amoeba
American Heritage Dictionary:
Top
a·moe·ba
also a·me·ba (ə-mē'bə) 
n., pl., -bas, also -bas, or -bae, also -bae (-bē).
Any of various one-celled aquatic or parasitic protozoans of the genus Amoeba or related genera, having no definite form and consisting of a mass of protoplasm containing one or more nuclei surrounded by a flexible outer membrane. It moves by means of pseudopods.
n., pl., -bas, also -bas, or -bae, also -bae (-bē).
Any of various one-celled aquatic or parasitic protozoans of the genus Amoeba or related genera, having no definite form and consisting of a mass of protoplasm containing one or more nuclei surrounded by a flexible outer membrane. It moves by means of pseudopods.
[New Latin, genus name, from Greek amoibē, change, from ameibein, to change.]
amoebic a·moe'bic (-bĭk) adj.
Top
is the customary UK spelling (US ameba). The plural is amoebas (US amebas); amoebae is only used in technical contexts.
| amid, amidst, amend, emend, amen | |
| amok, amuck, among, amongst, amount, number |
Top
(click to enlarge)
Amoeba (magnified). (credit: Russ Kinne/Photo Researchers)
Amoeba (magnified). (credit: Russ Kinne/Photo Researchers)
For more information on amoeba, visit Britannica.com.
Any protozoon moving by means of protoplasmic flow. In their entirety, the ameboid protozoa include naked amebas, those enclosed within a shell or test, as well as more highly developed representatives such as the heliozoians, radiolarians, and foraminiferans. Ameboid movement is accomplished by pseudopods—cellular extensions which channel the flow of protoplasm. Pseudopods take varied forms and help distinguish among the different groups. A lobe-shaped extension or lobopod is perhaps the simplest type of pseudopod. The shapelessness and plasticity of these locomotory organelles impart an asymmetric, continually changing aspect to the organism. Other, more developed, representatives have pseudopodial extensions containing fibrous supporting elements (axopods) or forming an extensive network of anastomosing channels (reticulopods). Though involved in locomotion, these organelles are also functional in phagocytosis—the trapping and ingesting of food organisms (usually bacteria, algae, or other protozoa) or detritus. See also Foraminiferida; Heliozoia; Phagocytosis; Radiolaria.
Amebas range from small soil organisms, such as Acanthamoeba (20 micrometers), to the large fresh-water forms Amoeba proteus (600 μm; see illustration) and Pelomyxa (1 mm, or more). Some types, such as Amoeba, are uninucleate; others are multinucleate. Reproduction is by mitosis with nuclear division preceding cytoplasmic division to produce two daughters. Multinucleate forms have more unusual patterns of division, since nuclear division is not immediately or necessarily followed by cytoplasmic division. Transformation of the actively feeding ameba into a dormant cyst occurs in many species, particularly those found in soil or as symbionts. The resting stages allow survival over periods of desiccation, food scarcity, or transmission between hosts. See also Reproduction (animal).
Phase-contrast photomicrograph of Amoeba proteus, a large fresh-water ameba. The organism is seen moving by means of a single lobose pseudopod.
Amebas are found in a variety of habitats, including fresh-water and marine environments, soil, and as symbionts and parasites in body cavities and tissues of vertebrates and invertebrates. Because of their manner of locomotion, amebas typically occur on surfaces, such as the bottom of a pond, on submerged vegetation, or floating debris. In soil, they are a significant component of the microfauna, feeding extensively on bacteria and small fungi. Amebas in marine habitats may be found as planktonic forms adapted for floating at the surface (having oil droplets to increase bouyancy and projections to increase surface area), where they feed upon bacteria, algae, and other protozoa. Several species of amebas may be found in the human intestinal tract as harmless commensals (for example, Entamoeba coli) or as important parasites responsible for amebic dysentery (E. histolytica).
Columbia Encyclopedia:
Top
Ameba
ameba or amoeba (both: əmē'bə), common name for certain one-celled organisms belonging to the phylum Sarcodina of the kingdom Protista. Amebas were previously classified as members of the animal kingdom. Most amebas are very small (from 5 to 20 microns in diameter) and contain a single nucleus. A. proteus averages 0.25 mm in length. Members of the genus Pelomyxa, however, may be well over a millimeter (up to 8 mm) in diameter and may contain hundreds of nuclei. The giant ameba Gromia sphaerica, found on the ocean floor, may reach 1.5 in. (40 mm) in diameter.
Amebas constantly change the shape of their bodies as a result of the phenomenon known as ameboid movement, involving the formation of temporary extensions (pseudopodia, or false feet) of the body. Pseudopodia, used in locomotion and feeding, may be rounded at the tip (lobopodia), pointed (filopodia), branched and fused together (rhizopodia), or somewhat rigid and pointed (axopodia).
Although simple in form, amebas are very successful organisms and are found abundantly in a variety of habitats all over the world. Amebas live in freshwater, the oceans, and in the upper layers of the soil, and many have adapted to a parasitic life on the body surface of aquatic animals or in the internal organs of both aquatic and terrestrial animals. Few animals escape invasion by some type of ameba. Some are harmless, but others are pathogenic and cause serious diseases; e.g., Entamoeba histolytica causes amebic dysentery, which is fatal if untreated. The many genera of amebas were given their common name because of their resemblance to the genus Amoeba (order Amoebida), which includes several large, common species of which the freshwater Amoeba proteus is the most familiar.
The term ameba is sometimes also used to refer to other unicellular protists (e.g., slime molds) that have ameboid features such as pseudopodia. Other ameboid protozoans of the phylum Sarcodina include the marine radiolarians, which form silicate skeletons; their freshwater counterparts, the heliozoans; and the shell-bearing foraminiferans.
Digestion and Respiration
In a process known as phagocytosis, amebas engulf their prey, or particles of appropriate size, with their pseudopodia, forming food vacuoles. Digestive enzymes, manufactured and secreted by the organism, are then poured into these vacuoles, and the particles are digested. Useful compounds are subsequently absorbed into the ameba's body. Useless residues remain in the vacuoles and are ultimately expelled (egested) as the vacuole comes in contact with the membrane at the body surface. Amebas can distinguish food (e.g., algae, diatoms, bacteria, and other protozoans) from other material and use different tactics in approaching different food. Freshwater amebas take up water constantly through the process of osmosis, and water content is regulated with a pulsating contractile vacuole. Marine amebas lack a contractile vacuole. Respiration is by diffusion of gases through the cell membrane.
Reproduction
Under favorable conditions amebas divide by binary fission (splitting) to produce two daughter amebas, the nucleus dividing by mitosis. When an ameba is divided artificially, the portion containing the nucleus forms a new cell membrane and continues as a whole animal, while the other portion lives only as long as its present food supply lasts, ultimately dying, since it cannot ingest food or reproduce. If conditions are unfavorable, e.g., in the absence of food and water, amebas secrete a firm protective covering and encyst until conditions are again favorable to active division.
Word Tutor:
Top
ameba
IN BRIEF: n. - Naked freshwater or marine or parasitic protozoa that form temporary pseudopods for feeding and locomotion.
LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!
Top
(uh-mee-buh)
The term amoeba is sometimes used to refer to something with an indefinite, changeable shape.
An animal composed of only one cell that has no fixed shape. It is the best known of the single-celled animals, or protozoa.
Top
or (US) ameba (pl. amoebae or amebae)
any of a genus (Amoeba) of naked rhizopod protozoans in the order Amoebida, characterized by a thin pellicle and frequent alterations in shape due to the constant formation and retraction of pseudopodia.
— amoebic or (US) amebic adj.
any of a genus (Amoeba) of naked rhizopod protozoans in the order Amoebida, characterized by a thin pellicle and frequent alterations in shape due to the constant formation and retraction of pseudopodia.
— amoebic or (US) amebic adj.
| amnion, amniocentesis, ammonotelic | |
| amoebapore, amoeboid, amorph |
Top
Pl. amobae, amobas [L.] a minute protozoan. The common laboratory example is Amoeba proteus.
Mosby's Dental Dictionary:
Top
amoeba
(əmē′bə)
n
n
A Rhizopod protozoa that uses extensions of its cytoplasm, called pseudopodia, to move. Some varieties of amoebae are implicated in human infection. Also spelled ameba(s).
Random House Word Menu:
Top
categories related to 'ameba'
- Diagnostic Terminology - ameba: single-celled, microscopic organism that may cause disease
- Genetics, Heredity, and Evolution - ameba: unicellular, microscopic protist that reproduces by fission
See crossword solutions for the clue Wikipedia on Answers.com:
Top
Amoeba
For other uses, see Amoeba (disambiguation).
| Amoeba | |
|---|---|
 |
|
| Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Protista |
| Phylum: | Tubulinea |
| Order: | Tubulinida |
| Family: | Amoebidae |
| Genus: | Amoeba Bory de Saint-Vincent, 1822 |
| Species | |
Amoeba (sometimes amœba or ameba, plural amoebae or amoebas) is a genus of Protozoa.[1] It is a shapeless unicellular organism.
|
Contents
|
History
The 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 (αμοιβή), meaning change.[4] Dientamoeba fragili was first described in 1918, and was linked to harm in humans.[5]
Anatomy
.svg)
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 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. 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 amoeba 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]
Genome
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]
Osmoregulation
Like most other protists, amoebas have a contractile vacuole complex. 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, therefore, 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 also attempt to 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
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 amoeba
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.
Video gallery
  |
 |
References
- ^ "Amoeba" at Dorland's Medical Dictionary
- ^ Leidy, Joseph (1878). "Amoeba proteus". The American Naturalist 12 (4): 235–238. doi:10.1086/272082. JSTOR 2463786.
- ^ Audouin, Jean-Victor; et al (1826). Dictionnaire classique d'histoire naturelle. Rey et Gravier. pp. 5. http://books.google.com/?id=1I8DAAAAQAAJ&pg=PA158&lpg=PA158&dq=%22bory+de+saint+vincent+jean+baptiste+genevieve+marcellin%22+amibe#PPA5,M1.
- ^ 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.
- ^ Eugene H. Johnson, Jeffrey J. Windsor, and C. Graham Clark Emerging from Obscurity: Biological, Clinical, and Diagnostic Aspects of Dientamoeba fragilis.
- ^ "Amoeba proteus". Amoebae on the Web. http://amoeba.ifmo.ru/species/amoebidae/aprot.htm. Retrieved 2009-10-08.
- ^ MacIver, Sutherland. "Isolation of Amoebae". The Amoebae. http://www.bms.ed.ac.uk/research/others/smaciver/Protocols/AmoebaProts/isolation_of_amoebae.htm. Retrieved 2009-10-08.
- ^ a b "Amoeba". Scienceclarified.com. http://www.scienceclarified.com/Al-As/Amoeba.html.
- ^ http://www.genomenewsnetwork.org/articles/02_01/Sizing_genomes.shtml
- ^ a b c d e Nishihara, E., Yokota, E., Tazaki, A., Orii, H., Katsuhara, M., Kataoka, K., et al. (2008). Presece of aquaporin and V-ATPase on the contractile vacuole of Amoeba proteus. Biology of the cell , 100 (3), 179-188.
- ^ Patterson, D.J. (1981). "Contractile vacuole complex behaviour as a diagnostic character for free living amoebae". Protistologica 17: 243–248.
External links
 |
Wikimedia Commons has media related to: Amoebozoa |
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Translations:
Top
Amoeba
Deutsch (German)
n. - Amöbe, Wechseltierchen
Ελληνική (Greek)
n. - (βιολ.) αμοιβάδα
Português (Portuguese)
n. - ameba (f) (Biol.)
中文（简体）(Chinese (Simplified))
变形虫, 阿米巴
中文（繁體）(Chinese (Traditional))
n. - 變形蟲, 阿米巴
العربيه (Arabic)
(الاسم) الأميبه, حييوين ويد الخليه يتغير شكله بإستمرار
עברית (Hebrew)
n. - אמבה, חילופית
If you are unable to view some languages clearly, click here.
Post a question - any question - to the WikiAnswers community:
Copyrights:
American Heritage Dictionary
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
Read more
Britannica Concise Encyclopedia
Britannica Concise Encyclopedia. © 1994-2012 Encyclopædia Britannica, Inc. All rights reserved. Read more
McGraw-Hill Science & Technology Encyclopedia
McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.
Read more
Columbia Encyclopedia
The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2012, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/.
Read more
Word Tutor
Copyright © 2004-present by eSpindle Learning, a 501(c) nonprofit organization. All rights reserved.
eSpindle provides personalized spelling and vocabulary tutoring online; sign up free. Read more
eSpindle provides personalized spelling and vocabulary tutoring online; sign up free. Read more
Dictionary of Cultural Literacy: Science
The New Dictionary of Cultural Literacy, Third Edition Edited by E.D. Hirsch, Jr., Joseph F. Kett, and James Trefil. Copyright © 2002 by Houghton Mifflin Company. Published by Houghton Mifflin. All rights reserved.
Read more
Oxford Dictionary of Biochemistry
Oxford University Press. Oxford Dictionary of Biochemistry and Molecular Biology © 1997, 2000, 2006 All rights reserved.
Read more
Saunders Veterinary Dictionary
Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.
Read more
Mosby's Dental Dictionary
Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved.
Read more
Random House Word Menu
© 2010 Write Brothers Inc. Word Menu is a registered trademark of the Estate of Stephen Glazier.
Write Brothers Inc.
All rights reserved.
Read more
Bradford's Crossword Solver's Dictionary
Collins Bradford's Crossword Solver's Dictionary © Anne Bradford, 1986, 1993, 1997, 2000, 2003, 2005, 2008
HarperCollins Publishers
All rights reserved.
Read more
Wikipedia on Answers.com
This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article Amoeba.
Read more
