RuBisCo is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCo carries out more carboxylation in C plants?
The enzyme RuBisCo is absent from the mesophyll cells of C plants. It is present in the bundle-sheath cells surrounding the vascular bundles. In C plants, the Calvin cycle occurs in the bundle-sheath cells. The primary CO acceptor in the mesophyll cells is phosphoenol pyruvate – a three-carbon compound. It is converted into the four-carbon compound oxaloacetic acid (OAA). OAA is further converted into malic acid. Malic acid is transported to the bundle-sheath cells, where it undergoes decarboxylation and CO fixation occurs by the Calvin cycle. This prevents the enzyme RuBisCo from acting as an oxygenase.