AMOEBA
An amoeba (/əˈmiːbə/; rarely spelled amœba, US English spelled ameba; plural am(o)ebas or am(o)ebae /əˈmiːbiː/),[1] often called amoeboid, is a type of cell or organism which has the ability to alter its shape, primarily by extending and retracting pseudopods.[2] Amoebas do not form a single taxonomic group; instead, they are found in every major lineage of eukaryoticorganisms. Amoeboid cells occur not only among the protozoa, but also in fungi, algae, and animals.[3][4][5][6][7]
Microbiologists often use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement.[8][9]
In older classification systems, most amoebas were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. Consequently,
Shape, movement and nutrition[edit]
Amoebae move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilamentspushing out the plasma membrane that surrounds the cell.[13] Typically, an amoeba moves by extending a pseudopod, attaching it to the substrate and filling it with cytosol, then releasing its rear portion from attachment to the substrate, which results in the organism being propelled forward.[14]
The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoebozoanspecies, such as those in the genus Amoeba, typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section. Cercozoan amoeboids, such as Euglypha and Gromia, have slender, thread-like (filose) pseudopods. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Some groups, such as the Radiolariaand Heliozoa, have stiff, needle-like, radiating axopodia (actinopoda) supported from within by bundles of microtubules.[3][15]
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Free-living amoebae may be "testate" (enclosed within a hard shell), or "naked" (a.k.a. gymnamoebae, lacking any hard covering). The shells of testate amoebae may be composed of various substances, including calcium, silica, chitin, or agglutinations of found materials like small grains of sand and the frustules of diatoms.[16]
To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell.[17] This organelle is necessary because freshwater has a lower concentration of solutes (such as salt) than the amoeba's own internal fluids (cytosol). Because the surrounding water is hypotonic with respect to the contents of the cell, water is transferred across the amoeba's cell membrane by osmosis. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst.
Marine amoebae do not usually possess a contractile vacuole because the concentration of solutes within the cell are in balance with the tonicity of the surrounding water.[18]
The food sources of amoebae vary. Some amoebae are predatory and live by consuming bacteria and other protists. Some are detritivores and eat dead organic material.
Amoebae typically ingest their food by phagocytosis, extending pseudopods to encircle and engulf live prey or particles of scavenged material. Amoeboid cells do not have a mouth or cytostome, and there is no fixed place on the cell at which phagocytosis normally occurs.[19]
Some amoebae also feed by pinocytosis, imbibing dissolved nutrients through vesicles formed within the cell membrane .[20]
Size range[edit]
The size of amoeboid cells and species is extremely variable. The marine amoeboid Massisteria voersi is just 2.3 to 3 micrometres in diameter,[21] within the size range of many bacteria.[22] At the other extreme, the shells of deep-sea xenophyophores can attain 20 cm in diameter.[23] Most of the free-living freshwater amoebae commonly found in pond water, ditches and lakes are microscopic, but some species, such as the so-called "giant amoebae" Pelomyxa palustris and Chaos carolinense, can be large enough to see with the naked eye.
| Species or cell type | Size in micrometres |
|---|---|
| Massisteria voersi[21] | 2.3 - 3 μm |
| Naegleria fowleri[24] | 8 - 15 μm |
| Neutrophil (white blood cell)[25] | 12 - 15 μm |
| Acanthamoeba[26] | 12 - 40 μm |
| Entamoeba histolytica[27] | 15 - 60 μm |
| Arcella vulgaris[28] | 30 - 152 μm |
| Amoeba proteus[29] | 220 - 760 μm |
| Chaos carolinense[30] | 700 - 2,000 μm |
| Pelomyxa palustris[31] | up to 5,000 μm |
| Syringammina fragilissima[32] | up to 200,000 μm |
Amoebae as specialized cells and life cycle stages[edit]
Some multicellular organisms have amoeboid cells only in certain phases of life, or use amoeboid movements for specialized functions. In the immune system of humans and other animals, amoeboid white blood cells pursue invading organisms, such as bacteria and pathogenic protists, and engulf them by phagocytosis.[33]
Amoeboid stages also occur in the multicellular fungus-like protists, the so-called slime moulds. Both the plasmodial slime moulds, currently classified in the class Myxogastria, and the cellular slime moulds of the groups Acrasida and Dictyosteliida, live as amoebae during their feeding stage. The amoeboid cells of the former combine to form a giant multinucleate organism,[34] while the cells of the latter live separately until food runs out, at which time the amoebae aggregate to form a multicellular migrating "slug" which functions as a single organism.[8]
Other organisms may also present amoeboid cells during certain life-cycle stages, e.g., the gametes of some green algae (Zygnematophyceae)[35] and pennate diatoms,[36] the spores (or dispersal phases) of some Mesomycetozoea,[37][38] and the sporoplasm stage of Myxozoa and of Ascetosporea.[39]
amoeboid organisms are no longer classified together in one group.[10]
Classification[edit]
Recent classification places the various amoeboid genera in the following groups:
| Supergroups | Major Groups and Genera | Morphology |
|---|---|---|
| Amoebozoa |
|
|
| Rhizaria |
| |
| Excavata |
| |
| Heterokonta |
|
|
| Alveolata |
|
|
| Nucleariid |
| |
| Ungrouped/ unknown |
|
It should be noted that some of the amoeboid groups cited (e.g., part of chrysophytes, part of xanthophytes, chlorarachniophytes) were not traditionally included in Sarcodina, being classified as algae or flagellated protozoa.
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