I'll try my best to make this as straightforward as possible.

In each cell in your body, Oxygen comes in in the form of 2 oxygen's doubly bonded, the 2 oxygen atoms are split apart, and combined with a carbon atom bonded to hydrogens. This results in a free hydrogen, which is pushed through some protein machinery to force a phosphate group onto adinosine diphosphate, creating adisonine triphosphate, otherwise known at ATP. ATP is the source of energy in the body, used for everything from moving your muscles to digesting food. The resulting CO2 molecule is expelled through your lungs.

This is why we breathe in oxygen, eat carbon based foods, and breathe out CO2. It's all about creating ATP, which is the source of energy for all biological processes.

Edit: other organisms make ATP differently, using light or non-carbon based processes. This is just what humans and other animals do.

Source: Took organic chemistry and biochemistry in college and I at least remember this intersting chemical process.

Nitrogen in its most common form N2 is very stable the triple bond between the two atoms takes a lot of energy to break. Compared to O2 which only has a double bond between them meaning it's much more reactive.

Both nitrogen and oxygen are necessary for life, but they have different chemical properties that mean they'll be used in different ways.

It doesn’t!

The first life on earth was anaerobic- it metabolized without oxygen. The evolution of oxygen producing life (photosynthesis) was disastrous for these early life forms and almost all of them were wiped out by the emergence of an oxygen rich atmosphere that was highly toxic to them.

We still have some life that can survive via anaerobic metabolism - for example Botulism can metabolize aerobically when there is oxygen available or anaerobically when there isn’t. For us the second path is terrible because it produces highly deadly (to us) chemicals in poorly preserved food.

Another, very well known, life form that uses anaerobic respiration area yeasts. In oxygen free environments they anaerobically metabolize sugars into alcohol and carbon dioxide. This is how we make all alcoholic beverages, and get bubbles in them!

Humans can, for a very limited time, metabolize anaerobically - typically for very short bursts of time when oxygen transport systems are unable to keep up with as very high intensity demand, like a 200 yard sprint. The byproducts of this are harmful to us though so our bodies will fairly promptly slam on the brakes and cause your muscles to seize up and force you to stop.

Anyway - why is oxygen such a preferred metabolic option? It enables a much more efficient production of energy than anaerobic options that we are aware of, and can output much higher energy - so lifeforms that we’re able to adapt and use it were able to do outperform any that weren’t.

>This results in a free hydrogen, which is pushed through some protein machinery to force a phosphate group onto adinosine diphosphate, creating adisonine triphosphate, otherwise known at ATP.

Ah the electron transport chain. It also binds leftover hydrogen atoms to oxygen make water as well right?

A better way to phrase it might be...why oxygen instead of Sulfur or Selenium? Because those are both in the same column on the Periodic Table, they have the same number of valence electrons and therefore can form double-bonds easily. However, the obvious answer comes to mind at the same time: they're both solid at room temperature. So...maybe...in a higher temperature environment, it might be possible for life to selectively choose gaseous Sulfur over Oxygen. But that's not how things work on Earth (most of the time)

Aerobic respiration releases about 10x more energy than anaerobic respiration from the same energy sources. Oxygen is one of the best oxidizers that is commonly available which is why it's able to do that. There are much stronger oxidizers known to chemistry which could release even more energy. However if you get much stronger than oxygen they start reacting so easily with other things, those chemicals would destroy all the other molecules that make up an organism.

Oh snap I forgot the water. Yes the electron transport chain!

I just had massive flashbacks to high school chemistry biology

How do you know this

Biochemistry and organic chemistry in college! I don't practice chemistry in my job, but I remember enough to text basic concepts. I figured I could give a high level explanation here.

Life existed before Oxygen was abundant, so no.

About 2.4 billion years ago, some bacteria figured out how to photosynthesise to get energy from sunlight. Photosynthesis produces Oxygen gas, which they didn't need, so they burped it out. Over hundreds of millions of years, the oxygen built up. Oxygen is quite toxic to life forms not used to it, so that led a mass extinction.

More info: https://en.wikipedia.org/wiki/Great_Oxidation_Event

The oxygen is converted to H2O not CO2 in aerobic respiration. The CO2 comes from the pyruvate/food.

They covered it in General Biology 1 at my college (the BIO101 course). The book even had cool pictures of the main protein involved in ATP synthesis, which spins around like a little motor or generator.

Check it out

I’ll try to go for a true ELI5 here. Life needs oxygen to burn food. Burning food gives living organisms a lot of energy, which they can use to move around, or grow fast, or break up some more food etc. it’s not strictly necessary though, life started out without oxygen, and there are many living things (bacteria and others) today that make do without it. There are other ways to get energy, doing it with oxygen is just much more effective. Organisms like us can’t live without oxygen because we came to rely on it.

For those wondering how oxygen is toxic:

https://byjus.com/question-answer/why-oxygen-is-toxic-to-anaerobic-bacteria/

Anaerobic metabolism means without free oxygen. Iirc the oxygen atom is vital to both forms of metabolism. You can't make glucose, pyruvate or hell even DNA without oxygen.

not all life requires oxygen, for some organisms it is toxic. oxygen is an extremely corrosive chemical that gives off energy (exothermic) when you combine it with just about anything. it is very abundant on earth and in the universe, but not always in a form that can be easily accessed. interestingly, the first organisms on earth used sulphur, which is directly below oxygen on the periodic table, so a lot of similarities.

>A better way to phrase it might be...why oxygen instead of Sulfur or Selenium? Because those are both in the same column on the Periodic Table, they have the same number of valence electrons and therefore can form double-bonds easily.

Certain bacteria for example, purple sulfur bacteria, use thiosulfate or hydrogen sulfide as a final electron acceptor instead of water. These are reduced into small grains of elemental sulfur instead. Others use various organic compounds like amino acids. These types of organisms are either obligate anaerobes which live in air-free hot springs that are rich in H2S, or else are microaerophilic, meaning they can tolerate low levels of oxygen but do not generally utilize it themselves. The latter live in relatively shallow, nutrient rich, stagnant ponds where there's an excess of organic materials. That is, the water has become stratified with a layer of oxygen poor, H2S rich water below a shallow oxygen rich layer on the surface.

These types of bacteria may predate the first oxygen-generating Cyanobacteria by hundreds of millions of years. The first photosynthetic organisms were non-oxygenic.

Selenium is vastly rarer in most environments. It wouldn't make sense to use it as a primary electron acceptor because it's rarity would be an extreme limiting factor on growth. Excess amounts of selenium are also toxic to most organisms. This is probably related to it's chemical similarity to arsenic.

>However, the obvious answer comes to mind at the same time: they're both solid at room temperature.

There are a number of bacteria that derive energy by oxidizing sulfur and/or various metal sulfide minerals into thiosulfate and sulfur dioxide. (Lithochemotrophs)

TIL

Selenium biochemistry is also an interesting topic. Trace amounts of selenium are essential to the diet of, at least to my knowledge, vertebrate animals. I'm not aware whether or not other kinds of animals use it. In several of the enzymes that metabolize iodine-containing thyroid hormones, the amino acid cysteine which contains the R-SH (thiol group) is replaced with R-SeH. Thus creating the unique amino acid "selenocysteine" at the active site of the enzymes. The -SeH seems to be important in handling iodine in thyroid hormones.

Thus, selenium deficiency can mimic symptoms of iodine deficiency. Although the former is rare in humans. Grazing mammals can get selenium deficiency in areas with certain kinds of clay soils.

I can't answer why it has to be oxygen, but the oxygen is used in the final step to keep your cells fed.

There is a molecule called ATP (adenosine triphosphate, for those of you over 5) that is what your cells run on, and your body produces it from sugar (glucose) and oxygen. That is the entire reason you need to breath, to bring in oxygen for the ATP cycle.

​

See here for details

https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/02%3A_Cell_Biology/2.31%3A_Anaerobic_and_Aerobic_Respiration#:~:text=Why%20oxygen%3F,many%20more%20ATP%20are%20made.

https://www.thoughtco.com/aerobic-vs-anaerobic-processes-1224566

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).

More like Toxygen, amirite?