= Motile cilium
= Motile cilia
{synonym}
A \b[motile cilium] is a <eukaryotic> <cilium> that actively bends or rotates to move the cell or the surrounding fluid. Its internal <axoneme> contains organized <microtubules> and motor machinery. In familiar bending cilia, <dynein> uses energy from <ATP> to drive relative sliding of neighboring microtubule doublets; constraints on that sliding convert motor activity into a change of shape.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 16, Cilia and Flagella: axonemal motors, sliding and bending.]{ref}
Many <motile cilia> have a 9+2 arrangement: nine outer doublets around two central singlets. The nine counts doublets, rather than nine individual outer tubules. Motility should not be defined solely by this pattern, since some motile cilia have other arrangements. Conversely, a ciliary-looking projection is not necessarily motile.
A classic experiment separated the force-producing mechanism from the intact structure's constraints. <K. E. Summers> and <I. R. Gibbons> isolated sea-urchin sperm <axonemes> and briefly treated them with trypsin. Adding <ATP> then caused active separation into tubules and groups of tubules. The experiment supported sliding between structural elements as a source of motion, rather than requiring each tubule to shorten like a contracting thread.<Adenosine triphosphate-induced sliding of tubules in trypsin-treated flagella of sea-urchin sperm by K. E. Summers and I. R. Gibbons>[Abstract: retained initial 9+2 architecture and ATP-induced disintegration after limited digestion.]{ref}
The long beating projection of a eukaryotic sperm cell and short <motile cilia> share this general mechanical framework. A <bacterial flagellum> has a different construction and motor. Comparing their functions alone would hide the difference between bending a microtubule assembly and rotating a bacterial filament.
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