细胞质膜结构和膜上运输

1、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)nonpolart。

2、ailsextendtotheinterioroftheplasmamembrane.Plasmamembraneproteinsmaybeperipheralproteinsorintegralproteins.Asidefromphospholipid,cholesterolisanotherlipidinanimalplasmamembranes;relatedsteroidsarefoundinplants.Cholesterolstrengthenstheplasmamembrane.Whenphospholipidshavecarbohydratechainsattached,theyarecalledglycolipids.Whenproteinshavecarbohydratechainsattached,theyarecalledglycoproteins.Carbohydratechainsoccuronlyontheexteriorsurfaceoftheplasmamembrane.Theoutsideandinsidesurfacesoftheplasmame。

3、mbranearenotidentical.ChannelproteinCarrierproteinCellrecognitionproteinInanimalcells,thecarbohydratechainsofcellrecognitionproteinsarecollectivelycalledtheglycocalyx.Theglycocalyxcanfunctionincell-to-cellrecognition,adhesionbetweencells,andreceptionofsignalmolecules.Thediversityofcarbohydratechainsisenormous,providingeachindividualwithauniquecellular“fingerprint”.ReceptorproteinEnzymaticproteinThePermeabilityofthePlasmaMembraneTheplasmamembraneisdifferentiallypermeable.Macromoleculescannotpasst。

4、hroughbecauseofsize,andtinychargedmoleculesdonotpassthroughthenonpolarinteriorofthemembrane.Small,unchargedmoleculespassthroughthemembrane,followingtheirconcentrationgradient.HowmoleculescrosstheplasmamembraneMovementofmaterialsacrossamembranemaybepassiveoractive.Passivetransportdoesnotusechemicalenergy;diffusionandfacilitatedtransportarebothpassive.Activetransportrequireschemicalenergyandusuallyacarrierprotein.Exocytosisandendocytosistransportmacromoleculesacrossplasmamembranesusingvesicleforma。

5、tion,whichrequiresenergy.DiffusionDiffusionisthepassivemovementofmoleculesfromahighertoalowerconcentrationuntilequilibriumisreached.Gasesmovethroughplasmamembranesbydiffusion.ProcessofdiffusionGasexchangeinlungsoccursbydiffusionOsmosisThediffusionofwateracrossadifferentiallypermeablemembraneduetoconcentrationdifferencesiscalledosmosis.Diffusionalwaysoccursfromhighertolowerconcentration.Waterenterscellsduetoosmoticpressurewithincells.OsmosisdemonstrationOsmosisincellsAsolutioncontainsasolute(soli。

6、d)andasolvent(liquid).Cellsarenormallyisotonictotheirsurroundings,andthesoluteconcentrationisthesameinsideandoutofthecell.“Iso”meansthesameas,and“tonocity”referstothestrengthofthesolution.OsmosisinplantandanimalcellsHypotonicsolutionscausecellstoswellandpossiblyburst.“Hypo”meanslessthan.Animalcellsundergolysisinhypotonicsolution.Increasedturgorpressureoccursinplantcellsinhypotonicsolutions.Plantcellsdonotburstbecausetheyhaveacellwall.Hypertonicsolutionscausecellstolosewater.“Hyper”meansmorethan;。

7、hypertonicsolutionscontainmoresolute.Animalcellsundergocrenation(shrivel)inhypertonicsolutions.Plantcellsundergoplasmolysis,theshrinkingofthecytoplasm.TransportbyCarrierProteinsSomebiologicallyusefulmoleculespassthroughtheplasmamembranebecauseofchannelproteinsandcarrierproteinsthatspanthemembrane.Carrierproteinsarespecificandcombinewithonlyacertaintypeofmolecule.Facilitatedtransportandactivetransportbothrequirecarrierproteins.FacilitatedtransportDuringfacilitatedtransport,substancespassthroughac。

8、arrierproteinfollowingtheirconcentrationgradients.Facilitatedtransportdoesnotrequireenergy.Thecarrierproteinforglucosehastwoconformationsandswitchesbackandforthbetweenthetwo,carryingglucoseacrossthemembrane.FacilitateddiffusionofglucoseActivetransportDuringactivetransport,ionsormoleculesaremovedacrossthemembraneagainsttheconcentrationgradient–fromanareaoflowertohigherconcentration.EnergyintheformofATPisrequiredforthecarrierproteintocombinewiththetransportedmolecule.ActivetransportCarrierproteins。

9、involvedinactivetransportarecalledpumps.Thesodium-potassiumpumpisactiveinallanimalcells,andmovessodiumionstotheoutsideofthecellandpotassiumionstotheinside.Thesodium-potassiumpumpcarrierproteinexistsintwoconformations;onethatmovessodiumtotheinside,andtheotherthatmovespotassiumoutofthecell.Thesodium-potassiumpumpExocytosisandEndocytosisDuringexocytosis,vesiclesfusewiththeplasmamembraneforsecretion.Somecellsarespecializedtoproduceandreleasespecificmolecules.Examplesincludereleaseofdigestiveenzymesf。

10、romcellsofthepancreas,orsecretionofthehormoneinsulininresponsetorisingbloodglucoselevels.ExocytosisEndocytosisDuringendocytosis,cellstakeinsubstancesbyinvaginatingaportionoftheplasmamembrane,andformingavesiclearoundthesubstance.Endocytosisoccursas:Phagocytosis–largeparticlesPinocytosis–smallparticlesReceptor-mediatedendocytosis–specificparticlesPhagocytosisPinocytosisReceptor-mediatedendocytosisSummaryThestructureoftheplasmamembraneallowsittobedifferentiallypermeable.Thefluidphospholipidbilayer,i。