Final answer:
When NAD+ becomes NADH, the coenzyme has been reduced, indicating it has gained electrons and hydrogen ions in metabolic reactions.
Step-by-step explanation:
When NAD+ becomes NADH, the coenzyme has been reduced. This reduction involves the acceptance of electrons and hydrogen ions (protons), specifically one hydrogen atom (with its accompanying electron) and an additional electron. In this process, the NAD+ gains two electrons and a proton to become NADH, thereby being reduced. The oxidized form, NAD+, acts as an electron acceptor in redox reactions, which is a crucial process in cellular metabolism including pathways like glycolysis and the Krebs cycle. In contrast, the reduction of NAD+ to NADH is central to many biochemical processes as it helps in transferring electrons, thus playing a significant role in the generation of energy within cells.