Mitochondria
Tiny former bacteria living inside nearly every cell you own, quietly paying the electric bill.
By Greg Goodwin · Published October 24, 2026
Cheat Sheet
- Own DNA: human mitochondrial DNA is a small circular genome of about 16,569 base pairs.
- Gene count: it holds 37 genes, of which 13 code for proteins used in oxidative phosphorylation and the rest for RNAs.
- Inheritance: mitochondrial DNA is typically passed down from the mother.
- Structure: two membranes, with the inner one folded into cristae to increase surface area.
- Product: the main output is ATP, the cell's usable energy currency, made by oxidative phosphorylation using oxygen.
- Theory of origin: the endosymbiotic theory, championed by Lynn Margulis in 1967, says mitochondria descend from engulfed bacteria.
The 60-Second Version
Mitochondria are small, bean-shaped compartments inside most of your cells, and the "powerhouse of the cell" line is a cliche that happens to be accurate. They take in nutrients derived from food and oxygen and use them to make ATP, the molecule cells spend as energy. Muscle, heart, brain and liver cells contain especially large numbers because they demand a lot of power. Unusually, mitochondria carry their own small genome, separate from the DNA in the nucleus, and they copy themselves independently. Scientists think they descend from free-living bacteria that were engulfed by an ancestral cell more than a billion years ago. Beyond energy, they also help regulate cell death, calcium signaling and heat production. When they malfunction, the result can be a group of conditions called mitochondrial diseases.
The Long Version
Anatomy of a Power Plant
Each mitochondrion is wrapped in two membranes. The smooth outer membrane encloses a space, and inside it lies a heavily folded inner membrane whose ridges, called cristae, greatly increase surface area. Within the innermost compartment, the matrix, enzymes break down fuel molecules. The folds hold the protein complexes that run the energy-making chain. Cells do not have a fixed number: a cell can contain from a few hundred to thousands, and they constantly fuse, split and are recycled. Mature red blood cells are a famous exception, having lost theirs.
How They Make Energy
Food is broken down in stages. Glucose is first split in the cytoplasm, and the fragments enter mitochondria, where the citric acid cycle (also called the Krebs cycle) strips off high-energy electrons. Those electrons pass along the electron transport chain embedded in the cristae, pumping protons across the inner membrane. The protons then flow back through an enzyme called ATP synthase, which uses that flow to build ATP. This whole process, oxidative phosphorylation, requires oxygen, which is the real reason you breathe: oxygen is the final electron acceptor at the end of the chain.
The Genome Within
Human mitochondrial DNA is a circular molecule about 16,569 base pairs long, tiny beside the roughly 3 billion base pairs in the nuclear genome. It contains 37 genes. Thirteen provide instructions for proteins involved in oxidative phosphorylation, and the others encode RNAs that help build those proteins. Yet mitochondria need far more proteins than this, so the vast majority are encoded in nuclear DNA and imported. mtDNA is also typically inherited from the mother, since egg cells contribute nearly all the mitochondria to an embryo, which makes it a useful tool for tracing maternal ancestry.
Where They Came From
In 1967 Lynn Margulis (then publishing as Lynn Sagan) argued for the endosymbiotic theory: mitochondria began as free-living bacteria swallowed by an ancestral cell, and both benefited. Evidence supports it. Mitochondria have their own circular DNA, divide in a bacteria-like way, have double membranes, and their ribosomes resemble bacterial ones. Over a very long time, most of their genes moved into the host nucleus. The theory is now textbook, and it helps explain why eukaryotic cells, the kind with nuclei, can be so large and complex.
When Mitochondria Go Wrong
Mutations in mtDNA or in nuclear genes that support mitochondria can cause mitochondrial diseases. Because the organs with the highest energy demands suffer most, symptoms can involve muscles, nerves, the brain, heart, eyes and more, and the severity ranges widely even within one family. Mitochondrial function is also studied in aging and in conditions such as neurodegeneration, though findings in those areas are still developing, and many supplements marketed as boosting mitochondria lack solid evidence. Diagnosis and management belong with specialist clinicians. This is general information, not medical advice.
Why People Care
Mitochondria connect biology classes, ancestry testing, and rare-disease research, and they are the target of a huge amount of wellness marketing. Knowing what they do, and what is still unproven, makes the headlines easier to judge.
Glossary
- ATP
- Adenosine triphosphate, the molecule cells use to store and deliver energy for work.
- Oxidative phosphorylation
- The process in which mitochondria use oxygen to produce most of a cell's ATP.
- Cristae
- The folds of the inner mitochondrial membrane where much of the energy-making machinery sits.
- mtDNA
- Mitochondrial DNA, the small genome inside mitochondria separate from the nuclear genome.
- Endosymbiosis
- A relationship in which one organism lives inside another; the origin proposed for mitochondria.
Go Deeper
More to Explore
- Epigenetics
Same script, different performances: how the cells in your body read the same DNA and end up as brain, skin, or liver.
- DNA and RNA
DNA is the master recipe book; RNA is the photocopy that actually makes it to the kitchen.
- Genetic Inheritance
Pea plants taught Gregor Mendel the basic rules of inheritance in the 1860s, decades before anyone had identified DNA as the carrier.
- Cell Biology
Your cells' power plants were probably once free-living bacteria, absorbed 1.5 to 2 billion years ago and never let go.
- Photosynthesis
Photosynthesis turns sunlight, water, and carbon dioxide into sugar, releasing the oxygen we breathe almost as an afterthought.