unicellular organism

Topics: Bacteria, Eukaryote, Organism Pages: 5 (1320 words) Published: December 4, 2013
Unicellular organism
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"Single-celled" redirects here. For prison cell assignment, see Single-celling.

Valonia ventricosa is among the largest unicellular species. A unicellular organism, also known as a single-celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of multiple cells. Historically the simple single celled organisms have sometimes been referred to as monads.[1] The main groups of unicellular organisms are bacteria, archaea, protozoa, unicellular algae and unicellular fungi. Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms.Unicellular organisms are believed to be the oldest form of life, possibly existing 3.8 billion years ago.[2] Prokaryotes, most protists, and some fungi are unicellular. Although some of these organisms live in colonies, they are still unicellular. These organisms live together, and each cell in the colony is the same. However, each cell must carry out all life processes in order for that cell to survive. In contrast, even the simplest multicellular organisms have cells that depend on each other in order to survive. Some organisms are partially uni- and multicellular, like Dictyostelium discoideum. Other can be unicellular and multinucleate, like Myxogastria and Plasmodium. ‘Candidatus Magnetoglobus multicellularis’, related to Deltaproteobacteria, is a multicellular prokaryote. It is neither unicellular, nor a colony. Most unicellular organisms are of microscopic size and are thus classified as microorganisms. However, some unicellular protists and bacteria are macroscopic and visible to the naked eye.[3] Examples include: Xenophyophores, protozoans of the phylum Foraminifera, are the largest examples known, with Syringammina fragilissima achieving a diameter of up to 20 cm.[4] Nummulite, foraminiferans

Valonia ventricosa, an alga of the class Chlorophyceae, can reach a diameter of 1 to 4 cm.[5][6] Acetabularia, algae.
Caulerpa, algae[7]
Gromia sphaerica, amoeba
Thiomargarita namibiensis is the largest bacterium, reaching a diameter of up to 0.75 mm. Epulopiscium fishelsoni, a bacterium.
Drosophila Mirkogaster, an amoeba.
Multicellular organism
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In this image, a wild-type Caenorhabditis elegans is stained to highlight the nuclei of its cells. Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. To form a multicellular organism, these cells need to identify and attach to the other cells.[1] Only a dozen or unicellular species have cells that can be seen individually with the naked eye. The rest of the nearly two million[citation needed] visible species are multicellular. In particular all species of animals, land plants and filamentous fungi are multicellular, as are many algae. Some organisms are partially uni- and multicellular, like Dictyostelium. Multicellular organisms —like plants, animals and brown algae— arise from a single cell and generate a multi-celled organism. Pluricellular organisms are the result of many-celled individuals joining together through colony formation, filament formation or aggregation. Pluricellularity has evolved independently in Volvox and some flagellated green algae.[2][3] Contents

1 Evolutionary history
2 Hypotheses for origin
2.1 The symbiotic theory
2.2 The cellularization (syncytial) theory
2.3 The colonial theory
3 Advantages
4 See also
5 References
6 External links
Evolutionary history[edit]
Multicellularity has evolved independently at least 25 times,[4] including in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae,...
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