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Thursday, September 30, 2010

Vacuoles

Thursday, September 30, 2010 - 0 Comments

They are membranous sacs that are part of cytomembrane system. Vacuoles occur in different shapes and sizes and have various functions. In animals like protozoa and sponges, contractile vacuoles are present that collect water from the cytoplasm and pump it outside to maintain organism’s interval environment. Other protozoa and sponges have vacuoles for storing food.

Centrioles and Microtubule Organizing Centres:

The specialized non membranous regions of cytoplasm near the nucleus are micro tubular organizing centres. These centres of dense material give rise to large number of microtubules with different functions in cytoskeleton. One such type of centre gives rise to centrioles that lie at right angles to each other. Centrioles are small hollow cylinders that occur in pairs in most animal and lower plant cells in distinctly staining region of the cytoplasm known as centrosome or centrosphere. They are located near the nuclear envelope and are double structures that lie at right angles to each other. Each centriole is about 300 – 500nm long and 200nm in diameter. Each centriole is composed of nine triplet microtubules that radiate from the centre like the spokes of a wheel. Adjacent triplets are attached to each other by fibrils. Centrioles are also present at the bases of cilia and flagella where they are known as basal bodies or kinetosomes. At the beginning of nuclear division, the centriole replicate and two new pairs migrate to opposite poles of the spindle, the structure on which the chromosomes become aligned. Centrioles help in the formation of poles during cell division mitosis and meiosis. Centrioles form the base of the cilia and flagella.

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