Amoeba are single celled invertebrates animals which belongs to Phylum Protozoa that catches food and moves about by extending fingerlike projections of protoplasm.
Before knowing about Amoeba let use first know briefly about phylum protozoa.
Protozoa mean “first animals” (Gr. Protos-first + Zoon-animal). Phylum Protozoa includes those animals which are generally not visible to the naked eye. They are studied with the help of a microscope.
Their size varies from 0.0001 to 5.0 mm. The body of these animals is made up of a single cell which is without a cell wall. These animals are generally called acellular or unicellular.
All the life activities such as digestion, excretion, reproduction etc. are performed in the single cell body of these animals.
They do not possess organs for different functions but their protoplasm assumes different shapes to perform different functions such as locomotion, food catching etc.
So far about one lakh species of protozoans are known. They are found in all types of waters, moist soil and as parasites in different animals. Nearly 50,000 species of protozoans are known.
Classification of Amoeba
Habits and Habitat
Amoeba was first observed by Roselvon Rosenhoff in 1755. He called it “the little proteus” after the mythical sea god who is believed to be capable of changing shape. H. I. Hirschfield in 1862 made a detailed study of Amoeba. Many species of Amoeba has been found but here we shall study Amoeba proteus only.
Amoeba proteus is found in the mud at the bottom of fresh-water ponds. It is also found attached to the leaves of water plants. Generally, it is present in those places in a pond where there is sufficient oxygen and plenty of food material.
Culture of Amoeba
It can be easily cultured in the laboratory by keeping water plants or mud from the bottom of ponds in a beaker with some water and allowing the contents to putrefy for a few days.
Structure of Amoeba
Its structure is studied with the help of a microscope. Its diameter is from 0.25 to 0.60 mm. (1/100 inch). The shape of its body is not definite because pseudopodia are continuously being formed in different directions and thus, the shape of the body keeps on changing.
The body of the Amoeba consists of the following parts.
- Plasmalemma– It is an elastic and tough covering found in its body, 1 µ to 2 µ thick. Normally, it protects the body from external damages and allows the exchange of water, oxygen and carbon dioxide because it is semipermeable. However, the protoplasm cannot come out of it. Amoeba is said to be “naked” like an ordinary animal cell.
- Protoplasm: Amoeba’s protoplasm is formed of many protein compounds-C, H, O, N, S and other elements plus water. The protoplasm is divided into two parts:
- The nucleus which is almost in the centre of the body, and
- Cytoplasm that surrounds the nucleus.
Cytoplasm-It is divided into two parts:
- Ectoplasm (Ectosarc)-It is present just below the plasmalemma. It is transparent and thick.
- Endoplasm (Endosare)-It is present below the ectoplasm and is thin and heterogeneous. The inner part of the endoplasm is known as the hyaline layer, therefore, two layers of cytoplasm are clearly seen.
The part of the endoplasm attached with the hyaline layer is known as plasma gel. Many granules are present in this part. It is a clear, colorless, transparent and glossy hyaline layer of watery fluid.
The inner part of endoplasm is called plasmasol. The structure of both the above parts is similar to each other and one can be changed into the other easily. There is a nucleus in the endoplasm whose position changes during locomotion.
A nuclear membrane is present around it. The nucleoplasm contains chromatin granules scattered in it.
The nucleus controls all the activities of the body and plays a very important role in reproduction and heredity. Its nucleoplasm contains several nucleoli and about 500 chromatin granules.
A small empty space. containing excretory products is present in the endoplasm near the nucleus. This is known as the contractile vacuole.
The liquid collected in it is expelled out of the body when this vacuole bursts and disappears. It takes part in respiration and excretion but its more important function is to control the amount of water in the cytoplasm since water continuously enters the body of A oeba by osmosis.
The amount of water becomes more than is required by the cytoplasm.
This extra water accumulates in contractile vacuole and when this is full it comes to the surface and bursts. Thus the extra water is removed out of the body. This process is known as osmoregulation.
Besides contractile vacuole, some other vacuoles filled with food, water and digestive juices, are found in the endoplasm. These vacuoles are called food vacuoles.
Besides these vacuoles, crystals, oil droplets, fat globules and other very minute structures which are not visible with an ordinary microscope are present in the cytoplasm. These cell inclusions are its stored food materials or such substances which are very necessary for metabolism.
Refractive Spherical Bodies
These are nutritive particles that are insoluble in water. According to Von Miller, they are protein granules.
Nutrition in Amoeba
Amoeba is a holozoic (zootrophic) animal with a complicated process of nutrition which includes ingestion of food, its digestion, assimilation of digested food and egestion of undigested food.
Food and Ingestion
The food consists of small protozoans, algae etc. It also takes living or dead organic substances but does not eat starch, salicylic acid, salts, iron etc. However, it readily takes globulin. glutin grains and tyrosine but shows little interest in fibrin, egg or milk albumin and protein.
The taking of food into the body is called ingestion.
Jennings in 1909 described this process in Amoeba in detail, which is as follows:
There is no mouth in Amoeba for ingestion of food. Therefore, it can be ingested at any place in its body.
When Amoeba comes in contact with a food particle while moving, it stops there and pseudopodia start forming at the place of contact of food with the body.
These pseudopodia gradually encircle the food between them and in the end, the pseudopodia of both sides meet taking the food into the body.
Some water also comes into the body along with the food and thus a food vacuole is formed.
The process of ingestion of food in Amoeba depends on the type of food and the circumstances in which it is found. It is mainly of the following four kinds:
This method of food ingestion is used when the prey is very active. During this process the pseudopodia of Amoeba do not come in contact with food, instead they surround it remaining away from it and form a cup-shaped structure which is known as food cup.
Gradually, the pseudopodia of the sides meet and enclose the food within them. In this process much water is taken along with food.
This process of food ingestion is called circumvallation by which Amoeba ingests living or moving food particles such as Paramecium during locomotion.
When Amoeba has to ingest an alga or stationary food particle, it first presses the food with its pseudopod after which it starts swallowing the food. This process is known as circumfluence and takes about one to two minutes to ingest the food. This method is used for ingesting less active food particles.
In this process the food starts sinking into the cytoplasm as soon as it comes in contact with Amoeba. This process is called import and in it Amoeba does not make any special effort for ingestion.
In this process the part of ectoplasm that comes in contact with food invaginates to form a groove. Later on, the outer part of the groove dissolves into the endoplasm and thus the part of to form the groove containing food reaches the endoplasm and forms a food vacuole. This process is called invagination.
An Amoeba can ingest many paramecia and flagellate protozoans etc. in a day.
Digestion occurs within food vacuole (gastric vacuoles). After the ingestion of food, the process of digestion starts. The cytoplasm forms digestive juices which are released into the food vacuole.
These juices or enzymes are acidic as well as basic and act in the following manner :
At first acidic stage remains for about 30 to 60 minutes during which the living food substance is killed by the effect of the acid. The acid is probably hydrochloric acid.
After this the medium becomes basic because the enzymes which digest proteins act in basic medium only. Various enzymes, such as proteases digest proteins, lipase digests fats and maltase digests sugar, act in this medium.
Holder and Dyle showed that starch digesting enzyme, amylopsin, is also present in Amoeba. Since digestion takes place in the single-celled body of Amoeba, it called intracellular digestion.
The digested food diffuses into the Assimilation endoplasm from the food vacuole and due to the continuous movement of these vacuoles in the endoplasm the food is distributed to different parts of the body. There it is assimilated.
It is a process by which the undigested food is removed out of the body. For egestion Amoeba has not vent (anus) or fixed place but as the animal moves forward, the undigested food accumulates in the hinder region.
Later on, the ectoplasm breaks at this place and a small aperture is formed through which all the undigested food goes out of the body. This aperture closes soon after the egestion is completed.
Excretion in Amoeba
Urea and carbon dioxide are the main waste substances which are formed as a result of many life activities in Amoeba. These substances diffuse out through the general surface of the body. However, some are also removed by the contractile vacuole.
The plasmalemma around Amoeba is semipermeable and as the water of the pond in which this animal lives is hypotonic to the cytoplasm of its body, according to the law of diffusion large amount of water enters the body.
If all this water is allowed to remain in the body, the body will swell and ultimately burst. Hence, a special arrangement is required for the removal of this excess water in the Amoeba and this is done by the contractile vacuole.
The vacuole bursts to the outside, disappears and soon reappears. This process goes on continuously.
There are two states of contractile vacuole in osmoregulation. 1. Diastole, and 2. Systole.
During this state many small vacuoles appear at the place where the previous contractile vacuole had burst and disappeared. Soon these small vacuoles unite together to form a large vacuole.
Excess water collects in the endoplasm in the form of small vacuoles and they unite with the large vacuole increasing the size of the latter. Thus the process of reformation of contractile is called vacuoles diastole.
The process of disappearance of contractile vacuoles after bursting is called systole. In this process the contractile vacuole, after attaining the maximum size, moves towards the lower surface and bursts to the outside and thus the systole is completed.
In this manner the contractile vacuole performs the function of osmoregulation in Amoeba. According to some scientists some waste products and carbon dioxide are also excreted by this vacuole.
Kitching in 1952 studied the process of osmoregulation in Amoeba.
Respiration in Amoeba
Like other living beings Amoeba also needs energy for its various life activities.
This energy is obtained by the oxidation of certain substances. The oxygen dissolved in water is used for oxidation, therefore, Amoeba is generally found in those waters which are rich in oxygen. The oxygen dissolved in water diffuses into the body and mixes with the cytoplasm.
The oxidation of some substance present in takes place as a result of which energy is released and CO2 is formed. The carbon dioxide diffuses out through the plasmalemma into the water of the pond.
Locomotion in Amoeba
In Amoeba the locomotion takes place with the help of pseudopodia. Several pseudopodia are formed in different directions but the animal moves in the direction of the longest pseudopodium.
The process of the formation of pseudopodia is very simple. First the hyaline layer of ectoplasm swells and extends on one side. Soon after a cap-like structure is formed at the end of this extension. Simultaneously, endoplasm fills into this swollen part and thus a new pseudopodium is formed.
Why and how a pseudopodium is formed ? What reactions take place in the body due to which the body is stretched in a particular direction ?
Different theories have been proposed to answer these questions.
1. Jenning’s Rolling Movement Theory
Jenning in 1904 held that locomotion in Amoeba takes place by the rolling of cytoplasm. In support of his view he gave the following experiment.
If a small carbon particle is placed on the upper surface of Amoeba verrucosa, it moves along with the cytoplasm and reaches the front end and remains stationary till whole of the cytoplasm passes over it.
2. Dellinger’s Contraction Theory
According to Dellinger (1906) a pseudopodium is formed in the direction in which Amoeba wants to move. The animal lifts it and places it a little forward on the base it is moving.
As soon as it rests on the surface, a contraction takes place in it and whole of the cytoplasm is pulled forward. In this way pseudopodia continue to form and the animal moves forward.
3. Surface Tension Theory
This theory was first put forward by Berthold in 1886. Later, Rhumbler and Butschli in 1898 supported this view.
According to this theory, due to reduction of surface tension at any place on the body surface of Amoeba the cytoplasm comes out in the form of pseudopodium at that place.
The supporters of this theory showed that a ball of mercury and drops of certain chemical mixtures flow like an Amoeba.
Some scientists objected to this theory on the ground that surface tension exists only on liquid surfaces, whereas the surface of Amoeba is in gel condition in the form of plasmalemma. Therefore, this theory is not acceptable fully.
4. Sol gel Theory
When cytoplasm was finally recongnised as a colloidal system Hymcan (1917) Schaudiun and others suggested the conversion of ectoplasm into endoplasm and vice versa.
The principle of sol gel was first proposed by Hyman (1917) and supported by Pantin in 1923-26 and Mast in 1925. They divided the body of moving Amoeba into four parts:
- Plasmalemma which is very thin and elastic.
- Non-granular ectoplasm.
- Plasmagel which is the outer part of endoplasm.
- Plasma sol which is the inner moving part of endoplasm.
According to them the locomotion in Amoeba is the result of four main processes.
- Amoeba sticks to the surface on which it is moving.
- During locomotion the plasmasol in the front part changes into plasmagel. This process is known as gelation.
- At the time the plasmagel in the hind part changes into plasmasol. This process is called solation.
- The plasmagel contracts and moves forward.
Since plasmasol is more liquid that than plasmagel, it flows quickly into the pseudopodium and is converted into plasmagel as soon as it reaches its end.
Some part of plasmagel moves backward along the sides while the remaining part forms a thin layer at the end of the pseudopodium due to which the plasmasol does not touch the plasmalemma.
This layer is called hyaline layer which contains ectoplasm at its foremost end. This cap-like structure is known as hyaline cap. The part of plasmagel starts changing into plasmasol when it reaches the hind end. The whole process continues.
The plasmasol keeps on flowing towards the pseudopodium because of the plasmagel in the direction opposite to that of locomotion and its changing into plasmasol.
Thus one or more pseudopodia keep on forming and disappearing moves forward.
Recently Allen (1962) and Renalidi and Jahn (1963) studied the details of amoeboid movement and confirmed the view of Pantin and Mast. Allen advanced altogether a different theory, not only for amoeboid locomotion, but also for cytoplasmic differentiation.
ATP AND ITS SIGNIFICANCE IN LOCOMOTION
Every animal needs energy to do work. This energy is stored in a chemical substance called Adenosine triphosphate (ATP) which is a nucleoprotein. Most of ATP is stored in mitochondria.
Three phosphate (PO4) radicals are attached to it by high energy bonds with a molecule of adenosine protein. To create working energy a radical of phosphate (PO4) is separated by some enzymes as a result of which large amount of energy is released.
ATP is changed into adenosine diphosphate which soon gets a phosphate radical from glucose and becomes ATP. The energy released by ATP is used by Amoeba for locomotion and other life activities.
Irritability in Amoeba
There is no nervous system in Amoeba but it responds to different kinds of external stimuli. This characteristic is known as irritability due to which all activities of the body go on properly between 20 to 25 centigrade.
All the activities are retarded if the temperature goes below this and if the temperature is raised to 35 or 40′ the Amoeba would die.
In Amoeba irritability is of two types- negative response and positive response. Negative response occurs when it strikes any object and moves away from it.
Positive response takes place when it touches some object and advances towards it. Bright light, chemicals, heat and electric stimuli produce negative response. While dim light and low temperature create positive response.
Amoeba generally responds to the following stimuli :
1. Reaction to mechanical contact or Thigmotaxis
Amoeba has a sense of touch. If it comes in contact with a sand particle or is touched with a needle, pseudopodia of that side disappear and the animal moves away from it showing negative response. But when it comes in contact with food, it shows positive response.
2. Reaction to temperature or Thermotaxis
With the increase in temperature Amoeba becomes more active but when temperature is lowered, its ingestion of food and locomotion slow down.
3. Reactions to Chemical stimuli or Chemotaxis
If undesired chemicals which are not found in its normal habitat are added into the water in which Amoeba is living, it shows negative response.
4. Reaction to Electric stimuli or Galvanotaxis
In electrically charged water Amoeba moves away from anode and goes towards cathode. However, it dies in high electric charge.
5. Reactions to Light or Phototaxis
There in no effect in the reactions of Amoeba by gradual changes in light but it moves away if suddenly bright light is thrown on it showing negative response.
What is Pinocytosis?
Brandt, Tera and Shoemaker in 1968 and Rustard in 1969 proved that like other cells Amoeba not only takes food but also drinks water. Very minute pockets are formed at the tip of pseudopodia and gradually these pockets sink into endoplasm forming vesicles. The process is known as pinocytosis.
Reproduction in Amoeba
Reproduction in Amoeba takes pleace by the following methods:
1. Binary Fission:
Amoeba reproduces by this method when the temperature is suitable and food is in plenty. Due to favourable conditions the size of Amoeba increases and when it reaches the maximum, division become necessary. Binary fission is Amoeba takes pleace by mitosis.
During this process first of all Amoeba rounded and many small pseudopodia are formed. In prophase stage the nuclear membrane breaks and the nucleus elongates becoming spindle-shaped. About 500 very minute but clear chromosomes appear in the form of chromatids by duplication.
In metaphase they arrange themselves on the equator of the spindle forming the metaphase plate.
In anaphase the chromatids separate and reach the opposite poles of the spindle. In the meantime the nucleus becomes dumbbell-shaped and later on divides into two daughter nuclei.
In telophase the pseudopodia start returning to normal condition. Thus nucleus divides into two by mitosis and at the same time cytoplasm also divides.
A groove is formed in plasmalemma and it slowly deepens to divide Amoeba into two parts, each of which has a nucleus.
The newly formed Amoeba soon start growing and become adult. Binary fission is then repeated in them. The reproductive cycle goes on indefinitely by this asexual method by which Amoeba does not die.
One cycle of binary fission is completed in about 30 minutes but the duration depends much on the temperature. One of the daughter cells has got original contractile vacuolee while other develops new one in it.
2. Encystment and Multiple Fission
This always takes place when the conditions are not favourable. When the temperature becomes high or low and when water or food becomes less in ponds, then the activity of cytoplasm stops and no difference remains in ectoplasm and endoplasm.
In adverse environmental conditions Amoeba withdraws its pseudopodia and becomes spherical. To face such conditions Amoeba stops at a place and secretes thick liquid around itself which upon drying forms a hard covering known as cyst.
The cyst becomes three-layered soon after. The entire process is known as encystment. Amoeba is actually in resting stage during the encystment and all its life activities become very slow. The dispersal of Amoeba also takes place in this stage.
During encysted stage the nucleus of Amoeba divides 500 to 600 times amitotically forming a large number of nuclei. A little cytoplasm gathers around each nucleus, and they gradually come on the peripheri.
Thus many new amoebae are formed. When the conditions become favorable, the cyst bursts and these amoebae or pseudopodiospores escape into water. Soon they grow, becoming adult and reproduce again.
There are no definite proofs of multiple fission within the cyst in Amoeba.
During the condition all life activities slow down and the weight of Amoeba lessens due to which the cysts are blown with the wind and dispersed here and there.
Thus encystment saves Amoeba from being destroyed and also helps in its dispersal.
Recently Taylor has described a new method of reproduction in Amoeba without encystment during unfavorable conditions.
This method called sporulation. Some species of Amoeba reproduce by sporulation either normally or regularly.
In this first of all the nuclear membrane disappears and then the nuclear material breaks into many small pieces called chromatin blocks.
A membrane then forms around each block, it is converted into a nucleus. A little cytoplasm gathers around each nucleus and hard covering is also formed around each structure.
The spores thus formed settle down at the bottom of the pond. When the conditions become favorable, an Amoeba comes out of each spore.
If Amoeba, is cut into small pieces then after some time each piece, which possesses a portion of nucleus, grows into new a Amoeba. This process is called regeneration. Those pieces which do not contain a part of nucleus soon die.
In this process two Amoebae of same type unite temporarily with each other but exchange of nuclear material does not take place between them.
This process has been observed in Amoeba diploidea during which the amoebae regain their lost vigor.
Inheritance in amoeba
Daniel and his colleagues have recently shown that inheritance of characters takes place through nucleus as well as cytoplasm in Amoeba.
They exchanged the nuclei between Amoeba proteus and a closely related species, Amoeba discoides, and also among Amoebae of the same species but of different clones. and cytoplasm of another.
Immortality in Amoeba
The life of most animals end with death but from the many cells produced by them some cells, i.e., germ cells do not die and form a new individual after development.
Welsmann in 1885 put forth this view in his theory of continuity of germplasm. According to this theory the development of a living being takes place from reproductive cells.
During development these cells give birth to two kinds of cells:
1. Somatic cells They are more numerous and form muscles, bones and other organs of the body.
2. Reproductive cells-They are fewer in number and take part in reproduction to form an offspring which is similar to the parent.. In this manner reproductive cells pass on from one generation to another.
Thus death means the end of somatic cells and since reproductive cells continuously form new individuals (at least theoretically) they are immortal. In acellular animals like Amoeba there is no differentiation between somatic and reproductive cells and natural death also does not occur. There are two main reasons for this.
1. Harmful substances formed as a result of various chemical reactions taking place in body do not accumulate in it.
2. The wear and tear taking place in the body are fully repaired.
However, this does not happen in multicellular animals in which due to incomplete removal of harmful substance the cytoplasm of their cells becomes weak and gradually they become unable to perform the life activities satisfactorily and in the end natural death occurs.
Thus death is the price paid for the so-called body.
Beside this, the reproduction in Amoeba takes place by the simplest process. Under favourable condition two daughter Amoebae are formed from the parent Amoeba by binary fission during which the nucleus divides mitotically.
With the formation of daughter Amoebae, the life of parent Amoeba ends; however, its protoplasm is not destroyed but reaches the daughter Amoebae.
Under unfavourable conditions Amoeba forms a cyst around itself which helps in its dispersal as well as to tide over unfavourable conditions.
Thus by encystment Amoeba continues its race. Amoeba regenerates its broken pieces if cut by accident. It also rejuvenates by conjugation with other Amoebae.
Thus there is no possibility of natural death in Amoeba which dies only accidentally. This shows that Amoeba is immortal.