Difference between revisions of "Protist" - New World Encyclopedia
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The protists are a paraphyletic grade, rather than a natural (monophyletic) group, and do not have much in common besides a relatively simple organization. Some call it the "left-overs" from the other eukaryotic kingdoms. | The protists are a paraphyletic grade, rather than a natural (monophyletic) group, and do not have much in common besides a relatively simple organization. Some call it the "left-overs" from the other eukaryotic kingdoms. | ||
| − | + | They effect humans indirectly through the important roles they play in the earth's [[ecosystem]]s and also directly as the cause of many [[disease]]s affecting both people and the cultivated plants we depend on for food. | |
==Traditional classification== | ==Traditional classification== | ||
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===Algae=== | ===Algae=== | ||
| − | Algae gather light energy through [[photosynthesis]]. They include many single-celled creatures that are also considered protozoa, such as ''Euglena'', which have acquired [[chloroplast]]s through secondary [[endosymbiosis]]. Others are non-motile, and some (called [[seaweed]]s) are truly multicellular, including members of the following groups: | + | [[Algae]], like plants, gather light energy through [[photosynthesis]] and convert it into chemical energy. They include many single-celled creatures that are also considered protozoa, such as ''Euglena'', which have acquired [[chloroplast]]s through secondary [[endosymbiosis]]. Others are non-motile, and some (called [[seaweed]]s) are truly multicellular, including members of the following groups: |
{|cellpadding="5" | {|cellpadding="5" | ||
| [[Green alga|Chlorophyte]]s || green algae, related to higher plants || e.g., ''Ulva'' | | [[Green alga|Chlorophyte]]s || green algae, related to higher plants || e.g., ''Ulva'' | ||
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Some algae exist in [[symbiosis|symbiotic]] relationships with other organisms. [[Lichen]]s are a partnership of [[fungus|fungi]] and algae. Some [[coral]]s, [[sponge]]s, and [[jellyfish]] host algae which provide them with food and in return receive protection. Agae also live in the hair of [[sloth]]s in [[South America]]n [[rain forest]]s. This relationship provides the algae with a home and the sloth with a green color for camouflage. | Some algae exist in [[symbiosis|symbiotic]] relationships with other organisms. [[Lichen]]s are a partnership of [[fungus|fungi]] and algae. Some [[coral]]s, [[sponge]]s, and [[jellyfish]] host algae which provide them with food and in return receive protection. Agae also live in the hair of [[sloth]]s in [[South America]]n [[rain forest]]s. This relationship provides the algae with a home and the sloth with a green color for camouflage. | ||
| − | [[Seaweed]]s are multi-cellular algae. Although they can grow very large they lack the complex structure of plants. The largest seaweed (and the largest protist) is the giant kelp, ''Macrocystis pyrifera'', found along the Pacific coast of North and South America. It can grow to a length of 60 meters (200 feet). | + | [[Seaweed]]s are multi-cellular algae. Although they can grow very large they lack the complex structure of plants. The largest seaweed (and the largest protist) is the giant kelp, ''Macrocystis pyrifera'', found along the Pacific coast of North and South America. It can grow to a length of 60 meters (200 feet). Some seaweeds are harvested by humans for food and other uses. |
===Protozoa=== | ===Protozoa=== | ||
| − | Protozoa are mostly single-celled, motile (can move around) protists that feed by | + | Protozoa are mostly single-celled, motile (can move around) protists that feed by phagocytosis, particles are enveloped by the cell membrane. There are numerous exceptions however. They are usually only 0.01-0.5 mm in size, generally too small to be seen without a [[microscope]]. Protozoa are ubiquitous throughout aqueous environments and the soil, commonly surviving dry periods as [[cyst]]s or [[spore]]s, and include several important [[parasite]]s. Based on [[locomotion]], protozoa are grouped into: |
| + | [[Image:Chaos diffluens.jpg|thumb|Amoeba (''Chaos diffluens'')]] | ||
{|cellpadding="5"| [[Flagellate]]s || with long [[flagellum|flagella]] || e.g., ''[[Euglena]]'' | {|cellpadding="5"| [[Flagellate]]s || with long [[flagellum|flagella]] || e.g., ''[[Euglena]]'' | ||
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===Fungus-like protists=== | ===Fungus-like protists=== | ||
| − | Various organisms with a protist-level organization were originally treated as fungi, because they produce sporangia. These include chytrids, slime molds, water molds, and Labyrinthulomycetes. | + | Various organisms with a protist-level organization were originally treated as fungi, because they produce sporangia, structures producing and containing spores. These include chytrids, slime molds, water molds, and Labyrinthulomycetes. |
| − | Chytrids can be single or multi-cellular. There are about 1,000 species, most living in water or soil. Most are decomposers. Some are parasites and can cause diseases in plants, including [[corn]], [[alfalfa]], and [[potato]]es. One species, ''Batrachochytrium dendrobatidis'', seems to be the cause of Chytridiomycosis, a disease of | + | Chytrids can be single or multi-cellular. There are about 1,000 species, most living in water or soil. Most are decomposers. Some are parasites and can cause diseases in plants, including [[corn]], [[alfalfa]], and [[potato]]es. One species, ''Batrachochytrium dendrobatidis'', seems to be the cause of Chytridiomycosis, a disease of [[frog]]s which is seriously affecting many wild frog populations around the world. |
[[Image:Slime_Mold_DP2.jpg|thumb|right|Slime mold on lawn, USA. Trail of movement can be seen.]] | [[Image:Slime_Mold_DP2.jpg|thumb|right|Slime mold on lawn, USA. Trail of movement can be seen.]] | ||
Revision as of 01:35, 16 July 2006
| Protists | ||||
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 Paramecium aurelia]], a ciliate | ||||
| Scientific classification | ||||
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| Typical phyla | ||||
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Protosts make up one of the kingdoms into which biologists divide living things. They are more complex than bacteria in that they have cell nuclei, but less complex than fungi, plants, and animals because they are mostly single-celled or if multi-celled they lack highly specialized tissues.
The protists are a paraphyletic grade, rather than a natural (monophyletic) group, and do not have much in common besides a relatively simple organization. Some call it the "left-overs" from the other eukaryotic kingdoms.
They effect humans indirectly through the important roles they play in the earth's ecosystems and also directly as the cause of many diseases affecting both people and the cultivated plants we depend on for food.
Traditional classification
Protists were traditionally (for the last 150 years) subdivided into several groups based on similarities to the higher kingdoms: the plant-like algae, the animal-like protozoa, and the fungi-like slime molds and water molds. These groups often overlap, and have been replaced by phylogenetic classifications. However, they are still useful as informal groups for describing the morphology and ecology of protists.
Algae
Algae, like plants, gather light energy through photosynthesis and convert it into chemical energy. They include many single-celled creatures that are also considered protozoa, such as Euglena, which have acquired chloroplasts through secondary endosymbiosis. Others are non-motile, and some (called seaweeds) are truly multicellular, including members of the following groups:
| Chlorophytes | green algae, related to higher plants | e.g., Ulva |
| Rhodophytes | red algae | e.g., Porphyra |
| Heterokontophytes | brown algae, diatoms, etc. | e.g., Macrocystis |
The green and red algae, along with a small group called the glaucophytes, appear to be close relatives of other plants, and so some authors treat them as Plantae despite their simple organization. Most other types of algae, however, developed separately. They include the haptophytes, cryptomonads, dinoflagellates, euglenids, and chlorarachniophytes, all of which have also been considered protozoans. Some protozoa host endosymbiotic algae, as in Paramecium bursaria or radiolarians, that provide them with energy but are not integrated into the cell.
Algae are found in many environments both in water and on land. They are most important as, along with cyanobacteria, a part of the phytoplankton in the world's oceans which is the basis of almost all marine life.
Some algae exist in symbiotic relationships with other organisms. Lichens are a partnership of fungi and algae. Some corals, sponges, and jellyfish host algae which provide them with food and in return receive protection. Agae also live in the hair of sloths in South American rain forests. This relationship provides the algae with a home and the sloth with a green color for camouflage.
Seaweeds are multi-cellular algae. Although they can grow very large they lack the complex structure of plants. The largest seaweed (and the largest protist) is the giant kelp, Macrocystis pyrifera, found along the Pacific coast of North and South America. It can grow to a length of 60 meters (200 feet). Some seaweeds are harvested by humans for food and other uses.
Protozoa
Protozoa are mostly single-celled, motile (can move around) protists that feed by phagocytosis, particles are enveloped by the cell membrane. There are numerous exceptions however. They are usually only 0.01-0.5 mm in size, generally too small to be seen without a microscope. Protozoa are ubiquitous throughout aqueous environments and the soil, commonly surviving dry periods as cysts or spores, and include several important parasites. Based on locomotion, protozoa are grouped into:
| Amoeboids | with transient pseudopodia | e.g., Amoeba |
| Ciliates | with multiple, short cilia | e.g., Paramecium |
| Sporozoa | non-motile parasites; form spores | e.g., Plasmodium |
Some protozoa are an important part of marine plankton as consumers of algae and other plant-like phytoplankton and as food for marine animals. They are also important as decomposers in land environments as well as in the ocean. Others are parasites of plants and animals.
Protozoa are the cause of many human diseases. The most serious is malaria which is caused by four species of the genus Plasmodium, protozoa whose life cycle makes them in turn parasites of both humans and mosquitoes. Malaria kills over one million people a year, most of them African children.[1] Other diseases caused by protozoa include: African sleeping sickness, amoebic dysentery, chagas' disease, giardiasis, leishmaniasis, primary amoebic meningoencephalitis (PAM) and vaginitis.
Fungus-like protists
Various organisms with a protist-level organization were originally treated as fungi, because they produce sporangia, structures producing and containing spores. These include chytrids, slime molds, water molds, and Labyrinthulomycetes.
Chytrids can be single or multi-cellular. There are about 1,000 species, most living in water or soil. Most are decomposers. Some are parasites and can cause diseases in plants, including corn, alfalfa, and potatoes. One species, Batrachochytrium dendrobatidis, seems to be the cause of Chytridiomycosis, a disease of frogs which is seriously affecting many wild frog populations around the world.
Slime molds are notable for their unusual life cycle. In some species individual single-celled organisms come together and fuse to form a giant cell with thousands of nuclei. This body, called a plasmodium, can move around consuming bacteria, fungi, and decaying plant matter. Slime molds are found world-wide.
Water molds thrive in water and wet soil. They are considered to be more closely related to plants than fungi since they have cellulose cell walls. They are single-celled. Most are parasites and can cause diseases in plants, fungi, and animals. One species Phytophthora infestans causes the potato blight, which led to the Irish potato famine.
Labyrinthulomycetes form a network of tubes or filaments over which the single cell organisms slide to gather food. They are mostly marine and are decomposers of dead plant material or parasites on plants and algae or some animals.
Phylogenetic classifications
The classification of protists is still changing. Newer classifications attempt to present monophyletic groups based on ultrastructure, biochemistry, and genetics. Because the protists as a whole are paraphyletic, such systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. The recent scheme by Adl et al. (2005) is an example which does not bother with ranks (phylum, class, etc.).
ReferencesISBN links support NWE through referral fees
- Cavalier-Smith, T. (2003) "Protist phylogeny and the high-level classification of Protozoa". European Journal of Protistology 39:338-348.
- Lipscomb, Diana, "Protozoa" accessed July 15, 2006 [2]
- Sina M. Adl, Alastair G. B. Simpson, Mark A. Farmer, Robert A. Andersen, O. Roger Anderson, John R. Barta, Samuel S. Bowser, Guy Brugerolle, Robert A. Fensome, Suzanne Fredericq, Timothy Y. James, Sergei Karpov, Paul Kugrens John Krug, Christopher E. Lane, Louise A. Lewis, Jean Lodge, Denis H. Lynn, David G. Mann, Richard M. Mccourt, Leonel Mendoza, Øjvind Moestrup, Sharon E. Mozley-Standridge, Thomas A. Nerad, Carol A. Shearer, Alexey V. Smirnov, Frederick W. Spiegel and Max F. J. R. Taylor. (2005) "The New Higher Level Classification of Eukaryotes with Emphasis on the Taxonomy of Protists". Journal of Eukaryotic Microbiology 52:(5) 399-451. [3]
- World Health Organization, Global Malaria Programme accessed July 14, 2006 [4]
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