Ultraefficient Bacterial Motors

bacterial flagellar motor


A topic that we will discuss in Physics 176 is how thermodynamic principles limit how efficiently one can convert energy of one kind, say work or chemical energy, into energy of another kind say kinetic energy in the form of translation or rotation. For example, nearly all electrical energy in the world is generated by using heat from coal, oil, or nuclear fission to convert water to steam and then use the steam to spin large turbines that rotate wires in magnetic fields to generate currents via Faraday's law. Why is our 21st-century world using 19th-century technology (steam and magnets) to generate energy? Can we do better?

One of the most efficient motors known, exceeding the efficiency of human electrical motors by a considerable amount, is the remarkable tiny (30 nanometer!) motor that bacteria use to spin their flagella (long thin whip-like threads that propel the bacterium through a fluid). It is currently not understood how these tiny motors convert energy stored in a gradient of protons (corresponding to a difference in the chemical potential across the cell membrane) to rotational energy with such high efficiency. These motors are truly impressive: they are 98% efficient in the conversion of chemical to rotational energy, they can spin up to 100,000 rpm, and they can reverse the direction of spinning almost instantaneously. Bacterial motors and related biological motors in eukaryotic cells represent an important frontier of biophysics, a frontier in which thermodynamics plays an essential role. By the way, similar chemical motors are also used to synthesize ATP, and their function in that context is also poorly understood.

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