Amoeba

An amoeba is a single-celled organism that has no fixed shape. As a protozoan and a member of the kingdom Protista that has animal-like qualities, an amoeba has to find and eat its food (since it is unable to make its own food as plantlike Protists do). Of the many species of amoeba, some live in water and soil, while others are parasites and live inside plants or animals.

Many biologists believe that the first protozoans were similar to today's amoeba. As members of the phylum (or primary division of the animal kingdom) Sarcodina, amoebas are distinct among protozoa in that they have no definite shape. Although they consist of a single cell surrounded by a membrane, they move by changing the shape of that membrane and therefore do not have one particular shape that makes them immediately recognizable. Some would say that they are recognizable because of this formlessness, while others say they look like a tiny bag of jelly. An amoeba actually moves in a very strange manner using pseudopods or "false feet."

When it wants to move, an amoeba turns its jelly-like body solid in a certain spot to form a temporary "foot" which it stretches in the direction it wants to move. The rest of the amoeba then flows into the pseudopod and basically changes its position to where its pseudopod had reached. This type of movement is called "streaming." Every time an amoeba does this it changes its shape. Since this is not the fastest way to move about, an amoeba only can move at a top speed of about 1 inch (2.54 centimeters) an hour.

An amoeba eats its food in much the same manner as it moves. It flows around and surrounds the food and then the amoeba totally engulfs it. The food is then held in a food vacuole, which is a specialized structure that digests the food. The amoeba mainly eats bacteria, algae, and other protozoans. Any waste that remains after the food is digested is released from a contractile vacuole (one that can open and close). Water

also flows into the amoeba by the process of osmosis (in which water flows through a membrane until the solutions on either side of it are at equal strength). When the amoeba needs to expel some water, it squeezes its contractile vacuole and squirts some out.

There are four types of protozoans that are called amoeboid protozoans. All use pseudopods to move about and to capture their food, and three of them are considered shelled amoeba since they are at least partially covered by a shell made up of the minerals they secrete. The first of the amoeba is the familiar "naked" amoeba called Amoeba proteus. While most of this type of amoeba live in water, some are parasitic and live in the human gut. The infected human then contracts the disease called amoebic dysentery. One of the shelled amoeba that lives in a tiny chambered sea shell and uses threadlike pseudopods to move about is called a foraminiferan. This amoeba can barely be seen with the naked eye. A second shelled amoeba is the freshwater heliozoan or "sun animal." It has thin pseudopods that look like needles that radiate from its body like rays from the Sun. The third and most intricate of the shelled amoebas is the radiolarian. These sea creatures have skeletons of silica, a mineral that does not dissolve in deep water, so that the deepest seabeds are covered with a thick layer of what is called radiolarian ooze.

Amoebas usually reproduce asexually by a process known as binary fission. In this form of reproduction, an amoeba splits in two after pinching in half and forms two smaller but identical cells. This occurs after the cell's nucleus duplicates its hereditary material and divides in two.

[See alsoCell; Protozoa; Reproduction, Asexual ]