Microtubule



Microtubules are long, vacant, unbranched cylinders about 25 nm in diameter and is equal several micrometers long. There are two rules of Microtubules in the cell: Microtubules can form an internal skeleton for some cells, known as the cytoskeleton, and they also acts as a configuration forth which motor proteins can move structures in the cell.

Microtubules are assembled from molecules of protein called tubulin. Tubulin is a dimer--a molecule made up of two monomers. The polypeptide monomers that make up this protein are known as alpha-tubulin and beta-tubulin. There are thirteen chains of tubulin dimers enclose the central cavity of the microtubule.

Tubulin dimers can be added or subtracted, especially at the plus end, and the microtubules can turn longer or shorter. This capability of change the length rapidly will makes the microtubules dynamic structures, and this dynamic property of microtubules is also can be seen in animal cells, where they are often found in parts of the cell that changes the shape.

Many microtubules radiate from a region of the cell called the microtubule organizing center. Tubule polymerization cause rigidity, and tubule depolymerization can destroyed this hard structure.

Microtubules help control the arrangement of the cellulose fibers in the cell wall of plants. Electron micrographs of plants often show microtubules lying just inside the plasma membrane of cells that are forming or extending their cell walls. Experimental alteration of the orientation of these microtubules leads to a similar change in the cell wall and a new shape for the cell.



Functions
In addition to being a major component in the cytoskeleton, microtubules also play a substantial role in separating chromosomes to daughter cells during cell division, and they are intimately associated with movable cell appendages: the flagella and cilia.(Purves 81)

Cytoskeleton

 * Main Articles: Cytoskeleton

Microtubules, microfilaments, and intermediate filaments are the three components of the cytoskeleton. The cytoskeleton is an important, complex, and dynamic cell component. It acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis, the uptake of external materials by a cell; and moves parts of the cell in processes of growth and motility. There are a great number of proteins associated with the cytoskeleton, each controlling a cell’s structure by directing, bundling, and aligning filaments.

Cytoskeleton is a set of long, thin fibers that contained in the eukaryotic cytoplasm, and it has many membrane-enclosed organelles. There are at least three important roles about cytoskeleton: Cytoskeleton maintains cell shape and support. Cytoskeleton provides for various types of cellular movement. Some of the fibers of cytoskeleton act as tracks or supports for motor proteins, which help move things within the cell.(Purves 79)

Flagellum and Cilia

 * Main Article: Flagellum and Cilia

Flagella and cilia are whip-like organelles that may push or pull a cell through its aqueous environment, or they may move surrounding liquid over the surface of the cell. Flagella are longer than cilia, and flagella are usually found singly or in pairs. Waves of bending propagate from one of a flagellum to the other in snake-like undulation.

Cilia and Flagella both contain micro tubes that are arranged in an outer ring of connected microtubules which are around a non connected pair of microtubules. This arrangement is located beneath the plasma membrane.

There are three difference between cilia and flagella: First of all, flagella has only one or two arms that attached to the cell surface; second, flagella are longer; and third, flagella makes force a different direction. Cilia are shorter than flagella and are usually present in great numbers. They beat stiffly in one direction and recover flexibly in the other direction, so that the recovery stroke does not undo the work of the power stroke.(Purves 81)

Cell Division

 * Main Articles: Cell Division

Cell division is the process by which cells multiply during the growth of tissues or organs. The type of cell division involved in the growth of the body is called mitosis. The cell division which produces the reproductive cells is called meiosis. The onset of cell division is marked by the condensation of the chromosomes visible under a standard light microscope. During the first stage of cell division (prophase) the centrosomes start to develop polar microtubules that later pull the chromosome pairs apart.(Purves 171-74)