1. Photorespiration & Alternative Methods of Carbon Fixation

2. Photorespiration Stomata are pores in the leaf surface, through which: Carbon dioxide enters the plant Carbon dioxide enters the plant Water evaporates from the plant Water evaporates from the plant Oxygen leaves the plant Oxygen leaves the plant

3. Photorespiration In hot, dry climates, plants close their stomata Pro: Pro: plant doesn’t lose as much water Cons: Cons: Plant doesn’t receive carbon dioxide Plant can’t get rid of oxygen

4. Photorespiration Rubisco: Enzyme that unites CO 2 and RuBP in the first step of the Calvin Cycle Enzyme that unites CO 2 and RuBP in the first step of the Calvin Cycle HOWEVER, it can also cause O 2 to join RuBP HOWEVER, it can also cause O 2 to join RuBP Higher concentration (O 2 or CO 2 ) “wins” and is joined to RuBP When O 2 is joined to RuBP, photorespiration occurs

5. Photorespiration as a Problem Photorespiration: Does not produce any ATP Prevents the synthesis of glucose Therefore, some plants in hot, dry climates have developed ways to combat photorespiration

6. Alternative Methods of Carbon Fixation

7. C 3 Pathway Uses only the Calvin Cycle Called C 3 because PGA is a 3-C compound Occurs in mesophyll cells

8. C 4 Pathway Different location within the leaf Mesophyll cells pump CO 2 into bundle- sheath cells, which are impermeable to CO 2 This increases the concentration of CO 2

9. C 4 Pathway, cont’d Since the concentration of CO 2 is greater than the concentration of O 2, the Calvin Cycle is favored over photorespiration Examples: Corn Corn Sugar cane Sugar cane

10. C 3 vs. C 4 Different LOCATIONS within the leaf

11. CAM pathway Occurs at a different time Stomata open at night and close during the day Pros: Pros: Prevents water loss Prevents CO 2 from leaving the leaves Con: Con: Plant grows slowly

12. CAM Plants, cont’d… CO 2 is stored overnight until the light reactions can supply ATP and NADPH to drive the Calvin Cycle Examples: Cacti Pineapples