The enzyme RuBisCo is absent from the mesophyll cells of `C_(4)` plants. It is present in the bundle-sheath cells surrounding the vascular bundles. In `C_(4)` plants, the Calvin cycle occurs in the bundle-sheath cells. The primary `CO_(2)` 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 `C_(O2)` fixation occurs by the Calvin cycle. This prevents the enzyme RuBisCo from acting as an oxygenase.
