The powerhouse of the cell
A mitochondrion is roughly 1–10 μm long, with a smooth outer membrane and a folded inner membrane (cristae), and energy-hungry cells in muscle, heart and brain contain thousands. Electrons from broken-down glucose and fatty acids pass along the electron transport chain in the inner membrane, pumping hydrogen ions outward; as the ions flow back through ATP synthase, it spins like a water wheel and makes ATP. Peter Mitchell proposed this chemiosmotic theory in 1961, was widely dismissed, and won the 1978 Nobel Prize in Chemistry. If chloroplasts store energy through photosynthesis, mitochondria are where it is spent.
Endosymbiotic theory
In 1967 American biologist Lynn Margulis argued that mitochondria were once free-living bacteria engulfed by another cell some 1.5–2 billion years ago. Her paper was rejected about 15 times, but the evidence piled up: mitochondria have double membranes, divide by binary fission like bacteria, and carry their own circular DNA and bacterial-type ribosomes. Today the theory is textbook science, and genetic data place their ancestors among the alphaproteobacteria. This merger is seen as the event that made complex life possible, a turning point in evolution alongside natural selection.
Maternal inheritance and health
Sperm mitochondria are mostly destroyed after fertilization, so people inherit mitochondrial DNA only from their mothers. In 1987 Rebecca Cann and colleagues compared mitochondrial DNA worldwide and traced all living humans to a female lineage in Africa about 200,000 years ago, popularly called "Mitochondrial Eve." Mutations in mitochondrial genes cause rare disorders of muscle and nerves, and in 2015 the UK became the first country to legalize "three-parent" IVF to prevent them. Mitochondrial function declines with age, and regular aerobic and strength exercise increases muscle mitochondria, which helps guard against sarcopenia.
FAQ
- Why do we inherit mitochondrial DNA only from our mothers?
- Sperm do contain mitochondria, but they are tagged and broken down inside the egg soon after fertilization. Very rare cases of paternal mitochondria persisting have been reported.
- Do mitochondria supplements work?
- Compounds like coenzyme Q10 are prescribed for some mitochondrial diseases, but evidence that they fight fatigue or aging in healthy people is weak. Exercise is the most reliable way to build mitochondria.
Sources reviewed: 2026-10-26
References
- Margulis, L. (Sagan, L.) (1967). On the origin of mitosing cells. Journal of Theoretical Biology, 14(3), 225–274.
- Mitchell, P. (1961). Coupling of phosphorylation to electron and hydrogen transfer by a chemi-osmotic type of mechanism. Nature, 191, 144–148.
- Cann, R. L., Stoneking, M., & Wilson, A. C. (1987). Mitochondrial DNA and human evolution. Nature, 325, 31–36.