CAM photosynthesis GO:0009761 biological_process The combination of atmospheric CO2 with a 3-carbon molecule phosphoenol pyruvate (PEP) to make malic acid. The malic acid is then passed into the vacuole where it is stored until daylight, when it is shuttled back out to be used as a substrate in the light reaction of photosynthesis. Wikipedia:Crassulacean_acid_metabolism carbon fixation photosynthesis, dark reaction