Nov 262012

Cilia and flagella are one of the most important parts of the cell. They are fine, hair-like protoplasmic processes of the cells which are similar to each other in structure. They are present in some motile cells. Both of them are the organs of locomotion in the cell. Flagella are longer than cilia and cilia are found more in number than flagella in a cell.

A flagellum is about 100 to 200 µm long. Only one to four flagella occur per cell. Many members of kingdom protista, motile algae, sperms of animals, bryophytes, pteridophytes, zoospore and motile gametes of algae etc. contain flagella in them.

Cilia are smaller parts of cells which are about 5 to 20 µm long and are present in high number. Ciliata of protozoa, flame cells of flatworms, larval bodies of many invertebrates, epithelium of respiratory tract, renal tubules, oviductal funnel etc. contain cilia.

Both cilia and flagella are made up of microtubules running longitudinally along its length which are covered by membrane extensions of plasma membrane.

Nov 232012

Centrioles are non-membranous organelles which are present in the cytoplasm near the nucleus of the cell. They are present in all animal cells and in flagellated motile cells of lower plants. They have ability to repcentrioleslicate and occur in pairs which are called diplosomes. In a diplosome, two centrioles lie at a right angle to each other. Diplosome is covered by cytoplasmic sheath which is known as centrosphere. The complex structure which is formed by centrospheres and centrioles is called centrosome.

Each centriole is cylindrical and about 500 nm long and 200 nm in diameter. Each centrole is composed of nine groups of microtubules arranged in a circular manner, i.e. 9 triplets+0 arrangement of microtubules. Each of nine groups of microtubules is a triplet which is composed of three microtubules at the periphery end and no microtubules at the centre. The adjacent triplets appear to be connected to each other by fibrils giving a cart-wheel structure of centrioles in transverse section (T.S.).

Centrioles perform different functions inside the cell. They help to form spindle fibres. During cell division, at the beginning of nuclear division, centrioles replicate, separate and migrate to opposite poles from where they give spindle fibres. Similarly, they also help to form basal body of cilia and flagella.

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