Preview Marks’ Basic Medical Biochemistry A Clinical Approach (5th Edition) by Michael A. Lieberman Alisa Peet (2017)

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Names:Lieberman,Michael,1950-author.|Peet,Alisa,author.
Title:Marks’basicmedicalbiochemistry:aclinicalapproach/MichaelLieberman,AlisaPeet;
illustrationsbyMatthewChansky.
Othertitles:Basicmedicalbiochemistry
Description:Fifthedition.|Philadelphia:WoltersKluwer,[2018]|Includesbibliographicalreferencesand
index.
Identifiers:LCCN2017016094|ISBN9781496324818
Subjects:|MESH:BiochemicalPhenomena|ClinicalMedicine

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PrefacetotheFifthEdition

Ithasbeen5yearssincethefourtheditionwascompleted.Thefiftheditionhas
somesignificantorganizationalchanges,assuggestedbyextensivesurveysof
facultyandstudentswhousedthefourtheditionintheirclassesandstudies.The
majorpedagogicfeaturesofthetextremain.Theyhavebeenenhancedbythe

followingchangesforthefifthedition:
1.Everypatienthistoryhasbeenreviewedandrevisedtoreflectcurrent
standardsofcare(asof2016).Thepatientnameshavealsobeenchanged
toafirstnameandlastinitial.Akeyindicatingthe“old”namesand
“new”namesisavailableintheonlinesupplementassociatedwiththe
text.
2.TheBiochemicalCommentsassociatedwitheachchapterhavebeen
updated,whereappropriate,toallowstudentstoexperiencewhere
currentresearcheffortsareheaded.
3.Thepresentationofmetabolismhasbeenalteredsuchthatglycolysisis
nowthefirsttopicdiscussed,followedbythetricarboxylicacidcycle,
andthenoxidativephosphorylation.Thecorrelationbetweenfourth
editionchaptersandfiftheditionchaptersareasfollows:
a.Chapters1through18,nochange
b.SectionIVisnowentitled“CarbohydrateMetabolism,Fuel
Oxidation,andtheGenerationofAdenosineTriphosphate”and
consistsofChapters19through28.
i.Chapter19ofthefifthedition(BasicConceptsintheRegulation


ofFuelMetabolismbyInsulin,Glucagon,andOtherHormones)
isbasedonChapter26ofthefourthedition.
ii.Chapter20ofthefifthedition(CellularBioenergetics:
AdenosineTriphosphateandO2)isbasedonChapter19ofthe
fourthedition.
iii.Chapter21ofthefifthedition(Digestion,Absorption,and
TransportofCarbohydrates)isbasedonChapter27ofthefourth
edition.
iv.Chapter22ofthefifthedition(GenerationofAdenosine
TriphosphatefromGlucose,Fructose,andGalactose:

Glycolysis)isbasedonChapter22ofthefourtheditionandalso
containspartsofChapter29ofthefourthedition(Pathwaysof
SugarMetabolism:PentosePhosphatePathway,Fructose,and
GalactoseMetabolism).
v.Chapter23ofthefifthedition(TricarboxylicAcidCycle)is
basedonChapter20ofthefourthedition.
vi.Chapter24ofthefifthedition(OxidativePhosphorylationand
MitochondrialFunction)isbasedonChapter21ofthefourth
edition.
vii.Chapter25ofthefifthedition(OxygenToxicityandFreeRadicalInjury)isbasedonChapter24ofthefourthedition.
viii.Chapter26ofthefifthedition(FormationandDegradationof
Glycogen)isbasedonChapter28ofthefourthedition.
ix.Chapter27ofthefifthedition(PentosePhosphatePathwayand
theSynthesisofGlycosides,Lactose,Glycoproteins,and
Glycolipids)isbasedonChapter30ofthefourthedition,along
withasection(ThePentosePhosphatePathway)ofChapter29
ofthefourthedition.ThisledtothedeletionofoldChapter29
fromtheTableofContentsofthefifthedition.
x.Chapter28ofthefifthedition(Gluconeogenesisand
MaintenanceofBloodGlucoseLevels)isbasedonChapter31
ofthefourthedition.
c.SectionV(LipidMetabolism)nowconsistsofthefollowing
chapters:


i.Chapter29ofthefifthedition(DigestionandTransportof
DietaryLipids)isbasedonChapter32ofthefourthedition.
ii.Chapter30ofthefifthedition(OxidationofFattyAcidsand
KetoneBodies)isbasedonChapter23ofthefourthedition.
iii.Chapter31ofthefifthedition(SynthesisofFattyAcids,

Triacylglycerols,andtheMajorMembraneLipids)isbasedon
Chapter33ofthefourtheditionandalsocontainsbasic
informationconcerningtheeicosanoidsfromChapter35ofthe
fourthedition.MaterialfromChapter35ofthefourthedition
thatwasnotincorporatedintoChapter31ofthefiftheditionis
availableasanonlinesupplement.Aseparatechapteron
eicosanoidmetabolismisnotpresentinthefifthedition.
iv.Chapter32ofthefifthedition(CholesterolAbsorption,
Synthesis,Metabolism,andFate)isbasedonChapter34ofthe
fourthedition.
v.Chapter33ofthefifthedition(MetabolismofEthanol)isbased
onChapter25ofthefourthedition.
vi.Chapter34ofthefifthedition(IntegrationofCarbohydrateand
LipidMetabolism)isbasedonChapter36ofthefourthedition.
d.SectionVI(NitrogenMetabolism)hasthesamechapterorderasin
thefourthedition,butbecausetwochaptershavebeendeleted
previouslyfromthetext,thechapternumbersinthefiftheditionare
twolessthaninthefourthedition.SectionVIinthefifthedition
comprisesChapters35through40,whereasinthefourthedition,itis
Chapters37through42.
e.SectionVII(TissueMetabolism)hasthesamechapterorderasinthe
fourthedition,butthechapternumbersinthefiftheditionaretwoless
thaninthefourthedition.SectionVIIinthefiftheditioncomprises
Chapters41through47,whereasinthefourthedition,itisChapters
43through49.
4.Thenumberofprintedreviewquestionsattheendofeachchapterhas
beenincreasedto10,upfrom5questionsperchapterinthefourth
edition(470totalquestions).Theonlinequestionbankassociatedwith
thetexthasalsobeenincreasedto560questions,ascomparedto468

questionsassociatedwiththefourthedition.Wherepossible,questions
arepresentedinNationalBoardofMedicalExaminersformat.
Asstatedinpreviouseditions,inrevisingatextgearedprimarilytoward
medicalstudents,theauthorsalwaysstrugglewithnewadvancesinbiochemistry
andwhethersuchadvancesshouldbeincludedinthetext.Wehavetakenthe
approachofonlyincludingadvancesthatwillenablethestudenttobetterrelate
biochemistrytomedicineandfuturediagnostictools.Althoughproviding
incomplete,butexciting,advancestograduatestudentsisbestfortheir
education,medicalstudentsbenefitmorefromamoredirectedapproach—one
thatemphasizeshowbiochemistryisusefulforthepracticeofmedicine.Thisis
amajorgoalofthistext.
Anyerrorsaretheresponsibilityoftheauthors,andwewouldappreciate
beingnotifiedwhensucherrorsarefound.
TheaccompanyingwebsiteforthiseditionofMarks’BasicMedical
Biochemistry:AClinicalApproachcontainstheaforementionedadditional
multiple-choicequestionsforreview,atablelistingpatientnamesforthefifth
editionandhowtheycorrespondtothoseofthefourthedition,summariesofall
patientsdescribedinthetext(patientcases),allchapterreferencesandadditional
reading(withlinkstothearticleinPubMed,whereapplicable),alistingof
diseasesdiscussedinthebook(withlinkstoappropriatewebsitesformore
information),andasummaryofallofthemethodsdescribedthroughoutthetext.


HowtoUseThisBook

Iconsidentifythevariouscomponentsofthebook:thepatientswhoare
presentedatthestartofeachchapter;theclinicalnotes,methodsnotes,
questions,andanswersthatappearinthemargins;andtheKeyConcepts,
ClinicalComments,andBiochemicalCommentsthatarefoundattheendof

eachchapter.
Eachchapterstartswithanabstractthatsummarizestheinformationsothat
studentscanrecognizethekeywordsandconceptstheyareexpectedtolearn.
ThenextcomponentofeachchapterisTheWaitingRoom,describingpatients
withcomplaintsanddetailingtheeventsthatledthemtoseekmedicalhelp.
indicatesafemalepatient
indicatesamalepatient
indicatesapatientwhoisaninfantoryoungchild
Aseachchapterunfolds,iconsappearinthemargin,identifyinginformation
relatedtothematerialpresentedinthetext:
indicatesaclinicalnote,usuallyrelatedtothepatientsinTheWaiting
Roomforthatchapter.Thesenotesexplainsignsorsymptomsofapatient
orgivesomeotherclinicalinformationrelevanttothetext.


indicatesamethodsnote,whichelaboratesonhowbiochemistryis
requiredtoperform,andinterpret,commonlaboratorytests.
Questionsandanswersalsoappearinthemarginandshouldhelptokeep
studentsthinkingastheyreadthetext:
indicatesaquestion
indicatestheanswertothequestion.Theanswertoaquestionisalways
locatedonthenextpage.Iftwoquestionsappearononepage,theanswers
aregiveninorderonthenextpage.
Eachchapterendswiththesethreesections:KeyConcepts,Clinical
Comments,andBiochemicalComments:
TheKeyConceptssummarizetheimportanttake-homemessagesfromthe
chapter.
TheClinicalCommentsgiveadditionalclinicalinformation,often
describingthetreatmentplanandtheoutcome.
TheBiochemicalCommentsaddbiochemicalinformationthatisnot

coveredinthetextorexploresomefacetofbiochemistryinmoredetailor
fromanotherangle.
Finally,ReviewQuestionsarepresented.Thesequestionsarewrittenina
UnitedStatesMedicalLicensingExamination–likeformat,andmanyofthem
haveaclinicalslant.Answerstothereviewquestions,alongwithdetailed
explanations,areprovidedattheendofeverychapter.


Acknowledgments

TheauthorswouldliketothankProfessorKentLittletonofBastyrUniversity,
forhiscarefulreadingofthefourtheditionandpointingoutmistakesanderrors
thatrequiredcorrectingforthefifthedition.Wegreatlyappreciatehiseffortsin
improvingthetext.Dr.BonnieBrehmwasinstrumentalinhelpingwiththe
nutritionaspectsofthetext,andDr.RickRicerwasinvaluableinwriting
questions,bothforthetextandtheonlinesupplement.Wewouldalsoliketo
acknowledgetheinitialcontributionsofDawnMarks,whosevisionofa
textbookgearedtowardmedicalstudentsledtothefirsteditionofthisbook.Her
visionisstillapplicabletoday.


Contents

SECTIONI
FuelMetabolism
1MetabolicFuelsandDietaryComponents
2TheFedorAbsorptiveState
3Fasting
SECTIONII
ChemicalandBiologicFoundationsofBiochemistry

4Water,Acids,Bases,andBuffers
5StructuresoftheMajorCompoundsoftheBody
6AminoAcidsinProteins
7Structure–FunctionRelationshipsinProteins
8EnzymesasCatalysts
9RegulationofEnzymes
10RelationshipbetweenCellBiologyandBiochemistry
11CellSignalingbyChemicalMessengers
SECTIONIII
GeneExpressionandtheSynthesisofProteins


12StructureoftheNucleicAcids
13SynthesisofDNA
14Transcription:SynthesisofRNA
15Translation:SynthesisofProteins
16RegulationofGeneExpression
17UseofRecombinantDNATechniquesinMedicine
18TheMolecularBiologyofCancer
SECTIONIV
CarbohydrateMetabolism,FuelOxidation,andtheGenerationof
AdenosineTriphosphate
19BasicConceptsintheRegulationofFuelMetabolismbyInsulin,
Glucagon,andOtherHormones
20CellularBioenergetics:AdenosineTriphosphateandO2
21Digestion,Absorption,andTransportofCarbohydrates
22GenerationofAdenosineTriphosphatefromGlucose,Fructose,and
Galactose:Glycolysis
23TricarboxylicAcidCycle
24OxidativePhosphorylationandMitochondrialFunction

25OxygenToxicityandFree-RadicalInjury
26FormationandDegradationofGlycogen
27PentosePhosphatePathwayandtheSynthesisofGlycosides,Lactose,
Glycoproteins,andGlycolipids
28GluconeogenesisandMaintenanceofBloodGlucoseLevels
SECTIONV
LipidMetabolism
29DigestionandTransportofDietaryLipids


30OxidationofFattyAcidsandKetoneBodies
31SynthesisofFattyAcids,Triacylglycerols,andtheMajorMembrane
Lipids
32CholesterolAbsorption,Synthesis,Metabolism,andFate
33MetabolismofEthanol
34IntegrationofCarbohydrateandLipidMetabolism
SECTIONVI
NitrogenMetabolism
35ProteinDigestionandAminoAcidAbsorption
36FateofAminoAcidNitrogen:UreaCycle
37SynthesisandDegradationofAminoAcids
38Tetrahydrofolate,VitaminB12,andS-Adenosylmethionine
39PurineandPyrimidineMetabolism
40IntertissueRelationshipsintheMetabolismofAminoAcids
SECTIONVII
TissueMetabolism
41ActionsofHormonesthatRegulateFuelMetabolism
42TheBiochemistryofErythrocytesandOtherBloodCells
43BloodPlasmaProteins,Coagulation,andFibrinolysis
44LiverMetabolism

45MetabolismofMuscleatRestandduringExercise
46MetabolismoftheNervousSystem
47TheExtracellularMatrixandConnectiveTissue
PatientIndex


SubjectIndex


FuelMetabolism

SECTION

I

nordertosurvive,humansmustmeettwobasicmetabolicrequirements:We
mustbeabletosynthesizeeverythingourcellsneedthatisnotsuppliedby
ourdiet,andwemustbeabletoprotectourinternalenvironmentfromtoxins
andchangingconditionsinourexternalenvironment.Tomeetthese
requirements,wemetabolizeourdietarycomponentsthroughfourbasictypesof
pathways:fueloxidativepathways,fuelstorageandmobilizationpathways,
biosyntheticpathways,anddetoxificationorwastedisposalpathways.
Cooperationbetweentissuesandresponsestochangesinourexternal
environmentarecommunicatedthroughtransportpathwaysandintercellular
signalingpathways(Fig.I.1).

I


Thefoodsinourdietarethefuelsthatsupplyuswithenergyintheformof

calories.Thisenergyisusedforcarryingoutsuchdiversefunctionsasmoving,
thinking,andreproducing.Thus,severalofourmetabolicpathwaysarefuel
oxidativepathwaysthatconvertfuelsintoenergythatcanbeusedfor
biosyntheticandmechanicalwork.Butwhatisthesourceofenergywhenweare
noteating,suchasbetweenmeals,andwhilewesleep?Howdoesapersonona
hungerstrikethatyoureadaboutinthemorningheadlinessurvivesolong?We
haveothermetabolicpathwaysthatarefuelstoragepathways.Thefuelsthatwe
storecanbemobilizedduringperiodswhenwearenoteatingorwhenweneed
increasedenergyforexercise.
Ourdietalsomustcontainthecompoundswecannotsynthesize,aswellas
allthebasicbuildingblocksforcompoundswedosynthesizeinourbiosynthetic
pathways.Forexample,wehavedietaryrequirementsforsomeaminoacids,but
wecansynthesizeotheraminoacidsfromourfuelsandadietarynitrogen
precursor.Thecompoundsrequiredinourdietforbiosyntheticpathwaysinclude
certainaminoacids,vitamins,andessentialfattyacids.


Detoxificationpathwaysandwastedisposalpathwaysaremetabolic
pathwaysdevotedtoremovingtoxinsthatcanbepresentinourdietsorintheair
webreathe,introducedintoourbodiesasdrugs,orgeneratedinternallyfromthe
metabolismofdietarycomponents.Dietarycomponentsthathavenovaluetothe
body,andmustbedisposedof,arecalledxenobiotics.
Ingeneral,biosyntheticpathways(includingfuelstorage)arereferredtoas
anabolicpathways;thatis,pathwaysthatsynthesizelargermoleculesfrom
smallercomponents.Thesynthesisofproteinsfromaminoacidsisanexample
ofananabolicpathway.Catabolicpathwaysarethosepathwaysthatbreakdown
largermoleculesintosmallercomponents.Fueloxidativepathwaysareexamples
ofcatabolicpathways.
Inhumans,theneedfordifferentcellstocarryoutdifferentfunctionshas
resultedincellandtissuespecializationinmetabolism.Forexample,ouradipose

tissueisaspecializedsiteforthestorageoffatandcontainsthemetabolic
pathwaysthatallowittocarryoutthisfunction.However,adiposetissueis
lackingmanyofthepathwaysthatsynthesizerequiredcompoundsfromdietary
precursors.Toenableourcellstocooperateinmeetingourmetabolicneeds
duringchangingconditionsofdiet,sleep,activity,andhealth,weneedtransport
pathwaysintothebloodandbetweentissuesandintercellularsignaling
pathways.Onemeansofcommunicationisforhormonestocarrysignalsto
tissuesaboutourdietarystate.Forexample,amessagethatwehavejusthada
meal,carriedbythehormoneinsulin,signalsadiposetissuetostorefat.
Inthefollowingsection,wewillprovideanoverviewofvarioustypesof
dietarycomponentsandexamplesofthepathwaysinvolvedinusingthese
components.Wewilldescribethefuelsinourdiet,thecompoundsproducedby
theirdigestion,andthebasicpatternsoffuelmetabolisminthetissuesofour
bodies.Wewilldescribehowthesepatternschangewhenweeat,whenwefast
forashorttime,andwhenwestarveforprolongedperiods.Patientswith
medicalproblemsthatinvolveaninabilitytodealnormallywithfuelswillbe
introduced.Thesepatientswillappearrepeatedlythroughoutthebookandwill
bejoinedbyotherpatientsaswedelvedeeperintobiochemistry.
Itisimportanttonotethatthissectionofthebookcontainsanoverviewof
basicmetabolism,whichallowspatientstobepresentedatanelementarylevel
andtowhetstudents’appetitesforthebiochemistrytocome.Thegoalisto


enablethestudenttotasteandpreviewwhatbiochemistryisallabout.Itisnot
designedtobeall-inclusive,asallofthesetopicswillbediscussedingreater
detailinSectionsIV,V,VI,VIIofthetext.Thenextsectionofthetext(Section
II)beginswiththebasicsofbiochemistryandtherelationshipofbasicchemistry
toprocessesthatoccurinalllivingcells.

MetabolicFuelsandDietary
Components

1

Foradditionalancillarymaterialsrelatedtothischapter,please
visitthePoint.

FuelMetabolism.Weobtainourfuelprimarilyfromthemacronutrients(i.e.,
carbohydrates,fats,andproteins)inourdiet.Asweeat,ourfoodstuffsare
digestedandabsorbed.Theproductsofdigestioncirculateintheblood,enter
varioustissues,andareeventuallytakenupbycellsandoxidizedtoproduce
energy.Tocompletelyconvertourfuelstocarbondioxide(CO2)andwater
(H2O),molecularoxygen(O2)isrequired.Webreathetoobtainthisoxygenand
toeliminatetheCO2thatisproducedbytheoxidationofourfoodstuffs.
FuelStores.Anydietaryfuelthatexceedsthebody’simmediateenergyneedsis
stored,mainlyastriacylglycerol(fat)inadiposetissue,asglycogen(a
carbohydrate)inmuscle,liver,andothercells,and,tosomeextent,asproteinin
muscle.Whenwearefasting,betweenmealsandovernightwhilewesleep,fuel
isdrawnfromthesestoresandisoxidizedtoprovideenergy(Fig.1.1).


FuelRequirements.Werequireenoughenergyeachdaytodrivethebasic
functionsofourbodiesandtosupportourphysicalactivity.Ifwedonot
consumeenoughfoodeachdaytosupplythatmuchenergy,thebody’sfuel
storessupplytheremainderandweloseweight.Conversely,ifweconsume
morefoodthanrequiredfortheenergyweexpend,ourbody’sfuelstoresenlarge
andwegainweight.
OtherDietaryRequirements.Inadditiontoprovidingenergy,thedietprovides
precursorsforthebiosynthesisofcompoundsnecessaryforcellularandtissue

structure,function,andsurvival.Amongtheseprecursorsaretheessentialfatty
acidsandessentialaminoacids(thosethatthebodyneedsbutcannot
synthesize).Thedietmustalsosupplyvitamins,minerals,andwater.
WasteDisposal.Dietarycomponentsthatwecanusearereferredtoasnutrients.
However,boththedietandtheairwebreathecontainxenobioticcompounds,
compoundsthathavenouseorvalueinthehumanbodyandmaybetoxic.
Thesecompoundsareexcretedintheurineandfecestogetherwithmetabolic
wasteproducts.

THEWAITING
ROOM
PercyV.isa59-year-oldschoolteacherwhowasingoodhealthuntilhis
wifediedsuddenly.Sincethattime,hehasexperiencedanincreasing
degreeoffatigueandhaslostinterestinmanyoftheactivitieshepreviously
enjoyed.Shortlyafterhiswife’sdeath,oneofhismarriedchildrenmovedfar


fromhome.Sincethen,Mr.V.hashadlittleappetiteforfood.Whenaneighbor
foundMr.V.sleepinginhisclothes,unkempt,andsomewhatconfused,she
calledanambulance.Mr.V.wasadmittedtothehospitalpsychiatryunitwitha
diagnosisofmentaldepressionassociatedwithdehydrationandmalnutrition.
OttoS.isa25-year-oldmedicalstudentwhowasveryathleticduringhigh
schoolandcollegebutisnowoutofshape.Sincehestartedmedical
school,hehasbeengainingweight.Heis5ft10intallandbeganmedical
schoolweighing154lb,withinhisidealweightrange.Bythetimehefinished
hislastexaminationinhisfirstyear,heweighed187lb.Hehasdecidedto
consultaphysicianatthestudenthealthservicebeforetheproblemgetsworse,
ashewouldliketoreducehisweight(at187lb,hisbodymassindex[BMI]is
27]tohispreviouslevelof154lb(whichwouldreducehisBMIto23,inthe
middleofthehealthyrangeofBMIvalues).

IvanA.isa56-year-oldaccountantwhohasbeenobeseforanumberof
years.Heexhibitsapatternofcentralobesity,calledan“appleshape,”
whichiscausedbyexcessadiposetissuebeingdisproportionallydepositedinthe
abdominalarea.HismajorrecreationalactivitiesarewatchingTVwhiledrinking
scotchandsodaanddoingoccasionalgardening.Atacompanypicnic,he
becamevery“winded”whileplayingsoftballanddecideditwastimefora
generalphysicalexamination.Attheexamination,heweighed264lbat5ft10
intall.Hisbloodpressurewaselevated,155mmHgsystolicand95mmHg
diastolic(forIvan’sage,hypertensionisdefinedas>140mmHgsystolicand
>90mmHgdiastolic).Foramaleoftheseproportions,aBMIof18.5to24.9
wouldcorrespondtoaweightbetween129and173lb.Mr.A.isalmost100lb
overweight,andhisBMIof37.9isintherangedefinedasobesity.
AnnR.isa23-year-oldbuyerforawoman’sclothingstore.Despitethe
factthatsheis5ft7intallandweighs99lb,sheisconvincedsheis
overweight.Twomonthsago,shestartedadailyphysicalactivityprogramthat
consistsof1hourofjoggingeverymorningand1hourofwalkingevery
evening.Shealsodecidedtoconsultaphysicianaboutweightloss.Ifpatients
areabove(likeIvanA.)orbelow(likeAnnR.)theiridealweight,thephysician,
ofteninconsultationwitharegistereddietitian,prescribesadietdesignedto
bringtheweightintotheidealrange.


I.DietaryFuels
Themajorfuelsweobtainfromourdietarethemacronutrients—namely,
carbohydrates,proteins,andfats.WhenthesefuelsareoxidizedtoCO2and
H2Oinourcells(theprocessofcatabolism),energyisreleasedbythetransfer
ofelectronstoO2.Theenergyfromthisoxidationprocessgeneratesheatand
adenosinetriphosphate(ATP)(Fig.1.2).CO2travelsinthebloodtothelungs
whereitisexpired,andwaterisexcretedinurine,sweat,andothersecretions.
Althoughtheheatthatisgeneratedbyfueloxidationisusedtomaintainbody

temperature,themainpurposeoffueloxidationistogenerateATP.ATPprovides
theenergythatdrivesmostoftheenergy-consumingprocessesinthecell,
includingbiosyntheticreactions(anabolicpathways),musclecontraction,and
activetransportacrossmembranes.Astheseprocessesuseenergy,ATPis
convertedbacktoadenosinediphosphate(ADP)andinorganicphosphate(Pi).
ThegenerationanduseofATPisreferredtoastheATP–ADPcycle.

TheoxidationoffuelstogenerateATPiscalledrespiration(Fig.1.3).Prior
tooxidation,carbohydratesareconvertedprincipallytoglucose,fattofatty
acids,andproteintoaminoacids.Thepathwaysforoxidizingglucose,fatty
acids,andaminoacidshavemanyfeaturesincommon.Theyfirstoxidizethe
fuelstoacetylcoenzymeA(acetyl-CoA),aprecursorofthetricarboxylicacid
(TCA)cycle.TheTCAcycleisaseriesofreactionsthatcompletestheoxidation
offuelstoCO2(seeChapter23).Electronslostfromthefuelsduringoxidative


reactionsaretransferredtoO2byaseriesofproteinsintheelectrontransport
chain(seeChapter24).TheenergyofelectrontransferisusedtoconvertADP
andPitoATPbyaprocessknownasoxidativephosphorylation.

Indiscussionsofmetabolismandnutrition,energyisoftenexpressedinunits
ofcalories.Acalorieinthiscontext(nutritionalcalorie)isequivalentto1
kilocalorie(kcal)inenergyterms.“Calorie”wasoriginallyspelledwitha
capitalC,butthecapitalizationwasdroppedasthetermbecamepopular.Thus,a
1-calsoftdrinkactuallyhas1kcalofenergy.Energyisalsoexpressedinjoules.
Onekilocalorieequals4.18kilojoules(kJ).Physicianstendtouseunitsof
calories,inpartbecausethatiswhattheirpatientsuseandunderstand.One
kilocalorieofenergyistheamountofenergyrequiredtoraisethetemperatureof
1Lofwaterby1°C.
A.Carbohydrates

Themajorcarbohydratesinthehumandietarestarch,sucrose,lactose,fructose,
maltose,galactose,andglucose.Thepolysaccharidestarchisthestorageformof