Cilia
Cilia are tiny, hairlike structures found on surfaces of many animal and protozoan cells as well as in some lower plants. They are very similar in both structure and function to flagella. Typically about 0.82 ft (0.25 m) in diameter and 33 ft (10 m) long, cilia are enclosed within the cellular (plasma) membrane and have an interesting and complex interior structure called the axoneme made of microtubules and a large number of associated macromolecular proteins. The basic function of cilia is to move in a whip-like motion that provides motile force to allow single cells to swim through a fluid medium or to move liquids or particles across the cell surface. Protozoans, for instance, use cilia to swim, as well as to capture food and move it into their gullet. Many epithelial tissues in higher animals have a carpet of cilia that moves mucus or particulate material across cell surfaces. For instance, respiratory passages are lined by cilia that push dust, pollen, and other contaminants up out of the lungs.
Similarly, the uterine tubes in females have a ciliary carpet that moves ova down to the uterus for implantation.
The microtubules in the ciliary axoneme are generally arranged in a circle of nine sets of fused doublets arrayed in a circle around two individual central microtubules. The central pair is enclosed in a spiral protein sheath and the outer doublets are attached to this central sheath by radial spokes in a wagon wheel-like structure. Curving arms made of an important protein called dyein reach from one doublet pair to the next, while a linking protein called nexin forms temporary bonds between adjacent doublets. Dyein is one of a family of molecular motors that use energy from adenosine triphosphate (ATP) molecules to change shape and move structures around within cells. In the case of cilia, the dyein molecules "walk" along microtubular surfaces and move one doublet set relative to the others. Coordinated attachment and release of many dyein arms throughout the cilium causes bends to move progressively along the length of a cilium and create the complex motions that allow beating and recovery strokes for individual cilia. How these complex motions are coordinated between the many cilia that cover many cell types is not yet known, but it is thought that ion channels in the surface membrane probably play a role in this process.
Interestingly, the sensory organs in higher animals often are derived from non-motile cilia. The light-sensitive rod and cone cells in the retina of the human eye, the chemical receptors of the olfactory cells in the human nose, and the sound sensitive hair cells in the human inner ear, for instance, all have highly modified, non-motile cilia as the basis of their sensory apparatus.
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