Distinguish C3, C4, and CAM photosynthesis and why plants use these pathways.

• A. C3 plants fix CO2 via RuBisCO in bundle-sheath cells.
• B. C4 concentrates CO2 in bundle-sheath cells to reduce photorespiration.
• C. CAM plants open stomata during the day to fix CO2 and store it as sugar.
• D. CAM plants do not use any form of carbon fixation.

Answer: (B)

This question hinges on how plants reduce wasteful photorespiration by altering where and when carbon is fixed. In the C4 pathway, CO2 is first fixed in mesophyll cells by the enzyme PEP carboxylase into a four-carbon compound, which is then transported to bundle-sheath cells where CO2 is released in a high-CO2 environment for fixation by Rubisco. This spatial separation keeps Rubisco working with plenty of CO2 and limits the oxygenase activity that leads to photorespiration, which is especially advantageous in hot, sunny environments where plants tend to keep stomata partly closed to conserve water. The result is more efficient photosynthesis under those conditions, which is why concentrating CO2 in bundle-sheath cells is the hallmark of this pathway.

In contrast, C3 plants fix CO2 directly with Rubisco in the mesophyll, which can lead to more photorespiration when stomata are closed or when temperature and oxygen levels promote the oxygenase reaction. CAM plants solve the same problem by opening stomata at night to fix CO2 into organic acids and storing it, then releasing CO2 for daytime fixation while the stomata remain closed to save water. This temporal separation is different from the spatial separation in C4.

So the choice describing the CO2 concentration in bundle-sheath cells to reduce photorespiration best captures the distinctive advantage of the C4 pathway. The other statements describe features that belong to C3 or CAM strategies or are incorrect about CAM.