1. PHOTORESPIRATION This process interferes with successful performance of the Calvin Cycle by preventing CO 2 from fixing and removing the RuBP.

2. What happens to photosynthesis if the CO 2 levels are low, yet there is abundant O 2 in the environment?

3. 1) If Oxygen levels are high, and CO2 levels are low, the Calvin cycle is bypassed to a degree. 2) Rubisco enzyme binds to oxygen, and RuBP (ribulose biphosphate) is converted to CO2. (This happens as the result of the 2C glycolate conversion outside the chloroplast.)

4. This results in a reduced rate of photosynthesis. Photosynthesis and photorespiration occur simultaneously, but their rates change in response to levels of oxygen and carbon dioxide.

5. PHOTOSYNTHESIS OR PHOTORESPIRATION

6. Comparison: Calvin to Photorespiration Evolution of Photorespiration: When the Calvin cycle evolved, there was little O 2 in the atmosphere. Binding of oxygen by Rubisco was not a problem. After millions of years, photosynthesis caused the build-up of oxygen levels in the atmosphere and the binding oxygen by Rubisco became the photorespiration problem of today. Perhaps it is mere evolutionary baggage, but its cost may be high because of reduced plant growth and reduced food production.