CAM pathway in plants
In some plants of arid region there is another variant of carbon concentrating mechanism, which operate to reduce the oxygenation of RUBISCO. This mechanism was initially observed in Kalanchoe pinnata (Bryophyllum calycinum) of the family Crassulaceae. Hence, the mechanism is recognized as 'crassulacean acid metabolism' (CAM). The mechanism is reported to occur in succulent plants such as cacti, agave, pineapple, orchids, etc., to overcome the seasonal water availability.
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| Opuntia; a CAM plant |
Diurnal pattern of stomatal movement in CAM plants efficiently fix atmospheric CO2 while restricting water loss.
In order to check the water loss, CAM plants of arid zones have, a) very thick waxy cuticle, b) reduced stomatal pores, and c) large vacuoles.
Uptake of CO2 from the atmosphere takes place only during the night hours, when transpiration is reduced because of the low temperature. The CO2 is first fixed by the PEPCase in a manner similar to the initial carboxylation in C4 cycle forming oxaloacetate and then malate. The CO2 fixed in the form of malate during the night is stored in the vacuole; a phenomenon known as "dark acidification".
During the day hours stomatal aperture remain closed to the check the transpiration. Malate stored in the vacuole exits in the cytosol and CO2 is released by NAD (or NADP) malic enzyme. This process is known as "light deacidification". 3-carbon acid leftover is converted into triose phosphate and then to the sucrose or starch. CO2 releases is then fixed via Calvin-Benson cycle.
In CAM pathway the activities of PEPCase and RUBISCO are separated 'temporally', whereas in C4 cycle these are separated'spatially'.
Uptake of CO2 from the atmosphere takes place only during the night hours, when transpiration is reduced because of the low temperature. The CO2 is first fixed by the PEPCase in a manner similar to the initial carboxylation in C4 cycle forming oxaloacetate and then malate. The CO2 fixed in the form of malate during the night is stored in the vacuole; a phenomenon known as "dark acidification".
During the day hours stomatal aperture remain closed to the check the transpiration. Malate stored in the vacuole exits in the cytosol and CO2 is released by NAD (or NADP) malic enzyme. This process is known as "light deacidification". 3-carbon acid leftover is converted into triose phosphate and then to the sucrose or starch. CO2 releases is then fixed via Calvin-Benson cycle.
In CAM pathway the activities of PEPCase and RUBISCO are separated 'temporally', whereas in C4 cycle these are separated'spatially'.
'Constitutive' and 'facultative' CAM plants.
Some plants, e.g., Kalanchoe always follow the diurnal pattern of CO2 fixation are recognized as 'obligate' or 'constitutive' CAM plants. There are some plants, e.g., ice plant (Mesembryanthemum crystallinum L.) and agave CAM pathway is adapted only on the water scarcity or salt stress. In normal conditions, these plants fix CO2 only via Calvin-Benson cycle. These are called as 'facultative' CAM plants.
Still there is another plant Clusia, which exhibits C3 pathway and CAM pathways in different leaves simultaneously.
Still there is another plant Clusia, which exhibits C3 pathway and CAM pathways in different leaves simultaneously.
First published on 13-04-2021
Last updated on 19-04-2021
Last updated on 19-04-2021
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