flagellum

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.