biochemistry - What does the human body use oxygen for besides the final electron acceptor in the electron transport chain?

Another small addition

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There is class of oxidoreductases called oxygenases which incorporate molecular oxygen into the substrates and not just use it as an electron acceptor like in oxidases (note that the terminal enzyme in ETC is an oxidase and there are other such oxidases). In other words, oxygen is not a cofactor but a co-substrate. Oxygenases are further classified into dioxygenases and monooxygenases which incorporate two oxygen atoms and one oxygen atom respectively. Examples:

• Cytochrome P450 family (monooxygenease): involved in detoxification of xenobiotics


• Cyclooxygenase (dioxygenase): involved in production of prostaglandins which are involved in pain and inflammation. Many NSAID painkillers like aspirin, paracetamol and ibuprofen target cyclooxygenase-2 (COX2)


• Lipoxygenase (dioxygenase): Involved in production of leukotrienes which are involved in inflammation.


• Monoamine oxidase (monooxygenase): Involved in catabolism of neurotransmitters such as epinephrine, norepinephrine and dopamine.

Does oxygen deprivation result in death just due to the halting of ATP
production, or is there some other reason as well?

Death predominantly occurs because of halt in ATP production. Some cells such as neurons (and also perhaps cardiac muscles) are highly sensitive to loss of oxygen (for energy requirements) and clinical death because of hypoxia usually occurs because of loss basic brain function.

What percentage of the oxygen we take in through respiration is
expelled later through the breath as carbon dioxide?

As already mentioned, it is said that there is a rough 1:1 ratio of CO₂ production and O₂ consumption. However, as indicated in a comment by CurtF, O₂ does not form CO₂; it forms water in the last reaction of ETC. CO₂ is produced in other reactions of Krebs cycle.

Glycolysis produces 32 molecules of ATP for 1 molecule of glucose via ETC (see here). There are three complexes in ETC and the third is dependent on oxygen; so you can assume that 1/2 a molecule of O₂ consumed for production of 3 ATP molecules. Therefore 32 molecules of ATP would consume 4 molecules of O₂. Seems like there is a 1:1 ratio of CO₂ production and O₂ consumption.

We can see it like this:

FADH₂ enters ETC at the second complex whereas NADH enters at the first. We can say that as long as NADH is present FADH₂ would not require an extra oxygen.

An NADH or a FADH₂ molecule would require 1/2 molecule of O₂. There are 8 molecules of NADH and 2 molecules of FADH₂ produced during glycolysis+krebs cycle which would require 10/2 = 5 molecules of O₂. Glycolysis produces 4 molecules of CO₂ during krebs cycle.

However, 2 cytosolic NADH molecules require 2 ATPs (in other words another NADH molecule) to be transported to mitochondria. So the net effect may be actually close to 1:1 O₂:CO₂.

Another factor to keep in consideration is that the three complexes do not actually produce ATP; they just pump proton to create a chemical potential. The F₀F₁-ATP synthase would probably work only after a threshold of H⁺ potential is established. The 1 ATP molecule per complex is most likely to be the mean value and not exactly what really happens per reaction.