The source of hydrogen for carbon assimilation is

Options

A. NADPH
B. `FADH_2`
C. RuBP
D. `CO_2`

Answer

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

### Step-by-Step Solution:

1. **Understand Carbon Assimilation**:
- Carbon assimilation refers to the process of carbon fixation, where inorganic carbon (like CO2) is converted into organic compounds. This is a crucial part of photosynthesis.

2. **Identify the Role of Photosynthesis**:
- Photosynthesis consists of two main stages: light reactions and dark reactions (Calvin cycle). During these stages, various molecules are produced and utilized.

3. **Focus on Light Reactions**:
- In the light reactions, sunlight is captured, and water is split (photolysis) to produce oxygen, ATP, and NADPH. NADP+ is reduced to NADPH, which is a key electron carrier.

4. **Examine the Function of NADPH**:
- NADPH, generated during the light reactions, serves as a reducing agent. It provides the necessary hydrogen (and electrons) for the reduction reactions that occur during the dark reactions (Calvin cycle).

5. **Consider Other Options**:
- Review the other options:
- FADH2: This is another electron carrier but is not primarily involved in carbon assimilation.
- RUBP (Ribulose bisphosphate): This is a substrate in the Calvin cycle but does not provide hydrogen.
- CO2: While it is the source of carbon, it does not provide hydrogen.

6. **Conclude the Correct Answer**:
- Based on the above analysis, the source of hydrogen for carbon assimilation is NADPH, as it is directly involved in the reduction of carbon compounds during the dark reactions of photosynthesis.

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

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