Flagellum

A flagellum (; pl.: flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores (zoospores), and from a wide range of microorganisms to provide motility. Many protists with flagella are known as flagellates. Across the three domains of Bacteria, Archaea, and Eukaryota, the flagellum has a different structure, protein composition, and mechanism of propulsion but shares the same function of providing motility. The Latin word flagellum means "whip" to describe its lash-like swimming motion. The flagella of bacteria and archaea, prokaryotes, are rotor-like structures that propel the organism via roatation. While very similar to bacterial flagella, the flagellum in archaea is called the archaellum to note its difference from the bacterial flagellum. Eukaryotic flagella differ from prokaryotic flagella as they are large membrane protrusions supported by the cytoskeleton, which wave back and forth rather than rotate. Eukaryotic cilia are very similar to flagella but are much shorter. Surface-attached cilia and flagella are used to swim or move fluid from one region to another. A cell may have zero, one, or many flagella depending on the species and cell type. A gram-negative bacterium Helicobacter pylori, for example, uses its flagella to propel itself through the stomach to reach the mucous lining where it may colonise the epithelium and potentially cause gastritis, and ulcers – a risk factor for stomach cancer. Flagella in some cases may have functions other than or in addition to motility. For example, Salmonella typhimurium can sense wetness of the surrounding environment using its flagella.