Question

CAM plants do not show photorespiration as they
A. Keep your stomata closed during the day.
B. Use PEP carboxylase.
C. Fix $C{O_2}$ into organic acid during the night and release $C{O_2}$ during the day.
D. Perform Calvin cycle at night.

Answer 1

CAM plants are plants that fix carbon dioxide through CAM pathways. CAM means Crassulacean Acid Metabolism. This pathway was first discovered in the plants of the Crassulaceae family. It is utilised during dry and arid environments. The CAM pathway is adapted by plants to reduce water loss and photorespiration. Some of the CAM plants include hydrilla, cactus, pineapple etc.

Complete explanation:
Option A Keep stomata closed during day time: The CAM plants have the ability to keep most of the leaf stomata closed during the day. By keeping the stomata closed during the hottest and driest part of the day, the plants are able to prevent loss of water through transpiration. However, this is not linked with photorespiration.
Option A is incorrect.
Option B Use PEP carboxylase: The CAM plants avoid photorespiration by separating the light-dependent reactions and the use of $C{O_2}$ in the Calvin cycle. They fix the $C{O_2}$ during the night with the help of PEP carboxylase which is used during the next day. Hence photorespiration does not take place.
Option B is correct.
Option C Fix $C{O_2}$ into organic acid during the night and release $C{O_2}$ during the day: The CAM plants does fix $C{O_2}$ in the form of organic acid during night but not without the help of PEP carboxylase.
Option C is incorrect.
Option D Perform Calvin cycle at night: This option is incorrect because Calvin cycle takes place during day. Only fixation of $C{O_2}$ into organic acids takes place during night.
Option D is incorrect.

Therefore, Option B) Using PEP carboxylase is the correct answer.

Note:
During night the CAM plants open their stomata and allow $C{O_2}$ to diffuse into the leaves. The diffused $C{O_2}$ is fixed into oxaloacetate with the help of PEP carboxylase. This is again converted into malate or another type of organic acid. The organic acid is stored in the vacuoles until the next day. During daylight, the CAM plants do not open their stomata, but can still perform photosynthesis. This is because the organic acids stored in the vacuole are broken down to release $C{O_2}$ which enters the Calvin cycle. This controlled release maintains a high concentration of carbon dioxide ($C{O_2}$) around rubisco.