Crassulacean acid metabolism

  Crassulacean acid metabolism, also known as CAM photosynthesis, is an elaborate carbon fixation pathway in some photosynthetic plants. CAM is usually found in plants living in arid conditions, including cacti and pineapples. It is named after the plant family in which it was first discovered, the Crassulaceae, which includes jade plants and sedums. Its metabolic pathways were described in the late 1950's by Ransom and Thomas at Newcastle.^[1]

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Synopsis

Plants adapted to thrive in dry climates are called xerophytes. Some have small, thick leaves with a low surface-area-to-volume ratio. Some also have a thick cuticle. Their stomata may be sunken into pits. Some shed their leaves during the dry season; others, such as cacti, orchids, and bromeliads store water in vacuoles. Some xerophytes perform photosynthesis using Crassulacean acid metabolism (or CAM).

CAM plants close their stomata during the day in order to conserve water by preventing evapotranspiration. Their stomata then open during the cooler and more humid nighttime hours, allowing uptake of carbon dioxide for use in carbon fixation. This is begun when the three-carbon compound phosphoenolpyruvate is carboxylated into oxaloacetate which is then reduced to form malate. CAM plants store these four-carbon intermediates and other simple organic compounds in their vacuoles. Malate is easily broken down into pyruvate and CO₂, after which pyruvate is phosphorylated to regenerate phosphoenolpyruvate (PEP). In the daytime, the malic acid is removed from the vacuoles and cleaved to produce CO₂ so that it can be utilized by the enzyme RuBisCO in the Calvin-Benson cycle in the chloroplast stroma. By thus reducing evapotranspiration rates during gas exchange, CAM allows plants to grow in environments that would otherwise be far too dry for plant growth or, at best, subject them to severe drought stress.   In some ways, CAM resembles C4 photosynthesis, except that CAM plants contain no bundle sheath cells. C₄ plants capture the CO₂ in one type of cell tissue (mesophyll) and then transfer it to another type of tissue (bundle sheath cells) so that carbon fixation may occur via the Calvin-Benson cycle. Furthermore, C₄ metabolism is continuous as long as there is light available, while CAM occurs only at night. Thus, C₄ metabolism physically separates CO₂ fixation from the Calvin cycle, while CAM metabolism temporally separates CO₂ fixation from the Calvin cycle.

CAM plants are very good at retaining water, and are also very efficient with the usage of nitrogen. They are ineffiecient at absorbing CO₂, however, so they are slow growing compared to other plants. In addition, CAM plants avoid photorespiration. The enzyme responsible for fixing carbon into the Calvin cycle, Rubisco, cannot distinguish CO₂ from oxygen. As a result, the plant uses energy to break down carbon compounds. This costly process occurs when the internal concentration of oxygen in the leaves is too high, particularly in C₃ plants.

References

1. ^ Ransom S. L.; Thomas M (1960). "Crassulacean acid metabolism". Annual Rev Plant Physiol 11: 81-110. doi:10.1146/annurev.pp.11.060160.000501.

List of plants known to exibit full or partial CAM metabolism