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Title: Protozoa  
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Subject: Kingdom (biology), Plant pathology, Cytostome, Karl Theodor Ernst von Siebold, Parasitism
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Leishmania donovani, (a species of Excavata) in a bone marrow cell

In some biological [1] but the term is no longer commonly used in modern taxonomy.

Historically, protozoa were defined as unicellular protists with animal-like behaviour, such as movement. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behavior, e.g. photosynthesis. The term protozoan has become highly problematic due to the introduction of modern ultrastructural, biochemical, and genetic techniques, which have showed that the group does not form a clade as required by modern classifications. Modern unicellular clades within Eukaryotes which may be viewed as approximately collectively replacing the class of protozoa include: Excavata, Amoeba, Chromalveolata and Rhizaria.

The term is still used informally, especially in high-school education, and today, protozoa are usually single-celled and parasites, and some are predators of faeces bacteria and algae. There are an estimated 30,000 protozoan species.[2]


Following the Greek root of the name, the singular form is protozoon (protos=first, zoon=animal). Its use has, however, partially been replaced by the word protozoan, which was originally only used as an adjective. In the same manner the plural form protozoans is sometimes used instead of protozoa.

In general, protozoa are referred to as animal-like protists because of movement (motility). However, both protozoa and protists are paraphyletic groups (not including all genetic relatives of the group). For example, Entamoeba is more closely related to humans than to Euglena. "Protozoa" is considered an outdated classification in more formal contexts. However, the term is still used in children's education.[3]

While there is no exact definition for the term protozoa, it is often referred to as a unicellular motile.

Protozoa was traditionally considered a subkingdom under Animalia, referring to unicellular animals, with Metazoa referring to multicellular animals.[4][5]


Protozoa commonly range in length between 10 to 52 flagellates (motile with flagella), ciliates (motile with cilia), and amoebas (motile by means of pseudopodia). Flagellates are the most numerous soil protozoa.

Motility and digestion

Tulodens are 2 of the slow-moving form of protozoa. They move around with whip-like tails called flagella (5-10 µm long), hair-like structures called cilia (20-30 µm long), or foot-like structures called pseudopodia (2 µm thick by 20 µm). Others do not move at all. Protozoa may absorb food via their cell membranes, some, e.g., amoebas, surround food and engulf it, and yet others have openings or "mouth pores" into which they sweep food,and that engulfing of food is said to be phagocytosis. All protozoa digest their food in stomach-like compartments called vacuoles.[6]


The pellicle is a thin layer supporting the hydrodynamic. They vary from flexible and elastic to rigid. Although somewhat stiff, the pellicle is also flexible and allows the protist to fit into tighter spaces. In ciliates and Apicomplexa, it is formed from closely packed vesicles called alveoli. In euglenids, it is formed from protein strips arranged spirally along the length of the body. Examples of protists with a pellicle are the euglenoids and the paramecium, a ciliate. In some protozoa, the pellicle consists of many bacteria that adhere to the surface by their fimbriae or "attachment pili".[7] Thus, attachment pili allow the organisms to remain in the broth, from which they take nutrients, while they congregate near air, where the oxygen concentration is greatest.

Ecological role

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. As predators, they prey upon unicellular or filamentous algae, bacteria, and microfungi. Protozoa are both herbivores and consumers in the decomposer link of the food chain. They also control bacteria populations and biomass to some extent. On average, Protozoa eat ~ 100 to 1,000 bacteria per hour. Protozoa such as the malaria parasites (Plasmodium spp.), trypanosomes and leishmania, are also important disease causing agents in humans. Protozoa can stimulate OM decomposition, digest cellulose in rumen of cows and termite guts, and play a role in nutrient mobilization.

Life cycle

Some protozoa have life stages alternating between proliferative stages (e.g., trophozoites) and dormant cysts. As cysts, protozoa can survive harsh conditions, such as exposure to extreme temperatures or 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 a 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. The conversion of a trophozoite to cyst form is known as encystation, while the process of transforming back into a trophozoite is known as excystation. Protozoa can reproduce by binary fission or multiple fission. Some protozoa reproduce sexually, some asexually, while some use a combination, (e.g., Coccidia). An individual protozoan is hermaphroditic.


Protozoa were previously often grouped in the Amoebas, paramecia, and trypanosomes are all examples of animal-like protists.


The classification of protozoa has been and remains a problematic area of taxonomy. Where they are available, DNA sequences are used as the basis for classification but for the majority of described protozoa such material is not available. They have been and still are mostly on the basis of their morphology and for the parasitic species their hosts. Protozoa have been divided traditionally on the basis of their means of locomotion.

As a phylum the Protozoa had been divided into four subphyla[8] reflecting the means of locomotion:

These systems are no longer considered to be valid. For an example of a system of classification of protozoa, see Kudo system.

Human disease

Some protozoa are human parasites, causing diseases. Examples of human diseases caused by protozoa:

Animal disease

The protozoan Ophryocystis elektroscirrha is a parasite of butterflies. It infects the butterfly entering the larval stage. The spores are found on the body of infected butterflies. These spores are passed, from female to caterpillar. Severely infected individuals are weak, unable to expand their wings, or unable to eclose, and have shortened lifespans, but parasite levels vary in populations. This is not the case in laboratory or commercial rearing, where after a few generations, all individuals can be infected.[9]Infection with this parasite creates an effect known as culling whereby infected migrating animals are less likely to complete the migration. This results in populations with lower parasite loads at the end of the migration.[10]

See also


  1. ^ I. Edward Alcamo; Jennifer M. Warner (28 August 2009). Schaum's Outline of Microbiology. McGraw Hill Professional. pp. 144–.  
  2. ^ Nyle C. Brady & Ray R. Weil (2009). Elements of the Nature and Properties of Soils (3rd Edition). Prentice Hall.  
  3. ^ Michelle Gunter (1 January 2008). Passing the North Carolina 8th Grade End of Grade Test of Science. American Book Company, Inc. pp. 196–.  
  4. ^ Kotpal, R. L. (2012). Modern Text Book of Zoology: Invertebrates. Rastogi Publications. pp. 151–.  
  5. ^ Haeckel, Ernst (1874). Anthropogenie oder Entwickelungsgeschichte des menschen (in German). Leipzig. p. 202. 
  6. ^ "Protozoa". MicrobeWorld. American Society for Chemistry. 2006. Archived from the original on 19 May 008. Retrieved 15 June 2008. 
  7. ^
  8. ^ Honigberg, B. M.; W. Balamuth; E. C. Bovee; J. O. Corliss; M. Gojdics; R. P. Hall; R. R. Kudo; N. D. Levine; A. R. Lobblich; J. Weiser (1964). "A Revised Classification of the Phylum Protozoa". Journal of Eukaryotic Microbiology 11 (1): 7–20.  
  9. ^ Leong, K. L. H.; M. A. Yoshimura, H. K. Kaya and H. Williams (1997). "Instar Susceptibility of the Monarch Butterfly (Danaus plexippus) to the Neogregarine Parasite, Ophryocystis elektroscirrha". Journal of Invertebrate Pathology 69 (1): 79–83.  
  10. ^ Bartel, Rebecca; Oberhauser, Karen; De Roode, Jacob; Atizer, Sonya (February 2011). "Monarch butterfly migration and parasite trasmission in eastern North America". Ecology 92 (2): page. 



  • Dogiel, V. A., revised by J. I. Poljanskij and E. M. Chejsin. General Protozoology, 2nd ed., Oxford University Press, 1965.
  • Hausmann, K., N. Hulsmann. Protozoology. Thieme Verlag; New York, 1996.
  • Jahn,T.L.- Bovee, E.C. & Jahn, F.F. How to Know the Protozoa. Wm. C. Brown Publishers, Div. of McGraw Hill, Dubuque, Iowa, 1979; 2nd ed.
  • Kudo, R.R. Protozoology. Springfield, Illinois: C.C. Thomas, 1954; 4th ed.
  • Lee, J.J., Leedale, G.F. & Bradbury, P. An Illustrated Guide to the Protozoa. Lawrence, Kansas, U.S.A: Society of Protozoologists, 2000; 2nd ed.
  • Manwell, R.D. Introduction to Protozoology, second revised edition, Dover Publications Inc., New York, 1968.
  • Patterson, D.J. Free-Living Freshwater Protozoa. A Colour Guide. Manson Publishing; London, 1996.
  • Patterson, D.J., M.A. Burford. A Guide to the Protozoa of Marine Aquaculture Ponds. CSIRO Publishing, 2001.
  • Roger Anderson, O. Comparative protozoology: ecology, physiology, life history. Berlin [etc.]: Springer-Verlag, 1988.
  • Sleigh, M., E. Arnold. The Biology of Protozoa. London, 1981.


  • Levandowski, M., S.H. Hutner (eds). Biochemistry and physiology of protozoa. Volumes 1, 2, and 3. Academic Press: New York, NY, 1979; 2nd ed.
  • Laybourn-Parry J. A Functional Biology of Free-Living Protozoa. Berkeley, California: University of California Press; 1984.
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