Cilia and flagella are two important cellular organelles with a similar microtubular structure. The basic structure comprises a basal body of nine sets of three microtubules ( triplets) arranged in a ring-like pattern--no tubules in the center of the circle( the 9+0 pattern). The central part of these organelles, the part above the plasma membrane, is called the axoneme. It contains 20 microtubules in a pattern of nine fused pairs (doublets) arranged in a circle. In addition the axoneme has an unfused pair of microtubules (singlets) in the center of the cylinder. Functional similarities notwithstanding, secondary cilia, and flagella have different functions in the human organism. The main function of human secondary cilia is to move fluids over the surfaces of cells. This is exemplified by the cilia of the epithelial lining of the trachea . The wave-like, coordinated beating of these cilia moves mucus secretions away from the lungs towards the mouth to be expectorated. Mucous fluids carry dirt, cellular debris and microorganisms which are potentially dangerous to healthy lung function . Therefore, the cilia of the respiratory tract serve a protective purpose. The only flagella in human organism are the tails of sperms. The basal body and and the axoneme of a spermatozoan are similar in structure to those of secondary cilia. However, the function of the human flagellum is to move cells through fluids and not to move fluids over cells. The secretions of the female genital tract are watery mucoid liquids. Sperms must travel through these fluids. Under favorable circumstances, the wiggling movements of sperm tails propel them from vagina, through cervix, and through uterus to arrive in fallopian tubes-- usually, in less than 40 minutes. At the end of that journey, sperms meet an oocyte in the distal end of the fallopian tube; if conditions are favorable, a sperm fertilizes the oocyte.
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Human motile cilia are only a few microns long and less than a micron in diameter. They comprise a set of 20 microtubule (9 doublets +2 singlets) which form the axoneme. the function of motile cilia is to move fluid over cells in one direction. This is accomplished by coordinated, wave-like motion of the cila. The motion is produced by the sliding and bending of microtubules in the axoneme. The microtubular motor proteins (kinesins and dyneins) link the microtubules and coordinate their sliding as well as their bending motions. In addition, dyneins and kinesins act as ATPases that liberate ATP- stored energy, which is necessary to power ciliary motion. In humans, motile cilia are found on cells in the respiratory tract, in the fallopian tubes, in the ventricles of the brain, and at other sites. Flagellar motion has been described as propulsive, undulating, wiggling , wriggling and serpentine. I Microscopic examination of human sperm movement under laboratory conditions show a clear side-to-side, lashing of sperm tails. The trajectory of the whole sperm may not be linear over the whole journey, but it is goal-directed. Microtubules and microtubular proteins play roles in flagellar movements and function as they do in ciliary function.