Which statement best describes C4 carbon fixation?

• A. It fixed carbon directly in the Calvin cycle.
• B. It fixes carbon at night to reduce water loss.
• C. It uses a three-carbon compound to minimize photorespiration.
• D. It uses a four-carbon compound to minimize photorespiration.

Answer: (D)

C4 carbon fixation concentrates CO2 by using a four-carbon intermediary as the first product. In these plants, CO2 is first fixed in mesophyll cells by PEP carboxylase to form a four-carbon molecule (like oxaloacetate, quickly converted to malate). This four-carbon compound is then shuttled to bundle-sheath cells, where CO2 is released and channeled into the Calvin cycle. This arrangement raises the CO2 level around Rubisco, reducing its tendency to fix O2 (photorespiration), which is especially advantageous under hot, dry conditions when stomata are closed.

So, using a four-carbon compound to begin the fixation process is the key feature that minimizes photorespiration. By contrast, fixing at night describes CAM plants, and fixed products of the Calvin cycle in C3 plants involve a three-carbon intermediate after Rubisco’s action, not the initial four-carbon step in C4 plants.