Most plants have their pores (stomata’s) open during the day to take in carbon dioxide, and use sunlight as a catalyst for the photosynthesis. But in the desert, plants with pores open during the hot days, lose much water through evapotranspiration. So, succulents use a modified version of photosynthesis called CAM (Crassulacean Acid Metabolism).
CAM plants open their stomata’s only at night when it is cooler so there is less evapotranspiration. Because there is no sunlight to act as a catalyst, carbon dioxide is stored as an organic acid, principally Malic Acid (C4H6O5). Carbon dioxide is
gradually released from the acid during the next day. CAM plants use about one-tenth the water to produce each unit of carbohydrate compared to standard photosynthesis. The price: a much slower growth rate.
Many plants contain malic acid, but usually in lesser quantities than found in cacti. Besides malic acid, succulents produce oxalic acid (C2H2O4), which is toxic, as another product of photosynthesis. “Its chief function seems to be sequestering metals, principally calcium. Calcium oxalates often occur as crystalline minerals within the cactus pulp. Their function seems to be aiding structural integrity and enzymatic processes. In fact two crystalline calcium oxalate minerals have been identified in all cacti tested: CaC2O4.2H2O (weddellite) and CaC2O4.H2O (whewellite).” Oxalates are also formed with heavy metals such as copper, perhaps to reduce toxicity to the plant.
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