Amoeba Proteus: Morphology, Locomotion and Reproduction | Protozoa
Amoeba proteus lives on the bottom of fresh water ponds, streams and ditches, gliding on the algae-covered mud or crawling on the surface of green submerged plants.
The name of this animal is derived from two Greek words— Amoeba meaning change, the proteus meaning a mythological sea- god who had the power of constantly changing his shape. To the naked eye the Amoeba is just visible as a minute speck of chalk particle. A large specimen, when fully expanded, measures about 0-01 inch (0-25 mm.) in length.
Under the microscope, the Amoeba appears to be an irregular mass of active jelly which is continually changing its shape by thrusting out and withdrawing blunt finger-like projections called pseudopodia (pseudo = false; podus = foot).
(i) A very thin and elastic limiting membrane called plasma lemma which encloses the entire surface of the animal.
(ii) A narrow outer zone of clear transparent ectoplasm which is totally free from granules and which lies just beneath the plasma lemma. The plasma- lemma may be regarded as the outermost layer of the ectoplasm.
(iii) Surrounded by the ectoplasm is the main mass of protoplasm called endoplasm. The endoplasm is granular and translucent. It consists of an outer stiff plasmagel, and an inner fluid plasmasol exhibiting streaming movements. The protoplasm varies in consistency according to the state of activity of the animal. Plasmagel may change into plasmasol and even ectoplasm may change into endoplasm.
(iv) Lying within the endoplasm there is a disc-like nucleus which is not easily seen in a living Amoeba. In a killed and stained specimen the nucleus is distinct. It is surrounded by a thin nuclear membrane and contains masses of chromatin granules which are suspended in the nuclear sap.
(v) A spherical vesicle, the contractile vacuole, is found within the endoplasm. It is filled with water and bubbles of gas. On being carefully observed for a time the contractile vacuole seems to expand slowly and then moving to the surface it suddenly bursts, discharging the contents into the surrounding water. A new contractile vacuole then reappears in its original position,
(vi) Lying scattered in the endoplasm there are several food vacuoles. These are of various sizes and contain morsels of food undergoing intracellular digestion.
(vii) Spherical water vacuoles are also seen scattered throughout the endoplasm and appearing as transparent drops of water. They never change their shape like the contractile vacuole,
(viii) Cytoplasmic inclusions such as mineral crystals, oil globules and granules of various sizes lie scattered in the endoplasm.
(ix) Blunt finger like projections or pseudopodia are protruded out from the surface of the body and drawn into it when not in use. Pseudopodia, therefore, are mere temporary structures.
(i) The plasma lemma retains the protoplasm within the cell body. It is permeable to water, oxygen and carbon dioxide,
(ii) The ectoplasm protects the more soft inner parts.
(iii) The endoplasm takes important part in movement by forming the pseudopodia; it is the structure which includes the nucleus and the vacuoles,
(iv) The nucleus regulates all the vital activities of the animalcule. It is particularly concerned with reproduction,
(v) The contractile vacuole is the organelle for respiration, excretion and fluid-regulation,
(vi) The food vacuoles serve as temporary stomachs where food is digested,
(vii) Water vacuoles along with the contractile vacuoles are responsible for fluid regulation,
(viii) The various inclusions are either reserve food materials for future use or excretory matter in the process of elimination,
(ix) The pseudopodia are used for locomotion and for capture of their prey. They are very sensitive and help the animal to avoid injurious objects.
It will not be out of place to mention here the relative importance of the nucleus and the cytoplasm. It is possible by micro dissection to take out the nucleus from the cytoplasm. It has been observed in such experiments that both the parts perish.
If on the other hand, an Amoeba is cut into two pieces, one-half containing only cytoplasm and the other containing the nucleus as well as a portion of the cytoplasm, then both the halves survive for a time.
The part composed of only cytoplasm is soon exhausted and dies; but the other part actively feeds, grows and leads a healthy life. This proves that although the nucleus governs the various vital activities of the cell body yet the nucleus and cytoplasm must function together.
All the vital activities that characterise a living animal may be observed in Amoeba on a miniature scale. It moves about in search
of food which is captured, ingested and digested. It absorbs and assimilates the digested food and egests undigested particles.
It secretes useful substances and excretes waste products. It breathes, repairs itself, grows, reproduces its own kind and responds adequately to changes in its environment. An Amoeba is immortal and has no natural death.
The Amoeba moves from place to place by forming temporary finger-like projections called pseudopodia. A pseudopodium may form at any place upon the surface of the body. It appears as a blunt projection of ectoplasm.
The sheet of plasmagel at the root of the ectoplasmic protrusion slowly thins out and is converted into plasmasol. The central granular part of the endoplasm, that is plasmasol, than flows into the ectoplasmic outgrowth by streaming movement through the gap in the plasmagel sheet.
Next to this, the stiff plasmagel at the opposite end of the animal is gradually converted into fluid plasmasol which rushes forward into the pseudopodium. While returning back along the sides of the finger-like tube, the plasmasol is again converted into plasmagel.
Many pseudopodia appear simultaneously but usually one of them is extended and the others are retracted. This sort of peculiar motion by thrusting out pseudopodia and then flowing into one of them is known as amoeboid movement.
Effective locomotion occurs only when the animal is in contact with the substratum and not when it is floating on the surface of the water. The tip of the pseudopodium may adhere to the substratum by secreting a sticky juice and the entire animal glides forward into the pseudopodium.
The speed of locomotion usually ranges from 0-5 to 5 micra per second. In favourable condition the speed may be as high as 1 inch (25 mm.) per hour. It varies with the temperature, increasing slowly up to a temperature of 30°C. Movement totally ceases at 33°C.
The food of Amoeba consists of small algae and other actively moving protozoa. The animal seems to have a choice in the selection of its food. It is always attracted by the movement of its prey.
Its dishes include Desmids, Oscillatoria, Paramoecium, Colpidium, and a small flagellate called Chilomonas. Food may be ingested anywhere on the surface of the body but it is usually taken in at the anterior end. The Amoeba sends out pseudopodia which engulf the prey by forming a food-cup.
When the prey is very swift, a large food-cup is formed and several victims are thus enclosed in a single cup without being touched. Gradually the food is completely surrounded on all sides along with a drop of water.
The result is a food vacuole whose walls are formed by the plasma- membrane and which contains the food suspended in water. The entire process of food-taking requires only a minute or two if the food is a favourite one. Chilomonas is ingested more rapidly than any of the others and the next choice is Colpidium.
Feeding occurs only when the animal is attached to a solid object and the rate of feeding is best at 25°C. The amoeba cannot feed when it is floating. When the temperature of the surrounding water is raised to 38°C or above, food-taking totally stops.
The food vacuole serves as a temporary stomach secreting digestive juice. The digestive juice contains free hydrochloric acid and appropriate enzymes. Later on, the reaction of the digestive juice becomes alkaline. Chilomonas and Colpidium remain alive vacuole for about 5 to 15 minutes and are usually digested in the course of a day.
The body of the prey swells up and gradually diminishes in size as the various enzymes break down the insoluble proteins, fats and carbohydrates to soluble forms. These are now absorbed into the cytoplasm and assimilated. The assimilated food is spent in providing energy for locomotion and the surplus is stored for future use.
The food vacuole gradually decreases in size with the progress of digestion, and at the end only indigestible residue is left behind. This is now egested out of the body. Indigestible particles are usually heavier than the protoplasm of Amoeba.
As such they lag behind as the animal moves forward and finally pass out from any point on the surface of the body near the posterior end. Thus in Amoeba, digestion takes place intracellularly, that is within the cell. As solid food is ingested, the mode of nutrition is holozoic.
|Amoeba Running After and Engulfing a Chilomonas|
The Amoeba forms nitrogenous waste products such as ammonia and urea, and other mineral substances in the course of its metabolism. A portion of these useless substances along with the excess of water in the body gradually accumulate in the contractile vacuole.
The latter dilates and ultimately bursts like a bubble, liberating its contents into the surrounding water. A new contractile vacuole soon appears in its original position. Major part of the waste products are, however, eliminated through the permeable plasma lemma from the surface of the body.
The Amoeba requires oxygen for the physiological burning of stored food, so that it may derive energy for its locomotion. The oxygen necessary for this purpose is procured from the dissolved air in the surrounding water.
The oxygen diffuses into the cytoplasm through the plasma lemma. The resulting CO collects as bubbles in the contractile vacuole and is expelled from the body when the latter bursts into the exterior. A part of the CO, however, diffuses out through the plasma lemma. Thus, in Amoeba, the mode of respiration is aerobic.
Any change in the environment acts as a stimulus and produces a response or reaction in Amoeba. As the animal has no nervous system these reactions are entirely due to the inherent irritability of its protoplasm.
The Amoeba reacts negatively, that is, it moves away from harmful stimuli like injurious chemicals, pricking with a needle, strong beam or light, increased temperature above 30°C, and lowered temperature nearing the freezing point. Beneficial stimuli, on the other hand, produce positive reactions and the animal moves towards the source of such stimuli.
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. If its forward progress is hampered by an obstacle, it is able to avoid the same by a process of repeated ‘trial and error’.
Response to changes in the temperature of the medium (thermo taxis) has already been mentioned in connection with amoeboid movement. The speed of locomotion increases steadily with a rise in temperature up to 30°C, but at 33°C locomotion abruptly ceases. 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.
There is, however, a positive response to weak chemicals diffusing from the surface of suitable food substances. These are quickly engulfed by the pseudopodia, and contact with the food is sometimes unnecessary. It is in this way that the Amoeba can discriminate a Chilomoias from a Colpidium and select its own food.
Sharp tapping of the coverslip produces rapid vibration of the medium, and to this the animal responds by withdrawing its pseudopodia (negative reaction). Weaker vibrations, such as those produced by a running ciliate, elicit formation of pseudopodia on the stimulated side (positive reaction). This is followed by the formation of a wide food-cup which is suitable for encircling a moving prey.
When Amoeba is subjected to weak electric current, the animal moves towards the cathode, and all pseudopodia are withdrawn from the side of the anode. Stronger currents produce cessation of movement, and ultimate disintegration of the body. Thus the Amoeba behaves exactly like other higher animals and reacts to states of pleasure, pain and hunger.
(1) Binary fission, and
(2) Multiple fission.
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. 48 A).
|Amoeba Undergoing Binary Fission|
Its nucleus swells up to divide by a special process of mitosis. The nucleus seems to elongate then becomes dumb-bell-shaped (Fig. 48 B, C.). 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. 48 D).
Usually binary fission is completed within half an hour at 24°C. If the temperature, is raised to 30°C the entire process is finished in 21 minutes. Fission is greatly retarded by low temperature, and the time required is slightly more than 4 hours at 4°C.
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 plasma lemma secretes a three-layered cyst within which the spherical body is safely housed. 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. proteus.
|Nuclear Fragmentation in an Encysted Amoeba|
|Formation of Pseudopodiospores|
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. Amoeba, therefore, is said to reproduce asexually.
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. Thus, E. histolytica causes amoebic dysentery and liver abscess in man.