Asked by: Andrew Davies, Caerphily
We use oxygen as part of an extremely complicated series of biochemical reactions that convert glucose, amino acids or fatty acids into the energy storage molecule Adenosine Triphosphate or ATP. This is called aerobic respiration. Most of our cells can also respire anaerobically (without oxygen), but the reaction is much less efficient.
Every molecule of glucose will yield 30 molecules of ATP under aerobic respiration, but just two with anaerobic respiration. The larger and more complex an organism, the more energy it needs to maintain its processes. There are no entirely anaerobic vertebrates for this reason. Even if we could somehow supply enough energy, without oxygen we wouldn't have evolved lungs and probably not a voice either, so we wouldn't look much like we do now. And this is just atmospheric oxygen. Even anaerobic organisms still have oxygen in their bodies as part of virtually every chemical compound. Protein, fat, carbohydrate, bone and water all involve oxygen atoms.