Amoeba: Behaviour and Nutrition | Protozoa | Microorganisms | Zoology

In this article we will discuss about the behaviour of amoeba. Also learn about the nutrition present in amoeba.

Behaviour of Amoeba:

1. Reaction to Mechanical Stimuli (Thigmotropism):

The response of Amoeba to contact varies according to the object of contact. When it is pricked with a needle the side in contact shrinks and opposite side gives out pseudopodia and the animal escapes. This is negative reaction –  A float Amoeba with spread pseudopodia responds positively to contact with solid object by fastening to it and soon starts creeping on it. Contact with the food also results in positive reaction. It behaves negatively to strong mechanical stimulus and positively to weak stimulus.

2. Reaction to Light (Phototropism):

When a beam of light is shone on the advancing pseudopodium of an amoeba, the rate of its movement is invariably slowed down, and if the light is strong enough, it is completely stopped. Movement later starts up in a new direction and in consequence the amoeba ultimately moves away from the light.

It must be noted that in its natural surrounding an amoeba is exposed to light of varying intensity. For this reason it does not respond to gradual changes in illumination. For any reaction it must be subjected to strong beam of light. Observations of some authors suggest that if A. proteus is exposed to light on any side, the plasmagel of that side thickens due to gelation of the adjacent plasmasol.

On this side, therefore, the plasmagel contracts and its elastic strength increase consequently forcing the pseudopodia to sprout on the opposite side.

3. Reaction to Chemical (Chemotropism):

Amoebae are indifferent to normal chemical constituents of water. Most dilute chemicals cause the pseudopodium to be pushed out at the point where the stimuli come in contact. The strong chemicals have adverse reaction, movement stops completely and the amoeba becomes rounded and later moves in the opposite direction.

4. Reaction to Temperature (Thermotropism):

Normally, the rate of locomotion and food-ingestion decreases in Amoeba with the fall in temperature and increases with rise in temperature. Temperature range of 20°C to 25°C is best suited (= optimum). Beyond 30°C, amoeba becomes spherical and stationary. Extreme fall or rise in temperature in fatal.

5. Reaction to Gravity (Geotropism):

It has been suggested that amoebae are positive to the force of gravity. It is for this reason that they are found attached to the bottom of the container.

6. Reaction to Electric Current (Galvanotropism):

If an electric current is passed into water containing amoebae, the amoebae move towards the cathode thus showing a negative response to the anode. This is brought about by the reversal of cytoplasmic streaming, brought about by the solation and consequently decreases in the elastic strength of plasmagel at the surface of body towards the cathode. If the intensity of the current is increased disintegration of the body begins at the surface towards the anode.

7. Reaction to Water Current (Rheotropism):

When floating freely in water the amoebae place in the line with the water current, i.e., they are positive to it. It is evident from the above that the responses by the amoeba to the stimuli are such as to benefit the individual and the species by avoiding unfavourable conditions and seeking those useful to it.

Nutrition in Amoeba:

Nutrition is holozoic, i.e. amoeba eats other organisms such as algae, other protozoans, rotifers and nematodes, dead protoplasm preferring small live flagellates and ciliates. It may eat several paramecia or several hundred small flagellates daily. It also exhibit choice in selecting food and is able to distinguish inert particles from the minute plants and animals upon which it feeds. Movements of the prey or substances diffusing from it attract the amoeba that always avoids highly active organisms.

There are no definite regions or organelles for food intake or ingestion. The food is captured by pseudopodia. A food cup is formed and pseudopodia are thrown around the sides and over the top of the object. In this way the food is held against the substratum, then completely surrounded by cytoplasm and finally incorporated into the main mass of the body.

Such a method is sometimes called circumfluence and is employed in the ingestion of less active prey. If the prey is active the pseudopodia are thrown out widely so that a wide food cup is formed and encloses the prey without touching or irritating it. This method of food capture is known as circumvallation.

In some cases the food is taken into the body upon contact with very little movement on the part of the amoeba. This method is described as feeding by import. Amoebae are also known to feed by invagination.

The amoeba touches and adheres to the food, and the ectoplasm in contact with it invaginates into the endoplasm as a tube carrying the food particle with it. The tube then dissolves leaving the food into the interior. Usually the prey is paralyzed on contact with the body. Sometimes active animals are ingested but they become inactive in the food vacuole.

The drop of water taken in along with the food forms a food vacuole in the endoplasm which secretes enzymes that enter the vacuole. In an actively feeding amoeba several of such food vacuoles may be seen, in food vacuole is the “digestive tract” of the amoeba in miniature.

Enzymes act best at a definite acidity or alkalinity, consequently the amoeba furnishes proper conditions for the proper functioning of the enzymes. Thus, in the food vacuole the reaction is at first acid, the acidity serving to kill the prey which often struggles for some time after being ingested. The reaction later becomes alkaline because the proteolytic (protein digesting) enzyme (like trypsin) of the amoeba is active only in an alkaline medium.

Later the food particles lose their sharp outlines, swell, become more transparent and reduce in amount as the products of digestion are assimilated by the general cytoplasm. Direct evidences showing the digestion of proteins and fats are available. For this purpose ferments like peptidase and proteinase are poured into the food vacuole by the endoplasm.

Holter and Doyle have recently demonstrated the presence of amylase in A. proteus, which means that the power to convert starch into soluble sugar exists in amoebae. According to Mast, Amoeba proteus is also able to digest the neutral fat globules of the food and break into fatty acids and glycerine.

The food so digested passes into the surrounding protoplasm from the food vacuoles. The digested food is distributed to all parts of the body because of the movements of the food vacuoles. The indigestible fragments are eliminated in a very simple way. They are extruded at any point where they happen to reach the surface.

Pinocytosis is cell-drinking so that Amoeba not only eats but also drinks. It has been shown that the process is induced by proteins, inorganic ions and certain dyes. When an Amoeba is placed in a suitable solution, say 1% sodium acetate solution, its surface gets wrinkled by folds and crevices.

The folds form little pockets or tubes leading inwards from the surface and each tube last for several minutes before it disappears. These tubes gulp in droplets of the solution. These droplets are punched off as vacuoles at the base of the tubes and enter the protoplasm. An amoeba drinks about 30% of its own volume of water in 3 hours.

The food vacuole, or the gastric vacuole, thus formed by the food engulfed in the cytoplasm along with some water, serves as temporary stomach or gut for digestion. A food vacuole is lined by plasmalemma and ectoplasm, but when it disintegrates the ectoplasm becomes absorbed in the surrounding endoplasm.

This goes to confirm that the difference between ectoplasm and the endoplasm is only temporary. An amoeba may contain several food vacuoles floating about freely in the streaming plasmasol and varying in shape and size according to the nature of their food contents.

The vacuolar contents gradually become indefinite in outline indicating digestion, and then smaller, indicating absorption of digested material. The solid food digested, that is, converted into diffusible liquid form, by the action of certain enzymes secreted by the surrounding cytoplasm.

Lysosomes containing the enzymes fuse with the food vacuoles. The reaction of the food vacuoles is first acidic, which serves to kill the prey, but later becomes alkaline when food is digested. Two enzymes have been demonstrated, amylase which digests starch and proteinase which digests proteins. Amoeba can readily digest protein, sugar and cellulose, while fats remain unaltered.

The digestion of food in Amoeba is said to be intracellular in contrast to the extracellular digestion in higher animals, like frog and earthworm, taking place outside the cells in the cavity of an alimentary canal.

The digested food is absorbed by the surrounding cytoplasm by a simple process of diffusion. A circulatory system is needless and therefore absent in amoeba, because the food vacuoles constantly move about in the streaming endoplasm by cyclosis and directly supply nourishment to all the parts of the cell. In the cytoplasm, the digested food gets assimilated to build new protoplasm.

The un-digestible food particles left after digestion, are gradually shifted backwards and left behind as faecal pellets as the amoeba moves on. There is no definite aperture for the egestion of the undigested residue which is eliminated through a temporary opening in the ectoplasm formed at any point on the surface.