1. 7. Photosynthesis: Light Reactions
Read till next time (incl. this lesson):
Biology of Plants 6th ed. pp th ed. pp
8th ed. pp
[Plant Physiology (Taiz & Zeiger) pp ]

2. Autotrophs & Heterotrophs
PHOTOSYNTHESIS
CO2 + H2O >>> C(H2O) + O2 6CO2 + 6H2O >>> C6H12O6 + 6O2   (glucose)
Other high- energy products
RESPIRATION
Autotrophs & Heterotrophs

3. Carbohydrate
(energy rich)
+ O2
Light
energy
Respiration
CO2 + H2O
(energy poor)
Energy for Biosynthesis, Active transport, Movement etc.
Photosynthesis

4. ‘Light energy’ = energy of photon = h c / λ
Quanta
Photons
Photosynthetically Active Radiation (PAR) = radiation ( nm)

5. So, all photons in PAR can give rise to photosynthesis…
But mostly blue and red photons are used in photosynthesis! (Why?)

7. So, all photons in PAR can give rise to photosynthesis…
But mostly blue and red photons are used in photosynthesis!
However SOME green (and any other) photons are absorbed too,
And IF ABSORBED, they too give rise to photosynthesis……

8. Fig. 2.3

9. So, all photons in PAR can give rise to photosynthesis…
But mostly blue and red photons are used in photosynthesis!
However SOME green (and any other) photons are absorbed too,
And IF ABSORBED, they too give rise to photosynthesis………
AS MUCH AS THE BLUE AND RED PHOTONS.
So, the quantum yield* for photosynthesis is the same for all photons IRRESPECTIVE of their energy. (How can this be?... Later!)
*quantum yield =
= photosynthesis performed per photon ABSORBED
(In percentage: can theoretically be 0-100% but is in reality 0 to 84%)

10. Quantum Yield 1
Quantum Yield

11. Quantum Yield (= photosynthesis per quantum absorbed)

12. An Overview of Photo-Synthesis
Light Reactions
“Dark” Reactions
CO2 Fixation

13. The Structure of the Chloroplast
Stroma
Granum

15. The PHOTO-reactions of photosynthesis
Light Reactions
“Dark” Reactions
CO2 Fixation

16. The Chlorophyll Structure
Chlorophyll b
Chlorophyll a
Hydrophobic

17. Chlorophyll a
MW ~ 950

18. The PHOTO-reactions of photosynthesis
Light Reactions
“Dark” Reactions
CO2 Fixation

19. Photoexcitation / De-Excitation of Chlorophyll
1 (~<1%)
2 (~<10%)
3 Energy Transfer
to Photosynthesis (~80-90%)

20. Photoexcitation / De-Excitation of Chlorophyll
1 (~<1%)
2 (~<10%)

21. So, all photons in PAR can give rise to photosynthesis…
But mostly blue and red photons are used in photosynthesis!
However SOME green (and any other) photons are absorbed too,
And IF ABSORBED, they too give rise to photosynthesis………
AS MUCH AS THE BLUE AND RED PHOTONS.
Therefore the quantum yield* for photosynthesis is the same for all photons IRRESPECTIVE of their energy.
How can this be?
*quantum yield =
= photosynthesis performed per photon ABSORBED
(In percentage: can theoretically be 0-100% but is in reality 0 to 84%)

22. (Heat)
Fluorescence
Energy transfer to Photosynthesis

23. Energy transfer and photochemistry Fluorescence Ph
Fig. 2.4
Chlorophyll
Heat
Energy transfer and photochemistry
Fluorescence Ph
Blue-photon excitation level
Red-photon excitation level
Ground state
red
orange
yellow
green
blue
Electron energy level

24. The Chlorophyll Structure
Chlorophyll b
Chlorophyll a
Hydrophobic

25. Phycobilisome Phycoerythrin Phycocyanin (Allo-phycocyanin)
Thylakoid membrane

26. Tail T H
Fig. 2.2
Head H

27. Fig. 4.3 Ch
PSII
APC PC PE
Thylakoid membrane

28. The ‘cluster’ of pigment molecules = photosystem
Chlorophyll b
Chlorophyll a
Hydrophobic

29. TWO photosystems (PS), PS I and PS II
Quinone

30. How light is harvested 3 1 4 2
Resonance
Energy Transfer

31. Photosystem ‘Antenna’ Resonance Energy Transfer Electron transfer
Reaction Centre 4
Electron acceptor

32. TWO photosystems (PS), PS I and PS II
Quinone

33. Primary Q
Quinone
P 680 / P 680+
In PS II
H2O

35. Primary Fd
Feredoxin
P 700 / P700+
In PS I
PS II (H2O)

36. -1.0V
-0.8V
Quinone
Red-Ox Potential
0.8V

37. (mid-point) Redox potential (V)
+0.8
+0.4
-0.4
-0.8
-1.0
PSII
(Pheophytin) QA QB
PSI
“A” PQ
Cyt b6/f PC
P700+
Strong oxidant
Strong reductant Fd
(mid-point) Redox potential (V)
(Thylakoid membrane) Ph
Stroma
Lumen
Fig. 5.3
NADP+
NADPH
H2O

38. NADP+ + 2e- + 2H+ > NADPH + H+
To OXIDISE to take away an electron(s) from another compound
לחמצן
To become oxidised to lose an electron(s) to another compound
להתחמצן
H2O > 2e- + 2H+ + ½ O2
2H2O > 4e- + 4H+ + O2
To REDUCE to give away an electron(s) to another compound
לחזר
To become reduced to gain an electron(s) from another compound
להתחזר
NADP+ + e- > NADPH
NADP+ + 2e- + 2H+ > NADPH + H+

39. 2 4 4

41. Why TWO Photosystems?
-1.0V
-0.8V
Red-Ox Potential
0.8V

42. A mechanical analogy for the Light Reaction

43. Complexes
-1.0V
-0.8V
Quinone
Red-Ox Potential 2 3 1
0.8V

44. PS-I
Cyt
PS-II
Complex 1
Complex 3
Complex 2
4e-
2H2O
O2 +4H+
2H2O

45. (mid-point) Redox potential (V)
+0.8
+0.4
-0.4
-0.8
-1.0
PSII
(Pheophytin) QA QB
PSI
“A” PQ
Cyt b6/f PC
P700+
Strong oxidant
Strong reductant Fd
(mid-point) Redox potential (V)
(Thylakoid membrane) Ph
Stroma
Lumen
Fig. 5.3
NADP+
NADPH
H2O

46. A mechanical analogy for the Light Reaction

47. -1.0V
-0.8V
Quinone
Red-Ox Potential
0.8V

48. To Calvin
Cycle
10nm

49. PQ > PQH2
> P680+
“Splitting of Water”,
“Photolysis”

50. Complex 4

51. pH ~ 8
pH ~ 4, Proton Motive Force (PMF)

52. Pi PSII PSI H2O 2e- 2H+ ½ O2 Cyt b6/f PQ H+ PQH2 H+ H+ H+ H+ H+ ADP+Pi
ATP
NADP+
NADPH+H+
PMF
pH ~4
Fig. 5.5
Stroma
pH ~8 Fd
Thylakoid membrane
ATP
synthase PC
Lumen

53. ATPase / ATP Synthase