Complex III, or cytochrome bc1 complex, is a membrane protein complex in the electron transport chain that passes electrons from ubiquinol to cytochrome c while helping build the proton gradient in mitochondria.
Complex III is the cytochrome bc1 complex in the inner mitochondrial membrane, and in Cell Biology it is one of the main electron-transfer stations of oxidative phosphorylation. It takes electrons from ubiquinol, also called QH2, and hands them off to cytochrome c. At the same time, it helps move protons into the intermembrane space, which adds to the gradient that ATP synthase later uses.
The step matters because the electron transport chain is not just about moving electrons around. It is also about converting the energy released by those transfers into a proton motive force. Complex III sits in the middle of that chain, between the earlier complexes that load up electron carriers and the later complex that passes electrons to oxygen.
A useful way to picture it is as a relay with two jobs. First, it accepts electrons from reduced coenzyme Q in the membrane. Then it passes those electrons one at a time to cytochrome c, a small mobile protein that floats along the outer surface of the inner mitochondrial membrane and carries electrons to Complex IV.
Complex III is also tied to the Q cycle, the mechanism that makes its proton-moving effect possible. Two electrons from QH2 do not travel as a simple pair to cytochrome c. Instead, the complex splits their path so electrons move through different redox centers, and that process helps release protons to the intermembrane space while regenerating oxidized and reduced forms of coenzyme Q.
The subunits matter too. Cytochrome b, cytochrome c1, and an iron-sulfur protein each handle part of the electron flow. You do not usually need to memorize every electron carrier for a basic class question, but you do need to know that this is a membrane-bound enzyme complex with internal redox centers, not a free-floating molecule like NADH or FADH2.
If Complex III is blocked, electrons back up in the chain, proton pumping drops, and ATP production falls. Because electrons can leak to oxygen when the chain gets jammed, inhibition can also increase reactive oxygen species. That is why this step shows up as both an energy-production checkpoint and a source of cell stress when it fails.
Complex III matters because it connects electron transfer to ATP production in a very direct way. In Cell Biology, you are often tracing how energy from food becomes usable cellular energy, and this complex is one of the main places where that conversion gets tied to a membrane gradient.
It also helps you understand the logic of the whole electron transport chain. Complex I, Complex II, and other carriers load electrons into the chain, but Complex III is where those electrons are passed to cytochrome c so the pathway can keep moving toward Complex IV and oxygen. Without that handoff, the later steps of oxidative phosphorylation stall.
This term also shows up when you explain why membranes matter. The inner mitochondrial membrane is not just a barrier, it is the surface that lets proton movement be controlled and turned into work. Complex III helps create the difference in proton concentration and charge that ATP synthase uses.
On top of energy production, the complex is a good checkpoint for interpreting what happens when respiration is disrupted. If a toxin, mutation, or experimental inhibitor affects Complex III, you can predict lower ATP output, slower electron flow, and more electron leakage. That makes it useful for linking mechanism to cell damage, metabolism questions, and lab-style reasoning about mitochondrial function.
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Visual cheatsheet
view galleryElectron Transport Chain
Complex III is one station in the electron transport chain, where electrons move step by step through membrane proteins. It sits between earlier carriers that feed electrons into the chain and later carriers that deliver them to oxygen. If you know the chain layout, Complex III makes more sense as a transfer point, not an isolated enzyme.
Oxidative Phosphorylation
Complex III is part of oxidative phosphorylation because it helps convert redox energy into a proton gradient. The electrons themselves do not make ATP directly. Instead, the movement of electrons through Complex III contributes to the proton motive force that powers ATP synthase.
Proton Motive Force
The proton motive force is the stored energy created by an uneven distribution of protons across the inner mitochondrial membrane. Complex III helps build that force by moving protons to the intermembrane space as electrons pass through. That gradient is what later drives ATP production.
cytochrome c
Cytochrome c is the mobile electron carrier that receives electrons from Complex III and delivers them to Complex IV. It is small, soluble, and moves along the membrane surface, which makes it the shuttle that keeps the chain flowing. If you mix these up, remember that Complex III is the membrane complex and cytochrome c is the carrier.
A quiz or lab question usually asks you to trace what happens to electrons and protons at Complex III. You might label it on a mitochondrion diagram, explain why cytochrome c is reduced there, or predict the effect of blocking the complex on ATP output.
For problem-style questions, follow the sequence: ubiquinol enters, electrons move through Complex III, cytochrome c picks them up, and protons accumulate in the intermembrane space. If a prompt mentions a respiratory inhibitor or a drop in ATP, Complex III is one of the first places to check.
In written responses, use the mechanism, not just the name. Say that Complex III transfers electrons from QH2 to cytochrome c and contributes to the proton gradient that powers ATP synthase. That shows you know how the step fits into respiration instead of just identifying a label.
These are easy to mix up because both are in the same part of the electron transport chain. Complex III is a large membrane protein complex that passes electrons and helps move protons, while cytochrome c is a small mobile protein that carries electrons from Complex III to Complex IV. One is the station, the other is the shuttle.
Complex III is the cytochrome bc1 complex in the inner mitochondrial membrane, and it passes electrons from ubiquinol to cytochrome c.
Its job is not only electron transfer, but also helping build the proton gradient that drives ATP synthase.
The Q cycle lets Complex III move protons across the membrane while handing electrons off one at a time.
Cytochrome c is the next carrier after Complex III, so this step connects the middle of the electron transport chain to Complex IV.
If Complex III is inhibited, ATP production drops and electron leakage can increase reactive oxygen species.
Complex III is a protein complex in the inner mitochondrial membrane that moves electrons from ubiquinol to cytochrome c. It also helps build the proton gradient used for ATP production in oxidative phosphorylation.
Complex III is a large membrane-bound enzyme complex, while cytochrome c is a small mobile electron carrier. Complex III passes electrons along and contributes to proton pumping, and cytochrome c ferries those electrons to Complex IV.
Complex III helps move protons into the intermembrane space as electrons are transferred through the complex. That adds to the proton motive force across the inner mitochondrial membrane, which ATP synthase later uses to make ATP.
Electron flow through the chain slows down, so the proton gradient weakens and ATP output drops. Blockage can also increase reactive oxygen species because electrons may leak when the chain is backed up.