Electron flow in thylakoid membrane from `PS-II rarr PSI` is prevented by

Options

A. 2,4,-D
B. Urea
C. DCMU
D. Paraquat

Answer

To solve the question regarding what prevents electron flow in the thylakoid membrane from PS-II (Photosystem II) to PS-I (Photosystem I), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Photosynthesis Process**:
- Photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). The light-dependent reactions involve the transfer of electrons through a series of proteins embedded in the thylakoid membrane.

2. **Identify the Components**:
- In the thylakoid membrane, there are two main photosystems: PS-II and PS-I. PS-II absorbs light and initiates the electron transport chain by splitting water molecules, releasing oxygen and transferring electrons.

3. **Electron Transport Chain**:
- Electrons flow from PS-II to plastoquinone and then to PS-I. This flow is crucial for the production of ATP and NADPH, which are essential for the Calvin cycle.

4. **Inhibitors of Electron Flow**:
- The question asks which substance prevents this electron flow from PS-II to PS-I. We need to evaluate the given options: 2,4-D, urea, DCMU, and paraquat.

5. **Analyze Each Option**:
- **2,4-D**: A herbicide that affects plant growth but does not specifically inhibit electron flow between PS-II and PS-I.
- **Urea**: A nitrogenous compound used as fertilizer; it does not inhibit electron transport.
- **DCMU (Dichlorophenyl dimethyl urea)**: This is a known inhibitor of PS-II. It blocks the transfer of electrons from PS-II to plastoquinone, thereby preventing the flow of electrons to PS-I.
- **Paraquat**: A herbicide that generates reactive oxygen species but does not directly inhibit the electron flow between PS-II and PS-I.

6. **Conclusion**:
- The correct answer is **DCMU**, as it specifically inhibits the electron transport from PS-II to plastoquinone, thus preventing the flow of electrons to PS-I.

### Final Answer:
**DCMU (Dichlorophenyl dimethyl urea)** prevents electron flow in the thylakoid membrane from PS-II to PS-I.

---