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CHAPTER 8 Photosynthesis: Energy from the Sun

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

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CHAPTER 8 Photosynthesis: Energy from the Sun

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  1. CHAPTER 8Photosynthesis: Energy from the Sun

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

  3. 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

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

  5. Photosynthesis • In plants, photosynthesis takes place in chloroplasts. 5

  6. 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

  7. figure 08-01.jpg Figure 8.1 Figure 8.1

  8. figure 08-02.jpg Figure 8.2 Figure 8.2

  9. 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

  10. figure 08-03.jpg Figure 8.3 Figure 8.3

  11. 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

  12. figure 08-03.jpg Figure 8.3 Figure 8.3

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

  14. figure 08-04.jpg Figure 8.4 Figure 8.4

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

  16. figure 08-05.jpg Figure 8.5 Figure 8.5

  17. 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

  18. figure 08-06.jpg Figure 8.6 Figure 8.6

  19. 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

  20. figure 08-07.jpg Figure 8.7 Figure 8.7

  21. figure 08-08.jpg Figure 8.8 Figure 8.8

  22. 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

  23. figure 08-09.jpg Figure 8.9 Figure 8.9

  24. 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

  25. figure 08-10.jpg Figure 8.10 Figure 8.10

  26. figure 08-11.jpg Figure 8.11 Figure 8.11

  27. 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

  28. figure 08-12a.jpg Figure 8.12 – Part 1 Figure 8.12 – Part 1

  29. figure 08-12b.jpg Figure 8.12 – Part 2 Figure 8.12 – Part 2

  30. 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

  31. figure 08-13.jpg Figure 8.13 Figure 8.13

  32. 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

  33. figure 08-14.jpg Figure 8.14 Figure 8.14

  34. 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

  35. Light-Dependent Reactions

  36. Photosynthesis begins when pigments in photosystem II absorb light, increasing their energy level. Photosystem II

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

  38. Enzymes on the thylakoid membrane break water molecules into: Photosystem II 2H2O Electroncarriers High-energy electron

  39. hydrogen ions • oxygen atoms • energized electrons Photosystem II + O2 2H2O Electroncarriers High-energy electron

  40. The energized electrons from water replace the high-energy electrons that chlorophyll lost to the electron transport chain. Photosystem II + O2 2H2O High-energy electron

  41. As plants remove electrons from water, oxygen is left behind and is released into the air. Photosystem II + O2 2H2O High-energy electron

  42. The hydrogen ions left behind when water is broken apart are released inside the thylakoid membrane. Photosystem II + O2 2H2O High-energy electron

  43. Energy from the electrons is used to transport H+ ions from the stroma into the inner thylakoid space. Photosystem II + O2 2H2O

  44. High-energy electrons move through the electron transport chain from photosystem II to photosystem I. Photosystem II + O2 2H2O Photosystem I

  45. Pigments in photosystem I use energy from light to re-energize the electrons. + O2 2H2O Photosystem I

  46. NADP+ then picks up these high-energy electrons, along with H+ ions, and becomes NADPH. + O2 2H2O 2 NADP+ 2 NADPH 2

  47. As electrons are passed from chlorophyll to NADP+, more H+ ions are pumped across the membrane. + O2 2H2O 2 NADP+ 2 NADPH 2

  48. Soon, the inside of the membrane fills up with positively charged hydrogen ions, which makes the outside of the membrane negatively charged. + O2 2H2O 2 NADP+ 2 NADPH 2

  49. The difference in charges across the membrane provides the energy to make ATP + O2 2H2O 2 NADP+ 2 NADPH 2

  50. H+ ions cannot cross the membrane directly. ATP synthase + O2 2H2O 2 NADP+ 2 NADPH 2

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