Crassulacean Acid Metabolism or CAM Cycle

Crassulacean Acid Metabolism (CAM) is a specialised carbon-fixation pathway found in succulent plants adapted to semi-arid or xerophytic (dry) habitats; it takes its name from the family Crassulaceae, where it was first described, in genera such as Bryophyllum, Sedum and Kalanchoe, though it is also found more broadly, in Agave, Opuntia, pineapple, and orchids among others. CAM's defining feature is a reversed, or "scotoactive," stomatal rhythm: unlike most plants, CAM plants keep their stomata closed during the day and open only at night. This reversed rhythm conserves water by preventing daytime transpiration through open stomata during the hottest, driest part of the day, but it also creates a genuine timing problem, since it means CO2 cannot be taken up from the atmosphere at the same time that light energy is actually available to drive the Calvin cycle. CAM plants solve this mismatch by separating CO2 capture and CO2 use into two different times of day rather than two different cell types (as C4 plants do). At night, with stomata open, atmospheric CO2 enters the leaf and is combined by PEP carboxylase with phosphoenolpyruvate to form oxaloacetate, which is then reduced (via NAD+-malic dehydrogenase) to malic acid; this malic acid is pumped into and stored in the cell's vacuole, causing a measurable rise in cellular acidity referred to as leaf acidification. During the day, stomata close (preventing water loss and further CO2 uptake), and the malic acid stored overnight is released from the vacuole and decarboxylated, using the enzyme NADP+-malic enzyme, into pyruvic acid and CO2, a step that lowers acidity again and is accordingly called deacidification; the CO2 released by this decarboxylation enters the ordinary Calvin cycle within the very same chloroplast, using the ATP and NADPH+H+ generated by that day's light reaction, to build carbohydrate. The overall benefit of CAM is threefold: it lets succulent plants obtain a workable CO2 supply, drawn from stored malic acid, even while their stomata remain fully closed; it lets photosynthesis continue productively t …