in the C4 pathway (Hatch and Slack pathway), the primary CO2 acceptor is a 3-carbon molecule called phosphoenol pyruvate (PEP). This reaction takes place in the mesophyll cells and is catalysed by the enzyme PEP carboxylase (PEPcase). In contrast, the primary CO2 acceptor in C3 plants is a 5-carbon molecule called Ribulose bisphosphate (RuBP) .