1. Reaction of mesophyll cells
(i) Under the action of carboxylase enzyme, phospho-enol pyruvic acid reacts with atmospheric CO2 to produce oxaloacetic acid.
(ii) Oxaloacetic acid is converted to malic acid (aspartic acid) with the help of malic dehydrogenase enzyme. At this time NADPH+H+ takes part in the reaction to produce NADP.
2. Reaction of bundlesheath cells
(iii) Pyruvic acid is produced from malic acid (aspartic acid) under the action of decarboxylase enzyme.
(iv) Phospho-enol pyruvic acid is produced from pyruvic acid under the action of pyruvic acid kinase enzyme. At this time, ATP takes part in the reaction and produces ADP.
Phospho-enol pyruvic acid then re-enters the Hatch-Slack cycle and keeps the cycle going.