Flagella & Cilia

Ultrastructure of Flagellum:-

Electron microscopy has shown that the flagellum has 3 parts:

1. Outer coat:- A contractile membranous sheath that is physically continuous with the cell membrane but it contains far less amount of protein than the latter.

2. Matrix:- The bounded space of flagellum contains a watery substance known as matrix. The axoneme is embedded into it.

3. Axoneme:- It is the inner core, composed of microtubules & other proteins.

Here microtubules are modified & arranged in a ring of 9 doublets surrounding a pair of central singlet. This arrangement is known as ‘9+2’ array.

Each of the central microtubule is complete & composed of 13 protofilaments. Both central microtubules are connected by a bridge & are enclosed in a common central sheath.

Each of 9 peripheral doublet consists of 2 microtubules, one is smaller (A) & complete, having 13 protofilaments & lying closer to axis ; the other microtubule (B) is larger & incomplete, having only 11 protofilaments.

• Major protein structures of axoneme:-

Axoneme component	Function
Tubulin	Principal component of microtubules.
Dynein	Project from microtubule doublets & interact with adjacent doublets to produce bending.
Nexin link	Hold adjacent microtubule doublets together.
Radial spokes	Extends from each of the 9 outer doublets inward to the central pair.
Sheath projections	Project as a series of side arms from the central pair of microtubules; together with the radial spokes these regulate the form of ciliary beats.

Flagellum arises from a basal body. The ultrastructure of basal body is like those of an axoneme except that the central singlets are absent & 9 fibrils in the outer circle are triplets. 2 of these microtubules are continuous with the doublet of the flagellum. Dynein arms are absent in triplets.

 Q. Mention the differences observed in cilium & flagellum.


Ans. :-
Morphologically & physiologically the cilia & flagella are identical structures but both can be distinguished from each other  by their no. , size & functions    
1. The flagella are less in no. (1-2) in no. than the cilia which may be numerous (3000-14000) in no.
2. Flagella occur at one end of the cell, while the cilia may occur throughout the surface of the cell.
3. The flagella are longer processes while the cilia are short appendages of the cytoplasm.
4. The flagella usually beat independently, while the cilia tend to beat in a co ordinated rhythm.
5. The flagella exhibit undulatory motion while the cilia move in a sweeping or pendular stroke.


17. Write a note on the basal body of cilium.
The basal body is a centriole like cellular organelle from which the cilium arises. It remains separated from cilium by a basal plate. The ultrastructure of basal body is like those of an axoneme except that the central singlets are absent & 9 fibrils in the outer circle are triplets. Each triplet contains one complete 13-protofilament microtubule, the A tubule, fused to the incomplete B tubule, which in turn is fused to the incomplete C tubule. 2 (A,B) of these microtubules are continuous with the doublet of the flagellum. Dynein arms are absent in triplets.


17. What is axoneme?
Axoneme is the inner core of cilia or flagella, composed of microtubules & other proteins. Here microtubules are modified & arranged in a ring of 9 doublets surrounding a pair of central singlets.


17. Mention different flagellar movements in mastigophorans.


Flagellar movement is characteristic of Mastigophorans which bear one or more flagellum. The flagellum requires liquid medium for movement.
3 types of flagellar movements have been recognized.


1. Paddle stroke:-
According to Ulhela & Krijsman (1925) the common movement of flagellum is sideways lash. It consists of an effective downstroke/ power stroke with flagellum held out rigidly & a relaxed recovery stroke with flagellum strongly curved. The power stroke produces more thrust in the backward direction & as a result the animal moves forward.


2. Undulating motion:-
Wave like undulations of the flagellum, when proceeds from tip to base, pull the animal forward. Backward movement is caused when undulations pass from base to tip.


3. Simple conical gyration;-
According to Butschli's screw theory the flagellum performs spiral turning like a screw. This exerts propelling action which pulls the animal forward through water with a spiral rotation as well as gyration around the axis of movement.


Q.   Describe the different types of flagella in protozoans.


 Flagella are the locomotor organelles of the mastigophoran protozoans. A typical flagella consists of central axoneme made up of 2 longitudinal microtubules enveloped by a central sheath & 9 pairs of peripheral longitudinal microtubules. All 20 fibres lie in a matrix of dense cytoplasm & covered by extension of cell membrane. They fuse at the base to join a basal granule or kinetosome.
              Flagella may be surrounded by very minute, fine, flexible lateral processes called mastigonemes.
   Flagella are classified based on the arrangement of mastigonemes and the nature of the axial  filament.
  There are five types of flagella. They are-
1.  Stichonematic:  When the  mastigonemes are present on one side of the flagellum, it is called stichonematic flagellum.  .
       e.g.  Euglena, Astasia
                                                     
2.  Pantonematic: When two or more rows of mastigonemes are  present on both sides of flagellum, it is called Pantonematic flagellum.
       e.g. Paranema ,Monas
                         
3.  Acronematic: When the mastigonemes are absent and the distal ends of the flagellum ends as a terminal, 'naked', axial  filament it is called acronematic flagellum.
        e.g.  Chlamydomonas, Volvox
                                           
4.  Pentachronematic: When the mastigonemes are present on 2 rows on the lateral sides of the flagellum but the flagellum ends in a terminal, naked, axial filament, it is called pentachronematic flagellum.
        e.g.  Urceolus
                                                
5. Anematic: When the flagellum is simple without mastigonemes and/or terminal naked filament are absent, it is called anematic flagellum.
        E.g.. Chilomonas, Chryptomonas.
Infraciliary system of ciliates:-


Modification of cilia:-


The cilia may form the following composite motile organelles.
1. Membranelles:- These are membranes formed by fusion of 2 or more adjacent transverse rows of short cilia. They are found in the peristome making powerful sweeps.
2. Undulating membranes:- These are made up of one or more longitudinal rows of cilia fusing together. They are found in the peristome or cytopharynx & are used for food collection eg. Vorticella.
                     the undulating membrane of Trypanosoma is only a web of ectoplasm, it is not made up of cilia & it is locomotory.
3. Cirri:- Cirri are formed by 2 or 3 rows of cilia on the ventral side of some ciliates. They are locomotory & they may also be tactile.


Mechanism of ciliary locomotion:-


       During swimming each cilium moves in a whip like motion.
               It first gives a forward active/effective stroke in which the cilium is fully extended beating against the surrounding liquid.
               It is followed by a recovery stroke in which the cilium returns to its original position with an unrolling movement that minimizes the viscous drag (drag:-resistance to motion: the resistance experienced by a body moving through a fluid medium).
              Cilia of the same transverse row beat together & those of the same longitudinal row beat one after the other. This coordinated movement pattern of the cilia is called metachronal rhythm. It can be compared to the passage of wind over a field of paddy.
              Such type of movement is regulated by a highly complicated neuro-motor system.