--- title: "NAD⁺ — AP Biology Definition & Exam Guide" description: "NAD⁺ is the electron-carrier coenzyme that grabs electrons during glycolysis and the Krebs cycle, becoming NADH to fuel ATP production in cellular respiration." canonical: "https://fiveable.me/ap-bio/key-terms/nad" type: "key-term" subject: "AP Biology" unit: "Unit 3" --- # NAD⁺ — AP Biology Definition & Exam Guide ## Definition NAD⁺ is a coenzyme that picks up electrons (and a hydrogen) during glycolysis and the Krebs cycle, becoming reduced to NADH so it can later deliver those electrons to the electron transport chain. ## What It Is NAD⁺ (nicotinamide [adenine](/ap-bio/key-terms/adenine "fv-autolink") dinucleotide) is the workhorse electron carrier of [cellular respiration](/ap-bio/unit-3/photosynthesis/study-guide/qIyyKCxB3XJI9oRI7yjl "fv-autolink"). Think of it as an empty shuttle bus. When a reaction strips electrons off glucose or its breakdown products, NAD⁺ pulls in two electrons plus a proton and becomes NADH. That's a reduction reaction, so NAD⁺ is the oxidized form and NADH is the reduced form. Per EK 3.5.B.1, [glycolysis](/ap-bio/key-terms/glycolysis "fv-autolink") converts glucose into pyruvate while turning NAD⁺ into NADH. Then per EK 3.5.B.2, when pyruvate gets oxidized in the mitochondrion and runs through the Krebs cycle, more NAD⁺ gets reduced to NADH (and FAD becomes FADH₂). The whole point is that the loaded NADH carries those high-energy electrons over to the electron transport chain, where they power ATP synthesis (EK 3.5.A.3). No NAD⁺ being recycled back from NADH, and the early steps of respiration grind to a halt. ## Why It Matters NAD⁺ lives in [Unit 3](/ap-bio/unit-3 "fv-autolink"): Cellular Energetics, specifically Topic 3.5 Cellular Respiration. It anchors learning objective [AP Bio](/ap-bio "fv-autolink") 3.5.B (how cells extract energy from macromolecules) and connects to AP Bio 3.5.A (how the mitochondrion and ETC use that energy). The bigger idea is energy transfer through coupled oxidation-reduction reactions, a theme that runs across the whole unit. NAD⁺ is the link that ties glycolysis and the Krebs cycle to the electron transport chain, so understanding it lets you trace electrons all the way from glucose to ATP. ## Connections ### [Electron Transport Chain (Unit 3)](/ap-bio/key-terms/electron-transport-chain) [NADH](/ap-bio/key-terms/nadh "fv-autolink") is basically the delivery truck for the ETC. The electrons NAD⁺ collected during glycolysis and the Krebs cycle get dropped off at the ETC, which uses them to pump protons and build the gradient that drives ATP synthase. ### [Lactic Acid Fermentation (Unit 3)](/ap-bio/key-terms/lactic-acid-fermentation) When oxygen runs out, the ETC can't accept electrons, so NADH piles up and NAD⁺ disappears. [Fermentation](/ap-bio/key-terms/fermentation "fv-autolink") exists to regenerate NAD⁺ so glycolysis can keep running. That's why a cell with no oxygen still has to recycle NAD⁺ to make even a little ATP. ### [Light-Dependent Reactions (Unit 3)](/ap-bio/key-terms/light-dependent-reactions) Photosynthesis uses [NADP⁺](/ap-bio/key-terms/nadp "fv-autolink") (NAD⁺'s close cousin) the same way, loading it into NADPH to carry electrons to the Calvin cycle. Same shuttle-bus logic, just a different pathway, which is a great way to remember both. ## On the AP Exam Expect NAD⁺ in multiple-choice questions about glycolysis, the Krebs cycle, and fermentation. A classic stem asks which molecule accepts electrons and becomes reduced (NAD⁺ to NADH, or FAD to FADH₂). Another common angle is fermentation: questions that ask what fermentation accomplishes, where the right answer involves regenerating NAD⁺ so glycolysis can continue. On free-response, you'd use NAD⁺ to explain how electrons move from glucose to the ETC, or to predict what happens to glycolysis if oxygen is removed. Be ready to identify NAD⁺ as oxidized and NADH as reduced, and to explain why the cell must keep recycling between the two forms. ## NAD⁺ vs NADH NAD⁺ is the empty, oxidized form that's ready to accept electrons; NADH is the loaded, reduced form carrying them. Same molecule, two states. NAD⁺ gets reduced to NADH during glycolysis and the Krebs cycle, then NADH gets oxidized back to NAD⁺ when it dumps electrons at the ETC. ## Key Takeaways - NAD⁺ is the oxidized electron carrier; it accepts electrons and a proton to become NADH, the reduced form. - Glycolysis and the Krebs cycle both reduce NAD⁺ to NADH (EK 3.5.B.1 and 3.5.B.2). - NADH carries its electrons to the electron transport chain, which uses them to drive ATP synthesis. - If NAD⁺ isn't regenerated, glycolysis stops, which is exactly why fermentation exists when oxygen is unavailable. - NADP⁺/NADPH does the same shuttle job in photosynthesis, so the reduction-oxidation logic transfers between units. ## FAQs ### What is NAD⁺ in AP Biology? NAD⁺ is a coenzyme that accepts electrons during glycolysis and the Krebs cycle, becoming reduced to NADH. It then carries those electrons to the electron transport chain so the cell can make ATP. ### What's the difference between NAD⁺ and NADH? NAD⁺ is the oxidized, 'empty' form ready to grab electrons; NADH is the reduced, 'loaded' form carrying them. NAD⁺ becomes NADH during glycolysis and the Krebs cycle, and NADH turns back into NAD⁺ at the electron transport chain. ### Is NAD⁺ reduced or oxidized? NAD⁺ is the oxidized form. When it gains electrons it gets reduced to NADH, which is the reduced form. Remembering that NAD⁺ is 'hungry' for electrons helps you keep them straight. ### Why does fermentation need NAD⁺? Without oxygen, the electron transport chain can't take electrons from NADH, so NAD⁺ runs out and glycolysis stalls. Fermentation regenerates NAD⁺ so glycolysis can keep producing a small amount of ATP. ### Is NAD⁺ the same as NADP⁺? No, but they're close cousins. NAD⁺ carries electrons in cellular respiration, while NADP⁺ (becoming NADPH) carries them in the light-dependent reactions of photosynthesis. They work the same way in different pathways. ## Related Study Guides - [3.5 Cellular Respiration](/ap-bio/unit-3/photosynthesis/study-guide/qIyyKCxB3XJI9oRI7yjl) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-bio/key-terms/nad#resource","name":"NAD⁺ — AP Biology Definition & Exam Guide","url":"https://fiveable.me/ap-bio/key-terms/nad","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-bio/key-terms/nad#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-11T05:27:37.911Z","isPartOf":{"@type":"Collection","name":"AP Biology Key Terms","url":"https://fiveable.me/ap-bio/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/ap-bio/key-terms/nad#term","name":"NAD⁺","description":"NAD⁺ is a coenzyme that picks up electrons (and a hydrogen) during glycolysis and the Krebs cycle, becoming reduced to NADH so it can later deliver those electrons to the electron transport chain.","url":"https://fiveable.me/ap-bio/key-terms/nad","inDefinedTermSet":{"@type":"DefinedTermSet","name":"AP Biology Key Terms","url":"https://fiveable.me/ap-bio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is NAD⁺ in AP Biology?","acceptedAnswer":{"@type":"Answer","text":"NAD⁺ is a coenzyme that accepts electrons during glycolysis and the Krebs cycle, becoming reduced to NADH. 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