What Is Cellular Respiration?
Cellular respiration is how your body turns food and oxygen into a form of energy your cells can actually spend. It is not the same as breathing, though the two are related. It is a set of chemical steps, most of them happening inside your mitochondria, that release the energy locked in glucose and repackage it as ATP.
The short answer
Cellular respiration is the controlled breakdown of nutrients, chiefly glucose, to produce ATP, the molecule cells use as ready cash for energy. In its most common aerobic form, the overall trade is simple to state: glucose and oxygen go in, and carbon dioxide, water, and usable energy come out. What makes it elegant is that the energy is not released all at once in a burst, which would waste most of it as heat, but in a series of small, managed steps that capture as much as possible in a usable form. Breathing supplies the oxygen and carries away the carbon dioxide, but the actual energy conversion happens deep inside your cells.
The three main stages
Aerobic cellular respiration is usually described in three stages. Each one hands off products to the next, and the payoff grows as you go.
1. Glycolysis
Glycolysis happens in the cytoplasm, the fluid part of the cell outside the mitochondria, and it does not require oxygen. Here a single six-carbon glucose molecule is split into two three-carbon molecules of pyruvate. The step costs a little ATP to get started but ends with a small net gain, a net of two ATP per glucose, along with two molecules of the electron carrier NADH. Glycolysis is ancient and universal; nearly every living thing on Earth does some version of it.
2. The citric acid (Krebs) cycle
If oxygen is available, pyruvate is transported into the mitochondrion and converted into a molecule called acetyl-CoA, which feeds into the citric acid cycle, also known as the Krebs cycle. Turning through this cycle strips carbon away as carbon dioxide (the gas you exhale) and, more importantly, loads up more electron carriers, NADH and FADH2. The cycle itself produces only a small direct amount of ATP, but the electron carriers it charges up are where the real value lies. They are about to be cashed in.
3. Oxidative phosphorylation
The final and largest stage takes place on the folded inner membrane of the mitochondrion. The NADH and FADH2 built earlier deliver their electrons to the electron transport chain, which uses their energy to pump protons and build a gradient. That gradient then drives a molecular turbine called ATP synthase to manufacture the bulk of the cell's ATP, and oxygen accepts the spent electrons at the end of the line to form water. We cover this step in depth in our companion explainer on oxidative phosphorylation. This is why oxygen is essential: it is the destination for electrons, and without it the whole line grinds to a halt.
How much energy comes out?
Add the stages together and a single glucose molecule yields, by modern estimates, roughly 30 to 32 ATP under aerobic conditions. Glycolysis and the Krebs cycle contribute only a handful of ATP directly; the overwhelming majority is produced in the final oxidative phosphorylation stage. Older textbooks quoted 36 to 38 ATP, but that figure did not subtract the small energy costs of moving molecules into the mitochondria, so the lower range is now regarded as more realistic. The headline point is that most of your usable energy is made inside mitochondria, which is exactly why mitochondrial upkeep gets so much attention.
What happens without oxygen?
When oxygen is scarce, for example during a hard sprint when your muscles outpace their oxygen supply, cells fall back on anaerobic pathways. Glycolysis still runs, but the pyruvate is converted into lactate (lactic acid) instead of entering the mitochondria. This lets glycolysis keep producing its small net of two ATP per glucose without oxygen, which buys the cell time. The catch is that it is far less efficient, yielding only a tiny fraction of the ATP that full aerobic respiration provides, and it cannot be sustained for long. This is the difference between a short, breathless burst of effort and steady endurance work that your aerobic system can support for much longer.
Where mitochondrial quality fits in
Because mitochondria carry out the most productive stages of respiration, their condition influences how efficiently the whole process runs. Mitochondria take on damage over time, partly because handling high-energy electrons and oxygen produces reactive byproducts. Healthy cells manage this with quality control, tagging their most worn mitochondria for recycling through a process called mitophagy and then building fresh replacements. Our overview of mitochondrial health walks through why this housekeeping matters as the years add up.
Urolithin A is studied in this context. It is a postbiotic that gut bacteria can produce from ellagitannins found in pomegranates, walnuts, and some berries, though only certain microbiomes convert it in meaningful amounts. In humans, a first-in-human trial (Andreux and colleagues, Nature Metabolism, 2019) found urolithin A safe and well tolerated up to 1,000 mg per day and reported a mitochondrial gene-expression signature. Randomized human trials have since 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 across 500 and 1,000 mg arms (Singh and colleagues, Cell Reports Medicine, 2022). An 8-week randomized trial in resistance-trained men (Journal of the International Society of Sports Nutrition, 2024) added to this picture. The specific connection between urolithin A and mitophagy was first shown in preclinical laboratory and animal research (Ryu and colleagues, Nature Medicine, 2016) and should be understood as such. A 2024 systematic review in Ageing Research Reviews described the human evidence as still emerging.
Practical takeaways
You do not need to manage cellular respiration; your cells handle the chemistry automatically. What you can influence is the environment your mitochondria work in. Regular aerobic exercise encourages your body to build more mitochondria and improve its capacity for aerobic respiration, which is part of why endurance improves with training. Enough sleep, sufficient protein, and a varied diet supply the raw materials. Supplements like SOMA HEALTH Urolithin A Gummies are meant to complement those basics, not stand in for them. Persistent, unexplained fatigue is not something to diagnose yourself; it has many possible causes and deserves a conversation with a qualified healthcare provider.
Frequently asked questions
Is cellular respiration the same as breathing?
No, though they are connected. Breathing is the movement of air in and out of your lungs. Cellular respiration is the chemistry inside your cells that uses the oxygen you breathe in and produces the carbon dioxide you breathe out. Breathing supplies and removes the gases; respiration does the energy conversion.
What are the three stages?
Glycolysis in the cytoplasm, the citric acid (Krebs) cycle in the mitochondria, and oxidative phosphorylation on the inner mitochondrial membrane. Each stage feeds products into the next, and the last stage makes the most ATP.
How much ATP does one glucose molecule produce?
By current estimates, roughly 30 to 32 ATP under aerobic conditions. Most of that comes from the final oxidative phosphorylation stage inside the mitochondria.
What happens during exercise when oxygen runs low?
Cells rely more on anaerobic glycolysis, which converts pyruvate into lactate and still makes a small amount of ATP without oxygen. It is quick but inefficient and cannot be sustained, which is why intense bursts leave you breathless.
Why do mitochondria matter so much here?
Two of the three stages of respiration, and the great majority of ATP production, happen inside mitochondria. Their condition affects how efficiently the whole process runs, which is why mitochondrial upkeep is a common focus.
How does urolithin A relate to cellular respiration?
It does not directly drive respiration. Urolithin A has been studied for supporting mitophagy, the recycling of worn-out mitochondria, which are where most respiration takes place. The human evidence is still emerging and this is described as structure and function support, not a treatment.
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*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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