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amoeba
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Amoeba live in moist surroundings – ponds, ditches, seas, and even damp soils. Some specialized species even live as parasites inside the bodies of other animals. The disease dysentery is caused by one such type of amoeba.
Locomotion
An amoeba moves by first advancing a pseudopodium from a point on its surface; the living matter then flows forward into the projection. The amoeba has a thin, semi-solid outer layer called the ectoplasm and an inner fluid called the endoplasm. Where a pseudopodium forms, the ectoplasm liquefies and flows forward by contraction of the sold ectoplasm elsewhere. The fluid in contact with the surface soon solidifies once more, but then the whole process is repeated.
Digestion
The formation of pseudopodia also enables amoeba to feed. It engulfs microscopic organisms with cup-shaped pseudopodia. Inside the body, the food is surrounded by digestive enzymes. Undigested residues are easily lost; the amoeba simply flows away from them.
Respiration
For respiration – the taking in of oxygen and removal of carbon dioxide – the gases simply diffuse through the whole of amoeba's permeable surface. Soluble nitrogen-containing wastes, produced by the amoeba's chemical activities are excreted in the same way.
Water removal
Under a microscope, a bubble of liquid can be seen to form periodically inside an amoeba's body. The bubble grows and grows until finally it bursts, releasing the water to the outside. This bubble is the contractile vacuole and its function is to remove excess water passing by osmosis into the animal. Otherwise the amoeba itself would burst.
Sensitivity
Amoeba has no special sense organs. Changes in the outside world are detected by all parts of the living material. In general, this sensitivity ensures favorable surroundings. For instance, amoeba quickly moves away from very bright light or strongly acidic or alkaline water.
The nucleus
The nucleus can be clearly seen with a microscope as a darkish spot inside the body. It controls the whole course of life and an amoeba deprived of its nucleus soon dies. The nucleus is especially important during reproduction. This takes place when food is plentiful and amoeba can grow to its full size. Then the nucleus divides followed by division of the rest of the cell. The two new amoeba increase in size and may themselves divide.
The spore state
During droughts or severe cold, amoeba withdraws all pseudopodia and secretes a tough coat or cyst. The cyst with its contents is called a spore. Each spore is very light and may be blown by the wind to new surroundings. Inside the cyst, the original animal divides to form numerous smaller individuals called amoebulae. In favorable conditions the cyst breaks down and releases them.
Related entry
• amoeboid
Related categories
• CELL BIOLOGY
• MICROBIOLOGY
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