5.12C: The Calvin Cycle - Biology LibreTexts

Key Points · The Calvin cycle refers to the light-independent reactions in photosynthesis that take place in three key steps. · Although the ... Skiptomaincontent KeyPointsKeyTermsTheCalvinCycleStage1:FixationStage2:ReductionStage3:Regeneration TheCalvincycleisorganizedintothreebasicstages:fixation,reduction,andregeneration. LearningObjectivesDescribetheCalvinCycle KeyPoints TheCalvincyclereferstothelight-independentreactionsinphotosynthesisthattakeplaceinthreekeysteps. AlthoughtheCalvinCycleisnotdirectlydependentonlight,itisindirectlydependentonlightsincethenecessaryenergycarriers(ATPandNADPH)areproductsoflight-dependentreactions. Infixation,thefirststageoftheCalvincycle,light-independentreactionsareinitiated;CO2isfixedfromaninorganictoanorganicmolecule. Inthesecondstage,ATPandNADPHareusedtoreduce3-PGAintoG3P;thenATPandNADPHareconvertedtoADPandNADP+,respectively. InthelaststageoftheCalvinCycle,RuBPisregenerated,whichenablesthesystemtoprepareformoreCO2tobefixed. KeyTerms light-independentreaction:chemicalreactionsduringphotosynthesisthatconvertcarbondioxideandothercompoundsintoglucose,takingplaceinthestroma rubisco:(ribulosebisphosphatecarboxylase)aplantenzymewhichcatalyzesthefixingofatmosphericcarbondioxideduringphotosynthesisbycatalyzingthereactionbetweencarbondioxideandRuBP ribulosebisphosphate:anorganicsubstancethatisinvolvedinphotosynthesis,reactswithcarbondioxidetoform3-PGA TheCalvinCycle Inplants,carbondioxide(CO2)enterstheleavesthroughstomata,whereitdiffusesovershortdistancesthroughintercellularspacesuntilitreachesthemesophyllcells.Onceinthemesophyllcells,CO2diffusesintothestromaofthechloroplast,thesiteoflight-independentreactionsofphotosynthesis.Thesereactionsactuallyhaveseveralnamesassociatedwiththem.Othernamesforlight-independentreactionsincludetheCalvincycle,theCalvin-Bensoncycle,anddarkreactions.Themostoutdatednameisdarkreactions,whichcanbemisleadingbecauseitimpliesincorrectlythatthereactiononlyoccursatnightorisindependentoflight,whichiswhymostscientistsandinstructorsnolongeruseit. Figure:LightReactions:Light-dependentreactionsharnessenergyfromthesuntoproducechemicalbonds,ATP,andNADPH.Theseenergy-carryingmoleculesaremadeinthestromawheretheCalvincycletakesplace.TheCalvincycleisnottotallyindependentoflightsinceitreliesonATPandNADH,whichareproductsofthelight-dependentreactions. Thelight-independentreactionsoftheCalvincyclecanbeorganizedintothreebasicstages:fixation,reduction,andregeneration. Figure:TheCalvinCycle:TheCalvincyclehasthreestages.Instage1,theenzymeRuBisCOincorporatescarbondioxideintoanorganicmolecule,3-PGA.Instage2,theorganicmoleculeisreducedusingelectronssuppliedbyNADPH.Instage3,RuBP,themoleculethatstartsthecycle,isregeneratedsothatthecyclecancontinue.Onlyonecarbondioxidemoleculeisincorporatedatatime,sothecyclemustbecompletedthreetimestoproduceasinglethree-carbonGA3Pmolecule,andsixtimestoproduceasix-carbonglucosemolecule. Stage1:Fixation Inthestroma,inadditiontoCO2,twoothercomponentsarepresenttoinitiatethelight-independentreactions:anenzymecalledribulosebisphosphatecarboxylase(RuBisCO)andthreemoleculesofribulosebisphosphate(RuBP).RuBPhasfiveatomsofcarbon,flankedbytwophosphates.RuBisCOcatalyzesareactionbetweenCO2andRuBP.ForeachCO2moleculethatreactswithoneRuBP,twomoleculesof3-phosphoglycericacid(3-PGA)form.3-PGAhasthreecarbonsandonephosphate.EachturnofthecycleinvolvesonlyoneRuBPandonecarbondioxideandformstwomoleculesof3-PGA.Thenumberofcarbonatomsremainsthesame,astheatomsmovetoformnewbondsduringthereactions(3atomsfrom3CO2+15atomsfrom3RuBP=18atomsin3atomsof3-PGA).ThisprocessiscalledcarbonfixationbecauseCO2is“fixed”fromaninorganicformintoorganicmolecules. Stage2:Reduction ATPandNADPHareusedtoconvertthesixmoleculesof3-PGAintosixmoleculesofachemicalcalledglyceraldehyde3-phosphate(G3P).Thisisareductionreactionbecauseitinvolvesthegainofelectronsby3-PGA.Recallthatareductionisthegainofanelectronbyanatomormolecule.SixmoleculesofbothATPandNADPHareused.ForATP,energyisreleasedwiththelossoftheterminalphosphateatom,convertingittoADP;forNADPH,bothenergyandahydrogenatomarelost,convertingitintoNADP+.Bothofthesemoleculesreturntothenearbylight-dependentreactionstobereusedandreenergized. Stage3:Regeneration Atthispoint,onlyoneoftheG3PmoleculesleavestheCalvincycleandissenttothecytoplasmtocontributetotheformationofothercompoundsneededbytheplant.BecausetheG3Pexportedfromthechloroplasthasthreecarbonatoms,ittakesthree“turns”oftheCalvincycletofixenoughnetcarbontoexportoneG3P.ButeachturnmakestwoG3Ps,thusthreeturnsmakesixG3Ps.OneisexportedwhiletheremainingfiveG3PmoleculesremaininthecycleandareusedtoregenerateRuBP,whichenablesthesystemtoprepareformoreCO2tobefixed.ThreemoremoleculesofATPareusedintheseregenerationreactions.