Flagella are tiny, hair-like appendages on the surface of many cell types that provide either the motile force for movement or that serve as sensory receptors for information about the cell's external world. There are two very different kinds of flagella in the biological world. Bacterial flagella are very thin, solid rods made up of a chain of globular molecules of a single protein called flagellin. These rods, which have a helical shape, move only when spun by molecular motors in the cell membrane. They allow bacterial cells to swim through a liquid medium.

In contrast, the flagella of eukaryotic cells (those with a true nucleus) are much larger and more complex than those of bacteria. Typically about 0.82 ft (0.25 m) in diameter and 66-164.05 ft (20-50 m) in length, eukaryotic flagella usually are very similar in structure and function to cilia. The most obvious differences between cilia and...