[matthewh (OP)]

Hao asked us:

Why do we need oxygen? Why do we not use carbon dioxide,helium or nitrogen?

Can you help?

[Kryptid]

Complex organisms like humans (and most other multicellular organisms) have high energy requirements. Oxygen, being a strong oxidizing agent, can release large amounts of energy when reacting with organic compounds like glucose. It therefore makes for an ideal reactant given its abundance in the atmosphere. Helium would do us no good because it doesn't react at all. Molecular nitrogen is held together by a strong triple bond, which means the majority of chemical reactions it is involved in consume energy instead of releasing it. Carbon dioxide is more reactive than the other two, but I don't think it will react with organic molecules to release energy (at least in most cases).

That's not to say that metabolism of glucose in oxygen is exactly like burning glucose in oxygen. It isn't. It occurs through a complex series of chemical reactions in the body and I believe that oxygen helps drive the process as an electron acceptor near the end of the reaction series. Others who know more about that may correct me on that.

[alancalverd]

Though interestingly, you can ignore all the boring biochemistry stuff about Krebs and suchlike, because the physics is remarkably consistent. Nutritionists use bomb calorimeters to measure the energy value of food by total oxidation, with a small (maybe 5%) correction for the residual combustibility of feces. Comparison with total body calorimetry or oxygen consumption suggests the technique is adequately accurate without having to calculate all the possible metabolic pathways!

That said, there are some living creatures that function anerobically, using e.g. sulfur as an oxidant, and all sorts of plants and algae actually exhale oxygen from carbon dioxide. The evolutionary argument is that oxygen users evolved along with oxygen producers to maintain a survivable balance for both.