Structure of unicellular organism Amoeba and its life cycle
Amoeba is the simplest unicellular microscopic organism present in nature. This article deals with the detailed structure of the Amoeba organism as it is observed under the microscope. This resource also provides a detailed account of various life activities exhibited by Amoeba. The most frequently asked questions related to this topic were also included along with this topic.
Amoeba is a unicellular, free-living freshwater organism. The name Amoeba is derived from a Greek word that means change. This organism has no permanent shape and quite often it changes its shape and thus got this name. The scientific name of Amoeba is Amoeba proteus. Proteus is after the name of a mythological sea God, famous for changing his body shape. Proteus animalcule is the common name of Amoeba proteus. Amoeba was first discovered and described by Roselvon Rosenholf in 1775. A detailed account of the Biology of Amoeba was given by H.I.Hirschfield (1962).
The Amoeba appears to be the simplest possible independent unicellular living organism with a nucleus and cytoplasm. Despite its simplicity in the bodily organization, it can locomote, capture food, digest and assimilate complex food, egest undigested residues, grow, respire, respond to external stimuli, do excretion, and reproduce the same kind like any other higher organisms.Amoeba culture preparation
Culture of Amoeba can be obtained by placing a few pondweeds in a petri-dish and add water to it. Place a few grains of wheat and cover the petri-dish with a lid. Leave the petri-dish in a dark room for a few days till brown scum forms over the water. The culture prepared like that contain many Amoebae in it. Freshly brought pond water also contains Amoebae in it. Usually under the surface of weeds of aquatic plants, one can obtain Amoeba.Microscopic structure of Amoeba
Amoeba proteus is a one-celled irregular shaped microscopic animal. It measures about 0.25 mm. to 0.60 mm. in diameter. Under a compound microscope, Amoeba appears to be an irregular, colorless, translucent mass of spec constantly changing its shape.
Microscopic Structure of Amoeba (Courtesy:-wikipedia.org)
The outer surface of the amoeba organism is externally bounded by a living selectively permeable membrane called Plasmalemma or Plasma membrane. This membrane is made up of proteins and lipids.
Next to Plasmalemma there present a granular semi-solid colloidal substance called cytoplasm in amoeba. This cytoplasm can be divisible into ectoplasm and endoplasm. The outer clear rigid non-granular part of the cytoplasm is known as ectoplasm or ectosarc. The inner fluid granular part of the cytoplasm is known as endoplasm or endosarc. The ectoplasm consists of a semi-solid substance called plasma gel and the endoplasm consists of a viscous fluid called plasma sol. Plasma gel and plasma sol are interconvertible. Pseudopodia are formed by the sol-gel transformation of the endoplasm (cytoplasm) of amoeba. The conversion of gel into sol and vice versa is a physiochemical phenomenon. The movement of granules of plasma sol causing streaming is known as Brownian movement.
The outer surface of the body of amoeba is provided with blunt finger-like projections called Pseudopodia or lobopodia.Pseudopodia are made of plasmalemma, ectoplasm, and endoplasm. These are not permanent structures and they quite often disappear. These projections of amoeba help them to capture food and help in locomotion.
A single biconvex disk-like nucleus is present at the center of the plasma sol region of amoeba organisms. The nucleus contains about 500-600 chromatin granules. The nucleus is covered by a double membraned nuclear membrane made of proteins and lipids. All the vital activities of amoeba are controlled by the nucleus. The nucleus takes an active part in the process of reproduction.
A single large contractile vacuole is in the outer part of endoplasm (plasma sol) of amoeba. Contractile vacuole in amoeba is mainly serving the purpose of Osmoregulation. Contraction and relaxation of contractile vacuole are called systole and diastole respectively. Contractile vacuoles store watery fluids of the amoeba's body. They aid in the removal of excess water and excretory fluids of the organism. They also help in the elimination of Co2 formed during respiration. If an amoeba is placed in marine water, it loses its contractile vacuole, and later on, plasmolysis occurs.
Inside the body of amoeba, we may find 2 or more food vacuoles containing food molecules inside it. Food vacuoles are temporarily formed vacuoles and are meant for collecting, digesting, and excreting undigested wastes.
In amoeba, other than the above vacuoles a large number of small vacuoles are present in the cytoplasm of the cells called water vacuoles. These water vacuoles help in maintaining the water balance of the body.
Cell organelles like mitochondria, Golgi bodies also can be seen in the cytoplasm of the amoeba under a high powered microscope. Golgi bodies in amoeba help in the secretion and excretion process of the body. Mitochondria in amoeba help in the respiratory activity of amoeba. The reserve food in amoeba is in the form of fats and glycogen.Locomotion in Amoeba
The displacement of the whole body from one point to another is known as locomotion. Amoeba shows its locomotion with the help of its temporarily formed pseudopodia for this purpose. Scientists put forth various theories to explain the locomotion of the simplest single-celled organism amoeba. The most popular theories among these are Sol-gel or change of viscosity theory by Hayman and later it was supported by Mast and Pantin.
The following steps are involved in the locomotion of Amoeba:-
Sol-gel theory in the Locomotion of Amoeba (Diagram by the Author)
The above amoeboid movements can be explained by the sol-gel theory as proposed by Hyman. According to this theory, the plasma gel of the cytoplasm of the amoeba changes into the plasma sol state and vice-versa for the formation of pseudopodia. At the extreme end of newly formed pseudopodia,
the plasma gel changes into a sol state by becoming thinner and weaker. The plasma sol change into rigid plasma-gel (gelates) at the anterior end and at the posterior end the plasma gel changes into plasma sol (solaces) causing forward streaming of the more fluid plasma sol. A pseudopodium in amoeba is formed by both ectoplasm and endoplasm through sol-gel transformations. The thickened ectoplasm at the tip of pseudopodium is called the hyaline cap.
Contraction theory was postulated by Dellinger. According to this theory, Amoeba walks on substratum using pseudopodia as legs.
Surface tension theory was proposed by Berthold. It was supported by Butschli and Rhumbler.
The rolling movement theory was put forward by Jennings. According to this theory Amoeba verrucosa rolls on substratum like a water drop.
Adhesion theory, Contraction theory, Rolling movement theory, Surface tension theory. Sol-gel theory is related to amoeboid movement.Nutrition in Amoeba
Although amoeba is the simplest single-celled organism, it follows the same steps of nutrition as in higher organisms. The mode of nutrition in amoeba is heterotrophic and holozoic in nature. It ingests solid food particles like bacteria, diatoms, and pieces of decaying plants and digests inside its body. So the type of digestion is said to be intracellular digestion in amoeba.
Nutrition process in Amoeba (Courtesy:-wikipedia.org)Ingestion
The pseudopodia encircle the organic food particles and draw them into temporarily formed food vacuoles.Digestion
The food vacuoles circulate in the endoplasm and slowly digestive enzymes secreted from the endoplasm are poured into them. With the help of these digestive enzymes, the complex organic food particles are broken into their simple form. Thus the process of digestion is accomplished with the help of chemical enzymes. The food vacuole of amoeba acts as a stomach of higher animals.Absorption
The fine digested particles in the food vacuoles are amoeba are absorbed into cytoplasm through diffusion from vacuoles.Assimilation
The absorbed fine particles now thoroughly get mixed with protoplasm and become a part of the proplasm. This step is known as assimilation.Egestion
The undigested food particles that remain in the vacuoles are now thrown out of the body by the bursting of these vacuoles at the surface of amoeba.Respiration in Amoeba
Amoeba mostly lives in ditches, ponds, and other bodies of standing water. The dissolved oxygen present in water bodies will be diffused into the body of amoeba through its general body surface by a simple diffusion method. By using this oxygen, the oxidation of food molecules occurs and thus ATP energy, Co2, and water are produced. The Co2 evolved during the oxidation of food is diffused out through its cell membrane.Irritability in Amoeba
The response to external stimuli is known as irritability and it is a characteristic feature of all living organisms. Amoeba responds to various external stimuli like temperature, chemicals, a strong beam of light, touch by a needle, etc. by moving away from the source of stimuli.Excretion in amoeba
Amoeba excretes its metabolic wastes formed inside its body by a simple diffusion method through its cell membrane by expelling them to outside water.Reproduction in Amoeba
Amoeba reproduces through two methods-Binary fission and Multiple fission.Binary fission
This is an asexual mode of reproduction that usually takes place under favorable conditions. The favorable conditions for binary fission are optimum temperature, plenty of water and oxygen availability, and a proper supply of food. 
Binary Fission in Amoeba (Diagram by the Author)Multiple Fission in Amoeba
This type of reproduction occurs under unfavorable conditions such as scarcity of food, lack of water, and high temperature.
Multiple Fission in Amoeba (Diagram by the Author)Encystment process in Amoeba
This process helps the amoeba to tide over unfavorable conditions like severe cold or hot, dryness, lack of food, lack of water, lack of oxygen, etc. The process occurs as follows:-Power of regeneration of Amoeba
Amoeba has great power of regeneration. By accident, if any amoeba cut into two bits and if these cut bits have small bits of nuclei in them and they can grow and transform into new individuals.Immortality of Amoeba
Amoeba has no natural death. Amoeba always reproduces through asexual means. The parent amoeba always divides and gives rise to two daughter cells through binary fission. The protoplasm of the parent cell always gets passed on to its daughter cells generations after generation and thus there is no natural death for the organism. Thus amoeba is considered to be an immortal organism.Most frequently asked questions:-
Ans. Plasmalemma or ell membrane
Ans. Contractile vacuole.
Ans. Heterotrophic and holozoic mode of nutrition.
Ans. Ingesting solid food particles and digesting solid food inside the body.
Ans. If a strong beam of light focused on an amoeba, it moves away from the source of light.
Ans. Ingestion
Ans. It undergo encystment.
Ans. Multiple fission.
Ans. Roselvon Rosenholf in 1775
Ans. Food vacuole.
Here below a video to know more about Amoeba:-