CAM photosynthesis at night. Illustration of the night-time part of the mechanism involved in CAM (crassulacean acid metabolism) photosynthesis, used by some plants to obtain energy from sunlight. CAM plants open their gas-exchange pores (stomata) at night, as shown here. Carbon from carbon dioxide (CO2) is fixed by reactions involving the PEP (phosphoenolpyruvate) carboxylase enzyme, oxaloecetate, malate, and malic acid. The 4-carbon compound produced is stored in the mesophyll cell's vacuole (yellow) to be used in the day in the chloroplast (green). For the daytime reaction, see image C023/8823. For this artwork without labels, see C023/8822.