In what respects are the photosynthesis adaptations of `C_(4)` plants and CAM plants similar?

Options

A. In both cases, the stomata normally close during the day
B. Both types of plants make their sugar without the Calvin-Benson cycle
C. In both cases, an enzyme other than rubisco carries out the first step in carbon fixation
D. Neither `C_(4)` plants nor CAM plants have grana in their chloroplasts

Answer

To answer the question regarding the similarities in the photosynthesis adaptations of C4 plants and CAM plants, we can break down the explanation into several key steps:

### Step-by-Step Solution:

1. **Understanding Carbon Fixation**:
- Both C4 and CAM plants utilize a process called carbon fixation to convert carbon dioxide (CO₂) from the atmosphere into organic compounds. This is the first step in photosynthesis.

2. **C3 Pathway Overview**:
- In typical C3 plants, carbon fixation occurs when CO₂ reacts with a 5-carbon compound called ribulose 1,5-bisphosphate (RUBP), producing a 3-carbon compound known as phosphoglyceric acid (PGA). This is known as the C3 pathway.

3. **C4 Pathway Mechanism**:
- In C4 plants (like maize and sugarcane), the carbon fixation process is different. CO₂ is first fixed to a 3-carbon compound called phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase, resulting in a 4-carbon compound called oxaloacetate. This adaptation helps to minimize photorespiration.

4. **CAM Plants Mechanism**:
- CAM (Crassulacean Acid Metabolism) plants also fix CO₂ to PEP, forming oxaloacetate, similar to C4 plants. However, CAM plants take up CO₂ at night to reduce water loss, storing it as malate, which is then used during the day for photosynthesis.

5. **Similarities in Adaptations**:
- Both C4 and CAM plants produce a 4-carbon compound (oxaloacetate) during the initial carbon fixation step.
- Both types of plants have adapted their photosynthetic processes to reduce photorespiration and enhance efficiency in carbon fixation, allowing them to thrive in environments with high temperatures and low water availability.
- Importantly, both C4 and CAM plants can produce sugars without relying on the Calvin-Benson cycle for the initial carbon fixation step.

### Conclusion:
In summary, the adaptations of C4 and CAM plants are similar in that both types of plants utilize a 4-carbon compound for initial carbon fixation, thereby avoiding the inefficiencies associated with the C3 pathway and the Calvin-Benson cycle.

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