1. 8. Photosynthesis: “Dark Reactions”
Read for this lesson:
Biology of Plants 6th ed. pp th ed. pp
8th ed. pp
(same as for last lesson)
Extra reading:
[Plant Physiology (Taiz & Zeiger) pp ]

2. An Overview of Photo-Synthesis
Light Reactions
“Dark” Reactions 1 3
CO2 Fixation 2

3. 1) Photoexcitation / De-Excitation of Chlorophyll and”Charge Separation”
1 (~<1%)
2 (~<10%)

4. Energy transfer and photochemistry Fluorescence Ph
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

5. Photoexcitation, Resonance Transfer and Charge Separation3 1 4 2
Resonance
Energy Transfer

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

7. Charge Separation
Primary Q
(phaeophytin)
P 680 / P 680+
In PS II
H2O

8. Charge Separation Primary Fd = Feredoxin P 700 / P700+ In PS I
PS II (H2O)

9. 2) Electron Transport in TWO Photosystems

10. (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
NADP+
NADPH
H2O

11. 3) Formation of ATP
-1.0V
-0.8V
Quinone
Red-Ox Potential
0.8V

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

14. Oxidative
phosphorylation
Photophosphorylation

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

16. “Dark” reactions

17. Light Reactions
“Dark” Reactions
Stroma
CO2 Fixation

19. The Calvin Cycle The C3 Cycle (of CO2 fixation and Reduction)
6CO2 + 6H2O >>> C6H12O6 + 6O2
Melvin Calvin

21. PGA = phosphoglyceric acid or phosphoglycerate
= a 3-carbon compound, therefore the C3 cycle.

22. Autochromatogram (from paper chromatogrphy
Lamp
Lollipop flask
Chlo-rella
Alco-hol
Sugars
14CO2
Autochromatogram (from paper chromatogrphy
PGA

24. The Calvin Cycle / C3 Cycle
Phosphoglyceric
acid (PGA)
Ribulose bis-phosphate
(RuBP)
Triose phosphate (TP)

25. The Calvin Cycle: Phase 1 Carbon Fixation
Phytobenthos C5
2 x C3
PGA P P P
RuBP

26. The Calvin Cycle: Phase 2, Carbon reduction
(PGA) P P P P P TP P

27. TP

28. The Calvin Cycle – C3 cycle
3 x 1C
3 x 5C
6 x 3C
5 x TP
1 x TP

29. The Calvin Cycle: Phase 3, Regeneration
RuBP P TP

30. 3 CO2 Rubisco 3 RuBP 6 PGA Calvin cycle 6 6 PGAL 5 TP 6 TP TP Sugars
6 ATP
Calvin cycle
3 ATP
6 NADPH 6
6 PGAL
5 TP
6 TP TP
Sugars

31. 3 CO2 C1 6 PGA 6 Triose-P (TP) C3 C3 3 RuBP C5 C3 C3 C3 C3 C3 C3
C Sugars
3 ATP
6 ATP 6 NADPH

32. 3 CO2 (C1)
6 PGA (C3) 6
6 TP
(C3)
3 RuBP (C5) TP
Sugars
5 TP
Rubisco
6 PGAL (C3)
6 ATP
3 ATP
6 NADPH C3 C5 C7 C6
2 C5 C4
Calvin cycle

33. 2H2O / 4e- / O2 / 2NADPH / CO2 / 3ATP needed / X photons
Stoichiometry for Photosynthesis
Light Reactions: O2
2 H2O (4H+) ~3 ATP
4 e-
2 NADP e NADPH (+2H+) 2ATP
“Dark” Reactions (Calvin cycle):
CO2 + RuBP [C6] 2 PGA PGAld (TP)
(C5) (C3) (C3)
5/ ATP /6 Sugars
2H2O / 4e- / O2 / 2NADPH / CO2 / 3ATP needed / X photons

35. Light reactions TP

36. Why are most terrestrial plants limited by today’s atmospheric CO2 concentration?

37. The Calvin Cycle: Phase 1
2 x C3
PGA P P P
RuBP
Rubisco
Ribulose bisphosphate carboxylase…

39. Rubisco, the “Dinosaur” enzyme: BIG: 8L 8S subunits, 560 kDa
Prof. J.A. Raven, FRS
“Dinosaur” enzyme:
BIG:
8L 8S subunits,
560 kDa
SLOW: (Rubisco)
Km (CO2) = mM
STUPID:
Rubisco acts also as an
Oxygenase!

41. Air

42. (mM)

43. Rubisco, Ribulose 1,5-bisposphate carboxylase/oxygenase
“Dinosaur” enzyme:
BIG:
8L 8S subunits,
560 kDa
SLOW: (Rubisco)
Km (CO2) = mM
STUPID:
Acts also as an
Oxygenase! (Rubisco)
Ribulose bisphospate
Carboxylase/oxygenas

44. Rubisco Function as an Oxygenase
PGA
Glycolate

45. CO2 /or O2 (2 x 3-C) (3-C) (2-C) Ribulose bisphosphate carboxylase =
Rubisc…o (Rubisco) = Ribulose bisphosphate carboxylase / oxygenase
(3-C)
(2-C)

46. Glycolate Metabolism in Photorespiration
Peroxisome
Mitochondrion
Chloroplast

47. Chloroplast
Peroxisome
NH3
Calvin
cycle
glyoxylate
glycine O2
CO2
2 x PGA
Rubisco O2
PGA +
glycolate
glycine
NH3
serine + CO2
glycerate
Mitochondrion

48. Rubisco Carboxylase vs. Oxygenase activity
AIR: CO O2
0.035% %
15 mM mM
SOLUBLE: 10 mM mM
Rubisco Carboxylase vs. Oxygenase activity
(activity = affinity x substrate concentration)
CO O2
Concentration 1 :
Affinity : (Km ~ 10 vs. 1,000 mM)
Activity : 25 48

49. [CO2] During Evolution
[O2] 21%
[CO2] 0.038% 49