Protozoa are single-celled eukaryotes belonging to the kingdom Protista.

They exist either as free-living organisms or as parasites.  More than 50,000 species, ranging in size from 2 to 70 microns (0.002 to 0.07 mm), are known.

They have a relatively complex internal structure and carry out complex metabolic activities.

The organelles of protozoa have functions like the organs of higher animals.

Some protozoa have specialized structures that help them in movements and propulsion. Cilia, flagella, and pseudopodia are examples of such structures.

General Characteristics of Phylum Protozoa

  • Kingdom: Protista
  • Habitat: mostly aquatic, either free living or parasitic
  • Grade of organization: protoplasmic grade of organization. A single cell performs all the vital activities. They are known as acellular or non-cellular organism as their body consists of only the mass of protoplasm.
  • Cover: body of protozoa is either naked or covered by a pellicle or chitin.
  • Locomotion: locomotory organ are pseudopodia, flagella, or cilia.
  • Nutrition: nutrition are holophytic, holozoic, saprophytic, or parasitic.
  • Digestion: digestion is intracellular and occurs in food vacuoles.
  • Respiration: respiration occurs through the body surface.
  • Osmoregulation: contractile vacuoles helps in osmoregulation (Figure 2).
  • Reproduction: asexual reproduction by binary fission or budding and sexual reproduction by syngamy or conjugation.

Classification of Phylum Protozoa

In 1985, the Society of Protozoologists published a taxonomic scheme that classified the Protozoa into four major classes shown below:

  1. Rhizopda
  2. Flagellata/Mastigophora
  3. Sporozoa
  4. Ciliata

Table comparing features of four classes of Protozoa

Features Rhizopoda Flagellata Sporozoa Ciliata
Locomotary organ Pseudopodia Flagella Absent Cilia
Habitat Mostly free living, some are parasitic Free living or parasite Exclusively endoparasites Free living or parasite
Reproduction Asexual by binary fission and sexual by syngamy Sexual reproduction by longitudinal fission Asexual reproduction by fission and sexual reproduction by spores Asexual reproduction by binary fission. sexual reproduction by conjugation
Body cover Absent Body covered with cellulose, chitin or silica Body covered with pellicle Body covered with pellicle
Examples Amoeba (Figure 2), Entamoeba Giardia, Euglena, Trypanosoma Plasmodium, Monocystis Paramecium (Figure 3), Voricella, Blantidium
Anatomy of Amoeba
Figure 2. Anatomy of Amoeba
Paramecium (Ciliata)
Figure 3. Anatomy of Paramecium (a Ciliata)


Protozoa are ubiquitous. They live in a wide variety of habitats including fresh water, marine habitat, and soil.

Their cysts can be found in even the most inhospitable parts of the biosphere. Because the group contains many unrelated or loosely related organisms, varieties in structure and form exists.

Most are free-living and eat bacteria, algae, or other protozoa. They are important components of aquatic and soil ecosystems.

For example, bacterivorous protozoa are abundant in activated sludge sewage treatment plants, whereas several protozoa such as Escherichia Coli (Figure 4), Entamoeba histolytica, Giardia lamblia are of medical and economic importance.

Escherichia coli bacterium (E.coli) gram-negative rod-shaped bacteria, part of intestinal normal flora and causative agent of diarrhea and inflammation
Figure 4. Escherichia coli bacterium (E.coli) gram-negative rod-shaped bacteria, part of intestinal normal flora and causative agent of diarrhea and inflammation

Life cycle

During its life cycle, a protozoan generally passes through several stages that differ in structure and activity.

Trophozoite is a general term given to the active, feeding, multiplying stage of a protozoan. In parasitic species, the trophozoites are associated with the pathogenesis.

In hemoflagellates (a kind of protozoa), the terms amastigote, promastigote, epimastigote, and trypomastigote represent trophozoite stages.

This stage differ in the absence or presence of a flagellum and in the position of the kinetoplast associated with the flagellum.

Some protozoa form cysts that contain one or more infective forms. Oocysts are stages resulting from sexual reproduction in some protozoa, while the oocysts of Plasmodium.

They develop in the body cavity of the mosquito vector (Figure 5).

Some protozoa have complex life cycles requiring two different host species, while others require only a single host to complete their life cycle.

Life cycle of Plasmodium Falciparum (Malaria)
Figure 5. Life cycle of Plasmodium Falciparum


Protozoa take a variety of nutrition:  holozoic (like animal), holophytic (like plant), saprophytic, or parasitic.

Amoeba engulf particulate food or droplets through mouth, perform digestion and absorption in a food vacuole, and eject the waste substances.

In many protozoa, ingested food passes through the cytosome or micropore and then gets enclosed in the food vacuoles.

Pinocytosis is a method of ingesting nutrient materials, whereby fluid is drawn through small, temporary openings present in the body wall of the organism.

Protozoa are also classified based on nutrition into three categories listed below:

  • Autotrophs
  • Heterotrophs
  • Chemoheterotrophs


Autotrophs, like some of the flagellates, synthesize carbohydrates from carbon dioxide and water using chlorophyll in presence of sunlight.

Most of the photoautotrophic flagellates (like Euglenida, Volvocida) tend to combine autotrophy with heterotrophy. Their source of  carbon are acetates, simple fatty acids, or alcohols.

While they are autotrophs in the light, these flagellates switch to heterotrophs in the dark.


Most of free living protozoa are heterotrophs. They depend on a wide range of food. Some feed on bacteria (microbivores), whereas others feed on algae.

The carnivorous forms feed on both herbivores and microbivores.


This group includes those that require energy and organic carbon sources.


Binary fission is the most common form of an asexual reproduction in protozoa. In binary fission, the organelles of the organism are duplicated and then the organism divides into two complete organisms.

Multiple Fission, plasmotomy, and budding are the other forms of asexual reproduction.

Reproduction in the protozoa may be asexual, as in the amoebas and flagellates (infect humans), or both asexual and sexual as in apicomplexa.

Sexual reproduction in Protozoa is primarily by:

  • syngamy (e.g., Chlamydomonas, Copromonas),
  • conjugation (e.g., Paramecium ( Figure 6), Vorti­cella), and
  • automixis (e.g., Actinosphaerium, Actinophrys).
  • plasmogamy (e.g., Rhizopoda and Mycetozoa),
  • regeneration, and
  • parthanogenesis ( e.g. Actinophrys, Chlamydomonas).

Division is longitudinal in flagellates, transverse in ciliates and endodyogeny is a form of asexual division seen in Toxoplasma.

Conjugation in paramecium under light microscopy
Figure 6. Conjugation in paramecium under light microscopy