Metabolism can be either aerobic (requiring oxygen) or anaerobic (occurring in the absence of oxygen). Anaerobic metabolism is the older process: Earth's atmosphere has contained molecular oxygen for less than half the planet's existence. For organisms like yeast that can operate in either mode, aerobic metabolism is generally the more efficient process, yielding tenfold more energy from the metabolism of a molecule of glucose than do anaerobic processes. But the efficiency that comes from the use of molecular oxygen as an electron acceptor carries a price. Molecular oxygen is easily transformed into toxic compounds. For example, hydrogen peroxide, H 2O 2, is used as a disinfectant, as is ozone, O 3. Furthermore, molecular oxygen can also oxidize metal ions, and that can cause problems. Iron‐containing enzymes and proteins use reduced iron, Fe(II) or Fe(I), and don't function if the iron atoms are oxidized to the stable Fe(III) form. Organisms must have means of preventing the oxidation of their iron atoms.
The third problem caused by the use of molecular oxygen as an electron acceptor is the fact that it really isn't very soluble in water. (If it were more soluble, people couldn't drown!) Multicellular organisms have evolved various oxygen transporters to solve the twofold problem of keeping oxygen tied up and less toxic as well as being able to deliver O 2 rapidly enough and in sufficient quantity to support metabolism. All animals (other than insects) with more than one kind of cell have evolved specialized proteins to carry oxygen to their tissues. The protein responsible for carrying oxygen in the blood of most terrestrial animals is hemoglobin. Within the tissues, especially muscle tissue, a related oxygen carrier, myoglobin, keeps molecular oxygen available for its final reduction to water as the end product of catabolism (nutrient utilization).