Neutrophil (white blood cell) engulfing anthrax bacteria 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.Īmoebae as specialized cells and life cycle stages At the other extreme, the shells of deep-sea xenophyophores can attain 20 cm in diameter. The marine amoeboid Massisteria voersi is just 2.3 to 3 micrometres in diameter, within the size range of many bacteria. The size of amoeboid cells and species is extremely variable. Diet įoraminifera have reticulose (net-like) pseudopods, and many species are visible with the naked eye 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. Without a contractile vacuole, the cell would fill with excess water and, eventually, burst. 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. This organelle is necessary because freshwater has a lower concentration of solutes (such as salt) than the amoeba's own internal fluids ( cytosol). ![]() To regulate osmotic pressure, most freshwater amoebae have a contractile vacuole which expels excess water from the cell. 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. "Naked" amoeba of the genus Mayorella (left) and shell of the testate amoeba Cylindrifflugia acuminata (right)įree-living amoebae may be " testate" (enclosed within a hard shell), or "naked" (also known as gymnamoebae, lacking any hard covering). Some groups, such as the Radiolaria and Heliozoa, have stiff, needle-like, radiating axopodia (actinopoda) supported from within by bundles of microtubules. Foraminifera emit fine, branching pseudopods that merge with one another to form net-like (reticulose) structures. Cercozoan amoeboids, such as Euglypha and Gromia, have slender, thread-like (filose) pseudopods. Amoebozoan species, such as those in the genus Amoeba, typically have bulbous (lobose) pseudopods, rounded at the ends and roughly tubular in cross-section. The appearance and internal structure of pseudopods are used to distinguish groups of amoebae from one another. Amoeba move and feed by using pseudopods, which are bulges of cytoplasm formed by the coordinated action of actin microfilaments pushing out the plasma membrane that surrounds the cell. The forms of pseudopodia, from left: polypodial and lobose monopodial and lobose filose conical reticulose tapering actinopods non-tapering actinopodsĪmoeba do not have cell walls, which allows for free movement. Other well known species include the so-called "brain-eating amoeba" Naegleria fowleri, the intestinal parasite Entamoeba histolytica, which causes amoebic dysentery, and the multicellular "social amoeba" or slime mould Dictyostelium discoideum. ![]() The best known amoeboid protists are Chaos carolinense and Amoeba proteus, both of which have been widely cultivated and studied in classrooms and laboratories. ![]() Consequently, amoeboid organisms are no longer classified together in one group. However, molecular phylogenetic studies have shown that Sarcodina is not a monophyletic group whose members share common descent. ![]() In older classification systems, most amoebae were placed in the class or subphylum Sarcodina, a grouping of single-celled organisms that possess pseudopods or move by protoplasmic flow. Microbiologists often use the terms "amoeboid" and "amoeba" interchangeably for any organism that exhibits amoeboid movement. Amoeboid cells occur not only among the protozoa, but also in fungi, algae, and animals. Amoebae do not form a single taxonomic group instead, they are found in every major lineage of eukaryotic organisms. Clockwise from top right: Amoeba proteus, Actinophrys sol, Acanthamoeba sp., Nuclearia thermophila., Euglypha acanthophora, neutrophil ingesting bacteria.Īn amoeba ( / ə ˈ m iː b ə/ less commonly spelled ameba or amœba plural am(o)ebas or am(o)ebae / ə ˈ m iː b i/), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopods.
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