What Is Oxidative Phosphorylation?
Oxidative phosphorylation is the last and largest energy-making step of cellular respiration. It is where the food you eat and the oxygen you breathe are finally converted into ATP, the small molecule your cells spend to do almost everything. It happens inside your mitochondria, and understanding it makes the idea of mitochondrial health far less abstract.
The short answer
Oxidative phosphorylation (often shortened to OXPHOS) is the process by which mitochondria use the energy stored in electrons to build ATP. Two things happen at once. First, electrons are passed step by step down a series of protein complexes called the electron transport chain. Second, the energy released along that chain is used to pump protons (positively charged hydrogen ions) across a membrane, creating a kind of chemical battery. The cell then lets those protons flow back through a molecular turbine called ATP synthase, and that flow drives the assembly of ATP. The word itself is a tidy summary: oxidative because oxygen is the final electron acceptor, and phosphorylation because the end result is adding a phosphate group to ADP to make ATP.
Where it happens: the mitochondrial inner membrane
Every step of oxidative phosphorylation is anchored in the inner membrane of the mitochondrion. That membrane is not smooth. It is folded into deep pleats called cristae, and those folds dramatically increase the surface area available to hold the machinery. A single muscle or heart cell, which has heavy energy demands, can contain thousands of mitochondria, each packed with these folded membranes. The more cristae surface a cell maintains, the more OXPHOS machinery it can run in parallel.
This is also why mitochondrial structure and mitochondrial function are so tightly linked. When cristae become disorganized or the membrane loses its integrity, the process becomes less efficient, and more electrons can leak out before they reach the end of the chain.
The electron transport chain
The electron transport chain is a relay of four main protein complexes, numbered I through IV, plus two mobile carriers that ferry electrons between them. The electrons themselves arrive on two carrier molecules, NADH and FADH2, that were produced earlier in cellular respiration during glycolysis and the citric acid (Krebs) cycle. As electrons move from one complex to the next, they drop to progressively lower energy states, releasing energy at each hand-off. Complexes I, III, and IV use that released energy to pump protons from the inner compartment (the matrix) into the narrow space between the inner and outer membranes. Oxygen sits at the very end of the line at Complex IV, accepting the spent electrons and combining with protons to form water. This is the precise reason you must keep breathing: without oxygen to catch the electrons, the entire chain backs up and stops.
Chemiosmosis and ATP synthase
All that proton pumping builds a steep concentration gradient, more protons on one side of the membrane than the other. Stored energy in a gradient like this is called the proton-motive force, and using it to make ATP is called chemiosmosis. The protons want to flow back toward the side with fewer of them, but the membrane blocks them everywhere except through one special channel: ATP synthase. As protons rush through, part of this enzyme physically rotates, like water turning a mill wheel, and that mechanical spin snaps a phosphate onto ADP to produce ATP. It is one of the few places in biology where you can genuinely describe a molecular machine as a spinning motor.
How much energy does it make?
Oxidative phosphorylation is by far the biggest ATP producer in the cell. Glycolysis and the Krebs cycle each yield only a small direct amount of ATP; the overwhelming majority comes from OXPHOS. Modern estimates put the total net yield from one glucose molecule at roughly 30 to 32 ATP, and the great bulk of that is generated here at the inner membrane. Older textbooks often cited a higher figure of 36 to 38, but those numbers did not account for the small energy costs of shuttling molecules into the mitochondria, so the lower range is now considered more accurate. Either way, the takeaway is the same: if oxidative phosphorylation falters, the cell's energy budget shrinks sharply.
Why oxidative phosphorylation can slow over time
Mitochondria take wear and tear. Because they handle high-energy electrons and oxygen, a small fraction of electrons inevitably escape and form reactive oxygen species, which can damage the very membranes and proteins the process depends on. Over years, some mitochondria accumulate enough damage that they run less cleanly and leak more. Healthy cells counter this with quality control: they tag and recycle their most worn-out mitochondria through a process called mitophagy, then build fresh ones. You can read more about that recycling step in our explainer on what mitophagy is, and about the bigger picture in our overview of mitochondrial health.
This is where urolithin A enters the conversation. Urolithin A is a postbiotic your gut bacteria can make from ellagitannins, the compounds in pomegranates, walnuts, and certain berries. Notably, only some people carry the microbes needed to produce meaningful amounts, which is part of why supplementing it has been studied. In humans, urolithin A has been evaluated for safety and biological activity: a first-in-human trial (Andreux and colleagues, Nature Metabolism, 2019) found it was safe and well tolerated at doses up to 1,000 mg per day and shifted mitochondrial gene-expression signatures. Later randomized human trials reported effects on muscle endurance in older adults (D'Amico and colleagues, JAMA Network Open, 2022) and on muscle strength and exercise performance in middle-aged adults (Singh and colleagues, Cell Reports Medicine, 2022). A 2025 randomized trial in Nature Aging reported support for immune-cell mitochondrial health in midlife adults. The mechanistic link between urolithin A and mitophagy was first established in preclinical work (Ryu and colleagues, Nature Medicine, 2016), which is laboratory and animal research and should be read as such. A 2024 systematic review in Ageing Research Reviews concluded the human evidence is still emerging.
Supporting your mitochondria in daily life
Oxidative phosphorylation is not something you consciously control, but the general conditions your mitochondria operate under are influenced by everyday habits. Regular aerobic and resistance exercise is one of the most reliable ways to prompt the body to build more mitochondria and maintain cristae. Adequate sleep, a diet with enough protein and micronutrients, and not smoking all support the broader system. Supplements such as SOMA HEALTH Urolithin A Gummies are intended to complement, not replace, those fundamentals. If you have persistent fatigue, breathlessness, or muscle weakness, those can have many causes and are worth discussing with a qualified healthcare provider rather than self-treating.
Frequently asked questions
Is oxidative phosphorylation the same as the electron transport chain?
Not quite. The electron transport chain is one half of oxidative phosphorylation, the part that moves electrons and pumps protons. Oxidative phosphorylation includes both that chain and the final ATP-building step performed by ATP synthase. The two are coupled: the chain builds the proton gradient, and ATP synthase spends it.
Why do we need oxygen for it?
Oxygen is the final electron acceptor at the end of the chain. Without it, electrons have nowhere to go, the chain backs up, and ATP production through this route stops. That is why cells switch to less efficient, oxygen-free pathways when oxygen runs short.
How much ATP does oxidative phosphorylation actually produce?
The great majority of the roughly 30 to 32 ATP generated from a single glucose molecule comes from oxidative phosphorylation. Glycolysis and the Krebs cycle contribute only a small direct amount each.
What does urolithin A have to do with it?
Urolithin A does not directly power oxidative phosphorylation. It has been studied for its role in supporting mitophagy, the cell's routine for recycling worn-out mitochondria, which are the structures where oxidative phosphorylation takes place. The human evidence is still emerging and is described as structure and function support, not a treatment.
Can lifestyle affect this process?
Indirectly, yes. Exercise, sleep, and a balanced diet influence how many mitochondria your cells maintain and how well their membranes are organized. You cannot consciously speed up the chemistry, but you can support the overall system.
Where exactly does oxidative phosphorylation happen?
On the inner mitochondrial membrane, which is folded into structures called cristae. Those folds pack in more of the protein machinery, which is why energy-hungry tissues like heart and muscle have so many of them.
Explore SOMA HEALTH Urolithin A Gummies
For a fuller picture of this postbiotic, see our complete urolithin A guide, our roundup of the best urolithin A gummies, and our note on urolithin A and energy.
*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.
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