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Scientific classification
Phylum: Amoebozoa
Subphylum: Sarcodina
Class: Tubulinea
Order: Tubulinida
Family: Amoebidae
Genus: Amoeba
Bery de St. Vincent 1822

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Alternate meanings: Amoeboid, Amoebozoa

Amoeba (sometimes amœba or ameba, plural amoebae) is a genus of protozoa that moves by means of temporary projections called pseudopods, and is well-known as a representative unicellular organism. The word amoeba or ameba is variously used to refer to it and its close relatives, now grouped as the Amoebozoa, or to all protozoa that move using pseudopods, otherwise termed amoeboids. The amoeba was first discovered by August Johann Rösel von Rosenhof in 1755.[1]

Habitat and study

Amoeba itself is found in decaying vegetation in fresh and salt water, wet soil, and animals. Due to the ease with which they may be obtained and kept alive they are common objects of study, both as representative protozoa and to demonstrate cell structure and function. The cells have several lobose pseudopods, with one large tubular pseudopod at the anterior and several secondary ones branching to the sides. The most famous species, Amoeba proteus, is 700-800 μm in length but amoebae vary from as large as a millimeter (Amoeba dubia which is visible to the naked eye) to far smaller than 700 μm. Its most recognizable features include a single nucleus and simple contractile vacuole to maintain osmotic pressure. The amoeba obtains its food through phagocytosis. Amoebas reproduce through binary fission.

Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus who could change his shape. The name "amibe" was given to it by Bery St. Vincent, from the Greek amoibè, meaning change.


Anatomy of an amoeba

An amoeba, from the order Amoebida, class Mastigophora phylum sarcodina protozoa,[2] is a single-celled organism. They live in freshwater stagnant ponds, soil, streams, the ocean, and the bodies of other organisms. Some of the largest amoebae are about 1mm across, which means a human being would barely be able to see it with the naked eye. The word amoeba means “to change” in Greek (Encyclopedia of Science, 1). An amoeba is composed of several different parts. One is a cell membrane, which is an amoeba’s outer covering. Then there is the nucleus, the central organelle, or brain, and the common animal cell organelles (Dery, 1). An amoeba also has endoplasm and ectoplasm, and the two specialized types of vacuoles (Dery, 1). See Figure 1 (Dery, 1).

The ectoplasm is the exterior gel of the amoeba and the endoplasm is the interior fluid. These two components are used for storing organelles and undergoing pseudopodial extension (see page seven, pseudopodial movement); locomotion of the amoeba and capturing food. See Figure 2. (Aardvark-Catalyst, 163-164). The two vacuoles are the digestive and food vacuoles. The food vacuole is formed when the amoeba undertakes the process of phagocytosis (pseudopodia surrounding food [Gale, 1]) (see Figure 3) (Aardvark-Catalyst, 163). Once the food vacuole is formed, it becomes a digestive vacuole, which is responsible for breaking down the food into energy (Encyclopedia of Science, 1). Also, an amoeba has a contractile vacuole, which is responsible for pumping water in and out of the amoeba. Amoebae are diverse in many ways. For example, they drastically range in size from 1mm across (Blake, 1). Some traits even change in different environments, making it hard to tell the amoebae apart (Blake, 1). Also, some amoebae are carnivorous, some are herbivorous, and some are even omnivorous. Then, there are the parasitic ones (Blake, 1), which can live in one’s liver, lungs, brain- even heart (Innvista, 1)! These will become a cyst until they go inside of you, then they become a trophozite, their replicating forms (1).


Hypertonic and hypotonic solutions

Like most pies, amoebae are adversely affected by excessive osmotic pressure, like very salty or very fresh water. When an amoeba is put into salt water with enough concentration, some of its organelles, like the contractile vacuole, are damaged. As the amoeba prevents the salt from entering, instead the solution will pull water out of the amoeba, concentrating the salts inside. When this happens the amoeba will appear to shrink.

If a brine amoeba is put into fresh water and it is not a cyst at the time, its contractile vacuole will burst (Do, 1). This is because the vacuole’s job is to create a solution isotonic to the amoeba's environment. If the salt concentration inside the vacuole is too high, it will trigger water absorption. (Do, 1). Soon, the vacuole will burst before it has reached equilibrium. This means the amoeba will also burst. See Figure 4. (Do, 1).

Also, some enzymes might be damaged in the process, including digestive enzymes used in phagocytosis. (Gale, 1). The amoeba will ingest food but be unable to digest it to extract energy. The amoeba then will perish.

Amoebaic cysts

Under adverse, incongruous, or unsuitable environments, an amoeba may turn into a cyst (Galileo, 1). This “encystment” occurs to keep the amoeba alive until it reaches a preferred area. Then the organism will secrete a special membrane. This membrane is called a cyst membrane and will enclose it thoroughly (Galileo, 1). Also, the amoeba will become spherical, and will lose a tremendous amount of the amoebae water (Galileo, 1). An adversative environment may mean an environment that’s too warm, cold, or salty for the amoeba. Cysts have a very similar function to the function of bacterial spores (Salyers/Dixie, 1). These both are defense mechanisms that help the organisms survive. In some adverse places where the organisms would typically expire in their reproducing form, the defenses are great and will keep the organism alive (Salyers/Dixie, 1). The amoeba will not be able to replicate in cyst form, however, and this can be a problem. If an amoeba is kept in an adverse environment as a cyst, the amoeba will perish and will not be able to reproduce other amoebae (Salyers/Dixie, 1). Amoebae reproduce with binary fission (Blake, 1). Their generation times can be very low. Some generation times can be about a day, others about seven hours. The lowest generation time ever recorded was about two hours. It’s also debated that some rare amoebae can reproduce both sexually and asexually.

New type of amoeba

Recently, a new type of marine amoeba was found. It was tested for salinity tolerance and it was found to withstand 0%c to 150%c salt without affecting reproduction (Hauer, 1). This amoeba was also able to grow within a range of 0%c to 138%c salt (Hauer, 1). This is unusual because some other amoebae couldn’t take that much at all. Some freshwater amoebae were destroyed at a very low level of concentration of salt. Other marine amoebae and a few freshwater amoebae could stand up to salt concentration in the hundred percentage range but then at 138% concentration salt the amoebae became domed, went into cyst form, or became wrinkled and were destroyed (Hauer, 1). This shows the diversity of amoebae, and that some amoebae will react differently to salt. Therefore the scientist(s) must choose one and only one type of amoeba if the scientist(s) wants accurate results.

Marine amoebae

Marine amoebae lack contractile vacuoles and their enzymes and organelles are not damaged by the salt water in the sea or ocean (Blake, 1). Some also live in salt swamps, salty lakes, and salty rivers or streams. When an amoeba is put on a microscope slide, it will usually tend to try to get away from the microscope light (Granville, 11/14/2006).

Amoebas pathogenic to humans


  1. Leidy, Joseph (1878). "Amoeba proteus". The American Naturalist. 12 (4): 235–238. Retrieved 2007-06-20. 
  2. McGrath, Kimberley (2001). Gale Encyclopedia of Science Vol. 1: Aardvark-Catalyst (2nd ed.). Gale Group. ISBN 078764370X. 

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