Which of the following processes occurs in C4 plants?

• A. Direct fixation of CO\(_2\) by Rubisco in mesophyll cells
• B. Photorespiration is the main pathway for carbon fixation
• C. Initial fixation of CO\(_2\) into a four-carbon compound in mesophyll cells
• D. Opening of stomata primarily at night to minimize water loss

Answer: (C)

Verified step by step guidance

1. Understand the unique adaptations of C4 plants: C4 plants have evolved a specialized mechanism to minimize photorespiration and efficiently fix CO\(_2\) in hot and dry environments.
2. Learn the role of mesophyll cells in C4 plants: In these cells, CO\(_2\) is initially fixed into a four-carbon compound (oxaloacetate) by the enzyme PEP carboxylase, which has a higher affinity for CO\(_2\) and does not bind oxygen, unlike Rubisco.
3. Recognize the importance of spatial separation: C4 plants separate the initial CO\(_2\) fixation in mesophyll cells from the Calvin cycle, which occurs in bundle sheath cells. This spatial separation reduces photorespiration.
4. Clarify why Rubisco does not directly fix CO\(_2\) in mesophyll cells: In C4 plants, Rubisco operates in the bundle sheath cells where CO\(_2\) is concentrated, ensuring efficient carbon fixation without interference from oxygen.
5. Understand stomatal behavior: C4 plants typically open their stomata during the day to allow CO\(_2\) uptake, unlike CAM plants, which open their stomata at night to conserve water.