NAD is a reusable carrier that shuttles hydrogen and electrons in respirationNAD (nicotinamide adenine dinucleotide) is a coenzyme that carries hydrogen and electrons in cellular respiration.It cycles between two forms so it can be used again and again: NAD⁺ (the empty carrier) and reduced NAD (the loaded carrier).Oxidation and reduction always happen togetherOxidation is the loss of electrons or hydrogen from a molecule.Reduction is the gain of electrons or hydrogen by a molecule.Oxidation and reduction occur together as paired redox reactions, so when one molecule is reduced another is oxidised.Oxidation and ReductionAnalogyRemember OIL RIG: Oxidation Is Loss, Reduction Is Gain.NAD picks up hydrogen when substrates are oxidisedNAD⁺ accepts hydrogen and electrons removed from respiratory substrates such as glucose.When it accepts hydrogen, NAD⁺ is reduced to reduced NAD, and the substrate is oxidised.Role of NAD in Aerobic RespirationNoteNAD⁺ is a coenzyme, so it helps enzymes but is not used up.It carries energy as electrons and hydrogen rather than releasing energy directly.NAD⁺ is reduced in three stages of respirationGlycolysis reduces NAD⁺ to reduced NAD in the cytoplasm.The link reaction reduces NAD⁺ in the mitochondrial matrix.The Krebs cycle reduces NAD⁺ several more times in the mitochondrial matrix.NAD to NADH in glycolysisNADH in Krebs cycleReduced NAD is re-oxidised at the electron transport chainReduced NAD later donates its electrons to the electron transport chain, which regenerates NAD⁺ for reuse.See C1.2.13 and C1.2.14 for how this energy is transferred and used to make ATP.NAD in ETCCommon MistakeWatch outDo not confuse the two forms of the carrier.NAD⁺ is the empty carrier, while reduced NAD is the loaded carrier ready to deliver electrons.AnalogyPicture NAD⁺ as an empty taxi that picks up passengers (electrons and hydrogen).Once loaded, it becomes reduced NAD, the full taxi.It drops the passengers at the electron transport chain, then returns empty as NAD⁺.Theory of KnowledgeHow does the idea of oxidation and reduction connect biology to chemistry and physics?What other systems can you think of where energy transfer is critical?Active recallDefine oxidation and reduction.What happens to NAD⁺ when it accepts hydrogen?Name the three respiration stages in which NAD⁺ is reduced.How is reduced NAD re-oxidised to NAD⁺?Why can NAD be used over and over?
