Cytoskeleton

The cytoskeleton of a cell refers to a dense network of protein fibers that provides the framework that supports the shape of the cell.3 The cytoskeleton also anchors organelles like the mitochondria to fixed locations within the cell interior. These protein fibers form a dynamic system as they are constantly being formed and disassembled. There are three different kinds of protein fibers that make up the cytoskeleton: microfilaments or actin filaments, microtubules and intermediate filaments. Actin filaments are long fibers about 7nm in diameter.3 Each filament is composed of two protein chains loosely twined together like two strands of pearls. The globular protein actin is the subunit on each of the chains. Actin filaments are found throughout the cell but are most highly concentrated just inside the plasma membrane. Actin filaments are responsible for cellular movements such as contraction during division and formation of cellular extensions. Microtubules are about 25nm in diameter and are hollow tubes composed of tubulin protein subunits that are arranged side by side to form the tube. In many cells, microtubules form from nucleation centers near the center of the cell and radiate toward the periphery. The ends of the microtubule are designated as “+” (away from the nucleation center) or “-” (toward the nucleation center).3 Microtubules are comparatively stiff cytoskeletal elements that serve to organize metabolism and intracellular transport in the non-dividing cell and to stabilize cell structure. Microtubules are the structures responsible for the movement of chromosomes during the process of mitosis. Intermediate filaments are made of overlapping staggered tetramers of protein, which themselves are then bundled into cables. This molecular arrangement allows for a rope-like structure that imparts tremendous mechanical strength to the cell. Intermediate filaments are intermediate in size between actin filaments and microtubules, with a diameter distance of 8 -10nm.3 Once formed, intermediate filaments are stable and usually do not break down. As such, they provide structural reinforcement to the cell and organelles. The cytoskeleton also lends itself as a scaffold for ribosomes to carry out protein synthesis as well as for enzymes to be localized within defined areas of the cytoplasm, in addition to its responsibility of maintaining the cell’s shape. By anchoring particular enzymes near one another, the cytoskeleton participates with organelles in organizing the cell’s activities.3

Centrioles are complex structures that assemble microtubules from tubulin subunits in the cells of animals and most protists. Centrioles occurs as a pair within the cytoplasm, usually located perpendicular to one another. They are commonly found near the nuclear envelope and are among the most structurally complicated microtubular assemblies of the entire cell. In cells that contain flagella or cilia, each flagellum or cilium is anchored by a form of centriole called a basal body. Most animal and protist cells have both centrioles and basal bodies. Higher plants and fungi lack these structures and have mechanisms to organize microtubules without such structures. Centrioles resemble spirochete bacteria in many ways, despite the obvious lack of a membrane around centrioles. Some biologists believe that centrioles, like mitochondria and chloroplasts, originated as symbiotic bacteria. Centrioles anchor and assemble microtubules. Centrioles are composed of nine triplets of microtubules and usually occur in pairs. All cell motion is, essentially, tied to the movement of actin filaments, microtubules, or both. Intermediate filaments act as intracellular tendons to prevent excessive stretching of the cells, while actin filaments play the role of determining the shape of cells. Actin filaments can form and dissolve readily, so they enable some cells to change shape quickly. Some eukaryotic cells contain flagella, which are threadlike organelles protruding from the cell surface. Some of the microtubules extend up into the flagellum, which consists of a circle of nine microtubule pairs surrounding two central ones. This 9 + 2 arrangement is a fundamental feature of eukaryotes and apparently evolved early in their history. In humans, we find a single long flagellum on each sperm cell that propels the cell in a swimming motion. If flagella are numerous and organized in dense rows, they are called cilia. Cilia do not differ from flagella in their structure, but cilia are usually short. Paramecium is a bacteria covered with cilia, giving it a furry appearance. Dense areas of cilia project from cells that line the trachea, to move mucus and dust particles out of the respiratory tract into the throat, in the human body. Although eukaryotic flagella serve a similar function as flagella found in prokaryotes, they are arranged different in their structure.

 

References

1) B Alberts, A. J. (2002). Molecular Biology of the Cell (4th ed.). New York: Garland Science.

2) O’Sullivan JM, P. D. (2013). The nucleolus: a raft adrift in the nuclear sea or the keystone in nuclear structure? Biomolecular Concepts, 277-86.

3) Hardin, J., Bertoni, G., & Kleinsmith, L. J. (2015). Becker’s World of the Cell (8th ed.). New York: Pearson.

4) Eurell, J. A., & al, e. (2006). Dellmann’s textbook of veterinary histology. Wiley-Blackwell.

5) Dorland, W. A. (2012). Dorland’s Illustrated Medical Dictionary (32 ed.). Elsevier.

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