After 50 years, the bacterial flagellar motor’s mechanism is resolved: proton currents pedal pentagonal stator turnstiles that drive a snap-switching C ring counterclockwise or clockwise.
Key Takeaways
Cryo-EM from 2020-2026 revealed stator 5:2 geometry: pentagonal rings of 5 proteins around 2 central proteins act as proton turnstiles, passing >2,000 protons/second.
Proton motive force (Peter Mitchell, Nobel 1978) is the universal cellular energy source; the flagellar motor is the clearest mechanical demonstration of it in biology.
Direction switching: phosphorylated CheY causes the 34-protein C ring to snap into an alternate conformation, reversing its contact from the outer to inner stator edge and tumbling the cell.
The motor rotates faster than a race car flywheel, self-assembles from 20 amino acids, processes chemical gradients, and switches directions without any external controller.
Final structural confirmation came March 2026; the system is still used, unchanged, by bacteria in virtually every gut and puddle on Earth.
Hacker News Comment Review
Commenters immediately flagged near-100% mechanical efficiency as the striking engineering detail: at the nanoscale, heat dissipation and friction that dominate human-scale motor design are essentially absent.
The proton motive force link to mitochondria was called the most important omission in the article: the flagellar “outside” maps directly onto the mitochondrial intermembrane space, making the flagellum a readable fossil of endosymbiosis.
Commenters reframed the creationist “irreducible complexity” argument as now fully collapsed: the stepwise assembly pathway is confirmed, not just theorized.
Notable Comments
@bacteriumiu: flags that the proton-pumping cycle described is exactly the mitochondrial ancestor mechanism, with “outside” now meaning inside the parent cell.
@zimpenfish: quantifies evolutionary compute: “a billion years at a 20 minute breeding cycle is 26.3 trillion generations.”
