AskDefine | Define protozoan

Dictionary Definition

protozoan adj : of or relating to the Protozoa [syn: protozoal, protozoic] n : any of diverse minute acellular or unicellular organisms usually nonphotosynthetic [syn: protozoon] [also: protozoa (pl)]

User Contributed Dictionary



  1. Any of the diverse group of eukaryotes, of the phylum Protozoa, that are primarily unicellular, existing singly or aggregating into colonies, are usually nonphotosynthetic, and are often classified further into phyla according to their capacity for and means of motility, as by pseudopods, flagella, or cilia.


  1. Of, pertaining to, or characteristic of a protozoan.

Derived terms


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

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