In case of C₄-pathway

In the case of the C₄-pathway, CO₂ combines with PEP.
The CO₂ combines with ribulose bisphosphate and goes through the Calvin cycle while the pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the C₄ cycle.
PEP carboxylase attaches an incoming carbon dioxide molecule to the three-carbon molecule PEP, producing oxaloacetate.