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why do we breathe oxygen and not nitrogen

There are two parts to this answer, and several answers have addressed one or both aspects but I figured I'd put it all in one place. 1) We use oxygen for a purpose that nitrogen is chemically useless for 2) While there is a different purpose we might want to use nitrogen for, it is something that is difficult to evolve (only bacteria have done it) and we can manage without. 1) We use oxygen because our metabolism uses it for energy. Our metabolism derives chemical energy from the breakdown of complex carbon molecules; this doesn't happen on its own and you need very reactive molecules to interact with those complex carbon molecules and break them down. All organisms do this step-by-step, using successive "electron acceptors" to basically strip electrons off of simpler-and-simpler molecules and thus break them down. Molecular oxygen is the most reactive molecule and greedy electron acceptor out there, and allows organisms that use it to get the most energy possible out of a given carbohydrate. That's why aerobic respiration is so useful, and that's what we use oxygen for.

Molecular nitrogen has completely different chemical properties; it isn't that electronegative (i. e. greedy for electrons) at all. There are other molecules that can be used as electron acceptors, and are used in various forms of anaerobic respiration: nitrate, sulfate, carbon dioxide. but molecular nitrogen isn't one of them. For various kinds of anaerobic respiration, see :
2) There IS a purpose for which one could use molecular nitrogen, which is to use it to build nitrogen-based molecules that our body depends on - like DNA, RNA and proteins, which basically do everything in a living organism. No organism uses molecular nitrogen as a source for these; it's much easier to use organic nitrogen compounds like nitrates and ammonia. It can seem silly that such compounds are so limiting, when nitrogen makes up most of the atmosphere! This is less of an issue for carnivores since we get all of our nitrogen needs from eating nitrogen-filled animals, but it's a huge issue for plants.

The need for such compounds (and, to a lesser extent, phosphates) is why agriculture needs fertilizer. So why can very very few organisms break down molecular nitrogen? Because it is a very stable molecule; if you've done chemistry you might know that the two nitrogen atoms in the nitrogen molecule are connected by a triple bond, which is very strong and hard to break. This may be a big reason why the metabolism to break that bond evolved only in bacteria, and all Eukaryotes get by using the bacteria themselves (nitrogen-fixing plants), absorbing nitrogen-filled organisms (carnivores, carnivorous plants - it's the reason they're carnivorous! ) or getting by on the organic nitrogen that naturally occurs in the ground thanks to nitrogen-fixing bacteria. As an aside, the fertilizer humans make uses the Haber process, which converts molecular nitrogen in the atmosphere to ammonia. If you look at the Wikipedia page by the way you'll get an idea of how hard it is to break that triple bond, between the catalysts and the high temperatures and pressures.

But through that process you could argue that humanity as a species does "breathe" nitrogen. So basically, tl;dr: 1) we don't need to breathe nitrogen 2) if we did our bodies still wouldn't because it's really hard to do; no eukaryote does it, except maybe humans themselves but only through technology. Mostly just to simplify the process so it s easier to understand. Oxygen is what our body needs, and we breathe in a cocktail of several different elements in the air. We exhale most of what we just inhaled, but we also exhale carbon dioxide, which is a waste product our bodies make when the oxygen we consume interacts with the carbon our bodies are made from. Oxygen is a very reactive element and it likes to combine with other elements. It s kind of like pooping. We poop out most of what we eat, but in a modified fom after our bodies have taken in what it finds useful in the food we eat. On a related note, because oxygen is so reactive, it needs a constant source of replenishment in order to continue to exist in its elemental form on Earth.

This is why vegetation is so important. Photosynthesis is effectively the opposite process and sends more O2 into the air after pulling the carbon out of CO2. Interestingly enough, this has become a telltale sign of extraterrestrial life for astrobiologists. Because chemistry works the same way everywhere in the universe, we know oxygen would have a hard time existing in its elemental form on any planet without some source of replenishment. Without some sort of photosynthetic or other process going on, all the oxygen on a planet would interact with other elements to creat CO2, NO2, etc. So if an astrobiologists finds sure signs of O2 on spectral images coming from an exoplanet, there s a pretty fair chance there s life on that planet. Unfortunately we haven t been able to do that yet. And spectral imaging on exoplanets is extremely difficult since they don t create their own source of light.

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