Crassulacean acid metabolism, also known as CAM photosynthesis, is an elaborate carbon fixation pathway in some plants. These plants fix carbon dioxide (CO₂) during the night, storing it as the four carbon acid malate. The CO₂ is released during the day, where it is concentrated around the enzyme RuBisCO, increasing the efficiency of photosynthesis. The CAM pathway allows stomata to remain shut during the day; tThe benefits of CAM

A great deal of energy is expended during CAM by the production and subsequent destruction of malate. This is in part countered by the increased efficiency of RuBisCO, but the more important benefit to the plant is the ability to leave leaf stomata closed during the day.^{[verification needed]} CAM plants are most common in arid environments, where water comes at a premium. Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evapotranspiration, allowing CAM plants to grow in environments that would otherwise be far too dry. C₃ plants, for example, lose 97% of the water they uptake through the roots to transpiration- a high cost avoided by CAM plants.

herefore it is especially common in plants adapted to arid conditions

 Historical background

CAM was first discovered in the late 1940s. It was observed by the botanists Ranson and Thomas, in the Crassulaceae family of succulents (which includes jade plants and sedums).^[1] Its name refers to acid metabolism in Crassulaceae, not the metabolism of Crassulacean acid.

 Overview of CAM: a two-part cycle

CAM is a mechanism whereby CO₂ is concentrated around RuBisCO by day, while the enzyme is operating at peak capacity. This concentration of CO₂ increases RuBisCO's efficiency, as it is prone to operate in the "reverse" direction via photorespiration - utilising oxygen to break down the reaction products the plant would rather it was producing. It differs from C₄ metabolism, which spatially concentrates CO₂ around RuBisCO.

 During the night

CAM plants open their stomata during the cooler and more humid night-time hours, permitting the uptake of carbon dioxide with the minimum water loss.

The carbon dioxide is converted to soluble molecules, which can be readily stored by the plant at a sensible concentration.

The precise chemical pathway involves a three-carbon compound phosphoenolpyruvate (PEP), to which a CO₂ molecule is added via carboxylation - forming a new molecule, oxaloacetate. This is then reduced, forming malate. Oxaloacetate and malate are built around a skeleton of four carbons - hence the term C₄. Malate can be readily stored by the plant in vacuoles within individual cells.

 The next day...

Malate can be broken down on demand, releasing a molecule of CO₂ as it is converted to pyruvate. The pyruvate can be phosphorylated (i.e. have a phosphate group added by the "energy carrier" ATP) to regenerate the PEP with which we started, ready to be spurred into action the next night. But it is the release of CO₂ that makes the cycle worth the plant's while. It is directed to the stroma of chloroplasts: the sites at which photosynthesis is most active. There, it is provided to RuBisCO in great concentrations, increasing the efficiency of the molecule, and therefore producing more sugars per unit photosynthesis

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