The Pasteur effect describes the inhibition of fermentation (glycolysis) by the presence of oxygen, first observed by Louis Pasteur in yeast. When oxygen becomes available, aerobic respiration (more efficient, producing ~30 ATP per glucose) suppresses fermentation (producing only 2 ATP per glucose). This occurs because aerobic metabolism produces more ATP per glucose, reducing the need for high glycolytic flux. Industrially, controlling aeration in yeast fermentation exploits the Pasteur effect.