Photosynthesis - Wikipedia

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, ... Photosynthesis FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Biologicalprocesstoconvertlightintochemicalenergy Schematicofphotosynthesisinplants.Thecarbohydratesproducedarestoredinorusedbytheplant. Compositeimageshowingtheglobaldistributionofphotosynthesis,includingbothoceanicphytoplanktonandterrestrialvegetation.Darkredandblue-greenindicateregionsofhighphotosyntheticactivityintheoceanandonland,respectively. Photosynthesisisaprocessusedbyplantsandotherorganismstoconvertlightenergyintochemicalenergythat,throughcellularrespiration,canlaterbereleasedtofueltheorganism'sactivities.Someofthischemicalenergyisstoredincarbohydratemolecules,suchassugarsandstarches,whicharesynthesizedfromcarbondioxideandwater–hencethenamephotosynthesis,fromtheGreekphōs(φῶς),"light",andsunthesis(σύνθεσις),"puttingtogether".[1][2][3]Inmostcases,oxygenisalsoreleasedasawasteproductthatstoresthreetimesmorechemicalenergythanthecarbohydrates.[4]Mostplants,algae,andcyanobacteriaperformphotosynthesis;suchorganismsarecalledphotoautotrophs.PhotosynthesisislargelyresponsibleforproducingandmaintainingtheoxygencontentoftheEarth'satmosphere,andsuppliesmostoftheenergynecessaryforlifeonEarth.[5] Althoughphotosynthesisisperformeddifferentlybydifferentspecies,theprocessalwaysbeginswhenenergyfromlightisabsorbedbyproteinscalledreactioncentersthatcontaingreenchlorophyll(andothercolored)pigments/chromophores.Inplants,theseproteinsareheldinsideorganellescalledchloroplasts,whicharemostabundantinleafcells,whileinbacteriatheyareembeddedintheplasmamembrane.Intheselight-dependentreactions,someenergyisusedtostripelectronsfromsuitablesubstances,suchaswater,producingoxygengas.Thehydrogenfreedbythesplittingofwaterisusedinthecreationoftwofurthercompoundsthatserveasshort-termstoresofenergy,enablingitstransfertodriveotherreactions:thesecompoundsarereducednicotinamideadeninedinucleotidephosphate(NADPH)andadenosinetriphosphate(ATP),the"energycurrency"ofcells. Inplants,algaeandcyanobacteria,sugarsaresynthesizedbyasubsequentsequenceoflight-independentreactionscalledtheCalvincycle.IntheCalvincycle,atmosphericcarbondioxideisincorporatedintoalreadyexistingorganiccarboncompounds,suchasribulosebisphosphate(RuBP).[6]UsingtheATPandNADPHproducedbythelight-dependentreactions,theresultingcompoundsarethenreducedandremovedtoformfurthercarbohydrates,suchasglucose.Inotherbacteria,differentmechanismssuchasthereverseKrebscycleareusedtoachievethesameend. Thefirstphotosyntheticorganismsprobablyevolvedearlyintheevolutionaryhistoryoflifeandmostlikelyusedreducingagentssuchashydrogenorhydrogensulfide,ratherthanwater,assourcesofelectrons.[7]Cyanobacteriaappearedlater;theexcessoxygentheyproducedcontributeddirectlytotheoxygenationoftheEarth,[8]whichrenderedtheevolutionofcomplexlifepossible.Today,theaveragerateofenergycapturebyphotosynthesisgloballyisapproximately130 terawatts,[9][10][11]whichisabouteighttimesthecurrentpowerconsumptionofhumancivilization.[12]Photosyntheticorganismsalsoconvertaround100–115billiontons(91–104Pgpetagrams,orbillionmetrictons),ofcarbonintobiomassperyear.[13][14]Thatplantsreceivesomeenergyfromlight–inadditiontoair,soil,andwater–wasfirstdiscoveredin1779byJanIngenhousz. Photosynthesisisvitalforclimateprocesses,asitcapturescarbondioxidefromtheairandthenbindscarboninplantsandfurtherinsoilsandharvestedproducts.Cerealsaloneareestimatedtobind3,825Tg(teragrams)or3.825Pg(petagrams)ofcarbondioxideeveryyear,i.e.3.825billionmetrictons.[15] Contents 1Overview 2Photosyntheticmembranesandorganelles 3Light-dependentreactions 3.1Zscheme 3.2Waterphotolysis 4Light-independentreactions 4.1Calvincycle 4.2Carbonconcentratingmechanisms 4.2.1Onland 4.2.2Inwater 5Orderandkinetics 6Efficiency 7Evolution 7.1Symbiosisandtheoriginofchloroplasts 7.2Photosyntheticeukaryoticlineages 7.3Cyanobacteriaandtheevolutionofphotosynthesis 8Experimentalhistory 8.1Discovery 8.2Refinements 8.3Developmentoftheconcept 8.4C3 :C4photosynthesisresearch 9Factors 9.1Lightintensity(irradiance),wavelengthandtemperature 9.2Carbondioxidelevelsandphotorespiration 10Seealso 11References 12Furtherreading 12.1Books 12.2Papers 13Externallinks Overview Photosynthesischangessunlightintochemicalenergy,splitswatertoliberateO2,andfixesCO2intosugar. Mostphotosyntheticorganismsarephotoautotrophs,whichmeansthattheyareabletosynthesizefooddirectlyfromcarbondioxideandwaterusingenergyfromlight.However,notallorganismsusecarbondioxideasasourceofcarbonatomstocarryoutphotosynthesis;photoheterotrophsuseorganiccompounds,ratherthancarbondioxide,asasourceofcarbon.[5]Inplants,algae,andcyanobacteria,photosynthesisreleasesoxygen.Thisoxygenicphotosynthesisisbyfarthemostcommontypeofphotosynthesisusedbylivingorganisms.Althoughtherearesomedifferencesbetweenoxygenicphotosynthesisinplants,algae,andcyanobacteria,theoverallprocessisquitesimilarintheseorganisms.Therearealsomanyvarietiesofanoxygenicphotosynthesis,usedmostlybybacteria,whichconsumecarbondioxidebutdonotreleaseoxygen. Carbondioxideisconvertedintosugarsinaprocesscalledcarbonfixation;photosynthesiscapturesenergyfromsunlighttoconvertcarbondioxideintocarbohydrate.Carbonfixationisanendothermicredoxreaction.Ingeneraloutline,photosynthesisistheoppositeofcellularrespiration:whilephotosynthesisisaprocessofreductionofcarbondioxidetocarbohydrate,cellularrespirationistheoxidationofcarbohydrateorothernutrientstocarbondioxide.Nutrientsusedincellularrespirationincludecarbohydrates,aminoacidsandfattyacids.Thesenutrientsareoxidizedtoproducecarbondioxideandwater,andtoreleasechemicalenergytodrivetheorganism'smetabolism.Photosynthesisandcellularrespirationaredistinctprocesses,astheytakeplacethroughdifferentsequencesofchemicalreactionsandindifferentcellularcompartments. ThegeneralequationforphotosynthesisasfirstproposedbyCornelisvanNielis:[16] CO2carbondioxide+2H2Aelectrondonor+photonslightenergy→[CH2O]carbohydrate+2Aoxidizedelectrondonor+H2Owater Sincewaterisusedastheelectrondonorinoxygenicphotosynthesis,theequationforthisprocessis: CO2carbondioxide+2H2Owater+photonslightenergy→[CH2O]carbohydrate+O2oxygen+H2Owater Thisequationemphasizesthatwaterisbothareactantinthelight-dependentreactionandaproductofthelight-independentreaction,butcancelingnwatermoleculesfromeachsidegivesthenetequation: CO2carbondioxide+H2Owater+photonslightenergy→[CH2O]carbohydrate+O2oxygen Otherprocessessubstituteothercompounds(suchasarsenite)forwaterintheelectron-supplyrole;forexamplesomemicrobesusesunlighttooxidizearsenitetoarsenate:[17]Theequationforthisreactionis: CO2carbondioxide+(AsO3−3)arsenite+photonslightenergy→(AsO3−4)arsenate+COcarbonmonoxide(usedtobuildothercompoundsinsubsequentreactions)[18] Photosynthesisoccursintwostages.Inthefirststage,light-dependentreactionsorlightreactionscapturetheenergyoflightanduseittomakethehydrogencarrierNADPHandtheenergy-storagemoleculeATP.Duringthesecondstage,thelight-independentreactionsusetheseproductstocaptureandreducecarbondioxide. Mostorganismsthatutilizeoxygenicphotosynthesisusevisiblelightforthelight-dependentreactions,althoughatleastthreeuseshortwaveinfraredor,morespecifically,far-redradiation.[19] Someorganismsemployevenmoreradicalvariantsofphotosynthesis.Somearchaeauseasimplermethodthatemploysapigmentsimilartothoseusedforvisioninanimals.Thebacteriorhodopsinchangesitsconfigurationinresponsetosunlight,actingasaprotonpump.Thisproducesaprotongradientmoredirectly,whichisthenconvertedtochemicalenergy.Theprocessdoesnotinvolvecarbondioxidefixationanddoesnotreleaseoxygen,andseemstohaveevolvedseparatelyfromthemorecommontypesofphotosynthesis.[20][21] Photosyntheticmembranesandorganelles Mainarticles:ChloroplastandThylakoid Chloroplastultrastructure:outermembraneintermembranespaceinnermembrane(1+2+3:envelope)stroma(aqueousfluid)thylakoidlumen(insideofthylakoid)thylakoidmembranegranum(stackofthylakoids)thylakoid(lamella)starchribosomeplastidialDNAplastoglobule(dropoflipids) Inphotosyntheticbacteria,theproteinsthatgatherlightforphotosynthesisareembeddedincellmembranes.Initssimplestform,thisinvolvesthemembranesurroundingthecellitself.[22]However,themembranemaybetightlyfoldedintocylindricalsheetscalledthylakoids,[23]orbunchedupintoroundvesiclescalledintracytoplasmicmembranes.[24]Thesestructurescanfillmostoftheinteriorofacell,givingthemembraneaverylargesurfaceareaandthereforeincreasingtheamountoflightthatthebacteriacanabsorb.[23] Inplantsandalgae,photosynthesistakesplaceinorganellescalledchloroplasts.Atypicalplantcellcontainsabout10to100chloroplasts.Thechloroplastisenclosedbyamembrane.Thismembraneiscomposedofaphospholipidinnermembrane,aphospholipidoutermembrane,andanintermembranespace.Enclosedbythemembraneisanaqueousfluidcalledthestroma.Embeddedwithinthestromaarestacksofthylakoids(grana),whicharethesiteofphotosynthesis.Thethylakoidsappearasflatteneddisks.Thethylakoiditselfisenclosedbythethylakoidmembrane,andwithintheenclosedvolumeisalumenorthylakoidspace.Embeddedinthethylakoidmembraneareintegralandperipheralmembraneproteincomplexesofthephotosyntheticsystem. Plantsabsorblightprimarilyusingthepigmentchlorophyll.Thegreenpartofthelightspectrumisnotabsorbedbutisreflectedwhichisthereasonthatmostplantshaveagreencolor.Besideschlorophyll,plantsalsousepigmentssuchascarotenesandxanthophylls.[25]Algaealsousechlorophyll,butvariousotherpigmentsarepresent,suchasphycocyanin,carotenes,andxanthophyllsingreenalgae,phycoerythrininredalgae(rhodophytes)andfucoxanthininbrownalgaeanddiatomsresultinginawidevarietyofcolors. Thesepigmentsareembeddedinplantsandalgaeincomplexescalledantennaproteins.Insuchproteins,thepigmentsarearrangedtoworktogether.Suchacombinationofproteinsisalsocalledalight-harvestingcomplex.[26] Althoughallcellsinthegreenpartsofaplanthavechloroplasts,themajorityofthosearefoundinspeciallyadaptedstructurescalledleaves.Certainspeciesadaptedtoconditionsofstrongsunlightandaridity,suchasmanyEuphorbiaandcactusspecies,havetheirmainphotosyntheticorgansintheirstems.Thecellsintheinteriortissuesofaleaf,calledthemesophyll,cancontainbetween450,000and800,000chloroplastsforeverysquaremillimeterofleaf.Thesurfaceoftheleafiscoatedwithawater-resistantwaxycuticlethatprotectstheleaffromexcessiveevaporationofwateranddecreasestheabsorptionofultravioletorbluelighttominimizeheating.Thetransparentepidermislayerallowslighttopassthroughtothepalisademesophyllcellswheremostofthephotosynthesistakesplace. Light-dependentreactions Mainarticle:Light-dependentreactions Light-dependentreactionsofphotosynthesisatthethylakoidmembrane Inthelight-dependentreactions,onemoleculeofthepigmentchlorophyllabsorbsonephotonandlosesoneelectron.Thiselectronistakenupbyamodifiedformofchlorophyllcalledpheophytin,whichpassestheelectrontoaquinonemolecule,startingtheflowofelectronsdownanelectrontransportchainthatleadstotheultimatereductionofNADPtoNADPH.Inaddition,thiscreatesaprotongradient(energygradient)acrossthechloroplastmembrane,whichisusedbyATPsynthaseinthesynthesisofATP.Thechlorophyllmoleculeultimatelyregainstheelectronitlostwhenawatermoleculeissplitinaprocesscalledphotolysis,whichreleasesadioxygen(O2)moleculeasahigh-energywasteproduct. Theoverallequationforthelight-dependentreactionsundertheconditionsofnon-cyclicelectronflowingreenplantsis:[27] 2H2O+2NADP++3ADP+3Pi+light→2NADPH+2H++3ATP+O2 Notallwavelengthsoflightcansupportphotosynthesis.Thephotosyntheticactionspectrumdependsonthetypeofaccessorypigmentspresent.Forexample,ingreenplants,theactionspectrumresemblestheabsorptionspectrumforchlorophyllsandcarotenoidswithabsorptionpeaksinviolet-blueandredlight.Inredalgae,theactionspectrumisblue-greenlight,whichallowsthesealgaetousetheblueendofthespectrumtogrowinthedeeperwatersthatfilteroutthelongerwavelengths(redlight)usedbyabove-groundgreenplants.Thenon-absorbedpartofthelightspectrumiswhatgivesphotosyntheticorganismstheircolor(e.g.,greenplants,redalgae,purplebacteria)andistheleasteffectiveforphotosynthesisintherespectiveorganisms. Zscheme The"Zscheme" Inplants,light-dependentreactionsoccurinthethylakoidmembranesofthechloroplastswheretheydrivethesynthesisofATPandNADPH.Thelight-dependentreactionsareoftwoforms:cyclicandnon-cyclic. Inthenon-cyclicreaction,thephotonsarecapturedinthelight-harvestingantennacomplexesofphotosystemIIbychlorophyllandotheraccessorypigments(seediagramatright).Theabsorptionofaphotonbytheantennacomplexloosensanelectronbyaprocesscalledphotoinducedchargeseparation.TheantennasystemisatthecoreofthechlorophyllmoleculeofthephotosystemIIreactioncenter.Thatloosenedelectronistakenupbytheprimaryelectron-acceptormolecule,pheophytin.Astheelectronsareshuttledthroughanelectrontransportchain(theso-calledZ-schemeshowninthediagram),achemiosmoticpotentialisgeneratedbypumpingprotoncations(H+)acrossthemembraneandintothethylakoidspace.AnATPsynthaseenzymeusesthatchemiosmoticpotentialtomakeATPduringphotophosphorylation,whereasNADPHisaproductoftheterminalredoxreactionintheZ-scheme.TheelectronentersachlorophyllmoleculeinPhotosystemI.Thereitisfurtherexcitedbythelightabsorbedbythatphotosystem.Theelectronisthenpassedalongachainofelectronacceptorstowhichittransferssomeofitsenergy.Theenergydeliveredtotheelectronacceptorsisusedtomovehydrogenionsacrossthethylakoidmembraneintothelumen.Theelectroniseventuallyusedtoreducetheco-enzymeNADPwithaH+toNADPH(whichhasfunctionsinthelight-independentreaction);atthatpoint,thepathofthatelectronends. Thecyclicreactionissimilartothatofthenon-cyclicbutdiffersinthatitgeneratesonlyATP,andnoreducedNADP(NADPH)iscreated.ThecyclicreactiontakesplaceonlyatphotosystemI.Oncetheelectronisdisplacedfromthephotosystem,theelectronispasseddowntheelectronacceptormoleculesandreturnstophotosystemI,fromwhereitwasemitted,hencethenamecyclicreaction. Waterphotolysis Mainarticles:PhotodissociationandOxygenevolution Linearelectrontransportthroughaphotosystemwillleavethereactioncenterofthatphotosystemoxidized.Elevatinganotherelectronwillfirstrequirere-reductionofthereactioncenter.Theexcitedelectronslostfromthereactioncenter(P700)ofphotosystemIarereplacedbytransferfromplastocyanin,whoseelectronscomefromelectrontransportthroughphotosystemII.PhotosystemII,asthefirststepoftheZ-scheme,requiresanexternalsourceofelectronstoreduceitsoxidized,high-energychlorophyllareactioncenter,calledP680+.[4]Thesourceofelectronsforphotosynthesisingreenplantsandcyanobacteriaiswater.Twowatermoleculesareoxidizedbytheenergyoffoursuccessivecharge-separationreactionsofphotosystemIItoyieldamoleculeofdiatomicoxygenandfourhydrogenions.Theelectronsyieldedaretransferredtoaredox-activetyrosineresiduethatisoxidizedbytheenergyofP680+.ThisresetstheabilityofP680toabsorbanotherphotonandreleaseanotherphoto-dissociatedelectron.TheoxidationofwateriscatalyzedinphotosystemIIbyaredox-activestructurethatcontainsfourmanganeseionsandacalciumion;thisoxygen-evolvingcomplexbindstwowatermoleculesandcontainsthefouroxidizingequivalentsthatareusedtodrivethewater-oxidizingreaction(Kok'sS-statediagrams).PhotosystemIIistheonlyknownbiologicalenzymethatcarriesouttheoxidationofwater.[4]ThehydrogenionsarereleasedinthethylakoidlumenandthereforecontributetothetransmembranechemiosmoticpotentialthatleadstoATPsynthesis.Oxygenisawasteproductoflight-dependentreactions,butthemajorityoforganismsonEarthuseoxygenanditsenergyforcellularrespiration,includingphotosyntheticorganisms.[28][29] Light-independentreactions Calvincycle Mainarticles:Light-independentreactionsandCarbonfixation Inthelight-independent(or"dark")reactions,theenzymeRuBisCOcapturesCO2fromtheatmosphereand,inaprocesscalledtheCalvincycle,usesthenewlyformedNADPHandreleasesthree-carbonsugars,whicharelatercombinedtoformsucroseandstarch.Theoverallequationforthelight-independentreactionsingreenplantsis[27]: 128  3CO2+9ATP+6NADPH+6H+→C3H6O3-phosphate+9ADP+8Pi+6NADP++3H2O OverviewoftheCalvincycleandcarbonfixation Carbonfixationproducesthethree-carbonsugarintermediate,whichisthenconvertedintothefinalcarbohydrateproducts.Thesimplecarbonsugarsproducedbyphotosynthesisarethenusedtoformotherorganiccompounds,suchasthebuildingmaterialcellulose,theprecursorsforlipidandaminoacidbiosynthesis,orasafuelincellularrespiration.Thelatteroccursnotonlyinplantsbutalsoinanimalswhenthecarbonandenergyfromplantsispassedthroughafoodchain. Thefixationorreductionofcarbondioxideisaprocessinwhichcarbondioxidecombineswithafive-carbonsugar,ribulose1,5-bisphosphate,toyieldtwomoleculesofathree-carboncompound,glycerate3-phosphate,alsoknownas3-phosphoglycerate.Glycerate3-phosphate,inthepresenceofATPandNADPHproducedduringthelight-dependentstages,isreducedtoglyceraldehyde3-phosphate.Thisproductisalsoreferredtoas3-phosphoglyceraldehyde(PGAL)or,moregenerically,astriosephosphate.Most(5outof6molecules)oftheglyceraldehyde3-phosphateproducedareusedtoregenerateribulose1,5-bisphosphatesotheprocesscancontinue.Thetriosephosphatesnotthus"recycled"oftencondensetoformhexosephosphates,whichultimatelyyieldsucrose,starchandcellulose.Thesugarsproducedduringcarbonmetabolismyieldcarbonskeletonsthatcanbeusedforothermetabolicreactionsliketheproductionofaminoacidsandlipids. Carbonconcentratingmechanisms Onland Mainarticles:C4carbonfixation,CAMphotosynthesis,andAlarmphotosynthesis OverviewofC4carbonfixation Inhotanddryconditions,plantsclosetheirstomatatopreventwaterloss.Undertheseconditions,CO2willdecreaseandoxygengas,producedbythelightreactionsofphotosynthesis,willincrease,causinganincreaseofphotorespirationbytheoxygenaseactivityofribulose-1,5-bisphosphatecarboxylase/oxygenaseanddecreaseincarbonfixation.SomeplantshaveevolvedmechanismstoincreasetheCO2concentrationintheleavesundertheseconditions.[30] PlantsthatusetheC4carbonfixationprocesschemicallyfixcarbondioxideinthecellsofthemesophyllbyaddingittothethree-carbonmoleculephosphoenolpyruvate(PEP),areactioncatalyzedbyanenzymecalledPEPcarboxylase,creatingthefour-carbonorganicacidoxaloaceticacid.OxaloaceticacidormalatesynthesizedbythisprocessisthentranslocatedtospecializedbundlesheathcellswheretheenzymeRuBisCOandotherCalvincycleenzymesarelocated,andwhereCO2releasedbydecarboxylationofthefour-carbonacidsisthenfixedbyRuBisCOactivitytothethree-carbon3-phosphoglycericacids.ThephysicalseparationofRuBisCOfromtheoxygen-generatinglightreactionsreducesphotorespirationandincreasesCO2fixationand,thus,thephotosyntheticcapacityoftheleaf.[31]C4plantscanproducemoresugarthanC3plantsinconditionsofhighlightandtemperature.ManyimportantcropplantsareC4plants,includingmaize,sorghum,sugarcane,andmillet.PlantsthatdonotusePEP-carboxylaseincarbonfixationarecalledC3plantsbecausetheprimarycarboxylationreaction,catalyzedbyRuBisCO,producesthethree-carbon3-phosphoglycericacidsdirectlyintheCalvin-Bensoncycle.Over90%ofplantsuseC3carbonfixation,comparedto3%thatuseC4carbonfixation;[32]however,theevolutionofC4inover60plantlineagesmakesitastrikingexampleofconvergentevolution.[30]C2photosynthesis,whichinvolvescarbon-concentrationbyselectivebreakdownofphotorespiratoryglycine,isbothanevolutionaryprecursortoC4andausefulCCMinitsownright.[33] Xerophytes,suchascactiandmostsucculents,alsousePEPcarboxylasetocapturecarbondioxideinaprocesscalledCrassulaceanacidmetabolism(CAM).IncontrasttoC4metabolism,whichspatiallyseparatestheCO2fixationtoPEPfromtheCalvincycle,CAMtemporallyseparatesthesetwoprocesses.CAMplantshaveadifferentleafanatomyfromC3plants,andfixtheCO2atnight,whentheirstomataareopen.CAMplantsstoretheCO2mostlyintheformofmalicacidviacarboxylationofphosphoenolpyruvatetooxaloacetate,whichisthenreducedtomalate.DecarboxylationofmalateduringthedayreleasesCO2insidetheleaves,thusallowingcarbonfixationto3-phosphoglyceratebyRuBisCO.CAMisusedby16,000speciesofplants.[34] Calciumoxalateaccumulatingplants,suchasAmaranthushybridusandColobanthusquitensis,showavariationofphotosynthesiswherecalciumoxalatecrystalsfunctionasdynamiccarbonpools,supplyingcarbondioxide(CO2)tophotosyntheticcellswhenstomataarepartiallyortotallyclosed.ThisprocesswasnamedAlarmphotosynthesis.Understressconditions(e.g.waterdeficit)oxalatereleasedfromcalciumoxalatecrystalsisconvertedtoCO2byanoxalateoxidaseenzymeandtheproducedCO2cansupporttheCalvincyclereactions.Reactivehydrogenperoxide(H2O2),thebyproductofoxalateoxidasereaction,canbeneutralizedbycatalase.Alarmphotosynthesisrepresentsaphotosyntheticvarianttobeaddedtothewell-knownC4andCAMpathways.However,alarmphotosynthesis,incontrasttothesepathways,operatesasabiochemicalpumpthatcollectscarbonfromtheorganinterior(orfromthesoil)andnotfromtheatmosphere.[35][36] Inwater Cyanobacteriapossesscarboxysomes,whichincreasetheconcentrationofCO2aroundRuBisCOtoincreasetherateofphotosynthesis.Anenzyme,carbonicanhydrase,locatedwithinthecarboxysomereleasesCO2fromdissolvedhydrocarbonateions(HCO−3).BeforetheCO2diffusesoutitisquicklyspongedupbyRuBisCO,whichisconcentratedwithinthecarboxysomes.HCO−3ionsaremadefromCO2outsidethecellbyanothercarbonicanhydraseandareactivelypumpedintothecellbyamembraneprotein.Theycannotcrossthemembraneastheyarecharged,andwithinthecytosoltheyturnbackintoCO2veryslowlywithoutthehelpofcarbonicanhydrase.ThiscausestheHCO−3ionstoaccumulatewithinthecellfromwheretheydiffuseintothecarboxysomes.[37]PyrenoidsinalgaeandhornwortsalsoacttoconcentrateCO2aroundRuBisCO.[38] Orderandkinetics Theoverallprocessofphotosynthesistakesplaceinfourstages:[14] Stage Description Timescale 1 Energytransferinantennachlorophyll(thylakoidmembranes) Femtosecondtopicosecond 2 Transferofelectronsinphotochemicalreactions(thylakoidmembranes) Picosecondtonanosecond 3 ElectrontransportchainandATPsynthesis(thylakoidmembranes) Microsecondtomillisecond 4 Carbonfixationandexportofstableproducts Millisecondtosecond Efficiency Mainarticle:Photosyntheticefficiency Plantsusuallyconvertlightintochemicalenergywithaphotosyntheticefficiencyof3–6%.[39][40] Absorbedlightthatisunconvertedisdissipatedprimarilyasheat,withasmallfraction(1–2%)[41]re-emittedaschlorophyllfluorescenceatlonger(redder)wavelengths.Thisfactallowsmeasurementofthelightreactionofphotosynthesisbyusingchlorophyllfluorometers.[41] Actualplants'photosyntheticefficiencyvarieswiththefrequencyofthelightbeingconverted,lightintensity,temperatureandproportionofcarbondioxideintheatmosphere,andcanvaryfrom0.1%to8%.[42]Bycomparison,solarpanelsconvertlightintoelectricenergyatanefficiencyofapproximately6–20%formass-producedpanels,andabove40%inlaboratorydevices. Scientistsarestudyingphotosynthesisinhopesofdevelopingplantswithincreasedyield.[40] Theefficiencyofbothlightanddarkreactionscanbemeasuredbuttherelationshipbetweenthetwocanbecomplex.[43]Forexample,theATPandNADPHenergymolecules,createdbythelightreaction,canbeusedforcarbonfixationorforphotorespirationinC3plants.[43]Electronsmayalsoflowtootherelectronsinks.[44][45][46]Forthisreason,itisnotuncommonforauthorstodifferentiatebetweenworkdoneundernon-photorespiratoryconditionsandunderphotorespiratoryconditions.[47][48][49] ChlorophyllfluorescenceofphotosystemIIcanmeasurethelightreaction,andinfraredgasanalyzerscanmeasurethedarkreaction.[50]Itisalsopossibletoinvestigatebothatthesametimeusinganintegratedchlorophyllfluorometerandgasexchangesystem,orbyusingtwoseparatesystemstogether.[51]InfraredgasanalyzersandsomemoisturesensorsaresensitiveenoughtomeasurethephotosyntheticassimilationofCO2,andofΔH2Ousingreliablemethods[52]CO2iscommonlymeasuredinμmols/(m2/s),partspermillionorvolumepermillionandH2Oiscommonlymeasuredinmmol/(m2/s)orinmbars.[52]BymeasuringCO2assimilation,ΔH2O,leaftemperature,barometricpressure,leafarea,andphotosyntheticallyactiveradiationorPAR,itbecomespossibletoestimate,"A"orcarbonassimilation,"E"ortranspiration,"gs"orstomatalconductance,andCiorintracellularCO2.[52]However,itismorecommontousedchlorophyllfluorescenceforplantstressmeasurement,whereappropriate,becausethemostcommonlyusedparametersFV/FMandY(II)orF/FM'canbemeasuredinafewseconds,allowingtheinvestigationoflargerplantpopulations.[49] GasexchangesystemsthatoffercontrolofCO2levels,aboveandbelowambient,allowthecommonpracticeofmeasurementofA/Cicurves,atdifferentCO2levels,tocharacterizeaplant'sphotosyntheticresponse.[52] Integratedchlorophyllfluorometer–gasexchangesystemsallowamoreprecisemeasureofphotosyntheticresponseandmechanisms.[50][51]WhilestandardgasexchangephotosynthesissystemscanmeasureCi,orsubstomatalCO2levels,theadditionofintegratedchlorophyllfluorescencemeasurementsallowsamoreprecisemeasurementofCCtoreplaceCi.[51][53]TheestimationofCO2atthesiteofcarboxylationinthechloroplast,orCC,becomespossiblewiththemeasurementofmesophyllconductanceorgmusinganintegratedsystem.[50][51][54] Photosynthesismeasurementsystemsarenotdesignedtodirectlymeasuretheamountoflightabsorbedbytheleaf.Butanalysisofchlorophyll-fluorescence,P700-andP515-absorbanceandgasexchangemeasurementsrevealdetailedinformationaboute.g.thephotosystems,quantumefficiencyandtheCO2assimilationrates.Withsomeinstruments,evenwavelength-dependencyofthephotosyntheticefficiencycanbeanalyzed.[55] Aphenomenonknownasquantumwalkincreasestheefficiencyoftheenergytransportoflightsignificantly.Inthephotosyntheticcellofanalga,bacterium,orplant,therearelight-sensitivemoleculescalledchromophoresarrangedinanantenna-shapedstructurenamedaphotocomplex.Whenaphotonisabsorbedbyachromophore,itisconvertedintoaquasiparticlereferredtoasanexciton,whichjumpsfromchromophoretochromophoretowardsthereactioncenterofthephotocomplex,acollectionofmoleculesthattrapsitsenergyinachemicalformaccessibletothecell'smetabolism.Theexciton'swavepropertiesenableittocoverawiderareaandtryoutseveralpossiblepathssimultaneously,allowingittoinstantaneously"choose"themostefficientroute,whereitwillhavethehighestprobabilityofarrivingatitsdestinationintheminimumpossibletime. Becausethatquantumwalkingtakesplaceattemperaturesfarhigherthanquantumphenomenausuallyoccur,itisonlypossibleoververyshortdistances.Obstaclesintheformofdestructiveinterferencecausetheparticletoloseitswavepropertiesforaninstantbeforeitregainsthemonceagainafteritisfreedfromitslockedpositionthroughaclassic"hop".Themovementoftheelectrontowardsthephotocenteristhereforecoveredinaseriesofconventionalhopsandquantumwalks.[56][57][58] Evolution Mainarticle:Evolutionofphotosynthesis LifetimelineThisbox:viewtalkedit−4500 —–—–−4000 —–—–−3500 —–—–−3000 —–—–−2500 —–—–−2000 —–—–−1500 —–—–−1000 —–—–−500 —–—–0 — Water Single-celledlife Photosynthesis Eukaryotes Multicellularlife Plants ArthropodsMolluscsFlowersDinosaurs MammalsBirdsPrimatesHadeanArcheanProterozoicPhanerozoic  ←Earthformed←Earliestwater←LUCA←Earliestfossils←LHBmeteorites←Earliestoxygen←Pongolaglaciation*←Atmosphericoxygen←Huronianglaciation*←Sexualreproduction←Earliestmulticellularlife←Earliestfungi←Earliestplants←Earliestanimals←Cryogenianiceage*←Ediacaranbiota←Cambrianexplosion←Andeanglaciation*←Earliesttetrapods←Karooiceage*←Earliestapes/humans←Quaternaryiceage*(millionyearsago)*IceAges Earlyphotosyntheticsystems,suchasthoseingreenandpurplesulfurandgreenandpurplenonsulfurbacteria,arethoughttohavebeenanoxygenic,andusedvariousothermoleculesthanwateraselectrondonors.Greenandpurplesulfurbacteriaarethoughttohaveusedhydrogenandsulfuraselectrondonors.Greennonsulfurbacteriausedvariousaminoandotherorganicacidsasanelectrondonor.Purplenonsulfurbacteriausedavarietyofnonspecificorganicmolecules.TheuseofthesemoleculesisconsistentwiththegeologicalevidencethatEarth'searlyatmospherewashighlyreducingatthattime.[59] Fossilsofwhatarethoughttobefilamentousphotosyntheticorganismshavebeendatedat3.4billionyearsold.[60][61]Morerecentstudiesalsosuggestthatphotosynthesismayhavebegunabout3.4billionyearsago.[62][63] ThemainsourceofoxygenintheEarth'satmospherederivesfromoxygenicphotosynthesis,andthefirstappearanceofthishigh-energymoleculeissometimesreferredtoastheoxygencatastrophe.Geologicalevidencesuggeststhatoxygenicphotosynthesis,suchasthatincyanobacteria,becameimportantduringthePaleoproterozoiceraaround2billionyearsago.Modernphotosynthesisinplantsandmostphotosyntheticprokaryotesisoxygenic,usingwaterasanelectrondonor,whichisoxidizedtomolecularoxygeninthephotosyntheticreactioncenter. Symbiosisandtheoriginofchloroplasts Plantcellswithvisiblechloroplasts(fromamoss,Plagiomniumaffine) Severalgroupsofanimalshaveformedsymbioticrelationshipswithphotosyntheticalgae.Thesearemostcommonincorals,spongesandseaanemones.Itispresumedthatthisisduetotheparticularlysimplebodyplansandlargesurfaceareasoftheseanimalscomparedtotheirvolumes.[64]Inaddition,afewmarinemollusksElysiaviridisandElysiachloroticaalsomaintainasymbioticrelationshipwithchloroplaststheycapturefromthealgaeintheirdietandthenstoreintheirbodies(seeKleptoplasty).Thisallowsthemolluskstosurvivesolelybyphotosynthesisforseveralmonthsatatime.[65][66]Someofthegenesfromtheplantcellnucleushaveevenbeentransferredtotheslugs,sothatthechloroplastscanbesuppliedwithproteinsthattheyneedtosurvive.[67] Anevencloserformofsymbiosismayexplaintheoriginofchloroplasts.Chloroplastshavemanysimilaritieswithphotosyntheticbacteria,includingacircularchromosome,prokaryotic-typeribosome,andsimilarproteinsinthephotosyntheticreactioncenter.[68][69]Theendosymbiotictheorysuggeststhatphotosyntheticbacteriawereacquired(byendocytosis)byearlyeukaryoticcellstoformthefirstplantcells.Therefore,chloroplastsmaybephotosyntheticbacteriathatadaptedtolifeinsideplantcells.Likemitochondria,chloroplastspossesstheirownDNA,separatefromthenuclearDNAoftheirplanthostcellsandthegenesinthischloroplastDNAresemblethosefoundincyanobacteria.[70]DNAinchloroplastscodesforredoxproteinssuchasthosefoundinthephotosyntheticreactioncenters.TheCoRRHypothesisproposesthatthisco-locationofgeneswiththeirgeneproductsisrequiredforredoxregulationofgeneexpression,andaccountsforthepersistenceofDNAinbioenergeticorganelles.[71] Photosyntheticeukaryoticlineages Symbioticandkleptoplasticorganismsexcluded: Theglaucophytesandtheredandgreenalgae—cladeArchaeplastida(uni-andmulticellular) Thecryptophytes—cladeCryptista(unicellular) Thehaptophytes—cladeHaptista(unicellular) ThedinoflagellatesandchromeridsinthesuperphylumMyzozoa—cladeAlveolata(unicellular) Theochrophytes—cladeHeterokonta(uni-andmulticellular) Thechlorarachniophytesand3speciesofPaulinellainthephylumCercozoa—cladeRhizaria(unicellular) Theeuglenids—cladeExcavata(unicellular) Exceptfortheeuglenids,whicharefoundwithintheExcavata,allofthesebelongtotheDiaphoretickes.ArchaeplastidaandthephotosyntheticPaulinellagottheirplastids,whicharesurroundedbytwomembranes,throughprimaryendosymbiosisintwoseparateevents,byengulfingacyanobacterium.Theplastidsinalltheothergroupshaveeitheraredorgreenalgalorigin,andarereferredtoasthe"redlineages"andthe"greenlineages".Indinoflagellatesandeuglenidstheplastidsaresurroundedbythreemembranes,andintheremaininglinesbyfour.Anucleomorph,remnantsoftheoriginalalgalnucleuslocatedbetweentheinnerandoutermembranesoftheplastid,ispresentinthecryptophytes(fromaredalga)andchlorarachniophytes(fromagreenalga).[72] Somedinoflaggelatesthatlosttheirphotosyntheticabilitylaterregaineditagainthroughnewendosymbioticeventswithdifferentalgae. Whileabletoperformphotosynthesis,manyoftheseeukaryoticgroupsaremixotrophsandpracticeheterotrophytovariousdegrees. Cyanobacteriaandtheevolutionofphotosynthesis Thebiochemicalcapacitytousewaterasthesourceforelectronsinphotosynthesisevolvedonce,inacommonancestorofextantcyanobacteria(formerlycalledblue-greenalgae),whicharetheonlyprokaryotesperformingoxygenicphotosynthesis.ThegeologicalrecordindicatesthatthistransformingeventtookplaceearlyinEarth'shistory,atleast2450–2320millionyearsago(Ma),and,itisspeculated,muchearlier.[73][74]BecausetheEarth'satmospherecontainedalmostnooxygenduringtheestimateddevelopmentofphotosynthesis,itisbelievedthatthefirstphotosyntheticcyanobacteriadidnotgenerateoxygen.[75]AvailableevidencefromgeobiologicalstudiesofArchean(>2500Ma)sedimentaryrocksindicatesthatlifeexisted3500Ma,butthequestionofwhenoxygenicphotosynthesisevolvedisstillunanswered.Aclearpaleontologicalwindowoncyanobacterialevolutionopenedabout2000Ma,revealinganalready-diversebiotaofcyanobacteria.CyanobacteriaremainedtheprincipalprimaryproducersofoxygenthroughouttheProterozoicEon(2500–543Ma),inpartbecausetheredoxstructureoftheoceansfavoredphotoautotrophscapableofnitrogenfixation.[citationneeded]GreenalgaejoinedcyanobacteriaasthemajorprimaryproducersofoxygenoncontinentalshelvesneartheendoftheProterozoic,butonlywiththeMesozoic(251–66Ma)radiationsofdinoflagellates,coccolithophorids,anddiatomsdidtheprimaryproductionofoxygeninmarineshelfwaterstakemodernform.Cyanobacteriaremaincriticaltomarineecosystemsasprimaryproducersofoxygeninoceanicgyres,asagentsofbiologicalnitrogenfixation,and,inmodifiedform,astheplastidsofmarinealgae.[76] Experimentalhistory Discovery Althoughsomeofthestepsinphotosynthesisarestillnotcompletelyunderstood,theoverallphotosyntheticequationhasbeenknownsincethe19thcentury. PortraitofJanBaptistvanHelmontbyMaryBeale,c.1674 JanvanHelmontbegantheresearchoftheprocessinthemid-17thcenturywhenhecarefullymeasuredthemassofthesoilusedbyaplantandthemassoftheplantasitgrew.Afternoticingthatthesoilmasschangedverylittle,hehypothesizedthatthemassofthegrowingplantmustcomefromthewater,theonlysubstanceheaddedtothepottedplant.Hishypothesiswaspartiallyaccurate–muchofthegainedmasscomesfromcarbondioxideaswellaswater.However,thiswasasignalingpointtotheideathatthebulkofaplant'sbiomasscomesfromtheinputsofphotosynthesis,notthesoilitself. JosephPriestley,achemistandminister,discoveredthatwhenheisolatedavolumeofairunderaninvertedjarandburnedacandleinit(whichgaveoffCO2),thecandlewouldburnoutveryquickly,muchbeforeitranoutofwax.Hefurtherdiscoveredthatamousecouldsimilarly"injure"air.Hethenshowedthattheairthathadbeen"injured"bythecandleandthemousecouldberestoredbyaplant.[77] In1779,JanIngenhouszrepeatedPriestley'sexperiments.Hediscoveredthatitwastheinfluenceofsunlightontheplantthatcouldcauseittoreviveamouseinamatterofhours.[77][78] In1796,JeanSenebier,aSwisspastor,botanist,andnaturalist,demonstratedthatgreenplantsconsumecarbondioxideandreleaseoxygenundertheinfluenceoflight.Soonafterward,Nicolas-ThéodoredeSaussureshowedthattheincreaseinmassoftheplantasitgrowscouldnotbedueonlytouptakeofCO2butalsototheincorporationofwater.Thus,thebasicreactionbywhichphotosynthesisisusedtoproducefood(suchasglucose)wasoutlined.[79] Refinements CornelisVanNielmadekeydiscoveriesexplainingthechemistryofphotosynthesis.Bystudyingpurplesulfurbacteriaandgreenbacteriahewasthefirsttodemonstratethatphotosynthesisisalight-dependentredoxreaction,inwhichhydrogenreduces(donatesitsatomsaselectronsandprotonsto)carbondioxide. RobertEmersondiscoveredtwolightreactionsbytestingplantproductivityusingdifferentwavelengthsoflight.Withtheredalone,thelightreactionsweresuppressed.Whenblueandredwerecombined,theoutputwasmuchmoresubstantial.Thus,thereweretwophotosystems,oneabsorbingupto600 nmwavelengths,theotherupto700 nm.TheformerisknownasPSII,thelatterisPSI.PSIcontainsonlychlorophyll"a",PSIIcontainsprimarilychlorophyll"a"withmostoftheavailablechlorophyll"b",amongotherpigments.Theseincludephycobilins,whicharetheredandbluepigmentsofredandbluealgae,respectively,andfucoxantholforbrownalgaeanddiatoms.TheprocessismostproductivewhentheabsorptionofquantaisequalinbothPSIIandPSI,assuringthatinputenergyfromtheantennacomplexisdividedbetweenthePSIandPSIIsystems,whichinturnpowersthephotochemistry.[14] RobertHillthoughtthatacomplexofreactionsconsistedofanintermediatetocytochromeb6(nowaplastoquinone),andthatanotherwasfromcytochromeftoastepinthecarbohydrate-generatingmechanisms.Thesearelinkedbyplastoquinone,whichdoesrequireenergytoreducecytochromef.FurtherexperimentstoprovethattheoxygendevelopedduringthephotosynthesisofgreenplantscamefromwaterwereperformedbyHillin1937and1939.Heshowedthatisolatedchloroplastsgiveoffoxygeninthepresenceofunnaturalreducingagentslikeironoxalate,ferricyanideorbenzoquinoneafterexposuretolight.IntheHillreaction:[80] 2H2O+2A+(light,chloroplasts)→2AH2+O2 Aistheelectronacceptor.Therefore,inlight,theelectronacceptorisreducedandoxygenisevolved.SamuelRubenandMartinKamenusedradioactiveisotopestodeterminethattheoxygenliberatedinphotosynthesiscamefromthewater. MelvinCalvinworksinhisphotosynthesislaboratory. MelvinCalvinandAndrewBenson,alongwithJamesBassham,elucidatedthepathofcarbonassimilation(thephotosyntheticcarbonreductioncycle)inplants.ThecarbonreductioncycleisknownastheCalvincycle,butmanyscientistsrefertoitastheCalvin-Benson,Benson-Calvin,orevenCalvin-Benson-Bassham(orCBB)Cycle. NobelPrize-winningscientistRudolphA.Marcuswaslaterabletodiscoverthefunctionandsignificanceoftheelectrontransportchain. OttoHeinrichWarburgandDeanBurkdiscoveredtheI-quantumphotosynthesisreactionthatsplitsCO2,activatedbytherespiration.[81] In1950,firstexperimentalevidencefortheexistenceofphotophosphorylationinvivowaspresentedbyOttoKandlerusingintactChlorellacellsandinterpretinghisfindingsaslight-dependentATPformation.[82] In1954,DanielI.Arnonetal.discoveredphotophosphorylationinvitroinisolatedchloroplastswiththehelpofP32.[83][84] LouisN.M.DuysensandJanAmeszdiscoveredthatchlorophyll"a"willabsorbonelight,oxidizecytochromef,whilechlorophyll"a"(andotherpigments)willabsorbanotherlightbutwillreducethissameoxidizedcytochrome,statingthetwolightreactionsareinseries. Developmentoftheconcept In1893,CharlesReidBarnesproposedtwoterms,photosyntaxandphotosynthesis,forthebiologicalprocessofsynthesisofcomplexcarboncompoundsoutofcarbonicacid,inthepresenceofchlorophyll,undertheinfluenceoflight.Overtime,thetermphotosynthesiscameintocommonusage.Laterdiscoveryofanoxygenicphotosyntheticbacteriaandphotophosphorylationnecessitatedredefinitionoftheterm.[85] C3 :C4photosynthesisresearch Inthelate1940sattheUniversityofCalifornia,Berkeley,thedetailsofphotosyntheticcarbonmetabolismweresortedoutbythechemistsMelvinCalvin,AndrewBenson,JamesBasshamandascoreofstudentsandresearchersutilizingthecarbon-14isotopeandpaperchromatographytechniques.[86]ThepathwayofCO2fixationbythealgaeChlorellainafractionofasecondinlightresultedina3carbonmoleculecalledphosphoglycericacid(PGA).Forthatoriginalandground-breakingwork,aNobelPrizeinChemistrywasawardedtoMelvinCalvinin1961.Inparallel,plantphysiologistsstudiedleafgasexchangesusingthenewmethodofinfraredgasanalysisandaleafchamberwherethenetphotosyntheticratesrangedfrom10to13μmolCO2·m−2·s−1,withtheconclusionthatallterrestrialplantshavethesamephotosyntheticcapacities,thatarelightsaturatedatlessthan50%ofsunlight.[87][88] Laterin1958–1963atCornellUniversity,fieldgrownmaizewasreportedtohavemuchgreaterleafphotosyntheticratesof40μmolCO2·m−2·s−1andnotbesaturatedatnearfullsunlight.[89][90]Thishigherrateinmaizewasalmostdoubleofthoseobservedinotherspeciessuchaswheatandsoybean,indicatingthatlargedifferencesinphotosynthesisexistamonghigherplants.AttheUniversityofArizona,detailedgasexchangeresearchonmorethan15speciesofmonocotanddicotuncoveredforthefirsttimethatdifferencesinleafanatomyarecrucialfactorsindifferentiatingphotosyntheticcapacitiesamongspecies.[91][92]Intropicalgrasses,includingmaize,sorghum,sugarcane,Bermudagrassandinthedicotamaranthus,leafphotosyntheticrateswerearound38−40μmolCO2·m−2·s−1,andtheleaveshavetwotypesofgreencells,i.e.outerlayerofmesophyllcellssurroundingatightlypackedcholorophyllousvascularbundlesheathcells.ThistypeofanatomywastermedKranzanatomyinthe19thcenturybythebotanistGottliebHaberlandtwhilestudyingleafanatomyofsugarcane.[93]PlantspecieswiththegreatestphotosyntheticratesandKranzanatomyshowednoapparentphotorespiration,verylowCO2compensationpoint,highoptimumtemperature,highstomatalresistancesandlowermesophyllresistancesforgasdiffusionandratesneversaturatedatfullsunlight.[94]TheresearchatArizonawasdesignatedaCitationClassicin1986.[92]ThesespecieswerelatertermedC4plantsasthefirststablecompoundofCO2fixationinlighthas4carbonsasmalateandaspartate.[95][96][97]OtherspeciesthatlackKranzanatomyweretermedC3typesuchascottonandsunflower,asthefirststablecarboncompoundisthe3-carbonPGA.At1000ppmCO2inmeasuringair,boththeC3andC4plantshadsimilarleafphotosyntheticratesaround60μmolCO2·m−2·s−1indicatingthesuppressionofphotorespirationinC3plants.[91][92] Factors Theleafistheprimarysiteofphotosynthesisinplants. Therearethreemainfactorsaffectingphotosynthesis[clarificationneeded]andseveralcorollaryfactors.Thethreemainare:[citationneeded] Lightirradianceandwavelength Carbondioxideconcentration Temperature. Totalphotosynthesisislimitedbyarangeofenvironmentalfactors.Theseincludetheamountoflightavailable,theamountofleafareaaplanthastocapturelight(shadingbyotherplantsisamajorlimitationofphotosynthesis),therateatwhichcarbondioxidecanbesuppliedtothechloroplaststosupportphotosynthesis,theavailabilityofwater,andtheavailabilityofsuitabletemperaturesforcarryingoutphotosynthesis.[98] Lightintensity(irradiance),wavelengthandtemperature Seealso:PI(photosynthesis-irradiance)curve Absorbancespectraoffreechlorophylla(blue)andb(red)inasolvent.Theactionspectraofchlorophyllmoleculesareslightlymodifiedinvivodependingonspecificpigment–proteininteractions. Theprocessofphotosynthesisprovidesthemaininputoffreeenergyintothebiosphere,andisoneoffourmainwaysinwhichradiationisimportantforplantlife.[99] Theradiationclimatewithinplantcommunitiesisextremelyvariable,inbothtimeandspace. Intheearly20thcentury,FrederickBlackmanandGabrielleMatthaeiinvestigatedtheeffectsoflightintensity(irradiance)andtemperatureontherateofcarbonassimilation. Atconstanttemperature,therateofcarbonassimilationvarieswithirradiance,increasingastheirradianceincreases,butreachingaplateauathigherirradiance. Atlowirradiance,increasingthetemperaturehaslittleinfluenceontherateofcarbonassimilation.Atconstanthighirradiance,therateofcarbonassimilationincreasesasthetemperatureisincreased. Thesetwoexperimentsillustrateseveralimportantpoints:First,itisknownthat,ingeneral,photochemicalreactionsarenotaffectedbytemperature.However,theseexperimentsclearlyshowthattemperatureaffectstherateofcarbonassimilation,sotheremustbetwosetsofreactionsinthefullprocessofcarbonassimilation.Thesearethelight-dependent'photochemical'temperature-independentstage,andthelight-independent,temperature-dependentstage.Second,Blackman'sexperimentsillustratetheconceptoflimitingfactors.Anotherlimitingfactoristhewavelengthoflight.Cyanobacteria,whichresideseveralmetersunderwater,cannotreceivethecorrectwavelengthsrequiredtocausephotoinducedchargeseparationinconventionalphotosyntheticpigments.Tocombatthisproblem,aseriesofproteinswithdifferentpigmentssurroundthereactioncenter.Thisunitiscalledaphycobilisome.[clarificationneeded] Carbondioxidelevelsandphotorespiration Photorespiration Ascarbondioxideconcentrationsrise,therateatwhichsugarsaremadebythelight-independentreactionsincreasesuntillimitedbyotherfactors.RuBisCO,theenzymethatcapturescarbondioxideinthelight-independentreactions,hasabindingaffinityforbothcarbondioxideandoxygen.Whentheconcentrationofcarbondioxideishigh,RuBisCOwillfixcarbondioxide.However,ifthecarbondioxideconcentrationislow,RuBisCOwillbindoxygeninsteadofcarbondioxide.Thisprocess,calledphotorespiration,usesenergy,butdoesnotproducesugars. RuBisCOoxygenaseactivityisdisadvantageoustoplantsforseveralreasons: Oneproductofoxygenaseactivityisphosphoglycolate(2carbon)insteadof3-phosphoglycerate(3carbon).PhosphoglycolatecannotbemetabolizedbytheCalvin-Bensoncycleandrepresentscarbonlostfromthecycle.Ahighoxygenaseactivity,therefore,drainsthesugarsthatarerequiredtorecycleribulose5-bisphosphateandforthecontinuationoftheCalvin-Bensoncycle. Phosphoglycolateisquicklymetabolizedtoglycolatethatistoxictoaplantatahighconcentration;itinhibitsphotosynthesis. Salvagingglycolateisanenergeticallyexpensiveprocessthatusestheglycolatepathway,andonly75%ofthecarbonisreturnedtotheCalvin-Bensoncycleas3-phosphoglycerate.Thereactionsalsoproduceammonia(NH3),whichisabletodiffuseoutoftheplant,leadingtoalossofnitrogen. Ahighlysimplifiedsummaryis: 2glycolate+ATP→3-phosphoglycerate+carbondioxide+ADP+NH3 ThesalvagingpathwayfortheproductsofRuBisCOoxygenaseactivityismorecommonlyknownasphotorespiration,sinceitischaracterizedbylight-dependentoxygenconsumptionandthereleaseofcarbondioxide. 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Furtherreading LibraryresourcesaboutPhotosynthesis Resourcesinyourlibrary Books BidlackJE,SternKR,JanskyS(2003).IntroductoryPlantBiology.NewYork:McGraw-Hill.ISBN 978-0-07-290941-8. BlankenshipRE(2014).MolecularMechanismsofPhotosynthesis(2nd ed.).JohnWiley&Sons.ISBN 978-1-4051-8975-0. Govindjee,BeattyJT,GestH,AllenJF(2006).DiscoveriesinPhotosynthesis.AdvancesinPhotosynthesisandRespiration.Vol. 20.Berlin:Springer.ISBN 978-1-4020-3323-0. ReeceJB,et al.(2013).CampbellBiology.BenjaminCummings.ISBN 978-0-321-77565-8. Papers GuptaRS,MukhtarT,SinghB(Jun1999)."Evolutionaryrelationshipsamongphotosyntheticprokaryotes(Heliobacteriumchlorum,Chloroflexusaurantiacus,cyanobacteria,Chlorobiumtepidumandproteobacteria):implicationsregardingtheoriginofphotosynthesis".MolecularMicrobiology.32(5):893–906.doi:10.1046/j.1365-2958.1999.01417.x.PMID 10361294.S2CID 33477550. RutherfordAW,FallerP(Jan2003)."PhotosystemII:evolutionaryperspectives".PhilosophicalTransactionsoftheRoyalSocietyofLondon.SeriesB,BiologicalSciences.358(1429):245–253.doi:10.1098/rstb.2002.1186.PMC 1693113.PMID 12594932. Externallinks Acollectionofphotosynthesispagesforalllevelsfromarenownedexpert(Govindjee) Indepth,advancedtreatmentofphotosynthesis,alsofromGovindjee ScienceAid:PhotosynthesisArticleappropriateforhighschoolscience Metabolism,CellularRespirationandPhotosynthesis–TheVirtualLibraryofBiochemistryandCellBiology OverallexaminationofPhotosynthesisatanintermediatelevel OverallEnergeticsofPhotosynthesis ThesourceofoxygenproducedbyphotosynthesisInteractiveanimation,atextbooktutorial MarshallJ(2011-03-29)."Firstpracticalartificialleafmakesdebut".DiscoveryNews.Archivedfromtheoriginalon2012-03-22.Retrieved2011-03-29. Photosynthesis–LightDependent&LightIndependentStagesArchived2011-09-10attheWaybackMachine KhanAcademy,videointroduction vteBotany History Outline Subdisciplines Archaeobotany Astrobotany Bryology Dendrology Ethnobotany Paleobotany Phycology Phytochemistry Phytogeography Geobotany Plantanatomy Plantecology Plantpathology Plantgroups Algae Archaeplastida Bryophyte Non-vascularplants Vascularplants Spermatophytes Pteridophyte Gymnosperm Angiosperm PlantanatomyPlantmorphology(glossary)Plantcells Cellwall Phragmoplast Plastid Plasmodesma Vacuole Tissues Cork Groundtissue Mesophyll Meristem Storageorgans Vasculartissue Vascularbundle Wood Vegetative Bulb Root Rhizoid Rhizome Shoot Bud Leaf Cataphyll Petiole Sessility Stem Reproductive(Flower) Archegonium Androecium Pollen Stamen Staminode Tapetum Flower Aestivation Flowerdevelopment Floraldiagram Floralformula Floralsymmetry Whorl Fruit Anatomy Berry Capsule Nut Pyrena Seed Dispersal Endosperm Gametophyte Gynandrium Gynoecium Ovary Locule Ovule Stigma Hypanthium(Floralcup) Inflorescence Bract Pedicellate Raceme Umbel Perianth Tepal Petal Sepal Plantembryo Receptacle Sporophyll Sporophyte Surfacestructures Cuticle Epicuticularwax Epidermis Nectar Stoma Thorns,spines,andprickles Trichome PlantphysiologyMaterials Aleurone Bulkflow Cellulose Nutrition Photosynthesis Chlorophyll Phytomelanin Planthormones Sap Starch Sugar Transpiration Turgorpressure Plantgrowthandhabit Habit Cushionplants Rosettes Shrubs Prostrateshrubs Subshrubs Succulentplants Trees Vines Lianas Herbaceousplants Secondarygrowth Woodyplants ReproductionEvolutionEcology Alternationofgenerations Doublefertilization Evolutionarydevelopment Evolutionaryhistory timeline Flora Germination Pollination Artificial Pollinators Pollentube Self Sporangium Microsporangia Microspore Megasporangium Megaspore Spore Planttaxonomy Biologicalclassification Botanicalnomenclature Botanicalname Correctname Authorcitation InternationalCodeofNomenclature(ICN) ICNforCultivatedPlants(ICNCP) Cultivatedplanttaxonomy Citrustaxonomy Cultigen Cultivar Group Grex Historyofplantsystematics Herbarium InternationalAssociationforPlantTaxonomy(IAPT) Planttaxonomysystems Taxonomicrank Practice Agronomy Floriculture Forestry Horticulture ListsRelatedtopics Botanicalterms Botanists byauthorabbreviation Botanicalexpedition Category  Plantsportal Commons WikiProject vteMetabolismmap Carbonfixation Photo-respiration Pentosephosphatepathway Citricacidcycle Glyoxylatecycle Ureacycle Fattyacidsynthesis Fattyacidelongation Betaoxidation Peroxisomal betaoxidation Glyco-genolysis Glyco-genesis Glyco-lysis Gluconeo-genesis Pyruvatedecarb-oxylation Fermentation Keto-lysis Keto-genesis feederstogluconeo-genesis Direct/C4/CAMcarbonintake Lightreaction Oxidativephosphorylation Aminoaciddeamination Citrateshuttle Lipogenesis Lipolysis Steroidogenesis MVApathway MEPpathway Shikimatepathway Transcription&replication Translation Proteolysis Glycosyl-ation Sugaracids Double/multiplesugars&glycans Simplesugars Inositol-P Aminosugars&sialicacids Nucleotidesugars Hexose-P Triose-P Glycerol P-glycerates Pentose-P Tetrose-P Propionyl-CoA Succinate Acetyl-CoA Pentose-P P-glycerates Glyoxylate Photosystems Pyruvate Lactate Acetyl-CoA Citrate Oxalo-acetate Malate Succinyl-CoA α-Keto-glutarate Ketonebodies Respiratorychain Serinegroup Alanine Branched-chainaminoacids Aspartategroup Homoserinegroup&lysine Glutamategroup&proline Arginine Creatine&polyamines Ketogenic&glucogenicaminoacids Aminoacids Shikimate Aromaticaminoacids&histidine Ascorbate(vitaminC) δ-ALA Bilepigments Hemes Cobalamins(vitaminB12) VariousvitaminBs Calciferols(vitaminD) Retinoids(vitaminA) Quinones(vitaminK)&tocopherols(vitaminE) Cofactors Vitamins&minerals Antioxidants PRPP Nucleotides Nucleicacids Proteins Glycoproteins&proteoglycans Chlorophylls MEP MVA Acetyl-CoA Polyketides Terpenoidbackbones Terpenoids&carotenoids(vitaminA) Cholesterol Bileacids Glycero-phospholipids Glycerolipids Acyl-CoA Fattyacids Glyco-sphingolipids Sphingolipids Waxes Polyunsaturatedfattyacids Neurotransmitters&thyroidhormones Steroids Endo-cannabinoids Eicosanoids Majormetabolicpathwaysinmetro-stylemap.Clickanytext(nameofpathwayormetabolites)tolinktothecorrespondingarticle.Singlelines:pathwayscommontomostlifeforms.Doublelines:pathwaysnotinhumans(occursine.g.plants,fungi,prokaryotes).Orangenodes:carbohydratemetabolism.Violetnodes:photosynthesis.Rednodes:cellularrespiration.Pinknodes:cellsignaling.Bluenodes:aminoacidmetabolism.Greynodes:vitaminandcofactormetabolism.Brownnodes:nucleotideandproteinmetabolism.Greennodes:lipidmetabolism. vteEcology:Modellingecosystems:TrophiccomponentsGeneral Abioticcomponent Abioticstress Behaviour Biogeochemicalcycle Biomass Bioticcomponent Bioticstress Carryingcapacity Competition Ecosystem Ecosystemecology Ecosystemmodel Keystonespecies Listoffeedingbehaviours Metabolictheoryofecology Productivity Resource Producers Autotrophs Chemosynthesis Chemotrophs 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kelpforests NorthPacificGyre SanFranciscoEstuary tidepool Processes Ascendency Bioaccumulation Cascadeeffect Climaxcommunity Competitiveexclusionprinciple Consumer–resourceinteractions Copiotrophs Dominance Ecologicalnetwork Ecologicalsuccession Energyquality EnergySystemsLanguage f-ratio Feedconversionratio Feedingfrenzy Mesotrophicsoil Nutrientcycle Oligotroph Paradoxoftheplankton Trophiccascade Trophicmutualism Trophicstateindex Defense,counter Animalcoloration Anti-predatoradaptations Camouflage Deimaticbehaviour Herbivoreadaptationstoplantdefense Mimicry Plantdefenseagainstherbivory Predatoravoidanceinschoolingfish vteEcology:Modellingecosystems:OthercomponentsPopulationecology Abundance Alleeeffect Depensation Ecologicalyield Effectivepopulationsize Intraspecificcompetition Logisticfunction Malthusiangrowthmodel Maximumsustainableyield Overpopulation Overexploitation Populationcycle Populationdynamics Populationmodeling Populationsize Predator–prey(Lotka–Volterra)equations 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