Mitochondria

Mitochondrion is Greek for thread-granule. Carl Benda coined it in Berlin in 1898 from mitos (μίτος, thread) and khondros (χόνδρος, granule), describing what the organelles looked like through a microscope. The name says nothing about what they are.

The earlier name was bioblast — Greek for life-unit. Richard Altmann at Leipzig had described the structures in 1890 in a monograph called Die Elementarorganismen (“The Elementary Organisms”). He proposed they were autonomous living entities residing inside cells. His colleagues dismissed him as a mystic. He died in 1900 without recognition. His hypothesis was correct: the units he saw were bacteria, still alive after a billion-plus years inside their hosts, dividing on their own schedule. His name for them was dropped. The merely descriptive name — Benda’s — survived.

The cell-biology vocabulary keeps this pattern. Cristae — Latin for crests, after the ridges on a Greek helmet. Cytochrome — Greek for cell color, because the iron-heme proteins are colored. Apoptosis — Greek for a falling off, the way leaves come off a tree. The words describe appearances and analogies. They are consistently silent about the most important fact, which is that mitochondria are, etymologically, descendants. They are not just thread-granules. They are children.

In 1905, Konstantin Mereschkowsky, a Russian botanist working in Kazan, published a paper in Biologisches Centralblatt arguing that chloroplasts — the green organelles in plant cells — had originated as free-living cyanobacteria that had been engulfed by ancestral plant cells and never digested. He coined the term symbiogenesis in a follow-up paper in 1910: origin from living together. The botanical establishment ignored him.

His life ended badly. Mereschkowsky had been forced out of Russia in 1914 under accusations of pedophilia. He wandered Europe — Crimea, Germany, France — for the next seven years, increasingly destitute. He took his own life in a hotel room in Geneva on January 9, 1921. His scientific work was forgotten. The idea of symbiogenesis sat dormant for half a century.

In late 1966, an American postdoctoral biologist submitted a paper called “On the Origin of Mitosing Cells” to a series of journals. The paper argued that mitochondria, chloroplasts, and basal bodies of cilia were all former free-living bacteria. The author had been born Lynn Petra Alexander in Chicago in 1938. She had married Carl Sagan in 1957, at nineteen, and had taken his name; she divorced him in 1965 and remarried in 1967 — taking the name Thomas Margulis would later be known by. The 1966 submission was made as Lynn Sagan. About fifteen journals rejected the paper. The Journal of Theoretical Biology eventually accepted it. It appeared in March 1967, volume 14, pages 225–274. By Margulis’s own later account, the rejections were often “without even being read.”

Initial reception was hostile. By 1985 the molecular evidence — circular mtDNA, bacterial ribosomes inside mitochondria, binary-fission division — had accumulated to the point that endosymbiosis entered the textbooks. By the 1990s it was canonical. The British biochemist Nick Lane (born 1967, the same year as the paper) has spent the 2010s and 2020s arguing that the mitochondrial endosymbiosis was a singular event in 4 billion years of Earth’s biological history — possibly the rate-limiting step for complex life anywhere in the universe.

Roughly 1.5 to 2 billion years ago, in a body of water somewhere on a Proterozoic Earth, an archaeon — single-celled, primitive, the kind of thing now found in deep-ocean hydrothermal vents — engulfed an alphaproteobacterium, a free-living single-celled organism related to the modern Rickettsia genus. The bacterium was not digested. It persisted inside the host. Over time it shed most of its genes — transferring them to the host’s nuclear genome — and retained the functions the host could not provide. Chiefly, oxidative phosphorylation: the production of ATP from oxygen and food.

The relationship was, in current biological terms, a permanent symbiotic fusion. The product was the first eukaryotic cell. Every plant, every animal, every fungus, every protist — every cell with a nucleus — descends from this event. The mitochondrion in your liver cell right now is the direct descendant, by roughly seventy billion cellular generations, of that engulfed bacterium.

Nick Lane’s argument, developed in Power, Sex, Suicide (2005) and The Vital Question (2015): this fusion is a singular event in the history of life on Earth. It has not been repeated. Bacteria and archaea have continued to diversify for the four billion years since life began, but neither has independently produced anything as structurally complex as a eukaryotic cell. Internal energy production via mitochondria freed the host from the surface-to-volume energy constraints that limit prokaryotic complexity. Without endosymbiosis, no animals. Without animals, no astronomers. The Fermi paradox — where is everybody? — may have a cellular answer. The energetic bottleneck was crossed exactly once.

In 1961, Peter Mitchell — a British biochemist who had been forced out of his University of Edinburgh position by illness and an institutional dispute — published in Nature the hypothesis that ATP synthesis was driven by a proton gradient across a membrane. Everyone in the field was searching for a high-energy chemical intermediate that would couple electron transport to phosphorylation. Mitchell said there wasn’t one. The energy of cellular respiration was stored as an electrochemical difference of hydrogen ions across the inner mitochondrial membrane — a kind of biological battery.

The field rejected the hypothesis. David Green at Wisconsin, who ran one of the largest mitochondrial laboratories in the world, was sharply opposed. Mitchell took inherited family money, renovated a Cornwall manor called Glynn House near Bodmin, and established a private research institute. He worked there with a small team — including his lifelong collaborator Jennifer Moyle — for the rest of his career. The position was unusual enough that the British press wrote profiles. He was, in the formal sense, outside the academy.

In 1966 the chloroplast biologist André Jagendorf at Cornell showed that an artificial pH gradient across thylakoid membranes drove ATP synthesis without any electron transport. The mechanism was proton-motive force. The chemical-intermediate hunt collapsed. Subsequent work by Efraim Racker, Paul Boyer, and John Walker filled in the structural details. Mitchell received the 1978 Nobel Prize in Chemistry as sole laureate.

ATP synthase — the enzyme that uses the proton gradient to make ATP — is a rotary motor. It spins. The F₀ subunit sits in the inner mitochondrial membrane like a turbine; the proton flow drives the rotation; the rotation drives conformational changes in the F₁ catalytic head that synthesize ATP. Physiological rotation rates of roughly one to three hundred revolutions per second have been measured. Three ATP molecules per full rotation.

Lewis Thomas — physician, immunologist, dean at NYU School of Medicine — wrote a series of essays in the New England Journal of Medicine between 1971 and 1973 that became The Lives of a Cell (1974, National Book Award). The opening essay said what was, in 1974, still controversial in working mitochondrial laboratories.

Cytochrome c is a small heme protein — twelve kilodaltons — normally lodged in the intermembrane space of a mitochondrion. Its day job, in roughly a thousand simultaneous copies per cell at this moment, is to shuttle electrons between Complex III and Complex IV of the electron transport chain. Without cytochrome c, no ATP.

When a cell receives a death signal — DNA damage that cannot be repaired, withdrawal of survival factors, a developmental program — pro-apoptotic proteins (Bax, Bak) form pores in the outer mitochondrial membrane. Cytochrome c leaks into the cytoplasm. There it binds Apaf-1. Seven Apaf-1 subunits, each bound to a cytochrome c, assemble into a wheel called the apoptosome. The apoptosome activates caspase-9, which activates downstream caspases that systematically dismantle the cell — fragmenting DNA, breaking down the cytoskeleton, packaging the remains for orderly disposal by neighboring cells.

From the cell’s perspective, the trigger for life and the trigger for death are interchangeable. What differs is whether cytochrome c is on the inside of the outer membrane or the outside. The most precisely choreographed event in cell biology — apoptosis, from Greek apo- + ptōsis, “a falling off,” like leaves from a tree — was named after the gentlest natural process the namers (Kerr, Wyllie, and Currie, British Journal of Cancer, 1972) could think of.

You inherit your mitochondria only from your mother. The egg’s mitochondria become the embryo’s mitochondria. The sperm’s mitochondria — densely packed in the midpiece, powering the swim to the egg — are tagged with ubiquitin during sperm maturation in the testis. When the sperm enters the egg, the egg’s autophagy machinery recognizes the ubiquitin marker and destroys the paternal mitochondria within hours. The body has engineered, at the molecular level, a single-sex inheritance pattern for one organelle.

The consequence: every human alive today inherits mitochondrial DNA through an unbroken matrilineal chain. Your mother. Her mother. Her mother. Every generation back. When the chains are traced — and they have been traced, in tens of thousands of full mitochondrial sequences — the lines converge.

In January 1987, Rebecca Cann, Mark Stoneking, and Allan Wilson at UC Berkeley published “Mitochondrial DNA and Human Evolution” in Nature. They had analyzed mitochondrial DNA from 147 individuals representing five geographic regions, most of the samples drawn from placentas collected at maternity wards. They reconstructed the phylogenetic tree. It rooted in Africa, in a single woman who lived roughly 155,000 to 200,000 years ago. The press dubbed her Mitochondrial Eve.

She was not the only woman of her time. She was not necessarily the most successful or numerous. She is simply the woman from whom all currently living humans inherit mtDNA. Many of her contemporaries also have descendants alive today — through the male line, through other genes. But matrilineal lines go extinct: a woman who has only sons does not pass her mtDNA forward. Over thousands of generations, all-but-one mtDNA lines are lost. She is the root of the data structure. She is not the founder of humanity. She is the woman whose mitochondria, in the lottery of who has daughters and who has sons, won.

In 1962 Rolf Luft at the Karolinska Institute in Stockholm described a 35-year-old Swedish woman who had presented for years with profound, unexplained metabolic abnormalities. She was perpetually overheated. She drank approximately ten liters of water per day to compensate for her sweating. She was thin, exhausted, constantly hungry. Muscle biopsy showed strange-looking mitochondria — abnormally numerous, abnormally shaped. Biochemical analysis showed her mitochondria were uncoupled — burning fuel furiously but failing to capture the energy as ATP. The condition came to be called Luft syndrome. It remains exquisitely rare; the original case is the only well-characterized one. It was the first identified mitochondrial disease.

By 2026 more than 350 are known. The major syndromes — MELAS, MERRF, LHON, Kearns-Sayre, Leigh syndrome — share a structural feature: they affect tissues with the highest metabolic demand first. Vision often goes early because retinal ganglion cells are among the most metabolically demanding cells in the body. Muscles fail because muscle is dense with mitochondria. The brain fails because the brain is dense with mitochondria. The clinical picture of mitochondrial disease is the clinical picture of distributed energy failure.

In February 2015, the UK Parliament approved mitochondrial donation — three-parent IVF. The procedure transfers the nuclear DNA of a mother carrying defective mitochondria into a donor egg whose own nuclear DNA has been removed. The resulting egg, with maternal nucleus and donor mitochondria, is then fertilized. The child has three biological parents: nuclear DNA from mother and father, mitochondrial DNA from the donor woman.

On April 6, 2016, the first child conceived this way was born in Mexico. The mother — a Jordanian woman who had lost two previous children to Leigh syndrome and had endured four miscarriages — was treated by John Zhang’s New York-based clinic. The procedure was performed in Mexico because the FDA prohibited it in the United States. The child carries less than two percent mutant mitochondrial DNA — well below the threshold at which Leigh syndrome typically manifests. He has been reported healthy in subsequent follow-ups.

The mitochondrion in your liver right now was, by an unbroken chain of division, the same bacterium-derived organelle that resided in your mother’s liver, and in her mother’s, and in her mother’s. The chain runs through your egg-line ancestors, back through the first hominids, through the first mammals, through the first vertebrates, through the first eumetazoans, through the first eukaryotes, to one fusion event between an archaeon and a bacterium roughly 1.5 to 2 billion years ago. None of the mitochondria in that chain has died in the sense that a cell dies. They have divided, partitioned, divided again — but never, in your particular lineage, fully terminated.

A Greek word that means “thread-granule,” coined in 1898 by Carl Benda. A Russian botanist who proposed the right answer in 1905 in Kazan and died unknown in a Geneva hotel on January 9, 1921. A young American who submitted a paper to fifteen journals in 1966 under a married name she had already given up, and one journal that finally said yes. A Cornwall manor where a man worked out how cells make energy and waited fifteen years to be believed. A circular DNA molecule 16,569 base pairs long that encodes thirty-seven genes and shares space with a thousand five hundred more proteins it has surrendered to the nucleus. A motor that spins three hundred times per second in your skeletal muscle at this moment. Fifty kilograms of ATP made and consumed today. A protein that delivers electrons to make the cell alive and that, released through a membrane pore, starts the cell’s orderly dismantling. One African woman from whom every living human’s matrilineal line descends. A Swedish woman in 1962 who drank ten liters of water a day. A baby in Mexico, April 2016, the first child whose matrilineal line was rewritten by a clinician.

The metabolism you call your own is the metabolism of a bacterium your great-(many times)-great-grandmother swallowed. Your sense of unitary self is, at the cellular level, a colonial arrangement that has been so successful for so long that the partners cannot be told apart. The nuclear genome has given the bacterium most of its genes; the bacterium runs the powerhouse. Neither can survive alone. There is no edge of self where you stop and the mitochondria start. The cell is the boundary. The cell contains both.