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A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Most prokaryotes are unicellular and are classified into bacteria and archaea.
The eukaryotes (/ j uː ˈ k ær i oʊ t s,-ə t s / yoo-KARR-ee-ohts, -əts) [4] constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus.All animals, plants, fungi, seaweeds, and many unicellular organisms are eukaryotes.
Clockwise from top right: Amoeba proteus, Actinophrys sol, Acanthamoeba sp., Nuclearia thermophila., Euglypha acanthophora, neutrophil ingesting bacteria. An amoeba (/ ə ˈ m iː b ə /; less commonly spelled ameba or amœba; pl.: amoebas (less commonly, amebas) or amoebae (amebae) / ə ˈ m iː b i /), [1] often called an amoeboid, is a type of cell or unicellular organism with the ability ...
Differing levels of biological organisation give rise to potentially different understandings of the nature of organisms. A unicellular organism is a microorganism such as a protist, bacterium, or archaean, composed of a single cell, which may contain functional structures called organelles. [22]
The definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology". [16] John Hogg's illustration of the Four Kingdoms of Nature, showing "Primigenal" as a greenish haze at the base of the Animals and Plants, 1860
The subunits of colonial organisms can be unicellular, as in the alga Volvox (a coenobium), or multicellular, as in the phylum Bryozoa. Colonial organisms may have been the first step toward multicellular organisms. [9] Individuals within a multicellular colonial organism may be called ramets, modules, or zooids.
Like bacteria, plant cells have cell walls, and contain organelles such as chloroplasts in addition to the organelles in other eukaryotes. Chloroplasts produce energy from light by photosynthesis, and were also originally symbiotic bacteria. [59] Unicellular eukaryotes consist of a single cell throughout their life cycle.
Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia. In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. For example, in wound healing in animals, white blood cells move to the wound site to kill ...