Cyclic photophosphorylation generates two ATP molecules per cycle.
Cyclic Photophosphorylation: An Overview
Cyclic photophosphorylation is a process that occurs in the thylakoid membrane of chloroplasts during photosynthesis. It involves only Photosystem I (PSI) and does not produce NADPH or oxygen. Instead, its primary function is to generate ATP.
Process Breakdown
- Light Absorption: PSI absorbs light energy, exciting electrons to a higher energy level.
- Electron Transport: These energized electrons are passed to ferredoxin (Fd), a mobile electron carrier.
- Cytochrome b6f Complex: Instead of being passed to NADP+ reductase (as in non-cyclic photophosphorylation), the electrons from Fd are transferred to the cytochrome b6f complex.
- Proton Gradient: As electrons move through the cytochrome b6f complex, protons (H+) are pumped from the stroma into the thylakoid lumen, creating a proton gradient.
- ATP Synthase: The proton gradient drives the synthesis of ATP by ATP synthase, as protons flow back into the stroma.
- Return to PSI: The electrons ultimately return to PSI, completing the cycle.
ATP Production
According to the reference material, cyclic photophosphorylation produces two ATP molecules per cycle. This ATP provides the necessary energy for carbon fixation in the Calvin cycle.
| Feature | Cyclic Photophosphorylation |
|---|---|
| Photosystem Involved | PSI Only |
| ATP Production | 2 ATP molecules per cycle |
| NADPH Production | None |
| Oxygen Production | None |
| Electron Pathway | Cyclic |
Why Cyclic Photophosphorylation?
Cyclic photophosphorylation becomes important when the plant cell needs more ATP than NADPH. This occurs particularly when:
- NADPH levels are already high.
- There is a high demand for ATP in processes like photorespiration.
- The plant is under stress conditions.
