1. 8. Photosynthesis: “Dark Reactions” (continued)
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 5 5 1 4 3
CO2 Fixation 2

3. 3 CO2 Rubisco 3 RuBP 6 PGA Calvin cycle 6 6 PGAL 5 TP 6 TP TP Sugars 4
6 ATP
Calvin cycle
3 ATP
6 NADPH 6
6 PGAL
5 TP
6 TP TP
Sugars

4. PHOTORESPIRATION Chloroplast Peroxisome NH3 Calvin cycle glyoxylate
glycine O2
CO2
2 x PGA
Rubisco O2
PGA +
glycolate
glycine
NH3
serine + CO2 5
glycerate
Mitochondrion

5. Repetition on the Calvin (C3) cycle:
This is the only1 (biochemical) way in which high-energy organic compounds (sugars) can be formed from low-energy compounds (CO2 and water) PGA (a C3 product) is formed as the first product of CO2 fixation;
The enzyme that catalyzes CO2 fixation in the Calvin cycle (Rubisco) can also fix O2 – this leads to photorespiration;
All plants1 have the Calvin (C3) cycle. Plants in which this cycle is the only2 cycle in which CO2 can be fixed are called C3 plants;
(This2 indicates that there are plants that have CO2 fixation cycles ADDITIONAL to the Calvin sycle!) 5

6. Repetition on the Calvin (C3) cycle:
This is the only (biochemical) way in which high-energy organic compounds (sugars) can be formed from low-energy compounds (CO2 and water) PGA (a C3 product) is formed as the first product of CO2 fixation;
The enzyme that catalyzes CO2 fixation in the Calvin cycle (Rubisco) can also fix O2 – this leads to photorespiration;
All plants1 have the Calvin (C3) cycle. Plants in which this cycle is the only cycle in which CO2 can be fixed are called C3 plants;
(This indicates that there are other cycles and plants other than C3 plants!)
In the 70’s, Kortchak repeated Calvin’s experiment on terrestrial plants (Sugar cane)
4-carbon compounds (malate, aspartate), and not the 3-carbon compound PGA, were formed as first products of CO2 fixation!
Plants that fix atmospheric CO2 into 4-carbon compounds are called C4 plants
(However, 1 dictates that also C4 plants have the Calvin cycle!) 6

7. Why are most terrestrial plants (C3 plants) limited by the aerial CO2 concentration?
2) Why are some plants (C4 plants and CAM plants) not limited by the aerial CO2 concentration? 7

8. The special leaf anatomy of C4 plants8

9. 9

10. C4 Plants 10

11. Carboxylation into C4 acids
CO2
C4 compounds
PEP carboxylase
a) HCO3- is fixed (not CO2), b) PEP carboxylase is NOT sensitive to O2 11

12. 12

13. A Plant Cell
10 μM CO2
CO2 + H2O > H2CO3 > HCO3- + H+ > CO32- + H+
pH 8.2: 10 μM μM μM

14. C4 metabolism is a CO2 concentrating mechanism (CCM)
C4 Plants
C4 metabolism is a CO2 concentrating mechanism (CCM) 14

15. Advantage of C4 metabolism:
PEP carboxylase: high affinity for CO2 (uses HCO3-)
No oxygenase activity (insensitive to O2)
C4 plants show No Photorespiration!
* C4 plants have a higher temperature optimum than C3 plants
* C4 plants can function at very low CO2 concentrations and thus “save water” in arid regions! 15

16. C4 C3 16

17. Advantage of C4 metabolism:
PEP carboxylase: high affinity for CO2 (uses HCO3-)
No oxygenase activity (insensitive to O2)
C4 plants show No Photorespiration!
C4 plants have a higher temperature optimum than
C3 plants
* C4 plants can function at very low CO2 concentrations and thus “save water” in arid regions! 17

18. C4 C3 18

19. C4 plants can partly close their stomates and still photosynthesise19

20. CAM plants (CAM = Crassulacean Acid Metabolism)
CAM is also a CCM! 20

21. 21

22. (WUE = CO2 fixed / water lost)
Why are most terrestrial plants (C3 plants) limited by the aerial CO2 concentration?
2) Why are some plants (C4 and CAM plants) not limited by the aerial CO2 concentration?
3) How much better are C4 and, especially, CAM plants regarding water use efficiency (WUE) than C3 plants?
(WUE = CO2 fixed / water lost) 22

23. Efficiency C3 CAM C4 C3 CAM C4 Close stomata
At day time
Grow slow C4
Tolerate higher Temp. & Drier conditions C3
CAM C C3
mol water lost / mol CO2 fixed
,100
Water Use Efficiency (mol CO2 fixed / mol water lost) 23

24. C4 and CAM Plants Sugarcane Pineapple C4 : Maize, Sugercane Atriplex
CAM: Cactus, Pineapple
Many succulent desert plants
C3: All trees, most crops,
most temperate plants
C4 : Maize, Sugercane
Atriplex
Many tropical grasses 24

25. C4 plants: Spatial division between primary fixation of atmospheric CO2 and its subsequent re-fixation and reduction in the Calvin cycle
CAM: Temporal division between primary fixation of atmospheric CO2 and its subsequent re-fixation and reduction in the Calvin cycle 25

26. 9. Marine Photosynthesis

27. Importance of Marine Photosynthesis:
1) Marine plant productivity “covers” some 72% of earth’s surface
2) Marine plants are responsible for about half of the world’s primary production (90% According to Encyclopedia Britannica) (mostly by Phytoplankton)
3) Benthic marine plants produce some 570 g C m-2 y-1 (as compared to 400 for terrestrial plants and 81 for phytoplankton)
Marine benthic plants have many other roles
apart from primary production!
There can be a very good correlation between photosynthetic rates and growth (e.g. Lipkin et al. 1986, Aquat. Bot. 26: ),

28. Facts and Experiences by
Photosynthesis in the
Marine Environment
Facts and Experiences by
Sven Beer and
John Beardall
Prof. John Raven, FRS
Prof. Paul Falkowski

29. Catalysed by carbonic anhydrase (CA)
0.035% = 350 ppm CO2
~ 15 μM
10 μM CO2
CO2 + H2O <> H2CO3 <> HCO3- + H+ <> CO32- + H+
SLOW reaction
Catalysed by carbonic anhydrase (CA)
pH 8.2: (200) μM

30. Diffusion
boundary layer
Plasma membrane
Cell wall
Inorganic carbon

31. Species Km(CO2) Rubisco Author
Giffordia μM Weidner & Kuppers
Bryopsis, μM Yamada et al. (in
Spatoglossum “ Johnston 1992)
Palmaria μM Cook & Colman
Gracilaria μM Israel & Beer
Ulva μM Beer et al.
Average ~51 μM
Terrestrial C ~10 μM

32. Air

33. There must be a CO2 concentrating mechanism (CCM)

34. Photosynthetic rates of many algae are NOT sensitive to O2, and CO2 compensation points are low…

35. C4 C3 35

36. = They have a CO2 concentrating mechanism (CCM).
Photosynthetic rates of many algae are NOT sensitive to O2, and CO2 compensation points are low…
= They have a CO2 concentrating mechanism (CCM).
So, perhaps C4 photosynthesis? Or CAM?

38. MOST Algae, Most Seagrasses
Plasma membrane
Diffusion boundary layer
Cell wall
MOST Algae, Most Seagrasses

39. Ulva

40. SOME Algae (Ulva spp.) Plasma membrane Diffusion boundary layer
Cell wall
SOME Algae
(Ulva spp.)

41. Red Blood Cell
OH-
HCO3-
in Algae

43. HCO3-
HCO3-
SOME Seagrasses

44. 2 1
HCO3- utilisation IS the CO2 Concentrating Mechanisms (CCM) in marine plants... 3

45. …that saturates Rubisco with CO2…
Km (CO2)

47. How to measure gas exchange…
“B”
IRGA
Air
“C”
Gas exchange column
Seawater
Seawater N2
Pump
Acid
‘Stirrer’
“A”
O2 metre
O2 electrode

49. Photosynthesis (P) (mmol O2 m-2 s-1)
Irradiance (I) (mmol photons m-2 s-1)

50. Photosynthesis (P) (mmol O2 m-2 s-1)
Pgross
Pnet
Irradiance (I) (mmol photons m-2 s-1)

51. Pmax b a
P (app) (O2 evolution or CO2 uptake)
Icomp Ik
Isat
I (mmol photons m-2 s-1) R

52. Fig. 3.2
Pmax(true) b
P(true)
Pmax(app) b
P(app) a
P (O2 evolution or CO2 uptake) a Ik
Isat
1,000
I (mmol photons m-2 s-1) R
Icomp

53. P(app) (mmol O2 or CO2 m-2 s-1)
Pmax(app) b a a
High-light plant
Pmax(app)
P(app) (mmol O2 or CO2 m-2 s-1) b
Low-light plant Ik
Isat
Isat
200
1,000
I (mmol photons m-2 s-1) Ik
Icomp L H

54. Pmax(true) b a a
High-light plant
Pmax(true)
P(true) (mmol O2 m-2 s-1) b
Low-light plant
Isat
Isat
200
1,000
I (mmol photons m-2 s-1) L H

55. 500 umol
100 umol
50 umol