Otto Heinrich Warburg (1883-1970) was awarded the 1931 Nobel Prize for Physiology or Medicine for his research on cellular respiration. He conducted detailed studies on the assimilation of carbon dioxide by plants, tumor metabolism, and the chemical constituent of the oxygen-transferring respiratory ferment. By looking for the cellular constituents involved in oxygen consumption, he identified the function of cytochromes, a family of enzymes in which the iron-containing heme group binds to molecular oxygen. His later studies at the Kaiser Wilhelm Institute led to the discovery that flavins and nicotinamide were the active groups of hydrogen-transferring enzymes. These studies, along with his earlier discovery of iron-oxygenase, gave a complete account of the oxidations and reductions in the living world.