C4 carbon fixation is the CO2 fixation mechanism used by the maize plant which harbors a kranz anatomy in its leaves. In maize it is carried out in two types of cells namely the mesophyll cell where Carbonic anhydrase fixes diffused CO2 to HCO3 and the enzyme PEP carboxylase converts HCO3 into oxaloacetic acid. Malate dehydrogenase catalyzes the conversion of oxaloacetic acid into malate in the mesophyll chloroplast. Malate is transported to the bundle sheath cell cytoplasm followed by the bundle sheath chloroplast where it is broken down into pyruvate and the CO2. This CO2 molecule is now fixed into 3-phosphoglycerate molecule by the active Rubisco enzyme. This process is different from the C3-carbon assimilation in rice which occurs only in the bundle sheath cells.