Locomotory Organelle and Locomotion in Protozoa
[I] Locomotory Organelles
Locomotory organelles in Protozoa include pseudopodia, flagella, cilia and pellicular contractile structures.
Pseudopodia or false feet are temporary structures formed by the streaming flow of cytoplasm. Sarcodina move with these structures. On the basis of form and structure, pseudopodia are of the following four types :
These are lobe-like pseudopodia with broad and rounded ends, as in Amoeba. These are composed of both ectoplasm as well as endoplasm. Lobopodia move by pressure flow mechanism.
These are more or less filamentous pseudopodia, usually tapering from base to the pointed tip, as in Euglypha. Unlike lobopodia, the filopodia are composed of ectoplasm only. Sometimes they may branch and form simple or complex networks.
The reticulopodia (rhizopodia or myxopodia) are also filamentous. Filaments are branched and interconnected profusely to form a network. This type occurs in foraminiferans (e.g. Globigerina). Reticulopodia display two-way flow of cytoplasm.
These are more or less straight pseudopodia radiating from the surface of the body. Each axopodia containing a central axial rod which is covered by granular & adhesive cytoplasm. Like reticulopodia, axopodia also display two-way flow of cytoplasm. Axopodia are characteristic of heliozoans, such as Actinosphaerium and Actinophrys.
Flagella are the locomotor organelles of flagellate Protozoa, like Euglena, Trypanosoma, etc. These are thread-like projections on the cell surface. A typical flagellum consists of an elongate, stiff axial filament, the axoneme, enclosed by an outer sheath. In axoneme, nine longitudinal peripheral paired fibres form a cylinder, which surrounds the two central longitudinal fibres, enclosed by a membranous inner sheath. Each of the peripheral pairs bears a double row of short arms. Axoneme arises from a basal granule, the blepharoplast or kinetosome. Mostly, it is a cylindrical body formed by the bases of peripheral fibres. Blepharoplasts are derived from centrioles, as the two structures are homologous.
Fibres of axoneme remain embedded in a fluid matrix. In between the outer ring of peripheral fibres and inner ring of central fibres, mostly occur nine accessory fibres. In certain groups of Mastigophora are found flagellar appedages or mastigonemes extending laterally from the outer sheath.
Number and arrangement of flagella vary in Mastigophora from to eight or more. Free-living species have usually one or two, while in parasitic species, the number ranges from one to many.
Cilia, characteristic of Ciliata, resemble flagella in their basic structure. These are highly vibratile small ectoplasmic processes. Electron microscope reveals the presence of an external membranous sheath, continuous with plasma membrane of cell surface and enclosing the fluid matrix. Running along the entire length of body of cilium are nine paired peripheral fibres and two central fibres, all embedded in a structureless matrix. Central fibres are enclosed within a delicate sheath. In between the outer and inner fibre rings are present nine spoke-like radial lamellae. In addition to these, one sub-fibre or microfibre of each peripheral pair bears a double row of short projections, called arms, all pointing in the same direction.
Each cilium arises from a thickened structure, the basal granule, basal body or blepharoplast. According to Lenhssek and Henneguy (1898), basal granules are centrioles or their derivatives. Basal granules show nine peripheral subfibre triplets, each disposed in a twist-like fashion. In many species, cilia become fused variously forming compound organelles, such as undulating membranes (Pleuronema), membranelle (Vorticella), and cirri (Euplotes).
Pellicular contractile structures:
In many Protozoa are found contractile structures, in pellicle or ectoplasm, called myonemes. These may be in the form of ridges and grooves (e.g. Euglena), or contractile myofibrils (e.g. larger ciliates), or microtubules (e.g. Trypanosoma).
[II] Methods of Locomotion
Basically there are four known methods by which Protozoa move- (1) Amoeboid movement, (2) Flagellar movement, (3) Ciliary movement, and (4) Metabolic movement. Speed of locomotion varies from 0.2µ to 3µ per second in amoeboid forms, 15µ to 300µ in flagellates, and 400µ to 2000µ in ciliates.
It is characteristic of all Sarcodina and certain Mastigophora and Sporozoa. It consists in the formation of pseudopodia by the streaming flow of cytoplasm in the direction of movement. Locomotion by pseudopodia is possible only over a surface. We still do not know precisely about the mechanism involved in the formation of pseudopodia, but the most convincing theory at present is that it depends upon active contraction of the ectoplasmic tube (plasmagel) at the posterior end of the body. This leads the endoplasm (plasmasol) to flow forward into the expanding pseudopodium. This process involves continuous solation at the posterior end and gelation at the anterior end. This is called sol-gel or change of viscosity theory by Mast and Pantin (1925).
It is characteristic of Mastigophora which bear one or more flagella. The flagella need liquid medium for movement locomotion. Three types of flagellar movements have been recognized:
Common movement of a flagellum is sideways lash, consisting of an effective down stroke with flagellum held out rigidly, and a relaxed recovery stroke in which flagellum, strongly curved, is brought forward again. As a result, the animal moves forward, gyrates and is also caused to rotate on its longitudinal axis.
Wave-like undulations in flagellum, when proceed from tip to base, pull the animal forward. Backward movement is caused when undulations pass from base to tip. When such undulations are spiral, they cause the organism to rotate in opposite direction.
Simple conical gyration:
Screw theory postulates a spiral turning of flagellum like a screw. This exerts propelling action, pulling the animal forward through water with a spiral rotation as well as gyration (revolving in circles) around the axis of movement.
The mechanism producing flagellar beat is not exactly known. It is believed that some or all of the axonemal fibres involved. According to the latest sliding tubule Theory of flagellar (or ciliary) movement, adjacent doublets slide past each other, causing the entire flagellum or cilium to bend. Cross bridges are formed and energy utilized for the process is supplied by adenosine triphosphate (ATP).
Most ciliates appear to move in a spiral path, rotating on their axis as they go. Spiral movement is due to in opposite directions on the two sides of the pseudopodial filaments oblique strokes of all body cilia working together and striking in the same direction. Coordination of ciliary movement is due to fact that basal bodies of all cilia are linked by kinetodesmata. Cilia also need liquid medium for their movements. Large ciliates are the swiftest swimmers, and the champion of them may be named Paramecium caudatum.
Ciliary action resembles the swing of a pendulum except that it is more rapid in one direction. Backward and forward vibrations produce a paddle stroke effect. Backward effective stroke is more active during which movement is brought about, while forward recovery stroke produces no significant movement. While moving, the succession of beats are coordinated in the well-known pattern of metachronal rhythm, conventionally compared to the passage of wind over a field of wheat.
4. Metabolic movements:
This is typical of certain flagellates (e.g. Euglena) and most sporozoans at certain stages of their life cycles. Such organisms- are seen to show gliding or wriggling or peristaltic movement. Contractile myonemes or microtubules, present in their pellicular walls, are responsible for this type of movement. Movements of this kind are usually also referred to as gregarine movements since they are characteristically exhibited by most gregarines.
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