What is the role of PEP carboxylase in C4 photosynthesis?

• A. Converts CO2 directly into glucose in mesophyll cells.
• B. Catalyzes the release of CO2 in the atmosphere.
• C. Fixes CO2 into oxaloacetate in mesophyll cells with high affinity for CO2 and no oxygenase activity.
• D. Breaks down CO2 into carbon monoxide and oxygen.

Answer: (C)

In C4 photosynthesis, the main idea is how CO2 is captured efficiently to feed the photosynthetic process without losing it to oxygenation by Rubisco. PEP carboxylase in the mesophyll cells fixes CO2 by attaching it to phosphoenolpyruvate, producing oxaloacetate, a four-carbon compound. This enzyme has a high affinity for CO2 and, unlike Rubisco, almost no oxygenase activity, so it can capture CO2 effectively even when CO2 levels are low or stomata are partly closed. The oxaloacetate is then converted to malate and transported to the bundle-sheath cells, where CO2 is released for use in the Calvin cycle by Rubisco, concentrating CO2 around Rubisco and reducing photorespiration. So the role of PEP carboxylase is to fix CO2 into oxaloacetate in mesophyll cells with high CO2 affinity and no oxygenase activity, setting up the efficient CO2 supply for subsequent carbon fixation.