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Cilia and flagella

Introduction:

  • Microtubules are also intimately associated with movable cell appendages: the cilia and flagella.
  • Many eukaryotic cells have one or both of these appendages.
  • Cilia are smaller than flagella—only 0.25 μm in length. They may move surrounding fluid over the surface of the cell (for example, protists or cells lining tubes through which eggs move, the oviducts).
  • Eukaryotic flagella are 0.25 μm in diameter and 100–200 μm in length. (The structure and operation of eukaryotic flagella are very different from those of prokaryotic flagella).
  • They may push or pull the cell through its aqueous environment (for example, protists or sperm).
  • Cilia and eukaryotic flagella are both assembled from specialized microtubules and have identical internal structures, but differ in their length and pattern of beating.

Detailed Explanation:
 

Flagella:

  • Eukaryotic flagella are usually found singly or in pairs.
  • Waves of bending propagate from one end of a flagellum to the other in a snakelike undulation.
  • Forces exerted by these waves on the surrounding fluid medium move the cell.

Cilia:

  • Cilia (singular cilium) are usually present in great numbers.
  • They beat stiffly in one direction and recover flexibly in the other direction (like a swimmer’s arm), so that the recovery stroke does not undo the work of the power stroke.
  • The power or effective stroke propels the cilium through the surrounding fluid, which allows the movement of the organism inside the water.
  • The cilium next bends along its length, while it is pulled forward during the recovery stroke in preparation for another effective stroke.

Ultrastructure:

  • Cilia and flagella are very similar in ultrastructure.
  • The electron microscopic study of a cilium or the flagellum shows that they are covered with plasma membranes.
  • Structurally cilia or flagella is composed of four parts.

(i) Basal body / kinetosome / basal granule / blepharoplast:

  • It is also known as basal granule which is embedded in the outer parts of the cytoplasm below the plasma membrane.
  • This structure is similar to the centriole with the nine triplet fibrils without the central fibril although a central proteinaceous hub is present here.
  • It is separated from the rest of the organelles by a basal plate.
  • From the basal body arises cilia and flagella .

(ii) Rootlets:

  • These are the striated fibrillar outgrowths that develop from the outer lower part of the basal body and are meant for providing support to the basal body.
  • They are made of bundles of microfilaments.

(iii) Basal plate :

  • The area of high density lies above the basal body at the level of the plasma membrane.
  • In this region, one subfibre of each peripheral fibril disappears and allows the development of central fibrils.

(iv) Shaft:

  • It is the projecting part of the flagellum.
  • It is composed of three parts.
    (i) Covering membrane (Extension of plasmalemma)
    (ii) Matrix
    (iii) Axoneme


    flagellum

     
  • Axoneme is the main part which possesses a number of microtubules running parallel to the long axis.
  • Nine peripheral doublet fibrils are tilted at 10º angle.
  • Nine peripheral doublet fibrils and two central singlet fibrils are composed of tubulin protein. Such an arrangement of axonemal microtubules is referred to as the 9+2 array.
  • The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by a radial spoke. Thus, there are nine radial spokes.
  • Each peripheral doublet fibril contains subfibrial A and B. A subfibril has two side arms or lateral arms composed of dynein protein.
  • Out of them the outer arm has a hook. Inner arm shows ATPase activity. It also generates force for the movement of cilia thus it is considered as a locomotory motor for cilia.
  • Two peripheral doublet fibrils are connected by A–B linker composed of nexin protein.
  • A spoke originates from each subfibril A and grows towards central part.
  • The tip of each spoke is swollen, that is called the head which is connected with central proteinaceous sheath by transitional junction.
  • Cilia and flagella are different in number, length, distribution and function.

The function of cilia and flagella :

  • These are the locomotory organs allowing the organism to show movement.
  • These structures create currents in the aquatic medium for obtaining food.
  • They also take part in capturing food in some protists and animals.
  • The animals of Porifera contain flagella in their collar cells or choanocytes which operate the canal system.
  • This organelle is involved in the circulation of food in the gastrovascular cavity of coelenterates.
  • The cilia help in the movement of food (egestion) in tunicates.
  • It generates water currents for renewal of oxygen supply and quick diffusion of carbon dioxide.
  • Cilia also allows the internal transportation of the eggs in the oviduct and the passage of excretory substances in the kidney.
  • These structures are sensitive to change in light, temperature, and contact.
  • The tips of these locomotory structures also secrete sticky substances which take part in the conjugation and fusion of gametes.

Difference between cilia and flagella:
 

Cilia Flagella
The number of cilia is very large (3000-14000) The number of flagella is usually less(1-4)
These structures are smaller These structures are longer.
The coordination of the cilia occurs either simultaneously or one after the other The flagellar movement occurs independently.
Cilia usually occurs throughout the major part of the surface of the cell. They are commonly found at one end of the cell.
This produces a sweeping/pendular stoke This produces an undulatory motion
Cilia help in locomotion, aeration, feeding circulation, Flagella helps in locomotion.


Frequently Asked Questions- FAQs:
 

Question 1. What are the functions of cilia and flagella?
Answer:

  • These are the locomotory organs allowing the organism to show movement.
  • These structures create currents in the aquatic medium for obtaining food.
  • They also take part in capturing food in some protists and animals.
  • The animals of Porifera contain flagella in their collar cells or choanocytes which operate the canal system.
  • This organelle is involved in the circulation of food in the gastrovascular cavity of coelenterates.
  • The cilia help in the movement of food (egestion) in tunicates.
  • It generates water currents for renewal of oxygen supply and quick diffusion of carbon dioxide.
  • Cilia also allows the internal transportation of the eggs in the oviduct and the passage of excretory substances in the kidney.
  • These structures are sensitive to change in light, temperature, and contact.
  • The tips of these locomotory structures also secrete sticky substances which take part in the conjugation and fusion of gametes.

Question 2. Explain the 9+2 organization of the axoneme of cilia and flagella.?
Answer:

  • Axoneme is the part of the shaft which forms the cilia and flagella.
  • Axoneme is the main part which possesses a number of microtubules running parallel to the long axis.
  • Nine peripheral doublet fibrils are tilted at 10º angle.
  • Nine peripheral doublet fibrils and two central singlet fibrils are composed of tubulin protein. Such an arrangement of axonemal microtubules is referred to as the 9+2 array.
  • The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by a radial spoke. Thus, there are nine radial spokes.
  • Each peripheral doublet fibril contains subfibrial A and B. A subfibril has two side arms or lateral arms composed of dynein protein.
  • Out of them the outer arm has a hook. Inner arm shows ATPase activity. It also generates force for the movement of cilia thus it is considered as a locomotory motor for cilia.
  • Two peripheral doublet fibrils are connected by A–B linker composed of nexin protein.
  • A spoke originates from each subfibril A and grows towards central part.
  • The tip of each spoke is swollen, that is called the head which is connected with central proteinaceous sheath by transitional junction.

Question 3. What is the difference between the cilia and flagella?
Answer:

 

Cilia Flagella
The number of cilia is very large (3000-14000) The number of flagella is usually less(1-4)
These structures are smaller These structures are longer.
The coordination of the cilia occurs either simultaneously or one after the other The flagellar movement occurs independently.
Cilia usually occurs throughout the major part of the surface of the cell. They are commonly found at one end of the cell.
This produces a sweeping/pendular stoke This produces an undulatory motion
Cilia help in locomotion, aeration, feeding circulation, Flagella helps in locomotion.


Question 4. What is the basal body? Mention its significance and organization.
Answer:

  • Basal bodies are also known as basal granule which is embedded in the outer parts of the cytoplasm below the plasma membrane.
  • This structure is similar to the centriole with the nine triplet fibrils without the central fibril although a central proteinaceous hub is present here.
  • It is separated from the rest of the organelles by a basal plate.
  • Cilia and flagella arise from the basal body.

Question 5: What is the difference between the structural organization of centriole and cilia/flagella?
Answer: Structure of centriole (9+0 organization):

  • A centriole possesses a whorl of nine peripheral fibrils.
  • Fibrils are absent in the centre and it shows 9 + 0 arrangement.
  • Fibrils run parallel to one another angled 40° to each other.
  • Each fibril is made up of three sub fibres. Therefore, it is called triplet fibril.
  • The three subfibres are in reality microtubules joined together by their margins and, therefore, sharing the common walls made of 2-3 protofilaments.
  • From outside to inside the three sub-fibres of a triplet fibril are named as C, B and A. Subfibre A is complete with 13 protofilaments while B and C subfibres are incomplete due to sharing of some microfilaments.
  • The adjacent triplet fibrils are connected by C—A proteinaceous linkers.
  • The centre of the centriole possesses a rod-shaped proteinaceous mass known as a hub.
  • From the hub, develops 9 proteinaceous strands towards the peripheral triplet fibrils. They are called spokes. Each spoke has a thickening called X before uniting with A sub-fibre.
  • Another thickening known as Y is present nearby and is attached both to X thickening as well as C—A linkers by connectives.
  • Due to the presence of radial spokes and peripheral fibrils, the centriole gives a cart wheel appearance.
  • On the outside of the centriole are present dense, amorphous, protoplasmic plaques in one or more series. They are called massules or pericentriolar satellites.

Structure of Cilia/ Flagella (9+2 organization):

  • Axoneme of the cilia/ flagella posses a number of microtubules running parallel to the long axis.
  • Nine peripheral doublet fibrils are tilted at 10º angle.
  • Nine peripheral doublet fibrils and two central singlet fibrils are composed of tubulin protein. Such an arrangement of axonemal microtubules is referred to as the 9+2 array.
  • The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by a radial spoke. Thus, there are nine radial spokes.
  • Each peripheral doublet fibril contains subfibrial A and B. A subfibril has two side arms or lateral arms composed of dynein protein.
  • Out of them the outer arm has a hook. Inner arm shows ATPase activity. It also generates force for the movement of cilia thus it is considered as a locomotory motor for cilia.
  • Two peripheral doublet fibrils are connected by A–B linker composed of nexin protein.
  • A spoke originates from each subfibril A and grows towards central part.
  • The tip of each spoke is swollen, that is called the head which is connected with central proteinaceous sheath by transitional junction.
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