Study Notes on Amoeba Proteus

In this article we will discuss about Amoeba Proteus:- 1. Habit and Habitat of Amoeba Proteus 2. Structure of Amoeba Proteus 3. Locomotion 4. Nutrition 5. Respiration 6. Excretion 7. Osmoregulation 8. Reproduction 9. Encystment 10. Sensitivity.

Contents:

1. Habit and Habitat of Amoeba Proteus
2. Structure of Amoeba Proteus
3. Locomotion of Amoeba Proteus
4. Nutrition in Amoeba Proteus
5. Respiration in Amoeba Proteus
6. Excretion in Amoeba Proteus
7. Osmoregulation in Amoeba Proteus
8. Reproduction in Amoeba Proteus
9. Encystment in Amoeba Proteus
10. Sensitivity of Amoeba Proteus

1. Habit and Habitat of Amoeba Proteus:

Amoeba proteus occurs abundantly in the bottom of pools, ponds, ditches. It is always found in association with aquatic vegetation’s. Successful culture of amoeba is made in the laboratory in different culture media.

2. Structure of Amoeba Proteus:

The body of amoeba proteus resembles a blob of an irregular jelly and measures about 600 micra in aver­age diameter (Fig. 10.15). The irregular shape is due to constant throwing of its own sur­face as pseudopodium.

The outer boundary of the body is made of the plasma membrane which is thin, elas­tic and selectively permeable. Recently, the existence of a very thin and flexible pellicle in amoeba has been reported and it has been possible to separate the pellicle from the plasma membrane by plasmolysis. Inside the plasma membrane are placed the nucleus and cytoplasm. The nucleus is disc-like and bi­concave.

The cytoplasm is differentiated into ectoplasm and endoplasm. The ectoplasm is less extensive, gel in the nature and non-granular though in electron microscopy it shows threads and particles of unknown significance. The endoplasm is divisible into two parts—the stiff region beneath the ecto­plasm is called plasmagel and an inner-fluid part called plasmasol. The plasmasol includes various organelles and inclusions.

These structures are:

1. Contractile vacuole:

Single, large and transparent; gradually increases in size and ultimately bursts.

2. Food vacuole:

One or more spherical vacuoles containing water and food particles at different phases of digestion.

3. Water vacuole or globule:

Several occur as perfectly transparent colour­less drops which do not change in size.

4. Stored food:

Numerous granules of the nature of fats and carbohydrates which are recognised by using special staining methods.

5. Mitochondria:

These are present in the form of rods or dots and are recognised by special stains.

6. Crystals:

Crystals of assorted sizes and shapes are seen which are probably metabolic wastes.

The role played by the above-mentioned structures is given below:

(i) Plasmalemma:

It retains the inner con­tent and is permeable to respiratory gases and water. It plays important part in pseudopodia formation and food capture.

(ii) Ectoplasm:

It is responsible for maintain­ing the shape and also protects the inner parts.

(iii) Endoplasm:

It houses different organelles including the nucleus. The conversion of plasmasol to gel and back is important in the process of pseudopodia formation.

(iv) Nucleus:

It regulates the working of all other organelles and takes part in reproduc­tion.

(v) Contractile vacuole:

It is involved in osmoregulation, respiration and excretion.

(vi) Food vacuoles:

These organelles are meant for nutrition.

(vii) Water vacuoles:

Numerous small water vacuoles control the water balance of the body.

(viii) Mitochondria:

These bodies are regarded as the ‘power house’ of the cell and are the seats of cellular respiration.

(ix) Various inclusions:

It includes reserve food materials and also different metabolic wastes.

3. Locomotion of Amoeba Proteus:

Locomotion in amoeba proteus is creeping in nature and is dependent upon an intimate and direct contact with a substratum. Creeping in amoeba involves the production of finger-like projections called the pseudopo­dia and the movement is called amoeboid movement.

During locomotion in Amoeba proteus one or more blunt finger-like pseu­dopodia are formed in the direction of move­ment. The projection grows more and more by accumulation of cytoplasm from other parts of the body.

In some cases Amoeba proteus has been seen to walk on the tips of the pseudopodia. A good many theories have been advocated to ex­plain the mechanism involved in the forma­tion of a pseudopodium. Mast (1925) has given a thorough and detailed description of pseudopodia formation (Fig. 10.16).

According to Mast, the body of the Amoeba is made up of four parts—the thin and elastic plasma membrane, the plasmagel, the plas­masol and a hyaline fluid in between the membrane and plasmagel. Actions and in­teractions between these four parts result in pseudopodium formation.

During the formation of a pseudopodium the plasma membrane gets attached to the substratum. A local and partial liquefaction occurs in the plasmagel at a point. The rest of the plasmagel exerts pressure on the weak­ened area to produce a bulge.

This pressure comes from osmotic and other forces. Posteriorly, the contracting plasmagel con­verts into plasmasol. Anteriorly, the plas­magel tube is continuously regenerated by gelation of plasmasol and the pseudopodium grows.

Thus the formation of pseudopodium and the resultant movement in amoeba are due to spontaneous and reversible sol-gel phe­nomenon.

4. Nutrition in Amoeba Proteus:

Amoeba is microphagous, i.e., ingestion of small food particles, animal and feeds on small organisms, such as bacteria, diatoms, small ciliates, tiny algal filaments and rotifers, etc. and the process is microphagy.

It is holozoic, i.e., feeding on the organic substances, in nutrition and has the capacity to discrimi­nate between nutritive and non-nutritive food. It has been shown that an amoeba can discriminate between Chilomonas and Monas and shows preference for the former.

Ingestion:

Ingestion commonly involves phagocytosis, i.e., engulfment of particles such as bacteria, protozoa by active out­growth of the plasma membrane of a cell, for this process Amoeba often chases its food from a considerable distance (Fig. 10.17). When it comes in contact with the food par­ticle, it extends pseudopodia on either sides above the particle and forms a concavity or food-cup.

Ultimately, the pseudopodia bend around the particle and their ends meet and fuse. The food particle along with some amount of water becomes enclosed in the food vacuole. The margin of the food-cup exerts sufficient force as indicated by the ‘biting’ of the prey into two parts. The whole process of ingestion takes about 2-3 minutes and several food vacuoles may be present at a time.

Digestion:

Digestive processes now start in the food vacuoles. The cytoplasm sur­rounding the vacuoles secretes HCl which kills the prey and makes the medium acidic which soon becomes alkaline as enzymes are secreted.

Among the enzymes pepsin and trypsin have been distinguished in Amoeba proteus. The end products of digestion which are proteins, carbohydrates and fats in simple form and some soluble minerals are absorbed by the surrounding cytoplasm.

Egestion:

Egestion or throwing-out of non-digested food particles is done through temporary openings in the ectoplasm at a spot near the food vacuole. Figure 10.18 illustrates the process of ingestion and egestion.

A process called pinocytosis or ‘drinking’ has been reported in Amoeba proteus. Pinocytosis involves the formation of a narrow canal on the surface of the body by invagination and then the pinching-off of the canal containing fluid. The importance of pinocytosis is yet to be evaluated.

5. Respiration in Amoeba Proteus:

Oxygen dissolved in water enters the body through the general body surface, and car­bon dioxide produced by oxidation goes out through the body surface by the process of diffusion.

6. Excretion in Amoeba Proteus:

By-products of dissimilation are urea and uric acid. They along with excess salt in solution, pass out of the body through plasmalemma by a physical process called diffusion.

7. Osmoregulation in Amoeba Proteus:

Excess of water which enters the body through body surface and during food-in- take is collected by the contractile vacuoles. A contractile vacuole is about 30-50 micra in diameter and contains accumulated fluid which is less dense than the surrounding cytoplasm.

A fully formed vacuole contracts and the fluid is forced out through the cell surface. The disappearance of one vacuole is followed by the production of a new one.

The rate of contraction of a vacuole varies from a few seconds to several minutes. The vacuolar activity increases when distilled water is injected and stops when the animal is put in sea water or is treated with potas­sium cyanide. Water discharged to the exterior by vacuoles contains traces of metabolic wastes and respiratory gases.

8. Reproduction in Amoeba Proteus:

Reproduction in amoeba proteus is asexual and is effected by Binary fission. The nuclear divi­sion is eumitotic type, i.e., there is distinct chromosome formation but the presence of 500 to 600 chromosomes makes the mitotic picture obscure. It has been shown that there is a definite correlation between the stages of nuclear division (Fig. 10.19) and the external morphological changes (Fig. 10.20).

During prophase the animal becomes round, stud­ded with fine pseudopodia and under reflected light presents a well-defined hya­line area at the centre. The hyaline area dis­appears in metaphase. During anaphase the pseudopodia become coarse and in telophase the body elongates, cleavage furrow appears and gradually the pseudopodia return to normal structure.

9. Encystment in Amoeba Proteus:

Encystment in Amoeba proteus has not yet been reported though it is a very common feature in other amoebae (Fig. 10.21).

In extremes of coolness or hotness and in other unfavourable conditions an amoeba encysts. During encystment the body becomes round and the pseudopodia are withdrawn. The food particles are either absorbed or thrown out and the contractile vacuoles dis­appear. The cytoplasm secretes a double- walled resistant envelope around it (Fig. 10.22).

On the return to favourable condition excystment occurs. The protoplasm inside the cyst comes out by breaking the cyst wall at a point.

The cyst in case of amoeba is protective in nature and not reproductive. Evidences in favour of amoeba undergoing nuclear divi­sion in encysted condition are very rare. It is to be noted that one amoeba comes out of one cysts.

10. Sensitivity of Amoeba Proteus:

Amoeba proteus gives negative response to mechanical obstacle. When it is pricked or touched by a rod it turns to avoid the obsta­cle. However, it can distinguish between a particle of no use and a particle of food.

It readily repels itself from salt, sugar, acid and alkali as it is unaccustomed to them. Amoeba proteus shows negative reaction to strong light and positive reaction to gravity. The responses of Amoeba proteus are so oriented as to benefit the in­dividual to the maximum.