A flagellum is a lash-like movable appendage - often used as a means of propulsion - which is attached to the cell body of many bacteria and some eukaryotic cells. There are some notable examples in plants (e.g. fern spores) and even mammals (e.g. sperm cells).

The flagellum was probably first noted and described shortly after the invention of the microscope, but its method of operation has still not yet been fully explained.

Many species of bacteria swim to find food or to avoid toxins. Swimming motility depends on helical flagella that act as propellers. Each flagellum is driven by a rotary molecular engine–the bacterial flagellar motor–which draws its energy from an ion flux entering the cell. Despite much progress, the detailed mechanisms underlying the motor's extraordinary power output, as well as its near 100% efficiency, have yet to be understood."

Source : Steps in the Bacterial Flagellar Motor PLOS Computational Biology

Also see : Mucociliary Clearanceplugin-autotooltip__plain plugin-autotooltip_bigMucociliary Clearance

"Mucociliary clearance is the first line of defense of the mammalian respiratory system. The mucociliary system consists of two major parts: mucus and cilia. Mucus is a thin fluid layer that coats the lung airway; cilia are hair-like organelles protruding from the cell surface and bathed in the mucus. With breathing, the mammalian respiratory tract is constantly in contact with particles in the air that could contain potentially infectious microorganisms or toxic substanc…

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