The CAM Cycle

A second, quite distinct strategy for improving photosynthetic efficiency under difficult environmental conditions is found in a group of plants adapted specifically to hot, arid habitats -- succulents such as cacti, Opuntia, Bryophyllum, and crops such as pineapple. These plants perform a pathway called Crassulacean Acid Metabolism, or CAM (named after the plant family Crassulaceae, in which it was first studied in detail), which achieves a broadly similar biochemical goal to the C4 pathway -- concentrating CO2 for RuBisCO and reducing water loss -- but does so through a fundamentally different strategy.

Where C4 plants separate the initial CO2-fixation step from the Calvin cycle proper spatially, using two distinct cell types (mesophyll and bundle sheath), CAM plants instead separate these same two steps temporally, carrying out both within one and the same mesophyll cell, but at two different times of day.

At night, when air temperatures are lower and the risk of water loss through open stomata is consequently much reduced, the stomata of a CAM plant open, allowing atmospheric CO2 to enter the leaf. This CO2 is fixed, exactly as in the C4 pathway, by PEP carboxylase acting on PEP to form oxaloacetic acid, which is then reduced to malic acid. Rather than being transported to a separate cell, however, this malic acid is simply accumulated and stored overnight within the unusually large central vacuole characteristic of these succulent cells -- a storage process that measurably increases the acidity of the cell sap (and of the whole leaf) by morning, a distinctive physiological feature that gave Crassulacean Acid Metabolism its name.

During the day, the situation reverses. Stomata close, drastically cutting down water loss through transpiration during the hottest, driest part of the day -- the central adaptive advantage of the whole strategy. With stomata closed, no fresh atmospheric CO2 can enter, but this is not a problem: the malic acid stored overnight in the vacuole is now broken down, releasing CO2 internally, directly within the same cell. This internally released CO2 is then fixed for a second time by RuBisCO, exactly as in the standard Calvin cycle, using the ATP and NADPH that only the daytime light reaction can supply. …