Flagella and cilia

Motility in eucaryotic cells may be achieved by means of flagella or cilia; cilia can be thought of as, essentially, short flagella. Both are enclosed within the plasma membrane and anchored by means of a basal body. Flagellated cells generally have a single flagel-lum, whereas cilia are often present in very large numbers on each cell. In the microbial world, flagella are found in protozoans and motile algal forms, whilst cilia are mostly found in a class of protozoans called the Ciliophora. Flagella and cilia are not found in members of the Fungi. Although they share the same thread-like gross morphology,

Figure 3.18 Eucaryotic flagella have a characteristic '9 + 2' structure. Although functionally analogous to their procaryotic counterparts, eucaryotic flagella differ appreciably in their fine structure. A membrane surrounds an arrangement of proteinaceous microtubules, in which nine pairs surround a single central pair. Movement of eucaryotic flagella is by means of an adenosine triphosphate-driven whiplike motion

Figure 3.18 Eucaryotic flagella have a characteristic '9 + 2' structure. Although functionally analogous to their procaryotic counterparts, eucaryotic flagella differ appreciably in their fine structure. A membrane surrounds an arrangement of proteinaceous microtubules, in which nine pairs surround a single central pair. Movement of eucaryotic flagella is by means of an adenosine triphosphate-driven whiplike motion eucaryotic flagella differ dramatically in their ultrastructure from those of procaryotes. Seen in cross-section, they have a very characteristic appearance, made up of two central microtubules, surrounded by a further nine pairs arranged in a circle (Figure 3.18). The microtubules are made of a protein called tubulin. Flagella in eucaryotes beat in waves, rather than rotating; cilia, present in large numbers, beat in a coordinated fashion so that some are in forward motion while others are in the recovery stroke (rather like a 'Mexican wave'!). In animals, ciliary motion has been adapted to move particulate matter across a tissue surface; ciliated cells of the respiratory tract, for example, act as a first line of defence in the removal of inhaled particles, such as bacteria from the airways.

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