 Assessment
16 List the three chemical structures that make up anucleotide
17 Compare and contrast the structures and functionsof DNA and RNA
18 A sample of nucleic acid was determined to con-tain adenine uracil cytosine and guanine Whattype of nucleic acid is this
19 Thinking Critically Analyze the structure ofnucleic acids to determine what structural featuremakes them acidic
20 Predicting What is the genetic code Predictwhat might happen to a protein if the DNA thatcoded for the protein contained the wrong basesequence
Figure 24-19
DNA and RNA are nucleic acidswith some important differencesin structure DNA containsthymine RNA contains uracilwhich lacks a methyl groupDNArsquos sugar deoxyribose has ahydrogen in one positioninstead of the hydroxyl groupthat ribose has
chemistrymccomself_check_quiz
null
10710045
792 Chapter 24 The Chemistry of Life
Section 245 Metabolism
Objectivesbull Distinguish between
anabolism and catabolism
bull Describe the role of ATP inmetabolism
bull Compare and contrast theprocesses of photosynthesiscellular respiration and fermentation
VocabularymetabolismcatabolismanabolismATPphotosynthesiscellular respirationfermentation
Yoursquove now studied the four major kinds of biological molecules and learnedthat they all are present in the food you eat What happens to these moleculesafter they enter your body
Anabolism and CatabolismMany thousands of chemical reactions take place in the cells of a livingorganism The set of reactions carried out by an organism is its metabolismWhy are so many reactions involved in metabolism Living organisms mustaccomplish two major functions in order to survive They have to extractenergy from nutrients in forms that they can use immediately as well as storefor future use In addition they have to use nutrients to make building blocksfor synthesizing all of the molecules needed to carry out their life functionsThese processes are summarized in Figure 24-20
The term catabolism refers to the metabolic reactions that break downcomplex biological molecules such as proteins polysaccharides triglyc-erides and nucleic acids for the purposes of forming smaller building blocksand extracting energy After you eat a meal of spaghetti and meatballs yourbody immediately begins to break down the starch polymer in the spaghettiinto glucose The glucose is then broken down into smaller molecules in aseries of energy-releasing catabolic reactions Meanwhile the protein poly-mers in the meatballs are catabolized into amino acids
The term anabolism refers to the metabolic reactions that use energy andsmall building blocks to synthesize the complex molecules needed by anorganism After your body has extracted the energy from the starch in pastait uses that energy and the amino-acid building blocks produced from themeat proteins to synthesize the specific proteins that allow your muscles tocontract catalyze metabolic reactions and carry out many other functionsin your body
Figure 24-20 shows the relationship between catabolism and anabolismThe simple building-block molecules that are listed on the right side of thediagram are used to build the complex molecules that are listed on the leftside of the diagram As the arrow moves from right to left anabolic reactions
Figure 24-20
A large number of differentmetabolic reactions take place inliving cells Some involve break-ing down nutrients to extractenergy these are catabolicprocesses Others involve usingenergy to build large biologicalmolecules these reactions areanabolic processes
Catabolism
Anabolism
Nucleic acids
Triglycerides
Polysaccharides
Proteins
Nucleotides
Fatty acids
Simple sugars
Amino acids
ADP PComplex molecules Building blocksATP
null
13876036
null
10318228
Figure 24-22
These giant seaweeds calledkelps must grow close to theoceanrsquos surface where light isavailable for photosynthesisThey cannot grow in the darkdepths where sunlight does notpenetrate the seawater
take place As the lower arrow moves from left to right catabolic processestake place and the complex molecules are broken down into their smallerbuilding blocks
ATP Catabolism and anabolism are linked by common building blocks thatcatabolic reactions produce and anabolic reactions use A common form ofpotential chemical energy also links the two processes ATP (adenosine triphos-phate) is a nucleotide that functions as the universal energy-storage moleculein living cells During catabolic reactions cells harness the chemical energy offoods and store it in the bonds of ATP When these bonds are broken the chem-ical energy is released and used by cells to drive anabolic reactions that mightnot otherwise occur Most cellular reactions have an efficiency of only about40 percent at best the remaining 60 percent of the energy in food is lost asheat which is used to keep your body warm
The structure of ATP is shown in Figure 24-21 During catabolic reac-tions cells produce ATP by adding an inorganic phosphate group to thenucleotide adenosine diphosphate (ADP) in an endothermic reaction Onemole of ATP stores approximately 305 kJ of energy under normal cellularconditions During anabolism the reverse reaction occurs ATP is brokendown to form ADP and inorganic phosphate in an exothermic reactionApproximately 305 kJ of energy is released from each mole of ATP
PhotosynthesisWhat is the source of the energy that fuels metabolism For most livingthings certain wavelengths of sunlight provide all of this energy Some bac-teria and the cells of all plants and algae including the brown algae shownin Figure 24-22 are able to capture light energy and convert some of it tochemical energy Animals canrsquot capture light energy so they get energy byeating plants or by eating other animals that eat plants The process that con-verts energy from sunlight to chemical energy in the bonds of carbohydratesis called photosynthesis During the complex process of photosynthesiscarbon dioxide and water provide the carbon hydrogen and oxygen atomsthat make up carbohydrates and oxygen gas which also is formed The fol-lowing net reaction takes place during photosynthesis
6CO2 6H2O light energy 0 C6H12O6 6O2
Carbon Water Glucose Oxygendioxide
Photosynthesis results in the reduction of the carbon atoms in carbon diox-ide as glucose is formed During this redox process oxygen atoms in waterare oxidized to oxygen gas
245 Metabolism 793
Figure 24-21
ATP is a nucleotide that containsan adenine nitrogen base aribose sugar and three phos-phate groups When the finalphosphate group is removedfrom ATP as modeled by the reddotted line ADP is formed andenergy is released
Ribose
Triphosphate
Adenine
ATP
P P P
null
17716411
Cellular RespirationMost organisms need oxygen to live Oxygen that is produced during photo-synthesis is used by living things during cellular respiration the process inwhich glucose is broken down to form carbon dioxide water and largeamounts of energy Cellular respiration is the major energy-producingprocess in living organisms Figure 24-23 shows one use of energy in thebody This energy is stored in the bonds of ATP and a maximum of 38 molesof ATP are produced for every mole of glucose that is catabolized Cellularrespiration is a redox process the carbon atoms in glucose are oxidized whileoxygen atoms in oxygen gas are reduced to the oxygen in water The netreaction that takes place during cellular respiration is
C6H12O6 6O2 0 6CO2 6H2O energyGlucose Oxygen Carbon Water
dioxide
Note that the net equation for cellular respiration is the reverse of the netequation for photosynthesis You will learn in Chapter 26 how these twoprocesses complement each other in nature
FermentationDuring cellular respiration glucose is completely oxidized and oxygen gas isrequired to act as the oxidizing agent Can cells extract energy from glucosein the absence of oxygen Yes but not nearly as efficiently Without oxygenonly a fraction of the chemical energy of glucose can be released Whereascellular respiration produces 38 moles of ATP for every mole of glucosecatabolized in the presence of oxygen only two moles of ATP are producedper mole of glucose that is catabolized in the absence of oxygen This pro-vides enough energy for oxygen-deprived cells so that they donrsquot die Theprocess in which glucose is broken down in the absence of oxygen is knownas fermentation There are two common kinds of fermentation In oneethanol and carbon dioxide are produced In the other lactic acid is produced
Alcoholic fermentation Yeast and some bacteria can ferment glucose toproduce the alcohol ethanol
C6H12O6 0 2CH3CH2OH 2CO2 energyGlucose Ethanol Carbon
dioxide
794 Chapter 24 The Chemistry of Life
Figure 24-23
These runners will need largeamounts of energy if they are tocomplete the race This energy isstored in the bonds of ATP intheir cells
Personal TrainerAre you interested in healthand fitness Are you good atmotivating people to do theirbest Then consider a career asa personal trainer
Personal trainers help peopleset up and follow exercise pro-grams The trainers design theprograms to get specificresults such as weight lossmuscle building improvedendurance or preparation fora sport They can create pro-grams for people who areolder or injured Trainers workin gyms health clubs and cor-porate fitness centers The fre-quency with which trainers seea client varies according to theclientrsquos needs
null
1529201
This reaction called alcoholic fermentation is important to certain segmentsof the food industry as you can see in Figure 24-24 Alcoholic fermentationis needed to make bread dough rise form tofu from soybeans and producethe ethanol in alcoholic beverages Another use of the ethanol that is pro-duced by yeast is as an additive to gasoline called gasohol You can observeyeast cells fermenting sugar in the CHEMLAB at the end of this chapter
Lactic acid fermentation Have you ever gotten muscle fatigue while run-ning a race During strenuous activity muscle cells often use oxygen fasterthan it can be supplied by the blood When the supply of oxygen is depletedcellular respiration stops Although animal cells canrsquot undergo alcoholic fer-mentation they can produce lactic acid and a small amount of energy fromglucose through lactic acid fermentation
C6H12O6 0 2CH3CH(OH)COOH energyGlucose Lactic acid
The lactic acid that is produced is moved from the muscles through the bloodto the liver There it is converted back into glucose that can be used in cata-bolic processes to yield more energy once oxygen becomes availableHowever if lactic acid builds up in muscle cells at a faster rate than the bloodcan remove it muscle fatigue results Buildup of lactic acid is what causes aburning pain in the muscle during strenuous exercise
245 Metabolism 795
Figure 24-24
Bread dough is heavy beforeyeast cells begin to ferment thecarbohydrates (left) After fer-mentation has produced carbondioxide the dough becomeslighter and fluffier as it rises(right)
Section 245 Assessment
21 Compare and contrast the processes of anabolismand catabolism
22 Explain the role ATP plays in the metabolism ofliving organisms
23 Decide whether each of the following processes isanabolic or catabolic
a photosynthesisb cellular respirationc fermentation
24 Thinking Critically Why is it necessary to usesealed casks when making wine
25 Calculating How many moles of ATP would ayeast cell produce if six moles of glucose wereoxidized completely in the presence of oxygenHow many moles of ATP would the yeast cellproduce from six moles of glucose if the cell weredeprived of oxygen For more help refer toArithmetic Operations in the Math Handbook onpage 887 of this textbook
chemistrymccomself_check_quiz
Topic Forensic AnthropologyTo learn more about forensicanthropology visit theChemistry Web site atchemistrymccomActivity Research forensicanthropology and explain atleast four ways that knowl-edge of the chemistry of lifehelps scientists learn aboutwhat ancient ice men andanimals did on their lastdays alive
null
9451008
796 Chapter 24 The Chemistry of Life
Pre-Lab
1 Reread the section of this chapter that describesalcoholic fermentation
2 Write the chemical equation for the alcoholic fer-mentation of glucose
3 Read the entire CHEMLAB
4 Prepare all written materials that you will take intothe laboratory Be sure to include safety precau-tions and procedure notes
5 Form a hypothesis about how the pressure insidethe test tube is related to the production of carbondioxide during the reaction Refer to the ideal gaslaw in your explanation
6 Why is temperature control an essential feature ofthe CHEMLAB
Procedure
1 Load the ChemBio program into your graphingcalculator Connect the CBL and calculator withthe link cable Connect the pressure sensor to theCBL with a CBL-DIN cable
2 Prepare a water bath using the 600-mL beaker Thebeaker should be about two-thirds full of waterThe water temperature should be between 36degCand 38degC
3 Set up the test tube ring stand and utility clamp asshown in the figure Obtain about 3 mL yeast sus-pension in a 10-mL graduated cylinder and pour itinto the test tube Obtain about 3 mL 5 sucrosesolution in a 10-mL graduated cylinder Add thesucrose solution to the yeast in the test tube Stir tomix Pour enough vegetable oil on top of the mix-ture to completely cover the surface
Safety Precautionsbull Always wear safety goggles and an apron in the labbull Do not use the thermometer as a stirring rod
ProblemWhat is the rate of alcoholicfermentation of sugar bybakerrsquos yeast
Objectivesbull Measure the pressure of
carbon dioxide produced bythe alcoholic fermentationof sugar by yeast
bull Calculate the rate of pro-duction of carbon dioxideby the alcoholic fermenta-tion of sugar by yeast
MaterialsCBL systemgraphing calculatorChemBio programVernier pressure
sensorlink cableCBL-DIN cabletest tube with 5
rubber-stopperassembly
5 sucrose solutionring stand
stirring rod600-mL beakerthermometerbasting bulbhot and cold wateryeast suspensionvegetable oilutility clamp10-mL graduated
cylinders (2)pipette
Alcoholic Fermentation inYeastYeast cells are able to metabolize many types of sugars In this
experiment you will observe the fermentation of sugar bybakerrsquos yeast When yeast cells are mixed with a sucrose solutionthey must first hydrolyze the sucrose to glucose and fructose Thenthe glucose is broken down in the absence of oxygen to form ethanoland carbon dioxide You can test for the production of carbon diox-ide by using a CBL pressure sensor to measure an increase in pressure
CHEMLAB CBL24
CHEMLAB 797
4 Place the stopper assembly into the test tube Makesure it has an airtight fit Leave both valves of theassembly open to the atmosphere
5 While one lab partner does step 5 the other partnershould do steps 6 and 7 Lower the test tube into thewater bath and allow it to incubate for 10 minutesKeep the temperature of the water bath between36degC and 38degC by adding small amounts of hot orcold water with the basting bulb as needed
6 Start the ChemBio program Choose 1SET UPPROBES under MAIN MENU Choose 1 for num-ber of probes Choose 3PRESSURE underSELECT PROBE Enter 1 for Channel Choose1USE STORED for CALIBRATION Choose1ATM for PRESSURE UNITS
7 Choose 2COLLECT DATA under MAIN MENUChoose 2TIME GRAPH under DATA COLLEC-TION Use time between sample seconds = 10Use number of samples = 60 (This will give you600 seconds or 10 minutes of data) Choose 1USETIME SETUP under CONTINUE Set Ymin = 08Ymax = 13 and Yscl = 01 Do not press ENTERuntil the test tube has finished incubating
8 After the test tube has incubated for ten minutesclose the valve attached to the stopper Make surethe valve near the pressure sensor is open to thesensor Start measuring the gas pressure by press-
ing ENTER Monitor the pressure reading on theCBL unit If the pressure exceeds 13 atm thestopper can pop off Open the air valve on thepressure sensor to release this excess gas pressure
9 After ten minutes the data collection will stopOpen the air valve on the stopper If needed youcan run a second trial by closing the air valve andchoosing 2YES to REPEAT If you are finishedpress 1NO
Cleanup and Disposal
1 Rinse out and wash all items
2 Rinse the yeast suspensionsucrosevegetable oilmixture down the sink with large amounts of water
3 Return all lab equipment to its proper place
Analyze and Conclude
1 Making and Using Graphs Choose 3VIEWGRAPH from the MAIN MENU Make a sketchof the graph (You also may want to record thedata table by using 4VIEW DATA)
2 Interpreting Data The rate of carbon dioxideproduction by the yeast can be found by calculat-ing the slope of the graph Return to the MAINMENU and choose 5FIT CURVE Choose 1LIN-EAR L1 L2 The slope will be listed under LIN-EAR as ldquoArdquo of Y = AX + B Record this value
3 Communicating How does your rate of carbondioxide production compare with the rates of othermembers of the class
4 Analyzing Why did you add vegetable oil to thetest tube in step 3
5 Suppose that the pressure doesnot change during a trial What might be some pos-sible reasons for this
Real-World Chemistry
1 Yeast is used in baking bread because the carbondioxide bubbles make the bread rise The otherproduct of alcoholic fermentation is ethanol Whycanrsquot you taste this alcohol when you eat bread
2 How would the appearance of a loaf of bread bedifferent if you used twice as much yeast as therecipe called for
Error Analysis
CHAPTER 24 CHEMLAB
Sense of SmellTake a deep breath What do you smell Maybe itrsquosthe eraser on your pencil some flowers outsideyour window an apple pie coming out of the ovenor even stinky gym shoes on the floor Your senseof smell otherwise known as olfaction tells you alot about the world around you
Fitting InHow exactly do you smell odors If you said thatyou smell with your nose you are only partiallycorrect The nose you see when you look in themirror doesnrsquot actuallydetect odors Its job is topull odor molecules knownas odorants into your nasalpassages when you breathe
The air you inhale carriesodorants to a small regionlocated high inside yournasal passage just below andbetween your eyes The tis-sues of this region containnerve cells that have hairlikefibers called cilia on one endWhen receptors located on the cilia are stimulatedby odorants the nerve cells send messages to thebrain The brain then identifies the odors you smell
The exact manner in which receptors detectodorants is not entirely understood Howeverresearchers have found evidence to suggest that itinvolves the shapes of both the receptors and theodorants Receptors are usually large globularprotein molecules that are similar in shape toenzymes Their surfaces contain small crevices Anodorant must fit into and bind with a crevice on thesurface of the receptor much as a substrate mole-cule fits into and binds with the active site on thesurface of an enzyme Each type of receptor has auniquely shaped crevice on its surface that bindsonly with an odorant of a particular shape
Not By Shape AloneThe process of smelling is not that simple how-ever Scientists have discovered that shape is notthe only property that determines whether an odor-ant will bind with a receptor Odorant moleculesmust be able to travel through air and they must beable to dissolve in the fluids of the nasal passagesIn addition some research suggests that odorreceptors can detect the energy levels of the odor-ants An odorant with exactly the right energy will
vibrate in such a way that itproduces a response in thereceptor This may be whatcauses the nerve cell to senda signal to the brain
So Many OdorsHow many odors do youthink you can recognize Ifyour olfactory system is atits best you can distinguishabout 10 000 different odorsDoes this mean that youhave more than 10 000 dif-
ferent types of olfactory receptors The answer isno In fact you have fewer than 1000 differenttypes of olfactory receptors
As researchers searched for an explanation ofhow the olfactory receptors recognize so manydifferent odors they found that the olfactory sys-tem uses a combination of receptors for each odorA single odorant can be recognized by more thanone receptor because different receptors respondto different parts of the same odorant moleculeRather than responding to a signal from a singlereceptor for each odor the brain interprets a com-bination of signals from several receptors in orderto identify a particular odor In this way a limitednumber of receptors can be used to identify a largenumber of odors
Everyday Chemistry
1 Relating Concepts How is the receptor-odorant relationship similar to an enzyme-substrate relationship
2 Applying How might fragrance designersbenefit from an understanding of how theshape of an odorant stimulates a receptor
798 Chapter 24 The Chemistry of Life
Study Guide 799
CHAPTER STUDY GUIDE24
Vocabulary
Summary241 Proteinsbull Proteins are biological polymers made of amino
acids that are linked together by peptide bonds
bull Protein chains fold into intricate three-dimensionalstructures
bull Many proteins function as enzymes which are high-ly specific and powerful biological catalysts Otherproteins function to transport important chemicalsubstances or provide structure in organisms
242 Carbohydratesbull Monosaccharides known as simple sugars are alde-
hydes or ketones that also have multiple hydroxylgroups
bull Bonding two simple sugars together forms a disac-charide such as sucrose or lactose
bull Polysaccharides such as starch cellulose and glyco-gen are polymers of simple sugars
bull Carbohydrates function in living things to provideimmediate and stored energy
243 Lipidsbull Fatty acids are long-chain carboxylic acids that usu-
ally have between 12 and 24 carbon atoms
bull Saturated fatty acids have no double bonds unsatu-rated fatty acids have one or more double bonds
bull Fatty acids can be linked to glycerol backbones toform triglycerides
bull The membranes of living cells have a lipid bilayerstructure
bull Steroids are lipids that have a multiple-ring structure
244 Nucleic Acidsbull Nucleic acids are polymers of nucleotides which
consist of a nitrogen base a phosphate group and asugar
bull DNA contains deoxyribose sugar and the nitrogenbases adenine cytosine guanine and thymine
bull RNA contains ribose sugar and the nitrogen basesadenine cytosine guanine and uracil
bull DNA functions to store the genetic information inliving cells and transmit it from one generation ofcells to the next RNA functions in protein synthesis
245 Metabolismbull Metabolism is the sum of the many chemical reac-
tions that go on in living cells
bull Catabolism refers to reactions that cells undergo toextract energy and chemical building blocks fromlarge biological molecules
bull Anabolism refers to the reactions through whichcells use energy and small building blocks to buildthe large biological molecules needed for cell struc-tures and for carrying out cell functions
bull During photosynthesis cells use carbon dioxidewater and light energy to produce carbohydratesand oxygen
bull During cellular respiration cells break down carbo-hydrates in the presence of oxygen gas to producecarbon dioxide and water Energy released is storedas chemical potential energy in the molecule ATP
bull In the absence of oxygen cells can carry out eitheralcoholic or lactic acid fermentation
bull active site (p 778)bull amino acid (p 776)bull anabolism (p 792)bull ATP (p 793)bull carbohydrate (p 781)bull catabolism (p 792)bull cellular respiration (p 794)bull denaturation (p 778)bull disaccharide (p 782)bull enzyme (p 778)
bull fatty acid (p 784)bull fermentation (p 794)bull lipid (p 784)bull metabolism (p 792)bull monosaccharide (p 781)bull nucleic acid (p 788)bull nucleotide (p 788)bull peptide (p 777)bull peptide bond (p 777)
bull phospholipid (p 786)bull photosynthesis (p 793)bull polysaccharide (p 782)bull protein (p 775)bull saponification (p 785)bull steroid (p 787)bull substrate (p 778)bull triglyceride (p 785)bull wax (p 787)
chemistrymccomvocabulary_puzzlemaker
800 Chapter 24 The Chemistry of Life
Go to the Chemistry Web site atchemistrymccom for additionalChapter 24 Assessment
Concept Mapping26 Complete the concept map using the following terms
lactic acid ATP cellular respiration metabolism alco-holic fermentation
Mastering Concepts27 What should you call a chain of eight amino acids A
chain of 200 amino acids (241)
28 Name five parts of your body that are made of struc-tural proteins (241)
29 Describe two common shapes found in the three-dimensional folding of proteins (241)
30 Name the organic functional groups in the side chainsof the following amino acids (241)
a glutamine c glutamic acidb serine d lysine
31 Explain how the active site of an enzyme functions(241)
32 Name the amino acids represented by each of the fol-lowing abbreviations (241)
a Gly d Pheb Tyr e Gluc Trp f His
33 Name an amino acid that has an aromatic ring in itsside chain (241)
34 Name two nonpolar and two polar amino acids (241)
35 Is the dipeptide lysine-valine the same compound asthe dipeptide valine-lysine Explain (241)
36 How do enzymes lower the activation energy for areaction (241)
37 The structure shown is tryptophan Describe some ofthe properties you would expect tryptophan to havebased on its structure In what class of large moleculesdo you think tryptophan is a building block (241)
38 Most proteins with a globular shape are oriented sothat they have mostly nonpolar amino acids on theinside and polar amino acids located on the outer sur-face Does this make sense in terms of the nature ofthe cellular environment Explain (241)
39 Classify the following carbohydrates as monosaccha-rides disaccharides or polysaccharides (242)
a starch e celluloseb glucose f glycogenc sucrose g fructosed ribose h lactose
40 Name two isomers of glucose (242)
41 What kind of bond is formed when two monosaccha-rides combine to form a disaccharide (242)
42 Give a scientific term for each of the following (242)
a blood sugar c table sugarb fruit sugar d milk sugar
43 Explain how the different bonding arrangements incellulose and starch give them such differentproperties (242)
44 The disaccharide maltose is formed from two glucosemonomers Draw its structure (242)
45 Hydrolysis of cellulose glycogen and starch producesonly one monosaccharide Why is this so Whatmonosaccharide is produced (242)
46 Digestion of disaccharides and polysaccharides cannottake place in the absence of water Why do you thinkthis is so Include an equation in your answer (242)
47 Draw the structure of the open-chain form of fructoseCircle all chiral carbons and then calculate the num-ber of stereoisomers with the same formula asfructose (242)
CHAPTER ASSESSMENTCHAPTER ASSESSMENT24
3 4
1
by
can be produced during
65
2
O
CH2
H2N OH
mdash mdash
CC
NHC
H
chemistrymccomchapter_test
Assessment 801
CHAPTER 24 ASSESSMENT
48 Compare and contrast the structures of a triglycerideand a phospholipid (243)
49 Predict whether a triglyceride from beef fat or atriglyceride from olive oil will have a higher meltingpoint Explain your reasoning (243)
50 Explain how the structure of soaps makes them effec-tive cleaning agents (243)
51 Draw a portion of a lipid bilayer membrane labelingthe polar and nonpolar parts of the membrane (243)
52 Where and in what form are fatty acids stored in thebody (243)
53 What type of lipid does not contain fatty acid chainsWhy are these molecules classified as lipids (243)
54 Draw the structure of the soap sodium palmitate(palmitate is the conjugate base of the 16-carbon satu-rated fatty acid palmitic acid) and label its polar andnonpolar ends (243)
55 What three structures make up a nucleotide (244)
56 Name two nucleic acids found in organisms (244)
57 Explain the roles of DNA and RNA in the productionof proteins (244)
58 Where in living cells is DNA found (244)
59 Describe the types of bonds and attractions that linkthe monomers together in a DNA molecule (244)
60 In the double helical structure of DNA the base gua-nine is always bonded to cytosine and adenine isalways bonded to thymine What do you expect to bethe relative proportional amounts of A T C and G ina given length of DNA (244)
61 One strand in a DNA molecule has the following basesequence What is the base sequence of the otherstrand in the DNA molecule (244)
C-C-G-T-G-G-A-C-A-T-T-A
62 Is digestion an anabolic or a catabolic processExplain (245)
63 Compare the net reactions for photosynthesis and cel-lular respiration with respect to reactants productsand energy (245)
Mastering ProblemsProtein Structure (241)64 How many peptide bonds are present in a peptide that
has five amino acids
65 How many peptides each containing four differentamino acids can be made from Phe Lys Pro andAsp List the peptides using the three-letter abbrevia-tions for the amino acids
66 The average molecular mass of an amino acid is 110gmol Calculate the approximate number of amino acidsin a protein that has a molecular mass of 36 500 gmol
Calculations with Lipids (243)67 The fatty acid palmitic acid has a density of 0853
gmL at 62degC What will be the mass of a 0886-Lsample of palmitic acid at that temperature
68 How many moles of hydrogen gas are required forcomplete hydrogenation of one mole of linolenic acidwhose structure is shown below Write a balancedequation for the hydrogenation reaction
CH3CH2CHCHCH2CHCHCH2CHCH(CH2)7COOHLinolenic acid
69 A chicken egg contains about 213 mg of cholesterolCalculate how many moles of cholesterol this repre-sents if the molecular mass of cholesterol is 386 amu
70 Calculate the number of moles of sodium hydroxideneeded in the saponification of 16 moles of atriglyceride
Working with DNA (244)71 The genetic code is a triplet code that is a sequence
of three bases in RNA codes for each amino acid in apeptide chain or protein How many RNA bases arerequired to code for a protein that contains 577 aminoacids
72 It has been calculated that the average length of a basepair in a DNA double helix is 34 Aring The humangenome (the complete set of all DNA in the nucleus ofa human cell) contains about three billion base pairs ofDNA In centimeters how long is the DNA in thehuman genome Assume that the DNA is stretched out and not coiled around proteins as it actually is in a living cell (1 Aring 10ndash10 m)
73 A cell of the bacterium Escherichia coli has about 42 106 base pairs of DNA whereas each humancell has about 3 109 base pairs of DNA What per-centage of the size of the human genome does theE coli DNA represent
Energy Calculations (245)74 Every mole of glucose that undergoes alcoholic fer-
mentation in yeast results in the net synthesis of twomoles of ATP How much energy in kJ is stored in twomoles of ATP Assume 100 efficiency
75 How many moles of lactic acid are produced whenthree moles of glucose undergo fermentation in yourmuscle cells Assume 100 completion of the process
802 Chapter 24 The Chemistry of Life
76 The synthesis of one mole of the fatty acid palmiticacid from two-carbon building blocks requires sevenmoles of ATP How many kJ of energy are required forthe synthesis of one mole of palmitic acid
77 How many grams of glucose can be oxidized completelyby 20 L of O2 gas at STP during cellular respiration
78 Calculate and compare the total energy in kJ that isconverted to ATP during the processes of cellular res-piration and fermentation
Mixed ReviewSharpen your problem-solving skills by answering thefollowing
79 Draw the carbonyl functional groups present in glu-cose and fructose How are the groups similar Howare they different
80 List the names of the monomers that make up pro-teins complex carbohydrates and nucleic acids
81 Describe the functions of proteins carbohydrateslipids and nucleic acids in living cells
82 Write a balanced equation for the hydrolysis of lactose
83 Write a balanced equation for photosynthesis
84 Write and balance the equation for cellular respiration
85 Write a balanced equation for the synthesis of sucrosefrom glucose and fructose
Thinking Critically86 Using Numbers Approximately 38 moles of ATP are
formed when glucose is completely oxidized during cel-lular respiration If the heat of combustion for 1 mole ofglucose is 282 103 kJmol and each mole of ATPstores 305 kJ of energy what is the efficiency of cellu-lar respiration in terms of the percentage of availableenergy that is stored in the chemical bonds of ATP
87 Recognizing Cause and Effect Some diets suggestseverely restricting the intake of lipids Why is it not agood idea to eliminate all lipids from the diet
88 Making and Using Graphs A number of sat-urated fatty acids and values for some of theirphysical properties are listed in Table 24-2
a Make a graph plotting number of carbon atomsversus melting point
b Graph the number of carbon atoms versus densityc Draw conclusions about the relationships between
the number of carbon atoms in a saturated fattyacid and its density and melting point values
d Predict the approximate melting point of a satu-rated fatty acid that has 24 carbon atoms
Writing in Chemistry89 Write a set of instructions that could be included in a
package of contact-lens cleaning solution containingan enzyme This enzyme catalyzes the breakdown ofprotein residues that adhere to the lenses Includeinformation about the structure and function ofenzymes and the care that must be taken to avoid theirdenaturation during use
90 Use the library or the Internet to research cholesterolWhere is this molecule used in your body What is itsfunction Why is too much dietary cholesterol consid-ered to be bad for you
Cumulative ReviewRefresh your understanding of previous chapters byanswering the following
91 a Write the balanced equation for the synthesis ofethanol from ethene and water
b If 448 L of ethene gas reacts with excess water atSTP how many grams of ethanol will be produced(Chapter 14)
92 Identify whether each of the reactants in these reac-tions is acting as an acid or a base (Chapter 19) a HBr H2O H3O
Br
b NH3 HCOOH NH4 HCOO
c HCO3 H2O CO3
H3O
93 What is a voltaic cell (Chapter 21)
CHAPTER ASSESSMENT24
Physical Properties of Saturated Fatty Acids
Number Density of Melting (gmL)
carbon point (values at Name atoms (degC) 60ndash80degC)
Palmitic acid 16 63 0853
Myristic acid 14 58 0862
Arachidic acid 20 77 0824
Caprylic acid 8 16 0910
Docosanoicacid 22 80 0822
Stearic acid 18 70 0847
Lauric acid 12 44 0868
Table 24-2
Standardized Test Practice 803
Use these questions and the test-taking tip to preparefor your standardized test
1 Which of the following is NOT true of carbohydrates
a Monosaccharides in aqueous solutions interconvertcontinuously between an open-chain structure anda cyclic structure
b The monosaccharides in starch are linked togetherby the same kind of bond that links the monosac-charides in lactose
c All carbohydrates have the general chemical for-mula Cn(H2O)n
d Cellulose made only by plants is easily digestibleby humans
2 All of the following are differences between RNA andDNA EXCEPT
a DNA contains the sugar deoxyribose while RNAcontains the sugar ribose
b RNA contains the nitrogen base uracil while DNAdoes not
c RNA is usually single-stranded while DNA is usu-ally double-stranded
d DNA contains the nitrogen base adenine whileRNA does not
3 Cellular respiration produces about 38 moles of ATPfor every mole of glucose consumed
C6H12O6 6O2 rarr 6CO2 6H2O 38ATP
If each molecule of ATP can release 305 kJ of energyhow much energy can be obtained from a candy barcontaining 130 g of glucose
a 274 kJ c 1159 kJb 836 kJ d 3970 kJ
4 The sequence of bases in RNA determines thesequence of amino acids in a protein Three bases codefor a single amino acid for example CAG is the codefor glutamine Therefore a strand of RNA 273 104
bases long codes for a protein that has
a 819 104 amino acidsb 910 103 amino acidsc 273 104 amino acidsd 455 103 amino acids
5 The equation for the alcoholic fermentation of glucoseis shown below
C6H12O6 rarr 2CH3CH2OH 2CO2 energy
The ethanol content of wine is about 12 Thereforein every 100 g of wine there are 120 g of ethanolHow many grams of glucose were catabolized to pro-duce these 120 g of ethanol
a 234 g c 470 gb 120 g d 270 g
Analyzing Tables Use the table below to answer thefollowing questionsNote of X of molecules of X in one DNA molecule
X
where X is any nitrogen base
6 What is the T of DNA D
a 284 c 716b 784 d 216
7 Every nitrogen base found in a DNA molecule is partof a nucleotide of that molecule The A nucleotide Cnucleotide G nucleotide and T nucleotide have molarmasses of 34722 gmol 32320 gmol 36323 gmoland 33821 gmol respectively What is the mass ofone mole of DNA A
a 279 105 g c 26390 105 gb 27001 105 g d 272 105 g
8 How many molecules of adenine are in one moleculeof DNA B
a 402 c 216b 434 d 175
of X of A of G of C of T
STANDARDIZED TEST PRACTICECHAPTER 24
Nitrogen Base Data for Four Different Double-Stranded DNA Molecules
DNA Molecule of A of G of C of T A G C T
A 165 231 208 292
B 402 325
C 194 234 227 273
D 266 203 284 216
chemistrymccomstandardized_test
Use The Process of Elimination On anymultiple-choice test there are two ways to find thecorrect answer to each question Either you canchoose the right answer immediately or you caneliminate the answers that you know are wrong