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My biology teachers never explained why animals need to breathe oxygen, just that we organisms die if we don't get oxygen for too long. Maybe one of them happened to mention that its used to make ATP. Now in my AP Biology class we finally learned the specifics of how oxygen is used in the electron transport chain due to its high electronegativity. But I assume this probably isn't the only reason we need oxygen.

What other purposes does the oxygen we take in through respiration serve? Does oxygen deprivation result in death just due to the halting of ATP production, or is there some other reason as well? What percentage of the oxygen we take in through respiration is expelled later through the breath as carbon dioxide?

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Doesn't the body use sunlight and oxygen to create Peroxide in the skin for immune reasons? I heard this once from somewhere –  Gabriel Fair Jan 9 '12 at 1:52
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Oxygen is actually highly toxic to cells and organisms – reactive oxygen species cause oxidative stress, essentially cell damage and contributing to cell ageing. A lot of anaerobic organisms have never learned to cope with this and die almost immediately when exposed to oxygen. One classical example of this is C. botulinum.

Oxygen is incorporated in several molecules in the cell (for instance riboses and certain amino acids) but as far as I know, all of this comes into the cell as metabolic products, not in the form of pure oxygen.

The oxygen (O2) we breathe is (almost) completely transformed into CO2 and exhaled. The stoichiometry is (again, almost) 1:1.

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Superoxide, O2 is created by the immune system in phagocytes (including neutrophils, monocytes, macrophages, dendritic cells, and mast cells) which use NADPH oxidase to produce it from O2 for use against invading microorganisms. However, under normal conditions, the mitochondrial electron transport chain is a major source of O2, converting up to perhaps 5% of O2 to superoxide. [1]

As a side note, there are two sides to this coin. While this is a useful tool against microorganisms, the formation of the reactive oxygen species has been incriminated in autoimmune reactions and diabetes (type 1). [2]

[1] Packer L, Ed. Methods in Enzymology, Volume 349. San Diego, Calif: Academic Press; 2002

[2] Thayer TC, Delano M, et al. (2011) Superoxide production by macrophages and T cells is critical for the induction of autoreactivity and type 1 diabetes,60(8), 2144-51.

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Never knew that, Thanks Jonsca! –  Gabriel Fair Jan 9 '12 at 1:50
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The overwhelming use of oxygen is to provide us (in combination with food) with energy. We have a great need for energy in our cells, which is why we have these lungs, diaphragms, red blood cells, etc.; they assure we get the oxygen to obtain the energy (via the electron transport chain).

The overall metabolism of glucose (C6H12O6) is a representative reaction:

 C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O + energy

You can see that just as much oxygen goes out as gaseous CO2 as came in as gaseous oxygen (O2).

The energy is temporarily kept in the form of the phosphate bond in ATP molecules so that it can be shuttled around the cell to the multitude of cellular processes that need energy.

Energy is so essential to the cellular processes that maintain animal cells that lack of that energy, which results quickly when there is oxygen deprivation, soon causes irreversible damage and death.

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You probably know by now that cytochrome c oxidase, the last complex of the electron transport chain, belongs to a class of enzymes called oxidoreductases, that use oxygen atoms as electron acceptors. One type of oxidoreductases are oxidases, enzymes that (at least in theory [1]) use molecular oxygen--O2, like in air--as their electron acceptor. From what I know, however, sometimes that isn't the case: xanthine oxidase, that converts xanthine to uric acid, gets its oxygen atoms from water [2]. Examples of the "true" oxidases include L-amino-acid oxidase and cytochrome P450 (aka. CYP family).

Despite cytochrome P450 being a numerous and important enzyme family, responsible for most of known drugs metabolism and some essential lipids transformations, it probably consumes only a fraction of oxygen that animals breathe in. I wasn't able to find any estimations, but would be surprised if it was more than perhaps 0,1%.


[1] Introduction to EC1 class

[2] Metz, S. & Thiel, W. A Combined QM/MM Study on the Reductive Half-Reaction of Xanthine Oxidase: Substrate Orientation and Mechanism. J. Am. Chem. Soc. 2009, 131, 14885–14902, PMID: 20050623.

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