Amoeba

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. The amoeba was first discovered by August Johann Rösel von Rosenhof in 1755.^[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 Bery St. Vincent, from the Greek amoibè, meaning change.^[2]

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Habitat and uses

Amoeba itself is found in decaying vegetation in fresh and salt water, wet soil, and animals.^[2] Due to the ease with which they may be obtained and kept alive, they are common objects of study as representative protozoa and to demonstrate cell structure and function.

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 a single nucleus and a simple contractile vacuole to maintain osmotic pressure. Food enveloped by the amoeba is stored and digested in vacuoles. Amoebae reproduce through binary fission. 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.^[3]

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.^[4] Adjusting the tonicity of the cytoplasm can also damage the digestive enzymes used in phagocytosis, allowing the amoeba to ingest food but not extract energy, resulting in death.^[2]

Amoebic cysts

Main article: Microbial cyst

In environments which are potentially lethal to the cell, some amoeba may become dormant by forming itself into a ball and secreting a protective membrane to become a cyst; the cell remains in this state until it encounters more favourable conditions.^[3] 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.

Amoebas pathogenic to humans

• Entamoeba histolytica
• Naegleria fowleri
• Acanthamoeba
• Balamuthia mandrillaris
• Hartmannella

References