life11e_ch10

CAM plants also use PEP carboxylase

Other plants besides the C₄ plants use PEP carboxylase to fix and accumulate CO₂. They include some water-storing plants (succulents) of the family Crassulaceae, many cacti, pineapples, and several other kinds of flowering plants. The CO₂ metabolism of these plants is called crassulacean acid metabolism, or CAM, after the family of succulents in which it was discovered. CAM is much like the metabolism of C₄ plants in that CO₂ is initially fixed into a four-carbon compound. But in CAM plants the initial CO₂ fixation and the Calvin cycle are separated in time rather than space.

At night, when it is cooler and water loss is minimized, the stomata open. CO₂ is fixed in mesophyll cells to form the four-carbon compound oxaloacetate, which is converted into malate and stored in the vacuole.
During the day, when the stomata close to reduce water loss, the accumulated malate is shipped from the vacuole to the chloroplasts, where its decarboxylation supplies the CO₂ for the Calvin cycle and the light reactions supply the necessary ATP and NADPH.