CHAPTER 8 Photosynthesis: Energy from the Sun

Chapter 8: Photosynthesis: Energy from the Sun Identifying Photosynthetic Reactants and Products The Two Pathways of Photosynthesis: An Overview Properties of Light and Pigments

Chapter 8: Photosynthesis: Energy from the Sun Light Reactions: Light Absorption Making Sugar from CO2: The Calvin–Benson Cycle Photorespiration and Its Evolutionary Consequences Metabolic Pathways in Plants

Photosynthesis Life on Earth depends on the absorption of light energy from the sun. 4

Photosynthesis In plants, photosynthesis takes place in chloroplasts. 5

Identifying Photosynthetic Reactants and Products Photosynthesizing plants take in CO2, water, and light energy, producing O2 and carbohydrate. The overall reaction is 6 CO2 + 12 H2O + light  C6H12O6 + 6 O2 + 6 H2O The oxygen atoms in O2 come from water, not from CO2. Review Figures 8.1, 8.2 6

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The Two Pathways of Photosynthesis: An Overview In the light reactions of photosynthesis, electron flow and photophosphorylation produce ATP and reduce NADP+ to NADPH + H+. Review Figure 8.3 9

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The Two Pathways of Photosynthesis: An Overview ATP and NADPH + H+ are needed for the reactions that fix and reduce CO2 in the Calvin–Benson cycle, forming sugars. Review Figure 8.3 11

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Properties of Light and Pigments Light energy comes in packets called photons, but it also has wavelike properties. Review Figure 8.4 12

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Properties of Light and Pigments Pigments absorb light in the visible spectrum. Review Figure 8.5 14

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Properties of Light and Pigments Absorption of a photon puts a pigment molecule in an excited state with more energy than its ground state. Review Figure 8.6 16

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Properties of Light and Pigments Each compound has a characteristic absorption spectrum which reveals the biological effectiveness of different wavelengths of light. Review Figures 8.7, 8.8 18

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Properties of Light and Pigments Chlorophylls and accessory pigments form antenna systems for absorption of light energy. Review Figures 8.7, 8.9, 8.11 21

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Light Reactions: Light Absorption An excited pigment molecule may lose its energy by fluorescence, or by transferring it to another pigment molecule. Review Figures 8.10, 8.11 24

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Electron Flow, Photophos-phorylation, and Reductions Noncyclic electron flow uses two photosystems: Photosystem II uses P680 chlorophyll, from which light-excited electrons pass to a redox chain that drives chemiosmotic ATP production. Light-driven water oxidation releases O2, passing electrons to P680 chlorophyll. Photosystem I passes electrons from P700 chlorophyll to another redox chain and then to NADP+, forming NADPH + H+. Review Figure 8.12 26

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Electron Flow, Photophos-phorylation, and Reductions Cyclic electron flow uses P700 chlorophyll producing only ATP. Its operation maintains the proper balance of ATP and NADPH + H+ in the chloroplast. Review Figure 8.13 29

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Electron Flow, Photophos-phorylation, and Reductions Chemiosmosis is the source of ATP in photophosphorylation. Electron transport pumps protons from stroma into thylakoids, establishing a proton-motive force. Proton diffusion to stroma via ATP synthase channels drives ATP formation from ADP and Pi. Review Figure 8.14 31

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Electron Flow, Photophos-phorylation, and Reductions Photosynthesis probably originated in anaerobic bacteria that used H2S as a source of electrons instead of H2O. Oxygen production by bacteria was important in eukaryote evolution. 33

Light-Dependent Reactions The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. The light-dependent reactions take place within the thylakoid membranes of chloroplasts.

Photosynthesis begins when pigments in photosystem II absorb light, increasing their energy level. The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. The light-dependent reactions take place within the thylakoid membranes of chloroplasts.

These high-energy electrons are passed on to the electron transport chain. Photosystem II Electron carriers High-energy electron

Enzymes on the thylakoid membrane break water molecules into: Photosystem II 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. Electron carriers High-energy electron

hydrogen ions oxygen atoms energized electrons Photosystem II + O2 The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. Electron carriers High-energy electron

The energized electrons from water replace the high-energy electrons that chlorophyll lost to the electron transport chain. Photosystem II + O2 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. High-energy electron

As plants remove electrons from water, oxygen is left behind and is released into the air. Photosystem II + O2 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. The light-dependent reactions take place within the thylakoid membranes of chloroplasts. High-energy electron

The hydrogen ions left behind when water is broken apart are released inside the thylakoid membrane. Photosystem II + O2 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. High-energy electron

Energy from the electrons is used to transport H+ ions from the stroma into the inner thylakoid space. Photosystem II + O2 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen.

High-energy electrons move through the electron transport chain from photosystem II to photosystem I. Photosystem II + O2 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. Photosystem I

Pigments in photosystem I use energy from light to re-energize the electrons. The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. Photosystem I

NADP+ then picks up these high-energy electrons, along with H+ ions, and becomes NADPH. 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. 2 NADP+ 2 2 NADPH

As electrons are passed from chlorophyll to NADP+, more H+ ions are pumped across the membrane. 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. 2 NADP+ 2 2 NADPH

Soon, the inside of the membrane fills up with positively charged hydrogen ions, which makes the outside of the membrane negatively charged. + O2 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. 2 NADP+ 2 2 NADPH

The difference in charges across the membrane provides the energy to make ATP 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. 2 NADP+ 2 2 NADPH

H+ ions cannot cross the membrane directly. ATP synthase + O2 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. 2 NADP+ 2 2 NADPH

The cell membrane contains a protein called ATP synthase that allows H+ ions to pass through it 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. 2 NADP+ 2 2 NADPH

As H+ ions pass through ATP synthase, the protein rotates. 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. 2 NADP+ 2 2 NADPH

As it rotates, ATP synthase binds ADP and a phosphate group together to produce ATP. The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. ADP 2 NADP+ 2 2 NADPH

Because of this system, light-dependent electron transport produces not only high-energy electrons but ATP as well. ATP synthase + O2 2H2O The light-dependent reactions use energy from sunlight to produce ATP, NADPH, and oxygen. ADP 2 NADP+ 2 2 NADPH

Making Sugar from CO2: The Calvin–Benson Cycle The Calvin–Benson cycle makes sugar from CO2. This pathway was elucidated through use of radioactive tracers. Review Figure 8.15 34

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Making Sugar from CO2: The Calvin–Benson Cycle The Calvin–Benson cycle has three phases: fixation of CO2, reduction and carbohydrate production, and regeneration of RuBP. RuBP is the initial CO2 acceptor, 3PG is the first stable product of CO2 fixation. Rubisco catalyzes the reaction of CO2 and RuBP to form 3PG. Review Figures 8.16, 8.17 36

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Photorespiration and Its Evolutionary Consequences Rubisco catalyzes a reaction between O2 and RuBP in addition to that of CO2 and RuBP. Photorespiration significantly reduces photosynthesis efficiency. Reactions that constitute photorespiration are distributed over chloroplast, peroxisome, and mitochondria organelles. 39

Photorespiration and Its Evolutionary Consequences At high temperatures and low CO2 concentrations, the oxygenase function of rubisco is favored. 40

Photorespiration and Its Evolutionary Consequences C4 plants bypass photorespiration. PEP carboxylase in mesophyll chloroplasts initially fixes CO2 in four-carbon acids, which diffuse into bundle sheath cells, where their decarboxylation produces locally high concentrations of CO2. Review Figures 8.19 41

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Photorespiration and Its Evolutionary Consequences CAM plants operate much like C4 plants, but their initial CO2 fixation by PEP carboxylase is temporally separated from the Calvin–Benson cycle, rather than spatially separated. Review Figure 8.21 43

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Metabolic Pathways in Plants Plants respire in light and darkness, but photosynthesize only in light. A plant must photosynthesize more than it respires, giving it a net gain of reduced energy-rich compounds. 45

Metabolic Pathways in Plants Photosynthesis and respiration are linked through the Calvin–Benson cycle, the citric acid cycle, and glycolysis. Review Figure 8.22 46

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