THE CYTOSKELETON

The cytoskeleton is a network of fibers suspended through the cytoplasm. There are three fibers in the cytoskeleton. These are microtubuies, microfilaments and intermediate filaments.

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Functions of Cytoskeleton –

The cytoskeleton performs following functions in the organisms:

  1. They give mechanical support to the cell and help to maintain its shape The animal cells lack cell wall. So cytoskeleton is especially important for animal cells.
  2. Cytoskeleton anchor and keep in place the organelles and cytoplasmic enzymes.
  3. Cytoskeleton also enables a cell to change its shape. The cytoskeleton can be broken in one cell and reassembled in a new location.
    1. The cytoskeleton is also associated with motility. They help in the movement of the entire cell or movement of organelles of the cell.
    2. The fibers of the cytoskeleton are involved in the contraction of cilia and flagella. They enable muscle cells to contract.
    3. The cytoskeleton extends the pseudopodia of Amoeba
    4. They cause the streaming movement of cytoplasm. This streaming movement circulates the material within large cells.
    5. Vesicles are moved by the cytoskeleton. The components of the cytoskeleton help the membrane to form food vacuoles during phagocytosis.

    T pes of Cytoskeleton

    T e cytoskeleton is constructed from three fibers. Microtubules are the thickest of th three types. Microfilaments (also called actin filaments) are the thinnest. In rmediate filaments are collection of fibers with diameters between the mi rotubules and microfilaments.

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(a M1CROTUBULES

Mi rotubules are straight, hollow rods. Their diameter is about 25nm. Their length is om 200 nm to 25 pm. The Wall of the hollow tube is composed of glObular pro ems called Tublin. There are two closely related kinds of Tublin: a-Tublin.A icrotubule can elongate by the addition of tubulin proteins to one end of the tub le. Microtubules can be broken and their tubulin can be used to build micro­tub les elsewhere in the cell. Microtubules are found in the cytoplasm of all eu ryotic cells. They radiate from a centrosome in many cells. They perform foil wing functions:

  1. ome microtubules arise from center. They support the cell.
    1. Bundles of microtubules are present near the plasma membrane. They ker – the shape of cell.
    2. The micro- tubules also acts as tracks. The organelles can move in the cell on these tracks. For example, microtubules guide secretary vesicles from the Golgi apparatus to the plasma membrane.
    3. Microtubules are also involved in the separation of chromosomes during cell division.

    S. A pair of centrioles is present within the centrosome of an animal cell. Each centriole is composed of nine sets of triplet microtubules. They are arranged in a ring.

    (a) INTERMEINATE FILAMENTS


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    They have intmidiate diameter between microfilament and microtubules Its diameter is 8 to 12 nm. Chemically, they are composed of heterogonous group of protein fibers. These proteins vary in different cell types. It is larger than the diameter of microfilaments. but smaller than that of microtubules. They perform

    following functions:         •

    1. Intermediate filaments give shape to cells. They fix the organelles in the cells.
    2. They fix the nucleus in its positions.
    3. They give support to nerve cells.

    (b), MICROFILAMENTS

    Microfilaments are solid rods. They are about 7 nm in diameter. They are butt from molecules of actin. Actin is a globular protein. The actin molecules are linked into chains. Two of these chains twisted around each other and form a helix microfilament. Microfilament performs following functions.

    1. They play role in contraction of muscles.
    2. They support the microvilli in small intestine.             •
    3. They play role in the formation of pseudopodia.
    4. Microfilaments function in the elongation and retraction of cells. Microfilaments play role in the streaming movement of the cytoplasm.

    Cilia and Flagella

    Cilia and flagella are elongated appendages. They are present on the surface of some cells. They help in the mo.vernent of cells. Some stationary cells also contain cilia. These cilia help in the movement of material over the surface of cell. Flagella are 5 to 20 times longer than cilia. Both have same internal structure. They are membrane bound cylinders. This membrane encloses a matrix. This matrix contains axonemes or axial filament. Axonemes consist of nine pairs of niicrotubules. These microtubules are arranged in a circle around two central tubules. This is called 9 + 2 pattern of microtubules. The microtubules slide over each other during movement of cilia and flagella. Each microtubule has two structures:

    nein arms: Each microtubule has a pair of dynein arms. These arms

    p ojects towards the neighboring doublets.

    • S okes: Each microtubule also has spokes. These spokes extends towards

    t e centre.

    Bas .l body

    Flag .Ila or cilia come out of basal body. Basal body is present in the cytoplasm.

    It is • omposed of microtubules. Its structure is similar to centrioles. The basal

    bod controls the growth of microtubules in cilia and flagella. The microtubules in

    the asal body form a 9 + 0 pattern.

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