1. Organisms capture and store free energy for use in biological processes
Calvin Cycle

2. Where does the Calvin Cycle take place?
Stroma of the chloroplast – the fluid filled area outside of the thylakoid membrane

3. How does CO2 enter the Calvin Cycle?
CO2 enters through the stomata – microscopic pores in leaves
Once in the leaf the CO2 diffuses into mesophyll cells where it can enter the chloroplast
Within the chloroplast carbon fixation takes place

4. Fig. 10-3a
Leaf cross section
Vein
Mesophyll
Stomata
CO2 O2
Chloroplast
Mesophyll cell
Figure 10.3 Zooming in on the location of photosynthesis in a plant
5 µm

5. What occurs during carbon fixation?
Carbon dioxide joins a five-carbon molecule called ribulose bisphophate (RuBP)
This reactions is catalyzed by RuBP carboxylase, aka Ribisco
Ribisco – the most abundant enzyme in nature
This enzyme often takes up 50% of the total chloroplast protein content
Ribisco is a slow – only catalyzing 3 molecules of substrate per second (compared to 1,000 per second)
Unstable 6 carbon compound is formed which splits to form 2 three carbon molecules of PGA (phosphoglycerate)

6. How is PGA turned into sugar?
Each molecule of PGA is systematically reduced by enzyme action.
NADPH provides the hydrogen atoms and ATP provides the energy for these reactions to occur. (NADPH and ATP from Light Reactions)
PGAL (phosphoglyceraldehyde), also called G3P (glyceraldehyde-3-phosphate) is the final product of the Calvin Cycle
G3P can be exported to the cytoplasm and combined to form fructose-6-phosphate and glucose 1-phosphate.
Fructose and glucose can join to form sucrose

7. How does the Calvin Cycle get back to 5-C RuBP?
For every 3 molecules of carbon dioxide fixed, 6 molecules of G3P are formed
Only 1 of the G3P exits the cycle
The other five G3P (3C) molecules are used to regenerate 3 molecules of RuPB (5C) using ATP from the Light Reactions

8. Figure 10.18 The Calvin cycle
Input 3
(Entering one
at a time)
CO2
Phase 1: Carbon fixation
Rubisco 3 P P
Short-lived
intermediate 3 P P 6 P
Ribulose bisphosphate
(RuBP)
3-Phosphoglycerate 6
ATP
6 ADP
3 ADP
Calvin
Cycle 6 3 P P
ATP
1,3-Bisphosphoglycerate 6
NADPH
Phase 3:
Regeneration of
the CO2 acceptor
(RuBP)
6 NADP+ 6 P i
Figure The Calvin cycle 5 P
G3P 6 P
Glyceraldehyde-3-phosphate
(G3P)
Phase 2:
Reduction 1 P
Glucose and
other organic
compounds
Output
G3P
(a sugar)

9. Alternative Carbon Fixation Mechanisms
Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes
Alternative Carbon Fixation Mechanisms

10. Why do plants need alternative mechanisms for carbon fixation?
Dehydration is a problem for 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 a seemingly wasteful process called photorespiration

11. What is photorespiration?
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound
In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

12. How do C4 plants avoid photorespiration?
C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
This step requires the enzyme PEP carboxylase
PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 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

13. C4 leaf anatomy The C4 pathway Mesophyll cell Mesophyll cell CO2
Fig
C4 leaf anatomy
The C4 pathway
Mesophyll
cell
Mesophyll cell
CO2
Photosynthetic
cells of C4
plant leaf
PEP carboxylase
Bundle-
sheath
cell
Oxaloacetate (4C)
PEP (3C)
Vein
(vascular tissue)
ADP
Malate (4C)
ATP
Pyruvate (3C)
Bundle-
sheath
cell
Stoma
CO2
Calvin
Cycle
Figure C4 leaf anatomy and the C4 pathway
Sugar
Vascular
tissue

14. How do CAM plants avoid photorespiration?
Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon
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

15. (a) Spatial separation of steps (b) Temporal separation of steps
Fig
Sugarcane
Pineapple C4
CAM
CO2
CO2
Mesophyll
cell 1
CO2 incorporated
into four-carbon
organic acids
(carbon fixation)
Night
Organic acid
Organic acid
Figure C4 and CAM photosynthesis compared
Bundle-
sheath
cell
CO2
CO2
Day 2
Organic acids
release CO2 to
Calvin cycle
Calvin
Cycle
Calvin
Cycle
Sugar
Sugar
(a) Spatial separation of steps
(b) Temporal separation of steps

16. Review
The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits
In addition to food production, photosynthesis produces the O2 in our atmosphere

17. Electron transport chain
Fig
H2O
CO2
Light
NADP+
ADP + P i
Light
Reactions:
Photosystem II
Electron transport chain
Photosystem I
RuBP
3-Phosphoglycerate
Calvin
Cycle
ATP
G3P
Figure A review of photosynthesis
Starch
(storage)
NADPH
Chloroplast O2
Sucrose (export)