Ciliary and Flagellar Movement
Ciliary and Flagellar
Movement
Cilia are minute, hairlike, motile processes that extend from the surfaces of the cells of many animals. They are a particularly distinctive feature of ciliate protistans, but except for nematodes in which motile cilia are absent and arthropods in which they are rare, cilia are found in all major groups of animals. Cilia perform many roles either in moving small organisms such as unicellular ciliates, flagellates, and ctenophores (Figure 31-12B) through their aquatic environment or in propelling fluids and materials across epithelial surfaces of larger animals.
Cilia are of remarkably uniform diameter (0.2 to 0.5 µm) wherever they are found. The electron microscope has shown that each cilium contains a peripheral circle of nine double microtubules arranged around two single microtubules in the center (Figure 31-11). (Several exceptions to the 9 + 2 arrangement have been noted; for example, sperm tails of flatworms have but one central microtubule, and sperm tails of a mayfly have no central microtubule.) Each microtubule is composed of a spiral array of protein subunits called tubulin. The microtubule doublets around the periphery are connected to each other and to the central pair of microtubules by a complex system of connective elements. Also extending from each doublet is a pair of arms composed of the protein dynein. The dynein arms, which act as cross bridges between the doublets, operate to produce a sliding force between the microtubules.
A flagellum is a whiplike structure longer than a cilium and usually present singly or in small numbers at one end of a cell. They are found in members of flagellate protistans, in animal spermatozoa, and in sponges. The main difference between a cilium and a flagellum is in their beating pattern rather than in their structure, since both look alike internally. A flagellum beats symmetrically with snakelike undulations so that water is propelled parallel to the long axis of the flagellum. A cilium, in contrast, beats asymmetrically with a fast power stroke in one direction followed by a slow recovery during which the cilium bends as it returns to its original position (Figure 31-12A). Water is propelled parallel to the ciliated surface (Figure 31-12B).
Although the mechanism of ciliary movement is not completely understood, it is known that microtubules behave as “sliding filaments” that move past one another much like the sliding filaments of vertebrate skeletal muscle described in the next discussion (sliding microtubule hypothesis,). During ciliary flexion, the dynein arms link to adjacent microtubules, then swivel and release in repeated cycles, causing microtubules on the concave side to slide outward past microtubules on the convex side. This process increases curvature of the cilium. During the recovery stroke microtubules on the opposite side slide outward to bring the cilium back to its starting position.
Cilia are minute, hairlike, motile processes that extend from the surfaces of the cells of many animals. They are a particularly distinctive feature of ciliate protistans, but except for nematodes in which motile cilia are absent and arthropods in which they are rare, cilia are found in all major groups of animals. Cilia perform many roles either in moving small organisms such as unicellular ciliates, flagellates, and ctenophores (Figure 31-12B) through their aquatic environment or in propelling fluids and materials across epithelial surfaces of larger animals.
Cilia are of remarkably uniform diameter (0.2 to 0.5 µm) wherever they are found. The electron microscope has shown that each cilium contains a peripheral circle of nine double microtubules arranged around two single microtubules in the center (Figure 31-11). (Several exceptions to the 9 + 2 arrangement have been noted; for example, sperm tails of flatworms have but one central microtubule, and sperm tails of a mayfly have no central microtubule.) Each microtubule is composed of a spiral array of protein subunits called tubulin. The microtubule doublets around the periphery are connected to each other and to the central pair of microtubules by a complex system of connective elements. Also extending from each doublet is a pair of arms composed of the protein dynein. The dynein arms, which act as cross bridges between the doublets, operate to produce a sliding force between the microtubules.
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| Figure 31-11
Cross section of a cilium showing the microtubules and connecting elements of the 9 2 arrangement typical of both cilia and flagella. |
A flagellum is a whiplike structure longer than a cilium and usually present singly or in small numbers at one end of a cell. They are found in members of flagellate protistans, in animal spermatozoa, and in sponges. The main difference between a cilium and a flagellum is in their beating pattern rather than in their structure, since both look alike internally. A flagellum beats symmetrically with snakelike undulations so that water is propelled parallel to the long axis of the flagellum. A cilium, in contrast, beats asymmetrically with a fast power stroke in one direction followed by a slow recovery during which the cilium bends as it returns to its original position (Figure 31-12A). Water is propelled parallel to the ciliated surface (Figure 31-12B).
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| Figure 31-12 A, Flagellum beats in wavelike undulations, propelling water parallel to the main axis of itself. Cilium propels water in direction parallel to the cell surface. B, Movement of cilia in comb plates of a ctenophore. Note how the waves of beating comb plates pass down a comb row, opposite the direction of the power stroke of individual cilia. The movement of one comb plate lifts the plate below it and so triggers the next lower plate and so on. |
Although the mechanism of ciliary movement is not completely understood, it is known that microtubules behave as “sliding filaments” that move past one another much like the sliding filaments of vertebrate skeletal muscle described in the next discussion (sliding microtubule hypothesis,). During ciliary flexion, the dynein arms link to adjacent microtubules, then swivel and release in repeated cycles, causing microtubules on the concave side to slide outward past microtubules on the convex side. This process increases curvature of the cilium. During the recovery stroke microtubules on the opposite side slide outward to bring the cilium back to its starting position.
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