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Amoeba is a single cell or unicellular organism which are small, usually microscopic, and cannot be seen with the naked eye. They are mostly found in ponds, river that are slow-moving, and lakes.
 
Amoeba has,
  • Cell membrane which is double-layered,
  • Round nucleus (dense),
  • Nucleolus,
  • Vacuoles - food vacuole and a contractile vacuole.
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Amoeba
 
It performs all the activities like digestion, respiration, locomotion and also reproduction. Amoeba swallows food present in water, and food digestion takes place in the food vacuole. They feed mostly on microscopic plants and animals.
 
It does not have any definite shape. The shape of amoeba keeps on changing constantly. Amoeba shape changes because it can make its cytoplasm flow in any direction it wants to. Amoeba has finger-like projections called pseudopodia or false feet.
The mode of nutrition is also holozoic. All the five steps of nutrition are carried out by the single cell of amoeba.
Pseudopodia
Amoeba moves one place to another by expanding and contracting pseudopodia. When they sense food around, with the help of pseudopodia, they surround, capture and engulf food.
When a food particle comes near the organism, the Amoeba produces two pseudopodia around the particle of food to surround it. The two pseudopodia join around the food particle and trap the food particle with some water to form a food vacuole.
Pseudopodia also helps in locomotion and movement.
Food vacuole and digestion
The food gets trapped in the segment called food vacuole. It is a fluid-filled membrane-bound structure present inside the cytoplasm of the cell. The fluid inside vacuole contains several enzymes that digest the food.
Food vacuole is a temporary stomach of the Amoeba. The digestive enzymes breaks the food particle into simpler substances. Thus the food is digested. Digested food is directly absorbed into the cytoplasm through the process of diffusion.
 
When a sufficient amount of undigested food is present in the amoeba, the cell membrane ruptures and is expelled outside from the amoeba. This type of digestion is called as intracellular digestion as it occurs within the cell.
 
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Digestion in Amoeba