Amoeba is the most popular, free-living available protozoan. It is commonly found on the bottom mud or on underside of aquatic vegetation in freshwater ponds, ditches, lakes, springs etc. It moves and feeds with the help of false or pseudopodia, formed as a result of streaming flow of cytoplasm.
Structure of an Amoeba:
Structure of an Amoeba: unicellular animal with pseudopods that lives in fresh or saltwater.
Pseudopodium: part of the amoeba used for locomotion.
Ectoplasm: vitreous superficial layer of an amoeba.
Endoplasm: central part of an amoeba.
Cell membrane: membrane covering an amoeba.
Contractile vacuole: cavity of the amoeba that is able to contract.
Food vacuole: cavity of the amoeba responsible for digestion.
Nucleus: central organelle for an amoeba.
Digestive vacuole: cavity of the amoeba responsible for digestion.
Locomotion in Amoeba:
The locomotion in the amoeba is effected by the formation of temporary finger-like processes of pseudompodia (false-feet, greek, pseudos, false + podos, foot). Amoebas have no distinct head or tail ends but have a surface which is everywhere the same, and any one point on this surface may flow out as pseudopod. The protoplasm that enters into it is withdrawn from other parts of the body, and therefore, if the formation of pseudopodia is mainly in other direction the amoeba moves to that side. But sooner or later another similar pseudopod forms at an adjacent point and the cytoplasm flows into it. In this manner the animal progresses in an irregular fashion, flowing first to one side, then to the other. It often alters its course by putting out pseudopod on the side opposite to the previous advances. As new pseudopods appear the old ones flow back in the general mass. Such movements are known as “amoeboid movements”. Amoeboid movement is very slow, and the animal does not proceed for long in any one direction.
Amoeboid movement occurs in other protozoa, and also in the amoebocytes of sponges and in white blood corpuscles of the vertebrates. Amoeboid movement has always excited great interest because it is presumed to be one of the most primitive types of animal locomotion. Apparently it is totally different from the muscular movement of complex animals. Bit it is probable that a through understanding of the mechanism of amoeboid movement may throw some light on the general nature of contractility and thus elucidate the nature of muscle contraction. That is why amoeboid movement has been the object of intensive investigation. Probably the amoeboid movement is the basic characteristic of unspecialized protoplasm and like most fundamental processed is difficult to explain.
Speculation about amoeboid movement goes back a hundred and thirty years to Ehrenberg, who first suggested that pseudopodia where hernia-like protrusions forced out by muscular contractions of hind part of the animal. Within a year, the alternative view was put forward that the pseudopodia were themselves inherently extensible, and pulled the rest of the animal after them, and ever since biologists have held essentially oen or the other of these views.
The elements of amoeboid movement are not difficult to grasp. At the tip of the advancing pseudopodium the plasmagel seems to spread out, while in the body of the pseudopodium the plasmasol flows forward. When the plasmasol reaches the very tip it fans out to form fresh pseudopodial wall. At the hind end of the animal, the plasmagel seems to draw itself together and get thicker while its surface becomes wrinkled.
Many theories have been put forward to explain amoeboid movements. Of these explanations only two are worth mentioning:
- The surface tension theory, and 2) The change of viscosity or “sol-gel’theory.
- Surface tension theory: It was put forward by Butschli and was widely accepted for long. According to this vies locomotion in amoeba is essentially klike the movement of a globule of mercury or other liquid produced by local reduction of surface of the fluid protoplasm that makes the mass spherical. From such a sphere an outflow will occur whereever the surface tension is locally lowered, either by internal or external changes. In such a projection the fluid will flow forward and the centre and backward along the sides. Such streaming movement can be seen in active pseudopods of some amoeboid forms. Further, drops of certain chemical mixtures have been shown to move in amoeboid fashion because of local cecrease in surface tension. It is very simple and attractive theory which has been extended in its application to various types of activity in amoeba (by Rhumbler). There are, however, a number of fully established facts which show conclusively that surface tension as applied in this theory plays but a very insignificant role in the process of movement and locomotion in amoeba.
The objections to this theory are as follows: 1. The upper surface in may species moves forward in places of backward. That is, it moves in a direction opposite to that produced in a globule of liquid by local reduction in surface tension. 2. Many amoebae are so rigid that local reduction in surface tension can not produce movement. 3. Amoebae sometimes cut specimens of paramecia in two pieces. This requires many times as much energy as can be developed by local reduction in surface tension. 4. According to this view the surface is assumed to be liquid whereas in most amoeboid forms it is gelatinized. This theory is consequently untenable.
Sol-gel theory: The sol-gel theory was first advocated by Hyman (1917) and has been adopted, among other, by Pantin (1923-1926) and Mast (1925).
This theory is based upon the fact that the plasmasol changes into the plasmagel and vice versa. The plasmasol changes into rigid plasmagel (gelates) at the anterior end and at the posterior end the plasmagel changes into plasmasol (solates) causing a forward streaming of the more fluid plasmasol. That is why in actively progressing specimens the plasmasol is continuously rapidly streaming forward, while the plasmagel is practically everywhere at rest forming, so to say, a tube within which the plasmasol flows. It the course or a granule or crystal fairly straight course either until it reaches or nearly reaches the inner surface of the plasmagel at the tip of an advancing pseudopod, then it deflects to the right or to the left, upward or downward and sooner or late, directly or indirectly comes into contact with plasmagel into which it finally changes. Other granules are similarly coming and changing it to plasmagel after attaching to it behind the crystal under observation. As more of these are coming in the same way the position of the first recedes from the anterior and approaches the posterior end of the amoeba. When it reaches this end it gradually moves inward and enters in the plasmasol, after which it moves forward and the whole processes is again repeated. This gives an idea about the movement of the plasmasol and plasmagel. From this, the phenomenon of locomotion can be easily deducted.
It is clearly understood that the plasmalemma and the plasmagel are elastic and that they are usually somewhat stretched and exert inward pressure, that is, the amoebae are usually turgid. In some specimens locomotion is accompanied with ‘ rolling movement’ of the surface. This generally occurs in the monopodal specimens. When such a specimen is about to move the plasmagel adjoining the plasmasol liquefies or sometimes at any one place resulting in a local cecrease in thickness and in elastic strength. The result is that the plasmagel bulges in this region, due to the elasticity of the plasmagel and turgidity of amoeba, and the plasmasol moves to this region. The principle involved in the formation is the same as the formation of a bulge on the surface of an inflated cycle tyre in which the wall has been locally weakened.
The bulge thus formed is the beginning of the pseudopod. The plasmagel is continuously stretched at the tip of the pseudopod, doe to the pressure of the plasmasol against it, and becomes a very thin sheet. The liquid form the plasmasol passes through it and forms a hyaline cap around the tip under the plasmalemma. The plasmasol gelates on the side forming the ‘the tube’ forward as rapidly s the psudopod extends. At the inner surface of the posterior end the plasmagel solates as a rapidly as it gelates at the anterior end and the process becomes continuous. The plasmalemma adheres to the substratum and to the adjoining plasmagel, the pressure of the plasmasol against the sheet of plasmagel under the hyaline cap pushes it forward, and as it pushes forward it draws the plasmalemma on the upper surface forward over the plasmagel, while it remains stationary below, producing locomotion with rolling movement of the surface.