Some bacteria swim using a propeller that puts human engineering to shame. The flagellar motor is a rotating structure a few tens of nanometres across, powered not by a chemical fuel but by a flow of protons across a membrane. It spins at hundreds of revolutions per second and can reverse direction in an instant. For fifty years, the details of how it generates torque resisted explanation.
The difficulty lay in scale. The motor's components shift position by fractions of a nanometre, and the forces involved are minuscule, making direct measurement extraordinarily hard. Advances in cryo-electron microscopy and single-molecule techniques have finally produced a coherent picture of the rotor, the stator and the charged residues that convert an electrical gradient into mechanical rotation.
The result is more than a satisfying answer to an old puzzle. The motor is a compact demonstration of the principle that living systems do not evade physics; they exploit it. Watching a cell turn an ion gradient into motion is, as columnist Natalie Wolchover puts it, a way of seeing the essence of life expressed in pure mechanism.
