End of electron transport chain: Oxygen accepts electrons then binds with H. H is from where?

[Gauss44]

At the end of the electron transport chain, electrons bind to oxygen and then oxygen forms water by binding to hydrogen. Where does THAT hydrogen directly come from?

Did it come into the matrix as part of NADH or FADH2 and stay in the matrix?

Did it come into the matrix as part of NADH and FADH2, get channeled into the inner membrane space, and then come back to the matrix?

What paths do hydrogen take through the mitochondria?

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[kraskadva]

H+ (from FADH2 and NADH) is pumped out of the matrix to the intermembrane space by Complexes 1, 3, & 4.
H+ then falls down the gradient through ATP synthetase and is captured by O-2.
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