Which of the following statements is incorrect ?

Options

A. In `C_(4)` plants, the site of RuBisCO activity is mesophyll cell
B. The substrate molecule for RuBisCO activity is a 5-carbon compound
C. RuBisCO action requires ATP and NADPH
D. RuBisCO is a bifunctional enzyme

Answer

To determine which statement is incorrect regarding Rubisco and its functions in photosynthesis, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Rubisco**:
- Rubisco stands for ribulose bisphosphate carboxylase/oxygenase. It is a crucial enzyme in the process of photosynthesis, specifically in the Calvin cycle.

2. **Analyzing Option 1**:
- **Statement**: "C4 plants, the site of Rubisco activity is mesophyll cell."
- **Evaluation**: In C4 plants, Rubisco is located in the bundle sheath cells, not in the mesophyll cells. Therefore, this statement is incorrect.

3. **Analyzing Option 2**:
- **Statement**: "The substrate molecule for Rubisco activity is a 5 carbon compound."
- **Evaluation**: Rubisco acts on ribulose bisphosphate (RuBP), which is a 5-carbon compound. This statement is correct.

4. **Analyzing Option 3**:
- **Statement**: "Rubisco action requires ATP and NADPH."
- **Evaluation**: While Rubisco itself does not directly require ATP and NADPH for its carboxylation reaction, the subsequent reactions in the Calvin cycle do. However, this statement can be misleading as it implies a direct requirement for Rubisco's action. Nevertheless, it is generally accepted that Rubisco is part of the process that ultimately involves ATP and NADPH, so this statement is considered correct in a broader context.

5. **Analyzing Option 4**:
- **Statement**: "Rubisco is a bifunctional enzyme."
- **Evaluation**: Rubisco has both carboxylase and oxygenase activities, making it a bifunctional enzyme. This statement is correct.

### Conclusion:
The incorrect statement is **Option 1**: "C4 plants, the site of Rubisco activity is mesophyll cell."