1. Calvin Cycle, C4 and CAM Plants
Chapter 8
Calvin Cycle, C4 and CAM Plants

2. You Must Know
How the Calvin cycle uses the energy molecules of the light reactions (ATP and NADPH) to produce carbohydrates (G3P) from CO2.
The difference between C3, C4, and CAM plants.
(Knowledge of C4/CAM photosynthesis is not required for the AP exam, but it is a nice example of competitive inhibition, an evolutionary relic, and evolutionary adaptations to arid conditions.)

3. anabolic H2O CO2 Light NADP ADP Calvin Cycle Light Reactions O2
Figure 8.UN03
anabolic
H2O
CO2
Light
NADP
ADP
Calvin
Cycle
Light
Reactions
ATP
Concept 8.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar
The Calvin cycle is anabolic. It builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH.
NADPH O2
[CH2O] (sugar) 3

4. For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2
Carbon fixation
3  5C
6  3C
Calvin
Cycle
Regeneration of
CO2 acceptor
5  3C
Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)
For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2
The Calvin cycle has three phases
Carbon fixation
Reduction
Regeneration of the CO2 acceptor
Reduction
1 G3P (3C) 4

5. (Note: for the next three slides, you only need to remember the names of molecules that are marked with a red arrow.)

6. R u BP? Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco RuBP Calvin
Figure
Input 3
as 3 CO2
Phase 1: Carbon fixation
Rubisco 3 P P 3 P P 6 P
RuBP
3-Phosphoglycerate
Calvin
Cycle
R u
BP?
Phase 1, carbon fixation, involves the incorporation of the CO2 molecules into ribulose bisphosphate (RuBP) using the enzyme rubisco 6

7. phosphorylation reduction Input 3 as 3 CO2 Phase 1: Carbon fixation
Figure
Input 3
as 3 CO2
Phase 1: Carbon fixation
Rubisco
phosphorylation 3 P P 3 P P 6 P
RuBP
3-Phosphoglycerate 6
ATP
6 ADP
reduction
Calvin
Cycle 6 P P
1,3-Bisphosphoglycerate 6
NADPH
6 NADP
6 P i
Phase 2, reduction, involves the reduction and phosphorylation of 3-phosphoglycerate to G3P. 6 P
G3P
Phase 2:
Reduction
Glucose and
other organic
compounds 1 P
G3P
Output 7

8. Phase 1: Carbon fixation Rubisco
Figure
Input 3
as 3 CO2
Phase 1: Carbon fixation
Rubisco 3 P P 3 P P 6 P
RuBP
3-Phosphoglycerate 6
ATP
6 ADP
3 ADP
Calvin
Cycle 6 P P 3
ATP
1,3-Bisphosphoglycerate 6
NADPH
Phase 3:
Regeneration
of RuBP
6 NADP
6 P i 5 P
Phase 3, regeneration, involves the rearrangement of G3P to regenerate the initial CO2 receptor, RuBP
You do not have to recreate the figure from memory, but you should be able to explain the process.
G3P 6 P
G3P
Phase 2:
Reduction
Glucose and
other organic
compounds 1 P
G3P
Output 8

9. The following two sides contain background Information on why C4 and CAM photosynthesis are adaptive in some environments.

10. Photosynthetic organisms evolved for roughly 800,000,000 years in a low O2 environment.
Life first evolved about 3.5 billion years ago. Very little O2 was available in the atmosphere until about 2.7 billion years ago. The 1st photosynthetic organisms evolved roughly 3.5 billion years ago. The 1st photosynthetic organisms oxygenated the atmosphere. (You won’t be expected to remember the dates from this slide)

11. What happens when a plant closes its stomata? Water loss decreases
Leaf cross section
Mesophyll
Stomata
Chloroplasts
Vein
CO2 O2
Figure 8.3a
What happens when a plant closes its stomata?
Water loss decreases
CO2 concentrations decreases
O2 concentrations increase
CO2 enters and O2 exits the leaf through microscopic pores called stomata.
Water also exits the leaf through the stomata.
Adaptation to dehydration is a problem for land plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis.
On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis.
The closing of stomata reduces access to CO2 and causes O2 to build up.
These conditions favor an apparently wasteful process called photorespiration.
H2O 11

12. competitive inhibition
In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound. It uses up ATP without producing any sugar.
Evolutionary Relic of low atmosphere
competitive inhibition
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate).
Photorespiration decreases photosynthetic output by consuming ATP, O2, and organic fuel and releasing CO2 without producing any ATP or sugar.
Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2 .
Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle.
Rubisco

13. An enzyme in the mesophyll cells has a high affinity for CO2 and can fix carbon into a four-carbon compound even when CO2 concentrations are low. C4
This takes energy! 1
CO2 C4
Mesophyll
cell
Organic
acid
CO2 2
C4 plants minimize the cost of photorespiration by incorporating CO2 into a four-carbon compound.
An enzyme in the mesophyll cells has a high affinity for CO2 and can fix carbon even when CO2 concentrations are low.
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle.
Bundle-
sheath
cell
Calvin
Cycle
Sugar
(a) Spatial separation of steps 13

14. C4 Plants
Sugarcane Corn Switchgrass

15. CAM Plants This takes energy!
CAM plants open their stomata at night, incorporating CO2 into organic acids.
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle.
This takes energy! 1
CO2
CAM
Organic
acid
Night
CO2 2
Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon.
Day
Calvin
Cycle
Sugar
(b) Temporal separation of steps 15

16. CAM Plants
CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce water loss, but open at night to collect carbon dioxide. The CO2 is stored as an organic acid , and then used during photosynthesis during the day. The pre-collected CO2 is concentrated around the enzyme rubisco, increasing photosynthetic efficiency.

17. Advantages? Disadvantages? CAM C3 C4
C4 and CAM plants have a competitive advantage over C3 plants under conditions of drought, high temperature, or CO2 limitation because they lose less water through their stomata when they fix CO2. The disadvantage is that it takes more ATP to make the same amount of G3P.
Advantages?
Disadvantages?