Photosynthesis

1. Photosynthesis Chapter 19

2. Plants • Have mitoch • Nutrient breakdown  • ets  • ATP production • Also have another ATP prod’n mech • Solar free energy trapped • Reduces carriers (NADPH), produces ATP • Side rxn: H2O  2 H+ (used in ATP prod’n) + ½ O2

3. Overall, light rxns: • 2 H2O + 3 ADP + 3 Pi + 2 NADP  O2 + 3 ATP + 2 NADPH • Dark rxns • Prod’s of light rxns + CO2 carbohydrates • CO2 + H2O  O2 + (CH2O)n (w/ light) • Source of plant CH’s in our diets

5. Light Rxns Similar to Mitoch ets • Both involve redox rxns • Both have membr-bound enz’s and proton gradients • Both have structures sim to Complex III (mitoch)

6. Similarities – cont’d • Both in dbl-membr organelles • Outer membr semipermeable • Inner membr impermeable • Both use ATP synthase complexes • Sim structures • Same rxn: ADP + Pi  ATP

7. Light Rxns Differ from Mitoch ets • e- Transfer • Mitoch e- from NADH to O2 NAD+ + H2O • Photosynth e- from H2O to NADP+  NADPH + O2 • Proton gradient • Mitoch incr’d [H+] in intermembr space • Photosynth incr’d [H] in lumen of thylakoid (analogous to mitoch matrix)

8. Differences – cont’d • Location of ATP synth’d • Mitoch ATP rel’d to matrix • Transporter moves ATP out • Photosynth ATP rel’d to chloroplast stroma (analogous to intermembr space in mitoch) • So synth’d ATP avail to cell w/out transporter

9. Chloroplast • Outer membr semipermeable • Intermembr space = stroma • Aqueous • Inner membr folded  thylakoids • “Stacks” of thylakoids = grana • Lumen = space inside thylakoid membr “loops”

11. Review of Physics of Light • Light energy = wave of particles • Particles = photons • l = wavelength of light • Visible range = 400 nm (violet)  700 nm (red) • Energy of photons inverse to l • Energy of 1 “mole” of photons = 170-300 kJ

13. Chromophores • Conjugated • Have “fluid” p electrons • Available for excitation by incident energy • Rel low energy needed for e-  excited state

14. Chromophore e- move to higher energy level • All or nothing • Photon energy level must match prescribed energy levels of chromophore mol electrons • = e- orbital levels • At higher energy level • e- excited, unstable • e- returns to lower level (ground state) for stability

15. Energy rel’d when e- falls back to ground state • = quantum • May be rel’d as light, heat, or chem energy • May be transferred to second chromophore

16. Chromophores in Photosynthesis = Pigments • Absorb radiant energy • Extensive conj’d db systems • Many fluid p electrons can move to higher energy levels • Absorb light energy of visible wavelengths • 2 Impt pigments: chlorophylls a, b • Structure sim to porphyrins • Where did you see porphyrin structure before?

18. Chlorophylls a, b – cont’d • Metal ion coordinated w/ structure = Mg • What was metal ion in previously studied porphyrins? • Hydrophobic side chain (called phytol) • How might this be related to its location? (Hint…) • In thylakoid membranes • In light-harvesting complexes (LHC’s) • Other impt proteins assoc’d • Arr’d in partic order

19. Other pigments – accessory pigments in LHC’s • Carotenoids (ex: b carotene) • Phycobilins (linear tetrapyrroles) • Lutein • Absorb light @ varied l • Match l of sunlight reaching earth • Different absorbance maxima • Different structures

23. Phycobilisome – A “Simple” Photosystem • Photosystem = light-harvesting pigment arrangement • Embedded in thylakoid membr • Phycobilisome in cyanobacteria, red algae • Phycobilin pigments complex w/ proteins • Phycoerythrin, phycocyanin, allophycocyanin • Analogous to accessory pigments, antenna molecules in higher plants

24. Final energy acceptor = chlorophyll a • Analogous to reaction center • Arranged in ordered complex

25. Incident light of 2 l ranges supply energy • Energy transferred pigment to pigment • Energy excites p electrons of each sequential pigment • “Exciton transfer” • Reaches chlorophyll a • Initiates redox rxn and electron transfer • Will be used to generate ATP

26. Photosystems in Higher Plants • ~200 chlorophyll molecules • Some make up Rxn Center • Some serve as antenna molecules • ~ 50 accessory pigments

27. Arrangement • Rxn Center • Surrounded by antenna molecules, accessory pigments • All embedded in thylakoid membr bilayer

29. Two types of rxn center • PS I • Mostly chl a’s, some chl b’s • Other specialized structures • Abs max = 700 nm • PS II • Chl’s a + b + c • Other specialized structures • Abs max = 680 nm

30. Photosystem Energy Transfer • Light energy strikes antenna molecule • Mostly chl a’s • Excites e- of 1st antenna mol to higher energy level

31. e- falls back to ground state • Releases energy • Energy avail to nearby antenna mol or accessory pigment

32. 2nd antenna mol/ accessory pigment accepts energy • Its e- excited to higher energy level (= exciton transfer) • e- falls back to ground state • Releases energy • Energy avail to nearby antenna mol or accessory pigment • 3rd antenna mol accepts energy, etc., etc.  Rxn Center

33. Energy Transfer to Rxn Center • Rxn Centers have special chlorophyll a • “Sandwiched” between 2 other rxn center structures • e- acceptor is “above” chl a • e- donor is “below” chl a

34. W/ energy transfer from antenna mol/ accessory pigment, e- @ special chl a excited

35. e- moves (physically transferred) to e- acceptor structure near chl a • Now acceptor structure has an extra e- • Takes on formal – charge • Now special chl a has no electron • Takes on formal + charge • Get “electron hole”

37. e- donor structure near chl a replaces e- in chl a • Now donor structure has no electron • Takes on formal + charge • Now chl a uncharged; lies between • e- acceptor structure (now – charged) • e- donor structure now (+ charged)

39. Have generated formal sep’n of charge in Rxn Center • REMEMBER: this is a highly energetic condition • Excited e- in rxn center -- good e- donor • Initiates redox chain among other structures in thylakoid membr

40. Pheophytin-Quinone – Simplified Rxn Center • In purple bacteria • “Special Chl a” = (Chl)2 • Excitons gen’d w/ incident light of l 870 nm • “e- acceptor” = Pheophytin • Chlorophyll w/out Mg

42. e- from pheo radical  quinone (Q) • Sim to Ubiquinone (=CoEnzyme Q) in mitoch • Can accept one or two reducing equivalents • Moves through thylakoid bilayer • Q  Cyt bc1 complex • Sim to Complex III in mitoch

43. Cyt bc1 complex transfers e-  Cyt c2 • Cyt c2 carries e- back to rxn center • Rxn center returned to neutral state to receive another exciton • Energy gen’d w/ e- transport • Can calc D G from voltage gen’d w/ e- transfer • (Chl)2’+  QH2 D G ~ -180 kJ/mole

44. Higher Plants Have 2 Rxn Centers • Sim Rxn Center, e- transport structures as bacteria • BUT others also, so more complex • PSII “first” Center • Like bacterial model • Pheophytin • Quinones (as Plastoquinones) • Cyt bf Complex (has a cyt f, not cyt c inc’d) • H+ gen’d, collects in thylakoid lumen

46. PSII “first” Center – cont’d • Not like bacterial model • Accepts incident light @ 680 nm • Cyt bf Complex transfers e-  Plastocyanin, not cyt c • Final acceptor transfers e- to rxn center of SECOND photosystem • So NOT a cycle, w/ rxn center regen’d w/ cycle • Rather, rxn center regen’d w/ e- from H2O splitting • 2 H2O  4 H+ + 4 e- + O2

47. Catalyzed by water splitting complex (= oxygen-evolving complex) • Requires 4 light photons • Cleaves water • AND transfers 4 e- one at a time to rxn center of PSII to regenerate rxn center • Mn impt to function • So light abs’d to: • Excite rxn center e- to initiate e- transfer, AND • Energize gen’n e- to regenerate rxn center electronically

49. PSII Summary • Light energy  accessory pigments, etc.  rxn center • Charge sep’n + excited “special” chl e-  e- transferred to pheophytin Cyanobacterium model

50. e- @ pheophytin  plastoquinones (2)  Cyt bf complex • Q cycle releases 1 e- at a time to Cyt bf complex • Generate H+  lumen • e- @ Cyt bf complex  plastocyanin • Plastocyanin travels to PSI w/ its e-