AskDefine | Define protozoon

Dictionary Definition

protozoon n : any of diverse minute acellular or unicellular organisms usually nonphotosynthetic [syn: protozoan]

User Contributed Dictionary



protozoon (Plural protozoa)
  1. protozoan.

Extensive Definition

Protozoa (in Greek protohi = first and zoa = animals) are unicellular eukaryotes, (singular protozoan). While there is no exact definition of the term, most scientists use protozoan to refer to a unicellular heterotrophic protist. The name 'protozoa' is used for the more animal-like single celled organisms like amoebas and ciliates. The term 'algae' is used for the more plant-like micro organisms. However, the distinction between the two is often vague. Dinobryon (top image) has chloroplasts for photosynthesis but it can also feed on organic matter. It is even able to swim. Such an organism can neither be animal nor plant. Nowadays, all these unicellular organisms that are neither animals, plants, bacteria or fungi are called 'protists‘.


Protozoa are around 10–50 μm, but can grow up to 1 mm and can easily be seen under a microscope. Protozoa exist throughout aqueous environments and soil. Protozoa occupy a range of trophic levels. As predators, they prey 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 populations 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 (Plasmodium spp.), trypanosomes and leishmania are also important as parasites and symbionts of multicellular animals.
Most protozoa exist in 5 stages of life which are in the form of trophozoites and cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures and harmful chemicals, or long periods without access to nutrients, water, or oxygen for a period of time. Being a cyst enables parasitic species to survive outside of the host, and allows their transmission from one host to another. When protozoa are in the form of trophozoites (Greek, tropho=to nourish), they actively feed and grow. The process by which the protozoa takes its cyst form is called encystation, while the process of transforming back into trophozoite is called excystation.
Protozoa can reproduce by binary fission or multiple fission. Some protozoa reproduce sexually, some asexually, and some both (eg. Coccidia). An individual protozoan is hermaphroditic.
Another name for protozoa is Acrita (R. Owen, 1861). It can also be considered ''Eukaryotic Cell Functionase


Protozoa were commonly grouped in the kingdom of Protista together with the plant-like algae and fungus-like water molds and slime molds. In the 21st-century systematics, protozoans, along with ciliates, mastigophorans, and apicomplexans, are arranged as animal-like protists. However, protozoans are neither Animalia nor Metazoa (with the possible exception of the enigmatic, moldy Myxozoa).


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

External links

protozoon in Arabic: أوليات
protozoon in Catalan: Protozou
protozoon in Czech: Prvoci
protozoon in Danish: Protozo
protozoon in German: Protozoen
protozoon in Modern Greek (1453-): Πρωτόζωο
protozoon in Esperanto: Protozooj
protozoon in Spanish: Protozoo
protozoon in Persian: تک‌یاختگان
protozoon in Finnish: Alkueläimet
protozoon in Indonesian: Protozoa
protozoon in Italian: Protozoa
protozoon in Japanese: 原生動物
protozoon in Latin: Protozoa
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protozoon in Macedonian: Праживотни
protozoon in Malay (macrolanguage): Protozoa
protozoon in Dutch: Protozoa
protozoon in Polish: Protisty zwierzęce
protozoon in Portuguese: Protozoário
protozoon in Romanian: Protozoar
protozoon in Simple English: Protozoa
protozoon in Slovenian: Praživali
protozoon in Serbian: Протозоа
protozoon in Swedish: Urdjur
protozoon in Thai: โพรโทซัว
protozoon in Vietnamese: Động vật nguyên sinh
protozoon in Walloon: Protozowaire
protozoon in Chinese: 原生动物门
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