Stage 2 – The Calvin Cycle

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

5. 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 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 • Eventually levels off • 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