CBSE Class 10 Answered

Desert plants which grow in semi-arid conditions fix atmospheric CO₂ to form malic acid in the dark. These plants accumulate organic acid during the night to be used during the daytime. This pathway of carbon fixation is called Crassulacean acid metabolism (CAM).

The stem, leaves and petioles of CAM plants are fleshy or succulent. They have reduced leaves, thick cuticle and sunken stomata. Their stomata remain closed during the day and open at night. This helps the plants to conserve water, but at the same time, cuts off CO₂ supply which is required for photosynthesis.

• In desert plants, the stomata are open at night and usually closed during the day.
• They take in CO₂ at night and store it in the form of an intermediate product called malic acid.
• During daytime, in the presence of light, deacidification takes place in which malic acid is decarboxylated to pyruvic acid by the malic enzyme and CO₂ is evolved. One molecule of NADP⁺ is reduced in this reaction.
• Pyruvic acid so formed may be oxidised to CO₂ by the Krebs cycle or reconverted to phosphoenol pyruvic acid.
• CO₂ released by deacidification is accepted by ribulose bisphosphate and fixed in the form of carbohydrate by the C₃ or Calvin cycle.