12.2.6—Krebs cycle redox reactions

NAD and FAD are reversible coenzymes that accept hydrogen/electrons during dehydrogenation. Their reduced forms temporarily carry this transferable reducing power from earlier respiratory reactions to the electron transport chain on the inner mitochondrial membrane.

• NAD: accepts hydrogen/electrons during oxidation of respiratory intermediates, becoming reduced NAD. Reduced NAD later donates its electrons/hydrogen to membrane electron carriers and is regenerated in its oxidised form.
• FAD: accepts hydrogen/electrons in the same reversible carrier pattern, becoming reduced FAD. Reduced FAD also transfers its electrons/hydrogen to the respiratory chain and is then reoxidised.
• Shared bridge: reduction captures transferable energy at dehydrogenation steps; later oxidation of the reduced coenzymes makes that reducing power available to the membrane electron-transfer system. This links matrix/cytoplasmic reactions to oxidative phosphorylation.
• Boundary: the source supports both coenzymes as carriers, but this card does not infer an unlisted energy ranking or detailed entry-point comparison. The chain mechanism and ATP accounting belong to the following cards.

Reaction → carrier → membrane: dehydrogenation removes hydrogen/electrons from a respiratory substrate; NAD or FAD accepts them and is reduced; the reduced coenzyme transfers them later and becomes oxidised again. Reoxidation matters because the oxidised coenzyme can accept more hydrogen/electrons in continuing respiratory reactions.

NAD and FAD are not ATP and do not release their stored energy directly as a complete ATP yield. They are reversible carriers that bridge earlier oxidation reactions to the inner-membrane electron-transfer stage; the proton gradient, ATP synthase and oxygen endpoint are taught separately.