1. Stage 2 – The Calvin Cycle
Series of reactions that produces a 3-carbon sugar
Enzymes help the cycle at each step
The 3-carbon sugar is used to make organic compounds - #1: glucose
This part of photosynthesis occurs in the stroma.

2. What is the role of CO2?
CO2 from the atmosphere is bonded or “fixed” into organic compounds
This process is called carbon fixation.

3. Steps of the Calvin Cycle
1. - CO2 diffuses into stroma from cytosol
- 3 CO2 molecules bind to 3 different molecules of ribulose biphosphate (RuBP)
- CO2 has 1 carbon but we have 3 molecules
- RuBP has 5 carbons, but we have 3 of those too
- each CO2 binds with a RuBP to get a very unstable intermediate molecule with 6 carbons (for a total of three intermediates)
- each intermediate breaks into 2 molecules of 3- phosphoglycerate (3-PGA); each 3-PGA has 3 carbons

4. # molecules
Molecule
# carbons per molecule
Total # carbons 3
CO2 1
RuBP 5 15
intermediates 6 18
PGA

5. 2. - The PGA molecules are converted into
2. - The PGA molecules are converted into glyceraldehyde 3-phosphate (G3P); ADP and NADP+ are produced in the process
- This happens in two main steps:
a. each PGA receives a PO4- (phosphate group)from ATP to make an intermediate compound
b. that intermediate receives a proton from NADPH and releases a phosphate group

6. 3. - one G3P leaves the cycle and is used to
3. - one G3P leaves the cycle and is used to make organic compounds (since C.Cycle incorporates only 1 molecule of CO2 {1 C} at a time, it takes 6 complete turns of the cycle to make 2G3P, that go into 1 glucose molecule.
4. - remaining 5 molecules of G3P converted back to RuBP through a series of steps that add phosphate groups to each from ATP
- the RuBP can enter the Calvin Cycle again

7. Diagram of Calvin Cycle

8. What are stomata? Pores on the underside of the leaf For gas exchange
Decrease water loss by partially closing
CO2 in leaf ↓
O2 ↑
Low CO2 and high O2 inhibit carbon fixation so alternative paths are necessary

9. Alternative Pathways
Used by plants in hot, dry climates that can rapidly lose water through stomata

10. C4 Fix CO2 into 4-C compounds
Keep stomata partially closed during hottest part of day
Certain cells have enzyme to fix CO2 into 4-C compounds even if levels are low
4-C compounds transported to other cells where CO2 is released and enters Calvin cycle

11. C4 Examples: corn, sugar cane, crab grass
Lose ½ as much water as C3 plants

12. CAM Crassulacean acid metabolism
Open stomata at night and close them during the day
Carbon fixation occurs at night
During the day, CO2 released from organic compounds and enters Calvin cycle
Examples: cactuses, pineapples
Lose less water than C3 and C4
Grow very slowly

13. Environmental factors that affect Photosynthesis
Light intensity
Light intensity increases, rate of photosynthesis increases
Eventually levels off
CO2 levels
CO2 levels increase, rate increases
Temperature
Temp. increases, rate increases
Peaks and decreases

14. Summary of Photosynthesis
In chloroplasts
Light reactions
Calvin cycle
Ongoing cycle
Many plants produce more carbs then needed
Stored as starch

15. How are light reactions and Calvin cycle related?
Work as continuous cycle
Can occur in every chloroplast
Calvin cycle doesn’t require light but usually takes place during the day