1. Lecture 44: Photosynthesis and Redox NO YES

2. In eukaryotes, photosynthesis occurs in chloroplasts

3. The major steps of photosynthesis Photon absorption excites an electron – Described in lectures 28 and 29 Electron flow allows H + to be pumped into a region of high concentration/positive charge – Focus of this lecture H + flow out of the high concentration/positive charge region drives ATP synthesis – Focus of Monday’s lecture ATP is used to power carbon fixation, generating e.g. sugars – Maybe next year?

4. Monday Today The major steps of photosynthesis

5. The photosynthesis electron transport chain

6. Why do electrons flow this way?

7. Outline for today’s lecture Photosynthesis – Overview and some cellular context – The photosynthesis electron transport chain Reduction and oxidation – Terminology – Role of electric potential – Predicting net direction of electron flow Coupling of electron flow to H + “pumping” – H + production from water splitting – The “Q cycle” mechanism

8. First, some key terminology: reduction and oxidation Reduction is the gain of an electron. Oxidation is the loss of an electron. The nomenclature reflects the tendency of oxygen, a highly electronegative atom, to partially or fully steal e - from other molecules. O2O2 e-e-

9. First, some key terminology: reduction and oxidation Reduction is the gain of an electron. Oxidation is the loss of an electron. Whenever one molecule is oxidized, another is reduced (unless the e - is truly liberated). The oxidized and reduced forms of a molecule constitute a redox pair. reduced form oxidized form

10. First, some key terminology: reduction and oxidation Reduction is the gain of an electron. Oxidation is the loss of an electron. When a molecule in water is reduced, it often picks up a H +, too: Adding/removing a net hydrogen (H + & e - ) is called hydrogenation/dehydrogenation

11. First, some key terminology: reduction and oxidation Fun fact: hydrogenation of fatty acids is used to make margarine

12. First, some key terminology: reduction and oxidation To understand the net direction of electron flow in the photosynthesis electron transport chain, we would like to know: What is the change in energy associated with a balanced redox reaction?, e.g.

13. Electrons can decrease their electric potential energy U e by moving to molecules where they experience a more positive electric potential  Electron flow and electric potential

14. Electrons experience electric potentials inside molecules Thorium nucleus (90 protons) Oxygen nucleus (8 protons) 97 other electrons?!

15. Electrons can minimize their electric potential energy U e by moving to molecules where they experience a more positive electric potential  We can measure relative values of  experimentally.

16. Electron flow between molecules, driven by electric potential differences, can be measured Metal #1 Metal #2

17. Electrons flow to molecules with more positive internal electric potentials

18. Magnesium Graphite

19. Measuring “reduction potential” of biological redox pairs The previous examples used the same solution in both cells but different electrode types. Now suppose we use identical electrodes, but different solutions. Electrons will flow toward the solution containing the “greedier” redox pair. We can use this electron flow to measure the electrical potential of a solution relative to a standard: we call this the “reduction potential”.

20. What will happen if A oxidized has a higher affinity for electrons than H + ? ? The electric potential on the left is more positive (reduction potential is positive)

21. What will happen if A oxidized has a lower affinity for electrons than H + ? ? The electric potential on the left is less positive (reduction potential is negative)

22. Reduction potential E o

23. Reduction potentials along the photosynthesis electron transport chain

24. Relationship between reduction potential and energy differences E 0 ’ = - 400 mV E 0 ’ = - 320 mV What is the change in electric potential energy  U e on transferring two electrons from ferredoxin (Fd 2- ) to NADP + at standard state?

25. Question 2: How is H + “pumping” driven by electron flow?

26. Proton “pumping” mechanism The Expectation The Reality

27. H + production from water splitting

28. Photosynthesis and aerobic respiration

29. Plastoquinone (Q) as a “proton pump” Photosystem II (initial site of excitation) Cytochrome b 6 -f complex

30. Plastoquinone (Q) as a “proton pump” Photosystem II (initial site of excitation) Cytochrome b 6 -f complex

31. Plastoquinone (Q) as a “proton pump” Photosystem II (initial site of excitation) Cytochrome b 6 -f complex

32. Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex

33. Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex

34. Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex

35. Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex

36. Plastoquinone (Q) as a “proton pump” Cytochrome b 6 -f complex This clever recycling of electrons in a loop is called the “Q cycle”

37. Electron transport chains in photosynthesis and respiration both use Q cycles

38. What we hope you learned today How a H + gradient is generated during photosynthesis – Why electrons move down an electron transport chain from carrier to carrier – How electron movement is coupled to proton “pumping” in the Q cycle How to describe redox reactions How to build a battery