amoeba

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Amoeba proteus
Amoeba (drawing from 1900) :
n : cell nucleus (nucleus)
wv : water vacuole
cv : contractile vacuole
fv : food vacuole .

The amoeba ( Gr. Αμοιβή amoibe , change ') or change animals are a large, diverse group of unicellular organisms that do not have a fixed body shape, but change their shape continuously through the formation of pseudopodia . Amoebas are a life form, not a family group ( taxon ).

to form

Amoebas are between 0.1 and 1 mm in size. Most of the species are naked; but there are also wired forms ( Thecamoeben ). In addition to the heterotrophic species that feed on phagocytosis , there are also amoeboid, motile unicellular organisms that contain chloroplasts and carry out photosynthesis . The best known are the "giant amoeba" of the genera Amoeba and Chaos .

Systematic dissemination

Amoeba-like life forms have evolved separately from one another in a wide variety of taxa . They therefore form a form of life or an organizational level , but not a taxonomic group.

The heterotrophic, so no photosynthesis operating amoebas (with the exception of the slime molds ) were traditionally counted among the root pods (Rhizopoda). According to today's nomenclature, the majority distributed to the Amoebozoa which (including slime molds) rhizaria (together with the other groups of the rhizopoda as Foraminiferen , Strahlentierchen and a large part of the heliozoa ) and the percolozoa within the excavata B. (eg. The vahlkampfia , Vahlkampfia ). In addition, there are various autotrophic (photosynthesis-driving) representatives of the chromalveolata , traditionally one of the algae . Also noteworthy are the Chlorarachniophyta , representatives of the Rhizaria, which were formerly part of the green algae because of their chloroplasts .

Occurrence

Amoebas can be found almost everywhere. Some genera are distributed globally from the Arctic to the Antarctic, and many can even be isolated from the air, which are mostly permanent stages ( cysts ). They are particularly abundant in moist soils and mud, but many genera are also common in freshwater and seawater.

Cell structure

Amoebas are usually transparent and can change their shape constantly. Inside the cell you can see the granular endoplasm (also: entoplasm) pulsing, which contains many small vesicles. Farther out is the structureless-looking ectoplasm . The cell nucleus is usually difficult to see.

Locomotion

For locomotion, amoeba form plasma processes, the pseudopods or pseudopods . This happens through local contraction of the cytoskeleton, which pressurizes the cytoplasm there. A cytoplasmic flow develops to areas of lower pressure, which leads to the formation of pseudopodia there. The pseudopodia have attachment points ( adhesive plaques ) in contact with the substrate.

Basically, there are three steps to getting around:

  1. Extension : The pseudopodia are formed in the direction of movement.
  2. Adhesion : The pseudopods are attached to the base through new adhesion points.
  3. Retraction : The rest of the amoeboid body is retracted.

This locomotion with continuous change in shape is called amoeboid .

Ingestion

Amoebas catch their prey, bacteria and smaller eukaryotic single-cell organisms by flowing their pseudopods around them and then trapping and digesting them in their bodies within food vacuoles . This type of ingestion of solid food particles is called phagocytosis . Inside the food vacuole, the food is broken up by digestive enzymes and converted into a water-soluble form. Valuable material is taken over into the cytoplasm through the vacuole membrane ; this process is called resorption .

There is also the absorption of liquids and substances dissolved in them in the form of pinocytosis . Often the amoebas form a longer pinocytosis channel, at the end of which a fluid-filled vesicle is constricted into the cell interior.

Water excretion

Freshwater amoebas have a contractile vacuole that regulates the water balance. Since amoeba constantly take in ions through food , the osmotic pressure inside them increases because water diffuses from the hypotonic environment into the more highly concentrated cytoplasm . The amoeba has to use energy to compensate for this in order not to burst. To do this, the pulsating vacuole pumps water out of the cell.

Reproduction

The amoebas reproduce asexually by simple division . However, parasexual activities also seem to be widespread , and there are isolated indications of genuine sexuality , which, however, have not yet been confirmed in any case. A number of species also form flagellated swarmers ( zoospores ).

Pathogenicity

Many amoebas are pathogenic , some of them can cause serious diseases in humans. Thus, the Magna-form calls from Entamoeba histolytica , the amoebic dysentery , a severe gastrointestinal disease forth. In addition, many types of amoeba harbor pathogenic bacteria such as Legionella . Some amoeba-like protozoa lead to severe meningoencephalitis in humans. These include the genera Naegleria , Balamuthia and Sappinia . Naegleria fowleri is mostly found in warm, standing freshwater bodies and in most cases it penetrates along the nasal mucous membrane to the brain and triggers purulent PAME ( primary amoebic meningoencephalitis ), which ends fatally in 95% of cases. The genus Acanthamoeba is found in fresh, salt and brackish water and in soils and typically leads to severe "Acanthamoeba keratitis" in contact lens wearers (80% of cases). It can also get to the brain along the nasal mucosa and trigger a mostly fatal meningoencephalitis there. Entamoeba gingivalis is often encountered in stubborn gingivitis and periodontitis.

Others

Web links

Individual evidence

  1. ↑ Kimberley A McGrath, Stacey Blachford: Gale Encyclopedia of Science Vol. 1: Aardvark-Catalyst . Ed .: Gale Group. 2nd ed. 2001, ISBN 0-7876-4370-X ., OCLC 46337140 .
  2. ^ Adl et al .: The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists. The Journal of Eukaryotic Microbiology 52 (5), 2005; Pages 399-451, cf. Bibliography in Amoebozoa .
  3. ^ Maciver, Ecology of the amobae
  4. ↑ Maciver, Sex and the single Amoeba , last update August 14, 2002
  5. ↑ Maciver, The Amoebae
  6. ↑ Facultatively pathogenic free-living amoeba
  7. ↑ Naegleria fowleri - Primary Amebic Meningoencephalitis (PAM) - Amebic Encephalitis
  8. ↑ Deadly parasite: Protozoa eats its way through the brain
  9. ↑ Acanthamoeba Keratitis
  10. ↑ X. Bao, R. Wiehe, H. Dommisch, AS Schaefer: Entamoeba gingivalis Causes Oral Inflammation and Tissue Destruction . In: Journal of Dental Research . tape 99 , no. 5 , May 2020, ISSN  0022-0345 , p. 561-567 , doi : 10.1177 / 0022034520901738 ( sagepub.com [accessed May 6, 2020]).