Crassulacean acid metabolism occurs in succulent
plants and gets completed in two phases :
Phase I : (a) Stomata are open at night. CO2 is absorbed from outside. With the help of PEP carboxylase, it is immediately fixed.
(b) The acceptor is Phosphoenol pyruvate or PEP.
(c) Malic acid is end product of dark fixation of CO2. It is stored inside cell vacuoles.
Phase II: (a) During day time, stomata are closed. However, light is available for photosynthesis.
(b) Malic acid moves out of the cell vacuoles.
(c) It is decarboxylated with the help of malic enzyme.
(d) Pyruvate is produced. It is metabolised.
Malic acid (malate) + NADP+ malic/Enzyme-------> Pyruvate + CO2 + NADPH
(e) The released CO2 is fixed again through Calvin cycle with the help of RuBP a'nd Rubisco.
(f) Crassulacean plants have developed a unique
mechanism to perform photosynthesis without much lose of water.