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Encyclopedia > Cilia
cross-section of two cilia, showing 9+2 structure

A cilium (plural cilia) is a fine projection from a eukaryotic cell that constantly beats in one direction. They are structurally identical to eukaryotic flagella, and the terms are often used interchangeably. In general, though, the term cilium is used when they are numerous, short and coordinated while flagellum is used when they are relatively sparse and long. The name cilium may also be used to emphasize their differences from bacterial flagella.

These structures are found in all animalia except arthropods and nematodes. They are rare in plants occurring most notably in cycads. Protozoans with cilia (ciliates) use them for either locomotion or to simply move liquid over their surface. Most other organisms that have cilia use them only to move liquid over their cell's surface. Cilia are almost never found alone, usually being present on a cell's surface in large numbers that beat in unified waves.

In humans, cilia are found for example in the lining of the windpipe, where they sweep mucus and dirt out of the lungs, and in the oviducts, where they move the ovum from the ovary to the uterus.

A cilium has an outer membrane that surrounds a matrix which contains nine pairs of microtubules around a central core with two additional microtubules. Biologists refer to this organization as a 9 + 2 structure.

  Results from FactBites:
Cilia and Flagella (645 words)
Cilia and flagella move liquid past the surface of the cell.
In the case of cilia and flagella, dynein powers the sliding of the microtubules against one another — first on one side, then on the other.
A primary cilium extends from the apical surface of the epithelial cells lining the kidney tubules and monitors the flow of fluid through the tubules.
Cilia, flagella, and centrioles (718 words)
Cilia and flagella are projections from the cell.
Cilia and flagella move because of the interactions of a set of microtubules inside.
Cilia and flagella are organized from centrioles that move to the cell periphery.
  More results at FactBites »



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