How can flowing ions turn a bacterium’s propeller?

A swimming Escherichia coli bacterium can turn long helical flagellar filaments using molecular rotary motors. The filaments rotate rather than waving like animal tails. Each connects through a hook to a motor embedded in the cell envelope, joining a chemical energy source to mechanical motion.

The immediate energy source is a proton electrochemical gradient across the inner membrane. Both proton concentration and electrical potential contribute. When protons pass through appropriate motor proteins in the energetically favorable direction, free energy becomes available. Metabolism must maintain the gradient; the motor does not create energy from an unchanging supply of ions.

Membrane-spanning stator complexes couple ion movement to interactions with the rotor. Their anchored portions provide a reaction support while molecular rearrangements generate torque on the rotating assembly. The diagram deliberately shows functional relationships rather than pretending to resolve each molecular power stroke.

An illustrative energy budget uses 100 protons crossing a proton-motive potential of 150 millivolts. Multiplying proton count, elementary charge and potential gives about 2.4 × 10⁻¹⁸ joule of available energy. Mechanical work cannot exceed that input. This budget neither specifies protons per revolution nor determines rotation speed: load, motor state and dissipative processes matter. Other bacteria can use sodium-driven motors, so the proton example is not a universal fuel specification for every flagellum.