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The Protozoa is a subkingdom of the kingdom Protista. However, in the classical system, they were placed in the kingdom Animalia.

Protozoa are unicellular eukaryotic microorganisms having a membrane-bound nucleus and other organelles within their cells. Despite their small size, they are incredibly diverse and exist in various aquatic and terrestrial environments worldwide. More than 50,000 species are known, most of which are free-living organisms.

Some common examples of protozoa are Amoeba proteus, Paramecium caudatum, Euglena gracilis, Plasmodium falciparum, and Trypanosoma brucei.


Classification of Protozoa

The modern classification of protozoa is based on the modified form proposed by Levine et al. in 1980. It is divided into 7 phyla, among which Sarcomastigophora, Apicomplexa, Microspora, and Ciliophora contain parasitic representatives. Two phyla—the Sarcomastigophora and the Apicomplexa- contain the most important species causing human diseases.


The phylum Sarcomastigophora includes a diverse group of protozoa primarily characterized by having flagella or pseudopods (amoeboid movement) as their modes of locomotion. With advancements in taxonomy, some of these organisms are reclassified into separate phyla.


  • Trypanosoma causing African sleeping sickness in humans and Chagas disease in animals
  • Giardia lamblia causing giardiasis in humans


In this phylum, obligate intracellular parasites reside, and most of its members are characterized by a specialized organelle called the apical complex. The apical complex aids in host cell invasion and is essential for the parasitic lifestyle of these protozoa. Many apicomplexans have complex life cycles involving multiple hosts.


  • Plasmodium causing malaria in humans
  • Toxoplasma gondii causing Toxoplasmosis in humans


The phylum Microspora consists of intracellular parasites that infect various animal hosts, including insects, crustaceans, and mammals. These protozoa have unique spore-like structures called microsporidia, which are used to infect host cells and spread to other individuals.


  • Nosema apis causing colony collapse disorder in honeybees
  • Encephalitozoon cuniculi found in rabbits and other mammals cause severe infections in the brain and other organs


The phylum Ciliophora comprises protozoa with hair-like structures called cilia, which they use for movement, feeding, and sensory functions. Ciliates are widespread in aquatic environments and are essential in nutrient cycling and aquatic food webs.


  • Paramecium caudatum
  • Stentor

Characteristics of Protozoa

Cell Size and Structure

The size and shape of Protozoa vary greatly, from 1µm to 50 µm. However, the largest, Balantidium coli, may measure 150 μm.

Protozoa are unicellular, having a eukaryotic cell structure with a well-developed nucleus and membrane-bound cell organelles such as mitochondria, Golgi bodies, and lysosomes.

The plasma membrane encloses the cytoplasm that is differentiated into outer ectoplasm and inner endoplasm. A central vacuole is present for osmoregulation that removes excess water. Some cells have a unique structure called cytostome for ingesting food.

How Do they Move

Most protozoa are motile, performing locomotion with the help of flagella, cilia, or pseudopodia.


Protozoa are found in a wide range of environments, from freshwater ponds and oceans to damp soil and even inside the digestive tracts of animals. Some even live in terrestrial habitats, such as soil, leaf litter, and decaying organic matter. Protozoa can also establish a symbiotic relationship with their host or exist as parasites, where they live at the expense of their host.

Feeding and Nutrition

Like all animals, protozoa cannot produce their own food and thus are heterotrophs. The feeding and nutrition strategies of protozoa are as diverse as their habitats. These remarkable microorganisms exhibit various feeding behaviors, allowing them to thrive in different environments. Some of the typical nutritional feeding strategies in protozoa are:

1. Phagotrophy

Phagotrophy is a feeding strategy in which protozoa ingest solid food particles, such as bacteria, algae, and other tiny organisms, by surrounding them with their cell membrane and forming a food vacuole. Once inside the vacuole, digestive enzymes break down the food particle, and the nutrients are absorbed into the cytoplasm.

Amoebas, such as Amoeba proteus and ciliates, like Paramecium caudatum perform phagotrophy.

2. Autotrophy

Certain protozoa have evolved the ability to perform photosynthesis, similar to plants and algae. They contain chloroplasts, specialized organelles that capture light energy and convert it to chemical energy (glucose) through photosynthesis.

Euglena gracilis perform photosynthesis.

3. Osmotrophy

Osmotrophy is a feeding strategy in which protozoa directly absorb dissolved organic matter and nutrients through their cell membrane. Instead of actively capturing solid particles, osmotrophs rely on the natural movement of dissolved substances.

Certain flagellates, like Euglena gracilis, can perform osmotrophy in addition to photosynthesis.

4. Mixotrophy

Mixotrophy combines phagotrophy and autotrophy (photosynthesis). Protozoa that exhibit mixotrophy can switch between consuming organic particles and photosynthesizing depending on the availability of light and nutrients.

Euglena gracilis perform mixotrophy.

5. Predation and Parasitism

Some protozoa are predators or parasites, obtaining nutrients by consuming other living organisms.

Like the ciliates, predatory protozoans actively hunt and consume bacteria, algae, and other small protists as their primary food source. In contrast, parasitic protozoans like Plasmodium species depend on host organisms for nutrition. They invade host cells, consuming nutrients from the host’s resources and causing harm.


Protozoa have evolved adaptations enabling them to thrive in their respective habitats. They are:

Encystment: Protozoa can form protective structures called cysts when they encounter adverse conditions such as desiccation (drying out), extreme temperatures, or a lack of nutrients.

Resistance to Environmental Stress: Many protozoa have developed mechanisms to resist environmental stress. Some species can tolerate high salinity, acidity, or pollution, allowing them to inhabit extreme environments inhospitable to other organisms.

Avoidance Responses: Certain protozoa have developed avoidance responses to harmful stimuli. For example, they can move away from areas of high toxicity or unfavorable conditions, allowing them to seek more suitable habitats for survival.

Various Forms of Locomotion: Protozoa have evolved different modes of locomotion, such as flagella, cilia, and pseudopods, enabling them to move efficiently through their environments in search of food and suitable conditions.

Rapid Reproduction: Many protozoa have short life cycles and can reproduce rapidly. It allows them to exploit favorable conditions and quickly increase their population. This rapid reproduction rate ensures their presence in environments that may experience resource fluctuations.

Predation and Defense Mechanisms: They have developed various predation and defense mechanisms to capture prey and evade predators. For example, some ciliates possess specialized structures called trichocysts that can discharge long, slender threads when threatened, which can deter predators.

Symbiotic Relationships: Protozoa can form symbiotic relationships with other organisms, providing them a survival advantage. In mutualistic relationships, they receive nutrients or protection from their host organisms. In contrast, commensal relationships can provide them with suitable habitats and access to resources without causing harm to the host.


Reproduction in protozoa can occur through both asexual and sexual processes, allowing them to adapt to changing environments.

1. Asexual Reproduction

It is the most common mode of reproduction in protozoa. In this process, a single parent cell divides to produce two or more genetically identical daughter cells. Asexual reproduction causes rapid population growth and is particularly advantageous when favorable environmental conditions.

The common forms of asexual reproduction are binary fission and multiple fission.

 2. Sexual Reproduction

Not all protozoa reproduce asexually. Few reproduce by sexual means. Sexual reproduction in protozoa involves the exchange of genetic material between individuals, resulting in genetically diverse offspring. This diversity is beneficial for adapting to changing environments and can lead to the emergence of new traits.

Conjugation and syngamy are two forms of sexual reproduction in protozoa.


Most protozoa’s life cycle alternates between the vegetative stage, the trophozoite, and the dormant cyst stage. During the trophozoite stage, the parasites actively divide, forming more cells. Some protozoa develop protective structures called cysts during unfavorable conditions or environmental stress. Once favorable conditions prevail, the cyst breaks open, and the protozoan emerges, resuming its normal life cycle.

Diseases Caused by Protozoa

Parasitic protozoa are associated with several diseases in humans and animals. Some of the common illnesses caused by protozoans are:

DiseaseCaused By
1.MalariaPlasmodium falciparumPlasmodium vivaxPlasmodium malariaePlasmodium ovale
2. Amoebiasis (Amoebic dysenter)Entamoeba histolytica
3. Trypanosomiasis (African Sleeping sickness)Trypanosoma brucei gambiense Trypanosoma brucei rhodesiense
4. TrichomoniasisTrichomoniasis vaginalis
5. ToxoplasmosisToxoplasma gondii
6. BalantidiasisBalantidium coli
7. GiardiasisGiardia lamblia or duodenalis
8. Leishmaniasis or Kala-azarLeishmania donovani

Bacteria vs. Protozoa

Bacteria are prokaryotic, single-celled organisms lacking membrane-bound cell organelles and a true nucleus. In contrast, protozoa are eukaryotic, single-celled organisms having membrane-bound cell organelles and a true nucleus.

Fungi vs. Protozoa

Fungi are multicellular eukaryotic organisms belonging to the Kingdom – Fungi. In contrast, protozoa are single-celled organisms belonging to the kingdom Protista.

Algae vs. Protozoa

The main difference between protozoa and algae is that protozoa are heterotrophic, animal-like organisms, whereas algae are autotrophic, plant-like organisms.


Q.1. Do protozoa have cell walls?

Ans. No, like all animal cells, protozoa lack cell walls.

Article was last reviewed on Thursday, August 3, 2023

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