Crassulacean acid metabolism or cam cycle

It occurs mostly in succulent plants which grow under semi-arid conditions. This mode of CO2 fixation takes place during night (dark) because the stomata of leaves of these plants remain open only during night. These plants absorb CO2 during night and convert it into malic acid which is then stored in vacuoles. During day time (light) decarboxylation of malic acid takes place and CO2 is released. This CO2 is utilized by C3-cycle. Since the cycle was first observed in the plants belonging to family Crassulaceae e.g. Bryophyllum, Sedum and Kalanchoe, etc. It was named as Crassulacean Acid Metabolism (CAM). Similar metabolism has been reported in the plants belonging to following families:

1. Dicot Families: Crassulaceae e.g. (sedum, Opuntia) Azoaceae, Asclepiadaceae,

Caryophyllaceae, Chenopodium, compositae, convolvulaceae, Euphoebiaceae, Vitaceae,

etc.

2. Monocot Families: Liliaceae, Orchidaceae.
3. Pteridophytes: Polypodiaceae.

Characteristic Features of CAM plants

1. The stomata remain closed during day (light) and open at night (dark).
2. CO2 fixation takes place in chlorophyll containing cells of leaves and stem during night (dark) and malic acid synthesis takes place.

3. Malic acid formed during dark (night) is stored in large vacuoles.
4. During day time decarboxylation of malic acid takes place and CO2 gas is released.

This CO2 is converted into sucrose and storage glucans (e.g. Starch) by C3-cycle.

Thus, CAM plants show diurnal cycle of organic acid formation i.e. they fix atmospheric CO2 during night by CAM and fix internally borne CO2 by C3-cycle during day time.

It is completed in following two parts:

1 Acidification and 2. Deacidification

1. Acidification: Acidification takes place during following steps:

(i) The stored carbohydrates are converted into phosphoenol pyruvic acid (PEP) through glycolysis. As stomata opens during night, the CO2 diffuses freely into the leaf through open stomata at night.

(ii) The CO2 combine with PEP in the presence of phosphoenol–carboxylase (PEP-C) enzyme to produce oxaloacetic acid (OAA).

(iii) The oxaloacetic acid (OAA) is not reduced into MALIC ACID in the presence of malic dehydrogenase enzymes.This reaction is facilitated in presence of reduced NADP+ ( =NADPH + H+) formed during glycolysis.This malic acid, thus produced in dark as a result of acidification is stored in the vacuoles. The oxaloacetic acid (OAA) may also be interconverted into aspartic acid

2.Deacidification: The decarboxylation of malic acid into pyruvicacid and CO2 in presence of light is called deacidification.

CO2 liberated is fixed by C3 cycle on coming next night this starch is converted into PEP, and is thus ready to accept atmospheric.