1. Chapter 6
Section 6.2

2. The Calvin Cycle Second set of reactions in photosynthesis
Produces organic compounds using the energy stored in ATP and NADPH during the light reactions
Named after Melvin Calvin (American scientist who identified the details of the pathway)

3. Carbon Fixation The incorporation of CO2 into organic compounds
The carbon atoms from CO2 become bonded or “fixed” into organic compounds

4. Steps of the Calvin Cycle
1. CO2 combines with RuBP to form two molecules of PGA
RuBP = 5-carbon carbohydrate
(Ribulose biphosphate)
PGA = 3-carbon molecule
(Phosphoglycerate)

5. 2. Each molecule of PGA is converted into a molecule of PGAL (3-phosphoglyceraldehyde)
Two part process: First, each PGA receives a phosphate group from a molecule of ATP.
Second, the resulting compound receives a proton from NADPH and releases a phosphate group producing PGAL

6. 3. Most of the PGAL is converted back into RuBP
Some PGAL is used to make a variety of organic compounds

8. Each turn of the Calvin cycle fixes one CO2 molecule
Since PGAL is a 3-carbon molecule, it takes three turns of the cycle to produce each molecule of PGAL

9. For each turn of the Calvin cycle, 2 ATP and 2 NADPH molecules are used in step 2
One more ATP molecule is used in step 3
Three turns of the Calvin cycle uses 9 molecules of ATP and 6 molecules of NADPH

10. Alternative Pathways
Calvin Cycle: the most common pathway for carbon fixation
C3 plants: fix carbon only through the Calvin cycle

11. Alternative pathways are found in plants from hot, dry climates
Water loss occurs through stomata (small pores) located on the underside of leaves
Stomata also allow CO2 to enter and O2 to leave

12. C4 Pathway Enables plants to fix CO2 into 4-carbon compounds
Stomata are partially closed during hottest part of the day
Examples: corn, sugar cane, crabgrass

13. The CAM Pathway
Found in plants that open stomata at night and close them during the day
Examples: Cacti and pineapples

14. Rate of Photosynthesis
Rate is affected by the plant’s environment
Most important factor is light intensity
Higher light intensity causes more electrons to get excited

15. Rate is also affected by CO2
Increased levels of CO2 stimulates photosynthesis

16. Temperature is another environmental factor that affects photosynthesis
When temperature rises, the rate of photosynthesis increases until it reaches a certain point
If temperature is too high, enzymes cannot function

17. The End