Hatch and Slack pathway is a cyclic process. The primary CO₂ acceptor is a three-carbon molecule phosphoenolpyruvate (PEP) and it is present in mesophyll cells. The enzyme that catalyses this CO₂ fixation is PEP carboxylase or PEPcase. The mesophyll cells pf C₄ plants lack the enzyme RuBisCO. The 4-carbon oxaloacetic acid (OAA) is formed in the mesophyll cells. It is then converted to other four-carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, these are then transported to the bundle sheath cells. In the bundle sheath cells, these C₄ acids are broken down to release CO₂ and three-carbon molecule. The CO₂ released in the bundle sheath cells enters the C₃ or the Calvin pathway. The bundle sheath cells are rich in an enzyme RuBisCO but lack PEPcase. The three-carbon molecule is transported back to the mesophyll cells where it is converted to PEP again, thus completing the cycle.