biological oxidation and reduction

[superduper12]

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ok this is really simple but I can't seem to get it.

oxidation: loss of e-, gain of oxygen
reduction: gain of e-, loss of oxygen

Mitochondria: In the electron transport chain we have the final reaction of

2H+ + 1/2 O2 --> H2O

thus oxygen is reduced. I don't understand this because from the reactant side, oxygen has to now give/share it's electrons with hydrogen. so it's losing some electrons...shouldn't oxygen be oxidized?

also why do we say that glucose is oxidized in glycolysis...I just don't see it.

 

[Quantum Chem]

Yeah sort of tricky when you put it how you explained it. But think about oxidation numbers. O2 has an oxidation state of 0, but oxygen in H2O has an oxidation state of -2, so the Oxygen has been reduced.

Glucose gets oxidized because the number of C-O bonds increases. For organic purposes you can think of the amount of C-O bonds increasing as being oxidized. The actual reason is that in the compound Carbon is loosing electron density, which means its being oxidized. Reduction then would be increasing Carbon's electron density, or in a simpler way decreasing the amount of C-O bonds.

 

[CdanceNS]

Ok, forget the whole gain and loss of oxygen thing and memorize that oxidation is loss of electrons and reduction is gain of electrons (which is always true in a redox reaction, even when no oxygen is involved). So now look at your equation. O2 has an oxidation number of 0 (has to otherwise it would have a charge). Now look at water. Here oxygen has an oxidation number of -2. It's the more electronegative atom so obviously you wouldn't think that it would have +2 and H would have -1. So oxygen went from oxidation number of 0 to -2, which means it gained electrons = reduction)