The flagellum is a long whip-like structure that consists of a thick and flexible microtubule plus an array of proteins. The microtubules are highly branched and have a diameter of about 20 nanometers.
The function of the flagellum is to propel bacteria through the water. The flagellum consists of two main parts: an outer protein structure called the filament, which is responsible for movement along the tubular body, and an inner protein structure called the basal body, which provides support and stability during movement.