Complex III is the cytochrome bc1 complex in the mitochondrial electron transport chain. It passes electrons from ubiquinol to cytochrome c while helping build the proton gradient used for ATP synthesis.
Complex III is the third major electron-transfer complex in the inner mitochondrial membrane of Biological Chemistry II, and it is usually called the cytochrome bc1 complex. Its job is to take electrons from ubiquinol, also written QH2, and pass them to cytochrome c.
That transfer is not just a handoff. As electrons move through Complex III, the complex helps move protons from the mitochondrial matrix into the intermembrane space. That proton movement adds to the proton gradient that powers ATP synthase later in oxidative phosphorylation.
The mechanism students usually need to know is the Q cycle. In simple terms, the two electrons carried by one ubiquinol do not go to the same place in the same way. One electron takes a path that eventually reduces cytochrome c, while the other goes through a different set of redox carriers and helps regenerate ubiquinone. That split path is what makes Complex III efficient at coupling electron flow to proton translocation.
Complex III contains redox-active parts such as heme groups and an iron-sulfur cluster. Those cofactors are not decoration, they are the steps in the electron-transfer path. The electrons move through these carriers in a controlled sequence, which lets the complex manage energy release instead of dumping it all at once as heat.
This complex sits between the earlier carriers in the ETC and Complex IV downstream. Electrons can come into the ubiquinone pool from Complex I, Complex II, and other metabolic routes, then Complex III pushes them onward toward oxygen reduction at Complex IV. So if you are tracing respiration, Complex III is one of the main checkpoints where electron flow is converted into a usable proton gradient.
A common mistake is to treat Complex III as only a transporter. In this course, you usually need to think of it as a coupled machine: electron transfer and proton movement happen together, and that coupling is what links substrate oxidation to ATP production.
Complex III matters because it sits at a major energy-conversion step in oxidative phosphorylation. If you can follow what happens here, you can explain why NADH and FADH2 ultimately lead to ATP production instead of just passing electrons around.
It also gives you a clean place to connect several ideas from Biological Chemistry II: redox chemistry, membrane transport, the proton motive force, and enzyme regulation. When a problem asks why the inner mitochondrial membrane needs a chain of carriers instead of one direct reaction, Complex III is part of the answer. The Q cycle shows how biology squeezes more useful work out of electron transfer.
This term also comes up when you analyze inhibitors or metabolic disruption. If Complex III is blocked, electrons back up, the proton gradient drops, and ATP output falls. That effect helps explain why mitochondrial defects can change cellular energy balance and even increase reactive oxygen species. In other words, this is one of the places where energy metabolism and cell stress meet.
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Visual cheatsheet
view galleryUbiquinone
Ubiquinone is the mobile lipid carrier that brings electrons to Complex III as ubiquinol, its reduced form. Complex III does not work in isolation, because it depends on the ubiquinone pool to collect electrons from upstream reactions and distribute them into the chain. When you trace electron flow, ubiquinone is the shuttle between membrane complexes.
Cytochrome c
Cytochrome c is the small soluble carrier that receives electrons from Complex III and delivers them to Complex IV. Unlike ubiquinone, it moves in the intermembrane space rather than inside the membrane. This makes it a good comparison point for understanding how electrons move through both membrane-bound and mobile carriers in the ETC.
Proton Motive Force
Complex III contributes to the proton motive force by helping move protons to the intermembrane space as electrons pass through. That gradient is the stored energy that ATP synthase uses to make ATP. If you are asked to connect electron transport to chemiosmosis, the proton motive force is the bridge concept.
Complex IV
Complex IV is the next major stop after Complex III, where electrons are ultimately passed to oxygen. Complex III feeds reducing power into the final electron acceptor step, so it helps determine whether the chain keeps moving smoothly. Thinking about both complexes together helps you trace the full path from ubiquinol to water.
A quiz question may ask you to identify what Complex III does in the electron transport chain, trace where its electrons come from, or explain how it supports ATP synthesis. In a diagram, you should be able to point to the inner mitochondrial membrane, show the transfer from ubiquinol to cytochrome c, and connect that transfer to the proton gradient. If a problem includes an inhibitor like antimycin A, the move is to predict slower electron flow, less proton pumping overall, and reduced ATP production. In essay or short-answer work, the strongest response links structure to function: redox cofactors enable the Q cycle, the Q cycle helps move protons, and the proton gradient powers ATP synthase downstream.
Complex II also feeds electrons into the ubiquinone pool, so it is easy to mix it up with Complex III. The difference is that Complex II transfers electrons to ubiquinone but does not pump protons, while Complex III takes electrons from ubiquinol and contributes to proton translocation. If a question asks which step directly builds the gradient more, Complex III is the better answer.
Complex III is the cytochrome bc1 complex in the inner mitochondrial membrane, and it passes electrons from ubiquinol to cytochrome c.
Its Q cycle lets one pair of electrons be handled in a way that supports proton movement into the intermembrane space.
Complex III helps build the proton motive force that later drives ATP synthase during oxidative phosphorylation.
The complex uses redox cofactors such as heme groups and an iron-sulfur cluster to move electrons in an ordered path.
If Complex III is inhibited, electron flow slows, ATP production drops, and reactive oxygen species can increase.
Complex III is the cytochrome bc1 complex of the mitochondrial electron transport chain. It moves electrons from ubiquinol to cytochrome c and helps create the proton gradient used for ATP synthesis. In this course, it is one of the main links between redox reactions and chemiosmosis.
The Q cycle splits the two electrons from ubiquinol into different paths. One electron is passed along to cytochrome c, while the other goes through a second route that helps regenerate ubiquinone and supports proton translocation. That split is why Complex III is so efficient at coupling electron flow to gradient formation.
Complex II transfers electrons into the ubiquinone pool, but it does not pump protons. Complex III takes electrons from ubiquinol and contributes to proton movement across the inner mitochondrial membrane. If you are tracing energy capture, Complex III is the step more directly tied to building the gradient.
If Complex III is inhibited, electrons cannot move forward efficiently through the ETC. That reduces proton gradient formation, lowers ATP production, and can increase electron leakage that leads to reactive oxygen species. In problem sets, this often shows up as an energy failure or backup in the chain.