This switch from aerobic to anaerobic metabolism, Pasteur's effect, maintains some, albeit markedly reduced, adenosine triphosphate (ATP) production. In severe hypoxia, when ATP production is inadequate to meet the energy requirements of ionic and osmotic equilibrium, cell membrane depolarization leads to uncontrolled Ca2+ influx and activation of Ca2+-dependent phospholipases and proteases. These events, in turn, cause cell swelling and ultimately cell necrosis.