Which is not ture regarding cyclic electron transfer system (ETP) in thylakoid membrane of higher plants ?

Options

A. Operates at low light intensity
B. Only PSI is involved
C. Act as a back up for ATP synthesis
D. External electron donor is required

Answer

To determine which statement is not true regarding the cyclic electron transfer system (ETP) in the thylakoid membrane of higher plants, we can analyze each option based on our understanding of the process.

### Step-by-Step Solution:

1. **Understanding Cyclic Electron Transfer System**:
- The cyclic electron transfer system, also known as cyclic photophosphorylation, is a process that occurs during the light reactions of photosynthesis. It primarily involves Photosystem I (PSI) and is responsible for the production of ATP.

2. **Evaluating the Statements**:
- **Statement 1**: "Operates at low light intensity."
- This statement is true. PSI can function effectively under low light conditions.

- **Statement 2**: "Only PS1 is involved."
- This statement is also true. In cyclic photophosphorylation, only Photosystem I is involved in the process of ATP generation.

- **Statement 3**: "Acts as a backup for ATP synthesis."
- This statement is true as well. The cyclic process provides an alternative mechanism for ATP synthesis, complementing other pathways.

- **Statement 4**: "External electron donor is required."
- This statement is not true. In cyclic photophosphorylation, there is no requirement for an external electron donor like water. This is a characteristic of non-cyclic photophosphorylation, where water acts as the electron donor.

3. **Conclusion**:
- The statement that is not true regarding the cyclic electron transfer system is: "External electron donor is required."

### Final Answer:
The statement that is not true regarding the cyclic electron transfer system in thylakoid membranes of higher plants is: **"External electron donor is required."**

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