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Mitochondria are more than powerhouses
More than energy
Mitochondria are best known as the powerhouses of the cell. They make adenosine triphosphate, the energy currency of life, through a cascade of chemical reactions that breaks down glucose and fat from food. Studies over the past decade have shown that they also process all sorts of molecules, which is why they directly affect what we experience.
Hormones and transmitters
They supply the raw materials for glutamate and acetylcholine, neurotransmitters that manage the brain, nerves, and muscles. The first step in making every steroid hormone, including cortisol, estrogen, testosterone, and progesterone, happens inside them, which makes them key regulators of sleep, sex, and stress.
Signals and genes
They also carry signals. They buffer and release calcium, which is involved in everything from muscle contraction to gene transcription, and reactive oxygen species, which activate immune cells and relay messages involved in cell growth. They help detect and react to stress, and they can trigger a cell to self-destruct when the damage is judged too great.
They even create the small molecules cells use to switch genes on and off as they grow, develop, and respond to their environment. That role makes them a link between a fixed genome and a changing environment.
How do mitochondria start hormone production?
Every steroid hormone is built from cholesterol, and the first step takes place on the inner membrane of the mitochondrion. A protein called StAR, identified in the 1990s, carries cholesterol into the organelle. There an enzyme known as P450scc converts it into pregnenolone. Pregnenolone is the precursor from which the body makes cortisol, estrogen, testosterone, and progesterone.
How do mitochondria set off cell self-destruction?
The trigger is a protein called cytochrome c. Inside healthy mitochondria it carries electrons along the chain that makes energy. When a cell is badly damaged, the mitochondrial membrane becomes leaky and cytochrome c escapes into the surrounding fluid of the cell. There it helps assemble a complex that activates enzymes called caspases, which dismantle the cell in an orderly way. Xiaodong Wang's lab described this pathway in 1996.
Why do mitochondria carry their own DNA?
Mitochondria descend from free-living bacteria that were swallowed by an ancestral cell more than a billion years ago. Lynn Margulis argued for this origin in a 1967 paper, and later genetic work supported it. The bacteria's old genome shrank over time but never vanished. Human mitochondrial DNA is a small ring of 16,569 base pairs carrying 37 genes.
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