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A bacterial flagellum is a rotary swimming appendage comprising a filament, a flexible hook and an envelope-associated bacterial flagellar motor. Rotation of the helical filament converts motor torque into propulsion through a liquid. It differs structurally from a eukaryotic cilium, which bends through the action of dynein on microtubules. In the classic tethered-cell experiment, attachment through a flagellar filament lets the cell body rotate instead, making otherwise hard-to-see motor rotation visible by microscopy. ^[ref]

A cellular projection whose structure and functions differ from the bacterial flagellar apparatus. Motile cilia can move fluid; a transport estimate must compare advection with diffusion.

A eukaryotic cilium contains an organized microtubule-based core. Motile forms use dynein-dependent sliding and bending, while many nonmotile primary cilia participate in sensing. These functions distinguish cilia from the rotary filament of a bacterial flagellum. ^[ref]

A 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.^[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.^[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.