In this article we will discuss about Amoeba:- 1. Introduction to Amoeba 2. Etymology of Amoeba 3. Structure 4. Osmoregulation 5. Sensitivity.
Contents:
- Introduction to Amoeba
- Etymology of Amoeba
- Structure of Amoeba
- Osmoregulation of Amoeba
- Sensitivity of Amoeba
1. Introduction to Amoeba:
The genera Amoeba included under the naked amoebas by Ruppert and Barnes (1994), was first discovered by Rossel Von Rosenhof in 1755. Later in 1962, H. I. Hirschfield provided a detailed account of its biology. Amoeba serves as an interesting and suitable material, and is universally studied as an introduction to protozoa.
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The genus Amoeba has a single nucleus. Pseudopodia may be extensions of considerable size (.Amoeba proteus) or small, wart-like elevations (A. verrucosa). Flagellated stages are unknown, but encystment has been reported in some species. There are several species of Amoeba, the most commonly studied species is Amoeba proteus.
2. Etymology of Amoeba:
Greek: amoibe, change.
The species name proteus is after the mythological Greek Sea God – Proteus, known for his power to change form variously.
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Habit and Habitat of Amoeba:
A. proteus is a minute, free living form, occurring abundantly in the bottom of fresh water ponds, ditches, lakes and spring-pools. They are always found in association with aquatic vegetation.
3. Structure of Amoeba:
External Structure:
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Body resembles a tiny mass of irregular jelly and measures about 0-25 mm in diameter, but some large species (A. proteus) may reach a diameter of 0-5 mm (Fig. 1.6). The irregular shape of the body is due to constant throwing of its own surface as pseudopodium.
Externally the body is covered by a very thin, clear, elastic and semipermeable layer of protoplasm, called the cell membrane or plasma membrane or plasma lemma. The thickness of plasma lemma may be between 0.5 µm to 2 µm. The existence of a very thin and flexible pellicle, covering the plasma lemma has been reported.
Internal Structure:
Inside the plasma membrane are placed a single nucleus and cytoplasm. Nucleus is disc-like and biconcave and occupies no fixed position in the endoplasm. Cytoplasm is differentiated into ectoplasm and endoplasm.
The ectoplasm is less extensive, gel in nature and non-granular, though in electron microscopy it shows thread and particles. The endoplasm is divisible into two parts — the stiff region beneath the ectoplasm, called plasmagel and an inner-fluid part called plasmasol. Embedded in the plasmasol are the following structures.
(a) Contractile Vacuole:
Single large and transparent vacuole, containing a space filled with a watery fluid. It gradually increases in size, travels to the surface and suddenly bursts to release its contents in the surrounding water and disappears. A new contractile vacuole is formed later.
(b) Food Vacuole:
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One or more, spherical, non-contractile food vacuoles, containing water and food particles, are present at different phases of digestion.
(c) Water Vacuole or Globule:
Several of these occur as transparent colourless drops which do not change in size.
(d) Stored Food:
Numerous granules of reserve food material of the nature of fats and carbohydrates are present.
(e) Mitochondria:
Present in the form of rods or more or less oval shape with tubular cristae.
(f) Crystals:
Different sizes and shapes of crystals are seen which are probably metabolic wastes.
Functions of the above mentioned structures are given below.
Plasma Lemma:
Being the outer membrane it retains the inner contents. It is permeable to respiratory gases and water. It plays important role in pseudopodia formation and food capture.
Ectoplasm:
It is responsible for maintaining the shape and also protects the inner parts.
Endoplasm:
It is the matrix, within which different organelles including the nucleus remain suspended.
Contractile Vacuole:
It is involved in osmoregulation, respiration and excretion.
Food Vacuoles:
These vacuoles are related with nutrition.
Water Vacuoles:
These control the water balance of the body.
Mitochondria:
These are considered as the ‘power house’ of the cell and are the seats of cellular respiration.
Locomotion:
Amoeba moves about by creeping and remains in direct contact with a substratum. This is performed by the production of finger-like projections called pseudopodia and the movement is called amoeboid movement.
Nutrition:
Nutrition in Amoeba is holozoic or zoo-trophic or heterotrophic. In this method animals and plants smaller than the body of the protozoa are used as food.
Respiration:
Dissolved oxygen in water enters the body through the general body surface and the carbon dioxide produced by oxidation goes out through the body surface by the process of diffusion.
Excretion:
The excretory bi-products are urea and uric acid. They pass out of the body through the plasma lemma by the process of diffusion.
4. Osmoregulation of Amoeba:
Water enters into the body by endosmosis through the general body surface. This water along with the water that is taken in during food intake is collected by the contractile vacuole. A contractile vacuole is about 20-30 µm in diameter and contains accumulated fluid which is less dense than the surrounding cytoplasm. A fully expanded vacuole contracts and the fluid is forced out through the cell surface.
As soon as the old vacuole disappears, its place is taken up by a new vacuole. The rate of contraction of a vacuole varies from a few seconds to several minutes. Schneider (1960) observed that the wall of a contractile vacuole is provided with contractile fibrils which bring about the contraction of the vacuole.
Water discharged to the exterior by vacuoles contains traces of metabolic wastes and respiratory gases. The vacuolar activity increases when distilled water is injected and stops when the animal is put in salty water or is treated with potassium cyanide.
5. Sensitivity of Amoeba:
Amoeba has no nervous system and special sense organs to perceive the stimuli, although it is highly sensitive. It gives negative response to mechanical obstacle. When pricked or touched by a rod it turns to avoid the obstacle. However, it is able to distinguish between a particle of food and a nonfood particle. It moves away from salt, sugar, acid and alkali to which it is unaccustomed.
Amoeba also avoids darkness as well as strong light, but shows positive response towards normal or diffused light. It becomes globular if an electric current is passed in the water which contains it. Even, extreme disturbance or injury causes it to become spherical and it remains motionless for some time.
It is most active at an optimal temperature of 20-25°C. Lower temperature retards its activity. Thus, the responses of Amoeba are so oriented so as to benefit the animal to the maximum.