There is double fixation of `CO_(2)` both in `C_(4)` and CAM plants. Which of the following statements brings out the main difference between the two ?

Options

A. Carboxylising enzymes are different
B. Fixation pathways are different
C. `CO_(2)` fixation in `C_(4)` plants is separated by space whereas in CAM plants, it is separated by time
D. The compensation point are different

Answer

To address the question regarding the double fixation of CO2 in C4 and CAM plants and to identify the main difference between the two, we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding CO2 Fixation**: Both C4 and CAM plants perform double fixation of CO2, but they do so in different ways. C4 plants fix CO2 in two distinct stages: first in the mesophyll cells and then in the bundle sheath cells. In contrast, CAM plants fix CO2 at night and store it as malate, which is then used during the day for photosynthesis.

2. **Carboxylation Enzymes**: Both C4 and CAM plants utilize different carboxylating enzymes. C4 plants primarily use PEP carboxylase for the initial fixation of CO2, while CAM plants also use PEP carboxylase but have a different temporal regulation of the process.

3. **Fixation Pathways**: The fixation pathways are indeed different; C4 plants have a spatial separation of the fixation process, while CAM plants have a temporal separation. This means that in C4 plants, the processes occur in different cells, whereas in CAM plants, the processes occur at different times of the day.

4. **Compensation Points**: The compensation point is the level of CO2 at which photosynthesis and respiration balance each other. C4 plants typically have a compensation point of around 0 to 10 ppm, while CAM plants have a lower compensation point, around 0 to 5 ppm. This difference is significant because it reflects the efficiency of CO2 utilization under different environmental conditions.

5. **Conclusion**: The main difference between C4 and CAM plants in terms of CO2 fixation is that in C4 plants, the fixation is separated by space (different cells), while in CAM plants, it is separated by time (night vs. day). Therefore, the correct statement that highlights the main difference is that "CO2 fixation in C4 plants is separated by space whereas in CAM plants it is separated by time."

### Final Answer:
The main difference between C4 and CAM plants regarding CO2 fixation is that CO2 fixation in C4 plants is separated by space, while in CAM plants, it is separated by time.

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