Nitrogen comprises around 78% of Earth's atmosphere, so why do most lifeforms breathe oxygen?
Oxygen is a very reactive element, so why do so many lifeforms breathe it? We think of oxygen as life, sustenance, a literal breath of fresh air. But it's actually a very reactive element. Anyone who's burned a log has witnessed this firsthand. So why do so many life-forms breathe oxygen? There are probably thousands of kinds of metabolisms, or chemical processes that maintain life, said Donald Canfield , a geobiologist at the University of Southern Denmark, but "virtually all eukaryotes" (life-forms whose cells contain a nucleus) and a vast array of prokaryotes (life-forms that lack a nucleus), use oxygen. Canfield is talking primarily about heterotrophs — organisms, including humans, that get their nutrients and energy by consuming other organic matter. Not all organisms do this exclusively. For example, "plants get their carbon from CO2 in the air," said Dan Mills , a postdoctoral researcher at the University of Munich. Heterotrophs break down organic matter in food by stripping electrons off of it. These are passed from one enzyme to another in the membrane of the mitochondria, generating a small current that pumps protons across this barrier. And given its high electronegativity, oxygen usually serves as the final station on this electron transport chain , accepting the electrons and picking up two protons to form water. The process essentially creates a reservoir of protons that then flood through a protein channel in the membrane like a tiny hydroelectric dam. And, like a turbine, the protein synthesizes energy in the form of adenosine triphosphate (ATP) as it spins, explained Nick Lane , a professor of evolutionary biochemistry at University College London, in…