Protozoans (Protozoa) are unicellular and heterotrophic organisms that feed on organic matter like other microorganisms or organic tissues or debris.

They belong to the protist group and are typically microscopic. Traditionally, the protozoa were regarded as single-celled animals due to their animal-like behaviors such as predation or motility, and a lack of cell was, as seen in plants and algae.

However, the practice of grouping protozoa with animals is no longer deemed valid, although the term continues to be used as a way of identifying unicellular organisms that can move independently and feed by heterotrophy.

Protozoa are eukaryotes, meaning that their nucleus is enclosed within a membrane. They do not possess filaments like organisms such as molds that have filaments called hyphae.

Protozoa can be found in moist and aquatic environments from the north pole to the south pole. Many protozoa are in a symbiotic relationship with other organisms, while others are parasites.

Protozoa are considered to be a subkingdom of the kingdom Protista, although the classical taxonomy placed them in the kingdom Animalia. Protozoa were first observed by Anton van Leeuwenhoek, using microscopes he constructed with simple lenses.

All humans have at some point in their lifetime have had protozoa living in or on their bodies, and many persons will be infected with one or more species of the organism throughout their life.

Some species are considered commensals – existing in a feeding relationship, which is beneficial and usually is not harmful to the host, whereas others are pathogens and often produce disease. Protozoan diseases range from very mild to severe and life-threatening.

People whose immune system is able to control but not eliminate a parasitic infection become carriers and constitute a source of infection for others.

In regions of high prevalence, well-tolerated infections are often not treated to eradicate the parasite because eradication would lower the individual’s immunity to the parasite and result in a high likelihood of reinfection.

Structure and Anatomy of Protozoa

Due to being eukaryotes, protozoa are larger cells compared to prokaryotes. They occur between 10 and 100 micrometers in diameter with a more complex structure.

They have a cell membrane that encloses the organelles and DNA that is also bound by a membrane. They also possess ribosomes, nucleoli, Golgi bodies, and multiple chromosomes.

There are also some of the organelles that are specific to protozoa; these include:

  • Contractile vacuoles
  • Trichocysts of Paramecium
  • Certain skeletal structures

Protozoa also have locomotory structures like flagella, pseudopodia, and cilia, which enable movement. The plasma membrane also surrounds these structures.

On the other hand, the pellicle found on some protozoa like Giardia is able to maintain sufficient rigidity to support and sustain a distinctive shape, which allows for twisting and bending while moving.

Given examples of protozoa that exhibit this characteristic include species of trypanosomes and Giardia.  Many parasitic protozoa found in humans are less than 50 μm in size.

The smallest is about 1 to 10 μm long. Some protozoa possess a cytosome or cell “mouth” for ingesting solid particles and fluids. The contractile vacuoles, organelles used for osmoregulation, occur in some protozoa such as Balantidium and Naegleria.


Based on Locomotion

Protozoa can be classified into sub-phyla based on their locomotive organelles. These are:


The organelles for movement in this sub-phylum may be flagella, pseudopodia, or none at all. Some of the classes that fall under this sub-phyla include

  • Mastigophora which possesses one or more flagella for locomotion. Examples include Giardia and Trypanosomes
  • Sarcodina which uses pseudopodia for movement and catching food. Examples include Amoeba
  • Sporozoa which lack locomotive structures. Examples include Plasmodium and Coccidian.


The protozoa in this sub-phyla use hair-like organelles called cilia or sucking tentacles in some stages or throughout their life span. Typical examples include Paramecia, Vorticella, and Coleps.


The locomotive structures present in this sub-phylum include pseudopodia or flagella. Here, the nuclei are also of one kind (monomorphic). Superclass Mastigophora, which falls under this sub-phyla are flagellates and thus use flagella for locomotion.

Based on Nutrition


Autotrophic protozoa synthesize carbohydrates from carbon dioxide and water using chlorophyll. Green chloroplasts present in these protozoan synthesizes Light energy from the sun.

Some photoautotrophic flagellates such as members of Cryptomonadida Euglenida and Volvocida also tend to combine heterotrophy with autotrophy. For this reason, they are often described as acetate flagellates. A given example is the Euglena viridis

Some of their sources of carbon include simple fatty acids, acetates as well as alcohols. While they are autotrophs in sunlight, these flagellates switch to heterotrophs in the dark.


Most of the free-living protozoa are classified under this category. They depend on a wide range of diets. Some protozoa, known as microbivores, feed on bacteria, while others feed on algae and are described as herbivores. The carnivorous protozoa feed on both of the microbivores and herbivores.

Independent living protozoa are also divided into two groups based on their morphology. Some possess a mouth or cytostome for the ingestion of food, while others lack a mouth or a definite point of entry and depend on the entire body to engulf their food before they can be digested. A given example of this is amoeba.

Life cycle

Protozoan typically passes through several stages that differ in structure, complexity, and activity during its life cycle. Trophozoite – which means “the animal that feeds” – is a general term for the active, feeding, and reproductive stage of most protozoa.

In parasitic protozoa, this is the stage usually associated with pathogenesis. A cyst is the dormant stage in the protozoan life cycles.

Some protozoa form cysts that contain one or more infective forms. Multiplication of these microorganisms occurs when the cysts burst and release many active protozoa. A given example is the trophozoite of Entamoeba histolytica, which first forms a cyst.

As the cyst matures nuclear division produces four nuclei via binary fusion, and during excystation (the escape of the parasite from the cysts), four uninucleate metacystic amebas appear.

Similarly, a freshly encysted Giardia lamblia has the same number of internal structures (organelles) in its active form. However, as the cyst matures the organelles multiply, two trophozoites are formed.

Cysts passed in the fecal matter have a protective wall that enables the parasite to survive harsh conditions in the outside environment for a period ranging from days to a year, depending on the species and existing environmental conditions.

Economic Importance of Protozoa

Protozoans play essential roles in the fertility of the soil. They help regulate microbial populations and enhance the rate at which bacteria decompose dead organic matter.

Protozoans also excrete phosphorus and nitrogen, essential elements needed for plant growth, as products of metabolism.

Protozoans are also vital in the treatment of wastewater in both activated sludge and filter plants. The liquid remaining after the removal of solid wastes is aerated and oxidized by aerobic ciliates that consume the organic wastes.

Protozoans probably play a similar role in contaminated natural ecosystems. During the bioremediation of contaminated environments, they feed on oil-degrading bacteria, increasing bacterial growth in the same way that they enhance the rates of decomposition in soils, thereby speeding up the breakdown of oil spillages.

Diseases caused by protozoa

Parasitic protozoa are known causal agents for several diseases. For example, protozoa from the genus, trypanosome cause a number of important diseases in humans.

Trypanosome brucei gambiense and Trypanosome brucei rhodesiense are the pathogens that cause African Sleeping Sickness, a disease that is ubiquitous in equatorial Africa.

The life cycle of this protozoa involves two hosts: the bloodsucking tsetse fly and human (or another mammal). The tsetse fly is the vector that transmits the parasite from one mammal to another.

Trypanosomes live in the blood plasma and the central nervous system of humans and have evolved ways of deceiving the immune system of the host.

Another significant disease caused by protozoa is Malaria. This disease caused by the protozoan Plasmodium remains a serious disease that affects developing countries, despite the availability of antimalarial drugs, and the measures taken to control and eradicate its vector, the mosquito.

The problems associated with the control of Malaria include the development of resistance of insecticide by mosquitoes and drug resistance by plasmodium.

Prophylactic medication taken before and during visits to areas where malaria is endemic may prevent a person with no resistance from being infected.

Other diseases caused by protozoa include:

  • Chagas disease
  • Babesiosis
  • Toxoplasmosis
  • Cyclosporiasis
  • Trichomoniasis
  • Giardiasis
  • Dientamoebiasis
  • Balantidiasis
  • Cryptosporidiosis