When NAD becomes NADH2 it has been?

When NAD (nicotinamide adenine dinucleotide) becomes NADH, it has been reduced, meaning it has gained electrons and a hydrogen ion (proton). This transformation occurs during metabolic processes, such as glycolysis and the Krebs cycle, where NAD serves as an electron carrier. The reduction of NAD to NADH is crucial for cellular respiration, as NADH ultimately donates electrons to the electron transport chain, facilitating ATP production.