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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. Important structural features of chloroplasts

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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. Important structural features of chloroplasts • 1) outer envelope • 2) inner envelope • 3) thylakoids: stromal membranes: most fluid known • PSI & ATP synthase are on outside • PSII is on inside of grana

  3. Light Rxns • 3 stages • 1) Catching a photon (primary photoevent) • 2) ETS • 3) ATP synthesis by chemiosmosis

  4. Light Reactions 1) Primary photoevent: pigment absorbs a photon

  5. 4 fates for excited e-: • 1) fluorescence • 2) transfer to another molecule • 3) Returns to ground state dumping energy as heat • 4) energy is transferred by inductive resonance • excited e- vibrates and induces adjacent e- to vibrate at same frequency

  6. 4 fates for excited e-: • 4) energy is transferred by inductive resonance • excited e- vibrates and induces adjacent e- to vibrate at same frequency • Only energy is transferred • e- returns to ground state

  7. Photosystems Pigments are bound to proteins arranged in thylakoids in photosystems arrays that channel energy absorbed byanypigment torxn center chlorophylls

  8. Photosystems Pigments are bound to proteins arranged in thylakoids in photosystems arrays that channel energy absorbed by any pigment to rxn center chls Need 2500 chlorophyll to make 1 O2

  9. Photosystems • Arrays that channel energy absorbed by any pigment to rxn center chls • 2 photosystems : PSI & PSII • PSI rxn center chl a dimer absorbs 700 nm = P700

  10. Photosystems • Arrays that channel energy absorbed by any pigment to rxn center chls • 2 photosystems : PSI & PSII • PSI rxn center chl a dimer absorbs 700 nm = P700 • PSII rxn center chl a dimer • absorbs 680 nm = P680

  11. Photosystems • Eachmayhave associated LHC (light harvesting complex)(LHC can diffuse within membrane) • PSI has LHCI: ~100 chl a, a few chl b & carotenoids

  12. Photosystems • Eachmayhave associatedLHC (light harvesting complex)(LHC can diffuse within membrane) • PSI has LHCI: ~100 chl a, a few chl b & carotenoids • PSII has LHCII: ~250 chl a, many chl b & carotenoids • Proteins of LHCI & LHCII also differ

  13. Photosystems Cyanobacteria & red algae associate phycobilisomes cf LHCII with PSII = proteins that absorb light & pass energy to rxn center chl Absorb 500-650 nm PE= phycoerythrin: Absorbs 500 & 570 PC= phycocyanin Absorbs 620 AP = allophycocyanin Absorbs 650

  14. Photosystems green sulfur bacteria absorb light with chlorosomes = mix of proteins, carotenoids and Bact Chl c that channel light to Bact Chl a (795 nm) then rxn center p840

  15. Photosystems Dinoflagellates absorb light with peridinin–chlorophyll a-proteins = mix of proteins & the carotenoid peridinin that absorb @ 480 & channel to Chl a

  16. Photosystems Result in very different absorption spectra

  17. Photosystems PSI performs cyclic photophosphorylation Absorbs photon & transfers energy to P700

  18. cyclic photophosphorylation Absorbs photon & transfers energy to P700 transfers excited e- from P700 to fd

  19. cyclic photophosphorylation Absorbs photon & transfers energy to P700 transfers excited e- from P700 to fd fd returns e- to P700 via PQ, cyt b6/f & PC

  20. cyclic photophosphorylation Absorbs photon & transfers energy to P700 transfers excited e- from P700 to fd fd returns e- to P700 via PQ, cyt b6/f & PC Cyt b6/f pumps H+

  21. Cyclic Photophosphorylation Transfers excited e- from P700 to fd Fd returns e- to P700 via cyt b6-f & PC Cyt b6-f pumps H+ Use PMF to make ATP

  22. Cyclic photophosphorylation first step is from P700 to A0 (another chlorophyll a) charge separation prevents e- from returning to ground state = true photoreaction

  23. Cyclic photophosphorylation first step is from P700 to A0 (another chlorophyll a) next transfer e- to A1 (a phylloquinone) next = 3 Fe/S proteins

  24. Cyclic photophosphorylation first step is from P700 to A0 (another chlorophyll a) next transfer e- to A1 (a phylloquinone) next = 3 Fe/S proteins finally ferredoxin

  25. Cyclic photophosphorylation Ferredoxin = branchpoint: in cyclic PS FD reduces PQ

  26. Cyclic photophosphorylation • Ferredoxin reduces PQ • PQH2 diffuses to cyt b6/f • 2) PQH2 reduces cyt b6 and Fe/S, releases H+ in lumen • since H+ came from stroma, transports 2 H+ across membrane (Q cycle)

  27. Cyclic photophosphorylation 3) Fe/S reduces plastocyanin via cyt f cyt b6 reduces PQ to form PQ-

  28. Cyclic photophosphorylation 4) repeat process, Fe/S reduces plastocyanin via cyt f cyt b6 reduces PQ- to form PQH2

  29. Cyclic photophosphorylation 4) repeat process, Fe/S reduces plastocyanin via cyt f cyt b6 reduces PQ- to form PQH2 Pump 4H+ from stroma to lumen at each cycle (per net PQH2)

  30. Cyclic photophosphorylation 5) PC (Cu+) diffuses to PSI, where it reduces an oxidized P700

  31. Cyclic photophosphorylation energetics: light adds its energy to e- -> excited state Eo' P700 = +0.48 V Eo' P700* = -1.3 V

  32. Cyclic photophosphorylation energetics: light adds its energy to e- -> excited state Eo' P700 = +0.48 V Eo' P700* = -1.3 V Eo' fd = - 0.42 V

  33. Cyclic photophosphorylation energetics: light adds its energy to e- -> excited state Eo' P700 = +0.48 V Eo' P700* = -1.3 V Eo' fd = - 0.42 V Eo' cyt b6/f = +0.3V

  34. Cyclic photophosphorylation energetics: light adds its energy to e- -> excited state Eo' P700 = +0.48 V Eo' P700* = -1.3 V Eo' fd = - 0.42 V Eo' cyt b6/f = +0.3V Eo' PC = +0.36V

  35. Cyclic photophosphorylation energetics: light adds its energy to e- -> excited state Eo' P700 = +0.48 V Eo' P700* = -1.3 V Eo' fd = - 0.42 V Eo' cyt b6/f = +0.3V Eo' PC = +0.36V e- left in excited state returns in ground state

  36. Cyclic photophosphorylation e- left in excited state returns in ground state Energy pumped H+

  37. Cyclic photophosphorylation Limitations Only makes ATP

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

  39. Photosystems PSI performs cyclic photophosphorylation Makes ATP but not NADPH: exact mech for PQ reduction unclear, but PQ pumps H+

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

  41. Photosystem II Evolved to provide reducing power Added to PSI rxn center absorbs 680 nm (cf 700 nm)

  42. Photosystem II rxn center absorbs 680 nm (cf 700 nm) can oxidize H2O redox potential of P680+ is + 1.1 V (cf + 0.82 V for H2O)

  43. Photosystem II rxn center absorbs 680 nm (cf 700 nm) can oxidize H2O redox potential of P680+ is + 1.1 V (cf + 0.82 V for H2O) Use e- from H2O to reduce NADP+ (the e- carrier used for catabolic reactions)

  44. Photosystem II rxn center absorbs 680 nm (cf 700 nm) can oxidize H2O redox potential of P680+ is + 1.1 V (cf + 0.82 V for H2O) Use e- from H2O to reduce NADP+ (the e- carrier used for catabolic reactions) use NADPH c.f. NADH to prevent cross- contaminating catabolic & anabolic pathways

  45. 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

  46. 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

  47. 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

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

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

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

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