Question

CAM-plants avoid photorespiration by

• A. performing Calvin cycle at night
• B. fixing CO₂ into organic acid during night and releasing CO₂ during day
• C. using PEP carboxylate to fix CO₂ to RuBP
• D. keeping the stomata closed during the day.

Answer 1

Answer: (B)

In members of Crassulaceae, Cactaceae, Agavaceae, Orchidaceae and Portulacaeae; CO₂ fixation occurs during night only. In succulents belonging to the above families, the stomata remain closed during day time, in order to reduce transpiration and open during night (scotoactive opening). Absorption of CO₂ during night and its storage as organic acid (malic .acid) is called acidification. Liberation of CO₂ from an organic acid during day time is called deacidification. The acidification and deacidification during the night and day time respectively is called CAM (crassulacean acid metabolism). In CAM plants, initial carboxylation and final carboxylation are separated by time to avoid photorespiration.