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 a public presentation. The cell can then use this packaged energy or send it off into the body to do things.
Life can use many other electron acceptors — like sulfate, nitrate and iron — but oxygen is the highest-energy acceptor available.
"The reduction of oxygen provides the largest free energy release per electron transfer, except for the reduction of fluorine and chlorine," University of Washington professor David Catling and his co-authors explained in a paper published in the journal Astrobiology.
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Chlorine and oxygen can generate similar amounts of energy. Fluorine could certainly provide more energy than oxygen, but "fluorine is [...] useless as a biological oxidant because it generates an explosion upon contact with organic matter," they wrote in the study. That's not a gas you'd want to breathe.
Chlorine and fluorine are also poisonous, which highlights another benefit of oxygen. Aerobic respiration doesn't produce any toxic compounds, just water and carbon dioxide. However, oxygen's reactivity can be an issue if it builds up in tissues, where it can damage cellular components like DNA and proteins. That's why antioxidants, in moderation, are good for our health.
Oxygen is also far more abundant than fluorine, chlorine or the myriad electron acceptors used in other forms of respiration. Despite its proclivity for forming compounds with other atoms, a copious amount of oxygen is constantly produced via photosynthesis. This enables it to accumulate in the atmosphere and dissolve in water, where it is readily available to life. And, as a gas, it's easy to transport across membranes, Canfield and Mills explained.
Speaking of abundance, why not use nitrogen, which comprises 78% of Earth's atmosphere?
"The main problem with nitrogen is that it's triple bonded," Canfield said. "And it's very, very difficult to break."
Nitrogen is an important component of many biologic compounds, and there are whole groups of organisms that specialize in the energy-intensive processes required to break nitrogen's strong bonds to make it bioavailable, Canfield said.
Oxygen's unique utility comes down to quantum physics. Oxygen in its normal ground state can only accept electrons in the same spin state, not as an electron pair, which is the usual currency of chemistry.
"So the real trick to oxygen is that it can accumulate to high levels without reacting, but releases a lot of energy (to pump protons) when it is fed electrons one at a time," Lane told Live Science in an email.
So it seems oxygen sits in a sweet spot of reactivity and availability. It's milder than halogens such as chlorine and fluorine, and it isn't bound too strongly, like nitrogen. But it's much more reactive than other electron acceptors, like sulfate and nitrate.
Oxygen is easy to acquire, and it doesn't generate toxic compounds that require further processing. What's more, plants produce copious amounts of this reactive gas through photosynthesis, enabling us to use it to fuel our own bodies.
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