The primary CO acceptor is a -carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase. It is important to register that the mesophyll cells lack RuBisCO enzyme. The C acid OAA is formed in the mesophyll cells.
It then forms other -carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, which are transported to the bundle sheath cells. In the bundle sheath cells these C acids are broken down to release CO and a -carbon molecule. The -carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle. The CO released in the bundle sheath cells enters the C or the Calvin pathway, a pathway common to all plants.
The bundle sheath cells are Figure . Diagrammatic representation of the Hatch and Slack Pathway rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO) , but lack PEPcase. Thus, the basic pathway that results in the formation of the sugars, the Calvin pathway, is common to the C and C plants. Did you note that the Calvin pathway occurs in all the mesophyll cells of the C plants?
In the C plants it does not take place in the mesophyll cells but does so only in the bundle sheath cells.