Flagella and Cilia | Manish Mevada

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CELL : STRUCTURE AND FUNCTIONS

Flagella and Cilia 

• They are fine hair like movable protoplasmic processes of the cells which are capable of producing a current in the fluid medium for locomotion and passage of substances.
• Flagella are longer ( 100-200 um ) but fewer.
• Only 1-4 flagella occur per cell, e.g. , many protists, motile algae, spermatozoa of animals, bryophytes and pteridophytes, choanocytes of sponges, gastrodermal cells of coelenterates, zoospores and gametes of thallophytes.
• Cilia are smaller (5–20 um) but are numerous.
• They occur in group ciliata of protista, flame cells of worms, larval bodies of many invertebrates, epithelium of respiratory tract, renal tubules, oviducal funnel, etc.
• Cilia present on the tracheal and bronchial epithelial cells are specialised to send back dust particles into the pharynx so that the lungs remain unharmed.
• However, cigarette smoking reduces / stops ciliary activity so that air borne dust particles pass into the lungs of smokers causing irrepairable harm.
• Both cilia and flagella are structurally similar and possess similar parts- basal body, rootlets, basal plate and shaft.
• Basal Body or Kinetosome
• It is also called basal granule or blepharoplast
• Basal body occurs embedded in the outer part of the cytoplasm below the plasma membrane.
• It is like a microcylinder which has a structure similar to a centricle with nine triplet fibrils present on the periphery without a central fibril, though a hub of protein is present here.
• Only subfibre A is complete (having 13 protofilaments) while subfibres B and C are incomplete as they share some of their protofilaments.
• Rootlets
• They are striated fibrillar outgrowths which develop from the outer lower part of the basal body and are meant for providing support to the basal body.

• The rootlets are made of bundles of microfilaments.
• Basal Plate
• It is an area of ​​high density which lies above the basal body at the level of the plasma membrane. 
• In the region of basal plate, one sub-fiber of each peripheral fibril disappears.
• The central fibrils develop in this area. 
• Shaft. 
• It is the hair - like projecting part of flagellum or cilium. 
• The length is 5 - 20 um in case of cilium and 100-200 um in case of flagellum. 
• The shaft is covered on the outside by a sheath which is the extension of plasma membrane. 
• In whiplash Flagellum, the sheath is smooth. 
• In tinsel flagellum, the sheath contains a number of thick hairy outgrowths called flimmers. 
• Internally, it contains a semifluid matrix having an axoneme of 9 peripheral doublet fibrils and 2 central singlet fibrils. 
• This arrangement is called 9 + 2 or 11- stranded. 
• However 9 + 1 (e.g., flatworm) and 9 +0 (e.g., eel, Asian Horseshoe Crab) arrangements have also been observed. 
• The two central singlet fibers are covered by a proteinaceous central sheath. 
• They are connected by a double bridge. 
• Each peripheral fibril consists of two microtubules or sub-fibers B and A.
• The sub-fiber A is slightly narrower.  It bears two bent arms, the outer one having a hook. 
• They are about 15 nm long and made up of protein dynein with ATP - ase activity. 
• Such activity is also present in central fibrils. 
• Movement of flagella or cilia occurs due to sliding motion in which dynein arm establishes temporary connection with subtubule B of adjacent doublet fiber. 
• The pe ripheral doublet fibrils as well as central singlet fibrils are made up of tubulin.
• Each sub-fiber or central singlet fibril contains thirteen protofilaments.  The peripheral doublet fibrils are interconnected by A - B linkers of protein nexin between B - subfiber of one and inner side arm of A - subfiber of adjacent fibril. 
• Each of their A sub-fibers sends a radial proteinaceous column to the center. It is called spoke. 
• The spokes are broader internally to form heads or knobs. 
• Head is connected to central proteinaceous sheath through transition junction.

• The cilia and Nagella move by sliding of the doublet fibrils against one another. 
• Energy is provided by ATP. 
• Flagella perform independent undulatory movements while cilia show rowing type of sweeping motion either simultaneously (isochronic or synchronous) or one after the other (metachronic).
• In a flagellum, sev eral symmetrical undulatory waves pass from base to the tip.
• This pushes the cell along. Undulations passing from tip to base pull the cell through water.
• In tinsel flagellum having a number of flimmers, the undulatory wave moving down from base to tip also pulls the cell along instead of pushing it.
• There is always a power stroke and a recovery or return stroke.
• The power stroke is able to move the fluid with a jerk in the direction of the stroke.
• The cell moves in the opposite direction, if it is motile.
• The recovery or return stroke is slow and without much force.
• Therefore, it does not cause much disturbance in the fluid medium.
• Rate of cili ary and flagellar movements is 10–40 strokes per second.
• Flagellate Monas stigmatica swims at the rate of 260 pm or 40 cell length/sec.
• It has the maximum speed per body length.
• Paramoecium caudatum has a speed of 1500 pm or 12 cell lengths sec.

Functions of Cilia and Flagella

• They help in locomotion in flagellate and ciliated organisms.
• They create current for obtaining food from aquatic medium.
• In some protists and animals , the organelles take part in capturing food .
• The canal system of porifers operates with the help of flagella present in their collar cells or choanocytes.
• In coelenterates , they circulate food in the gastrovascular cavity. In tunicates and lancelets, the cilia help in movement of food and its egestion.
• In aquatic organisms cilia create currents in water for renewal of oxygen supply and quick dif fusion of carbon dioxide.
• In land animals the cilia of the respiratory tract help in eliminating dust particles in the incom ing air.
• Internal transport of several organs is performed by cilia , e.g. passage of eggs in oviduct , passage of excretory substances in the kidneys , etc.
• Being protoplasmic structures they can function as sensory organs.
• Their tips secrete sticky substance to help in conjugation and fusion of gametes.
• In certain protistans, cilia fuse to form undulating membrane.
• Cilia and flagella show sensitivity to changes in light, temperature and contact.
• Ciliated larvae take part in dispersal of the species.

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Manish Mevada

M.Sc, M.Phil, B.Ed

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