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.

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

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

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

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

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

Diagram of Calvin Cycle

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

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

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

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

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

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

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

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