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Concept
Crassulacean acid metabolism
Summary
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions that allows a plant to photosynthesize during the day, but only exchange gases at night. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but they open at night to collect carbon dioxide () and allow it to diffuse into the mesophyll cells. The is stored as four-carbon malic acid in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to , which is then used during photosynthesis. The pre-collected is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. This mechanism of acid metabolism was first discovered in plants of the family Crassulaceae.
Observations relating to CAM were first made by de Saussure in 1804 in his Recherches Chimiques sur la Végétation. Benjamin Heyne in 1812 noted that Bryophyllum leaves in India were acidic in the morning and tasteless by afternoon. These observations were studied further and refined by Aubert, E. in 1892 in his Recherches physiologiques sur les plantes grasses and expounded upon by Richards, H. M. 1915 in Acidity and Gas Interchange in Cacti, Carnegie Institution. The term CAM may have been coined by Ranson and Thomas in 1940, but they were not the first to discover this cycle. It was observed by the botanists Ranson and Thomas, in the succulent family Crassulaceae (which includes jade plants and Sedum). The name "Crassulacean acid metabolism" refers to acid metabolism in Crassulaceae, and not the metabolism of "crassulacean acid"; there is no chemical by that name.
CAM is an adaptation for increased efficiency in the use of water, and so is typically found in plants growing in arid conditions. (CAM is found in over 99% of the known 1700 species of Cactaceae and in nearly all of the cacti producing edible fruits.
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