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Locomotion of microorganisms
Most motile protozoans, which are strictly aquatic animals, move by locomotion involving one of three types of appendages: flagella, cilia, or pseudopodia. Cilia and flagella are indistinguishable in that both are flexible filamentous structures containing two central fibrils (very small fibres) surrounded by a ring of nine double fibrils. The peripheral fibrils seem to be the contractile units and the central ones, neuromotor (nervelike) units. Generally, cilia are short and flagella long, although the size ranges of each overlap.
Many prokaryotes are motile by swimming, and this function is
due to a structure called the flagellum. The flagellum functions by rotation to push or pull the cellthrough a liquid medium.
Flagellar Structure
Flagella are not straight but are actually helical. When flattened,flagella show a constant distance between adjacent curves, called the wavelength, and this wavelength is characteristic for the fla-gella of any given species . The filament of a bacterial flagellum is composed of many copies of a protein called flagellin. The shape and wavelength of the flagellum are inpart determined by the structure of the flagellin protein and also to some extent by the direction of rotation of the filament.
A flagellum consists of several components and moves by rotation, much like a propeller of a boat motor. The base of the flagellum is structurally different from the filament.
Ciliary locomotion:
Cilia operate like flexible oars; they have a unilateral (one-sided) beat lying in a single plane. As a cilium moves backward, it is relatively rigid; upon recovery, however, the cilium becomes flexible, and its tip appears to be dragged forward along the body. Because the cilia either completely cover, as in ciliate protozoans, or are arranged in bands or clumps, the movement of each cilium must be closely coordinated with the movements of all other cilia. This coordination is achieved by metachronal rhythm, in which a wave of simultaneously beating groups of cilia moves from the anterior to the posterior end of the organism. In addition to avoiding interference between adjacent cilia, the metachronal wave also produces continuous forward locomotion because there are always groups of cilia beating backward. Moreover, because the plane of the ciliary beat is diagonal to the longitudinal axis of the body, ciliate organisms rotate during locomotion.
Although ciliar and flagellar locomotion are clearly forms of appendicular locomotion, pseudopodial locomotion can be classed as either axial or appendicular, depending upon the definition of the pseudopodium. Outwardly, pseudopodial locomotion appears to be the extension of a part of the body that anchors itself and then pulls the remainder of the body forward. Internally, however, the movement is quite different.
Two other types of locomotion are observed occasionally in protozoans. Some protozoans, usually flagellates, have along their bodies a longitudinal membrane that undulates, thereby producing a slow forward locomotion. A gliding locomotion is commonly seen in some sporozoans (parasitic protozoans), in which the organism glides forward with no change in form and no apparent contractions of the body. Initially, the movement was thought to be produced by ejecting mucus, a slimy secretion; small contractile fibrils have been found that produce minute contraction waves that move the animal forward.


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