Protozoa is a diverse group of unicellular eukaryotic organisms,[1] many of which are motile. Originally, protozoa had been defined as unicellular protists with animal-like behavior, e.g., movement. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behaviour, e.g., photosynthesis.

Characteristics

Protozoa commonly range from 10 to 52 micrometers, but can grow as large as 1 mm, and are seen easily by microscope. The largest protozoa known are the deep-sea dwelling xenophyophores, which can grow up to 20 cm in diameter.

They were considered formerly to be part of the protista family. Protozoa exist throughout aqueous environments and soil, occupying a range of trophic levels.

Motility and digestion

Tulodens are 2 of the slow-moving form of protozoa[citation needed] . They move around with whip-like tails called flagella, hair-like structures called cilia, or foot-like structures called pseudopodia. 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. All protozoa digest their food in stomach-like compartments called vacuoles.[5]

Pellicle

The pellicle is a thin layer supporting the cell membrane in various protozoa, protecting them and allowing them to retain their shape, especially during locomotion, allowing the organism to be more 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. The pellicle consists of many bacteria that adhere to the surface by their attachment pili. 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. Protozoa such as the malaria parasites (Plasmodium spp.), trypanosomes and leishmania, are also important as parasites and symbionts of multicellular animals.

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.

Classification

Protozoa were previously often grouped in the kingdom of Protista, together with the plant-like algae and fungus-like slime molds. As a result of 21st-century systematics, protozoa, along with ciliates, mastigophorans, and apicomplexans, are arranged as animal-like protists. With the possible exception of Myxozoa, protozoa are not categorized as Metazoa.[6] Protozoa are unicellular organisms and are often called the animal-like protists because they subsist entirely on other organisms for food. Most protozoa can move about on their own. Amoebas, Paramecia, and Trypanosomes are all examples of animal-like Protists.

Sub-groups

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 the their morphology and for the parasitic species their hosts.

Protozoa have been divided traditionally[citation needed] on the basis of their means of locomotion.

Flagellates (e.g., Giardia lamblia) Amoeboids (e.g., Entamoeba histolytica) Sporozoans (e.g., Plasmodium knowlesi)

o Apicomplexa o Myxozoa o Microsporidia

Ciliates (e.g., Balantidium coli)

As a phylum the Protozoa had been divided into four subphyla[7] reflecting the means of locomotion:

Subphylum Sarcomastigophora o Superclass Mastigophora (includes flagellates)o Superclass Sarcodinagdffgferclass Opalinata

Subphylum Sporozoa (includes apicomplexans) Subphylum Cnidospora

o Class Myxosporideao Class Microsporidea

Subphylum Ciliophora (includes ciliates)

These classification systems are no longer considered to be valid.

Human disease

Main article: Protozoan infection

Some protozoa are human parasites, causing diseases.

Examples of human diseases caused by protozoa:

Malaria Amoebiasis Giardiasis Toxoplasmosis Cryptosporidiosis Trichomoniasis Chagas disease Leishmaniasis Sleeping Sickness Dysentery

ProtozoaProtozoa are a varied group of single-celled animal-like organisms belonging to the kingdom Protista. More than 50,000 different types of protozoa have been described. Their name comes from two Greek words, protos, or "first," and zoön, or "animal." The vast majority of protozoa are microscopic, many measuring less than 1/200 millimeter. The largest, however, may reach 3 millimeters (0.1 inch) in length, large enough to be seen with the naked eye. Scientists have even discovered some fossil specimens that measured 20 millimeters (0.8 inch) in diameter.

Whatever their size, protozoa are well known for their diversity and the fact that they have evolved under so many different conditions. One of the basic requirements of all protozoa is the presence of water. Within this limitation, they may live in the sea; in rivers, lakes, or stagnant ponds of freshwater; in the soil; and even in decaying matter. Many are solitary organisms, but some live in groups. Some are free-living, while others are attached to other organisms. Some species are parasites of plants and animals, ranging from other protozoa to humans. Many protozoa form complex, exquisite shapes, although their beauty may be overlooked because of their very small size.

Life patterns

The vast majority of protozoa are heterotrophic. That is, they cannot manufacture their own food, but must obtain it by eating other organisms. A few protozoa contain the green pigment chlorophyll, which allows them to make their own food.

All protozoa reproduce asexually, by dividing into two parts at regular intervals. Some species, however, have evolved the ability to reproduce sexually also.

Words to Know

Chlorophyll: A green pigment that makes possible the conversion of carbon dioxide and water to complex carbohydrates.

Cilia: Tiny hairlike projections on the surface of a cell.

Cyst: A stage in a protozoan's life when it is covered by a tough outer shell and has become dormant.

Cytoplasm: The semifluid substance of a cell containing organelles and enclosed by the cell membrane.

Dormant phase: A period in which an organism is inactive.

Flagella: Whiplike structures used by some organisms for movement.

Heterotrophs: Organisms that cannot make their own food and that must, therefore, obtain their food from other organisms.

Parasitism: A situation in which one organism lives in connection with and at the expense of a second organism.

Protists: Members of the Kingdom Protista; primarily single-celled organisms that are not plants or animals.

Pseudopods: Extensions of an organisms cytoplasm used for movement and capturing food.

Symbiosis: A pattern in which two or more organism's live in close connection with each other, often to the benefit of both or all organisms.

Protozoa have evolved mechanisms that allow them to live under a great range of environmental conditions. When these conditions are unfavorable, most species are able to enter an inactive, or dormant, phase. They secrete a thick protective outer wall that prevents them from losing water and protects the cell from extreme temperatures. This tough little package, called a cyst, may also serve as a means of dispersal (to spread widely). Cysts are carried away on the wind or on the feet of animals. Once a cyst reaches a more favorable environment, its outer wall breaks down and the cell resumes normal activity.

Types of protozoa

Protozoa are classified according to the ways in which they move about. One phylum, the Sarcodina, moves by pushing out portions of their cytoplasm forming pseudopods, or "false feet." They capture their food by extending their pseudopods around it, engulfing it, and digesting it. Probably the best known example of the Sarcodina is the amoeba.

Members of the phylum Ciliophora get their name from tiny hair-like projections known as cilia on the surface of the cell. These protozoa swim around by waving their cilia back and forth, like the oars on a boat. Cilia are also used to obtain food. As they beat back and forth, the cilia create a whirlpoollike effect that brings food close enough for the organism to ingest. A common example of the ciliates is the paramecium.

The phylum Mastigophora consists of one-celled organisms that move about by means of flagella. Flagella are whiplike structures somewhat similar to cilia. The major difference between the two structures is that flagella are much larger than cilia. Also, flagellates have anywhere from one to several hundred flagella, while cilia never occur individually. The majority of flagellates live inside other organisms in either a symbiotic (mutually beneficial) or parasitic relationship. A well-known example of the flagellates is the organism that causes African sleeping sickness, the trypanosoma.

Members of the phylum Sporozoa have no means of movement. Instead, they form sporelike structures and attach themselves to other organisms. They are parasitic and depend on their hosts for all their food and survival. The best known of all Sporozoa are probably members of the genus Plasmodium. These organisms are responsible for malaria, a disease that is transmitted by the Anopheles mosquito.

protozoan (prō´təzō´ən), informal term for the unicellular heterotrophs of the kingdom Protista. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues. Formerly classified in the animal kingdom, they are now generally divided into five protist phyla: Mastigophora (the

flagellates), Sarcodina (the amebas), Ciliophora (the ciliates), Opalinida, and Sporozoa. Most are motile, and most ingest food, as do animals, rather than produce it themselves, as do plants. The 26,000 living species are cosmopolitan in distribution; they are found in freshwater and at all depths in the ocean; some live in soil. Some are parasites in the bodies of humans or other animals, sometimes causing diseases.

DigestionSome protozoans have complex digestive systems and feed on large food particles, such as other microorganisms. The food is digested by means of enzymes and the wastes transported to the cell surface or stored in vacuoles (bubblelike spaces in the cytoplasm). Others have no digestive system and absorb dissolved organic matter through the cell membrane.RespirationRespiration is accomplished by the diffusion of dissolved gases through the cell membrane. Oxygen diffuses into the cell, where it oxidizes food molecules, producing energy and the organic molecules used for the building and maintenance of the cell. Carbon dioxide and water, the waste products of this oxidation, diffuse out of the cell.ReproductionReproduction is usually asexual, occurring mostly by cell division, or binary fission; some forms reproduce asexually by budding or by the formation of spores (reproductive cells that give rise to a new organism without fertilization). In certain groups sexual reproduction sometimes also occurs. In these instances, cell division is preceded by the fusion of two individuals or, in ciliates, by conjugation and exchange of nuclear material.

Any memeber of the phylum Protozoa, which comprises unicellular organisms found worldwide in marine or freshwater, free-living and as parasites. These microscopic animals have the ability to move (by cilia or pseudopodia) and have a nucleus, cytoplasm and cell wall; some contain chlorophyll. Reproduction is by fission or encystment. Length: 0.3mm (0.1in). The 30,000 species divide into four classes: Flagellata, Cnidospora, Ciliophora, and Sporozoa.

Leishmania donovani, (a species of protozoa) in a bone marrow cell

A Plasmodium sporozoite traverses the cytoplasm of a mosquito midgut epithelial cell in this false-colour electron micrograph.