What is photosynthesis? | Live Science

Photosynthesis is the process used by plants, algae and certain bacteria to turn sunlight, carbon dioxide (CO2) and water into food (sugars) ... Skiptomaincontent LiveScience LiveScience Search Subscribe RSS (opensinnewtab) (opensinnewtab) (opensinnewtab) (opensinnewtab) HowItWorksMagazine(opensinnewtab)(opensinnewtab)Whysubscribe?Theultimateaction-packedscienceandtechnologymagazineburstingwithexcitinginformationabouttheuniverseEngagingarticles,amazingillustrations&exclusiveinterviewsIssuesdeliveredstraighttoyourdoorordeviceFrom$3.99(opensinnewtab)ViewDeal(opensinnewtab) News Space&Physics Health PlanetEarth StrangeNews Animals History Forums More Forums Tech Culture Reference AboutUs Magazinesubscriptions TrendingCOVID-19PregnancytipsFitnesstrackersHurricanesBacktoschool&collegedeals LiveScienceissupportedbyitsaudience.Whenyoupurchasethroughlinksonoursite,wemayearnanaffiliatecommission.Here’swhyyoucantrustus. Photosynthesistakesinthecarbondioxideproducedbyallbreathingorganismsandreintroducesoxygenintotheatmosphere. (Imagecredit:KPG_Payless|Shutterstock) Jumpto: Photosyntheticprocesses Photosynthesisequation Thecarbonexchange Howdoplantsabsorbsunlight? Locationofphotosynthesis Light-dependentreactions TheCalvincycle Photorespiration Typesofphotosynthesis Photosynthesis&climatechange Additionalresources Photosynthesisistheprocessusedbyplants,algaeandcertainbacteriatoturnsunlight,carbondioxide(CO2)andwaterintofood(sugars)andoxygen.Here'salookatthegeneralprinciplesofphotosynthesisandrelatedresearchtohelpdevelopcleanfuelsandsourcesofrenewableenergy. TypesofphotosyntheticprocessesTherearetwotypesofphotosyntheticprocesses:oxygenicphotosynthesisandanoxygenicphotosynthesis.Theybothfollowverysimilarprinciples,butoxygenicphotosynthesisisthemostcommonandisseeninplants,algaeandcyanobacteria. Duringoxygenicphotosynthesis,lightenergytransferselectronsfromwater(H2O)takenupbyplantrootstoCO2toproducecarbohydrates.Inthistransfer,theCO2is"reduced,"orreceiveselectrons,andthewateris"oxidized,"orloseselectrons.Oxygenisproducedalongwithcarbohydrates.OxygenicphotosynthesisfunctionsasacounterbalancetorespirationbytakingintheCO2producedbyallbreathingorganismsandreintroducingoxygentotheatmosphere.Anoxygenicphotosynthesis,meanwhile,useselectrondonorsthatarenotwateranddonotproduceoxygen,accordingto"AnoxygenicPhotosyntheticBacteria"byLibreTexts(opensinnewtab).Theprocesstypicallyoccursinbacteriasuchasgreensulfurbacteriaandphototrophicpurplebacteria. ThePhotosynthesisequationThoughbothtypesofphotosynthesisarecomplex,multistepaffairs,theoverallprocesscanbeneatlysummarizedasachemicalequation.Theoxygenicphotosynthesisequationis: 6CO2+12H2O+LightEnergy→C6H12O6+6O2+6H2OHere,sixmoleculesofcarbondioxide(CO2)combinewith12moleculesofwater(H2O)usinglightenergy.Theendresultistheformationofasinglecarbohydratemolecule(C6H12O6,orglucose)alongwithsixmoleculeseachofoxygenandwater.Similarly,thevariousanoxygenicphotosynthesisreactionscanberepresentedasasinglegeneralizedformula:CO2+2H2A+LightEnergy→[CH2O]+2A+H2OTheletterAintheequationisavariable,andH2Arepresentsthepotentialelectrondonor.Forexample,"A"mayrepresentsulfurintheelectrondonorhydrogensulfide(H2S),accordingtomedicalandlifesciencesnewssiteNewsMedicalLifeSciences(opensinnewtab). Howiscarbondioxideandoxygenexchanged?Stomataarethegatekeepersoftheleaf,allowinggasexchangebetweentheleafandsurroundingair. (Imagecredit:WaldoNell/500pxviaGettyImages)(opensinnewtab)PlantsabsorbCO2fromthesurroundingairandreleasewaterandoxygenviamicroscopicporesontheirleavescalledstomata.Stomataarethegatekeepersofgasexchangebetweentheinsideofplantsandtheexternalenvironment. Whenstomataopen,theyletinCO2;however,whileopen,thestomatareleaseoxygenandletwatervaporescape.Inabidtoreducetheamountofwaterlost,stomataclose,butthatmeanstheplantcannolongergainCO2forphotosynthesis.ThistradeoffbetweenCO2gainandwaterlossisaparticularproblemforplantsgrowinginhot,dryenvironments. Howdoplantsabsorbsunlightforphotosynthesis?Plantscontainspecialpigmentsthatabsorbthelightenergyneededforphotosynthesis.Chlorophyllistheprimarypigmentusedforphotosynthesisandgivesplantstheirgreencolor,accordingtoscienceeducationsiteNatureEducation(opensinnewtab).Chlorophyllabsorbsredandbluelighttouseinphotosynthesisandreflectsgreenlight.Chlorophyllisalargemoleculeandtakesalotofresourcestomake;assuch,itbreaksdowntowardstheendoftheleaf'slife,andmostofthepigment'snitrogen(oneofthebuildingblocksofchlorophyll)isresorbedbackintotheplant,accordingtoHarvardUniversity'sTheHarvardForest(opensinnewtab).Whenleaveslosetheirchlorophyllinthefall,otherleafpigmentssuchascarotenoidsandanthocyaninsbegintoshowtheirtruecolors.Whilecarotenoidsprimarilyabsorbbluelightandreflectyellow,anthocyaninsabsorbblue-greenlightandreflectredlight.Pigmentmoleculesareassociatedwithproteins,whichallowthemtheflexibilitytomovetowardlightandtowardoneanother.Alargecollectionof100to5,000pigmentmoleculesconstitutesan"antenna,"accordingtoanarticlebyWimVermaas(opensinnewtab),aprofessoratArizonaStateUniversity.Thesestructureseffectivelycapturelightenergyfromthesun,intheformofphotons.Thesituationisalittledifferentforbacteria.Whilecyanobacteriacontainchlorophyll,otherbacteria,forexample,purplebacteriaandgreensulfurbacteria,containbacteriochlorophylltoabsorblightforanoxygenicphotosynthesis,accordingto"MicrobiologyforDummies(opensinnewtab)"(ForDummies,2019). Related:Whatifhumanshadphotosyntheticskin?Whereintheplantdoesphotosynthesistakeplace?Plantsneedenergyfromsunlightforphotosynthesistooccur.(Imagecredit:Shutterstock)(opensinnewtab)Photosynthesisoccursinchloroplasts,atypeofplastid(anorganellewithamembrane)thatcontainschlorophyllandisprimarilyfoundinplantleaves.Double-membranedplastidsinplantsandalgaeareknownasprimaryplastids,whilethemultiple-membranedvarietyfoundinplanktonarecalledsecondaryplastids,accordingtoa2010articleinthejournalNatureEducation(opensinnewtab)byCheongXinChanandDebashishBhattacharya,researchersatRutgersUniversityinNewJersey. Chloroplastsaresimilartomitochondria,theenergycentersofcells,inthattheyhavetheirowngenome,orcollectionofgenes,containedwithincircularDNA.Thesegenesencodeproteins(opensinnewtab)thatareessentialtotheorganelleandtophotosynthesis.Insidechloroplastsareplate-shapedstructurescalledthylakoidsthatareresponsibleforharvestingphotonsoflightforphotosynthesis,accordingtothebiologyterminologywebsiteBiologyOnline(opensinnewtab).Thethylakoidsarestackedontopofeachotherincolumnsknownasgrana.Inbetweenthegranaisthestroma—afluidcontainingenzymes,moleculesandions,wheresugarformationtakesplace. Ultimately,lightenergymustbetransferredtoapigment-proteincomplexthatcanconvertittochemicalenergy,intheformofelectrons.Inplants,lightenergyistransferredtochlorophyllpigments.Theconversiontochemicalenergyisaccomplishedwhenachlorophyllpigmentexpelsanelectron,whichcanthenmoveontoanappropriaterecipient. Thepigmentsandproteinsthatconvertlightenergytochemicalenergyandbegintheprocessofelectrontransferareknownasreactioncenters.Thereactionsofplantphotosynthesisaredividedintotwomajorstages:thosethatrequirethepresenceofsunlight(light-dependentreactions)andthosethatdonot(light-independentreactions).Bothtypesofreactionstakeplaceinchloroplasts:light-dependentreactionsinthethylakoidandlight-independentreactionsinthestroma. Light-dependentreactionsWhenaplantabsorbssolarenergyitfirstneedstoconvertitintochemicalenergy. Whenaphotonoflighthitsthereactioncenter,apigmentmoleculesuchaschlorophyllreleasesanelectron.Thereleasedelectronmanagestoescapebytravelingthroughanelectrontransportchain,whichgeneratestheenergyneededtoproduceATP(adenosinetriphosphate,asourceofchemicalenergyforcells)andNADPH—bothofwhicharerequiredinthenextstageofphotosynthesisintheCalvincycle.The"electronhole"intheoriginalchlorophyllpigmentisfilledbytakinganelectronfromwater.Thissplittingofwatermoleculesreleasesoxygenintotheatmosphere.Light-independentreactions:TheCalvincyclePhotosynthesisinvolvesaprocesscalledtheCalvincycletouseenergystoredfromthelight-dependentreactionstofixCO2intosugarsneededforplantgrowth. (Imagecredit:NagendraYadavviaGettyImages)(opensinnewtab)TheCalvincycleusesenergystoredfromthelight-dependentreactionstofixCO2intosugarsneededforplantgrowth.AccordingtotheKhanAcademy(opensinnewtab),thesereactionstakeplaceinthestromaofthechloroplasts(opensinnewtab)andarenotdirectlydrivenbylight—hencetheirname"light-independentreactions."However,theyarestillrelatedtolightastheCalvincycleisfuelledbyATPandNADPH(bothfromthepreviouslymentionedlight-dependentreactions).Firstly,CO2combineswithribulose-1,5-bisphosphate(RuBP)whichisafive-carbonacceptor,accordingtotheKhanAcademy.Next,itsplitsintotwomoleculesofathree-carboncompound—3-phosphoglycericacid(3-PGA).ThereactioniscatalyzedbyanenzymecalledRuBPcarboxylase/oxygenase,alsoknownasrubisco. ThesecondstageoftheCalvincycleinvolvesconverting3-PGAintoathree-carbonsugarcalledglyceraldehyde-3-phosphate(G3P)—theprocessusesATPandNADPH.Finally,whilesomeG3Pmoleculesareusedtomakeglucose,othersarerecycledbacktomakeRuBP,whichisusedinthefirststeptoacceptCO2.ForeveryonemoleculeofG3Pthatmakesglucose,fivemoleculesarerecycledtogeneratethreeRuBPacceptormolecules. PhotorespirationAccordingtotheKhanAcademy,rubiscocansometimesfixoxygeninsteadofCO2(opensinnewtab)intheCalvincycle,whichwastesenergy—aprocessknownasphotorespiration.TheenzymeevolvedduringatimewhenatmosphericCO2levelswerehighandoxygenwasrare,soithadnoreasontodifferentiatebetweenthetwo,accordingtoresearchersinCanada(opensinnewtab). PhotorespirationisaparticularlybigproblemwhenplantshavetheirstomataclosedtoconservewaterandarethereforenottakinginanymoreCO2.Rubiscohasnootherchoicebuttofixoxygeninstead,whichinturnlowersthephotosyntheticefficiencyoftheplant.Thismeansthatlessplantfood(sugars)willbeproduced,whichcouldresultinaslowdownofgrowthandthereforesmallerplants. Thisisabigproblemforagriculture,assmallerplantsmeanasmallerharvest.Therearemountingpressuresontheagriculturalindustrytoincreaseplantproductivitytofeedourever-expandingglobalpopulation.Scientistsareconstantlylookingforwaystoincreasephotosyntheticefficiencyandreducetheoccurrenceofwastefulphotorespiration. TypesofphotosynthesisTherearethreemaintypesofphotosyntheticpathways:C3,C4andCAM.TheyallproducesugarsfromCO2usingtheCalvincycle,buteachpathwayisslightlydifferent.ThethreemaintypesofphotosyntheticpathwaysareC3,C4andCAM.MostplantsuseC3photosynthesis,includingriceandcotton. (Imagecredit:AndrewTBTanviaGettyImages)(opensinnewtab)C3photosynthesisMostplantsuseC3photosynthesis,accordingtothephotosynthesisresearchprojectRealizingIncreasedPhotosyntheticEfficiency(RIPE)(opensinnewtab),includingcereals(wheatandrice),cotton,potatoesandsoybeans.C3photosynthesisisnamedforthethree-carboncompoundcalled3-phosphoglycericacid(3-PGA)thatitusesduringtheCalvincycle.3-PGAisproducedwhenrubiscofixesCO2,formingthethree-carboncompound.C4photosynthesisPlantssuchasmaizeandsugarcaneuseC4photosynthesis.Thisprocessusesafour-carboncompoundintermediate(calledoxaloacetate)whichisconvertedtomalate(opensinnewtab),accordingtoBiologyOnline.MalateisthentransportedintothebundlesheathwhereitbreaksdownandreleasedCO2,whichisthenfixedbyrubiscoandmadeintosugarsintheCalvincycle(justlikeC3photosynthesis).C4plantsarebetteradaptedtohot,dryenvironmentsandcancontinuetofixcarbonevenwhentheirstomataareclosed(astheyhaveacleverstoragesolution),whichreducestheirriskofphotorespiration,accordingtoBiologyOnline. CAMphotosynthesisCrassulaceanacidmetabolism(CAM)isfoundinplantsadaptedtoveryhotanddryenvironments,suchascactiandpineapples,accordingtotheeducationalwebsite(opensinnewtab)KhanAcademy.WhenstomataopentotakeinCO2,theyrisklosingwatertotheexternalenvironment.Becauseofthis,plantsinveryaridandhotenvironmentshaveadapted.OneadaptationisCAM,wherebyplantsopenstomataatnight(whentemperaturesarelowerandwaterlossislessofarisk).AccordingtotheKhanAcademy,CO2enterstheplantsviathestomataandisfixedintooxaloacetateandconvertedintomalateoranotherorganicacid(likeintheC4pathway).TheCO2isthenavailableforlight-dependentreactionsinthedaytime,andstomataclose,reducingtheriskofwaterloss. HowphotosynthesiscouldcombatclimatechangePhotosyntheticorganismsareapossiblemeanstogenerateclean-burningfuelssuchashydrogen.AresearchgroupattheUniversityofTurkuinFinlandtappedintotheabilityofgreenalgaetoproducehydrogen.Greenalgaecanproducehydrogenforafewsecondsiftheyarefirstexposedtodark,anaerobic(oxygen-free)conditionsandthenexposedtolight.Theresearchersdevisedawaytoextendgreenalgae'shydrogenproductionforuptothreedays,asreportedintheir2018studypublishedinthejournalEnergy&EnvironmentalScience(opensinnewtab).Scientistshavealsomadeadvancesinthefieldofartificialphotosynthesis.Forinstance,agroupofresearchersfromtheUniversityofCalifornia,Berkeley,developedanartificialsystemtocaptureCO2usingnanowires,orwiresthatareafewbillionthsofameterindiameter.ThewiresfeedintoasystemofmicrobesthatreduceCO2intofuelsorpolymersbyusingenergyfromsunlight.Theteampublisheditsdesignin2015inthejournalNanoLetters(opensinnewtab). In2016,membersofthissamegrouppublishedastudyinthejournalScience(opensinnewtab)thatdescribedanotherartificialphotosyntheticsysteminwhichspeciallyengineeredbacteriawereusedtocreateliquidfuelsusingsunlight,waterandCO2.Ingeneral,plantsareonlyabletoharnessaboutonepercentofsolarenergyanduseittoproduceorganiccompoundsduringphotosynthesis.Incontrast,theresearchers'artificialsystemwasabletoharness10%ofsolarenergytoproduceorganiccompounds.In2019,researcherswroteintheJournalofBiologicalChemistry(opensinnewtab)thatcyanobacteriacouldboosttheefficiencyoftheenzymerubisco.ScientistsfoundthatthisbacteriaisparticularlygoodatconcentratingCO2initscells,whichhelpsstoprubiscofromaccidentallybindingtooxygen.Byunderstandinghowthebacteriaachievethis,scientistshopetoincorporatethemechanismintoplantstohelpboostphotosyntheticefficiencyandreducetheriskofphotorespiration. Continuedresearchofnaturalprocessesaidsscientistsindevelopingnewwaystoutilizevarioussourcesofrenewableenergy,andtappingintothepowerofphotosynthesisisalogicalstepforcreatingclean-burningandcarbon-neutralfuels.AdditionalresourcesDiscovermorefactsaboutphotosynthesiswiththeeducationalsciencewebsitesciencing.com(opensinnewtab).  ExplorehowleafstructureaffectsphotosynthesiswithTheUniversityofArizona(opensinnewtab). LearnaboutthedifferentwaysphotosynthesiscanbemeasuredwiththeeducationalsciencewebsiteScience&PlantsforSchools(opensinnewtab).  DaisyDobrijevicReferenceWriter,Space.comDaisyDobrijevicjoined Space.com(opensinnewtab) inFebruary2022asareferencewriterhavingpreviouslyworkedforoursisterpublication AllAboutSpace(opensinnewtab) magazineasastaffwriter.Beforejoiningus,DaisycompletedaneditorialinternshipwiththeBBCSkyatNightMagazineandworkedatthe NationalSpaceCentre(opensinnewtab) inLeicester,U.K.,wheresheenjoyedcommunicatingspacesciencetothepublic.In2021,DaisycompletedaPhDinplantphysiologyandalsoholdsaMaster'sinEnvironmentalScience,sheiscurrentlybasedinNottingham,U.K. LiveSciencenewsletter StayuptodateonthelatestsciencenewsbysigningupforourEssentialsnewsletter. 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