NADH and FADH2 in the electron transport chain do not directly donate electrons to oxygenWhy?

Answer 1

The temperature in cells is not high enough. Each of the redox reactions in the electron transport chain are catalyzed by an enzyme. NADH dehydrogenase transfers electrons from NADH to ubiquinone, and cytochrome c oxidase transfers electrons from cytochrome c to oxygen. So there is no enzyme to pass the electrons directly from NADH to oxygen.

Answer 2

Because that's the whole point of the ETC. NADH donates electron through three intermediates, generating 1 ATP per intermediate until it finally reaches O2. FADH2 only goes through ubiquinone or carrier Q before giving it to oxygen and thus creates 1 drop per electron with 2 electrons resulting in 2 ATP. If they donated directly to oxygen, they would bypass all these intermediates, thus bypassing the part where ATP is made, therefore yielding no ATP and defeating the purpose of the ETC. If your question is physically why oxygen is unable to just take it from NADH and FADH2, then I would say that it is because oxygen is never allowed to come into contact with anything other than the doners that are meant to give it to oxygen. It is probably physically separated in the mitochondrial membrane.

Answer 3

NADH and FADH2 donate electrons to the electron transport chain, specifically to complex I and complex II respectively. The electrons are then passed through a series of protein complexes, generating a proton gradient. Eventually, the electrons are transferred to oxygen at complex IV to form water.