Redox Reactions

Much of respiration's energy is captured not directly as ATP but first as reduced electron-carrier coenzymes, chiefly NAD+ (Nicotinamide Adenine Dinucleotide, oxidised form) and FAD (Flavin Adenine Dinucleotide). When NAD+ accepts two electrons and one proton it becomes $NADH + H^+$, and when FAD accepts two electrons and two protons it becomes $FADH_2$; both reactions are reversible, so the coenzymes can be re-oxidised back to NAD+ and FAD by donating those same electrons and hydrogens onward, most importantly to the electron transport chain. A reaction in which a molecule gains electrons is called reduction, and one in which it loses electrons is called oxidation -- together these paired gain-and-loss reactions are called redox (oxidation-reduction) reactions, and they occur repeatedly throughout glycolysis, the link reaction and the Krebs cycle. A simp …