CAM plants fix CO₂ at night, form malate which is stored in large vacuoles in mesophyll cells until next day. CAM plants use the enzyme PEPCO (PEP carboxylase). The malate formed at night releases CO₂ during the day to Calvin cycle within the same cell, which now has NADPH and ATP available from light dependent reactions. CAM represents Crassulacean acid metabolism. The crassulaceae is the family of flowering succulents. In CAM plants stomata open during night only and is an adaptation of xerophytic succulents. Photosynthesis in such cases is usually not as efficient as in C3 or C4 plants, but however it allows the CAM plants to live under stressful environment.

Cacti plants like Byrophyllum and some members of family Euphorbiaceae show CAM mechanism (Crassulacean acid metabolism). These have basically the C4 pathway of C fixation. Such plants contain large amount of organic acids like malic acids and oxalic acid. They fix carbon in the form of 4-C organic acids like C4 plants. But they fix CO₂ at night and reduce CO₂ via the Calvin cycle by using NADPH₂ form during the day. They keep their Stomata closed during the day and open during the night for inflow of CO₂. In this way they reduce the rate of transpiration. Such plants have separated the process of CO₂ fixation and electron transfer for reduction of organic acids. Thus, they are able to carry on one step during the day other during the night.

Cardinal points

Every factor influencing a physiological pro-cess has three principal values called cardinal points i.e. minimum, optimum and maximum. Minimum of a factor represents the value below which the process cannot take place. Optimum of the factor is the point where the process is taking place at its highest velocity. Maximum value of a factor is that value beyond which the process comes to stop.