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细胞质膜结构和膜上运输PlasmaMembraneStructureandFunctionTheplasmamembraneseparatestheinternalenvironmentofthecellfromitssurroundings.Theplasmamembraneisaphospholipidbilayerwithembeddedproteins.Theplasmamembranehasafluidconsistencyandamosaicpatternofembeddedproteins.Fluid-mosaicmodelofmembranestructureCellsliveinfluidenvironments,withwaterinsideandoutsidethecell.Hydrophilic(water-loving)polarheadsofthephospholipidmoleculeslieontheoutward-facingsurfacesoftheplasmamembrane.Hydrophobic(water-fearing)nonpolartailsextendtotheinterioroftheplasmamembrane.Plasmamembraneproteinsmaybeperipheralproteinsorintegralproteins.Asidefromphospholipid,cholesterolisanotherlipidinanimalplasmamembranes;relatedsteroidsarefoundinplants.Cholesterolstrengthenstheplasmamembrane.Whenphospholipidshavecarbohydratechainsattached,theyarecalledglycolipids.Whenproteinshavecarbohydratechainsattached,theyarecalledglycoproteins.Carbohydratechainsoccuronlyontheexteriorsurfaceoftheplasmamembrane.Theoutsideandinsidesurfacesoftheplasmamembranearenotidentical.ChannelproteinCarrierproteinCellrecognitionproteinInanimalcells,thecarbohydratechainsofcellrecognitionproteinsarecollectivelycalledtheglycocalyx.Theglycocalyxcanfunctionincell-to-cellrecognition,adhesionbetweencells,andreceptionofsignalmolecules.Thediversityofcarbohydratechainsisenormous,providingeachindividualwithauniquecellular“fingerprint”.ReceptorproteinEnzymaticproteinThePermeabilityofthePlasmaMembraneTheplasmamembraneisdifferentiallypermeable.Macromoleculescannotpassthroughbecauseofsize,andtinychargedmoleculesdonotpassthroughthenonpolarinteriorofthemembrane.Small,unchargedmoleculespassthroughthemembrane,followingtheirconcentrationgradient.HowmoleculescrosstheplasmamembraneMovementofmaterialsacrossamembranemaybepassiveoractive.Passivetransportdoesnotusechemicalenergy;diffusionandfacilitatedtransportarebothpassive.Activetransportrequireschemicalenergyandusuallyacarrierprotein.Exocytosisandendocytosistransportmacromoleculesacrossplasmamembranesusingvesicleformation,whichrequiresenergy.DiffusionDiffusionisthepassivemovementofmoleculesfromahighertoalowerconcentrationuntilequilibriumisreached.Gasesmovethroughplasmamembranesbydiffusion.ProcessofdiffusionGasexchangeinlungsoccursbydiffusionOsmosisThediffusionofwateracrossadifferentiallypermeablemembraneduetoconcentrationdifferencesiscalledosmosis.Diffusionalwaysoccursfromhighertolowerconcentration.Waterenterscellsduetoosmoticpressurewithincells.OsmosisdemonstrationOsmosisincellsAsolutioncontainsasolute(solid)andasolvent(liquid).Cellsarenormallyisotonictotheirsurroundings,andthesoluteconcentrationisthesameinsideandoutofthecell.“Iso”meansthesameas,and“tonocity”referstothestrengthofthesolution.OsmosisinplantandanimalcellsHypotonicsolutionscausecellstoswellandpossiblyburst.“Hypo”meanslessthan.Animalcellsundergolysisinhypotonicsolution.Increasedturgorpressureoccursinplantcellsinhypotonicsolutions.Plantcellsdonotburstbecausetheyhaveacellwall.Hypertonicsolutionscausecellstolosewater.“Hyper”meansmorethan;hypertonicsolutionscontainmoresolute.Animalcellsundergocrenation(shrivel)inhypertonicsolutions.Plantcellsundergoplasmolysis,theshrinkingofthecytoplasm.TransportbyCarrierProteinsSomebiologicallyusefulmoleculespassthroughtheplasmamembranebecauseofchannelproteinsandcarrierproteinsthatspanthemembrane.Carrierproteinsarespecificandcombinewithonlyacertaintypeofmolecule.Facilitatedtransportandactivetransportbothrequirecarrierproteins.FacilitatedtransportDuringfacilitatedtransport,substancespassthroughacarrierproteinfollowingtheirconcentrationgradients.Facilitatedtransportdoesnotrequireenergy.Thecarrierproteinforglucosehastwoconformationsandswitchesbackandforthbetweenthetwo,carryingglucoseacrossthemembrane.FacilitateddiffusionofglucoseActivetransportDuringactivetransport,ionsormoleculesaremovedacrossthemembraneagainsttheconcentrationgradient–fromanareaoflowertohigherconcentration.EnergyintheformofATPisrequiredforthecarrierproteintocombinewiththetransportedmolecule.ActivetransportCarrierproteinsinvolvedinactivetransportarecalledpumps.Thesodium-potassiumpumpisactiveinallanimalcells,andmovessodiumionstotheoutsideofthecellandpotassiumionstotheinside.Thesodium-potassiumpumpcarrierproteinexistsintwoconformations;onethatmovessodiumtotheinside,andtheotherthatmovespotassiumoutofthecell.Thesodium-potassiumpumpExocytosisandEndocytosisDuringexocytosis,vesiclesfusewiththeplasmamembraneforsecretion.Somecellsarespecializedtoproduceandreleasespecificmolecules.Examplesincludereleaseofdigestiveenzymesfromcellsofthepancreas,orsecretionofthehormoneinsulininresponsetorisingbloodglucoselevels.ExocytosisEndocytosisDuringendocytosis,cellstakeinsubstancesbyinvaginatingaportionoftheplasmamembrane,andformingavesiclearoundthesubstance.Endocytosisoccursas:Phagocytosis–largeparticlesPinocytosis–smallparticlesReceptor-mediatedendocytosis–specificparticlesPhagocytosisPinocytosisReceptor-mediatedendocytosisSummaryThestructureoftheplasmamembraneallowsittobedifferentiallypermeable.Thefluidphospholipidbilayer,i