Plants cannot afford to open stomata during the day to take in CO2, as they would lose too much water by transpiration. Instead, stomata open at night, when water evaporation is minimal, and take in CO2 by fixing with PEP to form oxaloacetate though PEP carboxylase. Oxaloacetate is converted to malate by malate dehydrogenase, and stored for use during the day when the light dependent reaction generates energy (mainly in the form of ATP) and reducing equivalents such as NADPH to run the Calvin cycle.[1][2][3]