Amoeba

Amoeba
Scientific classification
Domain: Eukaryota Kingdom: Amoebozoa Phylum: Tubulinea Order: Tubulinida Family: Amoebidae Genus: Amoeba Bory de Saint-Vincent, 1822
Species
Amoeba proteus Amoeba dubia

Amoeba (sometimes amœba or ameba, plural amoebae) is a genus of protozoa that moves by means of 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.

Contents 1 History 2 Anatomy 3 Reaction to stimuli 3.1 Hypertonic and hypotonic solutions 3.2 Amoebic cysts 3.3 Marine amoeba 3.4 Food scarcity 4 Pathogenic interactions with other organisms 5 References 6 External links

History

The amoeba was first discovered by August Johann Rösel von Rosenhof in 1757.^[1] 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 Bory de Saint-Vincent^[2], from the Greek amoibè (αμοιβή), meaning change.^[3]

Anatomy

The cell's organelles and cytoplasm are enclosed by a cell membrane, obtaining its food through phagocytosis. Amoebae have a single large tubular pseudopod at the anterior end, and several secondary ones branching to the sides. The most famous species, Amoeba proteus, is 700-800 μm in length but the species Amoeba dubia is as large as a millimeter, and visible to the naked eye. Its most recognizable features include one or more nuclei and a simple contractile vacuole to maintain osmotic equilibrium. Food enveloped by the amoeba is stored and digested in vacuoles. Amoebae, like other single-celled eukaryotic organisms, reproduce asexually via mitosis and cytokinesis, not to be confused with binary fission, which is how prokaryotes (bacteria) reproduce. In cases where the amoeba are forcibly divided, the portion that retains the nucleus will survive and form a new cell and cytoplasm, while the other portion dies. Amoebas also have no definite shape.^[4]

Reaction to stimuli

Hypertonic and hypotonic solutions

Like most cells, amoebae are adversely affected by excessive osmotic pressure caused by extremely saline or dilute water. Amoebae will prevent the influx of salt in saline water, resulting in a net loss of water as the cell becomes isotonic with the environment, causing the cell to shrink. Placed into fresh water, amoebae will also attempt to match the concentration of the surrounding water, causing the cell to swell and sometimes burst.^[5]

Amoebic cysts

Main article: Microbial cyst

In environments which are potentially lethal to the cell, an amoeba may become dormant by forming itself into a ball and secreting a protective membrane to become a microbial cyst. The cell remains in this state until it encounters more favourable conditions.^[4] While in cyst form the amoeba will not replicate and may die if unable to emerge for a lengthy period of time.

Marine amoeba

Marine amoeba lack contractile vacuoles and their enzymes and organelles are not damaged by the salt water found in seas, oceans, salt swamps, salty rivers and ponds. Most are microscopic, but some can grow as large as grapes ^[6]

Food scarcity

Research published in the journal PLoS Biology in November 2008 shows several unusual types of behavior by amoebas in times of food scarcity. Amoebas can seek out other amoebas which are genetically similar. They can then clump together to form multi-cellular organisms in which some amoebas will become spores which can be moved to other locations to find more food, or they can become stalks which provide some height for the spores to then spread out. Most would rather be a spore than a stalk. ^[7]

Pathogenic interactions with other organisms

Some species of amoeba can infect other organisms pathogenically (causing disease):

• Entamoeba histolytica is the cause of amoebiasis, or amoebic dystentery"
• Naegleria fowleri (the "brain-eating amoeba") is a fresh-water-native species that can be fatal to humans if introduced through the nose.
• Acanthamoeba can cause amoebic keratitis and encephalitis in humans.
• Balamuthia mandrillaris is the cause of (often fatal) primary amoebic meningoencephalitis

References

External links

• Amoebae: Protists Which Move and Feed Using Pseudopodia, David Patterson (Tree of Life)
• Wikibooks: compare size of cells
• Joseph Leidy's Amoeba Plates