Overview

Comprehensive Description

Description of Amoeba

Locomotive cells, often 220 to 760 µm in length, although A. diminutiva Bovee, 1972 is only 15 to 20 µm. Usually polypodial with one dominant cylindrical pseudopodium, often with ectoplasmic ridges. Resting, or well fed, cells irregularly rounded. Floating form with long, radiating pseudopodia. Cytoplasm granular in appearance often with truncated bipyramidal crystals. Posterior uroid morulate when present. Single nucleus, discoid or ovoidal, sometimes with peripheral granules. Nucleolar material often scattered. At the ultrastructural level, surface coat of A. proteus has filaments rising 230 nm above the plasma membrane whereas coat of A. lenigradensis amorphous and without filaments. Freshwater. The genus is best known for the species A. proteus which is a popular teaching tool and familiar to most students of biology.
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Ecology

Habitat

Depth range based on 1 specimen in 2 taxa.

Environmental ranges
  Depth range (m): 91 - 91
 
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Evolution and Systematics

Functional Adaptations

Functional adaptation

Cell coat is impenetrable: amoeba
 

Cell coat of amoeba is impenetrable due to a very thick layer of densely packed helical glycostyles with pentagonal glass-shaped tips.

   
  "The genus Paradermamoeba was established for a single species, Paradermamoeba valamo [1993]...Later a second species of this genus, P. levis, was discovered and described (Smirnov and Goodkov,1994). Amoebae of this genus have a very thick (up to 520 nm) cell coat, consisting of a layer of densely packed helical glycostyles with pentagonal glass-shaped tips. Perhaps, the impenetrability of the glycocalyx was the reason for numerous failed attempts to investigate the ultrastructure of these species." (Smirnov and Goodkov 2004:113)

"The cell coat of this species is about 520 nm of total thickness, and consists of a layer of regularly arranged tightly packed helical-shaped glycostyles. Coils of neighboring helices overlap. Each glycostyle terminates with a hollow straight funnel-shaped structure, which sometimes seems to be pentagonal in cross-section. The length of the spiral part of the glycostyles is about 400 nm and the diameter is about 100 nm; length of the straight terminal part of glycostyles is about 120 nm. Helices form 7-7,5 turns. They are embedded in an unstructured electron-transparent matrix which does not seem to prevent the neighboring glycostyles from moving in relation to each other." (Smirnov and Goodkov 1993:1)

[Note: The "impenetrability" appears to refer to the inability to determine the structure of the organism by using electron microscopy, which uses a beam of electrons to illuminate it.]

  Learn more about this functional adaptation.
  • Smirnov, A.; Goodkov, A. 2004. Ultrastructure and geographic distribution of the genus Paradermamoeba (Gymnamoebia, Thecamoebidae). European Journal of Protistology. 40: 113-118.
  • Smirnov & Goodkov. Paradermamoeba valamo Smirnov & Goodkov 1993.
    http://now.ifmo.ru/amecol/divers/pval.htm.
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Wikipedia

Amoeba

For other uses, see Amoeba (disambiguation).

Amoeba (sometimes amœba or ameba, plural amoebae or amoebas) is a genus of Protozoa[1] that consists of unicellular organisms which do not have a definite shape.

History[edit]

The microscopic amoeba was first discovered by August Johann Rösel von Rosenhof in 1757.[2] 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,[3] from the Greek amoibè (ἀμοιβή), meaning change.[4] Dientamoeba fragili was first described in 1918, and was linked to harm in humans.[5]

Anatomy[edit]

Anatomy of an amoeba.

The cell's organelles and cytoplasm are enclosed by a cell membrane; it obtains its food through phagocytosis. This makes amoebae heterotrophs. Amoebae have a single large tubular pseudopod at the anterior end, and several secondary ones branching to the sides. The cytoplasm of amoeba is divided into outer ectoplasm and inner endoplasm. The most famous species, Amoeba proteus, averages about 220-740 μm in length while in motion,[6] making it a giant among amoeboids.[7] A few amoeboids belonging to different genera can grow larger, however, such as Gromia, Pelomyxa, and Chaos.

Amoebae's most recognizable features include one or more nuclei and a simple contractile vacuole to maintain osmotic equilibrium - low water amounts in cell protecting cell from bursting with excess water. Food enveloped by the amoeba is stored and digested in vacuoles. Amoebae, like other unicellular 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 amoebae are forcibly divided, the portion that retains the nucleus will survive and form a new cell and cytoplasm, while the other portion dies. Amoebae also have no definite shape.[8]

Movement[edit]

Amoebae move by using pseudopodia or "false feet". Pseudopodia are formed by the amoeba by throwing out the cytoplasm, followed by endoplasm flowing inward.

Genome[edit]

The amoeba is remarkable for its very large genome. The species Amoeba proteus has 290 billion base pairs in its genome, while the related Polychaos dubium (formerly known as Amoeba dubia) has 670 billion base pairs. The human genome is small by contrast, with its count of 2.9 billion base pairs.[9] Unicellular budding yeast has an estimated 12 million pairs.[8]

Osmoregulation[edit]

Like most other protists, amoebae have a contractile vacuole. Amoeba proteus, a free-living, freshwater species of amoeba, has one contractile vacuole (CV), which is a membrane-bound organelle. The CV slowly fills with water from the cytoplasm (diastole), and, while fusing with the cell membrane, it quickly contracts releasing water to the outside (systole) by exocytosis. This process regulates the amount of water present in the cytoplasm of the amoeba; it is, a means of osmoregulation.

Immediately after the CV expels water, its membrane crumples, and soon afterwards, many small vacuoles or vesicles appear surrounding the membrane of the CV.[10] It is suggested that these vesicles split from the CV membrane itself. The small vesicles gradually increase in size as they take in water and then they fuse with the CV, which grows in size as it fills with water. Therefore, the function of these numerous small vesicles is to collect excess cytoplasmic water and channel it to the central CV. The CV swells for a number of minutes and then contracts to expel the water outside. The cycle is then repeated again.

The membranes of the small vesicles as well as the membrane of the CV have aquaporin proteins embedded in them.[10] These transmembrane proteins facilitate water passage through the membranes. The presence of aquaporin proteins in both CV and the small vesicles suggests that water collection occurs both through the CV membrane itself as well as through the function of the vesicles. However, the vesicles, being more numerous and smaller, would allow a faster water uptake due to the larger total surface area provided by the vesicles.[10]

The small vesicles also have another protein embedded in its membrane: Vacuolar-type H+-ATPase or V-ATPase.[10] This ATPase pumps H+ ions into the vesicle lumen, lowering its pH with respect to the cytosol. However, the pH of the CV in some amoebas is only mildly acidic, suggesting that the H+ ions are being removed from the CV or from the vesicles. It is thought that the electrochemical gradient generated by V-ATPase might be used for the transport of ions (it is presumed K+ and Cl-) into the vesicles. This builds an osmotic gradient across the vesicle membrane, leading to influx of water from the cytosol into the vesicles by osmosis,[10] which is facilitated by aquaporins.

Since these vesicles fuse with the central contractile vacuole, which expels the water, ions end up being removed from the cell, which is not beneficial for a freshwater organism. The removal of ions with the water has to be compensated by some yet-unidentified mechanism.

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 match the concentration of the surrounding water, causing the cell to swell and sometimes burst if the water surrounding the amoeba is too dilute.[11]

Amoebic cysts[edit]

Main article: Microbial cyst

In environments that 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.[8] While in cyst form the amoeba will not replicate and may die if unable to emerge for a lengthy period of time.

Nutrition in amoebae[edit]

Amoeba obtains its nutrition in a heterotrophic mode. Both the anabolic and catabolic functions are carried out in the same cell. Amoeba feeds on plankton and diatoms present in water. It can form arm- like structures called pseudopodia, extending from any part of its body as it is shapeless. When it senses food in its surroundings it extends its pseudopodia in that direction and moves towards it. Then it engulfs the food with its pseudopodia. When the food enters its body the amoeba forms a food vacuole around it which contains certain enzymes to digest the food. When the food is digested the unwanted waste is released through its body surface.

Reproduction of amoebae[edit]

Asexuality (reproduction by binary fission) is often thought to be a defining characteristic of amoebae.[12] However, analysis of 71 isolates of amoeba from the same geographical area along a river indicated that sex must be occurring in that particular lineage.[12] Recently, Lahr et al.[13] proposed that the majority of amoeboid lineages are, contrary to popular belief, at least anciently sexual, and that most current asexual groups have arisen recently and independently. In addition, recent evidence also indicates that several other single-celled eukaryotes that were previously regarded as asexual are, or were in the past, capable of sexual reproduction. These findings have led to the idea that sex was present in the earliest common ancestor of all eukaryotes.[14][15]

Video gallery[edit]

Amoeba proteus in motion
Amoeba engulfing a diatom

[16]

See also[edit]

References[edit]

  1. ^ "Amoeba" at Dorland's Medical Dictionary
  2. ^ Leidy, Joseph (1878). "Amoeba proteus". The American Naturalist 12 (4): 235–8. doi:10.1086/272082. JSTOR 2463786. 
  3. ^ Audouin, Jean-Victor; et al (1826). Dictionnaire classique d'histoire naturelle. Rey et Gravier. p. 5. 
  4. ^ McGrath, Kimberley; Blachford, Stacey, eds. (2001). Gale Encyclopedia of Science Vol. 1: Aardvark-Catalyst (2nd ed.). Gale Group. ISBN 0-7876-4370-X. OCLC 46337140. 
  5. ^ Johnson EH, Windsor JJ, Clark CG (July 2004). "Emerging from obscurity: biological, clinical, and diagnostic aspects of Dientamoeba fragilis". Clin. Microbiol. Rev. 17 (3): 553–70, table of contents. doi:10.1128/CMR.17.3.553-570.2004. PMC 452553. PMID 15258093. 
  6. ^ "Amoeba proteus". Amoebae on the Web. Retrieved 2009-10-08. 
  7. ^ MacIver, Sutherland. "Isolation of Amoebae". The Amoebae. Retrieved 2009-10-08. 
  8. ^ a b c "Amoeba". Scienceclarified.com. 
  9. ^ Winstead, Edward R. (12 February 2001). "Sizing up genomes: Amoeba is king". Genome News Network. 
  10. ^ a b c d e Nishihara E, Yokota E, Tazaki A, et al. (March 2008). "Presence of aquaporin and V-ATPase on the contractile vacuole of Amoeba proteus". Biol. Cell 100 (3): 179–88. doi:10.1042/BC20070091. PMID 18004980. 
  11. ^ Patterson, D.J. (1981). "Contractile vacuole complex behaviour as a diagnostic character for free living amoebae". Protistologica 17: 243–248. 
  12. ^ a b Hurst LD, Hamilton WD, Ladle RJ (May 1992). "Covert sex". Trends Ecol. Evol. (Amst.) 7 (5): 144–5. doi:10.1016/0169-5347(92)90205-P. PMID 21235987. 
  13. ^ Lahr DJ, Parfrey LW, Mitchell EA, Katz LA, Lara E (July 2011). "The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms". Proc. Biol. Sci. 278 (1715): 2081–90. doi:10.1098/rspb.2011.0289. PMC 3107637. PMID 21429931. 
  14. ^ Dacks J, Roger AJ (June 1999). "The first sexual lineage and the relevance of facultative sex". J. Mol. Evol. 48 (6): 779–83. doi:10.1007/PL00013156. PMID 10229582. 
  15. ^ Bernstein H, Bernstein C, Michod RE (2012). "Ch. 1: DNA repair as the primary adaptive function of sex in bacteria and eukaryotes". DNA Repair: New Research. Nova Science. pp. 1–49. ISBN 1621007561.  https://www.novapublishers.com/catalog/product_info.php?products_id=31918
  16. ^ Chakraborty, P. (2004). "Amoebae". Textbook of medical parasitology (2nd ed.). New Central Book Agency. p. 15. ISBN 8173814570. 
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