Photosynthesis

Another variant of photosynthesis was originally found in many plants of the Crassulaceae family. The photosynthetic leaves of these plants accumulate malic acid or isocitric acid at night and metabolize these acidic compounds during the day. This type of photosynthesis is referred to as Crassulacean Acid Metabolism or more simply, CAM photosynthesis.

During the night, the following reactions occur in plants with CAM photosynthesis: (a) they open up special pores in their leaves, referred to as stomata, and the leaves take in carbon dioxide from the atmosphere; (b) they metabolize some of their stored starch to PEP (phosphoenol pyruvate), a 3-carbon molecule; (c) they combine carbon dioxide with PEP to form malic acid or isocitric acid, 4-carbon molecules; (d) they accumulate large amounts of malic acid or isocitric acid in their leaves, so that they taste somewhat sour if sampled at night or early morning.

During the day, the following reactions occur in plants with CAM photosynthesis: (a) they close their stomata; (b) they release carbon dioxide from the accumulated malic acid or isocitric acid; (c) they combine this released carbon dioxide with RuBP and the Calvin cycle operates more or less as described above.

Most plants with CAM photosynthesis grow in deserts and other arid environments. In such environments, evaporative loss of water is lower in CAM plants because they close their stomata during the day.

Species from over 20 different plant families, including Cactaceae, Orchidaceae, Liliaceae, and Bromeliaceae have been identified as having CAM photosynthesis. Thus, plant physiologists believe that CAM photosynthesis evolved independently many times. Many CAM plants are succulents, plants with thick leaves and a high ratio of volume to surface area. Interestingly, while CAM photosynthesis is genetically determined, some plants can switch from C-3 photosynthesis to CAM photosynthesis when they are transferred to an arid environment.