Question

In the mesophyll cells of the CAM plants, $CO_2$ fixation during the day occur through
A) RuBP oxygenase
B) PEP carboxylase
C) RuBP carboxylase
D) Both RuBP carboxylase and PEP carboxylase

Answer 1

Hint: CAM stands for Crassulacean acid metabolism. The carbon dioxide fixation in these plants is adapted for arid conditions. The stomata remains closed during the day and open at night.

Complete Answer:
- Plants like pineapple belonging to the family Crassulaceae are adapted for dry conditions. They have a special mechanism to prevent the loss of water by evaporation.
- In these plants the stomata remain shut during the day time and open at night. Such plants are called CAM plants or Crassulacean Acid metabolism plants. At night the carbon dioxide is absorbed and diffuses to the mesophyll cells.
- The PEP carboxylase helps in binding of $CO_2$ with PEP to form oxaloacetate in the mesophyll tissues. The carbon dioxide is stored as malic acid in the vacuole.
- During the day time, the malate is transported to the chloroplast and is converted to carbon dioxide by the enzyme RuBP carboxylase. This $CO_2$ enters the Calvin cycle and photosynthesis is carried out.

Thus the correct answer is (C). RuBP carboxylase helps in fixing $CO_2$ during the day.

Note: The closure of stomata during the day helps the plant to save water due to transpiration. The absorbed carbon dioxide at night is stored in the mesophyll cell. During the day time this stored Carbon dioxide is released by the enzymatic activity (RuBPCo). This $CO_2$ enters the Celvin cycle and normal photosynthesis takes place.