Cytoskeleton

Mar 27, 2023, 16:45 IST

About Cytoskeleton

In eukaryotic cell, a framework of fibrous protein elements became necessary to support the extensive system of membranes. These elements collectively form cytoskeleton of the cell.

Types of Cytoskeleton.

Microtubules :

(i) Discovery : These were first discovered by De Robertis and Franchi (1953) in the axons of medullated nerve fibres and were named neurotubules.

(ii) Position : The microtubules are electron-microscopic structures found only in the eukaryotic cellular structures like cilia, flagella, centriole, basal-body, astral fibres, spindle fibres, sperms tail, neuraxis of nerve fibres etc. These are absent from amoebae, slime-moulds and prokaryotes.

(iii) Structure : A microtubule is a hollow cylindrical structure of about 250 Å in diameter with about 150 Å luman. Its wall is about 50Å thick. Its walls is formed of 13 parallel, proto-tubules, each being formed of a liner series of globular dimeric protein molecules.

(iv) Chemical composition : These are mainly formed of tubulin protein. A tubulin protein is formed of 2 sub-units : tubulin molecule and tubulin molecule which are alternatively in a helical manner.

Functions

(a) These form a part of cytoskeleton and help in cell-shape and mechanical support.

(b) The microtubules of cilia and flagella help in locomotion and feeding.

(c) The microtubules of asters and spindle fibres of the mitotic apparatus help in the movement of chromosomes towards the opposite poles in cell-division.

(d) These help in distribution of pigment in the chromatophores, so help in skin colouration.

(e) These also form micro-circulatory system of the cell which helps in intracellular transport.

(f) These control the orientation of cellulose microfibrils of the cell wall of plants.

Microfilament

(i) Position : These are electron-microscopic, long, narrow, cylindrical, non-contractile and proteins structures found only in the eukaryotic cytoplasm. These are present in the microvilli, muscle fibres (called myofilaments) etc. But these are absent from the prokaryotes. These are also associated with the pseudopodia, plasma membrane of fibroblats, etc. These are either scattered or organized into network or parallel arrays in the cytoplasmic matrix.

(ii) Discovery : These were discovered by Paleviz et. al. (1974).

(iii) Structure : Each microfilament is a solid filament of 50-60 Å diameter and is formed of a helical series of globular protein molecules. These are generally grouped to form bundles.

(iv) Chemical composition : These are mainly formed of actin-protein.

Functions

(a) The microfilaments forms a part of cytoskeleton to support the relatively fluid matrix.

(b) The microfilaments bring about directed movements of particles and organelles along them in the cell.

(d) The microfilaments also cause cleavage of animal cells which is brought about by contraction of a ring of microfilaments.

(e) The microfilaments also participate in gliding amoeboid motion shown by amoebae, leucocytes and macrophages.

(f) The microfilaments are also resoponsible for the change in cell shape curing development, motility and division.

(g) Myofilaments bring about muscle contraction.

(h) The microfilaments cause movements of villi to quicken absorption of food.

(i) The microfilaments are responsible for the movement of cell membrane during endocytosis and exocytosis.

(j) The microfilaments cause plasma membrane undulations that enable the firoblasts to move.

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Intermediate filaments

(i) Location : They are supportive elements in the cytoplasm of the eukaryotic cells, except the plant cells. They are missing in mammalian RBCes and in the prokaryotes.

(ii) Structure : The IFs are somewhat larger than the microfilaments and are about 10 nm thick. They are solid, unbranched and composed of nonmotile structural proteins, such as keratin, desmine, vimentin.