Sinh học - Chapter 7: Photosynthesis: Using light to make food

Chapter 7Photosynthesis: Using Light to Make Food0IntroductionPlants, algae, and certain prokaryotesconvert light energy to chemical energy andstore the chemical energy in sugar, made fromcarbon dioxide andwater.© 2012 Pearson Education, Inc.IntroductionAlgae farms can be used to produceoils for biodiesel orcarbohydrates to generate ethanol.© 2012 Pearson Education, Inc.Figure 7.0_1Chapter 7: Big IdeasAn Overview ofPhotosynthesisThe Light Reactions:Converting Solar Energy toChemical EnergyThe Calvin Cycle:Reducing CO2 to SugarPhotosynthesis Reviewedand ExtendedAN OVERVIEW OF PHOTOSYNTHESIS© 2012 Pearson Education, Inc.7.1 Autotrophs are the producers of the biosphereAutotrophsmake their own food through the process of photosynthesis,sustain themselves, anddo not usually consume organic molecules derived from other organisms.© 2012 Pearson Education, Inc.7.1 Autotrophs are the producers of the biospherePhotoautotrophs use the energy of light to produce organic molecules.Chemoautotrophs are prokaryotes that use inorganic chemicals as their energy source.Heterotrophs are consumers that feed onplants oranimals, ordecompose organic material.© 2012 Pearson Education, Inc.7.1 Autotrophs are the producers of the biospherePhotosynthesis in plantstakes place in chloroplasts,converts carbon dioxide and water into organic molecules, and releases oxygen.© 2012 Pearson Education, Inc.Figure 7.1A-D7.2 Photosynthesis occurs in chloroplasts in plant cellsChloroplasts are the major sites of photosynthesis in green plants.Chlorophyllis an important light-absorbing pigment in chloroplasts,is responsible for the green color of plants, andplays a central role in converting solar energy to chemical energy.© 2012 Pearson Education, Inc.7.2 Photosynthesis occurs in chloroplasts in plant cellsChloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf.Stomata are tiny pores in the leaf that allowcarbon dioxide to enter andoxygen to exit.Veins in the leaf deliver water absorbed by roots.© 2012 Pearson Education, Inc.Figure 7.2Leaf Cross SectionMesophyllCO2O2VeinLeafStomaMesophyll CellChloroplastThylakoidThylakoid spaceStromaGranumInner and outermembranesFigure 7.2_1Leaf CrossSectionMesophyllCO2O2VeinLeafStomaMesophyll CellChloroplast7.2 Photosynthesis occurs in chloroplasts in plant cellsChloroplasts consist of an envelope of two membranes, whichenclose an inner compartment filled with a thick fluid called stroma andcontain a system of interconnected membranous sacs called thylakoids.© 2012 Pearson Education, Inc.7.2 Photosynthesis occurs in chloroplasts in plant cellsThylakoidsare often concentrated in stacks called grana andhave an internal compartment called the thylakoid space, which has functions analogous to the intermembrane space of a mitochondrion.Thylakoid membranes also house much of the machinery that converts light energy to chemical energy.Chlorophyll moleculesare built into the thylakoid membrane andcapture light energy.© 2012 Pearson Education, Inc.Figure 7.2_2ChloroplastThylakoidThylakoid spaceStromaGranumInner and outermembranes7.3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopesScientists have known since the 1800s that plants produce O2. But does this oxygen come from carbon dioxide or water?For many years, it was assumed that oxygen was extracted from CO2 taken into the plant.However, later research using a heavy isotope of oxygen, 18O, confirmed that oxygen produced by photosynthesis comes from H2O.© 2012 Pearson Education, Inc.7.3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopesExperiment 1: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2Experiment 2: 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2© 2012 Pearson Education, Inc.Figure 7.3BReactants:Products:7.4 Photosynthesis is a redox process, as is cellular respirationPhotosynthesis, like respiration, is a redox (oxidation-reduction) process.CO2 becomes reduced to sugar as electrons along with hydrogen ions from water are added to it.Water molecules are oxidized when they lose electrons along with hydrogen ions.© 2012 Pearson Education, Inc.Figure 7.4ABecomes reducedBecomes oxidized7.4 Photosynthesis is a redox process, as is cellular respirationCellular respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule.This is accomplished by oxidizing the sugar and reducing O2 to H2O.The electrons lose potential as they travel down the electron transport chain to O2.In contrast, the food-producing redox reactions of photosynthesis require energy.© 2012 Pearson Education, Inc.7.4 Photosynthesis is a redox process, as is cellular respirationIn photosynthesis,light energy is captured by chlorophyll molecules to boost the energy of electrons,light energy is converted to chemical energy, andchemical energy is stored in the chemical bonds of sugars.© 2012 Pearson Education, Inc.Figure 7.4BBecomes reducedBecomes oxidized7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPHPhotosynthesis occurs in two metabolic stages.The light reactions occur in the thylakoid membranes. In these reactionswater is split, providing a source of electrons and giving off oxygen as a by-product,ATP is generated from ADP and a phosphate group, andlight energy is absorbed by the chlorophyll molecules to drive the transfer of electrons and H+ from water to the electron acceptor NADP+ reducing it to NADPH.NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle.© 2012 Pearson Education, Inc.7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPHThe second stage is the Calvin cycle, which occurs in the stroma of the chloroplast.The Calvin cycle is a cyclic series of reactions that assembles sugar molecules using CO2 and the energy-rich products of the light reactions.During the Calvin cycle, CO2 is incorporated into organic compounds in a process called carbon fixation.After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds.The Calvin cycle is often called the dark reactions or light-independent reactions, because none of the steps requires light directly.© 2012 Pearson Education, Inc.Figure 7.5_s1LightReactions(in thylakoids)NADP+ADPPH2OLightChloroplastFigure 7.5_s2LightReactions(in thylakoids)O2NADPHATPNADP+ADPPH2OLightChloroplastFigure 7.5_s3LightReactions(in thylakoids)CalvinCycle(in stroma)SugarO2NADPHATPNADP+ADPPH2OCO2LightChloroplastTHE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY© 2012 Pearson Education, Inc.7.6 Visible radiation absorbed by pigments drives the light reactionsSunlight contains energy called electromagnetic energy or electromagnetic radiation.Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths.Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves.© 2012 Pearson Education, Inc.7.6 Visible radiation absorbed by pigments drives the light reactionsLight behaves as discrete packets of energy called photons. A photon is a fixed quantity of light energy.The shorter the wavelength, the greater the energy.© 2012 Pearson Education, Inc.Figure 7.6AIncreasing energy105 nm103 nm1 nm103 nm106 nm1 m103 m650nm380400500600700750Wavelength (nm)Visible lightGammaraysMicro-wavesRadiowavesX-raysUVInfrared7.6 Visible radiation absorbed by pigments drives the light reactionsPigmentsabsorb light andare built into the thylakoid membrane.Plant pigmentsabsorb some wavelengths of light andreflect or transmit other wavelengths.We see the color of the wavelengths that are transmitted. For example, chlorophyll transmits green wavelengths.© 2012 Pearson Education, Inc.Figure 7.6BLightReflectedlightAbsorbedlightTransmittedlightChloroplastThylakoid7.6 Visible radiation absorbed by pigments drives the light reactionsChloroplasts contain several different pigments, which absorb light of different wavelengths.Chlorophyll a absorbs blue-violet and red light and reflects green.Chlorophyll b absorbs blue and orange and reflects yellow-green.Carotenoidsbroaden the spectrum of colors that can drive photosynthesis andprovide photoprotection, absorbing and dissipating excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules.© 2012 Pearson Education, Inc.7.7 Photosystems capture solar energyPigments in chloroplasts absorb photons (capturing solar power), whichincreases the potential energy of the pigment’s electrons andsends the electrons into an unstable state.These unstable electronsdrop back down to their “ground state,” and as they do,release their excess energy as heat.© 2012 Pearson Education, Inc.Figure 7.7AExcited stateHeatGround statePhotonof lightPhoton(fluorescence)Chlorophyllmolecule7.7 Photosystems capture solar energyWithin a thylakoid membrane, chlorophyll and other pigment moleculesabsorb photons andtransfer the energy to other pigment molecules.In the thylakoid membrane, chlorophyll molecules are organized along with other pigments and proteins into photosystems.© 2012 Pearson Education, Inc.7.7 Photosystems capture solar energyA photosystem consists of a number of light-harvesting complexes surrounding a reaction-center complex.A light-harvesting complex contains various pigment molecules bound to proteins.Collectively, the light-harvesting complexes function as a light-gathering antenna.© 2012 Pearson Education, Inc.Figure 7.7BPhotosystemLightLight-harvestingcomplexesReaction-centercomplexPrimary electronacceptorPigmentmoleculesPair ofchlorophyll a moleculesTransferof energyThylakoid membrane7.7 Photosystems capture solar energyThe light energy is passed from molecule to molecule within the photosystem.Finally it reaches the reaction center where a primary electron acceptor accepts these electrons and consequently becomes reduced.This solar-powered transfer of an electron from the reaction-center pigment to the primary electron acceptor is the first step in the transformation of light energy to chemical energy in the light reactions.© 2012 Pearson Education, Inc.7.7 Photosystems capture solar energyTwo types of photosystems (photosystem I and photosystem II) cooperate in the light reactions.Each type of photosystem has a characteristic reaction center.Photosystem II, which functions first, is called P680 because its pigment absorbs light with a wavelength of 680 nm.Photosystem I, which functions second, is called P700 because it absorbs light with a wavelength of 700 nm.© 2012 Pearson Education, Inc.7.8 Two photosystems connected by an electron transport chain generate ATP and NADPHIn the light reactions, light energy is transformed into the chemical energy of ATP and NADPH.To accomplish this, electrons areremoved from water,passed from photosystem II to photosystem I, andaccepted by NADP+, reducing it to NADPH.Between the two photosystems, the electronsmove down an electron transport chain andprovide energy for the synthesis of ATP.© 2012 Pearson Education, Inc.Figure 7.8ALightStromaPhotosystem IIThylakoidspaceThylakoid membranePrimaryacceptorPrimaryacceptorP680P700Photosystem ILightNADP NADPHElectron transport chainProvides energy forsynthesis of ATPby chemiosmosis21H2O312456HO2H2Figure 7.8A_1LightStromaPhotosystem IIThylakoidspaceThylakoid membranePrimaryacceptorP680Electron transport chainProvides energy forsynthesis of ATPby chemiosmosisO2  2 H21H2O3421Figure 7.8A_2Electron transport chainProvides energy forsynthesis of ATPby chemiosmosis456PrimaryacceptorP700Photosystem ILightNADP NADPHH+Figure 7.8BPhotosystem IPhotosystem IINADPHATPMillmakesATPPhotonPhoton7.8 Two photosystems connected by an electron transport chain generate ATP and NADPHThe products of the light reactions areNADPH,ATP, and oxygen. © 2012 Pearson Education, Inc.7.9 Chemiosmosis powers ATP synthesis in the light reactionsInterestingly, chemiosmosis is the mechanism thatis involved in oxidative phosphorylation in mitochondria andgenerates ATP in chloroplasts.ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions across a membrane.© 2012 Pearson Education, Inc.7.9 Chemiosmosis powers ATP synthesis in the light reactionsIn photophosphorylation, using the initial energy input from light,the electron transport chain pumps H+ into the thylakoid space, andthe resulting concentration gradient drives H+ back through ATP synthase, producing ATP.© 2012 Pearson Education, Inc.Figure 7.9H+ATP synthasePhotosystem IPhotosystem IIElectrontransport chainATPPADPNADPHNADP+LightLightChloroplastTo CalvinCycleStroma(low H+concentration)ThylakoidmembraneThylakoid space(high H+ concentration)H2OO2 + 212H+H+H+H+H+H+H+H+H+H+H+H+H+H+H+H+H+H+Figure 7.9_1H+H+H+H+H+H+H+H+H+H+H+H+H+H+H+H+ATP synthasePhotosystem IPhotosystem IIElectrontransport chainH+H+ATPPADPNADPHNADP+LightLightH+To CalvinCycleH2OO2 2127.9 Chemiosmosis powers ATP synthesis in the light reactionsHow does photophosphorylation compare with oxidative phosphorylation?Mitochondria use oxidative phosphorylation to transfer chemical energy from food into the chemical energy of ATP.Chloroplasts use photophosphorylation to transfer light energy into the chemical energy of ATP.© 2012 Pearson Education, Inc.THE CALVIN CYCLE: REDUCING CO2TO SUGAR© 2012 Pearson Education, Inc.7.10 ATP and NADPH power sugar synthesis in the Calvin cycleThe Calvin cycle makes sugar within a chloroplast.To produce sugar, the necessary ingredients areatmospheric CO2 andATP and NADPH generated by the light reactions.The Calvin cycle uses these three ingredients to produce an energy-rich, three-carbon sugar called glyceraldehyde-3-phosphate (G3P).A plant cell may then use G3P to make glucose and other organic molecules.© 2012 Pearson Education, Inc.Figure 7.10AInputOutput:G3PCalvinCycleCO2ATPNADPH7.10 ATP and NADPH power sugar synthesis in the Calvin cycleThe steps of the Calvin cycle includecarbon fixation,reduction,release of G3P, andregeneration of the starting molecule ribulose bisphosphate (RuBP).© 2012 Pearson Education, Inc.Figure 7.10B_s111PPP3363-PGARuBPCO2RubiscoInput:Step Carbon fixationCalvinCycleFigure 7.10B_s22211PPPPPATPADP33666666NADPHNADPG3P3-PGARuBPCO2RubiscoInput:Step ReductionStep Carbon fixationCalvinCycleFigure 7.10B_s3221133Glucoseand othercompoundsPPPPPPPATPADP3351666666NADPHNADPG3PG3PG3P3-PGARuBPCO2RubiscoInput:Output:Step Release of onemolecule of G3PStep ReductionStep Carbon fixationCalvinCycleFigure 7.10B_s422113443Glucoseand othercompoundsPPPPPPPATPATPADP3ADP33351666666NADPHNADPG3PG3PG3P3-PGARuBPCO2RubiscoInput:Output:Step Regeneration of RuBPStep Release of onemolecule of G3PStep ReductionStep Carbon fixationCalvinCycle7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climatesMost plants use CO2 directly from the air, and carbon fixation occurs when the enzyme rubisco adds CO2 to RuBP.Such plants are called C3 plants because the first product of carbon fixation is a three-carbon compound, 3-PGA.© 2012 Pearson Education, Inc.7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climatesIn hot and dry weather, C3 plantsclose their stomata to reduce water loss butprevent CO2 from entering the leaf and O2 from leaving.As O2 builds up in a leaf, rubisco adds O2 instead of CO2 to RuBP, and a two-carbon product of this reaction is then broken down in the cell.This process is called photorespiration because it occurs in the light, consumes O2, and releases CO2.But unlike cellular respiration, it uses ATP instead of producing it.© 2012 Pearson Education, Inc.7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climatesC4 plants have evolved a means of carbon fixation that saves water during photosynthesis whileoptimizing the Calvin cycle.C4 plants are so named because they first fix CO2 into a four-carbon compound.When the weather is hot and dry, C4 plants keep their stomata mostly closed, thus conserving water.© 2012 Pearson Education, Inc.7.11 EVOLUTION CONNECTION: Other methods of carbon fixation have evolved in hot, dry climatesAnother adaptation to hot and dry environments has evolved in the CAM plants, such as pineapples and cacti.CAM plants conserve water by opening their stomata and admitting CO2 only at night.CO2 is fixed into a four-carbon compound,which banks CO2 at night andreleases it to the Calvin cycle during the day.© 2012 Pearson Education, Inc.PHOTOSYNTHESIS REVIEWED AND EXTENDED© 2012 Pearson Education, Inc.7.12 Review: Photosynthesis uses light energy, carbon dioxide, and water to make organic moleculesMost of the living world depends on the food-making machinery of photosynthesis.The chloroplastintegrates the two stages of photosynthesis andmakes sugar from CO2.© 2012 Pearson Education, Inc.7.12 Review: Photosynthesis uses light energy, carbon dioxide, and water to make organic moleculesAbout half of the carbohydrates made by photosynthesis are consumed as fuel for cellular respiration in the mitochondria of plant cells.Sugars also serve as the starting material for making other organic molecules, such as proteins, lipids, and cellulose.Excess food made by plants is stockpiled as starch in roots, tubers, seeds, and fruits.© 2012 Pearson Education, Inc.Figure 7.12LightThylakoidsH2OCO2O2NADPADPPATPNADPHG3P3-PGARuBPChloroplastSugarsPhotosystem IIPhotosystem ILightReactionsElectrontransport chainCalvinCycle(in stroma)StromaCellularrespirationOther organiccompoundsCelluloseStarch