Crassulacean acid metabolism

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Crassulacean acid metabolism (CAM) is a carbon fixation pathway in some photosynthetic plants. CAM is usually found in plants living under arid conditions, including those found in the desert (for example, cacti or pineapple). It is named after the plant family it was first discovered in, the Crassulaceae.

Synopsis

Plants that are adapted to dry climates are called xerophytes. Some of these plants have small, thick leaves with a reduced surface area. They may also have a thickened cuticle to protect themselves from the environment. The stomata may be sunken into pits. Some xerophytes shed their leaves during the driest seasons and others can store water such as cacti. CAM plants uptake CO₂ at night and store it in the vacuoles, as malic acid that can be broken down during the day for sugars.

These plants close their stomata (tiny pores used for gas exchange) during the day in order to conserve water. Normally, they wouldn't be able to carry out photosynthesis, since carbon dioxide from the air wouldn't be available. Therefore, their stomata are open during the night, and it is then that they take in carbon dioxide. They store it as malate and other, simple organic compounds. During the day, these plants follow the C3 pathway therefore photosynthesis occurs and the malic acid is easily broken down into pyruvate, which can be phosphorylated into PEP and then be recycled to fix more carbon.

In some ways, CAM resembles C₄ metabolism, except that CAM plants contain no bundle sheaths around their veins, and C₄ metabolism is continuous (as long as there is light), while CAM occurs only at night. CAM metabolism is present in many plant families. Examples of these plants are cacti and other succulents.

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