Chilia and Flagella



CILIA AND FLAGELLA ( the organelles for motility): 

The Ultrastructure of prokaryotic flagellum like Paramecium  and Euglena swim in water with  the help of cilia and flagella respectively. In  multicellular organisms  some living tissues ( epithelial tissues) have cilia. They beat  and create a current  in the fluid in order to move in a given direction e.g.in the wide pipe ( trachea) to push out the mucus and dust particles. Cilia beat like tiny oars or pedals ( as in a boat) and flagella bring about whiplash like movement. The arrangement of the microtubes  is termed as 9+ 2, that is, two central microtubules and nine duplet sets  surrounding them.

        In Eukaryotes, the cilia and flagella are fine, vibratile,  cytoplasmic processes born by certain cell types. Both arise from the basal bodies, are similar in chemical composition,have basically identical Ultrastructure, and serve the same purpose, i.e.,their movement either propel the organism or move the medium past a fixed cell. However, the two are distinguishable by their number, size and mode of beating. 




Location:

The cilia are found in the protozoans of the class Ciliata, on the outer body surface of larvae and adults of certain animals, in the flame cells of flatworms, and on the epithelial lining of certain cavities) respiratory tract, parts of products  and renal tubules) in animal bodies. Nematodes and arthropods lack cilia.

 The cilia lining the respiratory tract move dust particles back into the pharynx to check their entry into the lungs. Tobacco smoke damages the ciliated epithelium, allowing the dust and smoke particles to reach and harm the lungs. Hence, smoking should be avoided. 


         The flagella are confined to the protozoans of the class Flagellata, choanocytes of sponges, gastrodermal cells of coelenterates, spermatozoa of most animals and lower plants,  unicellular algae and zoospores of algae.





Structure of the cilia and Flagella 

The cilia and flagella are enclosed by a unit membrane which is an extension of the  plasma membrane of the cell. Within the membrane, is a fluid matrix having a supporting axial shaft, or axoneme. The latter is about 2__ 10 ųm long in cilia and about 150 ųm long in flagella. The axoneme is generally connected to the surrounding plasma membrane by filaments called membrane links. The axoneme is composed of eleven  microtubules. 





Two microtubes are single and lie at the centre with a gap in between. They  are called central singlets. They are connected together by a double bridge. They are surrounded by a common central sheath. The two central singlet microtubules are known as C1 and C2, which are  distinguishables by fibres bound  only to C1. Each singlet microtubules is composed of 13 protofilaments arranged in a ring. Each protofilaments is a row of œ,ß_ tubulin dimers. The remaining nine microtubules are double and lie in  a ring around  the central microtubules. The two microtubules forming a doublet  are named A and B Subtubles. The subtuble and consists of 13 protofilaments like the central singlet microtubules. The subtubles B is C _ shaped and consists of 10  microfilaments only. The subtubles A gives off two thick, about 15nm long, processes, called the inner and outer arms, from one side. The arms of all the S subtubles run clockwise. A radial spoke extends from the central sheath to each A subtubule . Each radial spoke shows a thickening the spoke head, near the central sheath. All the  doublets are joined to one another by interdoublet links. 






The pattern of organisation of cilia and flagella is 9+ 2 as compared to 9+0 pattern of the basal bodies from which they  arises. The basal body is secured into the  cytoplasm by striated rootlets composed of microfilaments. At the level of the cell membrane is a dense basal plate, where one microtubule of each triple of the basal body ends and the central  microtubles of cilium/ flagellum start.

(i) Basal Body or Kinetosome: 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 micro_ cylinder having a structure similar to a centriole with triplet fibrils present on the periphery without a central fibril, though a hub of protein is present here.


(ii) Rootlets: They are striated fibrillar outgrowths developing from the outer lower part of the basal body are made of bundles of microfilaments. They are meant for providing support to the basal body.

(iii) Basal plate: Basal plate is a high density  area which lies above the basal body at the level of plasma membrane. In the region of basal plate, one sub_ fibre of each peripheral fibril disappears. The central fibrils develop in this area.

(iv) Shaft: Shaft is the hair _ like projecting  part. It is covered on the outside by a sheath. In whiplash flagellum this sheath  is smooth while in tinsel flagellum it contains  many hairy outgrowths. These hairy outgrowths are of two types:

(a) Filmmer filaments which increase  the surface area and efficiency of propulsion. 


(b) Mastigonemes which are able to reverse the thurst of the flagellum. Flagella with mastigonemes propel the organism in the same direction as they beat while naked flagella propel the organism in the opposite direction. This sheath basically  is the extension of plasma membrane and internally,it contains a semifluid matrix having an axonema of 9 peripheral double fibrils and 2 central singlet fibrils.This arrangement is called 9+ 2 or 11_ stranded in comparison to 9+ 0 arrangement of the centriole or basal body.The two central singlet fibres are covered by a proteinaceous central sheath and are connected by a double bridge. Each peripheral fibril consists of two microtubules or sub_ fibers, B and A. A contains 13 subunits while B contains 11 subunits as it lacks the wall adjacent to A. The subfibre A is slightly  narrower bearing two bent arms, the outer one having a hook. The side arms are made up of protein dynein with ATP_ ase activity. The peripheral double fibrils as well as central singlet fibrils are made up of tubulin. Each sub_  fibre or central singlet fibril contains thirteen protofilaments. 




Functions:

The cilia and flagella serve many functions __ 
(1): The cilia or flagella act as organelles of locomotion in spermatozoa, many protozoans, larvae of certain animals and some adult animals. 

(2): The cilia or flagella also help in capturing food in many protozoans and some animals.

3): The cilia or flagella create water currents in certain aquatic animals, such as sponges, mussels,  tuniccates, lancelets. The water current brings food and oxygen and removes faecal matter, carbon dioxide,  waste matter and gametes.

4): The flagella circulate food in the gastrovascular cavity of coelenterates. 

5): The cilia in the alimentary canal of tunicates and lancelets help in the movement of food and in egestion. 

6): The cilia of respiratory tract remove solid particles from it. Smoking damages the ciliated epithelium, allowing dust and smoke particles to enter the lung alveoli.

7): The cilia of urinary and genital tracts drive out urine and gametes.

8): The cilia are though to secrete a sticky material in ciliates to keep the conjugants together.

9): The cilia are probably sensory organelles too.

10): Cilia  and flagella help in locomotion in flagellate and  ciliated organisms. 

11): The canal system of porifers operates with the help of flagella present in their collar calls or choanocytes. 

12): Cilia  performs internal transport of several organs, e.g., passage of eggs on oviduct, passage  of excretory  substances in the kidneys, etc.

13): In certain protistans, cilia fuse to form undulating membrane.
 
14): They show sensitivity to changes in light, temperature and contact.

15): Ciliated larvae take part in dispersal  of the species. 

16) Flagella occur in spermatozoon of animals and provide motility. 

17): Cilia  occur in flame cells of worms.

In some cases cilia like structure  perform a sensory function, e.g., olfactory  cilia of nose.They may be  non__ motile but can be bent by external influences. 

Kinocilia: One  non_ motile kinocilium is present  on hair cells of internal  ear and it helps in perception of sound waves and kn sense of equilibrium. 

Stereocilia: are modified  microvilli  present on hair cells of internal ear and epithelium of epididymis. They help in absorption  of excess fluid in epididymis and perception of sound waves and sense of equilibrium in  internal ear.

Chemical Composition:

The microtubules, single as well as double are composed of the globular units of the protein tubulin.The arms of A microtubules contain a  protein dynein.The latter is ATP  ase  enzyme which catalyzes hydrolysis of ATP to ADP, and transfers the free energy released direct to ciliary / flagellar work. The interdoublet links and made of a protein nexin. The radial spokes and central sheath are formed of  protein of unknown nature.

Movement:

The movement of cilia and flagella are brought about by sliding of doublets past each other rather than by their contraction. With the energy released from ATP  by dynein protein,  the arms of A subtubles bend like small oars, producing force to push the doublets past each other in the flagellum.  The  sliding motion between doublets induces the flagellum to bend.



   
          
   Cilia: The cilia may beat in metachronous or synchronous ( isochronous) rhythm. In metachronous rhythm, the cilia of a row beat one after the other, whereas in synchronous rhythm all the cilia of a row beat simultaneously. 

    A ciliary beat comprises two phases: power, or effective, stroke and recovery stroke. During an effective stroke, the cilia become stiff and move almost as straight, rigid rods with force against the surrounding medium, say water. This pushes the water backward and the organism forward. During a recovery stroke, the cilia become limp and return to their original position in a curved state. This offers minimum resistance to water. This mechanism can be compared to the rowing of a boat, in which backward strokes of the oars through water push the boat forward.   ATP  is hydrolysed during both the power stroke and the recovery stroke. 





            
 Flagella: The flagella beat independently and in an undulatory manner,  generating planer or helical waves that may start at the base or the tip. The undulatory waves that pass from the base to the tip of the flagellum, push the organisms in the direction opposite to that of the flagellum. The wave which passes from the tip to the base of the flagellum, pulls the cell through the medium. Flagella are two types: 
Tinsel type flagellum with stiff hair, the flimmer filaments,  on its side and whiplash flagellum devoid of flimmer filaments. Spermatozoon moves by flagellar action with head forward in the female genital  tract.




 
Based body has no role in the generation of flagella or ciliary motility. This is in contrast to the basal body of prokaryotic flagella.
  
   The cilia and flagella beat at a rat of about 10__ 40 strokes or waves per second and propel the organisms fairly fast.

Speed Record Holder Protists

The flagellate Monos stigmatica and the ciliate Paramecium caudatum swim at a rate of 260 ųm per second and 1500 ųm per second respectively.

  

Difference between Cilia and Flagella  





Cilia: 

1): Cilia are short, hair __ like organelles, 2__ 20 ųm in average length. 

2): They occur in relatively large numbers per cell

3): They often cover the entire cell or the entire exposed surface of a cell.

4):  They beat coordinately in groups or rows.

5): They show sweeping or rowing motion.

6): Cilia fuse in some protozoans to form undulating membranes, membranelles or cirri.

7): Cilium lacks flimmer filaments. 

Flagella:

1): Flagella are long, whip__ like organelles that may be 10__ 200 ųm long.

2): They are usually fewer per cell.

3): They are often at one end of a cell. 

4): They usually beat independently. 

5): They show undulatory motion.

6): Flagella do not fuse.

7): Flagellum may bear stiff hair, the flimmer filaments, on the side.

Differences Between   Bacterial flagellum and Eukaryotic Flagellum 

Bacterial flagellum

1): Not covered by any sheath or extension of the  plasma membrane.

2): Length is 1_ 7 ųm and diameter 20 nm.

3): It is single stranded or monofibrillar.


4): It is made up of protein flagellin. 

5): ATP  _ ase is absent.

6): The motive force for the movement  lies at the base.

7): Flagella  rootlets are absent.


8): The basal body is attached to the plasma membrane and does not have 9+ 0 organisation. 

Eukaryotic Flagellum


1): Surrounded by a sheath derived from the plasma membrane.

2): It is 100__ 200 ųm long and 0.5 ųm in diameter . The cilium is shorter ( 5_ 20 ųm long).

3): It has 11 strands or is fibrillar with 9+ 2 arrangement. 

4): It consists  of proteins  ( tubulin, dynein and others), some lipids, carbohydrates and nucleotides. 

5): ATP_ ase is present.

6): The motive force resides inside the flagellum.

7): They are present  and provide support to basal body or blepharoplast.

8): The basal body  is embedded in the outer part of cytoplasm and has 9+ 0 organisation. 


































































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