What Is the Electron Transport Chain?
The electron transport chain is arguably the most important molecular machine you have never seen. Tucked inside your mitochondria, it produces the great majority of the ATP that powers your body. Understanding how it works reveals exactly why mitochondrial quality, and compounds like urolithin A that support it, have become such a focus of research.
The final step in turning food into energy
When you eat, your cells break food down through several stages to extract usable energy. The earlier stages, glycolysis and the citric acid cycle, do something clever but incomplete: rather than releasing all the energy at once, they carefully strip high-energy electrons from your food and load them onto two carrier molecules, NADH and FADH2. Those loaded carriers are like charged batteries. The electron transport chain is where that stored charge is finally converted into ATP.
The chain sits in the inner membrane of the mitochondrion, which is folded into deep pleats called cristae to pack in as much surface area, and therefore as much machinery, as possible. It consists of a series of large protein complexes, conventionally numbered Complex I through Complex IV, plus the ATP-making enzyme, ATP synthase. Electrons enter at one end and are passed from complex to complex like a bucket brigade, losing a little energy at each handoff.
Walking through the complexes
Complexes I and II: the entry points
NADH delivers its electrons to Complex I, and FADH2 delivers its electrons to Complex II. Both complexes pass those electrons onward, but there is a meaningful difference: Complex I uses the energy of electron transfer to pump protons across the membrane, while Complex II does not. This is a small detail with real consequences for how much ATP each electron carrier ultimately yields.
The mobile carriers and Complex III
Between the fixed complexes, two small mobile shuttles ferry electrons along. Coenzyme Q10 (ubiquinone) carries electrons from Complexes I and II to Complex III, and a small protein called cytochrome c carries them from Complex III to Complex IV. If the name coenzyme Q10 sounds familiar, it is because it is the same molecule sold as a supplement, precisely because of this central role. Complex III pumps additional protons as it passes electrons along.
Complex IV: where oxygen comes in
At the end of the line sits Complex IV, and here the breath you just took finally does its job. Complex IV hands the spent electrons to oxygen, the final electron acceptor, which combines with protons to form water. This is the fundamental reason you need to breathe: without oxygen waiting at the end of the chain, electrons have nowhere to go, the entire chain backs up, and ATP production by this route grinds to a halt. Complex IV also pumps protons.
The proton gradient and ATP synthase
All that proton pumping has a purpose. By moving protons from the inner compartment to the space between the mitochondrial membranes, Complexes I, III, and IV build up a difference in concentration and charge across the inner membrane. This stored potential is sometimes called the proton-motive force, and it is essentially a molecular battery.
The payoff comes from ATP synthase, one of the most remarkable enzymes in biology. It provides a channel through which the accumulated protons can flow back down their gradient, and as they rush through, they physically spin part of the enzyme like a rotary turbine. That mechanical rotation drives the chemical joining of ADP and phosphate into ATP. This entire oxygen-dependent process, the electron transport chain plus ATP synthase, is called oxidative phosphorylation, and it produces the overwhelming majority of the ATP your cells run on. Our companion page on ATP covers what the cell then does with all that energy.
Why the chain is a double-edged sword
The electron transport chain is extraordinarily efficient, but it is not perfect. Occasionally an electron escapes prematurely and reacts with oxygen to form a reactive oxygen species rather than being safely passed down the line. In a healthy, well-maintained mitochondrion this leakage is modest and well controlled. But as mitochondria age or sustain damage, the chain can become leakier, producing more reactive by-products while making ATP less efficiently. Our overview of antioxidants explains how cells keep that reactive chemistry in balance.
This is the crux of why the mitochondrial population matters, not just in number but in quality. A cell full of leaky, underperforming mitochondria is worse off than a cell with fewer but healthier ones. That insight is what makes cellular housekeeping so relevant.
Mitophagy: keeping the machinery in repair
Cells have a dedicated quality-control process, called mitophagy, for exactly this problem. When a mitochondrion becomes too damaged or its electron transport chain too dysfunctional, the cell tags it for removal, breaks it down, and recycles its components so that fresher, better-functioning mitochondria can take its place. Efficient mitophagy is one of the ways a cell keeps its energy machinery, the electron transport chain included, in good working order over time. Our detailed mitophagy guide unpacks how this recycling is triggered and carried out.
Where urolithin A fits
Urolithin A is of interest here precisely because of its studied relationship to mitophagy. It is a gut postbiotic: a compound your bacteria produce when they break down ellagitannins from pomegranates, walnuts, and certain berries. Because only some people's microbiomes convert it efficiently, taking it directly has become an area of active study.
We should describe the evidence carefully and label it honestly. Its most-studied action is support for mitophagy. In humans, the first-in-human trial by Andreux and colleagues (Nature Metabolism, 2019) found urolithin A safe and well tolerated up to 1,000 mg per day, with changes in mitochondrial gene-expression signatures. Randomized human trials have reported muscle-related effects: Liu and D'Amico (JAMA Network Open, 2022) in older adults on muscle endurance, and Singh (Cell Reports Medicine, 2022), testing 500 mg and 1,000 mg in middle-aged adults, on muscle strength and exercise performance. A 2025 randomized trial in Nature Aging reported support for immune-cell mitochondrial health in midlife adults. These are structure and function findings; none of them claim that urolithin A repairs your electron transport chain or treats disease. The foundational demonstration that urolithin A enhances mitophagy, along with its effects on muscle, comes from preclinical research in roundworms and rodents (Ryu, Nature Medicine, 2016) and should be read as early mechanistic science. A 2024 systematic review in Ageing Research Reviews characterizes the human aging evidence as still emerging.
In other words: the electron transport chain makes your ATP, its performance depends on healthy mitochondria, healthy mitochondria depend in part on recycling the worn-out ones, and urolithin A is studied for supporting that recycling step. It is a coherent, mechanism-based rationale expressed in structure and function terms, not a claim about fixing the machinery itself.
Practical support for your energy machinery
The habits that keep the electron transport chain and the mitochondria around it in good shape are the familiar ones, and they are backed by strong human evidence. Endurance and interval exercise are the most potent known stimulus for building more and healthier mitochondria. A varied, nutrient-dense diet supplies the cofactors, including B vitamins and minerals, that the complexes depend on to function. Good sleep supports repair. Supplements such as urolithin A are best thought of as sitting on top of that foundation, and our page on mitochondrial health ties these threads together. For the full picture on the compound, begin with the urolithin A guide, and if you are weighing options, compare formats in our best urolithin A gummies roundup.
A necessary note: this page explains normal cell biology and general wellbeing, not the treatment of any condition. Genuine metabolic or mitochondrial disorders are medical matters. If you have concerning or persistent symptoms, please see your doctor rather than rely on a supplement.
Frequently asked questions
What is the electron transport chain in simple terms?
It is a series of protein complexes in the inner membrane of your mitochondria that passes high-energy electrons from your food down a line, using the energy released to build a proton gradient. That gradient then powers ATP synthase to make ATP. It produces most of the energy your cells use.
Why do we need oxygen for it to work?
Oxygen is the final electron acceptor at Complex IV. It picks up the spent electrons at the end of the chain and forms water. Without oxygen waiting there, electrons have nowhere to go, the chain backs up, and this route of ATP production stops. That is the fundamental reason you breathe.
How is the electron transport chain related to ATP?
It does not make ATP directly. Instead, its complexes pump protons to build a gradient across the membrane. The enzyme ATP synthase then lets those protons flow back through, spinning like a turbine to forge ATP. Together this is called oxidative phosphorylation.
Is coenzyme Q10 part of the electron transport chain?
Yes. Coenzyme Q10, also called ubiquinone, is a mobile carrier that shuttles electrons from Complexes I and II to Complex III. That central role is why it is also sold as a dietary supplement, though supplementing it is a separate topic from how the chain functions.
Can the electron transport chain cause oxidative stress?
It can contribute to it. Occasionally electrons leak out early and form reactive oxygen species instead of moving down the chain. In healthy mitochondria this is modest and controlled, but damaged or aging mitochondria tend to leak more while producing ATP less efficiently.
How does urolithin A relate to the electron transport chain?
Only indirectly. Urolithin A is studied for supporting mitophagy, the recycling of worn-out mitochondria, which helps keep the overall mitochondrial population, and the electron transport chains within them, in better repair. We do not claim it repairs the chain itself; human trials report structure and function effects on muscle and mitochondrial biology.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This guide is educational and is not a substitute for advice from a qualified healthcare provider.
A clinically studied 1,000 mg dose of Urolithin A in one simple daily gummy — to support mitophagy, muscle strength, and cellular energy.
Shop SOMA Urolithin A Gummies →