Difference between Cilia and Flagella

Cilia and Flagella are fine hair like cytoplasmic processes arising from the free margins of the cell. These are structurally almost similar structures. However, they differ in size, number and mode of beating.

Distribution of the Cilia and Flagella

The Cilia occur in the protozoans of the class ciliate and members of other classes and ciliated epithelium of the metazoan. The cilia may occur on external body surface and may help in the locomotion of such animals as the larvae of certain platyhelminthes, echinodermata, mollusc and annelid. The nematode worms and arthropods have no cilia.

Cilia

Flagella

Except for sperm, the cilia in mammalian systems are not organelles of locomotion. But their effect is the same, that is, to move the environment with respect to the cell surface.

The flagella occur in the protozoans of the class Flagellata, choanocyte of the sponges, spermatozoa of the metazoan and among plants in the algae and gamete cells.

Structure of Cilia and Flagella

Both the cilia and flagella arise from a small granular structure called the basal body. The cilia and flagella are covered by a unit membrane which is an extension of the plasma membrane of the cell. They have a central filament called axoneme. The axoneme is formed of eleven microtubules. Two of them are central singlets (single microtubules) and nine peripheral doublets (pairs of microtubules). This gives a 9+2 microtubular arrangement. The axoneme get embedded in a fluid matrix and surrounded by the unit membrane. The sliding of the microtubules of the axoneme is responsible for the movement of cilia and flagella.

The axoneme of cilia has a variety of proteins such as α and β tubulins in the microtubules, dynein (the microtubule ATP ase), nexin and others (Table 1).

Table 1: Major protein structures of the axoneme of the cilia and Flagella
Serial No	Axoneme component	Function
1	Tubulin (8 nm)	Principal component of microtubules
2	Dynein (24 nm)	Project from microtubule doublets and interact with adjacent doublets to produce bending.
3	Nexin link (86 nm)	Hold adjacent microtubule doublets together.
4	Radial spokes (29 nm)	Extend from each of the nine outer doublets inward to the central pair.
5	Sheath  projections (14 nm)	Project as a series of side arms from the central pair of microtubules; together with the radial spokes these regulate the form of the ciliary beat

Ciliary and Flagellar movement

The ciliary beat consists of two strokes: Power stroke and Recovery stroke.

Four types of ciliary movements have been recognized which are as follows:

• The Pendulus ciliary movement
• The unciform ciliary movement
• The infundibuliform ciliary movement:
• The undulant movement

Difference between:

Cilia and Flagella

Table 2: Difference between Cilia and Flagella
Serial No	Cilia	Flagella
1	Numerous	Less in number
2	Short and hair like organelle (10µ)	Long whip like organelle (150μ)
3	Occur throughout the cell surface	Presence at one end
4	Beat in  coordination	Beat independently
5	Show sweeping movement or pendular stroke	Undulatory movement

Immotile Cilia Syndrome (Kartagenres’s Syndrome):

Cilary motion can be affected by many deficiencies in the protein composition of the organelle. For example, in immotile cilia syndrome, a condition characterized by severe respiratory difficulty and male sterility, the underlying genetic defect is the absence of inner and outer dynein arms on the peripheral doublets of both cilia and flagella. The symptoms of this syndrome result from the immobility of cilia in the respiratory tract and of the flagella in the sperm.

In Chlamydomonas several mutational defects have been studied in the axoneme of flagellum which may lead to paralysis of the flagellar function.

Functions of Cilia and Flagella:

• Serves as locomotory organs in ciliates, flagellates and other invertebrates.
• Help in capturing food in many protozoans and some metazoans.
• Helping in circulation, respiration and excretion in many organisms.
• The eggs of amphibians and mammals are driven out from the oviduct by the aid of vibratile cilia of the latter.

Thus, the cilia and flagella serve many physiological processes of the cell, such as locomotion, alimentation, circulation, respiration, excretion and perception of sense.