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Photosynthesis 1) Light rxns use light to pump H + use ∆ pH to make ATP by chemiosmosis

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. Light-independent (dark) reactions The Calvin cycle.

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Photosynthesis 1) Light rxns use light to pump H + use ∆ pH to make ATP by chemiosmosis

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  1. 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

  2. Light-independent (dark) reactions The Calvin cycle

  3. Light-independent (dark) reactions occur in the stroma of the chloroplast (pH 8) Consumes ATP & NADPH from light reactions regenerates ADP, Pi and NADP+

  4. Light-independent (dark) reactions Overall Reaction: 3 CO2 + 3 RuBP + 9 ATP + 6 NADPH = 3 RuBP + 9 ADP + 9 Pi + 6 NADP+ + 1 Glyceraldehyde 3-P

  5. Light-independent (dark) reactions 1) fixing CO2 2) reversing glycolysis 3) regenerating RuBP

  6. fixing CO2 1) RuBP binds CO2

  7. fixing CO2 • RuBP binds CO2 • 2) rapidly splits into two 3-Phosphoglycerate • therefore called C3 photosynthesis

  8. fixing CO2 • 1) CO2 is bound to RuBP • 2) rapidly splits into two 3-Phosphoglycerate • therefore called C3 photosynthesis • detected by immediately killing cells fed 14CO2

  9. fixing CO2 1) CO2 is bound to RuBP 2) rapidly splits into two 3-Phosphoglycerate 3) catalyzed by Rubisco (ribulose 1,5 bisphosphate carboxylase/oxygenase) the most important & abundant protein on earth

  10. fixing CO2 • 1) CO2 is bound to RuBP • 2) rapidly splits into two 3-Phosphoglycerate • 3) catalyzed by Rubisco (ribulose 1,5 bisphosphate carboxylase/oxygenase) • the most important & abundant protein on earth • Lousy Km

  11. fixing CO2 • 1) CO2 is bound to RuBP • 2) rapidly splits into two 3-Phosphoglycerate • 3) catalyzed by Rubisco (ribulose 1,5 bisphosphate carboxylase/oxygenase) • the most important & abundant protein on earth • Lousy Km • Rotten Vmax!

  12. Reversing glycolysis converts 3-Phosphoglycerate to G3P consumes 1 ATP & 1 NADPH

  13. Reversing glycolysis • G3P has 2 possible fates • 1) 1 in 6 becomes (CH2O)n

  14. Reversing glycolysis • G3P has 2 possible fates • 1) 1 in 6 becomes (CH2O)n • 2) 5 in 6 regenerate RuBP

  15. Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell)

  16. Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose

  17. Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose Pi/triosePO4 antiporter only trades DHAP for Pi

  18. Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose Pi/triosePO4 antiporter only trades DHAP for Pi mechanism to regulate PS

  19. Regenerating RuBP • G3P has 2 possible fates • 5 in 6 regenerate RuBP • necessary to keep cycle going

  20. Regenerating RuBP Basic problem: converting a 3C to a 5C compound feed in five3C sugars, recover three5C sugars

  21. Regenerating RuBP Basic problem: converting a 3C to a 5C compound must assemble intermediates that can be broken into 5 C sugars after adding 3C subunit

  22. Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C

  23. Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C 4C + 3C = 7C

  24. Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C 4C + 3C = 7C 7C + 3C = 5C + 5C

  25. Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C 4C + 3C = 7C 7C + 3C = 5C + 5C Uses 1 ATP/RuBP

  26. Light-independent (dark) reactions build up pools of intermediates , occasionally remove one very complicated book-keeping

  27. Light-independent (dark) reactions build up pools of intermediates , occasionally remove one very complicated book-keeping Use 12 NADPH and 18 ATP to make one 6C sugar

  28. Regulating the Calvin Cycle Rubisco is main rate-limiting step

  29. Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase : • uses ATP to activate rubisco

  30. Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma

  31. Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • a) pH: rubisco is most active at pH > 8 • (in dark pH is ~7.2)

  32. Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • a) pH • b) [Mg2+]: in light [Mg2+] in stroma is ~ 10x greater than in dark

  33. Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • a) pH • b) [Mg2+]: in light [Mg2+] in stroma is ~ 10x greater than in dark • Mg2+ moves from thylakoid lumen to stroma to maintain charge neutrality

  34. Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • a) pH • b) [Mg2+] • c) CO2 is an allosteric activator of rubisco that only binds at high pH and high [Mg2+] • also: stomates open in the light

  35. Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • Several other Calvin cycle enzymes (e.g.Fructose-1,6-bisphosphatase) are also activated by high pH & [Mg2+]

  36. Regulating the Calvin Cycle • Several Calvin cycle enzymes (e.g.Fructose-1,6-bisphosphatase) are also regulated by thioredoxin • contain disulfide bonds which get oxidized in the dark

  37. SH SH light oxidized enzyme (inactive) S - S reduced thioredoxin 2Fdox 2e- PSI + PSII S - S SH SH oxidized thioredoxin reduced enzyme (active) 2Fdred • Regulating the Calvin Cycle • Several Calvin cycle enzymes (e.g.Fructose-1,6-bisphosphatase) are also regulated by thioredoxin • contain disulfide bonds which get oxidized in the dark • in light, ferredoxin reduces thioredoxin, thioredoxin reduces these disulfide bonds to activate the enzyme

  38. Regulating the Calvin Cycle • Several Calvin cycle enzymes (e.g.Fructose-1,6-bisphosphatase) are also regulated by thioredoxin • contain disulfide bonds which get oxidized in the dark • in light, ferredoxin reduces thioredoxin, thioredoxin reduces these disulfide bonds to activate the enzyme • How light reactions talk to the Calvin cycle SH SH light oxidized enzyme (inactive) S - S reduced thioredoxin 2Fdox 2e- PSI + PSII S - S SH SH oxidized thioredoxin reduced enzyme (active) 2Fdred

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