Cilia (L. cilium =eye lash) and flagella (Gr. flagellum – whip) are fine hair-like protoplasmic outgrowths of cells and take part in cell motility.
These organelles were first reported by Englemann (1868). Cilia and flagella are basically similar but they vary in number, length and patterns of movement.
Cilia are smaller (5-10, wm) and numerous but flagella are longer (100-200fim) and fewer (1-4 per cell).
Cilia found in ciliate protozoans, flame cells of flatworms, larval forms of many invertebrates, epithelium of respiratory tract, fallopian tube, renal tubules, fern sperms, cycad sperms etc. On the other hand, flagella found in all flagellate protozoans, choanocytes of sponges, gastrodermal cells of coelenterate and sperms of animals. In plants flagella found in zoospores, gametes of seeral algae, primitive fungi, sperms of bryophytes and pteridophytes.
Under EM, a cilium or flagellum consists of four parts – basal body, rootlets, basal plate and shaft. In case of some flagella, the shaft contains hair-like lateral appendages called mastigonemes or flimmers.
(a) Basal body:
It is also called basal granule, kinetosome or blepharoplast. A basal body present in the outer cytoplasm at the base of each cilium or flagellum. Structurally, it is similar to a centriole having nine triplet microtubules arranged in the form of 9+ 0 pattern. But, the distal or upper half of the basal body back hub and radial spokes.
The lower end of basal body develops striated rootlets that penetrate deep into the cytoplasm. The rootlets are formed of bundles of microfilaments. They serve as anchors for the basal body.
(c) Based Plate:
It is a dense plate-like region lie at the level of plasma membrane between the basal body and the shaft. In the basal plate C-sub fibre disappears and the central singlet fibrils develop.
It is hair-like projecting part of cilium or flagellum. The shaft consists of axoneme embedded in the matrix and is surrounded by sheath which is continuous with the plasma membrane. Axonema or axial filament is a contractile micro-tubular frame work which consists of 11 fibrils arranged in 9 + 2 array [i.e. 9 peripheral doublet microtubules surrounding the 2 central singlet microtubules). The two central singlet microtubules are interconnected by a double bridge and is also enclosed by a central sheath.
Each peripheral doublet has pairs of dynein arms projecting from A- sub-fibre which act as ATPase for release of energy during movement of cilia or flagella. Each A-subfibre also sends a radial spoke which ends near the central sheath with a swollen head or knob. All the peripheral doublets are interconnected by A-B linkers composed of protein nexin.
Types of Cilia and Flagella:
Cilia are distinguished into two types – kinocilia and Stericilia, The kinocilia are motile and have the axonema whereas the steriocilia are non motile and lack the axonema. Flagella are of two types – tinsel flagellum and whiplash flagellum. The tinsel flagellum bears a number of hairy outgrowths on the sheath of shaft called mastigonemes while in whiplash flagellum the surface of sheath is smooth.
Movement of Cilia and Flagella (Fig. 3.46):
The movement of cilia and flagella are brought about by a sliding microtubule mechanism in which outer doublets slide past one another without their contraction. This causes bending of cilium or flagellum. For this act, ATP hydrolysis catalyzed by dynein arms provide free energy.
The movement of cilia is a type of rowing or sweep in motion where they beat simultaneously (synchronous or isochronous) or one after the other (metachronous). A ciliary beat has two distinct phases – power stoke (or effective stroke) and recovery stroke (or return stroke). During power stroke, the cilia become stiff and move almost as straight, rigid rods with force against surrounding medium (such as water). This force pushes water backward and ciliary organism forward. This action is followed by a return stroke. During return or recovery stroke, the cilia become limp and return to their regional position in a curved state.
The movements of flagella are independent and Un-adulatory (viz., move up and down like waves). If the Un-adulatory waves pass from base to the tip, the cell pushed along. If the waves pass from tip to base, the cell pulls through the water. In tinsel flagellum, the wave moving down from base to tip pulls the cell along instead of pushing it. The flagellar beat also involves a power stroke and a recovery strokes. The cilia and flagella beat at a rate of 10 – 40 strokes or undulatory waves per second.
1. In some algae and other protists, flagella are locomotory structures, propelling the organisms through the water.
2. They create current in the aquatic medium and help in capturing food by protozoans and some animals.
3. In coelenterates, they circulate food in the gastrovascular cavity.
4. The canal system of profiles operates with the help of flagella present in their collar cells.
5. The cilia of respiratory tract remove solid or dust particles from it.
6. In aquatic organisms cilia create currents in water for renewal of oxygen supply and quick diffusion of carbon dioxide.
7. Internal transport of several organs is performed by cilia, such as passage of eggs in the oviduct, passage of excretory substances in the kidneys etc.
8. In some organisms, they also function as sensory organs.
9. The ciliary tips secrete sticky substance in many cases that help in conjugation and fusion of gametes.
10. Cilia and flagella show sensitivity to changes in light, temperature and contact.