Biology Today — May 2017

Flagellar sheath

Outer side arm

Inner side arm

B

B

B

B

B

B

B

B B

Matrix

Transition junction

Fig.: Ultrastructure of flagellum

Central sheath

Central

singlet fibril

(tubulin)

Head

Spoke

(proteinaceous)

Peripheral doublet fibril (tubulin) Linker

A

A

A

A A

A

A

A

A

Protofilaments

Fig.: Doublet fibril

without arms

Table: Differences between cilia and flagella

Cilia Flagella

(i) The number of cilia per cell is usually very large (300-14000). The number of flagella per cell is usually 1-4.

(ii) They are smaller (5-20 μm in length), have smooth surfaces

and occur throughout or major part of the surface of a cell.

They are longer (100-200 μm in length), may have smooth

or possess flimmer surface and are commonly found at one

end of the cell.

(iii) They beat in a coordinated rhythm either simultaneously

(isochronic or synchronous rhythm) or one after the other

(metachronic rhythm).

The flagella beat independently.

(iv) Cilia produce a sweeping or pendular stroke. The flagella produce undulatory motion.

(v) Cilia help in locomotion, aeration, feeding, circulation, etc. Flagella help in locomotion.

(vi) Cilia may fuse to form undulating membrane. Fusion of flagella is unknown.

(vii) Flame cells of worms, larval bodies of many invertebrates,

epithelium of respiratory tract, renal tubules, oviducal funnel,

etc.

Many protists, motile algae, spermatozoa of animals,

bryophytes and pteridophytes, choanocytes of sponges,

gastrodermal cells of coelenterates, zoospores and gametes

of thallophytes.

Functions

• Help in locomotion.

• Create current for obtaining food from aquatic medium.

• Circulate food in the gastrovascular cavity of coelenterates and help in movement of food and egestion in tunicates.

• Help in oxygen supply and quick diffusion of carbon dioxide in aquatic organisms.

• Eliminate dust particles which come with inhaled air in the respiratory tract of land animals.

• Help in passage of eggs in oviduct, excretory substances in kidneys, etc.

• Function as sensory organs and help in fusion of gametes.

CENTRIOLES

• They are non-membranous organelles having (9 + 0) microtubular arrangement.

• They are approximately 0.3 – 0.5 μm in length and 0.15 μm in diameter.

• Usually they are arranged in a group of two at right angles to each other, forming a pair called diplosome.

• Diplosome is present in specialised part of cytoplasm called centrosphere or kinoplasm.

• Centrosome is a complex structure formed of centrioles and centrosphere.

• They are found in almost all eukaryotic animal cells, protozoan protists (except some forms like Amoeba), some fungi and the
cells of all those eukaryotic plants where flagellate structures are present in the life cycle. They are absent in angiosperms,
higher gymnosperms, some algae and fungi.