1. Converts light to chemical energy
Photosynthesis
Converts light to chemical energy
6 CO2 + 6 H2O + light energy <=> C6H12O6 + 6 O2

2. Photosynthesis
2 sets of rxns in separate
parts of chloroplast

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

4. 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

5. 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

6. Important structural features of chloroplasts
very large organelles: 5-10 µm long, 2-4 µm wide

7. Important structural features of chloroplasts
3 membranes
1) outer envelope
permeable to molecules up to 10 kDa due to porins

8. Important structural features of chloroplasts
3 membranes
1) outer envelope
2) inner envelope
impermeable: all import/export is via transporters

9. Important structural features of chloroplasts
1) outer envelope
2) inner envelope
3) thylakoids:
Stromal membranes

10. Important structural features of chloroplasts
3) thylakoids: Stromal membranes
a) grana: stacks of closely appressed membranes
b) stromal lamellae: single thylakoids linking grana

11. Important structural features of chloroplasts
All cp membranes have MGDG, DGDG & SL
thylakoids only have MGDG, DGDG, SL & PG
thylakoid lipids have many trienoic fatty acids
most fluid membranes known

12. Important structural features of chloroplasts
Stroma is pH 8.0 in light
thylakoid lumen is < 5
Stroma is full of protein
also contains DNA
& genetic apparatus

13. Light Rxns
3 stages
1) Catching a photon (primary photoevent)

14. Light Rxns
3 stages
1) Catching a photon (primary photoevent)
2) ETS

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

16. Catching photons
photons: particles of energy that travel as waves
Energy inversely proportional to wavelength ()
visible light ranges from nm

17. Catching photons
Photons: particles of energy that travel as waves
caught by pigments: molecules that absorb light

18. Pigments
Can only absorb certain photons

19. Pigments
Can only absorb certain photons
Photon has exact energy to push an e- to an outer orbital

20. Pigments
Can only absorb certain photons
Photon has exact energy to push an e- to an outer orbital
from ground to excited state

21. Pigments
Photon has exact energy to push an e- to an outer orbital
from ground to excited state
each pigment has an absorption spectrum: l it can absorb

22. Pigments
Chlorophyll a is most abundant pigment
chlorophyll a looks green
-> absorbs all l but green
Reflects green

23. Accessory Pigments
absorb l which chlorophyll a misses
chlorophyll b is an important
accessory pigment

24. Accessory Pigments
absorb l which chlorophyll a misses
chlorophyll b is an important accessory pigment
others include xanthophylls & carotenoids

25. Accessory Pigments
action spectrum shows use of accessory pigments
l used for photosynthesis

26. Accessory Pigments
action spectrum shows use of accessory pigments
l used for photosynthesis
plants use entire visible spectrum
l absorbed by chlorophyll work best

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

28. Light Reactions
1) Primary photoevent: pigment absorbs a photon
e- is excited -> moves to outer orbital

29. Light Reactions
4 fates for excited e-:
1) returns to ground state emitting heat & longer  light = fluorescence

30. Light Reactions
4 fates for excited e-:
1) fluorescence
2) transfer to another molecule

31. Light Reactions
4 fates for excited e-:
1) fluorescence
2) transfer to another molecule
3) Returns to ground state dumping energy as heat

32. 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

33. 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

34. 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

35. Photosystems
Pigments are bound to proteins arranged in thylakoids in photosystems
arrays that channel energy absorbed by any pigment to rxn center chlorophylls

36. 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

37. 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

38. 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

39. 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

40. 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

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

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

43. 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

44. 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+

45. 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

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

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

48. 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

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

50. 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)

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

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

53. 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)

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

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

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

57. 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

58. 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

59. 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

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

61. Cyclic photophosphorylation
Limitations
Only makes ATP

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

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

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

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

66. 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)

67. 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)

68. 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

69. 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

70. 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

71. 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

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

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

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

75. 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

76. 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

77. 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

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

79. 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

80. Z-scheme energetics

81. 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

82. 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+

83. 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

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

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

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

87. 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

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

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

90. 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

91. 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