(A) Nicotinamide adenine dinucleotide (NAD⁺) acts as a carrier of electrons in oxidation-reduction reactions by accepting electrons (e^-) to form NADH. (B) For example, NAD⁺ can accept electrons from one substrate (S1), yielding oxidized S1 plus NADH. The NADH formed in this reaction can then transfer its electrons to a second substrate (S2), yielding reduced S2 and regenerating NAD⁺. The net effect is the transfer of electrons (carried by NADH) from S1 (which becomes oxidized) to S2 (which becomes reduced).