FIG. 1.   The flagellum is the motor organelle for bacterial propulsion. Driven by a transmembrane proton gradient (H⁺H⁺), it rotates both CCW and CW; the filament is helical and so converts torque into thrust. The motor consists of stators or Mot complexes (red) and a rotor or C ring (green), which also serves as the CCWCW switch. As well as being the organelle of motility, the flagellum is a specialized type III export apparatus (lilac), translocating subunits of its substrates (pale blue) in an ATP-dependent manner across the plane of the cytoplasmic membrane (CM) and delivering them into a central channel in the basal body-hook-filament structure where they eventually reach their assembly point at the distal end of the structure. PG, peptidoglycan layer; OM, outer membrane.