A threadlike projection arising from the surface of motile unicellular algae, bacteria, and fungi and from spores and gametes. Flagella and cilia (see undulipodia) of eukaryotic cells have the same structure, cilia (5-10μm) being shorter than flagella (up to 150 μm). Cilia are usually numerous and their movements create currents that carry extracellular materials over the cell surface; flagella occur singly or in pairs and their activity moves the cell. Both arise from a *kinetosome
in the cytoplasm. Internally they consist of eleven fibres running lengthwise and constituting the axoneme, surrounded by a membrane continuous with the plasma membrane. Nine of the fibres form a peripheral outer cylinder and the remaining two are located in the centre, surrounded by a central sheath (the socalled 9 + 2 arrangement, see diagram). Each central fibre is a single *microtubule
, but the outer ones are paired microtubules, one of each pair being slightly wider than the other. At regular intervals along the length of the narrower tubule, short paired projections or arms arise, each positioned in a clockwise direction around the axoneme when viewed from the base. These consist of dynein, a protein with ATPase activity. The production of force in flagella depends on sliding movements of one peripheral fibre against its neighbour, energy being derived from dynein activity. Localized sliding of this kind, occurring in sequence around and along the axoneme produces linear forces that slightly distort the shape of the flagellum and induce bending to bring about the typical waves from base to tip. Flagella of prokaryotic cells have no membrane or axoneme. They are composed of three intertwined strands of the protein flagellin, which resembles the protein myosin in contractile muscle fibres.
Cross section of a eukaryotic flagellum.