No, not all cells require oxygen; in fact, some cells use a metabolic process called fermentation to produce energy, not oxygen. For example, nonoxygen-dependent (anaerobic) organisms include yeast and bacteria that are able to live in an environment with low levels of oxygen.

Overall, the majority of organisms are oxygen dependent (aerobic) because of the high ATP—the main source of energy for cells—yield that oxygen provides. Under some circumstances, oxygen-dependent cells can gather energy from fermentation for short periods of time—but this shortcut eventually results in a buildup of lactic acid, a toxic waste product, and produces little ATP. (For more about fermentation, see the chapter “Basics of Biology.”)