Explain why plants growing in deserts take CO₂ at night and utilize it during the day in the synthesis of carbohydrates.

In deserts, the climate is very hot and the amount of rainfall is very low. If desert plants take CO₂ during day, lots of water will be lost. To prevent the water loss some of the desert plants take CO₂ during night.

These plants undergo a different type of photosynthesis, known as crassulacean acid metabolism, or CAM. The cells of these plants contain enzymes that are able to metabolize carbon dioxide at night, so the plant can close the stomata during daylight hours and open them when temperatures are cooler and evaporation is reduced. Carbon dioxide is stored in organic acids at night and released within the plant during the day so the normal photosynthetic pathways can synthesize sugars using sunlight.

Benefits

The most important benefit of CAM to the plant is the ability to leave most leaf stomata closed during the day. Plants employing CAM are most common in arid environments, where water comes at a premium. Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evapotranspiration, allowing such plants to grow in environments that would otherwise be far too dry. Plants using only C3 carbon fixation, for example, lose 97% of the water they uptake through the roots to transpiration - a high cost avoided by plants able to employ CAM.