Introduction Crassulacean acid metabolism occurs in certain succulent plants of the family Crassulaceae. As they exhibit a special type of carbon dioxide assimilation, they are called CAM plants. In succulent plants growing in semiarid condition, carbon dioxide assimilation takes place at night.
Stomata of these plants are opened
in the night and closed in the day time. At night, the carbon dioxide absorbed is converted into malic acid inside vacuoles. This is decarboxylated in day time and the carbon dioxide released enters in C3 cycle in chloroplast.
Characteristic features of CAM plants The following characteristics are found in CAM plants:
Mechanism of CAM plants CAM cycle takes place in the following three steps. Firstly, at night, carbon dioxide molecules enter inside plant body through stomata. Inside green cells, carbon dioxide reacts with phosphoenol pyruvic acid (PEP) in the presence of enzymePEP carboxylase and produce oxaloacetic acid (OAA). In the second step, oxaloacetic acid (OAA) is reduced into malic acid in the presence of the enzymemalate dehydrogenase. One molecule of NADPH2 is consumed in this process. Malic acid molecules produced in this way are stored in cell vacuoles. In the day time, the malic acid slowly diffuses to the cytoplasm and decarboxylated to pyruvic acid. This reaction takes place in the presence of NADP and the enzyme acid decarboxylase. The carbon dioxide produced in the above reaction is used by the Calvin cycle where pyruvic acid is converted into PGA. It is ultimately converted to hexose sugar and stored in cells in the form of starch. At night, starch is again converted into PEP through the process of glycolysis.
Biological significances of CAM cycle The biological significances of the CAM cycle are as below:
Bacterial photosynthesis Certain bacteria are also capable of doing photosynthesis like green plants. These bacteria use infrared (invisible) light of sunlight as the source of energy for photosynthesis. The only difference in the bacterial photosynthesis is that substances other than water act as hydrogen donors and consequently liberation of oxygen does not take place in the process. These bacteria also have both pigment system, functioning independently. As they can photosynthesize in the presence of light, they are called photoautotrophic bacteria.
Chemosynthesis There are many microorganisms in nature which do not perform photosynthesis but they are autotrophs. These microbes use the energy released during oxidation of different inorganic compounds for carbon assimilation. Therefore, these are known as chemosynthetic bacteria and the process is known as chemosynthesis. The mechanism of carbon dioxide reduction, in these microbes, is similar to that of other plants.
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