Crassulacean acid metabolism
From Wikipedia, the free encyclopedia
 |
Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. |
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.
[edit] 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, orchids and bromeliads. CAM plants take up 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 would not be able to carry out photosynthesis during daytime because stomata of the CAM plants is then closed (in order to reduce transpiration and hence minimize water loss), since carbon dioxide from the air would not be available. Therefore, their stomata open during the night, and it is then that they take in carbon dioxide. The three-carbon compound phosphoenolpyruvate is carboxylated into oxaloacetate which is then reduced into malate. Xerophytes store the four-carbon intermediates as malate and other simple organic compounds. Malate in particular is easily broken down into pyruvate and CO2, the former being phosphorylated into phosphoenolpyruvate (PEP) and then be recycled to fix more carbon. In the daytime, the malic acid is removed from the vacuoles and cleaved to produce CO2 so that it can be utilized by RuBisCO to fix the carbon and covert the compounds into sugars.
Crassulacean acid metabolism allows plants to close their stomata (tiny pores used for gas exchange) during the day in order to conserve water and carry out photosynthesis with CO2 that is taken up during the cool, humid night. CAM metabolism allows plants to grow in environments that would be far too dry for plant growth or otherwise subject to severe droughts. When an environment is too dry for even CAM plants to grow, they can stay alive by re-fixing the same carbon over and over without ever having to open their stomata.
In some ways, CAM resembles C4 metabolism, except that CAM plants contain no bundle sheaths around their veins. 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 cycle. Furthermore, C4 metabolism is continuous (as long as there is availability of light), while CAM occurs only at night. 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 very efficient with nitrogen. The drawback is that they are slow growing.
