The cilia and flagella are fine, hairlike filamentous processes of cytoplasm which provide locomotion to the entire cell or the organism. Cilia and flagella are almost identical in their morphology and physiology but readily differ in length and number present per cell and movement pattern. Cilia are short and are present in large numbers on cell surface. The number of flagella is restricted to one or a pair.
Flagella are generally much larger than cilia. Cilia range from 0.2-10u in length and that of flagella from 10u to several mm. Both have a diameter of 0.2u. A cilium or a flagellum, consisting of a set of microtubules called axoneme, is enclosed by an extension of the plasmamembrane which forms a 95 Å thick unit membrane around it. Microtubules form the core of cilia and flagella and run along the length of these organelles.
A cross section shows nine pairs of microtubules are arranged in cylindrical fashion around two central microtubules (9+2 arrangement). Cilia and flagella perform their respective movements by the sliding of the microtubules they contain. At the base of each cilium or flagellum is present a structure in the cytoplasm called the basal body or kinetosome. The basal body has the same structure as a centriole. It shows 9+0 arrangement of the microtubules. The basal body serves as the starting point for the production of cilia and flagella.
The movement or beats of cilium constitutes two types of strokes Power stroke and recovery stroke. The power stroke of a cilium constitutes a bending motion. During this the cilium moves against the surrounding medium with force. Bending is achieved by lengthwise sliding of the microtubule doublets along each other. This causes movement of the medium in the direction of the stroke and that of the cell in opposite direction.
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But during the recovery stroke the cilium returns to its original position with a flexible movement. In this way the movement occurs like rowing of boat where the ores provide backward stroke that propels the boat forward and the ores are brought back to their original state by lifting them above water. But flagellum shows a number of simultaneous undulations passing from base to the tip during movement. This moves the organism in opposite direction of the flagellum.