Cilia (singular cilium) is an organelle projecting from a eukaryotic cell. They are similar to the flagellum, a whip like tail found on some cells that usually helps in propulsion. Some kinds of Cilia can move and bend, others just float and seem to be used for sensors.
Cilia, like mitochondria and the cell nucleus, are thought to be endosymbiotic or gained when a cell "eats" another organism which eventually takes up residence in the cell. Interestingly, in one study, cilia were singled out as not being possible to have originated in this way.
A type of motor protein called the axonemal dynein powers cilia. The axoneme is the contractile structure of cilia and flagella formed by a ring of nine pairs of microtubules outside a central pair and associated proteins Dynein and Nexin. The Dynein is a massive molecular motor that transports various cellular cargo throughout the cell along microtubule tracks. The movement of these proteins provides the movement of cilia and flagella. The axoneme is the 'skeleton' of the appendages that line, for example, the lungs, and fallopian tubes.
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