cilia and flagella
/ SIL-ee-uh and fluh-JEL-uh /
Think of the oars on a rowboat or the long tail a tadpole uses to swim. Cells have their own versions: thin, whip-like projections that beat to move fluid past the cell, or to move the cell itself through fluid. Short, numerous ones are called cilia; long, usually solitary ones are called flagella. They are how many single cells swim and how some of our own tissues sweep liquids along.
Despite different names and lengths, eukaryotic cilia and flagella share the same internal engine, called the axoneme: a ring of nine microtubule pairs around (usually) two central microtubules, the famous nine-plus-two arrangement. The motor protein dynein, anchored between neighboring microtubule pairs, walks along its neighbor and makes the pairs slide against each other; because they are tied together at the base, sliding turns into bending, and repeated bending produces the beat. Cilia tend to beat with a back-and-forth rowing stroke, while flagella often beat with a snake-like wave.
These structures show up in two kinds of jobs. Some move the cell: a sperm cell swims with a single flagellum, and many protists swim with cilia. Others move fluid past a stationary cell: ciliated cells lining your airways sweep mucus and trapped dust up out of the lungs, and cilia in the brain circulate cerebrospinal fluid. An important caution: the eukaryotic flagellum described here is completely different from the bacterial flagellum, which is a rotating protein propeller built on a different principle.
Smoking paralyzes and destroys the cilia lining the airways, which is why long-term smokers develop a chronic cough — the cilia can no longer sweep mucus and tar out of the lungs.
Airway cilia are a frontline defense; damaging them removes a key way the lungs clean themselves.
Eukaryotic cilia and flagella (microtubule-based, dynein-bent) are unrelated to the bacterial flagellum (a spinning protein motor); they only share the name, not the structure.