Difference between revisions of "Protozoa" - New World Encyclopedia

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'''Protozoa''' (in [[Greek language|Greek]] ''proto'' = first and ''zoa'' = animals) is a diverse group of single-[[cell (biology)|celled]], microscopic or near-microscopic [[eukaryote]]s (organisms whose cells have [[cell nucleus|nuclei]]) that commonly show characteristics usually associated with [[animal]]s, most notably [[mobility]] and [[heterotroph]]y. Among well-known protozoa are [[amoeba]]s, ciliates, [[paramecium|paramecia]], and dinoflagellates. Protozoans may live freely or be parasites, and may live as single cells or in simple colonies without any differentiation into tissues. While most are microscopic, some grow large enough to be seen unaided.
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[[Image:Paramecium.jpg|thumb|240px|right|''Paramecium aurelia'', a ciliated protozoa]]
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'''Protozoa''' (in [[Greek language|Greek]] ''proto'' = first and ''zoa'' = animals) is a diverse group of single-[[cell (biology)|celled]], microscopic or near-microscopic [[eukaryote]]s (organisms whose cells have [[cell nucleus|nuclei]]) that commonly show characteristics usually associated with [[animal]]s, most notably [[mobility]] and [[heterotroph]]y. An organism belonging to this group may be known as a '''protozoan''' (plural protozoans) or protozoon (plural protozoa). Both protozoa and protozoans are popular as the plural form. Among well-known protozoans are [[amoeba]]s, ciliates, [[paramecium|paramecia]], and dinoflagellates. Protozoa may live freely or be parasites, and may live as single cells or in simple colonies without any differentiation into tissues. While most are microscopic, some grow large enough to be seen unaided.
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While some are known for causing [[disease]] ([[malaria]] is caused by parasitic forms), protozoans are vitally important [[ecology|ecologically]] as key links in [[food chain]]s. Ubiquitous in aquatic environments and the [[soil]], protozoans prey upon [[algae]], [[bacteria]], and other organisms and are themselves consumed by animals such as micro[[invertebrate]]s.
  
 
Protozoa span several [[Taxonomy#Scientific or biological classification|phyla]], which are generally placed as part of the kingdom [[Protista]], rather than the kingdom [[Animalia]]. Protista (or Protoctista) comprises those eukaryotes that are not [[animal]]s, [[plant]]s, or [[fungi]], and is not a natural (monophyletic group), but rather the "left-overs" from other eukaryotic kingdoms. In some schemes, Protozoa is considered a sub-kingdom of Protista. There are also classifications in which the plant-like [[alga]]e and fungus-like water molds and slime molds are not considered part of Protista and the remaining forms then may be grouped as a kingdom labeled Protista. Often, protozoa is just an informal term for unicellular, heterotrophic protists.
 
Protozoa span several [[Taxonomy#Scientific or biological classification|phyla]], which are generally placed as part of the kingdom [[Protista]], rather than the kingdom [[Animalia]]. Protista (or Protoctista) comprises those eukaryotes that are not [[animal]]s, [[plant]]s, or [[fungi]], and is not a natural (monophyletic group), but rather the "left-overs" from other eukaryotic kingdoms. In some schemes, Protozoa is considered a sub-kingdom of Protista. There are also classifications in which the plant-like [[alga]]e and fungus-like water molds and slime molds are not considered part of Protista and the remaining forms then may be grouped as a kingdom labeled Protista. Often, protozoa is just an informal term for unicellular, heterotrophic protists.
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Under certain adverse conditions, some protozoa produce a protective capsule called a cyst. A cyst permits the organism to survive when food, moisture, or oxygen is lacking, when temperatures are not suitable, or when toxic chemicals are present. A cyst also enables a parasitic species to survive outside its host in order to get a new host. This is important because parasitic protozoa sometimes have to go through the excretory system of one host in order to find a new host. The cysts form in members of the phylum Apicomplexa is called an oocyst. It is a reproductive structure in which new cells are produced sexually.
 
Under certain adverse conditions, some protozoa produce a protective capsule called a cyst. A cyst permits the organism to survive when food, moisture, or oxygen is lacking, when temperatures are not suitable, or when toxic chemicals are present. A cyst also enables a parasitic species to survive outside its host in order to get a new host. This is important because parasitic protozoa sometimes have to go through the excretory system of one host in order to find a new host. The cysts form in members of the phylum Apicomplexa is called an oocyst. It is a reproductive structure in which new cells are produced sexually.
  
Many free-living protozoas have eye-spots that can detect changes in the quanity and quality of light, and some sense touch and chemical changes in their environment (Towle 1989).
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Many free-living protozoans have eye-spots that can detect changes in the quantity and quality of light, and some sense touch and chemical changes in their environment (Towle 1989).
  
 
==Types of protozoa==
 
==Types of protozoa==
Protozoan have traditionally been divided on the basis of their means of locomotion, although this is no longer believed to represent genuine relationships:
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Protozoans have traditionally been divided on the basis of their means of locomotion, although this is no longer believed to represent genuine relationships:
  
 
* [[Ciliate]]s
 
* [[Ciliate]]s

Revision as of 01:51, 15 February 2007


Paramecium aurelia, a ciliated protozoa

Protozoa (in Greek proto = first and zoa = animals) is a diverse group of single-celled, microscopic or near-microscopic eukaryotes (organisms whose cells have nuclei) that commonly show characteristics usually associated with animals, most notably mobility and heterotrophy. An organism belonging to this group may be known as a protozoan (plural protozoans) or protozoon (plural protozoa). Both protozoa and protozoans are popular as the plural form. Among well-known protozoans are amoebas, ciliates, paramecia, and dinoflagellates. Protozoa may live freely or be parasites, and may live as single cells or in simple colonies without any differentiation into tissues. While most are microscopic, some grow large enough to be seen unaided.

While some are known for causing disease (malaria is caused by parasitic forms), protozoans are vitally important ecologically as key links in food chains. Ubiquitous in aquatic environments and the soil, protozoans prey upon algae, bacteria, and other organisms and are themselves consumed by animals such as microinvertebrates.

Protozoa span several phyla, which are generally placed as part of the kingdom Protista, rather than the kingdom Animalia. Protista (or Protoctista) comprises those eukaryotes that are not animals, plants, or fungi, and is not a natural (monophyletic group), but rather the "left-overs" from other eukaryotic kingdoms. In some schemes, Protozoa is considered a sub-kingdom of Protista. There are also classifications in which the plant-like algae and fungus-like water molds and slime molds are not considered part of Protista and the remaining forms then may be grouped as a kingdom labeled Protista. Often, protozoa is just an informal term for unicellular, heterotrophic protists.

The name protozoa ("first animals") is misleading, since they are not animals (with the possible exception of the Myxozoa).

Overview

Most protozoans are too small to be seen with the naked eye—most are around 0.01–0.05 mm—but can easily be found under a microscope. However, forms that are up to 0.5 mm are still fairly common.

Free-living protozoa are ubiquitous throughout aqueous environments and the soil, and play an important role in their ecology. Many aquatic forms make up the zooplankton. Protozoa occupy a range of trophic levels. As predators upon unicellular or filamentous algae, bacteria, and microfungi, protozoa play a role both as herbivores and as consumers in the decomposer link of the food chain. Protozoa also play a vital role in controlling bacteria population and biomass. As components of the micro- and meiofauna, protozoa are an important food source for microinvertebrates. Thus, the ecological role of protozoa in the transfer of bacterial and algal production to successive trophic levels is important.

Protozoa such as the malaria parasites, trypanosomes, and leishmania are also important as parasites and symbionts of multicellular animals.

Most protozoa, whether free-living or parasitic, lack a protective outer covering (Towle 1989). Thus, a semipermeable cell membrane serves as the boundary between the environment and the cytoplasm, and minerals (calcium, potassium) and gases (oxygen, carbon dioxide) exchange accross this membrane (Towle 1989). Protozoa have at least one motile stage.

Under certain adverse conditions, some protozoa produce a protective capsule called a cyst. A cyst permits the organism to survive when food, moisture, or oxygen is lacking, when temperatures are not suitable, or when toxic chemicals are present. A cyst also enables a parasitic species to survive outside its host in order to get a new host. This is important because parasitic protozoa sometimes have to go through the excretory system of one host in order to find a new host. The cysts form in members of the phylum Apicomplexa is called an oocyst. It is a reproductive structure in which new cells are produced sexually.

Many free-living protozoans have eye-spots that can detect changes in the quantity and quality of light, and some sense touch and chemical changes in their environment (Towle 1989).

Types of protozoa

Protozoans have traditionally been divided on the basis of their means of locomotion, although this is no longer believed to represent genuine relationships:

  • Ciliates
  • Flagellates
  • Amoeboids
  • Sporozoans
    • Apicomplexa
    • Myxozoa
    • Microsporidia

The phyla into which protozoa fall vary with the taxonomic scheme. Three phyla that are commonly recognized are Sarcodina (those moving by cytoplasmic projection, or pseudopodia; the amoebas); Ciliophora (move by cilia; and Sporozoa (immobile and parasitic protozoans). Some other names used in different schemes for phyla include Zoomastigina (move by flagella), Mastigophora (another name for a group of flagellates), Sacromastigophora, Apicomplexa, Microspora, Myxozoa (spore-producing forms), and Opalinida.

There is a large variety of classifications. Towle (1989) recognizes four phyla following the classification above: Ciliophora, Zoomastigina, Sarcodina, and Sporozoa; in other words, those that move by cilia, flagella, pseudopodia, and are immobile and parasitic. Another scheme recognizes three phyla, Ciliophora (movement by cilia in at least one stage), Sarcomastigophora (locomotion by flagellum or pseudopodia or both), and Apicomplexa (cilia and flagella absent, parasitic, and cysts present), and place Mastiogophora (flagellates) and Sacrodina (move by pseudopodia) as subphyla of Sarcomastigophora (UM-CBS 2007).

References
ISBN links support NWE through referral fees

  • Hickman, C. P., and L. S. Roberts. 1994. Biology of Animals, Sixth Edition. Dubuque, IA: Wm.C.Brown Publishers.
  • Towle, A. 1989. Modern Biology. Austin: Holt, Rinehart and Winston. ISBN 0030139198.

Towle

  • University of Minnesota, The College of Biological Sciences (UM-CBS). 2007. Sub-kingdom: Protozoa]. Retrieved February 14, 2007.

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