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1、8885d_c20_751–7862/18/041:56PMPage751mac76mac76:385_reb:chapter20CARBOHYDRATEBIOSYNTHESISINPLANTSANDBACTERIA20.1PhotosyntheticCarbohydrateSynthesis751Plantsmustbeespeciallyversatileintheirhandling20.2PhotorespirationandtheC4andCAMofcarbohydrates,forseveralr
2、easons.First,plantsarePathways766autotrophs,abletoconvertinorganiccarbon(asCO2)intoorganiccompounds.Second,biosynthesisoccurs20.3BiosynthesisofStarchandSucrose771primarilyinplastids,membrane-boundedorganelles20.4SynthesisofCellWallPolysaccharides:Plantuniqu
3、etoplants,andthemovementofintermediatesbe-CelluloseandBacterialPeptidoglycan775tweencellularcompartmentsisanimportantaspectof20.5IntegrationofCarbohydrateMetabolisminthemetabolism.Third,plantsarenotmotile:theycannotPlantCell780movetofindbettersuppliesofwate
4、r,sunlight,ornutri-ents.Theymusthavesufficientmetabolicflexibilitytoallowthemtoadapttochangingconditionsintheplace...thediscoveryofthelong-livedisotopeofcarbon,wheretheyarerooted.Finally,plantshavethickcellwallsmadeofcarbohydratepolymers,whichmustbeassem-ca
5、rbon-14,bySamuelRubenandMartinKamenin1940bledoutsidetheplasmamembraneandwhichconstituteprovidedtheidealtoolforthetracingoftheroutealongasignificantproportionofthecell’scarbohydrate.whichcarbondioxidetravelsonitswaytocarbohydrate.Thechapterbeginswithadescrip
6、tionoftheprocess—MelvinCalvin,NobelAddress,1961bywhichCO2isassimilatedintotriosesandhexoses,thenconsidersphotorespiration,animportantsidere-actionduringCO2fixation,andthewaysinwhichcer-ehavenowreachedaturningpointinourstudyoftainplantsavoidthissidereaction.
7、WethenlookathowWcellularmetabolism.ThusfarinPartIIwehavede-thebiosynthesisofsucrose(forsugartransport)andscribedhowthemajormetabolicfuels—carbohydrates,starch(forenergystorage)isaccomplishedbymech-fattyacids,andaminoacids—aredegradedthroughcon-anismsanalogo
8、ustothoseemployedbyanimalcellstovergingcatabolicpathwaystoenterthecitricacidcy-makeglycogen.Thenexttopicisthesynthesisofthecleandyieldtheirelectronstotherespiratorychain,celluloseofplantcellwallsandthe
