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1、Chapter1026thFebruary,2007:FinalpagesMembraneStructure10Cellmembranesarecrucialtothelifeofthecell.TheplasmamembraneInThisChapterenclosesthecell,definesitsboundaries,andmaintainstheessentialdifferencesbetweenthecytosolandtheextracellularenvironment.Insideeucaryoticcells,THELIPIDBILAYERthemembraneso
2、ftheendoplasmicreticulum,Golgiapparatus,mitochondria,andothermembrane-enclosedorganellesmaintainthecharacteristicdiffer-MEMBRANEPROTEINSencesbetweenthecontentsofeachorganelleandthecytosol.Iongradientsacrossmembranes,establishedbytheactivitiesofspecializedmembranepro-teins,canbeusedtosynthesizeATP,
3、todrivethetransmembranemovementofselectedsolutes,or,asinnerveandmusclecells,toproduceandtransmitelec-tricalsignals.Inallcells,theplasmamembranealsocontainsproteinsthatactassensorsofexternalsignals,allowingthecelltochangeitsbehaviorinresponsetosignalsfromenvironmentalcues,includingsignalsfromotherc
4、ells;theseproteinsensors,orreceptors,transferinformation—ratherthanmolecules—acrossthemembrane.Despitetheirdifferingfunctions,allbiologicalmembraneshaveacommongeneralstructure:eachisaverythinfilmoflipid(fatty)andproteinmolecules,heldtogethermainlybynoncovalentinteractions(Figure10–1).Cellmem-brane
5、saredynamic,fluidstructures,andmostoftheirmoleculesmoveaboutintheplaneofthemembrane.Thelipidmoleculesarearrangedasacontinuousdoublelayerabout5nmthick.Thislipidbilayerprovidesthebasicfluidstruc-tureofthemembraneandservesasarelativelyimpermeablebarriertothepas-sageofmostwater-solublemolecules.Protei
6、nmoleculesthatspanthelipidbilayer(transmembraneproteins;seeFigure10–1)mediatenearlyalloftheotherfunctionsofthemembrane,transportingspecificmoleculesacrossit,forexam-ple,orcatalyzingmembrane-associatedreactionssuchasATPsynthesis.Intheplasmamembrane,sometransmembraneproteinsserveasstructurallinkstha
7、tconnectthecytoskeletonthroughthelipidbilayertoeithertheextracellularmatrixoranadjacentcell,whileothersserveasreceptorstodetectandtrans-ducechemicalsignalsinthecell’senvironment.Aswouldbeexpected,ittakesmanydiffe
