4. Rate of Photosynthesis in Response to
Differing Percentages of Atmospheric Oxygen

5. Rate of Photosynthesis in Response to Differing Amounts of Light

7. Photoinhibition
Rate of Photosynthesis
Light Intensity

8. Photorespiration occurs when the CO2 levels inside a leaf become low
Photorespiration occurs when the CO2 levels inside a leaf become low. This happens on hot dry days
On hot dry days the plant is forced to close its stomata to prevent excess water loss.
The plant continues fix CO2 when its stomata are closed, the CO2 will get used up and the O2 ratio in the leaf will increase relative to CO2 concentrations.

9. When the CO2 levels inside the leaf drop to around 50 ppm, Rubisco starts to combine O2 with RuBP instead of CO2
The net result of this is that instead of producing 2 3C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called phosphoglycolate is produced.

10. The plant must get rid of the phosphoglycolate since it is highly toxic.
It converts the molecule to glycolic acid.
The glycolic acid is then transported to the peroxisome and there converted to glycine.

11. All these conversions cost the plant energy and results in the net loss of CO2 from the plant
Photorespiration loses 25% of the carbon it takes from the Calvin cycle
To prevent this process, two specialized biochemical additions have been evolved in the plant world: C4 and CAM metabolism.

12. In C3 plants the photosynthesis, carbon fixation and Calvin cycle all occur in a single chloroplast.

13. In C4 plants the photosynthesis takes place in a chloroplast of a thin-walled mesophyll cell and a 4-carbon acid is handed off to a thick-walled bundle sheath cell where the Calvin cycle occurs in a chloroplast of that second cell. This protects the Calvin cycle from the effects of photorespiration.

14. In CAM plants the photosynthesis and initial carbon fixation occur at night and a 4-carbon acid is stored in the cell's vacuole. During the day, the Calvin cycle operates in the same chloroplasts.

15. CAM plants live in very dry condition and, unlike other plants, open their stomata to fix CO2 only at night.
Like C4 plants, the use PEP carboxylase to fix CO2, forming oxaloacetate.
The oxaloacetate is converted to malate which is stored in cell vacuoles. During the day when the stomata are closed, CO2 is removed from the stored malate and enters the Calvin cycle

18. Comparison between C3, C4, and CAM
Product
G3P
Day &night
Malate
Night only
Anatomy
No bundle sheet cell
Bundle sheet cell
No. of stomata
Photorespiration
Up to 40%
Not detectable
Species
Wheat, rice, potato
Sugar cane
Pineapple, vanilla, cacti