Protista


Protista

Protista

The Protista, or Protoctista, are a kingdom of simple eukaryotic organisms, usually composed of a single cell or a colony of similar cells. Protists live in water, in moist terrestrial habitats, and as parasites and other symbionts in the bodies of multicellular eukaroytes.

Other eukaryotic kingdomsthe Plantae, Fungi, and Animaliaare each believed to be monophyletic . That is, all plants evolved from one ancestral plant, all animals from one ancestral animal, and all fungi from one ancestral fungus. The Protista, however, are not; they are almost certainly polyphyletic and did not arise from a single ancestral protist. Rather, the Protista are a category of miscellaneous eukaryotes, not closely related to each other and not sharing many characteristics, but not fitting any other kingdom of life. Some authorities divide the Protista into as many as twenty-seven phyla, and some feel the Protista should be discarded as a kingdom name, and these organisms divided into as many as twelve kingdoms.

Historically, the Protista were divided into three main categories: the plantlike algae, animal-like protozoans, and funguslike slime molds. This classification persists in many elementary textbooks; however, current molecular evidence indicates that these are not natural groups related by common descent, but groups with merely superficial , deceptive similarities. Classifying them together is probably no more scientific than it would be to classify bees, birds, and bats in one group simply because they all have wings and fly. The two flagellated protozoan groups called trypanosomes and dinoflagellates, for example, are probably less related to each other than a human is to a fish. Genetic evidence (base sequences in their mitochondrial deoxyribonucleic acid [mtDNA] and ribosomal ribonucleic acid [rRNA]) now indicates that the following are more natural (evolutionarily related) groups of Protista.

Basal Protista

These are the most primitive protists. Some lack mitochondria and suggest what the first eukaryotes may have been like, while others have primitive mitochondria that closely resemble bacteria. Some basal Protista without mitochondria are Trichomonas, a vaginal parasite of humans; Giardia, an intestinal parasite; and Entamoeba, the cause of amoebic dysentery. The lack of mitochondria is not necessarily the primitive (original) condition of all these protists, however. Although Giardia lacks mitochondria, it does have mitochondrial genes. Apparently it once had mitochondria, and these genes transferred to its nuclear DNA before the mitochondria were lost.

Basal Protista with mitochondria include Trypanosoma, a genus of blood parasites that cause African sleeping sickness and other diseases; Euglena, a green freshwater flagellated protozoan with chloroplasts; and Physarum, a common terrestrial slime mold.

Alveolates

Alveolates are named for flattened sacs called alveoli just beneath their plasma membranes. They have mitochondria with tubular cristae rather than the flattened cristae typical of most mitochondria. Alveolates include dinoflagellates, aquatic forms with two flagella and a cell wall made of armorlike cellulose plates; Paramecium and other familiar ciliates; and the Apicomplexa, a group of intracellular parasites that includes Plasmodium, the cause of malaria, and Toxoplasma, the cause of toxoplasmosis.

Stramenopiles

Stramenopiles include water molds, golden and brown algae, and diatoms. The funguslike water molds (oomycetes) live in fresh water and soil, feeding on living or decaying organisms. Despite their name, some of them are important pests of row crops, including potato blight, downy mildew, and white rust. The golden algae (Chrysophyta ) and brown algae (Phaeophyta ) include many familiar seaweeds easily found on rocky coasts. Kelp is a gigantic marine brown alga (Macrocystus ) that grows up to 30 meters (100 feet) long and forms dense "forests" in some coastal waters. Diatoms are microscopic unicellular algae encased in siliceous (glasslike) walls, often with delicate lacy designs like tiny jewel boxes or Christmas ornaments.

Red Algae

The red algae (Rhodophyta ) include most seaweeds and are most abundant in tropical seas. Coral reefs are made not only by corals but also by coralline red algae that deposit calcium carbonate in the reef. Some red algae produce viscous polysaccharides such as agar and carrageenan, used to thicken ice cream, desserts, salad dressings, toothpaste, cosmetics, paints, and bacterial culture media.

Green Algae

The green algae (Chlorophyta ) include the single-celled Chlamydomonas, the spherical colonies of Volvox, and large seaweeds such as Codium magnum. Some unicellular green algae, notably Chlorella, live within the cells of animals, imparting a green color to some sponges, hydras, and flatworms. The plant kingdom probably evolved from a green alga.

The Study of Protista

Biologists in several subdisciplines of biology specialize in the Protista or have interests that overlap with this kingdom. Microbiologists study bacteria and some unicellular protists. Phycologists specialize in algae. Protozoologists study protozoans. Mycologists specialize in fungi but also often study water molds and slime molds, formerly classified as fungi. Parasitologists study disease-producing protists.

see also Algae; Coral Reef; Fungi; Mitochondrion; Plant Pathogens and Pests; Protozoa; Protozoan Diseases; Slime Molds

Kenneth S. Saladin

Bibliography

Margulis, L., J. O. Corlis, M. Melkonian, and D. J. Chapman. Handbook of Protoctista, Boston: Jones & Bartlett, 1990.

Margulis, L., and K. Schwartz. The Five Kingdoms, 3rd ed. New York: W. H. Freeman and Company, 1998.

Patterson, D. J. "The Diversity of Eukaryotes." American Naturalist 154 (1999): S96S124.

Show all research tools

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

Saladin, Kenneth S.. "Protista." Biology. 2002. Encyclopedia.com. 24 Apr. 2014 <http://www.encyclopedia.com>.

Saladin, Kenneth S.. "Protista." Biology. 2002. Encyclopedia.com. (April 24, 2014). http://www.encyclopedia.com/doc/1G2-3400700378.html

Saladin, Kenneth S.. "Protista." Biology. 2002. Retrieved April 24, 2014 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3400700378.html

Learn more about citation styles

Protista

Protista (prōtĬs´tə) or Protoctista (prō´tŏktĬs´tə), in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms. Protists, which are eukaryotes, have cells that have a membrane-bound nucleus, DNA that is associated with histone proteins, and organelles (e.g., mitochondria and chloroplasts). A recently proposed system of classification designates the eukaryotes as one of three great groups of life (beside bacteria and archaea) and places the protists within it.

It has been hypothesized that the organelles in protists descend evolutionarily from specialized symbiotic bacteria living within the cells of other bacteria, contributing at least in part to the transition from prokaryotic (bacterial) cells (the earliest form of life on the planet, dating back at least 3.5 billion years) to early eukaryotic cells (the cells that define protists, dating back 1.5 billion years) and the more complex life forms of later plants and animals.

The protists comprise a very diverse group of organisms. They include some algae, the protozoans, and multicellular or multinucleate autotrophs, such as the water molds. Many have flagella that enable them to move about. Before the advent of modern biochemistry and the electron microscope, these organisms were fit into the plant and animal kingdoms. It is now thought that, although green plants probably evolved from the green algae and animals from some other early forms, most modern protists have followed independent evolutionary lines. There are approximately 60,000 living species of protists.

Show all research tools

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Protista." The Columbia Encyclopedia, 6th ed.. 2013. Encyclopedia.com. 24 Apr. 2014 <http://www.encyclopedia.com>.

"Protista." The Columbia Encyclopedia, 6th ed.. 2013. Encyclopedia.com. (April 24, 2014). http://www.encyclopedia.com/doc/1E1-Protista.html

"Protista." The Columbia Encyclopedia, 6th ed.. 2013. Retrieved April 24, 2014 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-Protista.html

Learn more about citation styles

Protista

Protista (domain Eukarya) A paraphyletic kingdom comprising single-celled and multicelled eukaryotes that are not classified as animals, green plants, or true fungi. There are about 60 types and about 200000 species of protists. These include the Oomycota, parasitic and free-living protozoa, and various single-celled and multicelled algae (see ALGA). The multicellular organisms arose from the protists by various independent routes, possibly as many as 17. They were already a diverse group 1000 Ma ago. Protoctista is sometimes used as an alternative name. In earlier classifications the Protista formed a kingdom (or superphylum or phylum) of relatively simple, undifferentiated organisms.

Show all research tools

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

MICHAEL ALLABY. "Protista." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. 24 Apr. 2014 <http://www.encyclopedia.com>.

MICHAEL ALLABY. "Protista." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. (April 24, 2014). http://www.encyclopedia.com/doc/1O7-Protista.html

MICHAEL ALLABY. "Protista." A Dictionary of Plant Sciences. 1998. Retrieved April 24, 2014 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O7-Protista.html

Learn more about citation styles

Protista

Protista A kingdom (superkingdom Eukarya) comprising single-celled eukaryotes that are neither animals (developing from a blastula) nor plants (developing from an embryo). Animal-like members include naked and shelled amoebas, foraminiferans, zooflagellates, and ciliates; plant-like members include dinoflagellates, diatoms, and algae. In the widely used five-kingdom system of classification some multicellular organisms with protist affinities but previously classed as fungi or plants have been transferred into this kingdom and the name of the kingdom changed to Protoctista, which is now the name most often used.

Show all research tools

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

MICHAEL ALLABY. "Protista." A Dictionary of Zoology. 1999. Encyclopedia.com. 24 Apr. 2014 <http://www.encyclopedia.com>.

MICHAEL ALLABY. "Protista." A Dictionary of Zoology. 1999. Encyclopedia.com. (April 24, 2014). http://www.encyclopedia.com/doc/1O8-Protista.html

MICHAEL ALLABY. "Protista." A Dictionary of Zoology. 1999. Retrieved April 24, 2014 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O8-Protista.html

Learn more about citation styles

Protista

Protista(Protoctista) A kingdom in the domainEucaryota that comprises single-celled, eukaryotic organisms (see eukaryote) that may resemble animals or plants. Naked and shelled amoebas, foraminiferans, zooflagellates, ciliates, dinoflagellates, diatoms, and algae are protists. In an earlier five-kingdom classification, Protista was ranked as a kingdom; later some multicellular organisms with protist affinities but previously classed as fungi or plants were transferred into the Protista and the name was changed to Protoctista. Protoctista is now a synonym for Protista.

Show all research tools

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

MICHAEL ALLABY. "Protista." A Dictionary of Ecology. 2004. Encyclopedia.com. 24 Apr. 2014 <http://www.encyclopedia.com>.

MICHAEL ALLABY. "Protista." A Dictionary of Ecology. 2004. Encyclopedia.com. (April 24, 2014). http://www.encyclopedia.com/doc/1O14-Protista.html

MICHAEL ALLABY. "Protista." A Dictionary of Ecology. 2004. Retrieved April 24, 2014 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O14-Protista.html

Learn more about citation styles

Protista

Protista In some classifications, a kingdom containing unicellular eukaryotes that cannot be classified as animals, plants, or fungi. Originally proposed by Ernst Haeckel in 1866 to include the algae, bacteria, fungi, and protozoa, it was later restricted first to unicellular organisms, and then to protozoa, unicellular algae, and organisms then regarded as simple fungi. In most modern classifications it has been replaced by the Protoctista.

Show all research tools

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Protista." A Dictionary of Biology. 2004. Encyclopedia.com. 24 Apr. 2014 <http://www.encyclopedia.com>.

"Protista." A Dictionary of Biology. 2004. Encyclopedia.com. (April 24, 2014). http://www.encyclopedia.com/doc/1O6-Protista.html

"Protista." A Dictionary of Biology. 2004. Retrieved April 24, 2014 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O6-Protista.html

Learn more about citation styles

Protista

Protista See PROTIST.

Show all research tools

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

AILSA ALLABY and MICHAEL ALLABY. "Protista." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 24 Apr. 2014 <http://www.encyclopedia.com>.

AILSA ALLABY and MICHAEL ALLABY. "Protista." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (April 24, 2014). http://www.encyclopedia.com/doc/1O13-Protista.html

AILSA ALLABY and MICHAEL ALLABY. "Protista." A Dictionary of Earth Sciences. 1999. Retrieved April 24, 2014 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-Protista.html

Learn more about citation styles

Facts and information from other sites

Protista images
Protista. (Image by 天地水, GFDL)