Question

How is the photosynthetic efficiency higher in CAM plants?

Answer 1

CAM photosynthesis or Crassulacean Acid Metabolism is a carbon fixation pathway which is evolved in some plants as an adaptation to arid conditions. It allows a plant to photosynthesize during the day and only exchange gases during night. The CAM is an adaptation for increased efficiency in the use of water and hence, they are typically found in plants that are growing in arid conditions.

Complete answer:
In plants that use full CAM, the stomata in the leaves remain shut during day to reduce evapotranspiration, but they open during night to collect carbon dioxide, which allows it to diffuse into the mesophyll cells. The carbon dioxide is stored as the four-carbon malic acid in vacuoles at night, and during daytime, the malate is transported to chloroplasts where it is then converted back into carbon dioxide. This is then used during the photosynthesis process. The pre-collected carbon dioxide is concentrated around the RuBisCO enzyme which increases the photosynthetic efficiency.
The CAM photosynthetic plants have comparatively higher transpiration efficiencies than either $C3$ or $C4$ plants due to their stomata being open at night when vapor pressure differences between the leaf and the surrounding air are lowest, which reduces transpiration. The distribution patterns for CAM plants are reflected by this and are dominated by habitat aridity. Certain plants like the water storing cacti are adapted to enduring long periods with no precipitation. This is because these plants close their stomata even at night during prolonged periods of drought. They can refix carbon dioxide lost in the process of respiration before it diffuses out of the leaf or stem. Hence, some cactus species lose only very little biomass over many months without rain.

Note:
The plants using CAM often display other xerophytic characters such as thickness, reduced leaves with low surface area to volume ratio; thick cuticle; and stomata that are sunken into pits, since CAM is an adaptation to arid conditions. Some shed their leaves during the dry season and others store water in vacuoles. The CAM plants may have an increasingly sour taste during the night but can become sweet tasting during the day.