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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 various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologist Georg A. Goldfuss coined the word Protozoa to refer to organisms such as ciliates, [56] and this group was expanded until Ernst Haeckel made it a kingdom encompassing all single-celled eukaryotes, the Protista ...
The term continues to be used in a loose way to describe single-celled protists (that is, eukaryotes that are not animals, plants, or fungi) that feed by heterotrophy. [9] Traditional textbook examples of protozoa are Amoeba, Paramecium, Euglena and Trypanosoma. [10]
Cercozoa (now synonymised with Filosa) [2] is a phylum of diverse single-celled eukaryotes. [4] [5] They lack shared morphological characteristics at the microscopic level, [6] and are instead united by molecular phylogenies of rRNA and actin or polyubiquitin. [7]
A mesokaryote or mesokaryotic organism is a single-celled eukaryote that shows intermediate resemblance to both prokaryotes and 'higher' eukaryotes. The term originates from a 1965 hypothesis by John David Dodge, who proposed that certain eukaryotes (mainly dinoflagellates) with closed mitosis and other traits considered 'primitive' were an intermediate step between prokaryotes and the ...
The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter. [1] [page needed] A prokaryotic cell has three regions:
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. [1] [2] [3] They are estimated to constitute 1% of all described fungal species. [4]
The dinoflagellates (from Ancient Greek δῖνος (dînos) 'whirling' and Latin flagellum 'whip, scourge') are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata [5] and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also common in freshwater habitats.