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Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions that allows a plant to photosynthesize during the day, but only exchange gases at night. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but they open at night to collect carbon dioxide (CO) and allow it to diffuse into the mesophyll cells. The CO is stored as four-carbon malic acid in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to CO, which is then used during photosynthesis. The pre-collected CO is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. This mechanism of acid metabolism was first discovered in plants of the family Crassulaceae.