Objectives:
CH.6.whereitstartsphotosynthesis
1.Understandthemainpathwaysbywhichenergyfromthesunorfromspecificchemical reactions entersorganismsandpassesfromorganismtoorganismand/orbackintotheenvironment. 2.Knowthestepsofthelightdependentandlightindependentreactions.Knowtheraw materials neededtostarteachphase,andknowtheproductsfromeachphase. 3.Befamiliarwiththecomplexrelationshipbetweenphotosynthesis,CO2,andglobalwarming.
KeyTerms:
autotrophs heterotrophs photosynthesis electromagneticspectrum wavelength visiblelight photons pigments chlorophylla chlorophyllb accessorypigment carotenoids xanthophylls phycobilins anthocyanin lightdependentreactions lightindependentreactions chloroplasts stroma thylakoidmembrane photosystems reactioncenter electrontransferchains ATPsynthases photolysis cyclicpathway typeIphotosystem
chlorophyllP700 noncyclicpathway typeIIphotosystem CalvinBensoncycle rubisco RuBP PGA carbonfixation PGAL cuticle stomata C3plants C4plants CAMplants
LectureOutline:
Impacts,Issues:SunlightandSurvival A.Mostautotrophsmakeuseofphotosynthesistomakefoodoutofcarbondioxideandwater. 1.Plantsareatypeofphotoautotrophs,areselfnourishing;heterotrophsmustbenourishedby others. 2.Eachyearplantsproduceabout220billiontonsofsugarandreleaseoxygentothe atmosphere. B.Thefirstcellswerechemoautotrophsthatextractedenergyfromsimpleinorganicand organiccompoundsintheenvironment. 1.Ittookupwardsofabillionyearsforlighttrappingmetabolicpathwaystoevolve. 2.Oxygenenrichmentoftheearlyatmospherewasafunctionofthecollectiveoxygen productionoftheseevolvingphotoautotrophs. a.Theincreaseinabundanceofoxygencombiningwithfreeradicalsalteredthespeciesmix onearth. b.Organismsthatharvestedenergyviaaerobicrespirationputoxygensreactivepropertiesto use. C.Theevolutionofphotoautotrophsinthedistantpastmakemodernlifepossible. 6.1SunlightasanEnergySource A.PropertiesofLight 1.Visiblelightrepresentsapartoftheelectromagneticspectrum. a.Onepropertyofradiantenergyisthatittravelsinwavelengths.Wavelengthreferstothe distancebetweencrestsofthesewaves. b.Lightenergyispackagedasphotons,whichvaryinenergyasafunctionofwavelength. 2.Photoautotrophsuselightenergybetween380and750nanometersinwavelengthtobuild organicmolecules.
3.Portionsoftheelectromagneticspectrumbelow380nanometersareenergeticenoughtobe harmfultoDNAandproteins.UV(ultraviolet),xrays,andgammaraysareexamplesofthese shorterwavelengths. B.TheRainbowCatchers 1.Pigmentmoleculesareorganiccompoundsthatselectivelyabsorbslight.Wavelengthsnot absorbedarereflected. 2.Chlorophyllaisthemainphotosyntheticpigmentinplants,algae,andcyanobacteria. Itabsorbsvioletandredlight,soitappearsgreen. 3.Carotenoidsandotheraccessorypigmentsextendtherangeofwavelengthsusable forphotosynthesis. a.AccessorypigmentsdonotbreakdownasfastasChlorophyll,andinthefallweseetheir brightcolorsofyellow,orange,andred. b.Otheraccessorypigmentsactasantioxidants. 4.Thelighttrappingpartofallpigmentsconsistsofanarrayofatomswheresinglebonds alternatewithdoublebonds. a.Photonenergyfromsunlightexcitestheelectronsoftheseatomsincreasingtheirenergy level. b.Thisenergyisreleasedortransferred,returningtheseatomstoalowerenergystate. 6.2ExploringtheRainbowFocusonScience A. Priortothelate1800smostpeoplefeltplantsusedsubstancesinthesoiltomakefood. 1. Bythe1880safewscientistshadtheideathatplantsusedsunlight,water,andsomethingin theair. 2. TheodorEngelmann,abotanist,designedanexperimentusingaprismtotestwhatpartsof sunlightbacteriawereusingtoproduceoxygen. 3. Engelmannsexperimentallowedhimtodevelopanearlyabsorptionspectrumshowinghow absorptionvarieswiththewavelengthoflight. 6.3OverviewofPhotosynthesis A.Chloroplastsareorganellesofphotosynthesisinplantsandothereukaryoticorganisms thatharnesslight.Theirstructureplaysacriticalroleintheirfunction. 1.Chloroplastsinplantshavethreemembranes. a.Twomembranesencloseasemifluidmatrixcalledthestroma.Sugarsarebuiltin thestroma. b.Thethird,calledthethylakoidmembrane,isfoldedupintoflatteneddisksinsidethestroma. c.Thespacewithinthethylakoidmembraneiscontinuous,formingasinglecompartment. 2.Thethylakoidmembranecontainsclustersoflightharvestingpigments. a.Theseclustersrepresentantennasthatabsorbphotonsofdifferentenergy. b.Embeddedinthemembranearephotosystems,groupsofhundredsofpigmentsand moleculesthatfunctionasanenergyharvestingunit,wherethereactionofphotosynthesis begins.
c.ThephotosystemsaretermedPhotosystemIorII.Theyarenamedfortheirorderof discovery.Bothtypesharvestlightenergy. 3.Overall,theequationforglucoseformationiswritten: sunlight 12H2O+6CO26O2+C6H12O6+6H2O B.TwoStagesofReactions 1.LightdependentreactionsconvertlightenergytochemicalbondenergyofATP. a.Waterissplittoreleaseoxygen. b.NADP+picksupelectronstobecomeNADPHtobeusedlater. 2.ThelightindependentreactionsassemblesugarsandotherorganicmoleculesusingATP, NADPH,andCO2. 6.4LightDependentReactions A.CapturingEnergyforPhotosynthesis 1.Photonsstrikepigmentmolecules,increasingtheirenergystate. a.Inthethylakoidmembranethisenergyiscapturedbypigmentmoleculesthatpassthe energyfromcomplextocomplexuntilitabsorbsitforkeeps. 2.Thecenterofeachphotosystemhasapairofchlorophyllamolecules. a.InphotosystemI,thepairisoptimizedtoabsorbenergyof700nanometersandiscalled P700. b.InphotosystemII,thepairisoptimizedtoabsorbenergyof680nanometersandiscalled P680. c.Theenergylevelofthechlorophyllamoleculesatthecenterisraisedtosuchahighlevel thattheelectronspoprightoff. B.ElectronFlowinaNoncyclicPathway 1.Electronsdrivenoutofaphotosystemjourneythroughanoncyclicalorcyclicalpathwayof ATPformation.ThisisthedominantmodelofATPproductioninchloroplasts. 2.UsingElectronstoMakeATP. a.Electronslostfromphotosystemsimmediatelyenteranelectrontransferchainthatis embeddedinthethylakoidmembrane. b.Electrontransferchainsareorganizedarraysofenzymes,coenzymes,andotherproteins thatacceptanddonateelectronsinturn. c.Theentryoftheelectronintothischainisthefirststepinthelightdependentreactions.Here lightenergyisconvertedtochemicalenergy. d.Electronsreleaseenergybitbybitastheymovealongthechain.Componentsofthechain usereleasedenergytomoveH+ionsacrossthethylakoidmembrane. e.AsthisprocessprogressesanH+ionconcentrationgradientisestablishedacrossthe thylakoidmembrane.H+ionsmovebackacrossthemembranethroughtheinteriorofATP synthases.TheflowcausestheseproteinstoattachphosphategroupstoADP,creatingATPin thestroma. 3.ReplacingLostElectrons
a.Electronslostbyphotosystemsarenotunlimitedandneedtobereplaced. moleculesthendisassociateintohydrogenionsandoxygen. b.Photolysisisaprocessthatpullsreplacementelectronsfromwatermolecules.Thewater c.ThehydrogenionscontributetothegradientsthatdriveATPproduction;oxygenisdiffused outofthecell. d.Photolysisisuniquetononcyclicalpathwaysofphotosynthesis;itisnamedforthefactthat theelectronsgivenupdonotreturn. e.FollowingtransferthroughtypeIItotypeI,theelectronsendupinNADPH. C.ElectronFlowinaCyclicPathway A.WhenNADPHisnotbeingused,itbacksupandstallsthenoncyclicalpathway. B.WhenthishappensacyclicpathwayrunindependentlyintypeIphotosystemswillkickin andcontinuemakingATP. a.InthiscaseelectronslostfromphotosystemIgetcycledbacktoit. b.AstheelectronscyclebacktheypassthroughanelectrontransferchainthatshuttlesH+ ionsintothethylakoidcompartment,creatingagradient. c.ThisgradientdrivesATPformationjustasinthenoncyclicalpathway,butNADPHdoesnot form. 6.5EnergyFlowinPhotosynthesisFocusonScience A.Photophosphorylationdescribesanylightdrivenreactionthatattachesphosphatetoa molecule. 1.Thecyclicpathwayevolvedfirstinanaerobicorganisms. a.Itissimplerandlessenergyefficient. b.Itstilloperatesinnearlyallphotoautotrophs. c.NoNADPHformsandoxygenisnotreleased.ElectronslosttophotosystemIarecycled backandreturned. 2.ThenoncyclicalpathwayevolvedlaterandphotosystemIIbecamepartofit. a.Thiscreatedasequenceofreactionspowerfulenoughtostripelectronsfromwater, oxidizingit. b.ThereactionyieldsoxygenandH+ions.Theelectronsthatleavedonotreturn. 3.Inbothpathwaysenergyassociatedwithelectronsflowingthroughelectrontransferchains shuttleH+acrossthethylakoidmembranesettingupaconcentrationgradientthatdrivesATP production. B.Today,differentbacteriahaveeithertypeIortypeIIphotosystemsintheirplasma membranes.Cyanobacteria,plants,andallphotoautotrophicprotistshavebothtypes. 1.ThepathwaythatdominatesdependsupontheorganismsdemandforATPandNADPH. 2.Havingthealternatepathwaysisenergyefficient.Cellsmaydirectenergytowardproducing ATPandNADPHorsimplyATPalone. 6.6LightIndependentReactions:TheSugarFactory A.ThesereactionsconstituteapathwayknownastheCalvinBensoncycle. 1.Theparticipantsandtheirrolesinthesynthesisofcarbohydratesare:
a.ATP,whichprovidesenergy; b.NADPH,whichprovideshydrogenatomsandelectrons;and c.Atmosphericair,whichprovidesthecarbonandoxygenfromcarbondioxide. 2.Thereactionstakeplaceinthestromaofchloroplastsandarenotdependentonsunlight. B.Carbondioxidediffusesintoaleaf,acrosstheplasmamembraneofaphotosyntheticcell. 1.RubiscojoinscarbondioxidetoRuBPtoproduceanunstableintermediatethatsplitstoform twomoleculesofPGA. 2.EachPGAthenreceivesaPifromATPplusH+andelectronsfromNADPHtoformPGAL (phosphoglyceraldehyde). 3.MostofthePGALmoleculescontinueinthecycletofixmorecarbondioxide,buttwo PGALjointoformasugarphosphate,whichwillbemodifiedtosucrose,starch,andcellulose. C.Mostoftheglucoseproducedisconvertedatoncetosucroseorstarchbyotherpathways that concludethelightindependentreactions. 1.Sucroseisthetransportableformofcarbohydrateinplants. 2.Excessglucoseisstoredasstarchgrainsinthestromaofchloroplasts. 3.Whensugarisneededinotherpartsoftheplants,starchisconvertedtosucroseand exported. 6.7Adaptations:DifferentCarbonFixingPathways A.Gasesdiffuseintoandoutofplantsviastomata,smallopeningsacrossthesurfaceofthe leaforgreenstem. 1.Plantsinhot,dryenvironmentsclosetheirstomatatoconservewater,butinsodoingretard carbondioxideentryandpermitoxygenbuildupinsidetheleaves. 2.Thisiswhy85percentofallplantsdonotgrowwellindryclimates.Theseplantsarecalled C3plantsbecausetheyfixcarbonwiththeCalvinBensoncycleinwhichthreecarbonPGAisthe firststableintermediate. 3.AthighO2levelstheenzymerubiscoattachesoxygeninsteadofcarbontoRuBPina wastefulpathwaycalledphotorespiration. a.CO2becomesaproductofphotorespiration,causingtheplanttoloosecarboninsteadof gainingit. b.ATPandNADPHareusedtofuelthisprocessinsteadofbeingusedtomakesugars.Thus sugarproductionislimitedinC3plantsonhot,drydays. c.Plantscompensatebyproducinglotsofrubisco,itisthemostabundantproteinonEarth. B.Bamboo,corn,andotherplantsthatevolvedinthetropicsalsoclosetheirstomatawhenitis hotanddry,buttheirsugarproductiondoesnotcease. 1.WecalltheseplantsC4plants.Theyfixcarbontwice(inmesophyllcells,theninbundle sheathcells)toproduceoxaloacetate,afourcarboncompound,whichcanthendonatethe carbondioxidetotheCalvinBensoncycle. 2.TheC4cyclekeepsCO2levelshighnearrubiscoandminimizesphotorespiration.
C.CAMisanotherevolutionaryadaptationofplantsinhot,dryclimates.CAMplantssuchas cacti,opentheirstomataandfixCO2onlyatnight,storingtheintermediateproductforusein photosynthesisthenextday. 6.8ABurningConcernFocusontheEnvironment A.Photoautotrophsarethesourceforthecarbonatomsinourbodies. B.Theseatomslikelypassedfromanautotrophthroughaseriesofheterotrophsbeforethey becamepartofus. C.Photosynthesisisthemechanismbywhichcarbonisremovedfromtheatmosphereand lockedinsideoforganicmolecules. 1.Whenaerobicorganismsbreakdownorganiccompoundsforenergy,carbonisreleasedin theformofCO2. 2.Sincephotosynthesisevolved,therehasbeenaroughbalancebetweencarbonuptakeand release. 3.Humanshavealteredthatbalance,increasingthecarbonreleaseabovethelevelthatplants takecarbonup. D.Increasedburningofwoodandcarbonfuelssuchascoal,petroleum,andnaturalgas contributetothisincreasedreleaseofcarbonintotheatmosphere. 1.IcecoresfromAntarcticaindicatethatatmosphericcarbonlevelsremainedfairlystablefor the10,000yearspriorto1850whentheindustrialrevolutionbegan. 2.SincethenatmosphericCO2levelshavebeensteadilyrising. 3.Todaywereleaseabout26billiontonsofcarbontotheatmosphere,mostfromtheburning offossilfuels.Thisis10timeswhatwereleasedintheyear1900. E.Increasedatmosphericcarbonlevelsarehavinganeffectonourglobalclimate. 1.Earthisnowwarmerthatithasbeenin12,000years. 2.Climatechangeinfluencesallbiologicalsystems. 3.Evidenceofthisimpactcanbeseeninalteredegglayingtimesinbirds,floweringtimesin plants,aswellasmigrationpatterns 4.Oneconcernforscientistsisthatthesechangesarehappeningataratethatexceedsthe capacityfororganismstoadapt. F.UndernormalconditionsextraCO2intheatmospherestimulatesphotosynthesis. 1.Alteredclimatepatternsareminimizingthisincreasedphotosyntheticrate. 2.Evidenceindicatesthatclimatechangesareharmfultoplantsandotherphotosynthesizers. 3.Currentresearchtargetsthedevelopmentofalternatefuelsthatmimictheefficiencyof photosystemIIorimprovetheefficiencyofrubisco.