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 by any pigment to rxn 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
Each may have associated LHC (light harvesting complex) (LHC can diffuse within membrane)
PSI has LHCI: ~100 chl a, a few chl b & carotenoids

12. Photosystems
Each may have associated LHC (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 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 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 = V
Eo' P700* = -1.3 V

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

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

34. Cyclic photophosphorylation
energetics:
light adds its energy to e-
-> excited state
Eo' P700 = V
Eo' P700* = -1.3 V
Eo' fd = 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 = V
Eo' P700* = -1.3 V
Eo' fd = 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 V for H2O)

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

51. 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

52. 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

53. 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

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

55. Ultimate e- source None water O2 released? No yes
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

56. Z-scheme energetics

57. 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

58. 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+

59. 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

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

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

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

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

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

65. 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

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

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

68. 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

69. 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