Sinh học - Chapter 10: Photosynthesis

Chapter 10PhotosynthesisPhotosynthesis: The Energy Transfer Process That Feeds the BiospherePhotosynthesis is an energy transfer process that converts solar energy into chemical energy.Directly or indirectly, photosynthesis nourishes almost the entire living world.Photosynthesis is the key to Earth’s food and atmospheric oxygen.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsAutotrophs “self feeders” make their own food. Autotrophs sustain themselves without eating anything derived from other organisms.Autotrophs are the producers of the biosphere, producing organic molecules (C-H-O) from CO2 and other inorganic molecules.Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules (C-H-O) from H2O and CO2Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Autotrophs = Producers(a) Plants(c) Unicellular protist10 µm1.5 µm40 µm(d) Cyanobacteria(e) Purple sulfur bacteria(b) Multicellular algaHeterotrophs obtain their organic material from other organisms. They eat (-vores)Heterotrophs are the consumers of the biosphere.Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsPhotosynthesis converts light energy to the chemical energy of food (C-H-O)Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria: cyanobacteria. The structural organization of these cells allows for the chemical reactions of photosynthesis.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsChloroplasts: The Sites of Photosynthesis in PlantsLeaves are the major locations of photosynthesis.Their green color is from chlorophyll, the green pigment within chloroplasts.Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast.CO2 enters and O2 exits the leaf through microscopic pores called stomata.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsChloroplasts are found mainly in cells of the mesophyll, the interior tissue (layers) of the leaf.A typical mesophyll cell has 30–40 chloroplastsIn the chloroplast, the chlorophyll is in the membranes of thylakoids (connected sacs); thylakoids may be stacked in columns called grana.Chloroplasts also contain stroma, a dense fluid region.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsChloroplastsZooming in on the location of photosynthesis in a plant5 µmMesophyll cellStomataCO2O2ChloroplastMesophyllVeinLeaf cross sectionTracking Atoms Through Photosynthesis: Scientific InquiryPhotosynthesis can be summarized as the following equation:6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2Oor6 CO2 + 6 H2O + Light energy  C6H12O6 + 6 O2 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsThe Splitting of Water = PhotolysisIn the light dependent reactions, chloroplasts split H2O into hydrogen and oxygen.In the light independent reactions, the chloroplasts incorporate the hydrogen from water into sugar molecules.Photosynthesis is a redox process in which H2O is oxidized to O2 and CO2 is reduced to C6 H12 O6Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsThe Two Stages of Photosynthesis: A PreviewPhotosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)The light reactions (in the thylakoids):Split H2ORelease O2Reduce NADP+ to NADPHGenerate ATP by photophosphorylationCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsThe Calvin cycle (in the stroma) forms sugar C-H-O from CO2, using ATP and NADPH.The Calvin cycle begins with carbon fixation, which incorporates CO2 into organic molecules and then reduction produces sugar C-H-O.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsLightFig. 10-5-1H2OChloroplastLightReactionsNADP+PADPi+LightLight ReactionsH2OChloroplastLightReactionsNADP+PADPi+ATPNADPHO2LightH2OChloroplastLightReactionsNADP+PADPi+ATPNADPHO2CalvinCycleCO2LightPhotosynthesisH2OChloroplastLightReactionsNADP+PADPi+ATPNADPHO2CalvinCycleCO2[CH2O](sugar)Light Reactions convert Solar Energy to the Chemical Energy of ATP and NADPHChloroplasts are solar-powered chemical factories.Their thylakoids transform light energy into the chemical energy of ATP and NADPH.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsThe Nature of SunlightLight is a form of electromagnetic energy, also called electromagnetic radiation.Like other electromagnetic energy, light travels in rhythmic waves.Wavelength is the distance between crests of waves.Wavelength determines the type of electromagnetic energy.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsThe electromagnetic spectrum is the entire range of electromagnetic energy, or radiation. Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see.Light also behaves as though it consists of discrete energy particles, called photons.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsUVVisible lightInfraredMicro-wavesRadiowavesX-raysGammarays103 m1 m(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm380450500550600650700750 nmLonger wavelengthLower energyHigher energyShorter wavelengthElectromagnetic Spectrum Photosynthetic Pigments: The Light ReceptorsPigments are substances that absorb visible light.Different pigments absorb different wavelengths.Wavelengths that are not absorbed are reflected or transmitted.Leaves appear green because chlorophyll reflects green light. We see reflected light.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsWhy leaves are green: interaction of light with chloroplastsReflectedlightAbsorbedlightLightChloroplastTransmittedlightGranumA spectrophotometer measures a pigment’s ability to absorb various wavelengths. This machine sends light through pigments and measures the fraction of light transmitted at each wavelength. Transmitted light is light that is not absorbed.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsDetermining an absorption spectrumGalvanometerSlit moves topass lightof selectedwavelengthWhitelightGreenlightBluelightThe low transmittance(high absorption)reading indicates thatchlorophyll absorbsmost blue light.The high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.RefractingprismPhotoelectrictubeChlorophyllsolutionTECHNIQUE1234An absorption spectrum is a graph plotting a pigment’s light absorption vs. wavelength.The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis.An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process such as photosynthesis.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsWavelength of light (nm)Action spectrum Absorption spectra Engelmann’s experimentAerobic bacteriaRESULTSRate of photosynthesis(measured by O2 release)Absorption of light bychloroplast pigmentsFilamentof algaChloro- phyll aChlorophyll bCarotenoids500400600700700600500400The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann.In his experiment, he exposed different segments of a filamentous alga to different wavelengths.Areas receiving wavelengths favorable to photosynthesis produced excess O2He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsChlorophyll a is the main photosynthetic pigment.Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis.Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsChlorophyllPorphyrin ring:light-absorbing“head” of molecule;note magnesiumatom at centerin chlorophyll aCH3Hydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts; H atoms notshownCHOin chlorophyll bExcitation of Chlorophyll by LightWhen a pigment absorbs light, it goes from a ground state to an excited state, which is unstable.When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence.If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsExcitation of isolated chlorophyll by light(a) Excitation of isolated chlorophyll moleculeHeatExcitedstate(b) FluorescencePhotonGroundstatePhoton(fluorescence)Energy of electrone–ChlorophyllmoleculeA Photosystem: A Reaction-Center Complex Associated with Light-Harvesting ComplexesA photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes.The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsA primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a.Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsHow aphotosystem harvests lightTHYLAKOID SPACE(INTERIOR OF THYLAKOID)STROMAe–PigmentmoleculesPhotonTransferof energySpecial pair ofchlorophyll amoleculesThylakoid membranePhotosystemPrimaryelectronacceptorReaction-centercomplexLight-harvestingcomplexesThere are two types of photosystems in the thylakoid membranes.Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm.The reaction-center chlorophyll a of PS II is called P680.Photosystem I (PS I) is best at absorbing a wavelength of 700 nm.The reaction-center chlorophyll a of PS I is called P700. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsLinear Electron Flow: Non-Cyclic Flow is Normal. During the light reactions, there are two possible routes for electron flow: cyclic and linear (non-cyclic).Linear electron flow, non-cyclic electron flow, is the primary pathway. Non-cyclic flow involves both photosystems II & I, and produces ATP and NADPH using light energy.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsIn Photosystem II electron flow, P680+ (P680 that is missing an electron) is a very strong oxidizing agent.H2O is split (photolysis) by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680.O2 is released as a by-product of this reaction.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsPigmentmoleculesLightP680e–Primaryacceptor21e–e–2 H+O2+3H2O1/24PqPcCytochromecomplexElectron transport chain5ATPPhotosystem II(PS II)In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor.P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain, ETC.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsEach electron “falls” down an ETC from the primary electron acceptor of PS I to the protein ferredoxin (Fd).The electrons are then transferred to NADP+ and reduce it to NADPH.The electrons of NADPH are available for the reactions of the Calvin cycle in the stroma.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsPigmentmoleculesLightP680e–Primaryacceptor21e–e–2 H+O2+3H2O1/24PqPcCytochromecomplexElectron transport chain5ATPPhotosystem I(PS I)LightPrimaryacceptore–P7006FdElectron transport chainNADP+reductaseNADP++ H+NADPH87e–e–6  Photosystems II and I: Non-Cyclic Electron Flow:  Produces: O2 NADPH and ATPPhotosystem II(PS II)A mechanical analogy for the light reactionsMillmakesATPe–NADPHPhotone–e–e–e–e–PhotonATPPhotosystem IIPhotosystem Ie–Cyclic Electron Flow: not the normal flowCyclic electron flow uses only photosystem I and produces ATP, but not NADPH.Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCyclic Electron Flow makes only ATPATPPhotosystem IIPhotosystem IPrimary acceptorPqCytochromecomplexFdPcPrimaryacceptorFdNADP+reductaseNADPHNADP++ H+A Comparison of Chemiosmosis in Chloroplasts and MitochondriaChloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy.Mitochondria transfer chemical energy from food to ATP : oxidative phosphosphorlation.Chloroplasts transform light energy into the chemical energy of ATP: photophosphorylation.Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsIn mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix.In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Proton Gradient drives ATP Synthesischemiosmosis in mitochondria and chloroplastsKeyMitochondrionChloroplastCHLOROPLASTSTRUCTUREMITOCHONDRIONSTRUCTUREIntermembranespaceInnermembraneElectrontransportchainH+DiffusionMatrixHigher [H+]Lower [H+]StromaATPsynthaseADP + PiH+ATPThylakoidspaceThylakoidmembrane ATP and NADPH are produced on the side facing the stroma where the Calvin cycle takes place.In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH.O2 , ATP, and NADPH are produced in the Light Reactions.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsLightFdCytochromecomplexADP + iH+ATPPATPsynthaseToCalvinCycleSTROMA(low H+ concentration)ThylakoidmembraneTHYLAKOID SPACE(high H+ concentration)STROMA(low H+ concentration)Photosystem IIPhotosystem I4 H+4 H+PqPcLightNADP+reductaseNADP+ + H+NADPH+2 H+H2OO2e–e–1/2123Photochemical = Light ReactionsThe Calvin Cycle uses ATP and NADPH to convert CO2 to sugarThe Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle.The Calvin Cycle builds sugar from smaller molecules using: CO2 , ATP, and NADPH.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCarbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) a triose sugar.For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2The Calvin cycle has three phases:Carbon fixation (CO2 attaches to RuBP catalyzed by rubisco)Reduction “sugar making”Regeneration of the CO2 acceptor (RuBP).Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCalvin CycleRibulose bisphosphate(RuBP)3-Phosphoglycerate Short-livedintermediate Phase 1: Carbon fixation(Entering oneat a time)RubiscoInputCO2P3633PPPPATP66 ADPPP61,3-Bisphosphoglycerate6PP666 NADP+NADPHiPhase 2:Reduction:“sugar making”Glyceraldehyde-3-phosphate(G3P)1POutputG3P(a triose sugar)Glucose andother organiccompoundsCalvinCycle33 ADPATP5P Phase 3:Regeneration of the CO2 acceptor RuBPG3PAlternative mechanisms of carbon fixation have evolved in hot, arid climatesDehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis.On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesisThe closing of stomata reduces access to CO2 and causes O2 to build up.These conditions favor a seemingly wasteful process called photorespiration.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsPhotorespiration: An Evolutionary Relic?In most plants, C3 plants, initial fixation of CO2 by rubisco forms a three-carbon compound.In photorespiration, rubisco bonds with and adds O2 instead of CO2 in the Calvin cycle.Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar. Wasteful.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsPhotorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle. This slows / harms plant growth significantly.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings C4 Plants = Most Efficient at Carbon FixationC4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells.This step requires the enzyme PEP carboxylasePEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low.These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsC4 leaf anatomyMesophyll cellPhotosyntheticcells of C4plant leafBundle-sheath cellVein(vascular tissue) StomaThe C4 pathwayMesophyllcellCO2PEP carboxylaseOxaloacetate (4C)Malate (4C)PEP (3C)ADPATPPyruvate (3C)CO2Bundle-sheathcellCalvinCycleSugarVasculartissue Carbon fixation occurs in Bundle Sheath, a region with low O2 CAM Plants: Stomates Open at Night Some plants, including succulents, use crassulacean acid metabolism (CAM) for carbon fixation.CAM plants open their stomata at night, incorporating CO2 into organic acids.Stomata close during the day, and CO2 is released from organic acids and used in the Calvin Cycle.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsCO2SugarcaneMesophyllcellCO2C4Bundle-sheathcellOrganic acidsrelease CO2 to Calvin cycleCO2 incorporatedinto four-carbonorganic acids(carbon fixation)PineappleNightDayCAMSugarSugarCalvinCycleCalvinCycleOrganic acidOrganic acid(a) Spatial separation of steps (b) Temporal separation of stepsCO2CO212The Importance of Photosynthesis: A ReviewThe energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds.Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells.Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits.In addition to food production, photosynthesis produces the O2 in our atmosphere.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsPhotosynthesisLightReactions: Photosystem II Electron transport chain Photosystem I Electron transport chainCO2NADP+ADPPi+RuBP3-PhosphoglycerateCalvinCycleG3PATPNADPHStarch(storage)Sucrose (export)ChloroplastLightH2OO2Photochemical = Light ReactionsCO2NADP+reductasePhotosystem IIH2OO2ATPPcCytochromecomplexPrimaryacceptorPrimaryacceptorPhotosystem INADP++ H+FdNADPHElectron transportchainElectron transportchainO2 H2OPqCalvin Cycle = Light Independent ReactionsRegeneration ofCO2 acceptor1 G3P (3C)ReductionCarbon fixation3 CO2CalvinCycle6  3C5  3C3  5CYou should now be able to:Describe the structure of a chloroplast.Describe the relationship between an action spectrum and an absorption spectrum.Trace the movement of electrons in linear, noncyclic electron flow.Trace the movement of electrons in cyclic electron flow.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin CummingsDescribe the similarities and differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. .;Describe the role of ATP and NADPH in the Calvin cycle.Describe the major consequences of photorespiration.Describe two important photosynthetic adaptations that minimize photorespiration.Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings