Why are C4 plants able to photosynthesize with little to no apparent photorespiration?

• A. They have a higher rate of transpiration, which dilutes the concentration of O\(_2\) in the leaf.
• B. They use a different enzyme than Rubisco for carbon fixation, which is not affected by O\(_2\).
• C. They only open their stomata at night, preventing oxygen from entering the leaf.
• D. They spatially separate carbon fixation and the Calvin cycle, concentrating CO\(_2\) around Rubisco and minimizing oxygenation reactions.

Answer: (D)

Verified step by step guidance

1. Understand the problem: The question is asking why C4 plants are able to minimize photorespiration, a process that occurs when Rubisco, the enzyme responsible for carbon fixation, reacts with O\(_2\) instead of CO\(_2\). This leads to a wasteful process that reduces photosynthetic efficiency.
2. Step 1: Recall the key difference in C4 plants. Unlike C3 plants, C4 plants have a specialized mechanism to minimize photorespiration. They achieve this by spatially separating the initial carbon fixation and the Calvin cycle into different cell types.
3. Step 2: Identify the role of PEP carboxylase. In C4 plants, the enzyme phosphoenolpyruvate (PEP) carboxylase is used for the initial fixation of CO\(_2\) in mesophyll cells. This enzyme has a high affinity for CO\(_2\) and is not affected by O\(_2\), unlike Rubisco.
4. Step 3: Explain the transport of the fixed carbon. The CO\(_2\) is fixed into a 4-carbon compound (e.g., oxaloacetate or malate) in the mesophyll cells. This 4-carbon compound is then transported to bundle-sheath cells, where it releases CO\(_2\) for the Calvin cycle.
5. Step 4: Highlight the concentration of CO\(_2\) around Rubisco. By releasing CO\(_2\) in the bundle-sheath cells, C4 plants create a high concentration of CO\(_2\) around Rubisco. This minimizes the likelihood of Rubisco binding to O\(_2\), thereby reducing photorespiration and increasing photosynthetic efficiency.