Phosphoenolpyruvate or PEP is the 3C compound and acts as the primary carbon dioxide acceptor in the mesophyll cells of C4 plants, leading to the formation of OAA, C4 acid. Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) is an enzyme found in the mesophyll cells present within the C_{3 }plants. In C_{4 }plants, this enzyme is found in the bundle sheath cells.