· Amoeba proteus is the most common species of Amoeba.

· Amoeba proteus (Gr. Amoeba - change; proteus = mythical sea god who could change shape) is regarded as one of the lowest forms of life. It is versatile in comparison to other organisms. It lives in freshwater, moist soil, or the sea.

· Amoeba proteus: Freshwater Amoeba
· Amoeba verrucosa: Freshwater Amoeba
· Amoeba limax: Marine water Amoeba
· Amoeba terrestis: Muddy water Amoeba

General morphology

· Microscopic; unicellular, acellular or non-cellular and without fixed body shape (irregular). i.e. Amoeba has no skeleton.
· The outermost covering of the body is the plasma membrane called plasmalemma. It is very thin and selectively permeable.
· Plasmalemma is excretory (ammonia diffuses out through it), respiratory (diffusion of oxygen and carbon dioxide occurs through it), and protective in nature.
· Inside plasmalemma, there is a dense mass of cytoplasm containing several cell organelles.
· Cytoplasm is differentiated into an outer ectoplasm and an inner endoplasm.
· Ectoplasm is clearly visible at the tip of the pseudopodium where it forms a hyaline cap.
· Ectoplasm internally surrounds endoplasm, which occurs in two colloidal states, i.e., a peripheral viscid state called plasma gel and a central following state called plasmasol.

· Locomotory organelle in Amoeba is pseudopodia.
· At the anterior region, Amoeba forms new functional pseudopodia while the posterior region is marked by a wrinkled region called uroid.
· Pseudopodia in Amoeba are lobe-like with the blunt end without polar filament and are called lobopodia.
· Lobopodium is formed by conversion of gel into sol and vice-versa.
· Amoeba proteus is monopodial (many lobopodia are formed but only one proceeds forward).

Theories on pseudopodia formation

· According to surface tension theory (Berthold, 1886AD), the difference in surface tension in body surface and substratum is the reason for movement.
· Sol-gel theory (Hyman): It is the most accepted theory. It was supported by Mast and Pantin.
· According to this theory, amoeboid movement is due to a change in the viscosity of the cytoplasm. [BPKIHS 2000]
· The conversion of plasmasol into a gel and vice-versa is a physicochemical phenomenon. Sol-gel conditions are due to the contraction and relaxation of long-chain proteins.
· Locomotion of Amoeba is known as an amoeboid movement.
· Contraction-hydraulic theory (Rinaldi and Jahn, 1963 AD)

· Mode of nutrition is holozoic (zootrophic) and omnivorous.
· Ingestion occurs with the help of pseudopodia. Amoeba ingests food by circumvallation, circumfluence, invagination, and import.
· Circumvallation is the engulfment of active prey like ciliate or flagellate.
· Circumfluence is the ingestion of less active or motionless organisms like bacteria.
· Invagination is the formation of a tube, e.g. microscopic and filamentous algae.
· Import (passive- food ingestion): It is the passive sinking of food into the body by rupture of plasmalemma, e.g. algae.
· Digestion is intracellular.
· Food vacuole of the Amoeba is analogous to the gastrovascular cavity of the Hydra and alimentary canal of an animal. The contents of food vacuole in Amoeba are first acidic and then alkaline.
· Egestion occurs from the body surface either through the temporary or permanent pores.

Respiration and excretion
· Both respiration and excretion takes place through general body surface (plasmalemma) (IE 2004)
· Amoeba is ammonotelic as the chief excretory product is ammonia.
· The contractile vacuole is single, rounded, and filled with watery fluid; found in between nucleus and uroid. It expands and contracts rhythmically (called systole and diastole) to discharge its watery fluid outside and helps in osmoregulation and excretion.

Points to remember

· Pseudopodia in Amoeba are organs of locomotion as well as food capture.
· Pseudopodium in Amoeba is composed of both ectoplasm and endoplasm. It is a temporary locomotory structure.
· Pseudopodium at its forward end gets its firm consistency by the hyaline cap, which is made of ectoplasm alone.
· Amoeba can't digest fat.
· Contractile vacuole is found only in freshwater forms; absent in marine and parasitic forms.
· If an Amoeba is placed in distilled water, its contractile vacuole works faster and if placed in saltwater, its contractile vacuole will disappear. When a freshwater Amoeba is kept in seawater, it loses its contractile vacuole but, when a marine Amoeba is kept in freshwater, it develops a contractile vacuole.
· The main difference between Amoeba and Entamoeba is the contractile vacuole (Entamoeba lacks contractile vacuole).
· Contractile vacuole of Amoeba is analogous (similar in function) to uriniferous tubules of the frog.

· Amoeba shows positive phototaxis to dim light and negative to strong light and darkness.
· Amoeba shows negative chemotaxis.
· Amoeba shows positive geotaxis as it is the bottom dweller.
· Amoeba shows positive thermotaxis at 20-25°C; negative to both high and low temperatures.
· Amoeba shows positive thigmotaxis to those objects upon which it glides and rests but negative to any foreign body/obstacle/needle.
· Amoeba shows positive rheotaxis as it swims against water current.
· Amoeba shows positive galvanotaxis in the weak electric field as it gathers at the cathode however in a strong electric field, it becomes inactive.

· Amoeba reproduces by asexual mode only. It occurs by various methods like binary fission, multiple fission, sporulation, and encystment.
· Binary fission is the most common method of reproduction and follows a simple pattern of division, i.e. all stages of karyokinesis of mitosis. It takes place when food is abundant and temperature is suitable and is completed in 20-30 minutes.
· An Interesting feature of mitosis during binary fission in Amoeba is that multiple nuclear spindles are formed which is reduced to the unipolar nuclear spindle at the end of the anaphase stage.
· Multiple fission occurs during adverse environmental conditions. First of all, three layers of the cyst (of chitin) are secreted inside which nucleus divides repeatedly/amitotically to form several (500-600) daughter nuclei.
· These daughter nuclei acquire some portion of cytoplasm and are now called pseudopodiospores/ amoebulae. In favourable conditions, each amoebula grows into an Amoeba
· Sporulation also occurs during adverse conditions but without encystations.

· Encystment: During the unfavourable conditions, it secretes cyst and when the favourable condition is available, the cyst breaks to give rise to a single Amoeba.

Points to remember

· Amoeba is immortal and can undergo rejuvenation for regaining energy.
· Amoeba regenerates from nucleated bits.
· Nucleus of Amoeba is solid and biconcave.
· Amoeba is cultured in the laboratory by the ‘Hay infusion method’.

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