1. Alternative Mechanisms of Carbon Fixation (5.4)

2. RuBisCO RuBisCO is the most abundant protein on Earth catalyzes the first reaction of the Calvin cycle

3. Photorespiration in the first reaction of the Calvin cycle, CO 2 is “fixed”, forming 2 molecules of 3-PGA O 2 competes with CO 2 for the active site of RuBisCO (ribulose-1,5-bisphosphate carboxylase oxidase)

4. Photorespiration RuBP is combined with O 2 produces only 1 molecule of 3-PGA also produces a molecule that is toxic to the cell that must be removed in a series of reactions, which use ATP release 1 CO 2

5. Photorespiration

6. In hot & dry climates… stomata close during the day to reduce water loss so less CO 2 is present, less is fixed into G3P, and more photorespiration occurs How have plants in warmer climates evolved to cope with RuBisCO’s affinity for O 2 ?

7. C 4 Photosynthesis CO 2 is combined with PEP to form 4-carbon oxaloacetate (enzyme PEP carboxylase) in mesophyll cells oxaloacetate is converted to malate, then transported into bundle-sheath cells malate is decarboxylated forming pyruvate, and CO 2 enters the Calvin cycle

8. C 4 Photosynthesis

10. C 4 Plants

11. CAM Plants crassulacean acid metabolism (CAM), first discovered in Crassulacea family of plants stomata are closed during the day, so CO 2 is only absorbed at night C 4 pathway at night, C 3 pathway (Calvin cycle) during the day

12. CAM Plants

13. CAM plants

14. Animation Narrated animation of C 4 photosynthesis and CAM plants: http://www.uic.edu/classes/bios/bios100/l ectures/c4.htm