北京大学细胞生物学、物质的跨膜运输-医学资料

Chapter5LearningObjectives:1.Principlesofmembranetransport;2.Passivetransportandactivetransport;3.Twomainclassesofmembranetransportproteins:CarriersandChannels;4.Theiontransportsystems;5.EndocytosisandPhagocytosis:cellularuptakeofmacromoleculesandparticles.A.TheMovementofSubstancesAcrossCellMembranesAmotorneuroncellbodyinthespinalcord.(A)Manythousandsofnerveterminalssynapseonthecellbodyanddendrites.Thesedeliversignalsfromotherpartsoftheorganismtocontrolthefiringofactionpotentialsalongthesingleaxonofthislargecell.(B)Micrographshowinganervecellbodyanditsdendritesstainedwithafluorescentantibodythatrecognizesacytoskeletalprotein(green).Thousandsofaxonterminals(red)fromothernervecells(notvisible)makesynapsesonthecellbodyanddendrites;theyarestainedwithafluorescentantibodythatrecognizesaproteininsynapticvesicles.1.PrinciplesofmembranetransportA.TheplasmamembraneisaselectivelypermeableB.barrier.ItallowsforseparationandexchangeofC.materialsacrosstheplasmamembrane.Figure11-1Therelativepermeabilityofasyntheticlipidbilayertodifferentclassesofmolecules.Thesmallerthemoleculeand,moreimportant,thefewerhydrogenbondsitmakeswithwater,themorerapidlythemoleculediffusesacrossthebilayer.B.Theprotein-freelipidbilayersarehighlyimpermeabletoions.Ifunchargedsolutesaresmallenough,theycanmovedowntheirconcentrationgradientsdirectlyacrossthelipidbilayerbysimplediffusion.Mostsolutescancrossthemembraneonlyifthereisamembranetransportproteintotransferthem.Passivetransport,inthesamedirectionasaconcentrationgradient.Activetransport,ismediatedbycarrierproteins,againstaconcentrationgradient,requireaninputofenergy.DiffusionofsmallmoleculesacrossphospholipidbilayersFigure11-2Permeabilitycoefficients(cm/sec)forthepassageofvariousmoleculesthroughsyntheticlipidbilayers.Therateofflowofasoluteacrossthebilayerisdirectlyproportionaltothedifferenceinitsconcentrationonthetwosidesofthemembrane.Multiplyingthisconcentrationdifference(inmol/cm3)bythepermeabilitycoefficient(cm/sec)givestheflowofsoluteinmolespersecondpersquarecentimeterofmembrane.Aconcentrationdifferenceoftryptophanof10-4mol/cm3(10-4/10-3L=0.1M),forexample,wouldcauseaflowof10-4mol/cm3x10-7cm/sec=10-11mol/secthrough1cm2ofmembrane,or6x104molecules/secthrough1microns2ofmembrane.C.Theenergeticsofsolutemovement:Diffusionisthespontaneousmovementofmaterialfromaregionofhighconcentrationtoaregionoflowconcentration.Thefree-energychangeduringdiffusionofnonelectrolytesdependsontheconcentrationgrdient.Thefree-energychangeduringdiffusionofelectrolytesdependsontheelectrochemicalgrdient.D.Transportprocesseswithinaneukaryoticcell2.PassivetransportandactivetransportA.Comparisonoftwoclassesoftransport.Figure11-7Kineticsofsimplediffusioncomparedtocarrier-mediateddiffusion.Whereastherateoftheformerisalwaysproportionaltothesoluteconcentration,therateofthelatterreachesamaximum(Vmax)whenthecarrierproteinissaturated.Thesoluteconcentrationwhentransportisathalfitsmaximalvalueapproximatesthebindingconstant(KM)ofthecarrierforthesoluteandisanalogoustotheKMofanenzymeforitssubstrate.Thegraphappliestoacarriertransportingasinglesolute;thekineticsofcoupledtransportoftwoormoresolutes(seetext)aremorecomplexbutshowbasicallysimilarphenomena.B.TwoclassesofmembranetransportproteinsCarrierproteinsareresponsibleforboththepassiveandtheactivetransport.Channelproteinsareonlyresponsibleforpassivetransport.Figure11-8Threetypesofcarrier-mediatedtransport.Theschematicdiagramshowscarrierproteinsfunctioningasuniports,symports,andantiports.Carrierproteinsbindoneormoresolutemoleculesononesideofthemembraneandthenundergoaconformationalchangethattransferthesolutetotheothersideofthemembrane.Thecarrierprotein,theGlucosetransporter(GluT1)intheerythrocytePM,alterconformationtofacilitatethetransportofglucose.Facilitatediffusion:Protein-mediatedmovement,movementdownthegradientMostofthechannelproteinsareionchannels,includingthreetypes,withionchannelsthattheycanbeopenedandclosedFigure11-36.Amodelforthestructureoftheacetylcholinereceptor.Fivehomologoussubunits(a,a,b,g,d)combinetoformatransmembraneaqueouspore.Theporeislinedbyaringoffivetransmembraneahelices,onecontributedbyeachsubunit.Initsclosedconformation,theporeisthoughttobeoccludedbythehydrophobicsidechainsoffiveleucines,onefromeachahelix,whichformagatenearthemiddleofthelipidbilayer.Thenegativelychargedsidechainsateitherendoftheporeensurethatonlypositivelychargedionspassthroughthechannel.Bothoftheasubunitscontainanacetylcholine-bindingsite;whenacetylcholinebindstobothsites,thechannelundergoesaconformationalchangethatopensthegate,possiblybycausingtheleucinestomoveoutward.电压门控离子通道：铰链细胞失水应力激活的离子通道：2X1013N，0.04nm3.Activetransport:Carrierprotein-mediatedmovementupthegradientA.Thisprocessdiffersfromfacilitateddiffusionintwocrucialaspects:Activetransportmaintainsthegradientsforpotassium,sodium,calcium,andotherionsacrossthecellmembrane.Alwaysmovessolutesupaconcentrationorelectrochemicalgradient;ActivetransportcouplesthemovementofsubstancesagainstgradientstoATPhydrolysis.i.eAlwaysrequirestheinputofene