Crassulacean Acid Metabolism: The Complete mechanism
Crassulacean Acid Metabolism
Ramson and Thomas in the year of 1940 coined the term Crassulacean Acid Metabolism Pathway (CAM Pathway). It was 1st discovered in Crassulaceae family members of Angiosperms, hence called as crassulacean acid metabolism. The first observation about CAM where made by de saussure in 1804 in his book Rechercges Chimiques Sur La Vegetation.
Despite the name CAM the pathway does not confined to only Crassulaceae family members there are other Angiospermic plant families found in desert which also shows Crassulacean Acid Metabolism Pathway.
CAM pathway occurs in xerophytic plants like Aloe vera, Bryophyllum, Opuntia Sedum, Kalanchoe etc.
CAM plants don’t show kranz anatomy like C4 plants but show slightly similarities with C4 mechanism.
The mechanism of CAM pathway helps plant to increase water use efficiency. Let’s see this by comparing CAM plants with C3 and C4 plants:
From above comparison it would clear that CAM plants (CAM pathway) are way better competitors than C3 and C4 plants when it comes to water use efficiency and hence in xerophytic conditions CAM plants have competitive advantage over C3 and C4 plants.
[The distinct difference between CAM and C4 pathway has given at the end of this article.]
This high water efficiency by CAM plants is result of opening their stomata during low temperature period in night time and closing them in high temperature period hot and dry day time.
Mechanism of crassulacean acid metabolism pathway:
The mechanism of CAM plants is very much similar to C4 cycle mechanism formation of C4 acid in C4 plant is takes place in mesophyll cell and it is spatially separated from the carboxylation reaction of C4 acid. While Re-fixation of CO2 released from C4 acid is done in bundle sheath cells by C3 cycle ( Celvin Cycle).
On the other hand CAM plants shows both temporal and spatial separation of C4 acid during dark (night) CO2 is captured by PEP carboxylase in cytosol by incorporating CO2 into PhosphoEnolPyruvate PEP (3C) and produce oxaloacetic acid OAA (4C). This OAA is further reduced into Malate (4C) and transfer to store into large vacuole. [Note: large vacuoles are typical character of leaf cells of CAM plants,but it us not compalsary.]
The accumulation of considerable amount of malic acid similar to the amount of CO2 assimilate at night it is recognised as acidification of leaf. This phenomenon is given by Bonner & Bonner in 1948 ans called as Nocturnal Acidification of leaf.
During light (day) the malic acid stored in vacuole transport to chloroplast. Hence leaf cells get deacidify as the reserves of vacuolar malic acid are consumed. Hence the process is known as deacidification of leaf cells.
Drincoovich et.al. in 2001 sound that maleic acid transferred in chloroplast undergoes decarboxylation and this is usually achieved by NADP – Malic enzyme and produce CO2 and pyruvate.
The released CO2 transferred to C3 Cycle ( Celvin Cycle) to to continue the process of photosynthesis which is the ultimate aim of CAM pathway. The remaining pyruvate is again converted back into sucrose and starch, which are the source of original carbon acceptor (PEP).
Special case :
Plants with crassulacean acid metabolism pathway shows long term regulation and are able to adjust their behaviour of CO2 uptake to environmental condition. The plats which alters their behaviour with environmental conditions are called as facultative CAM plants.
Mesembryanthemum crystallinum shows C3 mechanism under unstress , normal conditions. While in response to dry, heat, water and salt stress it shows CAM mechanism.
Differance between C4 and CAM Pathway:
Crassulacean acid metabolism pathway shows some significant differences which has taken place in plants during the course of evolution. Below is the image containing table with 6 distinct difference between CAM pathway and C4 Pathway.
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