Flagella microtubules

Microtubules, an integral component of the cellular cy-toskeleton, consist of cytoplasmic tubes 25 nm in diameter and often of extreme length. Microtubules are necessary for the formation and function of the mitotic spindle and thus are present in all eukaryotic cells. They are also involved in the intracellular movement of endocytic and exocytic vesicles and form the major structural components of cilia and flagella. Microtubules are a major component of axons and dendrites, in which they maintain structure and participate in the axoplasmic flow of material along these neuronal processes. 

Motion. Muscle contraction occurs when filaments containing the proteins actin and myosin slide along one another. In cells with cilia or flagellae, microtubules, which contain tubulin protein, slide along each other, facilitating movement. 

See also Motions of Cilia and Flagella, Microtubule Systems... 

Figure 2. Electron micrograph of cross section of flagellum of mouse sperm, taken near the tip. The axoneme contains nine outer pairs of doublet microtubules and two central singlet microtubules. Several dynein arms and the fibrous sheath of the sperm are also shown.

An isolated flagellum will continue to bend actively, indicating that this function is linked to its intrinsic structure. Treatment of cilia from the protozoan Tetra-hymena with the proteolytic enzyme trypsin selectively dissolves the nexin links and radial spokes but leaves unaffected the microtubules and dynein arms. If such a preparation is treated with a small amount of ATP, the loosened microtubule doublets slide against each other and through longitudinal overlap, extend for a distance that is up to nine times the original length of the cilium (Warner and Mitchell, 1981). 

It is informative to compare these dimensions with those of the smallest structures visible in cells for example, a bacterial flagellum is -13 nm in diameter and a cell membrane -8-10 nm in thickness. Bricks of the size of the 300-residue polypeptide could be used to assemble a bacterial flagellum or a eukaryotic microtubule. Helical polypeptides may extend through cell membranes and project on both sides, while a globular protein of the same chain length may be almost completely embedded in the membrane. 

Cilia and flagella are stable microtubule-based structures which project from the plasma membranes of particular eukaryotic cells. The energy-dependent oscillations of these structures can drive material over the surface of a cell or propel the cell along. For example, the whip-like motions of cilia on the cells at the head of the fallopian tube draw newly released ova from the ovaries into and along the oviduct. The snake-like movements of the flagellum on a sperm provide these cells with movement. 

Fig. S-36 Representations of ccntrioles. basal bodies and axonemes within a cilium. la) Ccntriole/basal body showing the nine sets of triplet microtubules, (h) Cross-section of a cilium/flagellum showing the 9 + 2" array of microtubules and accessory proteins of the axoneme. (c) View of a cell indicating the organizing role of ccntrioles and basal bodies.

The bacterial flagellum is a right-hand helical fiber, composed almost entirely of the fibrous protein flagellin. That is, the flagellum does not contain microtubules, actin, myosin (see here), or any contractile system. The flagellum itself rotates as a means of propelling the bacterium (Figure 8.29). 

A variety of human cells have cilia and flagella, hairlike projections from the surface that have a strokelike motion. These projections contain a flexible organized array of microtubules. Fluid or mucus is propelled over the surface of ciliated epithelial cells by the coordinated beating of cilia. A sperm cell swims by means of a flagellum. 

Swimming by a bending-and-pushing flagellum ATP Active sliding of microtubules, carried out by attachment/ detachment of the dynein arms Spermatozoa of most species 7... 

The polymerization of flagellin to flagellum and that of tubulin to microtubule are also endothermic. The monomer-monomer bond free energy is of the same order as that in F-actin, although the dependency on the salt concentration is different in each. 

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