The limiting step in photosynthesis is the rate of

Options

A. `O_2` evolution
B. light reaction
C. dark reaction
D. `CO_2` diffusion to photosynthetic site

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the limiting step in photosynthesis, which is the step that determines the overall rate of the process.

2. **Defining Limiting Step**: In biochemical reactions, the limiting step (or rate-determining step) is the slowest step in a series of reactions that dictates the speed of the overall process.

3. **Identifying the Options**: The options provided are:
- A) Rate of oxygen evolution
- B) Light reaction
- C) Dark reaction
- D) Carbon dioxide diffusion to the photosynthetic site

4. **Analyzing Each Option**:
- **Oxygen Evolution**: This occurs during the light reactions of photosynthesis and is relatively fast.
- **Light Reaction**: This phase captures light energy and converts it into chemical energy (ATP and NADPH), which is also relatively fast.
- **Dark Reaction**: This phase, also known as the Calvin cycle, involves the fixation of carbon dioxide into carbohydrates. This process is slower compared to the light reactions.
- **Carbon Dioxide Diffusion**: This is a physical process and is not considered a reaction step in photosynthesis.

5. **Conclusion**: The dark reaction is the slowest step in the process of photosynthesis, making it the rate-limiting step. Therefore, the correct answer is **C) Dark reaction**.

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