--- title: "NADH Dehydrogenase | General Biology I" description: "NADH dehydrogenase is Complex I of the electron transport chain, moving electrons from NADH to ubiquinone and building the proton gradient in General Biology I." canonical: "https://fiveable.me/college-bio/key-terms/nadh-dehydrogenase" type: "key-term" subject: "General Biology I" unit: "Unit 7" --- # NADH Dehydrogenase | General Biology I ## Definition NADH dehydrogenase is Complex I of the electron transport chain. In General Biology I, it oxidizes NADH to NAD+ while passing electrons to ubiquinone and helping build the proton gradient used for ATP production. ## What It Is NADH dehydrogenase is the first major electron carrier protein complex in the mitochondrial electron transport chain, also called Complex I. In General Biology I, you usually meet it when the class shifts from making NADH in glycolysis and the citric acid cycle to using that NADH to drive oxidative phosphorylation. Its job is to take electrons from NADH and hand them off to ubiquinone, also called coenzyme Q. When NADH gives up its electrons, it becomes NAD+, which matters because cells need NAD+ available so glycolysis and other pathways can keep running. The electron transfer is not just a simple handoff. As electrons move through Complex I, the complex uses that energy to pump protons, H+, from the mitochondrial matrix into the intermembrane space. That creates an electrochemical gradient, often called the proton motive force. That gradient is the real payoff. The cell stores energy in the difference in proton concentration and charge across the inner mitochondrial membrane, and ATP synthase later uses that stored energy to make ATP. So NADH dehydrogenase does not make ATP directly, but it helps set up the conditions that let ATP be made efficiently. A common way to picture it is as the start of a chain reaction. NADH brings high-energy electrons to Complex I, Complex I passes those electrons to ubiquinone, and the membrane gets loaded with protons in the process. If Complex I slows down or is damaged, the whole flow of cellular respiration gets less efficient, which can lower ATP output and disturb the balance between NADH and NAD+. ## Why It Matters NADH dehydrogenase is one of the clearest places where the big idea of cellular respiration becomes visible in General Biology I. Earlier steps like glycolysis and the citric acid cycle mostly store energy in NADH, but Complex I is where that stored energy starts being converted into a proton gradient that can power ATP synthesis. This term also helps you connect metabolism across the whole cell. If NADH cannot pass electrons efficiently, NAD+ is not regenerated as quickly, and pathways that depend on NAD+ slow down. That means one protein complex affects both energy production and the pace of upstream reactions. It also gives you a concrete way to explain why the inner mitochondrial membrane matters. The membrane is not just a boundary, it is the surface that makes proton pumping useful. Without that membrane separation, the gradient could not build up, and oxidative phosphorylation would lose most of its power. In class, this term often shows up when you are tracing a diagram, labeling the electron transport chain, or explaining what happens after NADH is produced. It is one of those concepts that connects structure, energy flow, and membrane function in a single step. ## Connections ### Electron Transport Chain NADH dehydrogenase is the first entry point for electrons from NADH into the electron transport chain. If you are tracing the path of electrons across the inner mitochondrial membrane, Complex I is where that path begins for NADH-derived electrons. It starts the chain reaction that eventually helps power ATP synthesis. ### Oxidative Phosphorylation Complex I is part of oxidative phosphorylation because it helps create the proton gradient that ATP synthase uses. The term matters anytime you need to explain how electron transfer is linked to ATP production. NADH dehydrogenase does not add phosphate to ADP itself, but it helps drive the process that makes ATP possible. ### Ubiquinone Ubiquinone is the electron acceptor that receives electrons from NADH dehydrogenase. It is mobile in the membrane, so it can carry electrons from Complex I to the next complexes in the chain. If you are following the route of electrons, ubiquinone is the shuttle after Complex I. ### [intermembrane space](/college-bio/key-terms/intermembrane-space) The intermembrane space is where protons are pumped during Complex I activity. That buildup of H+ there helps create the concentration and charge difference needed for ATP synthase to work. If a question asks where the gradient forms, this is the side of the inner membrane that gains protons. ## On the AP Exam A diagram question may ask you to identify where NADH dehydrogenase sits in the mitochondrion and what it does next. You should be able to say that it oxidizes NADH, reduces ubiquinone, and pumps protons into the intermembrane space. In a short answer or lab question, you might trace how electron transfer through Complex I contributes to the proton gradient that later powers ATP synthase. If the prompt asks what happens when the complex fails, connect it to lower ATP yield and reduced NAD+ regeneration. A good response shows the sequence, not just the label. ## NADH dehydrogenase vs Complex II Complex I and Complex II both feed electrons into the electron transport chain, but they are not the same step. NADH dehydrogenase is Complex I and takes electrons from NADH while pumping protons. Complex II takes electrons from FADH2 and does not pump protons, so it contributes differently to the gradient. ## Key Takeaways - NADH dehydrogenase is Complex I of the electron transport chain in the inner mitochondrial membrane. - It oxidizes NADH to NAD+ and transfers the electrons to ubiquinone, which keeps electron flow moving down the chain. - As electrons pass through Complex I, protons are pumped into the intermembrane space, helping build the proton gradient used for ATP synthesis. - The enzyme connects earlier energy capture in glycolysis and the citric acid cycle to later ATP production in oxidative phosphorylation. - If Complex I is disrupted, the cell makes less ATP and has a harder time regenerating NAD+, which can slow other metabolic pathways. ## FAQs ### What is NADH dehydrogenase in General Biology I? NADH dehydrogenase is Complex I of the electron transport chain. It takes electrons from NADH, passes them to ubiquinone, and helps pump protons across the inner mitochondrial membrane. That makes it a starting point for oxidative phosphorylation. ### Is NADH dehydrogenase the same as Complex I? Yes, NADH dehydrogenase is another name for Complex I. In biology classes, you may see either name on diagrams or test questions. The term Complex I is usually used when the focus is on the whole membrane complex and its place in the chain. ### What does NADH dehydrogenase do to NADH? It oxidizes NADH to NAD+. That means NADH loses electrons, which are then moved into the electron transport chain. Regenerating NAD+ matters because other cellular pathways need it to keep running. ### How is NADH dehydrogenase different from Complex II? Complex I accepts electrons from NADH and pumps protons, while Complex II accepts electrons from FADH2 and does not pump protons. That difference changes how much each complex contributes to the proton gradient. Students often mix them up because both feed electrons into the same overall pathway. ## Related Study Guides - [7.4 Oxidative Phosphorylation](/college-bio/unit-7/4-oxidative-phosphorylation/study-guide/n5kDP1dgY0R12fMc) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/college-bio/key-terms/nadh-dehydrogenase#resource","name":"NADH Dehydrogenase | General Biology I","url":"https://fiveable.me/college-bio/key-terms/nadh-dehydrogenase","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/nadh-dehydrogenase#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:10.328Z","isPartOf":{"@type":"Collection","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/college-bio/key-terms/nadh-dehydrogenase#term","name":"NADH dehydrogenase","description":"NADH dehydrogenase is Complex I of the electron transport chain. 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