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The CAM plants are mostly succulent xerophytes such as plants of families Crassulaceae, Euphorbiaceae, Cactaceae, etc. The stomata in these plants remain closed during the day. This helps to check transpiration and open them during the cool night for gaseous exchange.

These plants are characterized by the presence of Crassulacean acid metabolism (CAM) involving night photosynthetic activity. This is a desert adaptation where stomata must remain closed during the day.

The CAM plants have the dark reaction very much similar to the C₄ plants ( i.e., they have both C₄ and C₃ pathways.) However, they differ from C₄ plants because, in CAM plants, both pathways occur in mesophyll cells only. Moreover, C₄ pathway occurs during the night when stomata are open (night photosynthetic activity) and C₃ pathway is completed during day time when the stomata are closed.

At night, atmospheric carbon dioxide is taken in through the open stomata. As in the C₄ pathway, carbon dioxide combines with 3-C PEP to form 4-C OAA. This is then converted to malic acid and stored in the cells. Thus, organic acids accumulate in the dark.

During the day, when stomata are closed and atmospheric carbon dioxide is not available, the 4-C malic acid undergoes decarboxylation to form carbon dioxide and 3-C pyruvic acid. Carbon dioxide thus released enters the Calvin cycle (C₃ cycle) to form sugars.

Chapter 5