CAM — Crassulacean Acid Metabolism

Crassulacean Acid Metabolism (CAM) is a further alternative pathway of carbon fixation, first described in the plant family Crassulaceae (from which it takes its name) and typically found in desert plants. In CAM plants, the stomata are 'scotoactive' — they open at night rather than during the day — so the initial fixation of CO2 happens at night rather than in daylight.

Chemically, CAM uses the same PEP-carboxylase/malic-acid/decarboxylation chemistry as the C4 pathway, but instead of splitting the two steps between two different cell types (mesophyll versus bundle sheath, as C4 plants do), CAM plants perform both steps, at different times, within the very same mesophyll cell — CAM plants have no Kranz anatomy, and both PEP carboxylase and Rubisco are present together in the mesophyll cell. At night, stomata open and CO2 entering the leaf is fixed by PEP carboxylase into a 4-carbon organic acid (malic acid); this step, called acidification, is stored in the cell's vacuole overnight. During the day, when stomata close (conserving water in the hot, dry conditions these plants live in), the stored malate is released from the vacuole and decarboxylated, releasing CO2 for the ordinary C3 (Calvin) pat …