Cyclic photophosphorylation is carried out by
(a) P S I only
(b) P S II only
(c) Both a and b
(d) Photolysis and P S II

Options

A. `PS` I only
B. `PS` II only
C. Both A and B
D. Photolysis and `PS` II.

Answer

**Step-by-step Solution:**

1. **Understanding Cyclic Photophosphorylation**:
- Cyclic photophosphorylation is a process that occurs during the light-dependent reactions of photosynthesis. It specifically involves the generation of ATP without the production of NADPH.

2. **Identifying the Photosystems**:
- There are two main photosystems involved in photosynthesis: Photosystem I (PS I) and Photosystem II (PS II).
- PS I is also referred to as P700 because it absorbs light at a wavelength of 700 nm.
- PS II is referred to as P680 because it absorbs light at a wavelength of 680 nm.

3. **Role of Photosystem I in Cyclic Photophosphorylation**:
- Cyclic photophosphorylation is carried out exclusively by Photosystem I (PS I).
- When PS I absorbs light, it becomes excited and releases electrons. These electrons are then transferred through a series of electron carriers and eventually return to PS I, completing the cycle.

4. **ATP Production**:
- The movement of electrons through the electron transport chain leads to the synthesis of ATP via chemiosmosis.
- Importantly, NADPH is not produced during cyclic photophosphorylation; only ATP is generated.

5. **Evaluating the Options**:
- (a) PS I only: Correct, as cyclic photophosphorylation occurs only in PS I.
- (b) PS II only: Incorrect, as PS II is not involved in cyclic photophosphorylation.
- (c) Both a and b: Incorrect, since only PS I is involved.
- (d) Photolysis and PS II: Incorrect, as photolysis is part of the non-cyclic pathway and PS II is not involved in cyclic photophosphorylation.

6. **Conclusion**:
- The correct answer is (a) PS I only.

**Final Answer**: (a) PS I only

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