Energy and Metabolism

ID #1084

I understand that the reason fermentation occurs during anaerobic respiration is to regenerate the cofactors. What I was wondering though, is what these cofactors actually do in anaerobic respiration since there is no ETC for them to deliver electrons to. Basically, why do the cofactors need to regenerated during anaerobic respiration if they don't serve the purpose of delivering electrons to the ETC.

The answer to the question about the role of cofactors in anaerobic respiration is that they only exist to give you back what is necessary for glycolysis to continue. Glycolysis is dependent on the availability of NAD+, and if NAD+ disappears, glycolysis shuts down. If glycolysis shuts down, the cell dies. It's that simple. In aerobic respiration, the regeneration of the cofactor to its oxidized state happens for you automatically when NADH drops off the electrons at the ETC. In the absence of oxygen, you need some molecule--any molecule--which will accept the electrons from NADH as part of a coupled reaction pair. This electron transfer involves the breaking of covalent bonds between a hydrogen and the rest of the NADH molecule, so it is understandably exergonic. It would be great if we could directly or indirectly harness that released energy, as we do in the ETC, but it is just as important to get back the NAD+. Even if that means using the energy to build a molecule that you don't really need (ethanol or lactic acid), which is the result of fermentation. It really just serves as a means to regenerate NAD+.

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