Ultrastructure and Mechanics of Flagellar/Ciliary Locomotion

Ultrastructure and Mechanics of Flagellar/Ciliary Locomotion

Flagella and cilia are both hair-like, motile cell extensions used for locomotion in protozoans, and both share the same fundamental internal architecture, called the axoneme.

The axoneme is built on the classic "9+2" arrangement: nine outer doublet microtubules arranged in a ring around a central pair of two single microtubules, the whole assembly enclosed within an extension of the cell membrane. Each outer doublet microtubule bears a pair of dynein arms — motor protein complexes that use the energy of ATP hydrolysis to generate a sliding force between adjacent microtubule doublets. It is this ATP-powered sliding of the doublets past one another, constrained and converted into a bending motion by the structural links holding the axoneme together, that produces the whip-like beating or undulating movement of the flagellum or cilium.

In ciliates, the many cilia covering the cell surface are not scattered at random but are typically arranged in longitudinal rows; each such row, together with its associated basal bodies and connecting fibrils just beneath the cell surface, is called a kinety. The kinety system helps coordinate the beating of the many cilia in a row so that they work together rather than independently.

Ciliary beating itself can be coordinated in two contrasting patterns: synchronous beating, in which all the cilia across the cell surface beat in unison, moving together at exactly the same phase of the beat cycle; and metachronous beating, in which the cilia beat in a coordinated, sequential wave, with each cilium slightly out of phase with its neighbour, so that a wave-like pattern of movement appears to sweep across the ciliated surface. Metachronous beating (seen, for example, in Paramecium) is generally more effective at producing smooth, continuous, directional movement than fully synchronous beating.