Amoeba was discovered by Russel von Resenhoff in 1755. The most common species of Amoeba is proteus. It lives in freshwater, moist soil or sea and can be cultured in laboratory by 'Hay infusion method'. Its body is covered by plasmalemma. It is a trilaminar and selectively permeable membrane. It is excretory, ammonia diffuses out through it.
The type of pseudopodium found in Amoeba proteus is lobopodium. They are composed of both ectoplasm and endoplasm.
Pseudopodium at its forward end gets its firm consistency by hyaline cap which is made of ectoplasm. Endoplasm is divided into outer plasma gel and inner plasma sol.
Sol-GEL theory of
amoeboid movement was first given by Hyman supported by Pantin (1923) and Mast (1926).
According to this theory amoeboid locomotion is due to change in the viscosity of cytoplasm. The conversion of plasmasol into gel and vice versa is a physicochemical phenomenon. Sol-gel conditions are due to contraction and relaxation of long chains of proteins.
Contractive vacuole is found only in freshwater
forms. It is absent in marine and parasitic forms. Difference between amoeba and Entamoeba is in contractile vacuole. If an amoeba is placed in distilled water, its contractile vacuole works faster. If an amoeba is placed in salt water, its contracile vacuole will disappear. contractile vacuole of amoeba is analogous (similar in function) to uriniferous tubules of frog. mode of nutrition in Amoeba is heterotrophic
Amoeba ingest food by
import, circumfluence, circumvallation or invagination. Import involves passive sinking of food into body by rupture of plasmalemma, e.g., Ingestion of algae. Circumfluence is the ingestion of less active or motion less organisms like bacteria. Circumvallation is the engulfment of active prey like ciliate or flagellate.
Amoeba responds to environmental conditions. Response to the
stimuli is called taxis (movement), positive towards the stimulus and negative, away from the stimulus. Different taxis are thermotaxis (temperature), phototaxis (light), thigmotaxis (touch) ,chemotaxis (chemical), galvanotaxis (electric current), geotaxis (gravity) and rheotaxis (water current). Optimum temperature 25*C, negative to strong and positive to dim lights avoid solid substance, attracted by food reacts negative to all chemicals, moves to cathode, moves to gravity, going according to water current.
Amoeba stops moving and rounds off.
The nucleus begins to divide.
The nucleus has divided and the cytoplasm starts to constrict. 4 & 5 The constriction continues to divide the cytoplasm.
6. The daughter amoebae separate. This is a form of asexual reproduction called binary fission. There is no evidence of sexual reproduction in this species of Amoeba.