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简介：INTRODUCTIONThealternatingaccesstransportermodel(AATM)wasinitiallyconstructedtorationalizeandexplainafewreadilyquantifiableandreproducibleexperimentalfindings.TheAATM,asoriginallyconceived,consistsofsinglespecificbindingsitecentrallysituatedwithinthecellmembrane.Thesitealternatelyfacesinwardsandoutwards.Duringtheinversionprocess,thesitecantransportsugaracrossthemembrane,anddissociationintothealternatebathingsolutionresultsinnettransport.ReturnoftheemptysitereinitiatesthetransportcycleasillustratedinFig.1A.Glucosetransport,whichwasdemonstratedfirstinhumanredcells(LeFevreandLeFevre1952)andsheepplacenta(Widdas1951),hashighstereospecificityforD-pyranosesugars,likeD-glucoseandD-xyloseinpreferencetoL-sugars,e.g.L-fructoseornon-transportedL-glucose.Thistransportprocesshassimilaritiestoenzymekinetics:bothhavesaturationkinetics,theKmbeingtheconcentrationatwhichhalfmaximaltransportvelocity,Vmaxobtained,isameasureofapparentaffinityofligandforthetransporter(Fig.1B).Theprocessispassiveinthehumanredcell,sinceatequilibriumtheglucoseconcentrationsintheextraandintracellularsolutionsarethesame;i.e.netuphillaccumulationdoesnotoccur.

简介：AuxindistributionduringembryogenesisandseedgerminationwerestudiedwithtransgenicArabidopsisplantsexpressingGUSgenedrivenbyasyntheticDR5promoter,anauxinresponsivepromoter.TheresultsshowedthatGUSactivityishigherinendsofhypophysisandcotyledonprimordiaofheart-,torpedo-andcotyledon-stageembryos,leaftiparea,lateralrootprimordia,rootapexandcotyledonofyoungseedlings.AndGUSaccumulatedinrootapexoftheseedlingsgrownonauxintransportinhibitorcontainingmedia.Allthesesuggestedthatabove-mentionedpartoftheorgansandtissueshaveahigherlevelofauxin,andauxinpolartransportinhibitorcouldcausetheaccumulationofauxininrootapex.AndauxintransportinhibitoralsoresultedinaberrationofArabidopsisleafpatternformation,rootgravitropismandelongation.

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