Differentiate Between Cyclic and Non-Cyclic Photophosphorylation
Photophosphorylation is the process of synthesizing ATP from ADP and inorganic phosphate using light energy during photosynthesis. It occurs in the thylakoid membranes of chloroplasts and can follow two distinct pathways: cyclic and non-cyclic.
Cyclic Photophosphorylation
- Involves only Photosystem I (PSI).
- Electrons are excited by light and passed through an electron transport chain, eventually returning to PSI.
- Generates ATP but not NADPH or O₂.
- Serves as a mechanism to balance the ATP/NADPH energy budget.
- Occurs under conditions where NADP⁺ is in short supply.
Non-Cyclic Photophosphorylation
- Involves both Photosystem I (PSI) and Photosystem II (PSII).
- Electrons are excited in PSII, passed to PSI, and finally transferred to NADP⁺ to form NADPH.
- Electrons lost from PSII are replaced by photolysis of water, releasing O₂ as a byproduct.
- Generates both ATP and NADPH, which are essential for the Calvin cycle.
Summary Table
| Feature | Cyclic | Non-Cyclic |
|---|---|---|
| Photosystems Involved | PSI only | PSI and PSII |
| Products | ATP only | ATP, NADPH, O₂ |
| Electron Pathway | Circular (returns to PSI) | Linear (ends in NADPH) |
These two pathways together help maintain energy balance and ensure sufficient production of ATP and NADPH for photosynthesis.