NADH is the reduced form of the coenzyme NAD+, an electron carrier that picks up high-energy electrons during glycolysis and the Krebs cycle and delivers them to the electron transport chain to drive ATP production.
NADH is a coenzyme that acts as an electron shuttle. Think of it as a rechargeable battery for electrons. When NAD+ grabs two electrons (and a proton), it becomes NADH, the "loaded" form carrying energy. When NADH drops those electrons off, it turns back into NAD+, ready to be reused.
This loading and unloading is a redox reaction. NAD+ getting reduced to NADH happens during the early, electron-harvesting steps of cellular respiration: glycolysis, the conversion of pyruvate to acetyl-CoA, and the Krebs cycle. NADH then hands those electrons off to the electron transport chain, where the energy stored in them is used to pump protons and ultimately make ATP. So NADH doesn't make much ATP directly; it's the delivery truck that brings the fuel to where ATP actually gets built.
NADH lives in Topic 3.6, Cellular Respiration, and it ties together every stage of the process. Glycolysis, pyruvate oxidation, and the Krebs cycle all produce NADH; oxidative phosphorylation spends it. Without NADH connecting those phases, the electron transport chain has nothing to work with.
The College Board wants you to trace energy through a pathway and explain what happens when a step is blocked. NADH is the through-line that makes that possible. It also shows up in fermentation, because the whole point of fermentation is to regenerate NAD+ from NADH so glycolysis can keep running without oxygen. That cross-pathway role is exactly what exam questions probe.
Keep studying AP Biology Unit 3
NAD+ and Redox Reactions (Unit 3)
NADH and NAD+ are the same molecule in two states. NAD+ is the empty carrier, NADH is the loaded one. Reduction (gaining electrons) turns NAD+ into NADH; oxidation (losing them) turns it back. If you understand this one redox flip, you understand why NADH is central to respiration.
Electron Transport Chain and Oxidative Phosphorylation (Unit 3)
NADH is where the ETC gets its electrons. As electrons pass down the chain, protons get pumped across the inner mitochondrial membrane, building the gradient that ATP synthase uses. No NADH delivery means no electron flow, no proton gradient, and almost no ATP.
Acetyl-CoA and the Krebs Cycle (Unit 3)
The conversion of pyruvate to acetyl-CoA produces NADH, and the Krebs cycle pumps out even more. A 2019 FRQ on the pyruvate dehydrogenase complex hinges on this step, so knowing that this stage loads NADH lets you reason about what fails downstream.
Fermentation and Anaerobic Respiration (Unit 3)
When oxygen is gone, the ETC can't accept electrons, so NADH piles up and NAD+ runs out. Fermentation solves this by oxidizing NADH back to NAD+, letting glycolysis continue. The whole reason fermentation exists is to keep the NAD+ supply flowing.
MCQs love to block the NADH-to-ETC handoff and ask what happens next. One practice stem inhibits the transfer of electrons from NADH to the electron transport chain and asks how ATP production changes (it crashes, because the proton gradient can't form). Others ask which substance to add to a stalled mitochondria experiment, where NADH or its precursors restore electron flow. On FRQs like the 2019 question on the pyruvate dehydrogenase complex, you may need to explain that a blocked step means less NADH, which means a weaker proton gradient and less ATP. The skill being tested is tracing cause and effect through the pathway, so always be ready to say where NADH is made and where it gets spent.
NADH and NADPH are close cousins but work in opposite directions. NADH is made in cellular respiration to break glucose down and is mostly spent powering the ETC. NADPH is made in photosynthesis (and some anabolic pathways) and is used to build molecules, like fixing carbon in the Calvin cycle. NADH unloads energy; NADPH delivers it for construction.
NADH is the reduced, electron-loaded form of NAD+, and the two interconvert through redox reactions throughout cellular respiration.
NADH is produced in glycolysis, pyruvate oxidation, and the Krebs cycle, then spent at the electron transport chain.
NADH does not make ATP directly; it delivers electrons to the ETC, which builds the proton gradient that ATP synthase uses.
If electron transfer from NADH to the ETC is blocked, the proton gradient collapses and ATP production drops sharply.
Fermentation exists to oxidize NADH back into NAD+ so glycolysis can keep running when oxygen is unavailable.
Don't confuse NADH (respiration, energy release) with NADPH (photosynthesis and biosynthesis, energy delivery for building).
NADH is a coenzyme and electron carrier, the reduced form of NAD+. It picks up high-energy electrons during glycolysis and the Krebs cycle and delivers them to the electron transport chain, where the energy is used to make ATP.
No. NADH carries electrons to the electron transport chain, and it's the ETC and ATP synthase that actually produce ATP. NADH is the delivery truck, not the factory, so it indirectly powers most of the cell's ATP.
They're the same molecule in two states. NAD+ is the empty carrier; NADH is NAD+ after it gains two electrons and a proton (gets reduced). NADH unloads those electrons to become NAD+ again, so it cycles back and forth constantly.
NADH is made during cellular respiration and is mostly used to power the electron transport chain, releasing energy. NADPH is made during photosynthesis and is used to build molecules, like in the Calvin cycle. NADH releases energy; NADPH supplies it for construction.
Fermentation regenerates NAD+ by oxidizing NADH. Without oxygen, the electron transport chain can't accept electrons, so NADH builds up and NAD+ runs out. Fermentation dumps those electrons elsewhere to restock NAD+, keeping glycolysis going.