Flagelles

Flagelles are slender, whip-like appendages that enable movement in a wide range of cells and organisms. The term is commonly used to describe locomotory organelles in bacteria, archaea, and many eukaryotic microorganisms, including algae and protozoa. In prokaryotes, flagella rotate to propel the cell, while in most eukaryotes, flagella beat in a whip-like fashion to generate propulsion. Flagelles are distinct from cilia, though in many eukaryotes they share an identical internal structure.

In bacteria, flagelles consist of a basal body embedded in the cell envelope, a flexible hook, and a long filament made of flagellin protein. The motor at the base uses ion gradients, typically a proton motive force, to rotate the filament like a rotor. This rotation enables the bacterium to swim and to navigate via chemotaxis toward nutrients or away from harmful substances. Different bacterial species display various flagellation patterns, such as peritrichous, polar, or lophotrichous arrangements, influencing movement behavior.

In eukaryotes, flagella are generally longer and fewer in number and display coordinated beating. They possess a 9+2 axoneme of microtubules powered by dynein motors, producing bending waves that push the cell through the surrounding fluid. This structure is shared with cilia, though flagella typically lie longer and beat with different patterns. Eukaryotic flagella are found in many organisms, including the green alga Chlamydomonas and the protozoan Trypanosoma. Archaea have a related but distinct structure, the archaellum, which is powered by ATP hydrolysis rather than the same motor mechanism used by bacteria.