flagellum

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The bacterium Proteus vulgaris (greatly magnified) showing flagella (credit: © Lee D. Simon — Photo Researchers)

Hairlike structure that acts mainly as an organelle of movement in the cells of many living organisms. Characteristic of the protozoan group Mastigophora, flagella also occur on the sex cells of algae, fungi ( fungus), mosses, and slime molds. Flagellar motion causes water currents necessary for respiration and circulation in sponges and cnidarians. Most motile bacteria move by means of flagella. The structures and patterns of movement of flagella in prokaryotes differ from those in eukaryotes. cilium.

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1. the specialized locomotory appendage of bacteria of certain taxonomic groups, consisting of a noncontractile, filiform extension through the cell surface, borne singly, in groups, or as a covering layer. It is commonly 3 — 20 μm long and 12 — 25 nm in diameter, is built up of several (often three) longitudinally arranged chains of flagellin subunits (often in a spiral), and is anchored to the cell envelope by a basal body (def. 1), which imparts rotatory motion to it. Bacterial flagella carry the H (or flagellar) antigens, which are useful in some instances, e.g. in Salmonella spp., for classification.
2. the specialized locomotory appendage of the motile cells of unicellular eukaryotes of certain taxonomic groups (e.g. some algae and protozoa) and also of the male gametes (spermatozoa) of most groups of animals. Usually borne singly or in small groups, it is a whiplike structure, similar to, but generally longer than a cilium, commonly about 40 μm long and 200 — 300 nm in diameter. It is built up of an outer membranous sheath, which is a continuation of the cell membrane, and an inner axoneme, which terminates within the cell at a basal body (def. 2). Eukaryotic flagella often exhibit undulatory motion, which in multiflagellate organisms can occur as coordinated beating.

Pl. flagella [L.] a long, mobile, whiplike appendage arising from a basal body at the surface of a cell, serving as a locomotor organelle; the only known example in biology of a rotatory motion. In eukaryotic cells, flagella contain nine pairs of microtubules arrayed around a central pair; in bacteria, they contain tightly wound strands of flagellin.

Hairlike projections that extend from some unicellular organisms and aid in their movement.

• Cell Structure and Function - flagella: threadlike organelles used for locomotion and feeding by some eukaryotic organisms