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Protozoans – Structure, Classification, Characteristics, Examples

Last Updated : 24 Jul, 2022
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A two-kingdom classification system, the Plantae and Animalia kingdoms, was created during Linnaeus’ time and comprised all plants and animals, respectively. This system was unable to differentiate between the prokaryotes and eukaryotes, the single-celled and multiple-celled organisms both non-photosynthetic (such as green algae) and photosynthetic (fungi) organisms. It was simple to divide organisms into plants and animals. done and was simple to comprehend, yet there were numerous organisms. neither of the two categories. Hence the division into two kingdoms. the long-used system was proven to be insufficient. Additionally, gross morphology.

It was also considered that additional properties, such as cell structure, the type of wall, the habitat, the mechanisms of reproduction, relationships in evolution, etc. systems of classification for living things, Consequently, organisms have changed significantly over time. In 1969, R.H. Whittaker proposed a classification of five kingdoms. He designated five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. Cell type is one of the key classification factors he uses. structure, physical make-up, dietary practices, reproductive methods, and phylogenetic connections.



Protozoa are heterotrophic, eukaryotic, unicellular creatures. Either they live freely or they are parasites. Protozoan species are divided into approximately 65000 distinct categories. Their cell wall is absent. Numerous different cell organelles carry out the diverse functions carried out by various organs in higher animals.


Protozoa are eukaryotes with only one cell. The nucleus is surrounded by a membrane, as it is in all eukaryotes. Other than ciliates, all protozoa have vesicular nuclei, which are characterized by dispersed chromatin that gives the nucleus a diffuse appearance. One variety of vesicular nuclei has an endosome or karyosome, which is a somewhat central body. In trypanosomes and parasitic amebas, the endosome is devoid of DNA. On the other side, the vacuolar nuclei of the phylum Apicomplexa contain single or even more nucleoli that carry DNA. The macronucleus and micronucleus of the ciliates are both present, and their composition seems to be relatively uniform.

The functions of protozoa’s organelles are comparable to those of higher animals’ organs. Along with covering the cytoplasm, the plasma membrane also covers the protruding locomotory elements like flagella, cilia, and pseudopodia. Some protozoa have an exterior surface layer called a pellicle that is sufficiently hard to maintain a unique shape, such as trypanosomes and Giardia. Therefore, while migrating across their surroundings, these organisms may easily bend and twist. The cytoplasm of most protozoa is divided into two layers: the ectoplasm, which is the exterior, transparent layer, and the endoplasm, which is the innermost lining containing organelles. Species with protruding pseudopodia, such as amebas, make it easiest to observe the cytoplasm’s structure. Several protozoa have such a cytosome or cellular “mouth” where they can consume liquids or solids.

For osmoregulation, some organisms, like Naegleria and Balantidium, have contractile vacuoles. Subpellicular microtubules are found in many protozoa; which lack external organelles for motility, these allow for sluggish movement. The body wall and a flagellum of the trypanosomes are separated by a recognizable undulating membrane. The Golgi apparatus, lysosomes, feeding vesicles, and other specialized structures are among the numerous additional structures seen in parasitic protozoa. In order to see the intricate features of the protozoal structure, electron microscopy is necessary.


Protozoa would be present in aquatic environments, which is their habitat. They are marine or freshwater creatures. Some are parasitic on plants and animals, while others live freely. Although they are mostly aerobic, some of them can also be found in the human intestine or rumen.

  • Hot springs, for example, are home to some of the species. To survive in arid settings, some of them produce resting cysts.
  • Size and Shape: Protozoa come in a wide range of sizes and shapes, from microscopic (1 mm) to large enough to be seen with the unaided eye. A unicellular foraminifera’s shell can be 20 cm in diameter.
  • Protozoans would be flexible and come in a variety of shapes because they don’t have a solid cell wall. It has a membrane that would be thin and encloses each individual cell. On their outside, some would possess a shell that would be hard. It also has a hard pellicle it may be hard or kind of flexible, supports the cell in various protozoans, most notably in ciliates, gives the organisms a distinct form, and aids in motility.
  • They have a single eukaryotic cell and a unicellular cellular structure. Specific internal structures carry out the metabolic processes.
  • They typically have a single cell with a membrane-bound nucleus.
  • Due to the dispersed chromatin, the nucleus appears diffused. The vesicular nucleus possesses an endosome or nucleoli as its primary body. Apicomplexan nucleoli contain DNA, whereas amoeboids’ endosomes do not.
  • Micronuclei and macronuclei are seen in ciliates.
  • In the cytoplasm also the flagella, pseudopodia, and cilia are enclosed by the plasma membrane.
  • The pellicle, which is a membranous envelope seen in some genera, gives the cell a distinct form. Epibiotic bacteria adhere to the pellicle of some protozoans by their fimbriae.
  • Cystames are used by some protozoa to consume food. Food that has been consumed is contained in food vacuoles. Ciliates have an externally accessible body chamber called a gullet. The water loaded with food is directed into the gullet by the rhythmic movements of rows of cilia.
  • For osmoregulation, which eliminates excess water, the vacuole which would be in the center is necessary.
  • There are numerous membrane-bound cell organelles, including lysosomes, Golgi bodies, mitochondria, and other specialized structures.
  • Protozoa have holozoic feeding and are heterotrophic. They phagocytose their food before eating it. The cytostome is a specialized structure used for phagocytosis by some protozoan species.
  • Most would have the flagella, cilia, or pseudopodia for locomotion. Despite having no locomotory structure, Sporozoa have subpellicular microtubules that aid in their slow locomotion.

Life Cycle

  • Most protozoa rotate between a latent cyst stage and a vegetative stage that is actively growing, such as trophozoites.
  • The cyst stage is resilient and can endure extreme conditions without food or water. It can get transmitted and survive outside the host for a longer time.
  • They eat and proliferate during the contagious trophozoite stage.


  • They often reproduce asexually. They can divide into binary, longitudinal, transverse, or budding fission.
  • There is sexual reproduction in some of the species. Conjugation, syngamy, or the generation of gametocytes are all methods of sexual reproduction.


A phylum known as protozoa contains monocellular heterotrophs. It belongs to the kingdom of protozoa. Based on their anatomy and the parts of their bodies that are engaged in motility, protozoa are categorized into four main groups:

Flagellated protozoans, also known as Mastigophora,

  • Either parasitic or free-living.
  • To move, they have flagella.
  • They have a pellicle covering their body.
  • Forms seen in freshwater have a contractile vacuole.
  • Trypanosoma, Giardia, etc. are a few examples of organisms that reproduce by binary fission (longitudinal division).


  • They are Sarcodina or amoeboids and can be found in freshwater, the sea, or damp soil.
  • Pseudopodia are used to move. They use pseudopodia to catch their prey.
  • There is no clear shape and no pellicle.
  • Here amoeboids would live in the water which is fresh and have contractile vacuoles.
  • Cyst formation and binary fission are the means of reproduction.
  • Entamoeba, for instance, or amoeba.


  • Also known as Sporozoans, are endoparasitic.
  • They lack a specially designed organ for locomotion.
  • There is a pellicle and its subpellicular microtubules aid in motility.
  • Sporozoites are formed during reproduction.
  • Myxidium, Globidium, etc. are some examples


  • They are aquatic protozoans known as Ciliophora or ciliated protozoans, and their cilia assist them to move actively.
  • Because of the pellicle’s coating, they have a set shape.
  • In the subclass Suctoria, for example, they may have tentacles.
  • There are contractile vacuoles.
  • Trichocysts are a type of defensive organ found in several species.
  • They are propelled by cilia, so the movement would be of the cilia aids in bringing food into the gullet.
  • They create cysts in addition to transverse division reproduction.
  • Examples include Balantidium, Vorticella, and Paramecium.

Diseases caused by the protozoans in the case of Humans

Protozoa are thought to be responsible for the majority of human protest diseases. When protozoa transform into human parasites, they cause disease in people. Most common and deadly illnesses in humans, including malaria, amoebic dysentery, and African sleeping sickness, are brought on by protozoan infections.

These can grow in numbers inside of humans, aiding in their survival and allowing deadly diseases to originate from a single entity merely. Protozoa discovered in human intestines are typically transmitted between humans by the fecal-oral route, such as contaminated water, food, or direct touch. An arthropod vector transmits the protozoa found in human tissue or blood to other humans.

Frequently Asked Questions

Question 1: Importance of osmoregulation in protozoans?


Osmoregulation inwhich the body’s water balance is preserved. Animal osmoregulation in protozoans is conducted by contractile vacuoles. The contractile vacuole is missing in marine speciesand also the parasitic protozoans. Osmoregulation in the case of amoebasis accomplished by contractile vacuoles.

Question 2: How does the sporozoan aid in motility?


Sporozoans would be endoparasites and they would have a specialized layer called pellicle also the microtubules would an aid in the motility.

Question 3: Use of Gullet in ciliates?


Due to the abundance of many cilia, these are watery, moving animals. They have a gullet (cavity) that protrudes from the cell surface. The water loaded with food is directed into the gullet by the rhythmic movements of rows of cilia.

Question 4: How do protozoans play an important role in phagocytosis?


A cell’s cytostome, which often takes the shape of a microtubule funnel or groove, is specialized for phagocytosis. Food enters the cytostome and is vacuolized there.

Question 5: Examples for Sarcodina?


Amoeba is an example of Sarcodina. Pseudopodia are used to move. They use pseudopodia to catch their prey. There is no clear shape and no pellicle.

Question 6: Mention any two protozoan illnesses?


Malaria and toxoplasmosis are two diseases attributed to protozoans. Plasmodium spp., a spore-forming protozoan, is the cause of malaria, which is defined by recurrent fever bouts. The spore-forming protozoan Toxoplasma gondii is the cause of toxoplasmosis. Man is susceptible to illnesses from cats, dogs, sheep, and other animals.

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