Bài giảng Biology - Chapter 10: Photosynthesis

Chapter 10PhotosynthesisOverview: The Process That Feeds the BiospherePhotosynthesisIs the process that converts solar energy into chemical energyPlants and other autotrophsAre the producers of the biospherePlants are photoautotrophsThey use the energy of sunlight to make organic molecules from water and carbon dioxideFigure 10.1PhotosynthesisOccurs in plants, algae, certain other protists, and some prokaryotesThese organisms use light energy to drive the synthesis of organic molecules from carbon dioxideand (in most cases) water. They feed not onlythemselves, but the entire living world. (a) Onland, plants are the predominant producers offood. In aquatic environments, photosyntheticorganisms include (b) multicellular algae, suchas this kelp; (c) some unicellular protists, suchas Euglena; (d) the prokaryotes calledcyanobacteria; and (e) other photosyntheticprokaryotes, such as these purple sulfurbacteria, which produce sulfur (sphericalglobules) (c, d, e: LMs).(a) Plants(b) Multicellular algae(c) Unicellular protist10 m40 m(d) Cyanobacteria1.5 m(e) Pruple sulfurbacteriaFigure 10.2HeterotrophsObtain their organic material from other organismsAre the consumers of the biosphereConcept 10.1: Photosynthesis converts light energy to the chemical energy of foodChloroplasts: The Sites of Photosynthesis in PlantsThe leaves of plantsAre the major sites of photosynthesisVeinLeaf cross sectionFigure 10.3MesophyllCO2O2StomataChloroplastsAre the organelles in which photosynthesis occursContain thylakoids and granaChloroplastMesophyll5 µmOutermembraneIntermembranespaceInnermembraneThylakoidspaceThylakoidGranumStroma1 µmTracking Atoms Through Photosynthesis: Scientific InquiryPhotosynthesis is summarized as6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O The Splitting of WaterChloroplasts split water intoHydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules6 CO212 H2OReactants:Products:C6H12O66 H2O6 O2Figure 10.4Photosynthesis as a Redox ProcessPhotosynthesis is a redox processWater is oxidized, carbon dioxide is reducedThe Two Stages of Photosynthesis: A PreviewPhotosynthesis consists of two processesThe light reactionsThe Calvin cycleThe light reactionsOccur in the granaSplit water, release oxygen, produce ATP, and form NADPHThe Calvin cycleOccurs in the stromaForms sugar from carbon dioxide, using ATP for energy and NADPH for reducing powerAn overview of photosynthesisH2OCO2LightLIGHT REACTIONSCALVINCYCLEChloroplast[CH2O](sugar)NADPHNADP ADP+ PO2Figure 10.5ATPConcept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPHThe Nature of SunlightLightIs a form of electromagnetic energy, which travels in wavesWavelengthIs the distance between the crests of wavesDetermines the type of electromagnetic energyThe electromagnetic spectrumIs the entire range of electromagnetic energy, or radiationGammaraysX-raysUVInfraredMicro-wavesRadiowaves10–5 nm10–3 nm1 nm103 nm106 nm1 m106 nm103 m380450500550600650700750 nmVisible lightShorter wavelengthHigher energyLonger wavelengthLower energyFigure 10.6The visible light spectrumIncludes the colors of light we can seeIncludes the wavelengths that drive photosynthesisPhotosynthetic Pigments: The Light ReceptorsPigmentsAre substances that absorb visible lightReflect light, which include the colors we seeLightReflectedLight ChloroplastAbsorbedlight GranumTransmittedlight Figure 10.7The spectrophotometerIs a machine that sends light through pigments and measures the fraction of light transmitted at each wavelengthAn absorption spectrumIs a graph plotting light absorption versus wavelengthFigure 10.8WhitelightRefractingprismChlorophyllsolutionPhotoelectrictubeGalvanometerSlit moves topass lightof selectedwavelength GreenlightThe high transmittance(low absorption)reading indicates thatchlorophyll absorbsvery little green light.The low transmittance(high absorption) readingchlorophyll absorbs most blue light.Bluelight123401000100The absorption spectra of chloroplast pigmentsProvide clues to the relative effectiveness of different wavelengths for driving photosynthesisThe absorption spectra of three types of pigments in chloroplasts Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below.EXPERIMENTRESULTSAbsorption of light bychloroplast pigmentsChlorophyll a(a) Absorption spectra. The three curves show the wavelengths of light best absorbed by  three types of chloroplast pigments.Wavelength of light (nm)Chlorophyll bCarotenoidsFigure 10.9The action spectrum of a pigmentProfiles the relative effectiveness of different wavelengths of radiation in driving photosynthesisRate of photosynthesis(measured by O2 release)Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids.(b)The action spectrum for photosynthesisWas first demonstrated by Theodor W. Engelmann400500600700Aerobic bacteriaFilamentof algaEngelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most.Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b.(c) Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis.CONCLUSIONChlorophyll aIs the main photosynthetic pigmentChlorophyll bIs an accessory pigmentCCHCH2CCCCCCNNCH3CCCCCCCCCNCCCCNMgHH3CHCCH2CH3HCH3CHHCH2CH2CH2HCH3COOOOOCH3CH3CHOin chlorophyll ain chlorophyll bPorphyrin ring:Light-absorbing“head” of moleculenote magnesiumatom at centerHydrocarbon tail:interacts with hydrophobicregions of proteins insidethylakoid membranes ofchloroplasts: H atoms notshownFigure 10.10Other accessory pigmentsAbsorb different wavelengths of light and pass the energy to chlorophyll aExcitation of Chlorophyll by LightWhen a pigment absorbs lightIt goes from a ground state to an excited state, which is unstableExcitedstateEnergy of electionHeatPhoton(fluorescence)ChlorophyllmoleculeGroundstatePhotone–Figure 10.11 AIf an isolated solution of chlorophyll is illuminatedIt will fluoresce, giving off light and heatFigure 10.11 BA Photosystem: A Reaction Center Associated with Light-Harvesting ComplexesA photosystemIs composed of a reaction center surrounded by a number of light-harvesting complexesPrimary electionacceptorPhotonThylakoidLight-harvestingcomplexesReactioncenterPhotosystemSTROMAThylakoid membraneTransferof energySpecialchlorophyll amoleculesPigmentmoleculesTHYLAKOID SPACE(INTERIOR OF THYLAKOID)Figure 10.12e–The light-harvesting complexesConsist of pigment molecules bound to particular proteinsFunnel the energy of photons of light to the reaction centerWhen a reaction-center chlorophyll molecule absorbs energyOne of its electrons gets bumped up to a primary electron acceptorThe thylakoid membraneIs populated by two types of photosystems, I and IINoncyclic Electron FlowNoncyclic electron flowIs the primary pathway of energy transformation in the light reactionsProduces NADPH, ATP, and oxygenFigure 10.13Photosystem II(PS II)Photosystem-I(PS I)ATPNADPHNADP+ADPCALVINCYCLECO2H2OO2[CH2O] (sugar)LIGHTREACTIONSLightPrimaryacceptorPqCytochromecomplexPCeP680e–e–O2+H2O2 H+LightATPPrimaryacceptorFdee–NADP+reductaseElectronTransportchainElectron transport chainP700LightNADPHNADP++ 2 H++ H+15723468A mechanical analogy for the light reactionsMillmakesATPATPe–e–e–e–e–PhotonPhotosystem IIPhotosystem Ie–e–NADPHPhotonFigure 10.14 Cyclic Electron FlowUnder certain conditionsPhotoexcited electrons take an alternative pathIn cyclic electron flowOnly photosystem I is usedOnly ATP is producedPrimaryacceptorPqFdCytochromecomplexPcPrimaryacceptorFdNADP+reductaseNADPHATPFigure 10.15Photosystem IIPhotosystem INADP+A Comparison of Chemiosmosis in Chloroplasts and MitochondriaChloroplasts and mitochondriaGenerate ATP by the same basic mechanism: chemiosmosisBut use different sources of energy to accomplish thisThe spatial organization of chemiosmosisDiffers in chloroplasts and mitochondriaKeyHigher [H+]Lower [H+]MitochondrionChloroplastMITOCHONDRIONSTRUCTUREIntermembrancespaceMembranceMatrixElectrontransportchainH+DiffusionThylakoidspaceStromaATPH+PADP+ATPSynthaseCHLOROPLASTSTRUCTUREFigure 10.16In both organellesRedox reactions of electron transport chains generate a H+ gradient across a membraneATP synthaseUses this proton-motive force to make ATPThe light reactions and chemiosmosis: the organization of the thylakoid membraneLIGHTREACTORNADP+ADPATPNADPHCALVINCYCLE[CH2O] (sugar)STROMA(Low H+ concentration)Photosystem IILIGHTH2OCO2CytochromecomplexO2H2OO211⁄22Photosystem ILightTHYLAKOID SPACE(High H+ concentration)STROMA(Low H+ concentration)ThylakoidmembraneATPsynthasePqPcFdNADP+reductaseNADPH+ H+NADP+ + 2H+ToCalvincycleADPPATP3H+2 H++2 H+2 H+Figure 10.17Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugarThe Calvin cycleIs similar to the citric acid cycleOccurs in the stromaThe Calvin cycle has three phasesCarbon fixationReductionRegeneration of the CO2 acceptorThe Calvin cycle(G3P)Input(Entering oneat a time)CO23RubiscoShort-livedintermediate3 PP3 PPRibulose bisphosphate(RuBP)P3-PhosphoglycerateP6 P61,3-Bisphoglycerate6 NADPH6 NADPH+6 PP6Glyceraldehyde-3-phosphate(G3P)6 ATP3 ATP3 ADPCALVINCYCLEP5P1G3P(a sugar)OutputLightH2OCO2LIGHTREACTIONATPNADPHNADP+ADP[CH2O] (sugar)CALVINCYCLEFigure 10.18O26 ADPGlucose andother organiccompoundsPhase 1: Carbon fixationPhase 2:ReductionPhase 3:Regeneration ofthe CO2 acceptor(RuBP)Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climatesOn hot, dry days, plants close their stomataConserving water but limiting access to CO2Causing oxygen to build upPhotorespiration: An Evolutionary Relic?In photorespirationO2 substitutes for CO2 in the active site of the enzyme rubiscoThe photosynthetic rate is reducedC4 PlantsC4 plants minimize the cost of photorespirationBy incorporating CO2 into four carbon compounds in mesophyll cellsThese four carbon compoundsAre exported to bundle sheath cells, where they release CO2 used in the Calvin cycleC4 leaf anatomy and the C4 pathwayCO2Mesophyll cellBundle-sheathcellVein(vascular tissue)Photosyntheticcells of C4 plantleafStomaMesophyllcellC4 leaf anatomyPEP carboxylaseOxaloacetate (4 C)PEP (3 C)Malate (4 C)ADPATPBundle-SheathcellCO2Pyruate (3 C)CALVINCYCLESugarVasculartissueFigure 10.19CO2CAM PlantsCAM plantsOpen their stomata at night, incorporating CO2 into organic acidsDuring the day, the stomata closeAnd the CO2 is released from the organic acids for use in the Calvin cycleThe CAM pathway is similar to the C4 pathwaySpatial separation of steps. In C4 plants, carbon fixation and the Calvin cycle occur in differenttypes of cells.(a)Temporal separation of steps. In CAM plants, carbon fixation and the Calvin cycle occur in the same cellsat different times.(b)PineappleSugarcaneBundle-sheath cellMesophyll CellOrganic acidCALVINCYCLESugarCO2CO2Organic acidCALVINCYCLESugarC4CAMCO2 incorporatedinto four-carbonorganic acids(carbon fixation)NightDay12Organic acidsrelease CO2 toCalvin cycleFigure 10.20The Importance of Photosynthesis: A ReviewA review of photosynthesisLight reactions:• Are carried out by molecules in the thylakoid membranes• Convert light energy to the chemical energy of ATP and NADPH• Split H2O and release O2 to the atmosphere Calvin cycle reactions:• Take place in the stroma• Use ATP and NADPH to convert CO2 to the sugar G3P• Return ADP, inorganic phosphate, and  NADP+ to the light reactionsO2CO2H2OLightLight reactionCalvin cycleNADP+ADPATPNADPH+ P 1RuBP3-PhosphoglycerateAmino acidsFatty acidsStarch(storage)Sucrose (export)G3PPhotosystem IIElectron transport chainPhotosystem IChloroplastFigure 10.21Organic compounds produced by photosynthesisProvide the energy and building material for ecosystems