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1、c10Lipids.qxd12/6/124:26PMPageS-112chapter10Lipids1.OperationalDefinitionofLipidsHowisthedefinitionof“lipid”differentfromthetypesofdefinitionsusedforotherbiomoleculesthatwehaveconsidered,suchasaminoacids,nucleicacids,andproteins?AnswerTheterm“lipid”doesnotspecifyapartic
2、ularchemicalstructure.Whereasonecanwriteageneralformulaforanaminoacid,nucleicacid,orprotein,lipidsaremuchmorechemicallydiverse.Compoundsarecategorizedaslipidsbasedontheirgreatersolubilityinorganicsolventsthaninwater.2.MeltingPointsofLipidsThemeltingpointsofaseriesof18-c
3、arbonfattyacidsare:stearicacid,69.6C;oleicacid,13.4C;linoleicacid,5C;andlinolenicacid,11C.(a)Whatstructuralaspectofthese18-carbonfattyacidscanbecorrelatedwiththemeltingpoint?(b)Drawallthepossibletriacylglycerolsthatcanbeconstructedfromglycerol,palmiticacid,andolei
4、cacid.Ranktheminorderofincreasingmeltingpoint.(c)Branched-chainfattyacidsarefoundinsomebacterialmembranelipids.Wouldtheirpresenceincreaseordecreasethefluidityofthemembranes(thatis,givethemalowerorhighermeltingpoint)?Why?Answer(a)Thenumberofcisdoublebonds(stearicacid,18:
5、0;oleic,18:1;linoleic,18:2;linolenic,18:3).Eachcisdoublebondcausesabendinthehydrocarbonchain,andbentchainsarelesswellpackedthanstraightchainsinacrystallattice.Thelowertheextentofpacking,thelowerthemeltingtemperature.(b)Sixdifferenttriacylglycerolsarepossible:onewithglyc
6、erolandonlypalmiticacid(PPP);onewithglycerolandonlyoleicacid(OOO);andfourwithglycerolandamixtureofoleicandpalmiticacids.Fourmixedtriacylglycerolsarepossible,becausethethreecarbonsofglycerolarenotequivalent:thusOOPandOPOarepositionalisomers,asarePOPandOPP.Thegreatertheco
7、ntentofsaturatedfattyacid(P),thehigherthemeltingpoint.Thus,theorderofmeltingpointsisOOOOOPOPOPOPOPPPPP.SeeTable10–1andFigure10–3forinformationonhowtodrawthetriacylglycerols.(c)Branched-chainfattyacidswillincreasethefluidityofmembranes(i.e.,lowertheirmeltingpoint)be
8、causetheydecreasetheextentofpackingpossiblewithinthemembrane.Theeffectofbranchesissimilartothatofbendscausedby
