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The amoeba is a group of single celled organisms that are called protozoa or protista. The word protozoa means first animal and since these organisms have an affinity with plants the term protista means first creature, which is the preferred name of today’s scientist. Like all cells the amoeba consist basically of protoplasm. In the middle of this cell, surrounded by protoplasm is the nucleus. The nucleus is a body which can be thought of as a blueprint for the organization of the cells activities. When you cut an amoeba in half, the half with the nucleus may survive and reproduce. On the other hand the other half will move around but being unable to digest its food, will soon die. It was once thought that the protoplasm was a jelly but it is now known that protoplasm has a very complicated structure. Protoplasm consist largely of an intricately folded and changing double layered membrane that will form itself into a variety of tiny structures that perform the cells diverse functions.
Amoeba, as a name, is applied to a wide range of different protista that live in the ocean, fresh water, damp soil and the bodies of larger animals. There are half a dozen species living in the human mouth and digestive system. The textbook amoeba is called the amoeba proteus and measures about 1\2 millimeter long. It is barely visible to the naked eye and can at times be found in fairly still water. When it moves about it extends a finger of protoplasm that is called a pseudopodium or false foot. When the pseudopodium enlarges the cells protoplasm and nucleus flow into it as the remainder of the cell contracts behind. Amoebas are known to have no definite shape but they are not just shapeless sacs of protoplasm since it has a permanent hind end and tends to form its false foot in a characteristic pattern according to its species.
In most cases the amoeba feeds mainly on other protista by flowing around them. When the protoplasm completely surrounds their food it is enclosed in a food vacuole containing the fluid in which the prey was swimming. Like most other organisms digestion juices are secreted into the food vacuole and the parts that can be digested are broken down and absorbed. Parts that cannot be digested are simply left behind as the amoeba swims. When the amoeba drinks fluids they are drawn into the channels and absorbed into the protoplasm in the same way digested foods are. This allows the cell to absorb fluids in bulk. The amoeba is continually drawing water in and passing it through the cell membrane. The excess water is pumped out by contractile vacuoles which fill with water and then collapse causing the water to discharge to the outside.
Amoebas reproduce themselves by dividing into two equal parts. This process is known as binary fission and takes less than one hour. When it begins the amoeba becomes spherical as the nucleus divides into two parts. The two halves then move apart and the cell splits down the middle. In some cases, such as the amoeba proteus, the reproduction is done in a different manner. Their nucleus will divide into hundreds of smaller nucleus, which is then surrounded by a little cytoplasm and a tough wall. All of this is done within the original cell. These minute cyst will survive if the water dries up and are disbursed to start new populations.