Flagellum

A flagellum is a long, whip-like appendage that propels a cell through liquid. Despite the shared name, the flagella of bacteria, archaea and eukaryotes are built from different proteins and move in different ways, and they evolved separately.

A bacterial flagellum is a rigid helical filament of the protein flagellin attached by a hook to a rotary motor embedded in the cell envelope. The motor is powered by the flow of protons, or in some bacteria sodium ions, down their gradient, as in chemiosmosis, and it turns at about a hundred to several hundred revolutions per second in Escherichia coli. Cells swim in straight runs and tumble by reversing the motor, and they bias their path toward food through chemotaxis.

Archaea have a similar-looking structure called an archaellum, which is driven by ATP and is related to the machinery of type IV pili. The eukaryotic flagellum, also called a cilium when short, has a bundle of nine pairs of microtubules around two central ones, called the axoneme. The motor protein dynein slides the pairs against each other to bend it, creating a wave. It is covered by an extension of the plasma membrane.

Sperm cells swim with a single flagellum, and cilia in the airway sweep mucus. Many protozoa, such as trypanosomes, and algae use flagella to move, and pathogens use them to reach host tissues. In some bacteria, a variant of the apparatus injects proteins into host cells. The study of flagella has illuminated the evolution of complex molecular machines in bacteria and archaea.

• Dynein — the motor that bends eukaryotic flagella
• Microtubule — the structural fibers in the eukaryotic flagellum
• Chemiosmosis — the energy source of the bacterial motor