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Photosynthesis--thelegendofPrometheus

OutlineBiochemistryofPhotosynthesisThelightreaction:harvestinglightenergyThedarkreaction:CO2fixationC4PhotosynthesisPhotorespirationC4plants–asolutionoflowCO2environmentCAMPhotosynthesis–adaptationtodroughtCanC4andCAMphotosynthesispathwaysexistinthesameplant?Photosynthesis:Lightand“Dark”Reactions6CO2+12H2O

C6H12O6+6H2O+6O2Photosynthesis:Lightand“Dark”ReactionsPhotonAbsorptionbyPigmentsFourstagesofthelightreactionsPrimaryPhotoevent–lightabsorptionbyapigmentandanelectronisexcited ChargeSeparation–reactioncentertransfersanelectrontoanacceptormoleculeElectrontransport–excitedelectronshuttledfromonecompoundtothenextresultingintheformationofaprotongradientChemiosmosis–theprotongradientisusedtoformATPHundredsofpigmentmoleculesareclusteredinantennacomplexes

(680nm)(700nm)11433222H2O+2NADP++2ADP+2Pi2NADH2+2ATP+O28hυP700+P680Pi+Φa[O2]max=0.125LightReactionsofPhotosynthesisLightenergyisAbsorbedElectronstransported(RedoxReaction)FromH2OToNADPH

ATPisformedOxygenisproducedLightindependentor“Dark”ReactionsRequirestheproductsoftheLightReactionsStroma(inChloroplast)CarbondioxidefixationSugarformationEnzymemediatedMultiplevariationsRibulose1,5-bisphosphateCarboxylaseOxygenaseRubisCOnCO2+nAcc+2nNADPH2+2nATP(CH2O)2+2nNADP++2nADP+2nPi+nAcc+nH2OEnzymePhotosynthesiscapturesenergyfromsunlightforthebiosphere–Prometheus,whobroughtthehumanbeingsfiretheconversionoflight-energytochemical-energyinthechloroplastsofeukaryoticcellshighefficiency(20-30%ofabsorbedlight),using2-3%ofirradiationgenerates120billionmetrictonsofsugarannually-ultimatelynearlyallofthecarboninthebiosphere!istheprocessbywhichenergyentersthebiosphereHasproducedalloftheOxygenintheAtmosphereC4PhotosynthesisThefirstproductofcarbonfixationhas4carbonatomsC4photosynthesisaddoneCO2concentratingsystemonthebaseofC3photosynthesisAdaptationtolowatmosphereCO2concentrationinearthhistoryTHEDUALCATALYTICNATUREOFRUBISCORUBISCOCO2O2CO2PhotorespirationPhotosynthesisRuBPRuBPPGA2PGAATPsugarATPNADPHATP+NADPHPGA+PGATPCatalyzesCO2+RuBP2PGAAlsocatalyzes02+RuBPPGA+PGSubstratelimitedatcurrentCO2levelsInefficient,unusuallyslowturnoverrateLarge,expensiveenzyme,15%to30%ofallleafnitrogeninC3plantsTheonlycarboxylasethatcatalyzesnetfixationofcarbonfrominorganictoorganicformsinallphotosyntheticorganisms.Rubisco

Ribulose-1,5-bisphosphatecarboxylase/oxygenase

Thisisnotquiteascrazyasitseems,remembertheatmosphereis210,000ppmO2andonly360ppmCO2!

At25°CCaboxylation:Oxygenation≈4:1TheRelativeRateofPhotorespirationasaFunctionofCO2andTemperatureLongTermCO2Record550myrs“SnowBallEarth”Event440myrsFirstlandplant400myrsFirsttreelikeplant360myrsFirstSoilLargeglacialeventSpreadofvascularplantsSolutionstothePhotorespiratoryProblem1.EvolveanewRubisco2.OpentheStomata,lettingmoreCO2in3.Reduceleaftemperature3.ReduceO2aroundRubisco4.ConcentrateCO2aroundRubiscoASchematicofC4PhotosynthesisCO2HCO3-PEPCO2PCRcyclesugarsxylemphloemExportOAAC4acidDCC3acidPyruvateATPAMPPPi2PiCytosolMesophyllTissueBundleSheathTissuePPDKRUBISCORUBISCORUBISCOPco2~150μbarPco2~1500μbarCHLPEPCAbbreviations:DC,decarboxylatingenzyme;PEPC,PEPcarboxylase;PPDK,pyruvate,phosphatedikinaseC3leafcrosssectionMesophyllcellsBundlesheathcellsC4leafcrosssectionMesophyllcellsLightUseEfficiencyofC3

versusC4Plants800Myrs600MyrsC4speciesandC3–C4intermediatesExamplesofC4species:Corn,sorghum,millets,mostsummerweeds,tropicalandsubtropicalgrassesHot,highlighttropical,subtropicalandsummertemperatehabitatsAlligatorweedproject:thegenusAlternantherahasC3(A.sessilis),C4(A.pungen)andC3-C4(A.ficoides,A.tenella)species.Weproposetotestingwhether(A.philoxeroides)canchangefromC3toC4photosynthesisinhot,dry,highlightenvironmentCAM(CrassulaceanAcidMetabolism)PhotosynthesisChemicalreactionsareverysimilartoC4plantCAMplantssegregateC4andC3cyclesintimePEPcarboxylationoccursatnightandOAAisstoredinvacuoleCO2releasedandfixedbyRubiscoduringthedayAdaptationtodrought,mainlyoccursindesertplantsandepiphytesIncompatibilitybetweenC4andCAMphotosynthesisDifferentphysiological/biochemicalregulationDifferentrequirementinstructureEvolutionpathwayCurrently,wedonotfindanyC4-CAMplantPEPCarboxylaseRegulationC4PhotosynthesisCAMPhotosynthesisLight~CircadianoscillatorPEPCasekinasePEPCase-oPEPCase-o-PPhosphatase-2APEPCaseActivationCyclemoreactivemalateinsensitiveG6PsensitivelessactivemalatesensitiveG6PinsensitivetonoplastmalatepermeabilitycytosolicmalatecontentnuclearsignalsproteinsynthesisATPandNADPHcontentPGAexportfrombundlesheathcytosolicpHcalciumchannelactivitycalcium-dependent