Question

Why does Rubisco act only as a carboxylase in C4 plants?

Answer 1

In C4 plants an additional pathway called PEP carboxylase is present. An additional layer of bundle sheath chloroplasts is present. mesophyll bundles convert PEP to malate which is a 4 carbon compound and transfers it to bundle sheath chloroplast. Here malate is converted to 3 carbon pyruvate yielding carbon dioxide. This maintains a continuous supply of Carbon dioxide , hence Rubisco functions exclusively as a carboxylase fixing this carbon.