RuBP carboxylase is a less efficent enzyme for `CO_(2)` fixation in photosynthesis than PEP carboxylase because:

Options

A. RuBP is less predominant than PEP in a mesophyll cell
B. RuBP is a 4-carbon sugar phosphate while PEP is a phosphorylated 3-carbon organic acid.
C. Km value of RuBP carboxylase for `CO_(2)` is higher than that of PEP carboxylase
D. The molecular weight of RuBP carboxylase is larger than that of PEP carboxylase

Explanation

### Step-by-Step Solution:

1. **Understanding the Enzymes**:
- RuBP carboxylase (also known as Rubisco) and PEP carboxylase are both enzymes involved in the fixation of carbon dioxide during photosynthesis.
- Rubisco is primarily found in the chloroplasts of plants and is responsible for the first major step of carbon fixation in the Calvin cycle.

2. **Efficiency Comparison**:
- Rubisco is known to be a less efficient enzyme compared to PEP carboxylase.
- The efficiency of an enzyme can be assessed by its turnover number (how quickly it can convert substrate into product) and its affinity for its substrate (often represented by the Km value).

3. **Turnover Rate**:
- Rubisco has a relatively low turnover rate, fixing only about three molecules of CO2 per second.
- In contrast, PEP carboxylase has a higher turnover rate, allowing it to fix CO2 more rapidly.

4. **Km Value**:
- The Km value indicates the concentration of substrate at which the reaction rate is half of its maximum. A higher Km value means that the enzyme requires a higher concentration of substrate to achieve its maximum activity.
- Rubisco has a higher Km value for CO2 compared to PEP carboxylase, indicating that it is less efficient in CO2 fixation.

5. **Conclusion**:
- Based on the above points, we can conclude that RuBP carboxylase is less efficient than PEP carboxylase primarily because the Km value of RuBP carboxylase for CO2 is higher than that of PEP carboxylase.

### Final Answer:
Hence, the correct answer is: **The Km value of RuBP carboxylase for CO2 is higher than that of PEP carboxylase.**

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