Some plants that grow in very hot environments have evolved strategies to overcome water loss and fix carbon efficiently. Two examples are corn plants that use the C4 pathway and cacti that use the Crassulacean acid metabolism or CAM pathway. In C4 plants, carbon fixation occurs in the mesophyll cells. The enzyme PEP carboxylase combines carbon dioxide with phosphoenolpyruvate to form oxaloacetate. Oxaloacetate is then enzymatically converted to malate or another organic acid. Next, these organic acids are transported into the bundle sheath cells to be broken down into CO2 and pyruvate. The CO2 enters the Calvin cycle and pyruvate replenishes the PEP. Unlike C4 plants where CO2 fixation and the Calvin cycle are separated in space, CAM plants separate these two steps in time. CAM plants only open their stomata for CO2 intake at night to prevent water loss. The carbon is fixed into a molecule of malate and stored in the vacuoles of mesophyll cells until daylight. The CO2 is released during the day time and the Calvin cycle proceeds along with the light reactions.