Comparison of Photosynthesis and Photorespiration in C3/C4/CAM Plants — Tropical Plant Photosynthesis Adaptations

Key Points

• Photorespiration is fundamentally inefficient due to the Rubisco enzyme acting on Oxygen instead of the desired Carbon Dioxide
• Most plants are ”non-tropical” — C3 plants.
• Tropical plants — C4
• Minimise photorespiration by physically seperating light-dependent reactions from the Calvin Cycle
• Desert plants — CAM (Crassulacean Acid Metabolism)
• Minimise photorespiration by performing the calvin cycle during the day.
• It is favourable to have a high CO2:O2 ratio so that rubisco will be more likely to act on CO2 (during the calvin cycle)

Comparisons

C3 Plants — Cool and Wet

1. CO2 enters the mesophyll cell
2. Rubisco converts CO2 → 3-PGA
3. 3-PGA (3-Carbon) → Various sugars.

Approximately 85% of all plants are C3.

Stomata are open during the day, and close at night.

C4 Plants — Hot, Sunny, Tropical

Physical seperation — Mesophyll and neighbouring bundle-sheath cell. Stomata open during the day, closed at night

1. CO2 enters mesophyll cell.
2. PEP + CO2 → Oxaloacetate → Malate
3. Malate is pumped into the bundle-sheath cell in order to ensure a high CO2:O2 ratio.
4. Malate → CO2 + Pyruvate
5. Pyruvate ferried back to mesophyll and converted to PEP
6. This ferrying process is ATP-Dependent
7. CO2 → Sugars via Rubisco.

CAM Plants — Very Hot and Dry

Time-seperation — Stomata open during the night to allow CO2 into the leaf, close during the day to ensure high CO2:O2 ratio

1. CO2 enters mesophyll cell.
2. PEP + CO2 → Oxaloacetate → Malate → Various organic acids.
3. Organic acids pumpedto neighbouring cell to increase the CO2:O2 ratio
4. PEP replenished from the Calvin cycle during the day.
5. This process is ATP-dependent