Why does life require oxygen and not some other gas like nitrogen for example.
Is there something specific in oxygen that life needs to survive?
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All organisms need energy and they do this through chemical reactions called redox reactions. Redox is short for reduction (which means an atom/molecule gains electrons) – oxidation (which means an atom/molecule loses electrons). Some elements like oxygen are really hungry for electrons, while others (carbon and hydrogen) are relatively happy to give up electrons. When a redox reaction occurs that makes everyone happier, energy is released. Oxygen is important for life because it wants electrons more than anything else that is readily available. Because oxygen wants electrons so bad, more energy is released when it gets electrons. If a reaction gave electrons to sulfur rather than oxygen, less energy would be released.
A common redox reaction is respiration, where sugars (mostly carbon and hydrogen) react with oxygen (O2) to make water (H2O) and carbon dioxide (CO2). O2 oxygen is unhappy because there are two oxygen atoms stuck together, and each of them are trying to hog all of the electrons. Carbon and hydrogen don’t pull on electrons very hard, so sugar essentially has an excess of electrons. When the redox reaction occurs and these atoms are rearranged, you end up with oxygen bonded to carbon or hydrogen. The oxygen gets to hog the electrons in those bonds, and the carbon and hydrogen are generally pretty happy with that arrangement. Organisms use a complicated series of steps that allows them to capture the energy from this reaction in the form of ATP, which is then used to power all kinds of cellular processes.
Some of the other comments have mentioned that there are lots of organisms that do not use oxygen. These organisms use redox reactions to get energy, but they use sulfur (or actually sulfate), manganese, iron or other things in place of oxygen. When oxygen is present, organisms that use oxygen tend to dominate, because they can get way more energy than organisms that do not use oxygen.
Photosynthesis is also a redox reaction, but it runs in the opposite direction as respiration. Plants use energy from the sun to power reactions that turn water and carbon dioxide into sugar and oxygen (O2).
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