SugarsandLipids(Lecture6):Sugarfunctions:energystorage,buildingblockfornucleicacids,structuralcomponent(wood).Disaccharide:tablesugar,madeoffructoseandglucoseThetypicalstructureisamultipleofCH2O,e.g.C3H6O3Youalwaysfindtwofunctionalgroupsinasugar:onecarbonylgroupandseveralhydroxylgroups.Aldoseandketoseareisomers,sameformula,butdifferentstructure.Importanttokeepinmind:allisomersdifferintheircovalentbonds(thatis,opticalisomersalsohaveadifferentstructure).Thiscanbeillustratedbydrawingtwoopticalisomersandtryingtorotateonetomakeitoverlaptheother.Thisisimpossible,justlikeyourtwohandscanneveroverlap.Glucoseandgalactosearetwodifferentopticalisomersofthis6-carbonsugar.Insolution,straight-chainglucoseformsanothercovalentbondtobecometheringformofglucose.BecausethisconvertsC-1fromasymmetrictoanasymmetriccarbonatom(C-1intheringformhasfourdifferentgroupsattachedtoit,versus3inthestraightchainform),youcangettwoglucoseisomers,alpha-andbeta-glucose.Sugarpolymers(polysaccharides)areformedbycovalentbondscalledglycosidiclinkagesbetweentheC-1ofonesugarwithanyOH-groupofthesecondsugar,butonlytwolinksareinterestingtous:alpha1,4givingrisetomaltoseandeventuallystarch,andbeta1,4givingrisetocellobiose,andeventuallycellulose.Instarch,bulkyCH2OHgroupsareallonthesameside,bendingthepolymerresultingintheshapeofaspiral.Ontheotherhand,thesecondglucoseincellobioseisflipped(withtheCH2OHgrouppointingdownwards)comparedtostarch,whichhashugestructuralimplications.Younowgetaverysymmetrical,straightmolecule.Anotherreminderaboutstructuralformulas:theyareoftensimplifiedbyomittingCandHatoms:eachcornercorrespondstoacarbonatombondedto2hydrogenatomsifnothingelseisindicated.Lipids:canbeallkindsofstructures:theonlythingtheyhaveincommonisthattheyareinsolubleinwater.

Themostimportantlipidsarefats(solid)andoils(liquid),whichconsistofthreefattyacidsandoneglycerolconnectedbycovalentbonds.Fattyacidsontheirownhaveonecarboxylgroupandalonghydrocarbonchain.Fattyacidsareamphiphilic(hydrophiliccarboxylgroupplushydrophobichydrocarbonchain),andarefoundinsoap.Fatsprovidemoreenergyperweightthanstarch,buttakelongertomobilize;long-termstorage.Phospholipidsarealsoamphiphilic:ahydrophilicheadgroupandtwohydrophobicfattyacidtailsattachedtoglycerol.Phospholipidshavetheuniquecapabilitytoself-assembleintolipidbilayerswiththehydrophobicfattyacidtailspointinginwardawayfromwater.Toavoidexposededges,lipidbilayerswillassembleintoaglobularcompartment.Thisisenergeticallymostfavorable,asnomorehydrophobicpartsareexposedtowater.Nocomplicatedmechanismneededtoinvokeevolutionofaprimitivecellenclosedbyalipidbilayer;phospholipidspreferentiallyassembleintoasealedcompartment.Lipidbilayers(cellmembranes)areveryfluid,withphospholipidsinconstantlateralmotion.Somefattyacidscontaindoublebonds;thentheyarecalledunsaturatedfattyacids.Doublebondscausekinksinfattyacidtails.Thereforetheycannotbepackedascloselytogetherasstraight,saturatedfattyacids.Thisisthereasonbutterissolidatroomtemperature(onlystraight-chainfattyacidsinbutterfat),andoilsarefluid(manyunsaturatedfattyacids:doublebondsmakingkinks).Unsaturatedfattyacidsinlipidbilayersincreasethefluidityandpermeabilityofthemembrane.Fishandplantsadjusttheamountofdoublebondsinphospholipidstokeepmembranefluiditystableoverawidevarietyoftemperatures.