Crassulacean Acid Metabolism, or CAM photosynthesis, is a specialized form of photosynthesis. This pathway allows plants to adapt to challenging environmental conditions, particularly those characterized by limited water availability. Unlike other photosynthetic methods, CAM photosynthesis involves a unique temporal separation of gas exchange, enabling plants to thrive where water is scarce.
How CAM Photosynthesis Works
CAM photosynthesis separates the two main stages of carbon dioxide uptake and fixation into different times of the day. During cooler night hours, plant stomata open to absorb carbon dioxide from the atmosphere. This absorbed carbon dioxide is fixed by the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase) into a four-carbon compound, oxaloacetate.
Oxaloacetate is converted into malic acid, which accumulates and is stored in large vacuoles within the plant’s mesophyll cells. This storage allows the plant to collect carbon dioxide when water loss through transpiration is minimal. As dawn approaches and temperatures rise, the stomata close, effectively sealing in the stored carbon.
During daylight hours, when sunlight is available for photosynthesis, the stored malic acid is transported out of the vacuoles. It undergoes decarboxylation, releasing the stored carbon dioxide. This released carbon dioxide is fed into the Calvin cycle, where it is converted into sugars using energy from the light reactions. This temporal separation ensures efficient carbon fixation while minimizing water loss.
Why CAM Plants Thrive in Dry Climates
The main advantage of CAM photosynthesis is its water-conservation strategy. By opening their stomata exclusively at night, CAM plants reduce water loss through transpiration. During the day, when temperatures are higher and the air is drier, stomata remain closed, preventing excessive water evaporation. This is a stark contrast to C3 plants, which can lose up to 97% of absorbed water through transpiration.
This nocturnal gas exchange allows CAM plants to maintain a favorable internal water balance, even in arid and semi-arid environments where water is a limiting resource. The ability to collect and store carbon dioxide overnight, when humidity is higher and temperatures are lower, means the plant can photosynthesize during the day without opening its stomata and risk dehydration. This mechanism enables CAM plants to survive and flourish in conditions that would be inhospitable to many other plant types. CAM plants often exhibit additional adaptations, such as thick cuticles and succulent tissues, which further aid in water retention.
Common CAM Plants and Their Habitats
CAM photosynthesis is found in many plant species. Many familiar plants employ this specialized pathway, including most succulents like cacti, agaves, and jade plants. Other examples include pineapples and many species of orchids. This pathway is also observed in some species of Euphorbia and Bromelioideae.
These plants are commonly found in environments where water availability is a major challenge. Deserts and semi-deserts are typical habitats for cacti and agaves, where their CAM metabolism allows them to endure extreme heat and drought. Orchids, found as epiphytes in tropical regions, also utilize CAM because they lack direct access to soil water, making their aerial environments functionally dry. The distribution of CAM plants across various families and habitats underscores the effectiveness of this metabolic pathway in supporting plant life in dry conditions.