The liberated CO 2 is refixed in the chloroplasts by the bifunctional enzyme ribulose-bisphostate carboxylase/oxygenase (Rubisco). In the following light period, the stomata are maintained closed, and vacuolar malic acid is remobilized into the cytoplasm (returning to the malate form) and decarboxylated, releasing CO 2 (a process mediated by malic enzyme, ME-type, or phosphoenolpyruvate carboxykinase, PEPCK-type, enzymes, depending on the plant species) and causing the deacidification of the cells. This transport into the vacuole is mediated by an active process of proton pumping through H +-V-ATPases in the tonoplast and an organic acid anion channel. OAA is then reduced by malate dehydrogenase (MDH) to malate, which is subsequently transported into the vacuole and stocked in the form of malic acid, generating the typical nocturnal acidification of CAM plants. In general, CAM photosynthesis consists of the nocturnal carboxylation of phosphoenolpyruvate (PEP) by using atmospheric or respiratory CO 2, giving rise to oxaloacetate (OAA), a reaction mediated by the enzyme phosphoenolpyruvate carboxylase (PEPC). Nowadays, we know that CAM is present in plant families other than Crassulaceae, being found in about 20,000 terrestrial and aquatic species, with representatives in 343 genera of 35 families. The term Crassulacean Acid Metabolism (CAM) was introduced in the 1940s as a result of observations in Bryophyllum calycinum, a crassulacean plant, which showed prominent diel variations in leaf acid content, with increases at night followed by daytime deacidification.
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