What do mitochondria do? More than the powerhouse of the cell

Mitochondria are the small compartments inside most of your cells that do the last and biggest part of breaking down food: they use oxygen to pull the remaining energy out of sugar and fat and capture it in ATP, the cell's energy carrier. They also make heat, buffer calcium, help build some molecules, and trigger a cell's death when it is damaged. They are the descendants of bacteria, and they still carry a little DNA of their own.

In short

• Mitochondria make most of the ATP in an animal cell, by pumping protons across a membrane and letting them flow back through a turbine-like enzyme.
• They have other jobs: heat in brown fat, calcium signaling, and the decision for a damaged cell to self-destruct.
• They began as free-living bacteria swallowed by another cell, and keep their own small, circular genome, inherited from the mother.

What is a mitochondrion, and where is it?

A mitochondrion (plural mitochondria) is an organelle, a working part of a cell with its own membrane. It is found in almost every eukaryotic cell, the kind with a nucleus that makes up animals, plants and fungi; bacteria do not have them. Each one is wrapped in two membranes. The outer one is smooth and leaky to small molecules. The inner one is folded into ridges called cristae, which pack more working surface into the same space, and inside it sits a dense fluid called the matrix.

How many a cell has depends on how much energy it spends. A mature red blood cell has none, which is why it can't use the oxygen it carries. A human liver cell has about 1,000 to 2,000, filling about a fifth of its volume, according to Wikipedia's summary of the research; heart muscle cells, which never stop working, are packed with them too. They are not fixed beans, as diagrams show them: in a living cell they move, fuse into networks and split apart again.

How do mitochondria make energy?

Mitochondria finish what digestion and the first steps of metabolism start. Sugar has already been split in half in the cell's fluid; fats arrive as fatty acids. Inside the matrix, enzymes take these pieces apart completely, down to carbon dioxide, which is what you breathe out. The energy is not released as a flame. It is handed to carrier molecules, NADH and FADH₂, as high-energy electrons.

The electrons then go down the electron transport chain, a row of protein complexes in the inner membrane. A useful picture is a hydroelectric dam. Each time an electron steps down to a lower-energy protein, the complex uses the drop to pump a proton, a hydrogen ion, out across the membrane, piling protons up on one side like water behind a dam. At the bottom of the chain, the electrons are handed to oxygen, which combines with protons to make water. That is the main reason you need to breathe oxygen: most of the oxygen you take in ends up as the final catcher at the end of this chain.

The protons can only flow back through one gate, an enzyme called ATP synthase. As they pass, part of it physically rotates, and as it turns it snaps phosphate onto ADP to make ATP, three molecules per full turn, as Nature Education's Scitable describes. The whole process is called oxidative phosphorylation. It makes the large majority of the roughly 30 ATP a cell gets from one glucose molecule; the count, and why textbooks disagree about it, is worked through step by step in the note on what ATP is.

The picture stops being true in one place: the dam is not perfectly sealed. Some protons leak back without passing through ATP synthase, and their energy becomes heat. That leak turns out to be one of the organelle's jobs.

What else do mitochondria do?

Making ATP is the headline, but the list of jobs is longer.

• Making heat. Brown fat, common in babies and present in smaller amounts in adults, has mitochondria fitted with a protein called UCP1, or thermogenin, that opens a deliberate leak in the dam. Protons rush back without making ATP, and the energy comes out as heat. This is how a newborn keeps warm without shivering.
• Handling calcium. Calcium ions are one of the cell's main signals: a pulse of them tells a muscle to contract or a nerve ending to release its messengers. Mitochondria take up and release calcium, shaping those pulses, and the calcium in turn speeds up their own energy-making enzymes when demand rises.
• Deciding when a cell dies. When a cell is badly damaged, the outer mitochondrial membrane becomes leaky and releases cytochrome c, a small protein normally used in the electron transport chain. Out in the cell's fluid, cytochrome c helps assemble a structure that starts apoptosis, an orderly self-destruction that keeps a damaged cell from harming its neighbors or turning cancerous.
• Building molecules. Several construction jobs begin or end inside mitochondria, including steps in making heme, the iron-holding part of hemoglobin, and in liver cells the first steps of turning ammonia, a toxic waste from protein, into urea.

Mitochondria also act as alarms for the immune system. Because they descend from bacteria, their DNA looks bacterial, and if it leaks out of a damaged cell, the body's defenses react to it as if to an infection.

Why do mitochondria have their own DNA?

Mitochondria carry their own small genome, separate from the 46 chromosomes in the nucleus. MedlinePlus, from the US National Library of Medicine, gives its size in humans as about 16,500 base pairs, holding 37 genes: 13 for parts of the energy-making machinery, and the rest for the RNA molecules needed to build those parts. That is a tiny fraction of the cell's DNA: the nuclear genome is about 3 billion base pairs, so one copy of mitochondrial DNA is roughly 0.0005% of it, though each cell holds hundreds to thousands of copies. Everything else a mitochondrion needs, hundreds of proteins, is now coded in the nucleus and imported.

The genome is there because mitochondria were once bacteria. The endosymbiotic theory, set out early in the twentieth century and argued with microbiological evidence by Lynn Margulis in 1967, holds that more than 1.5 billion years ago a larger cell engulfed an oxygen-using bacterium, and instead of digesting it, kept it. The guest got shelter and food; the host got a far better way to use oxygen. Over time the guest handed most of its genes to the host's nucleus and became an organelle.

The evidence is the kind anyone can check off.

Feature of mitochondria	What it matches
Two membranes, the inner one like a bacterial membrane	A bacterium wrapped in the membrane of the cell that swallowed it
Their own DNA, in a single circle	The circular chromosome of bacteria, unlike the nucleus's linear chromosomes
Their own ribosomes, closer to bacterial ribosomes than to the cell's own	Bacterial ribosomes, which is why some antibiotics can affect mitochondria
They divide by splitting in two, on their own schedule	Binary fission, the way bacteria reproduce
Their genes' closest relatives in DNA comparisons	The alphaproteobacteria, a group of living bacteria

Why do you inherit mitochondria from your mother?

In humans, a baby's mitochondria come from the egg. The sperm's few mitochondria enter the egg but are broken down soon after fertilization, so mitochondrial DNA passes down the maternal line almost unchanged except for occasional mutations. That makes it useful for tracing ancestry: comparing it across living people is how geneticists map the migrations of human populations, and how they identified remains such as those of Russia's last royal family.

It also matters for disease. Mutations in mitochondrial DNA, or in nuclear genes that mitochondria need, can starve the most energy-hungry tissues, the brain, muscles, heart and eyes. Newcastle University, where much of the research is done, gives the figure that around one in 5,000 children is born each year with mitochondrial DNA mutations that can cause serious disease. In 2015 the UK became the first country to legalize mitochondrial donation, an IVF technique that swaps in a donor's healthy mitochondria, and on 16 July 2025 the Newcastle team reported eight babies born that way, all with no signs of the disease, though three carried low levels of the mutation. This note explains the biology; it isn't medical advice.

Why are mitochondria called the powerhouse of the cell?

The phrase was popularized by the biologist Philip Siekevitz in a 1957 Scientific American article titled "Powerhouse of the Cell". It stuck in textbooks, and from the 2010s, as a phrase millions of students had to memorize, it became an internet joke: "mitochondria is the powerhouse of the cell" stands for the one thing everyone remembers from school biology.

The phrase is right as far as it goes. A powerhouse is where energy is converted, not made from nothing, which is exactly what mitochondria do with the energy in food. What it leaves out is that they are also a thermostat, a signal relay, a factory and a self-destruct switch, with a past as independent living things, which links them to the question of what counts as life at all.

Check yourself

• Without looking back, explain what oxygen does inside a mitochondrion, and what happens to the electrons if it is missing.
• How does brown fat turn the mitochondrion's proton leak into a way to stay warm?
• Name three pieces of evidence that mitochondria were once bacteria.

Sources

• Mitochondrial DNA — MedlinePlus, US National Library of Medicine
• Mitochondria — Nature Education, Scitable, 2014
• Mitochondrion — Wikipedia
• Symbiogenesis (endosymbiotic theory) — Wikipedia
• Eight babies born after mitochondrial donation — Newcastle University, 16 July 2025