Amongst the prokaryotes, flagella are organelles (specialized subunits within a cell) that provide some species of bacteria with a method of motility through their environment. Although several different flagella systems have been described in different bacterial species, these systems share a common structure. At the base of the flagella is the SMC ring-complex, a rivet-like structure that spans the inner membrane, cell wall and outer membrane of the cell. At the centre of the ring lies a rotor, which is turned by the SMC ring-complex motor by the proton motive forces generated by electrochemical potential differences between the two sides of the cells membrane. On the outside of the cell, the rotor is joined to a hook structure, which as the name suggests, is a bent tube that in turn is connected to a hollow, helical, filament. When the rotor is turned by the proton motor, the filament transforms converts the rotational motion into thrust for propulsion. Bacteria can regulate the rate and direction of the flagella rotation, which allows it control over the speed and direction of the movement. Different types of bacteria have different numbers and positions of their flagella. Some bacterial species only have one flagella, while others have multiple flagella projecting in all directions. The speed and agility of bacteria movement will also depend on both the number and arrangement of these flagella.The bacterial flagella is widely thought to have evolved from another type of organelle commonly found in bacteria and used in extracellular secretion, and still shares many structural similarities. Not all species of bacteria possess flagella, however. These species are not all immotile, as some have evolved the ability to move trough their environment by gliding on the secretions that they produce.

Flagella (that's the plural of flagellum) are used for movement. They look like hairs, but are actually powered for beating in a sort of spiral motion.They can be used to let a single cell "swim" through the water. Some bacteria have one flagellum, others have more.

The structure of the flagellum in prokaryotes is simpler than that in eukaryotes, but both types have the same function.

Why do you think bacteria need to swim?