PinkMonkey Online Study Guide-Biology
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3. CAM plants (Crassulacean acid metabolism)
The CAM plants are mostly succulent xerophytes such as
plants of families Crassulaceae, Euphorbiaceae, Cactaceae, etc. The stomata
in these plants remain closed during the day. This helps to check transpiration
and open them during the cool night for gaseous exchange.
These plants are characterized by the presence of Crassulacean acid metabolism (CAM) involving night photosynthetic activity. This is a desert adaptation where stomata must remain closed during the day.
The CAM plants have the dark reaction very much similar
to the C4 plants ( i.e., they have both C4 and C3
pathways.) However, they differ from C4 plants because, in
CAM plants, both pathways occur in mesophyll cells only. Moreover, C4
pathway occurs during the night when stomata are open (night photosynthetic
activity) and C3 pathway is completed during day time when
the stomata are closed.
At night, atmospheric carbon dioxide is taken in through
the open stomata. As in the C4 pathway, carbon dioxide combines
with 3-C PEP to form 4-C OAA. This is then converted to malic acid and
stored in the cells. Thus, organic acids accumulate in the dark.
During the day, when stomata are closed and atmospheric
carbon dioxide is not available, the 4-C malic acid undergoes decarboxylation
to form carbon dioxide and 3-C pyruvic acid. Carbon dioxide thus released
enters the Calvin cycle (C3 cycle) to form sugars.