Ameba or Amoeba

The most common form of ameba is the Ameba proteus, named from the mythological Greek sea god, Proteus, who could change his shape at will.

An Ameba
Taxonomic Hierarchy
Kingdom Protozoa
Phylum Protozoa
Subphylum Sarcodina
Superclass Rhizopoda
Class Lobosa – amoebas, amibes
Direct Children:
Order Amoebida
Order Arcellinida
Order Euamoebida

The simplest of all the microscopic, single cell lifeforms. Once classed as an animal, the protozoa are now classified in the Kingdom Protozoa. The Amoebae form a group of single cell organisms called Protozoa or Protista.

Protozoa means "first animals". Protozoa also share many similar characteristics with plants, such as photosynthesis among some species, so the term Protista meaning "first creatures" is often used.

The name amoeba is not exclusively applied to members of the genus Amoeba but to a range of different types of Protista with pseudopodia. Some live in salt water, others in fresh water, perhaps on wet stems of plants which grown around wet environments. Others thrive in damp soil and others as parasitic in bodies of animals. They include some with shells, like Arcella, and also the half-dozen species that live in the human mouth and digestive system. One of these species is the cause of amoebic dysentery (Entamoeba). Some amoebae contain several nuclei which includes the large Chaos chaos, which may become six times the length of Amoeba proteus, which probably is the most well known species.

The amoeba's cell body is of an indefinate shape, changing constantly as it moves by means of pseudopodia (meaning "false feet"). The endoplasm is thrust outward from any point of its cell body, the liquid protoplasm (life material) within flows slowly into them. Afterward the rest of the cell contracts behind, creating motion and forward movement. Its basic structure consists of a thin, elasticized external membrane which holds the protoplasm safely inside the cell. Directly inside the membrane is a narrow layer of clear material, or ectoplasm, which aides in the cell's ability to possess form. Ectoplasm surrounds the main body mass of granular ectoplasm, which is differentiated into a more solid outer portion, the plasmagel, and more fluid inner portion, the plasmasol. Contained within the endoplasm is the roughly spherical nucleus which are associated with life fuctions of the cell.

Ameba Side View on Slide
An Ameba from Side View on Microscope Slide
This is a diagram of what was captured under a modern microscope. This side view, appears to be of an amoeba moving to the right with an extended "false foot", (or pseudopodium) and seemingly elevated on small protoplasmic pegs.

The ameba's body consists of a single cell which outermost film is called the cell membrane. This encloses a semi-liquid substance called cytoplasm, which is a special form of protoplasm. Though the ameba has no permanenly definitive shape, it does have a permanent hind end and forms its pseudopodium in a characteristic pattern, depending on the species.

Within the cytoplasm the following are included

  • a vacuole or spherical liquid-containing space which regulates water content and pressure and aides in regulation of the excretion of some liquid wastes.
  • a spherical, central mass called the nucleus, which regulates vital functions of the cell
  • granules of variegated substances Food vacuoles enclose whatever the ameba has consumed.

The ameba was once classified among the animal kingdom because it feeds, excretes, respires and reproduces in many of the same fundamental ways that many animals do, not excluding man.

It feeds mainly on other protista, algae and bacteria, by utilizing its pseudopodia to surround and engulf a food particle. Water is engulfed with the food and the two form a vacuole within the cytoplasm. Digestion is a similar process to that which occurs with most other organisms, that is, digestive fluids are secreted into the food vacuole and digestible portions broken down and absorbed. The particle is digested within the vacuole and asorbed into the cell. This process is known as phagocytosis, which comes from Greek meaning, "eating by cells".
The contractile vacuole, bubblelike in appearance regulates water in the cell and may also serve the function of excretion. Food vacuoles are formed as the ameba ingest nutrients. Food vacuoles become smaller as digestion proceeds until the unused residue is excreted and vacuole disappears.

Excretion is accomplished through the cell membrane by osmosis (the passage of a liquid through a semi-permeable membrane) and perhaps also by the contractile vacuole that regulates water pressure. The Ameba discards fecal materials by merely flowing away from indigestible particles.

Respiration is achieved through osmosis as well. The ameba takes oxygen from surrounding water and releases carbon dioxide by a reverse process.

Reproduction is accomplished by the cell dividing into two equal halves of a whole, mature ameba which has reached maximum size. During this process, elements of the nucleus separate into identical halves, which insure the chromatin of the nucleus is evenly divided between the two resulting individuals. This process is called binary fission and takes less than an hour to complete. It is the usual method of reproduction. The chromatin of the nucleus likely carry the genetic and heredity factors. This complicated process which results in successfully dividing a single chromosome into two individuals is called mitosis.
The entire reproduction process begins with the amoeba becoming spherical in shape and the nucleus divides into two, the halves move apart, then the cell splits down the center.
Sometimes Amoeba proteus reproduce differently. The nucleus divides into hundreds of smaller ones which become surrounded by cytoplasm and a protective wall. All this takes place in the original single cell. These cysts can survive through droughts but also serve the purpose of transplant into new locations. Large cysts are formed when the whole cell surrounds itself with a thick shell. Some species of Amoeba do reproduce sexually.

If threatened the ameba has the ability to form a tiny spherical shaped droplet, and if the water supply in its environment is evaporated, it secretes about its body a thick impervious shell, called a cyst. While in this state it is able to withstand death by desiccation until the availability of more favorable environmental conditions. Encysted amebas can be carried by wind and therefore are widely distributed. Other protozoans are also known to use this same survival mechanism.

The average ameba is between 5 to 200 microns in diameter or an average size of about 1/1000th of an inch. The largest amebas are only about twice the diameter of a coarse human hair. The smallest species may be about 1/20th of the same hair. Amoeba proteus, measures ½ mm., and is just large enough to be visible to the naked eye.

There are many known species, some of which are:

  • A. proteus
  • A. limax, called limax because its body takes on the form of a slug
  • A. guttula, a notably small species

Amebas are not only free-roaming, but some are parastic in both humans, animals and plants. Some are harmless whilst others may cause serious disease or potentially fatal. One serious human parasite is Endamoeba histolytica which lives in the intestine and is the cause of amebic dysentery. See Amebiasis. Once considered serious but now is readily controlled by drugs, such as an alkaloid called emetine, derived from ipecac.

These creatures, protozoa, named so as "first animals," consist of but a single cell and may serve as a glimpse, an insight into what possibly may have been the first animals to inhabit the planet, and give some indication as to what kind of common ancestor in the past gave rise to all living things.

Some permissible variations on spelling include

  • amoeba
  • amœba
  • endamoeba
  • entamœba

Classification of Amoeba proteus from, Amoeba proteus (Pal.), Taxonomic Serial No.: 43854

Diagram of Amoeba proteus

Ameoba proteus
Amoeba proteus

Taxonomic Hierarchy
Kingdom Protozoa
Phylum Protozoa
Subphylum Sarcodina
Superclass Rhizopoda
Class Lobosa – amoebas
Order Amoebida
Family Amoebidae
Genus Amoeba Ehrenberg, 1930
Species Amoeba proteus (Pal.)


References and Further Reading

  • Funk and Wagnalls Encyclopedia, ©1950
  • The American Peoples Encyclopedia, ©1960
  • Encyclopedia International, ©1966 (Grolier Inc.)
  • Funk and Wagnalls Wildlife Encyclopedia, ©1975
  • Ameba, Definition
  • Lobosa, Taxonomic Serial No.: 43850
  • Protozoa, Definition
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