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What is Protozoa?

Protozoa or Protozoan (plural- Protozoans) are single-celled eukaryotes that can be either parasitic or free-living. These are the unicellular or one-celled and heterotrophic groups of organisms that feed on organic matter that can be other microorganisms/debris/organic tissues. Over 6.5 K species of protozoans can be categorized in varied groups. Protozoans possess behavior like animals for predation and motility; they lack a cell wall. Protozoa belongs to a high-level taxonomic group and it was first introduced in the year 1818 by Georg Goldfuss. 

Characteristics of Protozoa

The general characteristics of Protozoa are explained as follows:

  1. Shape and Size

Protozoans vary to a great extent for their shape and size; these can be from 1microbial or micrometer to larger sizes that can be seen with naked eyes. The shell of one of the protozoa named foraminifera possesses a diameter of 20 cm. Their cells are enclosed within a thin plasma membrane and bear a hard shell on the outer surface. In ciliates, the cell is supported by Pellicle and its function is to provide the organisms with a definite shape and help in locomotion as it is flexible and rigid. These are flexible and available in varied shapes and their habitat is covered in the next point. 

  1. Habitat (Mode of Living)

These organisms are present in aquatic environments such as oceans or freshwater. These are free-living and many of these are parasitic in nature feeding on plants and animals. Most of the protozoa are aerobic, however, some of them are anaerobic and found in the rumen or human intestine. Their habitat also includes extreme atmospheres like hot springs and some form resting cyst and overcome dry environments.

  1. Structure of Cell

As we discussed, protozoans are unicellular and consist of a eukaryotic cell. The specialized internal structures are present in the cell which perform metabolic functions. They have generally one membrane-bound nucleus in the cell which has diffused appearance due to chromatin which is scattered everywhere. The vesicular nucleus of the cells contains an endosome or nucleoli which is a central body. The ciliates comprise macronucleus and micronucleus; the plasma membrane is also enclosed with locomotory projections such as cilia, flagella, and pseudopodia.

The pellicle is a membranous envelope that some of the Protozoans Genera consist of and it gives a definite cell shape. In some protozoans, epibiotic bacteria stick to the pellicle by their fimbriae.

The cytoplasm of Protozoans is differentiated into the inner endoplasm and outer ectoplasm and the cell organelles are present in the endoplasm and the ectoplasm is transparent.

Cytostome is structures in some of the protozoans for the purpose of ingesting food. Many food vacuoles are present where ingested food arrives. Ciliates possess a gullet which is a body cavity that opens outside. Protozoans can perform osmoregulation with the presence of a central vacuole and thus remove excess water. Protozoans have multiple membrane-bound organelles that include Golgi bodies, mitochondria, lysosomes, and other specialized structures.

  1. Mode of Nutrition

Protozoans have holozoic nutrition and belong to the heterotrophic category of organisms. They perform phagocytosis to ingest food and some of the groups have specialized structures known as cytostome for phagocytosis. In amoeba, their pseudopodia help in catching the prey and they have numerous cilia present in ciliates to help the food-laden water into the gullet.

Lysosomal enzymes help in digesting the ingested food through the food vacuole and hence the food gets distributed throughout the cell.

  1. Locomotion

Protozoa move from one place to another with the help of cilia, flagella or pseudopodia. Sporozoa are among the group which does not have any locomotory structure; instead, they possess subpellicular microtubules that help in the slow movement.

  1. Life Cycle of Protozoans

They have an alternate life cycle between the dormant cyst stage and proliferating vegetative stage. An example where this kind of life cycle takes place is Trophozoites.  The cyst stage is strong enough to survive harsh conditions in the absence of nutrients and water; this stage is usually dormant. It can remain without the host for a longer period of time and get transmitted. The other one, the trophozoite stage is known to be infectious as the organisms feed and multiply during this stage and also can cause diseases.

  1. Reproduction

Protozoans follow the asexual method of reproduction and they also multiply with processes like binary fission, transverse fission, longitudinal fission, or budding. Some of the species also undergo sexual reproduction and it is done by conjugation, syngamy, or by the formation of gametocytes.

Classification of Protozoans

Protozoans fall under the kingdom Protista and are divided into four major groups depending upon their structure and locomotory behavior. Following are the four protozoa classification:

  1. Flagellates (Mastigophora): These can be free-living or parasites and consist of flagella for locomotion. Their body is covered by pellicle or cuticle and freshwater organisms have a contractile vacuole. They undergo reproduction by binary fission. Some of the examples include Trichomonas, Leishmania, Trypanosoma, etc. 

  2. Amoeboids (Sarcodina): These kinds of protozoans live in freshwater, moist soil, or sea. They move and capture food or prey with the help of pseudopodia. They don’t have a definite shape as the pellicle is absent. Reproduction is performed by binary fission and cyst formation. Examples of these categories include Entamoeba and Amoeba.

  3. Sporozoans: These are endoparasitic and do not possess any specialized organ for movement. They have pellicles that help them in movement and reproduction takes place by sporozoite formation. These protozoans examples include Myxidium, Plasmodium, Globidium, Nosema, etc.

  4. Ciliates (Ciliophora): These organisms are aquatic and move actively with the presence of numerous cilia. Pellicle helps them to have a fixed shape and they have tentacles, for example: sub-class Suctoria. They have contractile vacuoles. Trichocysts are the organs that some of the species possess and they move with the help of cilia and their reproduction is through transverse division thereby forming cysts. Examples include Vorticella, Paramoecium, and Balantidium.

FAQ (Frequently Asked Questions)

1. Name the Diseases Caused by Protozoa.

Many of the protozoans are disease pathogens and some of the diseases caused by protozoans in animals and humans include Plasmodium or malarial parasite, Trypanosoma, or sleeping sickness, Trichomoniasis, etc.

2. What is Balantidiasis?

Balantidiasis is a disease caused in pigs because of protozoa Balantidium coli. In this, excystation occurs in the small intestine where sporozoites migrate to the colon and its symptoms include ulcers due to lesion in the colon, mucus in the stool, and colitis.