Complex II is succinate dehydrogenase in the inner mitochondrial membrane. In General Biology I, it connects the citric acid cycle to the electron transport chain by oxidizing succinate and passing electrons to ubiquinone.
Complex II is the mitochondrial enzyme complex that carries electrons from succinate to coenzyme Q during cellular respiration. In General Biology I, you usually see it as the spot where the citric acid cycle and the electron transport chain meet.
Its other name, succinate dehydrogenase, matters because it tells you what the complex does in the citric acid cycle. It oxidizes succinate to fumarate, which is one of the steps that regenerates the cycle and produces a reduced electron carrier pathway for the next stage of respiration.
Complex II sits in the inner mitochondrial membrane, but unlike Complexes I, III, and IV, it does not pump protons into the intermembrane space. That makes it unusual. It still helps build the electron flow needed for oxidative phosphorylation, but it does so by feeding electrons into the chain rather than directly adding to the proton gradient.
The electrons leave Complex II and move to ubiquinone, also called coenzyme Q. When ubiquinone picks up those electrons, it becomes ubiquinol and can move within the membrane to deliver them to Complex III.
Complex II contains protein subunits and cofactors, including flavin and iron-sulfur centers, that pass electrons step by step. In class diagrams, this is the part of respiration that is easy to miss because it seems smaller than the proton-pumping complexes, but it is the bridge that keeps the whole process moving.
A useful way to picture it is this: the citric acid cycle loads the electrons onto Complex II, then the electron transport chain continues the route. If Complex II is blocked or damaged, succinate builds up, fumarate production drops, and the respiratory chain gets fewer electrons from that pathway.
Complex II matters because it shows how cellular respiration is not a set of separate chapters, it is one connected process. In General Biology I, you need to see how the citric acid cycle supplies electrons to oxidative phosphorylation, and Complex II is one of the clearest links between those two stages.
It also helps explain why not every complex in the electron transport chain behaves the same way. Many students assume every ETC complex pumps protons, but Complex II does not. That difference shows up in energy yield, because electrons entering through Complex II contribute less to the proton gradient than electrons that start at Complex I.
This term also gives you a better read on mitochondrial diagrams and respiration summaries. If you can identify where succinate becomes fumarate and where ubiquinone picks up electrons, you can trace the path of energy instead of memorizing isolated names.
In a broader biology class, Complex II is a good example of enzyme dual function, since one protein complex participates in both a metabolic cycle and membrane electron transfer. That kind of connection comes up again and again when you study metabolism.
Keep studying General Biology I Unit 7
Visual cheatsheet
view galleryCitric Acid Cycle
Complex II is part of the citric acid cycle as succinate dehydrogenase. It catalyzes the conversion of succinate to fumarate, so it sits right inside the cycle instead of only acting in the membrane chain. That makes it one of the clearest links between carbon metabolism and electron transfer.
Electron Transport Chain
Complex II feeds electrons into the electron transport chain by reducing ubiquinone. It is part of the chain, but it does not pump protons the way some other complexes do. If you are tracing electron flow, Complex II is one of the entry points into the membrane pathway.
Complex I
Complex I and Complex II both pass electrons to ubiquinone, but they start with different electron donors. Complex I receives electrons from NADH and pumps protons, while Complex II receives electrons from succinate and does not pump protons. That difference affects how much ATP the cell can make.
Complex III
Complex III is the next major stop after ubiquinol leaves Complex II. Once ubiquinone has been reduced to ubiquinol, it carries those electrons through the membrane to Complex III, where more proton pumping helps strengthen the gradient used for ATP synthesis.
ATP Synthase
Complex II does not make ATP directly, but the electrons it feeds into the chain help maintain the proton gradient that drives ATP synthase. If you are looking at the whole pathway, Complex II contributes upstream to the proton motive force that ATP synthase uses to build ATP.
A quiz question may ask you to label Complex II on a mitochondrion diagram, identify its substrate and product, or decide whether it pumps protons. You may also be asked to trace what happens when succinate is oxidized, or to compare the ATP yield from electrons that enter through Complex II versus Complex I. In lab or worksheet problems, this term often shows up in pathway charts, membrane diagrams, or questions about what happens to electron flow if a respiratory complex is inhibited. If you can say, “Complex II is succinate dehydrogenase and transfers electrons from succinate to ubiquinone without pumping protons,” you can handle most of those prompts.
Complex II is often confused with Complex I because both feed electrons into the electron transport chain and both connect to ubiquinone. The difference is the electron source and the proton pumping. Complex I takes electrons from NADH and pumps protons, while Complex II takes electrons from succinate and does not pump protons.
Complex II is succinate dehydrogenase in the inner mitochondrial membrane.
It oxidizes succinate to fumarate and passes the electrons to ubiquinone.
Unlike Complexes I, III, and IV, Complex II does not pump protons across the membrane.
It connects the citric acid cycle to the electron transport chain, so it links two major parts of cellular respiration.
If Complex II is damaged or blocked, the cell loses one route for feeding electrons into oxidative phosphorylation.
Complex II is succinate dehydrogenase, a membrane enzyme in the inner mitochondrial membrane. It takes electrons from succinate, turns it into fumarate, and transfers those electrons to ubiquinone in the electron transport chain.
No. Complex II is the odd one out in the electron transport chain because it does not pump protons across the inner mitochondrial membrane. It still helps build the proton gradient indirectly by feeding electrons into the chain.
Both complexes pass electrons to ubiquinone, but they do not start from the same place. Complex I gets electrons from NADH and pumps protons, while Complex II gets electrons from succinate and does not pump protons.
That name comes from the reaction it catalyzes in the citric acid cycle. It removes electrons from succinate, converting it to fumarate, so the enzyme has a metabolic role in addition to its role in oxidative phosphorylation.