Amoeboid locomotion due to both non-extracellular and extracellular stimuli represents one of the most widespread forms of cell motility and constitutes the typical way of locomotion in a broad range of adherent and the suspended eukaryotic cell types. How does amoeba move? How does the movement due to pseudopodia occurs? How the cytoplasmic flow is effected and the pseudopodia are formed? Rosel von Rosenhof first observed the amoeboid movement in , but during those times the reason for such a kind of movement was not known.
Since the first observation of amoeboid movement, many researchers have put forward many theories to explain these above mentioned queries. A brief description of 5 of these theories are provided below :. He was on the view that the ectoplasm of amoeba undergoes contraction at the posterior end causing protoplasmic currents to flow forward, with these the pushing of the fluid-like endoplasm forward also occurs.
Thus, with the forward pushing of ectoplasm and endoplasm results in the forward growth of pseudopodium and propelling the amoeba ahead in the medium. This theory was stated by Berthold in This theory is also supported by Rhumbler and Butshi It states that the amoeba is fluid like and moves due to the difference between the physical characteristics of body surface and the substratum. This theory is not universally accepted to all the species of Amoeba as it is known that a few like Amoeba verrucosa is devoid of pseudopodia.
Dellinger examined an Amoeba proteus not from the top, but from the side view, and explained how the pseudopodia become attached to the substratum and then due to the contraction of the contractile vacuole, the body gets pulled forward. Thus, the animal is pulled from in front and pushed from behind due to contractions of a contractile vacuole. In this way, the Amoeba actually walks putting one foot out, then another.
This theory was first put forward by Hyman and later supported Pantin and mast According to this theory, the cytoplasm of Amoeba is divided into ectoplasm and endoplasm. As the plasmasol changes into plasmagel at the anterior end, the plasmagel tube extends forwards and is converted into plasmasol at the posterior end, the plasmagel tube drives the plasmasol forwards to form a pseudopodium. Pseudopodia are formed because plasmagel is elastic and under tension, it is pushed out where the elastic strength is the lowest.
Amoeba is a unicellular, microscopic animalcule that simply measures about to microns in maximum diameter only. Explanation: Amoebas develop pseudopodium False foot for the purpose of movement. Related questions How do sodium-potassium pumps support the efficient functioning of cells? Why is the sodium potassium pump an example of active transport?
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What molecules require active transport across cell membranes? As the animal has no nervous system these reactions are entirely due to the inherent irritability of its protoplasm. Response to light photo taxis is negative when the rays are very strong; but the animal may react positively to very weak light. The Amoeba moves away and repeatedly changes its course when strong light is flashed on it from different directions.
Response to contact thigmotaxis is variable. When touched or pricked by a needle, a creeping Amoeba reacts negatively by withdrawing its pseudopodia, and then moving away in a different direction. A floating Amoeba, on the other hand, adheres to a solid object when it is lightly touched by the same, and the reaction, therefore, is positive. Response to changes in the temperature of the medium thermo taxis has already been mentioned in connection with amoeboid movement. Similarly, there is no locomotion when the temperature is brought down to the freezing point.
Response to chemical changes in the medium chemo taxis depends upon the nature of the stimulus. The animal reacts negatively and moves away from substances such as acetic acid, common salt, and cane sugar.
These are quickly engulfed by the pseudopodia, and contact with the food is sometimes unnecessary. This is followed by the formation of a wide food-cup which is suitable for encircling a moving prey. Thus the Amoeba behaves exactly like other higher animals and reacts to states of pleasure, pain and hunger. Reproduction in Amoeba Proteus: Amoeba reproduces asexually by two different methods:.
Binary fission is the usual mode of reproduction by which a full-grown parent individual divides into two daughter Amoebae. During binary fission, the Amoeba becomes spherical and is covered with small pseudopodia Fig. Its nucleus swells up to divide by a special process of mitosis. The nucleus seems to elongate then becomes dumb-bell-shaped Fig. Finally it divides into two daughter nuclei. Meanwhile the body of the animal narrows down in the middle. Lastly, this constriction cuts through completely and the two halves separate, each containing a nucleus of its own Fig.
Each amoebula feeds voraciously and grows to normal size within three days, when it again becomes ready for binary fission. As the animal reproduces by fission, there is no death from old age and the Amoeba, therefore, is potentially immortal. But the amoebae, like all ether living organisms, are liable to death by accident or other means.
When water and food become scarce or when unfavourable environment threatens with death, Amoeba reproduces by multiple fission or sporulation. This, therefore, is a means for tiding over a critical period in its life-history, and not a normal method of reproduction. Sporulation is preceded by encystment. The Amoeba retracts all pseudopodia and becomes spherical. The cyst is very light and may be carried by wind to a favourable environment.
In any case, the tough and impervious wall of the cyst affords shelter and protection till the return of normal condition. In the meantime, the nucleus of the encysted individual undergoes rapid division to produce a large number of daughter nuclei.
Later on, the cytoplasm divides, and a fragment of cytoplasm surrounds each daughter nucleus. A large number of pseudopodiospores are formed in this way.
With the return of favourable conditions, the cyst wall breaks and the pseudopodiospores come out by extending pseudopodia. Each pseudopodiospore is thus converted into a small amoebula.
In this manner, a large number of amoebulae are formed out of a single encysted parent individual. They feed actively and are soon converted into adult amoebae which reproduce by binary fission.
Encystment followed by sporulation have been described in other free-living Amoebae, but this process of reproduction has not yet been confirmed in A. It is to be noted that reproduction in Amoeba is not effected by the union of germ cells or gametes, and only one individual can reproduce its own kind. Animals closely resembling Amoeba proteus live as parasites in the alimentary canal of man and other higher animals. They are included in the genus Entamoeba. They feed on the nutritive materials in the gut of their host and occasionally invade and destroy tissues.
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