What Mitochondria Actually Do
Power, signalling, life-and-death decisions
Mitochondria are the bacteria-derived organelles inside virtually every cell. Their famous job — producing ATP, the cellular energy currency — is just one of three. They also generate reactive-oxygen-species signals that regulate cell behaviour, AND decide when a cell should die. Healthy mitochondria are why you have energy, clear thinking, and resilience.
10× more in muscle, 100× more in heart
Muscle cells contain ~1,000 mitochondria each. Heart muscle contains ~5,000 — they make up nearly 40% of cardiac cell volume. This is why aerobic training (which builds mitochondria) protects the heart so well, and why even brief mitochondrial dysfunction shows up first as fatigue, then as heart-failure symptoms in serious cases.
They have their own DNA
Mitochondria carry their own circular DNA (mtDNA) — a relic of their bacterial origin. mtDNA mutates ~10× faster than nuclear DNA because it sits right next to the reactive-oxygen production line. Accumulated mtDNA damage is one of the original 9 hallmarks of aging.
Match mitochondrial role to symptom
Connect each mitochondrial function to the symptom that shows up first when it fails.
60 kg of ATP per day
An adult turns over roughly **65 kg of ATP every 24 hours** — close to your body weight. Each ATP molecule is recycled thousands of times by spinning rotors in the inner mitochondrial membrane (the F1F0 ATP synthase, which literally rotates at ~7,000 RPM). This is why even brief mitochondrial dysfunction is felt so quickly: there's no buffer; you're always making ATP just-in-time.
Counter-intuitive: ROS is not just damage
Conventional wisdom: "reactive oxygen species (ROS) cause aging — quench them with antioxidants." Reality: **mitochondrial ROS is a signal**, not a poison. Brief, controlled ROS bursts during exercise tell cells to upregulate antioxidant enzymes, build new mitochondria, and adapt. This is **mitohormesis**. High-dose antioxidant supplementation (vitamin E, C in mega-doses) blunts the training response and has been associated with *worse* cardiovascular outcomes in some trials. The dose makes the medicine.
Inherited from your mother only
Mitochondrial DNA is inherited **exclusively from the maternal line**. The sperm's mitochondria are tagged for destruction during fertilisation, so paternal mtDNA is normally erased from the embryo. This is why mitochondrial-disease genealogies trace through mothers and grandmothers, and why "three-parent IVF" — replacing the mother's defective mtDNA with a donor's — became legal in the UK in 2015 for families with severe mitochondrial disease.
Order the energy pathways by ATP yield per glucose
Drag from lowest to highest ATP output.
Key Takeaway
Mitochondria do far more than 'make energy.' They turn over ~65 kg of ATP a day, send hormetic signals through controlled ROS bursts, decide when cells die, and pass along a maternally inherited genome that mutates 10× faster than your nuclear DNA. Building mitochondrial health is upstream of aerobic fitness, metabolic flexibility, and cellular resilience.