Can humans make a gas better than oxygen to use as a substitute for life? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Can humans make a gas better than oxygen to use as a substitute for life?

Bacteria beat us to this, long before there were humans. In fact, they did it long before oxygen was abundant on the planet.

Oxygen’s key role is to function as a terminal electron acceptor. In other words, it is like the positive terminal on a battery. Food (like carbohydrates) function as the negative terminal. Electrons flow from food to oxygen, and cells capture some of this energy along the way, similar to how a light bulb captures the energy in an electrical circuit.

A variety of other compounds can function as electron acceptors in bacteria and archaea: nitrate, sulfate, even carbon dioxide [1]. But are they “better” than oxygen?

From the standpoint of energy gained, that is the difference in redox potential between the electron donor (food), and the electron acceptor, no. They are weaker electron acceptors than oxygen.

Both chlorine and fluorine gas are stronger oxidizers than oxygen [2]. In principle, we could get more energy from our food by using them as terminal electron acceptors.

In practice, of course, both gasses are deadly poisons and chlorine gas in particular is a notorious chemical warfare agent. These strongly oxidizing gasses do not react in a controlled manner with our food as oxygen does, but with all of our tissues. They essentially burn us up at room temperature.

As did oxygen gas 2.5 billion years ago. Cyanobacteria produced enough oxygen through photosynthesis that it reached toxic levels. All organisms at that point had evolved in a low-oxygen atmosphere and were not capable of controlling and detoxifying molecular oxygen. The Great Oxygenation Event gave rise to organisms - our direct ancestors - that could control and then use oxygen.

Oxygen levels are still tightly controlled in your cells and tissues in order to prevent oxidative damage. One of the most abundant proteins in your blood, catalase, serves to reverse the oxidation of water to hydrogen peroxide that would otherwise quickly kill you.

So yes, we can make (and have made) better electron acceptors than oxygen gas. But we are not adapted to use them and they are poisons.

Footnotes

[1] Electron Donors and Acceptors in Anaerobic Respiration - Boundless Open Textbook

[2] Comparing Strengths of Oxidants and Reductants

This question originally appeared on Quora. More questions on Quora:

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