SM_c20CarbohydrateBiosynthesisinPlantsandBacteria

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1、c20CarbohydrateBiosynthesisinPlantsandBacteria.qxd12/7/127:33PMPageS-238chapterCarbohydrateBiosynthesisinPlants20andBacteria1.SegregationofMetabolisminOrganellesWhataretheadvantagestotheplantcellofhavingdifferentorganellestocarryoutdifferentreactionsequencesthatshareinte

2、rmediates?AnswerWithinorganelles,reactionintermediatesandenzymescanbemaintainedatdiffer-entlevelsfromthoseinthecytosolandinotherorganelles.Forexample,theATP/ADPratioislowerinmitochondriathaninthecytosolbecausetheroleofadeninenucleotidesinthemitochondrialmatrixistoaccepta

3、phosphorylgroup,whereastheroleinthecytosolisto++donateaphosphorylgroup.Similarly,differentNADH/NADandNADPH/NADPratiosreflectthereductive(biosynthetic)functionsofthecytosolandtheoxidative(catabolic)functionsofthemitochondrialmatrix.Bysegregatingreactionsequencesthatsharei

4、ntermediates,thecellcanregulatecatabolicandanabolicprocessesseparately.2.PhasesofPhotosynthesisWhenasuspensionofgreenalgaeisilluminatedintheabsenceofCO2141414andthenincubatedwithCO2inthedark,CO2isconvertedto[C]glucoseforabrieftime.Whatisthesignificanceofthisobservationwi

5、thregardtotheCO2-assimilationprocess,andhowisitrelatedto1414thelightreactionsofphotosynthesis?WhydoestheconversionofCO2to[C]glucosestopafterabrieftime?AnswerThisobservationsuggeststhatphotosynthesisoccursintwophases:(1)alight-dependentphasethatgeneratesATPandNADPH,whicha

6、reessentialforCO2fixation,and(2)alight-independent(dark)phase,inwhichtheseenergy-richcomponentsareusedforsynthesisofglucose.Intheabsenceofadditionalillumination,thesuppliesofNADPHandATPbecomeexhaustedandCO2fixationceases.3.IdentificationofKeyIntermediatesinCO2Assimilatio

7、nCalvinandhiscolleaguesusedtheunicellulargreenalgaChlorellatostudythecarbon-assimilationreactionsofphotosynthesis.They14incubatedCO2withilluminatedsuspensionsofalgaeandfollowedthetimecourseofappearanceof14Cintwocompounds,XandY,undertwosetsofconditions.Suggesttheidentitie

8、sofXandY,basedonyourunderstandingoftheCalvincycle.14(a)IlluminatedChlorellaweregrownwithunlabeledCO2,th