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Lecture 6 - Photosynthesis: The Light Reactions

Lecture 6 - Photosynthesis: The Light Reactions. Chem 454: Regulatory Mechanisms in Biochemistry University of Wisconsin-Eau Claire. Introduction. Photosynthesis represents for the biosphere the major source of Free energy (10 17 kcal/year stored) Carbon (10 10 tons/year assimilated)

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Lecture 6 - Photosynthesis: The Light Reactions

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  1. Lecture 6 - Photosynthesis:The Light Reactions • Chem 454: Regulatory Mechanisms in Biochemistry • University of Wisconsin-Eau Claire

  2. Introduction • Photosynthesis represents for the biosphere the major source of • Free energy (1017 kcal/year stored) • Carbon (1010 tons/year assimilated) • Oxygen

  3. Overview • Analogous to the combination of oxidative phosphorylation and the citric acid cycle.

  4. 1. Chloroplasts • Like oxidative phosphorylation, photosynthesis is compartmentalized

  5. 1 Mitochondria • Mitochondria are bound by a double membrane

  6. 2. Electron Transfer • Light absorption by chlorophyll induces electron transfer

  7. 2. Electron Transfer • Absorption of light leads to photoinduced charge separation

  8. 2. Electron Transfer • Absorption of light leads to photoinduced charge separation

  9. 2.1 Bacteria vs. Green Plants • Plants have two photosystems • Photosystem I • 13 polypeptides chains • 60 Chlorophylls • 3 4Fe-4S centers • 1 Quinone • Photosystem II • 10 polypeptide chains • 30 Chlorophylls • 1 Non-heme Fe • 4 Mn+2 (Mn+2, Mn+3, Mn+4 , Mn+5)

  10. 2.1 Bacteria vs. Green Plants • Bacteria have a single system • 4 Bacteriochlorophylls b • 2 Bacteriopheophytin b • 2 Quinones • 1 Fe2+

  11. 2.1 Bacteria vs. Green Plants • Bacteria have a single system

  12. 2.2 The Bacterial Photosynthetic Reaction Center

  13. 2.3 Electron Transfer • The rate of electron transfer is dependent up two factors: Distance Driving Force

  14. 3.3 Q-Cytochrome c Oxidoreductase • The Q cycle:

  15. 3. Two Photosystems of Plants • In green plants there are two photosynthetic reaction centers.

  16. 3.1 Photosystem II • The core of PSII is similar to the bacterial system.

  17. 3.1 Photosystem II

  18. 3.1 Photosystem II • Four photons are required to generate one oxygen molecule

  19. 3.1 Photosystem II • The 4 Manganese center is where the electrons are extracted from the water.

  20. 3.1 Photosystem II • An equivalent of 4 H+ are moved across the thylakoid membrane by PSII

  21. 3.2 Cytochrome bf • The cytochrome bf complex transfers the electrons from plastoquinone to platocyanin:

  22. 3.2 Cytochrome bf

  23. 3.3 Q-Cytochrome c Oxidoreductase • The Q cycle:

  24. 3.3 Photosystem I • Though larger, the core of PSI is also similar to the bacterial system.

  25. 3.3 Photosystem I • The receptor of the the electrons from PSI is the small one-electron redox protein, Ferredoxin.

  26. 3.3 Photosystem I

  27. 3.3 Photosystem I • The Z-scheme of photosynthesis

  28. 3.4 Ferredoxin-NADP+ Reductase • Ferredoxin-NADP+ reductase

  29. 3.4 Ferredoxin-NADP+ Reductase • Ferredoxin-NADP+ reductase contains the coenzyme FAD.

  30. 3.4 Ferredoxin-NADP+ Reductase • Ferredoxin-NADP+ reductase contains the coenzyme FAD.

  31. 4. The Proton Gradient • PSII, Cytochrome bf and ferredoxin-NADP+ reductase each contribute to the proton gradient.

  32. 4. The Proton Gradient • André Jagendorf's demostration • 1966, was one of the earliest pieces of evidence to support Peter Mitchell's chemiosmotic hypothesis.

  33. 4.1 ATP Synthase • The ATP synthase closely resembles the ATP synthase of mitochondria

  34. 4.1 ATP Synthase • Comparing photosynthesis to oxidative phosphorylation:

  35. 4.2 Flow of Electrons Through Photosystem I • Normally photosynthesis produces both ATP and NADPH. • When there is not NADP+ available, cyclic flow of electrons through PSI can be used to produce ATP without reducing NADP+

  36. 4.2 Flow of Electrons Through Photosystem I • Cyclic flow of electrons through PSI:

  37. 4.3 Stoichiometry of Photosynthesis • 12 protons are expected to be used for one turn of the ATP synthase, therefore, producing 3 AtP's from 3ADP's and 3Pi's

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