Here is your short essay on Cilia and flagella

Cilia and flagella are specially differentiated, fine, hair like, filamentous extensions of cytoplasm associated with either the locomotion of cells (e.g. sperms) or to move fluid past the cells (e.g. ciliated epithelial cells).

Both cilia and flagella are constructed from microtubules and both are identical in their morphology and physiology. They differ in their lengths, numbers per cell and patterns of movement.

Flagella are longer (10 microns to several millimeters) where as cilia are shorter (0.2 to 10 microns) in length. Only one or pair of flagella is present in a cell, whereas cilia are much more in umber per cell.

Each cilium or flagellum consists of a shaft constituted by bundle of microtubules called axoneme entirely unsheathed by the extension of plasma membrane. The axoneme is made of a cylindrical array of 9 evenly spaced microtubules each with a partial microtubule attached to it.

This gives the structure a “Figure 8” appearance. Two single microtubules run up through the centre of the bundle of 9 microtubules so as to form a “9 +2” arrangement. The motion of the cilia and flagella is created by the microtubules sliding past one another.

Each cilium or flagellum grows out from, and remains attached to a basal body or kinetosome or basal granule embedded in the outer part of cytoplasm. Basal bodies have the same structures as the centriole, with triplets of microtubules present at the periphery without central fibril (9 + 0 arrangement)

Rootlets develop from the outer lower part of basal body to provide support to it. Rootlets are striated fibrilar outgrowths made of bundles of microfilaments.

Some vertebrate cells carry a single cilium which lacks the central pair of microtubules in the axoneme (9+0 arrangement). These are called primary cilia. As they lack the central pair of micro filaments, they cannot beat. These cilia are involved in sensory reception.

The primary cilium extending from apical surface of epithelial cells lining the kidney tubules monitor the flow of fluid in Kidney tubules and act as Mechanoreceptors. Inherited defects in their formation cause polycystic kidney disease in man.

The primary cilium of olfactory neurons acts as chemoreceptor in detecting odor. The outer segments of the rod-cells in the vertebrate retina are derived from primary cilia acting as photoreceptors.

The movement or beats of cilia always involve two types of strokes – Power stroke and recovery stroke the power stroke is by a bending motion that moves the surrounding fluid with a jerk in the direction of the stroke. As a result the cell moves in opposite direction.

The bending is achieved by the length-wise sliding of microtubule doublets along each other. During the recovery stroke the cilium returns to its original position with a flexible and slow movement without causing much disturbance in the surrounding fluid medium.

This action is analogous to rowing a boat where the ores provide backward stroke that propels the boat forward and the ores are brought back to their original state by them above water. But the flagellum shows a number of simultaneous undulating waves moving up from base to tip and pulling the cell along instead of pushing it.

Functions

1. Motility: Protozoa and some have characteristic cilliary locomotion. Invertebrate larvae move in water with the help of cilia. Sperms have flagella for movement.

2. Nutrition: Fresh water mussels, rotifers, herdmania, amphioxus etc. have characteristic cilliary feeding.

3. Respiration: The activity of flagella and cilia cause water current that helps in gaseous exchange.

4. Circulation: In many annelids and starfishes, movement of coelomic fluid is facilitated by flagellar activity.

5. Excretion: Movement of material in uriniferous tubules of kidney and genital duct is aided by cilia/flagella. Ciliated cells near the oral end of sea-anemones remove undesirable particles.

6. Cilia and flagella act as sensory structures.