a) Initial fixation of CO2 occurs in mesophyll cells.
(b) The primary acceptor of CO2 is phosphoenol
pyruvate or PEP. It combines with CO2 in the
presence of PEP carboxylase to form oxalo-acetic acid or oxaloacetate.
PEP + CO2 + H2O —> Oxalo-acetic acid + H3PO4
Oxaloacetic acid is reduced to malic acid or transaminated to form aspartic acid.