The source of hydrogen for carbon assimilation is

Options

A. `NADPH`
B. `FADH_(2)`
C. `H_(2)O`
D. `RuBP`

Answer

To determine the source of hydrogen for carbon assimilation in photosynthesis, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Photosynthesis**: Photosynthesis consists of two main phases: the light-dependent reactions and the light-independent reactions (Calvin cycle or dark phase).

2. **Identifying Key Products**: During the light-dependent reactions, two important molecules are produced: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). ATP provides energy, while NADPH serves as a reducing agent.

3. **Role of NADPH**: NADPH is crucial in the Calvin cycle, where it donates electrons and hydrogen ions (H+) to convert carbon dioxide (CO2) into glucose. This process is known as carbon assimilation or CO2 fixation.

4. **Evaluating the Options**:
- **NADPH**: This is the correct answer as it provides the hydrogen needed for carbon assimilation.
- **FADH2**: This is primarily involved in cellular respiration and not in photosynthesis.
- **H2O**: While water is split during the light reactions to release oxygen, it does not directly provide hydrogen for carbon assimilation.
- **RUBP (Ribulose 1,5-bisphosphate)**: This is a substrate in the Calvin cycle but not a source of hydrogen.

5. **Conclusion**: The source of hydrogen for carbon assimilation in photosynthesis is NADPH.

### Final Answer:
The source of hydrogen for carbon assimilation is **NADPH**.

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