Chapter5LearningObjectives:1.Principlesofmembranetransport;2.Passivetransportandactivetransport;3.Twomainclassesofmembranetransportproteins:CarriersandChannels;4.Theiontransportsystems;5.EndocytosisandPhagocytosis:cellularuptakeofmacromoleculesandparticles.TheMovementofSubstancesAcrossCellMembranes1.PrinciplesofmembranetransportA.Theplasmamembraneisaselectivelypermeablebarrier.Itallowsforseparationandexchangeofmaterialsacrosstheplasmamembrane.B.Theprotein-freelipidbilayersarehighlyimpermeabletoions.C.Theenergeticsofsolutemovement:Diffusionisthespontaneousmovementofmaterialfromaregionofhighconcentrationtoaregionoflowconcentration.Thefree-energychangeduringdiffusionofnonelectrolytesdependsontheconcentrationgrdient.Thefree-energychangeduringdiffusionofelectrolytesdependsontheelectrochemicalgradient.D.Transportprocesseswithinaneukaryoticcell2.PassivetransportandactivetransportA.IonicdifferentiationinsideandoutsidecellB.Comparisonoftwoclassesoftransport.SolutescrossmembranebysimplediffusionDiffusionofsmallmoleculesacrossphospholipidbilayersIfunchargedsolutesaresmallenough,theycanmovedowntheirconcentrationgradientsdirectlyacrossthelipidbilayerbysimplediffusion.Mostsolutescancrossthemembraneonlyifthereisamembranetransportproteintotransferthem.Passivetransport,inthesamedirectionasaconcentrationgradient.Activetransport,ismediatedbycarrierproteins,againstaconcentrationgradient,requireaninputofenergy.C.TwoclassesofmembranetransportproteinsCarrierproteinsareresponsibleforboththepassiveandtheactivetransport.Channelproteinsareonlyresponsibleforpassivetransport.Carrierproteinsbindoneormoresolutemoleculesononesideofthemembraneandthenundergoaconformationalchangethattransferthesolutetotheothersideofthemembrane.Thecarrierprotein,theGlucosetransporter(GluT1)intheerythrocytePM,alterconformationtofacilitatethetransportofglucose.Facilitatediffusion:Protein-mediatedmovement,movementdownthegradientMostofthechannelproteinsareionchannels,includingthreetypes,withionchannelsthattheycanbeopenedandclosedThechannelproteins,incontrast,interactwiththesolutetobetransportedmuchmoreweakly.ThechannelproteinsfacilitatediffusionbyforminghydrophilictransmembranechannelsTransportthroughchannelproteinsoccursatamuchfasterratethantransportmediatedbycarrierproteins.3.Activetransport:Carrierprotein-mediatedmovementupthegradientA.Thisprocessdiffersfromfacilitateddiffusionintwocrucialaspects:Activetransportmaintainsthegradientsforpotassium,sodium,calcium,andotherionsacrossthecellmembrane.Alwaysmovessolutesupaconcentrationorelectrochemicalgradient;ActivetransportcouplesthemovementofsubstancesagainstgradientstoATPhydrolysis.i.eAlwaysrequirestheinputofenergy.B.CellscarryoutactivetransportinthreemainwaysCoupletheuphilltransportofonesoluteacrossmembranetothedownhilltransportofanother.CoupleuphilltransporttothehydrolysisofATPMainlyinbacteria,coupleuphilltransporttoaninputofenergyfromlight.C.DirectactivetransportdependsonfourtypesoftransportATPasesThefourclassesofATP-poweredtransportproteins:“P”typestandsforphosphorylation;ABC(ATP-bindingCassette)superfamily,bacteria—humans.Twotransmembrane(T)domainsandtwocytosolicATP-binding(A)domainsTheNa+-K+ATPase---AcouplingactivetransporttoATPhydrolysis.TheactivetransportofNa+/K+ATPaseisusedtomaintainselectrochemicaliongradients,andtherebymaintainscell’sexcitability.TheNa+/K+pumoisrequiredtomaintainosmoticbalanceandstabilizecellvolumeThebiologicalfunctionsofNa+/K+pumpformingaphosphorylatedproteinintermediateAModelMechanismfortheNa+/K+ATPaseOtherP-typepunps:includingH+andCa+ATPases,andH+/K+ATPasesPlantcellshaveaH+-transpotingplasmamembranepump.Thisprotonpumpplaysakeyroleinthesecondarytransportofsolutes,inthecontrolofcytosolicpH,andpossiblyincontrolofcellgrowthbymeansofacidificationoftheplantcellwall.Ca2+pump:Ca2+-ATPasepresentinboththeplasmamembraneandthemembranesoftheER.Itcontain10transmembranehelices.ThisCa2+pumpfunctionstoactivelytransportCa2+outofthecytosolintoeithertheextracellularspaceorthelumenoftheER.H+/K+ATPases(epithelialliningofthestomach):whichsecretesasolutionofconcentratedacid(upto0.16NHCl)intothestomuchchamber.TheV-typepump:utilizetheenergyofATPwithoutformingaphosphorylatedproteinintermediate.Vacuolar(V-type)pumpactivelytransportH+acrossthewallsofcytoplasmicorganellesandvacuoles.Theyprecentinlysosomes,secretorygranules,andplantcellvacuoles,havealsobeenfoundintheplasmamembranesofavarietyofcells(kidneytubules).TheABCtransporters(ATP-bindingcassette):Constitutethelargestfamilyofmembranetransportproteins.Inbacteria,ABCtransporteruseATPbindingandhydrolysistotransportmoleculesacrossthebilayer.TheeucaryoticABCtransporterpumphydrophobicdrugsoutofthecytosol.MDR(multidrugresistance)proteinoverexpressioninhumancancercells(40%).Inyeasts,ABCtransporterisresponsibleforexportingamatingpheromone.Inmostvertebratecells,ABCinERmembraneactivelytransportsawidevarietyofpeptidesfromthecytosolintotheER.4.IndirectactivetransportisdrivenbyIongradients-----CotransportA.Sugars,aminoacids,andotherorg