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Photosynthesis LEARNING OBJECTIVES 1. Understand that ENERGY can be transformed from one form to another. 2.Know that energy exist in two forms; free energy - available for doing work or as heat - a form unavailable for doing work. 3.Appreciate that the Sun provides most of the energy needed for life on Earth. The biomass (dry weight) of a tree comes primarily from –soil. –water. –light. –organic fertilizer (manure, detritus). –air. • Autotrophs sustain themselves without eating anything derived from other organisms • Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules • Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules • Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes • These organisms feed not only themselves but also most of the living world • Heterotrophs obtain their organic material from other organisms • Heterotrophs are the consumers of the biosphere • Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2 • The Earth’s supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of years ago • In a sense, fossil fuels represent stores of solar energy from the distant past Photosynthesis converts light energy to the chemical energy of food • Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria • The structural organization of these cells allows for the chemical reactions of photosynthesis The organic carbon in a tree comes primarily from –soil. –water. –light. –organic fertilizer (manure, detritus). –air. Chloroplasts: The Sites of Photosynthesis in Plants • Leaves are the major locations of photosynthesis • Their green color is from chlorophyll, the green pigment within chloroplasts • Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf • Each mesophyll cell contains 30–40 chloroplasts Light Reflected light Transmitted light Chloroplast Absorbed light • The location and structure of chloroplasts Chloroplast LEAF CROSS SECTION MESOPHYLL CELL LEAF Mesophyll CHLOROPLAST Intermembrane space Outer membrane Granum Grana Stroma Inner membrane Stroma Thylakoid Thylakoid compartment WHY ARE PLANTS GREEN? Different wavelengths of visible light are seen by the human eye as different colors. Gamma rays X-rays UV Infrared Visible light Wavelength (nm) Microwaves Radio waves Why are plants green? Transmitted light What colors of light will drive photosynthesis by green plants most efficiently? a) b) c) d) e) red only yellow only green only blue only red and blue Chloroplast Pigments • Chloroplasts contain several pigments – Chlorophyll a – Chlorophyll b – Carotenoids Figure 7.7 Chlorophyll a & b •Chl a has a methyl group •Chl b has a carbonyl group Porphyrin ring delocalized e- Phytol tail Different pigments absorb light differently Excitation of chlorophyll in a chloroplast e 2 Excited state Heat Light Light (fluorescence) Photon Ground state Chlorophyll molecule (a) Absorption of a photon Loss of energy due to heat causes the photons of light to be less energetic. Less energy translates into longer wavelength. Transition toward the red end of the visible spectrum. AN OVERVIEW OF PHOTOSYNTHESIS • The light reactions convert solar energy to chemical energy Light Chloroplast NADP ADP +P – Produce ATP & NADPH • The Calvin cycle makes sugar from carbon dioxide – ATP generated by the light reactions provides the energy for sugar synthesis – The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose Light reactions Calvin cycle Figure 10.5 Reactants: Products: 6 CO2 C6H12O6 12 H2O 6 H2O 6 O2 Which experiment will produce 18O2? 1. experiment 1 2. experiment 2 3. both experiment 1 and experiment 2 4. neither experiment Photosynthesis as a Redox Process • Photosynthesis reverses the direction of electron flow compared to respiration • Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced • Photosynthesis is an endergonic process; the energy boost is provided by light becomes reduced Energy 6 CO2 6 H2O C6 H12 O6 6 O2 becomes oxidized The Two Stages of Photosynthesis: A Preview • Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) • The light reactions (in the thylakoids) – – – – Split H2O Release O2 Reduce NADP+ to NADPH Generate ATP from ADP by photophosphorylation • The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH • The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes Photosystem Lightharvesting complexes Thylakoid membrane • A 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) transfer the energy of photons to the reaction center Photon Reactioncenter complex STROMA Primary electron acceptor e Transfer of energy Pigment Special pair of molecules chlorophyll a molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a) How a photosystem harvests light • There are two types of photosystems in the thylakoid membrane • 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 Linear Electron Flow e e e e Mill makes ATP e NADPH e e ATP Photosystem II Photosystem I Photosynthetic bacteria that have only photosystem I 1. 2. 3. 4. can split water molecules to produce oxygen. cannot fix carbon dioxide. generate ATP but not NADPH. can reduce NADP+ to NADPH but cannot make ATP through photophosphorylation. 5. can reduce NADP+ to NADPH and make ATP through cyclic photophosphorylation. Cyclic Electron Flow – only PSI Primary acceptor Primary acceptor Fd Fd Pq NADP reductase Cytochrome complex NADPH Pc Photosystem I Photosystem II ATP NADP + H Which evolutionary development caused the initial oxygenation of Earth’s atmosphere? 1. the evolution of the earliest photosynthetic organisms that had only PSI 2. the evolution of the first land plants 3. the evolution of the woody plants 4. the evolution of flowering plants 5. the evolution of cyanobacteria with PSI + PSII Figure 10.17 Chloroplast Mitochondrion CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE H Intermembrane space Inner membrane Diffusion Electron transport chain Thylakoid space Thylakoid membrane ATP synthase Matrix Stroma ADP P i Key [H ] Higher Lower [H ] H ATP Figure 10.18 STROMA (low H concentration) Photosystem II 4 H+ Light Cytochrome complex Photosystem I Light NADP reductase 3 NADP + H Fd Pq H2O NADPH Pc 2 1 THYLAKOID SPACE (high H concentration) 1/ 2 O2 +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H concentration) ATP synthase ADP + Pi ATP H+ The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar • The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave • 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 CO2 • The Calvin cycle has three phases – Carbon fixation (catalyzed by rubisco) – Reduction – Regeneration of the CO2 acceptor (RuBP) In this diagram, compound X is the CO2 acceptor. If CO2 is cut off, then 1. X and 3PG will both increase. 2. X will increase, 3PG decrease. 3. X will decrease, 3PG increase. 4. X and 3PG will both decrease. In this diagram, compound X is the CO2 acceptor. If light is cut off, then 1. X and 3PG will both increase. 2. X will increase, 3PG decrease. 3. X will decrease, 3PG increase. 4. X and 3PG will both decrease. • The production of ATP by chemiosmosis in photosynthesis Thylakoid compartment (high H+) Light Light Thylakoid membrane Antenna molecules Stroma (low H+) ELECTRON TRANSPORT CHAIN PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE Alternative mechanisms of carbon fixation have evolved in hot, arid climates • Dehydration 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 photosynthesis • The closing of stomata reduces access to CO2 and causes O2 to build up • These conditions favor an apparently wasteful process called photorespiration Photorespiration occurs because a) rubisco can use oxygen as a substrate when CO2 levels are low and oxygen levels are high. b) linear electron flow cannot provide the Calvin cycle with enough ATP. c) leaf cells use photorespiration to make ATP for cellular work outside the chloroplasts. d) C4 plants operate a CO2 shuttle at a cost of extra ATP, provided by photorespiration. e) plants need a way to consume the oxygen they produce. • In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3phosphoglycerate) • In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound • Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar • Photorespiration 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 C4 Plants • C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells • This step requires the enzyme PEP carboxylase • PEP 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 Figure 10.20 The C4 pathway C4 leaf anatomy Mesophyll cell PEP carboxylase Mesophyll cell Photosynthetic cells of C4 Bundleplant leaf sheath cell Oxaloacetate (4C) Vein (vascular tissue) PEP (3C) ADP Malate (4C) Stoma Bundlesheath cell CO2 ATP Pyruvate (3C) CO2 Calvin Cycle Sugar Vascular tissue • In the last 150 years since the Industrial Revolution, CO2 levels have risen greatly • Increasing levels of CO2 may affect C3 and C4 plants differently, perhaps changing the relative abundance of these species • The effects of such changes are unpredictable and a cause for concern CAM Plants • Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon • 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