LOCOMOTION IN PROTOZOA

1.5. LOCOMOTION IN PROTOZOA

• Locomotor Organelles. The locomotor organelles of protozoans may be long filaments called flagella, short hair-like processes termed cilia, or flowing extensions of the body known as pseudopodia. The pseudopodia are of four types: lobopodia filopodia, axopodia or actinopodia and reticulopodia.

(a) Lobopodia. These are thick, finger-like outgrowths with rounded, blunt tip. They consist of both ectoplasm and endoplasm. These are found in Amoeba, Entamoeba and Arcella.

(b) Filopodia. These are slender processes with pointed tip, and consist of ecoplasm only. They have a tendency to branch and radiate in all directions. These are seen in Acanthometra. (c) Axopodia. These are stiff, ray-like processes from the ecotplasm supported by a firm axial filament (bundle of microtubules) from the endoplasm. They radiate fom the spherical body in all directions. These occur in Actinophyrs.

(d) Reticulopodia. These are fine, thread-like processes that branch and anastomose freely to form a network. These are met with in Elphidium and other forms.
Here we discuss the locomotion in Amoeba (amoeboid locomotion) as a reprentative of pseudopodial locomotion.

1. Amoeboid Locomotion

Amoeba moves from place to place by means of temporary finger-like projections, the pseudopodia or false feet (Gr. pseudos = false, podos = foot). This mode of locomotion is known as the pseudopodial locomotion or amoeboid locomotion. Pseudopodia, being thick, blunt and composed of both ectoplasm and endoplasm, are described as the lobopodia. Several theories have been put forward for the formation of pseudopodia. Most zoologists at present accept the change in viscosity, or solgel theory. According to this theory, the pseudopodia result from the changes in the coloidal state of the peripheral cytoplasm from gel to sol and sol to gel.

The ectoplasm consists of a network of actin microfilaments that give it a gel-like nature. Filamin and α-actinin proteins link the actin microfilaments together into a network. Myosin also occurs but as isolated molecules rather than assembled into filaments. Some other proteins are also present. At the site where chemical signals from the medium bind to the receptors of the cell membrane, the gelled ectoplasm solates, that is, becomes fluid. Solation results from depolymerizaion of actin microfilaments composing the ectoplasm. Gelsolin and villin proteins break up the actin network. The depolymerization increases the number of particles (actin subunits), and this particle increase raises the osmotic pressure of the solated ectoplasm, and water from endoplasm flows to this region. The inflow of water from the endoplasm causes the extension of the solated region as a pseudopodium. Around the periphery of the pseudopodial tip, the endoplasm gelates to form ectoplasm, thus building up and extending the sides of the pseudopodium. In the gelation of endoplasm, the actin-subunits repolymerise and bond to one another at more or less right angles, forming a mesh of microfilaments. The actin mesh gives the ectoplasm its firm gelatinous nature. The small mesh size excludes the organelles, giving the ectoplasm its hyaline look.

At the hind end of the body, the ectoplasm changes into endoplasm by depolymerization of its actin mesh. The endoplasm so formed flows forwards. This results in the withdrawal of pseudopodia from the hind end, and ensures a continuous supply of endoplasm to the developing pseudopodia at the advancing end.

In depolymerization at the hind end, a part of cell membrane is removed as the membrane length decreases here due to withdrawal of pseudopodia, and new cell membrane is added at the pseudopodial tip to provide for its elongation.

By forming pseudopodia continually in one direction in the manner explained above, Amoeba slowly changes its position as well as its shape. It can cover a distance of about 25 mm. in an hour by pseudopodial locomotion.

There are two views about the generation of force which makes the endoplasm flow into the pseudopodium. Probably both the mechanisms move the endoplasm.

(i) The actin microfilaments at the hind end slide over each other through the action of myosine cross-bridges, making this end narrow (contract). Contraction pushes the fluid endoplasm toward the advancing end.

(ii) The actin microfilaments and myosine molecules linked to endoplasmic structures slide over the microfilaments held firmly in the ectoplasm. Thus, sliding occurs at the interface between ectoplasm and endoplasm . This sliding causes contraction of ectoplasm, thereby moving the endoplasm forward.

It may be emphasized that all movements brought about by microfilaments result from sliding mechanism involving a coordinated interaction of actin and myosin proteins in both nonmuscle and muscle cells.

The energy for the movement, depolymerization and repolymeri-zation of actin subunits and for sliding of microfilaments is provided by ATP.

2. Flagellar locomotion

Flageller locomotion is seen in photosynthetic protists and flagellated protozoans. We describe the mechanism with the help of Euglena.Euglena has two modes of locomotion : swimming or flagellar and creeping or euglenoid.

(i) Swimming . Swimming is fairly rapid, about 3.6 mm per minute. It is brought about by the flagellum. For swimming, the flagellum trails obliquely backward, and works on the principle of an airplane propeller. Waves of spiral undulations pass over the flagellum from the base to the tip at the rate of 12 per second. A single wave, or beat, of the flagellum, describes a helix. The force generated by the beat pushes the water backward, and propels the organism for-ward. Spiral beating of the flagellum makes the organism progress in a spiral path around a straight line, and, at the same time, rotate about its own long axis. Rotation occurs in such a way that same surface of the body always faces the central axis of the spiral path.

The velocity and amplitude of the waves in-crease as they pass towards the tip of the flagellum. This shows that the energy needed for its action is expended by the flagellum, itself. This is proved by the fact that a flagellum cut off om the body continues its lashing, provided its blepharoplast is with it. Moreover, if the energy for each wave came from the attached end of flagellum, its velocity and amplitude would decrease towards the tip.

Euglena is unable to swim backward as the direction of flagellar motion cannot be reversed. It can, however, quickly change the direction by violent flexure of the anterior part of the body.

Mechanism of Flagellar Motion.

Flagellar motion results from sliding of a doublet over a doublet. The dynein arms produce the force for sliding of doublets in the axoneme by hydrolysing ATP. The spokes and nexin links hold the doublets together and cause the flagellum to bend in order to accommodate the internal displacement of the doublets during sliding. In the presence of ATP, the dynein arm of one doublet binds to the adjacent doublet and flexes, causing the doublets to slide past each other by one step. ATP molecule is hydrolysed in this process. Successive attachments and flexes of dynein arm cause the doublets to slide smoothly past one another ewer a distance sufficient to bend the flagellum.

The flagellum of Euglena, being directed backward during locomotion, pushes the organism forward. It is, therefore, called a pulsellum. Flagellates often swim with flagellum directed forward. They are pulled by the flagellum, which is termed a tractellum. The flagellum of a sper-matozoon is also a pulsellum as it is located behind the head.

(ii) Creeping. Creeping is a far slower mode of locomotion than swimming. It is brought about by worm-like movements, in which waves of contraction and expansion pass over the body from the anterior to the posterior end more or less like peristalsis of a vertebrate intestine. These movements, called euglenoid movements or metaboly, as already mentioned, cause temporary changes in the shape of the body, and result in slow-progression. They are due to the contractility of the epiplasmic microfilaments.

3. Ciliary Locomotion

Ciliary locomotion is seen in phylum Ciliophora. We describe the locomotion of Paramecium as a representative.

Locomotion

Paramecium shows two modes of locomotion : swimming and creeping. The cilia act as locomotory organelles in both.

1. Swimming. Paramecium swims in water rather rapidly by beating of cilia. A ciliary beat comprises two phases: an effective or power stroke and a recovery or return stroke. During an effective stroke, the cilia stretch out, become stiff and move almost as straight rigid rods from a forward to a backward position to become parallel to the body surface. This oar-like movement of cilia pushes the water backward and the organism forward.

During a recovery stroke, the cilia become limp(Acellular fungi), flex and return to their original upright position in a plane nearly parallel to the body surface. This movement of cilia offers minimum resistance to water. The cilia neither beat simultaneously nor one by one. Instead, they beat in metachronous rhythm . Each cilium beats a little in advance of the one anterior to it in the line. As a result of this, waves of ciliary activity appear to pass along the body from front to rear, much like the lashing of the tall wheat plants in a strong wind. Beating of cilia is possibly coordinated by the kinetodesmata but there is no evidence for this.

The movements of adjacent cilia are coupled due to the interference effects of the surrounding water layers. Thus, the hydrodynamic forces provide coordination to the cilia.

The mechanism of ciliary motion resembles that of the flageller motion.

Paramecium does not swim in a straight line. It follows a spiral path and at the same time rotates about its own longitudinal axis. Such a locomotion results from 3 processes-

(a) Forward Push. Since the cilia beat backward, the organism is pushed forward in the same way as the backward strokes of the oars push the boat forward.

(b) Axial Rotation. The cilia do not beat straight backward. Instead, they beat obliquely to the right. This makes the organism roll over to the left. As a result of rolling, the organism, rotates on its long axis anticlockwise as it swims forward.

(c) Swerving of the Anterior End to the Aboral Side. The effective strokes of the cilia lining The effective strokes of the cilia lining the oral groove are more powerful than those of the body cilia. This makes the anterior end of the body swerve (turn) continually away from the oral side. Since the body is rotating on its long axis, swerving occurs alternately to all sides. In the beginning, the oral side is downward and the body bends upward ; a little later (due to rotation), the oral side comes to face the right and the body turns to the left; still later, the oral side gets directed upward and the body bends downward; finally, the oral side shifts to the left and the body turns to the right. The same process is repeated over and over again. As the swerving in one direction is compensated by an equal swerving in the opposite direction, the organism progresses along a spiral path around a straight axis . The spiral is anticlockwise when seen from behind. The body rotates once for every spiral revolution so that the same surface of the body always faces the centre of the spiral.

Paramecium can swim backward also. For this, the cilia beat forward and all the processes are reversed. The backward movement is associated with the so called avoiding reaction discussed ahead.

Paramecium swims at the rate of about 1 mm. per second. Its speed seems to be much faster under a microscope, as the microscope magnifies the speed as much as it magnifies the object.

(ii) Creeping. Paramecium can glide over a surface with the help of cilia of the oral surface.