Question

why does rubisco act only as carboxylase in c4

Answer 1

In C4 plants, along with Rubisco there is another enzyme present called phosphoenol pyruvate carboxylase or PEP carboxylase. These plants have an additional ring of specialized cells called bundle sheath in their leaves. PEP carboxylase converts oxaloacetate into malate in the leaf mesophyll cells and transfer into bundle sheath cells. Here, 4 carbon malate is converted into 3 carbon pyruvate to release carbon dioxide. This keeps the level of carbon dioxide high and provides continuous supply. For fixing this carbon Rubisco acts as carboxylase only in C4 plants.