Crassulacean acid metabolism, or CAM, is a photosynthetic process that allows plants to conserve water and survive in arid or semi-arid environments.
Plants from the Crassulaceae, Liliaceae, Cactaceae families, and some of the Compositae, Osteraceae species, etc. have adapted to carry photosynthesis even in regions with drought and deserts.
These plants fixate carbon dioxide in the same manner as C4 plants do. But for the two-stage photo-synthesis is specific: one during the night and another during the day.
The stomata of these plants are open, particularly during the night when most of the gas exchange processes happen. During daylight, they close reducing thus the loss of water through stomatal transpiration.
The reaction of CO2 fixation by the phosphoenolpyruvic acid (first carboxylation reaction) takes place in the cytoplasm in the presence of phosphoenolpyruvate carboxylase.
As a result of this reaction oxaloacetate forms which, under the action of NADP+H+ dependent malate-dehydrogenase, is reduced with the formation of malic acid which accumulates in the vacuoles.
The transport of malate through the tonoplast is passive, according to the electrochemical gradient generated by the accumulation of protons in the vacuoles by means of active transport by proton pumps.
Accumulation of malate in cell vacuoles determines an increase in their acidity. During the daytime, when stomata are closed, malic acid diffuses through the tonoplast into the cytoplasm, where it is decarboxylated, resulting in pyruvic acid and carbon dioxide.
CO2 is transported in chloroplasts and used in phosphoglyceric acid synthesis reactions by means of the C3 photosynthetic path, the second reaction of carboxylation being performed in this manner.
Thus, the biochemical mechanism of performing C4 and CAM photosynthesis is identical; it only differs by the isolation in time (C4) and in space (CAM) of the two carboxylation reactions.
In some species with CAM or C4 photosynthesis, under favorable life conditions (CO2 and water), photosynthesis takes place according to the C3 photosynthetic type and vice versa.
It is considered that these spatial and time variations in photosynthesis emerged as an adaptation to environmental conditions.
