The combination of atmospheric CO2 with a 3-carbon molecule phosphoenol pyruvate (PEP) in the mesophyll cells to make a 4-carbon acid which is immediately converted to malic acid. The malic acid is then passed across to the bundle sheath cells where it is broken down again to pyruvic acid and CO2. The acid is passed back to the mesophyll cells to be used again, while the CO2 is fed into the reductive pentose-phosphate cycle (Calvin cycle) and converted into sugar and starch.