Understanding Photosynthesis: Light and Dark Reactions

2. Photosynthesis 1) Light rxns use light to pump H+ use ∆ pH to make ATP by chemiosmosis 2) Light-independent (dark) rxns use ATP & NADPH from light rxns to make organics only link: each provides substrates needed by the other

3. Cyclic photophosphorylation Limitations Only makes ATP Does not supply electrons for biosynthesis = no reducing power

4. Photosystem II Evolved to provide reducing power -> added to PSI

5. PSI and PSII work together in the “Z-scheme” - a.k.a. “non-cyclic photophosphorylation” General idea: ∆ redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 steps

6. PSI and PSII work together in the “Z-scheme” General idea: ∆ redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 steps each step uses a photon

7. PSI and PSII work together in the “Z-scheme” General idea: ∆ redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 steps each step uses a photon 2 steps = 2 photosystems

8. PSI and PSII work together in the “Z-scheme” 1) PSI reduces NADP+

9. PSI and PSII work together in the “Z-scheme” 1) PSI reduces NADP+ e- are replaced by PSII

10. PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI

11. PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI Each e- drives cyt b6/f

12. PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI Each e- drives cyt b6/f Use PMF to make ATP

13. PSI and PSII work together in the “Z-scheme” - a.k.a. “non-cyclic photophosphorylation” General idea: ∆ redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 steps

14. PSI and PSII work together in the “Z-scheme” General idea: ∆ redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 steps each step uses a photon

15. PSI and PSII work together in the “Z-scheme” General idea: ∆ redox potential from H2O to NADP+ is so great that must boost energy of H2O e- in 2 steps each step uses a photon 2 steps = 2 photosystems

16. PSI and PSII work together in the “Z-scheme” 1) PSI reduces NADP+

17. PSI and PSII work together in the “Z-scheme” 1) PSI reduces NADP+ e- are replaced by PSII

18. PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI

19. PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI Each e- drives cyt b6/f

20. PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI Each e- drives cyt b6/f Use PMF to make ATP

21. PSI and PSII work together in the “Z-scheme” 2) PSII gives excited e- to ETS ending at PSI Each e- drives cyt b6/f Use PMF to make ATP PSII replaces e- from H2O forming O2

22. PSI and PSII work together in the “Z-scheme” Light absorbed by PS II makes ATP Light absorbed by PS I makes reducing power

23. cyclic non-cyclic Ultimate e- source None water O2 released? No yes Terminal e- acceptor None NADP+ Form in which energy is ATP ATP & temporarily captured NADPH Photosystems required PSI PSI & PSII

24. Z-scheme energetics

25. Physical organization of Z-scheme • PS II consists of: P680 (a dimer of chl a) • ~ 30 other chl a& a few carotenoids • > 20 proteins • D1 & D2 bind P680 & all e- carriers

26. Physical organization of Z-scheme • PSII has 2 groups of closely associated proteins • 1) OEC (oxygen evolving complex) • on lumen side, near rxn center • Ca2+, Cl- & 4 Mn2+

27. Physical organization of Z-scheme • PSII also has two groups of closely associated proteins • 1) OEC (oxygen evolving complex) • on lumen side, near rxn center • Ca2+, Cl- & 4 Mn2+ • 2) variable numbers of LHCII complexes

28. Physical organization of Z-scheme D1 & D2 bind P680 & all e- carriers Synechoccous elongatus associates phycobilisomes cf LHCII with PSII

29. Physical organization of Z-scheme D1 & D2 bind P680 & all e- carriers Synechoccous elongatus associates phycobilisomes cf LHCII with PSII

30. Physical organization of Z-scheme 2 mobile carriers plastoquinone : lipid similar to ubiquinone

31. Physical organization of Z-scheme 2 mobile carriers 1) plastoquinone : lipid similar to ubiquinone “headgroup” alternates between quinone & quinol

32. Physical organization of Z-scheme 2 mobile carriers 1) plastoquinone : lipid similar to ubiquinone “headgroup” alternates between quinone & quinol Carries 2 e- & 2 H+

33. Physical organization of Z-scheme 2 mobile carriers 1) plastoquinone : hydrophobic molecule like ubiquinone “headgroup” alternates between quinone and quinol Carries 2 e- & 2 H+ diffuses within bilayer

34. Physical organization of Z-scheme • 2 mobile carriers • 1) plastoquinone • 2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumen

35. Physical organization of Z-scheme • 2) plastocyanin (PC) : peripheral membrane protein of thylakoid lumen • Cu is alternately oxidized & reduced • carries 1 e- & 1 H+

36. Physical organization of Z-scheme • 3 protein complexes (visible in EM of thylakoid) • 1) PSI • 2) PSII • 3) cytochrome b6/f • 2 cytochromes & an Fe/S protein

37. Physical organization of Z-scheme • 2 mobile carriers • 1) plastoquinone • 2) plastocyanin (PC) • 3 protein complexes • 1) PSI • 2) PSII • 3) cytochrome b6/f • ATP synthase (CF0-CF1 • ATPase) is also visible • in E/M

38. Physical organization of Z-scheme • 3 protein complexes (visible in EM of thylakoid) • 1) PSI • 2) PSII • 3) cytochrome b6/f • 2 cytochromes & an Fe/S protein

39. Physical organization of Z-scheme • 2 mobile carriers • 1) plastoquinone • 2) plastocyanin (PC) • 3 protein complexes • 1) PSI • 2) PSII • 3) cytochrome b6/f • ATP synthase (CF0-CF1 • ATPase) is also visible • in E/M

40. Physical organization of Z-scheme • Complexes are arranged asymmetrically! • PSII is in appressed regions of grana

41. Physical organization of Z-scheme • Complexes are arranged asymmetrically! • PSII is in appressed regions of grana • PSI and ATP synthase are found in exposed regions (ends & margins of grana, and stromal lamellae)

42. Physical organization of Z-scheme • Complexes are arranged asymmetrically! • PSII is in appressed regions of grana • PSI and ATP synthase are in exposed regions • cytochrome b6/f, PC and PQ are evenly dispersed

43. Physical organization of Z-scheme • Complexes are arranged asymmetrically! • PSII is in appressed regions of grana • PSI and ATP synthase in exposed regions • cytochrome b6/f, PC and PQ are evenly dispersed • why PC and PQ must be mobile

44. Physical organization of Z-scheme • Complexes are arranged asymmetrically! • PSII is in appressed regions of grana • PSI and ATP synthase in exposed regions • cytochrome b6/f, PC and PQ are evenly dispersed • why PC and PQ must be mobile • why membrane must be very fluid

45. PSII Photochemistry 1) LHCII absorbs a photon 2) energy is transferred to P680

46. PSII Photochemistry 3) P680* reduces pheophytin ( chl a with 2 H+ instead of Mg2+) = primary electron acceptor

47. PSII Photochemistry 3) P680* reduces pheophytin ( chl a with 2 H+ instead of Mg2+) = primary electron acceptor charge separation traps the electron

48. PSII Photochemistry 4) pheophytin reduces PQA(plastoquinone bound to D2) moves electron away from P680+ & closer to stroma

49. PSII Photochemistry 5) PQA reduces PQB (forms PQB- )

50. PSII Photochemistry 6) P680+ acquires another electron , and steps 1-4 are repeated