Stage I: Mesophyll Cells

The C4 pathway's first stage takes place in the mesophyll cells, specifically in their stroma. Here, the CO2 acceptor is not RuBP (as in the C3 cycle) but a 3-carbon compound, phosphoenolpyruvate (PEP); the enzyme PEP carboxylase - which, critically, has an extremely high affinity for CO2 and, unlike RuBisCO, no sensitivity to oxygen at all - combines PEP with a CO2 molecule to form the 4-carbon compound oxaloacetic acid (OAA): $PEP\ (3C) + CO_2 \xrightarrow{PEP\ carboxylase} OAA\ (4C)$. This oxaloacetic acid is not fixed further inside the mesophyll cell itself; instead, it is quickly converted into either malic acid or (in some species) aspartic acid, and this 4-carbon acid is then transported out of the mesophyll cell …