Sinh học - Chapter 010: Photosynthesis

PhotosynthesisChapter 10Overview: The Process That Feeds the BiospherePhotosynthesis is the process that converts solar energy into chemical energyDirectly or indirectly, photosynthesis nourishes almost the entire living world© 2011 Pearson Education, Inc.Autotrophs sustain themselves without eating anything derived from other organismsAutotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic moleculesAlmost all plants are photoautotrophs, using the energy of sunlight to make organic molecules© 2011 Pearson Education, Inc.Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotesThese organisms feed not only themselves but also most of the living world© 2011 Pearson Education, Inc.(a) Plants(b)Multicellularalga(c)Unicellularprotists(d) Cyanobacteria(e)Purple sulfurbacteria10 m1 m40 mFigure 10.2Heterotrophs obtain their organic material from other organismsHeterotrophs are the consumers of the biosphereAlmost all heterotrophs, including humans, depend on photoautotrophs for food and O2© 2011 Pearson Education, Inc.The Earth’s supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of years agoIn a sense, fossil fuels represent stores of solar energy from the distant past© 2011 Pearson Education, Inc.Concept 10.1: Photosynthesis converts light energy to the chemical energy of foodChloroplasts are structurally similar to and likely evolved from photosynthetic bacteria The structural organization of these cells allows for the chemical reactions of photosynthesis© 2011 Pearson Education, Inc.Chloroplasts: The Sites of Photosynthesis in PlantsLeaves are the major locations of photosynthesisTheir green color is from chlorophyll, the green pigment within chloroplastsChloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leafEach mesophyll cell contains 30–40 chloroplasts© 2011 Pearson Education, Inc.CO2 enters and O2 exits the leaf through microscopic pores called stomataThe chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called granaChloroplasts also contain stroma, a dense interior fluid © 2011 Pearson Education, Inc.Figure 10.4MesophyllLeaf cross sectionChloroplastsVeinStomataChloroplastMesophyllcellCO2O220 mOutermembraneIntermembranespaceInnermembrane1 mThylakoidspaceThylakoidGranumStromaTracking Atoms Through Photosynthesis: Scientific InquiryPhotosynthesis is a complex series of reactions that can be summarized as the following equation:6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O © 2011 Pearson Education, Inc.The Splitting of WaterChloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product© 2011 Pearson Education, Inc.Figure 10.5Reactants:Products:6 CO26 H2O6 O212 H2OC6H12O6Photosynthesis as a Redox ProcessPhotosynthesis reverses the direction of electron flow compared to respirationPhotosynthesis is a redox process in which H2O is oxidized and CO2 is reducedPhotosynthesis is an endergonic process; the energy boost is provided by light© 2011 Pearson Education, Inc.Figure 10.UN01Energy  6 CO2  6 H2OC6 H12 O6  6 O2becomes reducedbecomes oxidizedThe 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 from ADP by photophosphorylation© 2011 Pearson Education, Inc.The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPHThe Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules© 2011 Pearson Education, Inc.LightLightReactionsCalvinCycleChloroplast[CH2O](sugar)ATPNADPHNADPADP+ P iH2OCO2O2Figure 10.6-4Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPHChloroplasts are solar-powered chemical factoriesTheir thylakoids transform light energy into the chemical energy of ATP and NADPH© 2011 Pearson Education, Inc.The Nature of SunlightLight is a form of electromagnetic energy, also called electromagnetic radiationLike other electromagnetic energy, light travels in rhythmic wavesWavelength is the distance between crests of wavesWavelength determines the type of electromagnetic energy© 2011 Pearson Education, Inc.The 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 seeLight also behaves as though it consists of discrete particles, called photons© 2011 Pearson Education, Inc.Figure 10.7GammaraysX-raysUVInfraredMicro-wavesRadiowavesVisible lightShorter wavelengthLonger wavelengthLower energyHigher energy380450500550600650700750 nm105 nm103 nm1 nm103 nm106 nm(109 nm)103 m1 mPhotosynthetic Pigments: The Light ReceptorsPigments are substances that absorb visible lightDifferent pigments absorb different wavelengthsWavelengths that are not absorbed are reflected or transmittedLeaves appear green because chlorophyll reflects and transmits green light© 2011 Pearson Education, Inc.ChloroplastLightReflectedlightAbsorbedlightTransmittedlightGranumFigure 10.8A 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© 2011 Pearson Education, Inc.Figure 10.9WhitelightRefractingprismChlorophyllsolutionPhotoelectrictubeGalvanometerSlit moves topass lightof selectedwavelength.GreenlightHigh transmittance(low absorption):Chlorophyll absorbsvery little green light.BluelightLow transmittance(high absorption):Chlorophyll absorbsmost blue light.TECHNIQUEAn absorption spectrum is a graph plotting a pigment’s light absorption versus wavelengthThe absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesisAn action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process© 2011 Pearson Education, Inc.(b) Action spectrum(a)AbsorptionspectraEngelmann’sexperiment(c)Chloro-phyll aChlorophyll bCarotenoidsWavelength of light (nm)Absorption of light bychloroplast pigmentsRate of photosynthesis (measured by O2 release)Aerobic bacteriaFilamentof alga400500600700400500600700400500600700RESULTSFigure 10.10Chlorophyll a is the main photosynthetic pigmentAccessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesisAccessory pigments called carotenoids absorb excessive light that would damage chlorophyll© 2011 Pearson Education, Inc.Figure 10.11Hydrocarbon tail(H atoms not shown)Porphyrin ringCH3CH3 in chlorophyll aCHO in chlorophyll bExcitation of Chlorophyll by LightWhen a pigment absorbs light, it goes from a ground state to an excited state, which is unstableWhen excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescenceIf illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat© 2011 Pearson Education, Inc.Figure 10.12ExcitedstateHeatePhoton(fluorescence)GroundstatePhotonChlorophyllmoleculeEnergy of electron(a) Excitation of isolated chlorophyll molecule(b) FluorescenceA 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 complexesThe light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center© 2011 Pearson Education, Inc.Figure 10.13(b) Structure of photosystem II(a) How a photosystem harvests lightThylakoid membraneThylakoid membranePhotonPhotosystemSTROMALight-harvestingcomplexesReaction-centercomplexPrimaryelectronacceptorTransferof energySpecial pair ofchlorophyll amoleculesPigmentmoleculesTHYLAKOID SPACE(INTERIOR OF THYLAKOID)ChlorophyllSTROMAProteinsubunitsTHYLAKOIDSPACEeFigure 10.13a(a) How a photosystem harvests lightThylakoid membranePhotonPhotosystemSTROMALight-harvestingcomplexesReaction-centercomplexPrimaryelectronacceptorTransferof energySpecial pair ofchlorophyll amoleculesPigmentmoleculesTHYLAKOID SPACE(INTERIOR OF THYLAKOID)eA primary electron acceptor in the reaction center accepts excited electrons and is reduced as a resultSolar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions© 2011 Pearson Education, Inc.There are two types of photosystems in the thylakoid membranePhotosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nmThe reaction-center chlorophyll a of PS II is called P680© 2011 Pearson Education, Inc.Photosystem I (PS I) is best at absorbing a wavelength of 700 nmThe reaction-center chlorophyll a of PS I is called P700© 2011 Pearson Education, Inc.Linear Electron FlowDuring the light reactions, there are two possible routes for electron flow: cyclic and linearLinear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy© 2011 Pearson Education, Inc.A photon hits a pigment and its energy is passed among pigment molecules until it excites P680An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)© 2011 Pearson Education, Inc.Figure 10.14-1PrimaryacceptorP680LightPigmentmoleculesPhotosystem II(PS II)12eP680+ is a very strong oxidizing agentH2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680O2 is released as a by-product of this reaction© 2011 Pearson Education, Inc.Figure 10.14-2PrimaryacceptorH2OO22 H+1/2P680LightPigmentmoleculesPhotosystem II(PS II)123eeeEach electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS IEnergy released by the fall drives the creation of a proton gradient across the thylakoid membraneDiffusion of H+ (protons) across the membrane drives ATP synthesis© 2011 Pearson Education, Inc.Figure 10.14-3CytochromecomplexPrimaryacceptorH2OO22 H+1/2P680LightPigmentmoleculesPhotosystem II(PS II)PqPcATP1235Electron transport chaineee4In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptorP700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain© 2011 Pearson Education, Inc.Figure 10.14-4CytochromecomplexPrimaryacceptorPrimaryacceptorH2OO22 H+1/2P680LightPigmentmoleculesPhotosystem II(PS II)Photosystem I(PS I)PqPcATP12356Electron transport chainP700Lightee4eeEach electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)The electrons are then transferred to NADP+ and reduce it to NADPHThe electrons of NADPH are available for the reactions of the Calvin cycleThis process also removes an H+ from the stroma© 2011 Pearson Education, Inc.Figure 10.14-5CytochromecomplexPrimaryacceptorPrimaryacceptorH2OO22 H+1/2P680LightPigmentmoleculesPhotosystem II(PS II)Photosystem I(PS I)PqPcATP1235678Electron transport chainElectron transport chainP700Light+ HNADPNADPHNADPreductaseFdeeee4eePhotosystem IIPhotosystem IMillmakesATPATPNADPHeeeeeeePhotonPhotonFigure 10.15Cyclic Electron FlowCyclic electron flow uses only photosystem I and produces ATP, but not NADPHNo oxygen is releasedCyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle© 2011 Pearson Education, Inc.Figure 10.16Photosystem IPrimaryacceptorCytochromecomplexFdPcATPPrimaryacceptorPqFdNADPHNADPreductaseNADP+ HPhotosystem IIA Comparison of Chemiosmosis in Chloroplasts and MitochondriaChloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energyMitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATPSpatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities© 2011 Pearson Education, Inc.In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrixIn chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma© 2011 Pearson Education, Inc.MitochondrionChloroplastMITOCHONDRIONSTRUCTURECHLOROPLASTSTRUCTUREIntermembranespaceInnermembraneMatrixThylakoidspaceThylakoidmembraneStromaElectrontransportchainHDiffusionATPsynthaseHADP  P iKeyHigher [H ]Lower [H ]ATPFigure 10.17ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes placeIn summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH© 2011 Pearson Education, Inc.Figure 10.18STROMA(low H concentration)STROMA(low H concentration)THYLAKOID SPACE(high H concentration)LightPhotosystem IICytochromecomplexPhotosystem ILightNADPreductaseNADP + HToCalvinCycleATPsynthaseThylakoidmembrane213NADPHFdPcPq4 H+4 H++2 H+H+ADP+P iATP1/2H2OO2Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugarThe Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycleThe cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH© 2011 Pearson Education, Inc.Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2The Calvin cycle has three phasesCarbon fixation (catalyzed by rubisco)ReductionRegeneration of the CO2 acceptor (RuBP)© 2011 Pearson Education, Inc.Input3(Entering oneat a time)CO2Phase 1: Carbon fixationRubisco3PPP6Short-livedintermediate3-Phosphoglycerate66 ADPATP6PP1,3-BisphosphoglycerateCalvinCycle6 NADPH6 NADP6 P i6PPhase 2: ReductionGlyceraldehyde 3-phosphate(G3P)P5G3PATP3 ADPPhase 3:Regeneration ofthe CO2 acceptor(RuBP)3PPRibulose bisphosphate(RuBP)1PG3P(a sugar)OutputGlucose andother organiccompounds3Figure 10.19-3The Importance of Photosynthesis: A ReviewThe energy entering chloroplasts as sunlight gets stored as chemical energy in organic compoundsSugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cellsPlants store excess sugar as starch in structures such as roots, tubers, seeds, and fruitsIn addition to food production, photosynthesis produces the O2 in our atmosphere© 2011 Pearson Education, Inc.LightLightReactions:Photosystem IIElectron transport chainPhotosystem IElectron transport chainNADPADP+ P iRuBPATPNADPH3-PhosphoglycerateCalvinCycleG3PStarch(storage)Sucrose (export)ChloroplastH2OCO2O2Figure 10.22Figure 10.UN02PrimaryacceptorPrimaryacceptorCytochromecomplexNADPreductasePhotosystem IIPhotosystem IATPPqPcFdNADP+ HNADPHH2OO2Electron transportchainElectron transportchainRegeneration ofCO2 acceptorCarbon fixationReductionCalvinCycle1 G3P (3C)5  3C3  5C6  3C3 CO2Figure 10.UN03Figure 10.UN05Figure 10.UN06Figure 10.UN07Figure 10.UN08