The motor that spins the flagellum is a complicated molecular machine made of many copies of dozens of different proteins, spanning the cell envelope with components in the cytoplasm, the periplasm (in diderms), and outside the cell, as you can see in this Bdellovibrio bacteriovorus. To get a closer look, we can average the individual motors from many cells (⇩). Broadly, the motor consists of stationary “stators” that drive rotation of the “rotor” to spin the filament. The torque for spinning the flagellum comes from small movements in the stators that kick the rotor in a circle. The energy for these movements comes from the ion potential across the cell membrane that we discussed in Chapter 2; the stators provide a conduit for protons (in most species) or sodium ions (in some marine species) to diffuse down their chemical gradient into the cytoplasm, powering a conformational change in the stators in the process. The energy demands of the machine are high: a single rotation requires about 1,000 protons to flow through the stators, and the motor may spin at more than 100 rotations per second. The fact that cells pay this energetic cost indicates a strong evolutionary selection for motility, or in other words, the powerful advantage your cell can gain by learning to swim.
While the basic plan of the motor is the same in different species, there are structural differences that reflect the different environments those species encounter (⇩).