NAD accepts hydrogen during oxidation reactions and carries high-energy electrons to later respiration stages, supporting usable cellular work and measurable biological activity.
Coverage 2014–2023 · Updated 15 Jul 2026
NAD is a hydrogen and electron carrier: it is reduced when it accepts hydrogen during dehydrogenation and later oxidized when it donates electrons.
Oxidation is electron loss. Removing hydrogen together with its electron oxidizes a respiratory substrate, while NAD gains that reducing power; oxidation and reduction therefore occur together in a redox reaction.
Carrier cycle: substrate loses hydrogen/electrons → NAD is reduced → reduced NAD transfers a pair of electrons to the mitochondrial electron transport chain → NAD is regenerated for further dehydrogenation.
Reduced NAD made during glycolysis, the link reaction or the Krebs cycle is reoxidized when its electrons enter the chain, allowing those pathways to keep accepting hydrogen.
NAD is not the terminal electron acceptor. In aerobic respiration oxygen ultimately accepts the electrons, while NAD cycles between oxidized and reduced forms.
This objective is assessed through multiple choice, commonly using Identify.
Identify
Build the answer around this relationship: NAD is reduced when it accepts hydrogen or electrons.
Representative question
The diagram shows some reactions occurring during respiration in the mitochondrion.
Energy that is released by oxidation reactions in the mitochondrial matrix is carried to the cristae of the mitochondria. How is this energy carried?
As ATP
As glucose
In lysed water
As reduced NAD
In HL respiration, carbon and hydrogen are followed separately. Glycolysis turns glucose into pyruvate, net ATP, and reduced NAD. Without oxygen, pyruvate becomes lactate in humans or ethanol and carbon dioxide in yeast to regenerate NAD. With oxygen, pyruvate enters the link reaction, forming acetyl-CoA, carbon dioxide, and reduced NAD. The Krebs cycle releases more carbon dioxide and reduced coenzymes. Electron transport uses reduced NAD/FAD to pump protons, chemiosmosis through ATP synthase makes ATP, and oxygen accepts electrons and protons to form water. Substrate comparisons depend on ATP yield, oxygen demand, water production, and speed. Regenerated NAD allows glycolysis to continue. Fermentation regenerates NAD for glycolysis and is used in baking and brewing. Electron transfers release energy while coenzymes are reoxidized.
