Select the correct statements about photophoshorylation :

Options

A. Cyclic photophosphorylation involves photolysis of water
B. Cyclic photophosphorylation occurs in granal thylakoid
C. External electron donor is required in cyclic photophosphorylation
D. Only PS-I is involved in cyclic photophosphorylation

Answer

**Step-by-Step Solution:**

1. **Understanding Photophosphorylation:**
- Photophosphorylation is the process of ATP formation using light energy during photosynthesis. It occurs in two forms: cyclic and non-cyclic photophosphorylation.

2. **Cyclic Photophosphorylation:**
- In cyclic photophosphorylation, only Photosystem I (PSI) is involved. It occurs in the stromal lamellae of chloroplasts.
- Electrons are excited by light energy and flow through a series of proteins (cytochromes) and return to PSI, creating a cycle. No external electron donor is required.

3. **Photolysis of Water:**
- Photolysis of water (the splitting of water molecules into hydrogen ions, electrons, and oxygen) occurs during non-cyclic photophosphorylation, not cyclic.
- In non-cyclic photophosphorylation, electrons lost from Photosystem II (PSII) are replaced by electrons derived from the photolysis of water.

4. **Evaluating Statements:**
- **Statement 1:** "Cyclic photophosphorylation involves photolysis of water." - This is incorrect.
- **Statement 2:** "Cyclic photophosphorylation occurs in the granulothylakoid." - This is also incorrect; it occurs in the stromal lamellae.
- **Statement 3:** "External electron donor is required in cyclic photophosphorylation." - This is incorrect; no external donor is needed as electrons cycle back to PSI.
- **Statement 4:** "Only PSI is involved in cyclic photophosphorylation." - This statement is correct.

5. **Conclusion:**
- The only correct statement about photophosphorylation is that only PSI is involved in cyclic photophosphorylation.

**Final Answer:**
- The correct statement about photophosphorylation is: "Only PSI is involved in cyclic photophosphorylation."

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