Z-scheme in thylakoid membrane is concerned with

Options

A. Reduction of `NAD^(+)`
B. Reduction of ` CO_(2)`
C. Electron transfer
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding the Z-Scheme**: The Z-scheme is a model that describes the flow of electrons during the light-dependent reactions of photosynthesis, particularly in the thylakoid membranes of chloroplasts. It illustrates how light energy is converted into chemical energy.

2. **Identifying Components**: The Z-scheme involves two photosystems: Photosystem II (PS II) and Photosystem I (PS I). PS II absorbs light and initiates the process by splitting water molecules (photolysis), releasing oxygen and providing electrons. These electrons are then transferred through an electron transport chain to PS I.

3. **Electron Transfer Process**: The electrons lost from PS II are replaced by electrons derived from the splitting of water. As electrons move through the electron transport chain, they release energy, which is used to pump protons into the thylakoid lumen, creating a proton gradient that drives ATP synthesis.

4. **NADP+ Reduction**: At PS I, the electrons are re-energized by light absorption and are eventually used to reduce NADP+ to NADPH. This process is crucial for the subsequent Calvin cycle, where CO2 is fixed into organic molecules.

5. **Evaluating the Options**:
- **Reduction of NAD+**: This is incorrect; the Z-scheme involves the reduction of NADP+ to NADPH.
- **Reduction of CO2**: This is also incorrect; the reduction of CO2 occurs later in the Calvin cycle, not during the Z-scheme.
- **Electron Transfer**: This is correct; the Z-scheme primarily describes the transfer of electrons through the photosystems and the electron transport chain.
- **All of These**: This is incorrect as not all options are valid.

6. **Conclusion**: The Z-scheme in the thylakoid membrane is primarily concerned with electron transfer during the light-dependent reactions of photosynthesis.

### Final Answer:
The correct answer is **electron transfer**.

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