Photorespiration remains an enigma to researchers today- it seems to serve no purpose. It takes place in C3 plants on hot, dry days when their stomata close to preserve water (but at the same time preventing CO2 from entering). This process yields no sugar and no energy and even drains as much as 50% of the carbon fixed by the Calvin cycle.
In contrast, CAM plants have a special photosynthetic adaptation for arid conditions. These plants open their stomata only at night. Because of this, CO2 only enters the plant at night and conserves more water than if their stomata were open at night. Similar to the adaptation in C4 plants, the CO2 obtained in CAM plants is fixed into a four-carbon compound to hold the carbon for the night. Then, at daytime, the carbon atoms are released for the Calvin cycle. This way, photosynthesis can still proceed in the day even with the stomata closed. Dissimilar to C4 plants, carbon fixation and the Calvin Cycle occur in the same types of cells in CAM plants but at different times. C4 plants have these processes take place in different types of cells.
This adaptation is common in water storing plants such as cacti, pineapples, ice plants, and jade plants. All of these plants are adapted to arid conditions. CAM stands for Crassulacean Acid Metabolism, named after the plant family Crassulacean (the plant family where the adaptation was first found).