--- title: "NAD+/NADH — AP Biology Definition & Exam Guide" description: "NAD+/NADH is the rechargeable electron carrier that ferries electrons from glycolysis and the Krebs cycle to the ETC, powering ATP synthesis in cellular respiration." canonical: "https://fiveable.me/ap-bio/key-terms/nad-nadh" type: "key-term" subject: "AP Biology" unit: "Unit 3" --- # NAD+/NADH — AP Biology Definition & Exam Guide ## Definition NAD+/NADH is an electron carrier pair: NAD+ is the empty form that picks up electrons during glycolysis and the Krebs cycle to become NADH, which then delivers those electrons to the electron transport chain to drive ATP production. ## What It Is Think of NAD+ and [NADH](/ap-bio/key-terms/nadh "fv-autolink") as the same molecule wearing two outfits. NAD+ is the "empty" version, and when it grabs electrons (along with a [hydrogen](/ap-bio/unit-1/elements-life/study-guide/kLZ8GN081XmAmZpivYFN "fv-autolink")) during glucose breakdown, it becomes the "loaded" version, NADH. It's basically a rechargeable battery for electrons. Per EK 3.5.B.1, glycolysis releases [energy](/ap-bio/unit-3/environmental-impacts-on-enzyme-function/study-guide/Q8PevM3BI76060aoWtit "fv-autolink") from glucose to form ATP, pyruvate, and NADH from NAD+. Then in the Krebs (citric acid) cycle, pyruvate is oxidized and even more NAD+ gets reduced to NADH (EK 3.5.B.2). All that stored energy doesn't stay in NADH, though. NADH carries its electrons to the electron transport chain, where they're passed down a series of oxidation-reduction reactions to build the electrochemical gradient that powers ATP synthase (EK 3.5.A.3). NADH does the delivering; the ETC does the cashing-in. ## Why It Matters NAD+/NADH sits at the center of [Unit 3](/ap-bio/unit-3 "fv-autolink"): Cellular Energetics, specifically topic 3.5 Cellular Respiration. It directly supports [AP Bio](/ap-bio "fv-autolink") 3.5.A (how mitochondria use energy stored in macromolecules) and AP Bio 3.5.B (how cells obtain energy from those macromolecules). Without it, the whole energy-flow story falls apart, because NADH is the messenger that connects glycolysis and the Krebs cycle to the ETC. On the exam, the recurring theme is energy transfer and transformation, and NAD+/NADH is the literal vehicle for that transfer. ## Connections ### [Electron Transport Chain (Unit 3)](/ap-bio/key-terms/electron-transport-chain) NADH is useless on its own; it only matters because it hands electrons to the ETC. NADH is the delivery truck, and the ETC is the loading dock where electrons get unloaded to build the [proton gradient](/ap-bio/key-terms/proton-gradient "fv-autolink"). ### [Lactic acid fermentation (Unit 3)](/ap-bio/key-terms/lactic-acid-fermentation) When oxygen is gone, the ETC stops accepting electrons, so NADH piles up and NAD+ runs out. [Fermentation](/ap-bio/key-terms/fermentation "fv-autolink") exists purely to dump those electrons and regenerate NAD+ so glycolysis can keep making ATP. ### [Light-Dependent Reactions (Unit 3)](/ap-bio/key-terms/light-dependent-reactions) Photosynthesis uses NADP+/NADPH, a close cousin that does the same electron-carrying job in reverse, storing energy from light to power [carbon fixation](/ap-bio/key-terms/carbon-fixation "fv-autolink"). Same trick, opposite direction. ### [ATP yield (Unit 3)](/ap-bio/key-terms/atp-yield) Count the NADH and you can estimate the ATP. Each NADH delivered to the ETC drives the synthesis of multiple ATP, so tracking NADH production is how you reason about total energy output. ## On the AP Exam Expect NAD+/NADH in multiple-choice questions about why glycolysis stalls without oxygen and in any question tracing electrons through respiration. A classic setup: yeast in a glucose-rich, oxygen-free environment keep making ATP but pile up ethanol. The reasoning you need is that fermentation regenerates NAD+ from NADH, and without that recycled NAD+, glycolysis would run out of its electron acceptor and stop. The skill being tested is causal reasoning, so connect NAD+ supply to whether glycolysis can continue. No released FRQ has used "NAD+/NADH" verbatim, but it shows up constantly in the logic of cellular respiration prompts, so be ready to explain the recharge cycle in words. ## NAD+/NADH vs FAD/FADH2 Both are electron carriers feeding the ETC, but they're not interchangeable. NAD+ becomes NADH in glycolysis AND the Krebs cycle, while FAD becomes FADH2 only in the Krebs cycle (EK 3.5.B.2). FADH2 also enters the ETC at a later point, so it drives slightly less ATP per molecule than NADH. ## Key Takeaways - NAD+ is the empty electron carrier and NADH is the loaded one; the molecule cycles back and forth between these two forms. - NADH is produced in glycolysis and the Krebs cycle, then delivers its electrons to the electron transport chain to drive ATP synthesis. - Under anaerobic conditions, fermentation regenerates NAD+ from NADH so glycolysis can keep running. - If NAD+ runs out, glycolysis stops, because it needs NAD+ as the electron acceptor. - More NADH delivered to the ETC means more ATP, which is why counting NADH helps you reason about energy yield. ## FAQs ### What is NAD+/NADH in AP Biology? NAD+/NADH is an electron carrier pair. NAD+ picks up electrons during glycolysis and the Krebs cycle to become NADH, then NADH delivers those electrons to the electron transport chain to help make ATP (EK 3.5.A.3, EK 3.5.B.1). ### Does NADH directly make ATP? No. NADH itself doesn't make ATP. It carries electrons to the electron transport chain, and the energy released as those electrons move down the chain builds the gradient that ATP synthase uses to make ATP. ### Why does glycolysis stop without NAD+? Glycolysis needs NAD+ to accept electrons from glucose. If all the NAD+ is locked up as NADH and can't be recharged (like when oxygen is absent and the ETC is backed up), glycolysis has nothing to pass electrons to, so it shuts down. Fermentation fixes this by regenerating NAD+. ### How is NAD+/NADH different from FAD/FADH2? Both feed electrons into the ETC, but NAD+ becomes NADH in both glycolysis and the Krebs cycle, while FAD becomes FADH2 only in the Krebs cycle. FADH2 also enters the chain at a later point and yields slightly less ATP per molecule. ### Is NAD+ the same as NADP+ in photosynthesis? They're close cousins, not the same. NAD+/NADH carries electrons in cellular respiration, while NADP+/NADPH does the parallel job in the light-dependent reactions, storing energy to power carbon fixation. ## 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-nadh#resource","name":"NAD+/NADH — AP Biology Definition & Exam Guide","url":"https://fiveable.me/ap-bio/key-terms/nad-nadh","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-bio/key-terms/nad-nadh#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-11T05:27:38.967Z","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-nadh#term","name":"NAD+/NADH","description":"NAD+/NADH is an electron carrier pair: NAD+ is the empty form that picks up electrons during glycolysis and the Krebs cycle to become NADH, which then delivers those electrons to the electron transport chain to drive ATP production.","url":"https://fiveable.me/ap-bio/key-terms/nad-nadh","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+/NADH in AP Biology?","acceptedAnswer":{"@type":"Answer","text":"NAD+/NADH is an electron carrier pair. 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