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, cactus). It is named after the plant family it was first discovered in, the crassulaceae.

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. Malate in particular 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, while CAM only occurs at night.

The carbon stored during CAM eventually enters the Calvin cycle. The Calvin cycle's main enzyme, rubisCO can either create PGAL by utilizing carbon, or create glycolate by utilizing oxygen (a process called photorespiration). Glycolate isn't as useful to plants as PGAL is, since they create food and structures out of PGAL, while glycolate just gets sent off to peroxisomes. When stomata are closed, waste oxygen builds up in the leaf, causing rubisco to leave off making PGAL in favor of glycolate. CAM and C₄ are both ways that plants help supplement their carbon levels to prevent this from happening.