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 The cytoskeleton is a network of fibers that organizes structures and activities in the cell. It is cellular skeleton contained within a cell's cytoplasm.

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Presentation on theme: " The cytoskeleton is a network of fibers that organizes structures and activities in the cell. It is cellular skeleton contained within a cell's cytoplasm."— Presentation transcript:

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3  The cytoskeleton is a network of fibers that organizes structures and activities in the cell. It is cellular skeleton contained within a cell's cytoplasm.  The cytoskeleton is present in prokaryotic and eukaryotic cells.  It forms structures such as flagella and cilia and plays important roles in both intracellular transport (the movement of vesicles and organelles) and cell division.  In 1903 Nikolai K Koltsov proposed that the shape of cells was determined by a network of tubules that he termed the cytoskeleton.  Cytoskeleton is composed of microtubules, microfilaments and intermediate filaments.

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6 COMPONENTS OF CYTOSKELETON

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10  Microtubules are thickest and their role is compression resistance.  Hollow rods about 25nm in diameter and 200nm to 25 μm in length.  The wall of hollow tube is made of globular protein called tubulin  Tubulin protein is a dimer consists of α-tubulin and β-tubulin.  They grow by the addition of tubulin dimer.  Due to the orientation of tubulin dimer there are two ends of microtubles; one end can accumulate and release tubulin dimer with much higher rate thus grows and shrinks faster with the celluler activities. It is plus end.  Other end has slow rate of on and off.

11  Microtubules shape and support the cell  It serves as track along which organelles equipped with motor proteins can move.  Guide secretory vesicles from golgi apparatus to plasma membrane.  Involved in separation of chromosomes during cell division.

12  Centrosomes: Microtubules grow out from centrosome located near the nucleus and known as microtubule organizing center.  Microtubules are known as girders of cytoskeleton.  Centrioles: Within the centrosome is a pair of centrioles. Each composed of nine sets of triplet microtubules arranged in a ring. Before the division of animal cell centriole replicates. In plant cell centrosomes lack centrioles. Centrioles are not present in the cells of higher plants. One of the important functions that centrioles perform is the generation of cilia and flagella for cells. These are surface features that cells use for movement.  The cells of higher plants have no centrioles because they do not make cilia or flagella. A cell can make a spindle without centrioles, but it can't make cilia or flagella without centrioles.

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15  Cilia and Flagella: In eukaryotes special arrangement of microtubule is responsible for the beating of flagella and cilia.  Unicellular organisms and sperms of algae move via flagella or cilia. They can move in a wave to propelled the surrounding liquid thus cause movement. i.e. Cilia lining of windpipe sweeps mucus out of lungs. The cilia lining of oviducts help move an egg towards uterus.  Cilia are large in number, 0.25 μm in ф and 2-20 μm in length.  Flagella are 0.25 μm in ф and 10-200 μm in length, usually one or few per cell.

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18  Flagella and cilia differ in beating pattern.  Flagellum has an undulating motion that generates force in the same direction of its axis. Cilia work with alternating power strokes generating force perpendicular to the axis of cilium.  Cilia act as antenna to receive signals for the cell. Usually one/cell. Membrane proteins on cilia transmit molecular signals from the environment of the cell to the interior.  Flagella and cilia are similar in ultrastructure. Nine doublets of microtubules make a ring having two single microtubles in the center. Cross linking motor proteins along the length of flagellum and cilium connect the outer neighboring doublet. Centriole like basal body anchored the flagellum or cilium in the cell. Basal body of sperm in animals/humans enter the egg and become centriole.

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24  The motor protein extends from one doublet to another is know dynein. Dynein arms perform movements cause by conformational changes in proteins at the expense of ATP. Like a cat climbing on tree the arms of one doublet attach to the arms of the other and pull to slide the doublet. Then arms release and attach further. Walking of doublets.  For bending flagellum/cilium, the two doublets can not slide but held in, thus cause the bending i.e. bending of knee (bone and microtuble)

25  Microfilaments: (Actin filaments): These are solid rods about 7nm in ф in all eukaryotic cells. Made up of actin a globular protein. They are twisted double chain of actin subunits. They form structural network due to the proteins that bind along the side of an actin filament and extend a new filament as branch.  Microfilament in cytoskeleton bears tension (pulling force).  They for 3-D network in cytoplasm, thus supports the cell’s shape. This network gives the cortex, a semisolid like a gel.  In animal cell they help in transport of material across the plasma membrane i.e. In intestinal cells they form the core of microvilli.

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28  Microfilament are involved in cell motility i.e. contratile apparatus of muscle cells.  Thousands of actin filament are arranged parallel along the muscle cell interdigitating with myosin protein filament. Myosin act as motor protein by means of projections that walk along the actin filaments. Contraction of the muscle cell results from the sliding of actin and myosin filaments.  Actin-myosin aggregate cause localized contractions of the cells i.e. furrow formation in the dividing cell to form two daughter cells.  Actin-myosin contractions results in ameoboid movements by extending pseudopodia (toothpast release from tube). Another example is white blood cells.  Cytoplasmic streaming in large plant cells is caused by microfilaments for even distribution materials within the cell.

29  Intermediate Filaments: They have intermediate ф of 8-12 nm. Specialized for bearing tension.  They are diverse class of cytoskeleton elements, constructing from a different protein subunits of keratins.  They are more permanent elements.  They persists after cell death. Outer layer of skin is made up of keratin. They do not dissolve in chemicals that remove microtubules and microfilaments.  They give shape to the cell and fix the positions of cell organelles. i.e. nucleus sits within the cage of IF.  They make the nuclear lamina.  Long extensions of nerve cell contain intermediate filaments.

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31  Cytoskeleton gives mechanical support to the cell and maintain its shape. Specially in animal cells which lack cell walls.  Architecture of cytoskeleton is like a dome tent and stabilized by the balance between the opposing forces exerted by its elements.  It provides anchorage for many organelles and cytosolic enzyme molecules.  It is very dynamic and may dismantled in one part of the cell and reassembled in new location, changing the shape of the cell

32  It plays a role in cell motility and movements of cell organelles.  Cell motility requires the interaction of cytoskeleton with motor proteins work together with plasma membrane molecules to allow whole cell to move along fibers outside the cell.  Motor proteins cause the bending of the cilia and flagella by gripping microtubules and sliding them against each other.  Same mechanism involves the muscle cell contraction.

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35  Inside cell vesicles and other organelles use motor protein as “feet to walk” along a track provided by cytoskeleton  This is how the vesicles containing neurotransmitter molecules migrate to the tips of axons, the long extensions of nerve cells that release these molecules as chemical signals to adjacent nerve cells.  Cytoskeleton manipulates plasma membrane to form phagocytic vesicles.  Cytoplasmic streaming that circulate materials within the plant cells is due to cytoskeleton.

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