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Protozoa Definition

What is Protozoa and its Significance?

Protozoa are single-celled organisms with more than 65,000 identified species. These organisms are one of the most diverse groups on earth, which come in different shapes as well as sizes and have been around for millions of years.

These are tiny organisms that serve as an essential food source for human beings and other creatures by consuming bacteria and algae in water and soil. This process mainly helps to recycle organic matter, which is significant to sustaining life on Earth.

Protozoa Definition

Protozoans are essential in research labs for studying biological processes like cell division, gene expression, and protein synthesis. They are also very useful for researchers to know about how cells function at the molecular level. In this guide, we will get information about Protozoa, their Role in Maintaining Ecological Balance, Significance and other facts.

What do you mean by Protozoa?

Protozoa are single-celled animals found in every habitat on earth, with over 50 thousand species. They can be herbivores, carnivores or omnivores. These organisms have existed on earth since the Pre-Cambrian era and can be found in the fossil record. Some types of Protozoa live on human bodies, either as commensals (not harmful) or pathogens (causing disease).

Protozoa-related diseases are commonly found in individuals with AIDS. Some examples include Rhizopoda, Sarcodina, amoeba, and Mycetozoa.

Protozoa Structure

Protozoa Definition

Protozoa are tiny parasites that can measure less than 50 μm in size, with some intracellular forms being only 1 to 10 μm long. The largest known species, Balantidium Coli, can reach up to 150 μm in size.

In addition, Protozoa are small and unicellular eukaryotes, which means they have a true nucleus enclosed in a membrane. The nucleus also contains chromatin, which is scattered throughout the cell. Protozoa possess numerous organelles that perform vital functions identical to organs in higher animals.

One of the most notable characteristics of protozoa is their cytosome. This structure allows them to ingest food particles and other small organisms into their cells for nourishment. Ciliates are another type of protozoan that possess both macronuclei and micronuclei, which play important roles in reproduction and gene expression.

Despite being single-celled organisms, protozoa exhibit similarities to multicellular animals. For example, some species form colonies or aggregates for protection or feeding purposes. Others can differentiate into different forms based on environmental cues, such as light or nutrient availability.

What are the General Characteristics of Protozoa?

Protozoa are aquatic organisms that can be free-living or live in damp places. Some protozoa also live inside or on the bodies of animals and plants as parasites or commensals.

Protozoa Definition
  • They are very small, typically microscopic and not visible without a microscope.
  • Protozoa are single-celled organisms with one or more nuclei and organelles, lacking distinct tissues or organs.
  • They occur alone or in loose groups where individuals are independent and similar.
  • Protozoa have a vegetative body that is either naked or has a pellicle, and it may be equipped with simple to complex cells or exoskeletons.
  • The shape of a cell is typically constant but can change due to environmental factors or age.
  • The single cell is responsible for all vital activities, so there is no physiological division of labour.
  • Locomotion organs may take the form of finger-like pseudopodia, whip-like flagella, and hair-like cilia or be absent altogether.
  • Protozoa obtain nutrition through various methods, including holozoic, halophytic, saprophytic or parasitic. They may have oral and anal apertures or not. Digestion takes place inside food vacuoles within the cell.
  • Excretion in organisms can happen through their general surface or via contractile vacuoles, which also help with osmoregulation.

Process of Reproduction in Protozoa

Most protozoa reproduce asexually, but some also reproduce sexually.

1. Asexual Reproduction

Protozoa can reproduce asexually through various ways such as binary fission, multiple fission, as well as budding.

i) Binary Fission

Protozoa Definition
  • Protozoans reproduce asexually through binary fission, where the body divides to create two daughter bodies of equal or unequal size.
  • The process of binary fission typically involves transverse division, though certain protozoa use longitudinal division.
  • Amoebas undergo binary fission, with the process being transverse and involving the withdrawal of pseudopodia and rounding of the body. During this process, the nuclear membrane remains intact while mitosis occurs in the nucleus.
  • During cytokinesis in amoeba, the cell elongates, and a constriction appears between the two daughter nuclei. This constriction deepens until it bisects the body into two identical halves, which then pull apart to produce two distinct daughter amoeba from a single parent amoeba.
  • Ciliates undergo binary fission in a transverse plane and contain two types of nuclei, which divide differently. The micronucleus divides by regular mitosis while the macronucleus elongates and pinches into two daughter fragments. Each daughter ciliate receives copies of both nuclei.
  • Flagellated protozoa also undergo binary fission but typically along the long axis of the body, with flagella that is regenerated from basal bodies which divide before nuclear division begins.

ii) Multiple Fission

Protozoa can reproduce asexually through multiple fission or Schizogamy, where the nucleus divides to create many nuclei before the body splits.

During multiple fission, a daughter protozoan is formed from each nucleus and its surrounding cytoplasm before they separate. This process has been studied in Plasmodium, the malarial parasite, as well as certain amoebae like foraminiferans and radiolaria.

iii) Budding

Protozoa can reproduce asexually through budding, where daughter nuclei move into a cytoplasmic protrusion and then separate from the parent protozoan through fission.

2. Sexual Reproduction

  • Protozoans rarely engage in sexual reproduction, with conjugation being the most common method. It involves paired protozoa exchanging gametes of complementary mating types.
  • During conjugation, ciliates like Tetrahymena and Paramecium unite by fusing their pellicles at the contact point. In Tetrahymena, both macronuclei degrade as a result of this process.
  • The micronuclei divide and form haploid pronuclei, with one dividing again to create two gametic nuclei. These nuclei fuse to form a diploid zygote nucleus that undergoes three rounds of mitosis.
  • During sexual reproduction in protozoans, eight nuclei are produced in each conjugant.
  • One of these nuclei becomes a micronucleus, three degrade, and the remaining four fuses form a macronucleus. The conjugants then undergo binary fission to produce new protozoan offspring.

Significant Advantages of Protozoa

Protozoa, also known as phylum protozoa, have important ecological and industrial benefits. They play a significant role in helping the environment and industry. Some of their key benefits are listed below:

1) Sanitation

One of the most crucial roles played by protozoa is in sanitation. Protozoans enter polluted water sources and feed on bacteria present there. By killing these bacteria, they purify the water making it safe to drink. This process helps maintain hygiene levels and provides clean drinking water essential for human survival.

2) Food

Protozoans also act as a source of food for both humans and fishes. Planktonic protozoa provide food directly or indirectly to small crustaceans, larvae, worms etc., which are then eaten by fish species found in oceans. Humans can consume these fish as part of their diet.

3) Digestion

In addition to this, some species of protozoa help in digestion processes too. Termites host different kinds of symbiotic protozoa like Trichonympha and Colonymphya that live within their guts, helping them digest cellulose efficiently.

Apart from ecological benefits, protozoans have industrial importance too as after their death they left skeletal deposits at the bottom layers of seas. These skeletal deposits are made up of calcium carbonate or silica that turn into sedimentary rocks over time with commercial uses.

4) Scope in Education

Protozoans have been instrumental in scientific research for many years. They are favoured by several scientists and geneticists for various biological laboratory studies because of their tiny structures and fast reproduction rates. These protozoans have been utilised to study the evolution of living organisms, genetic modifications and disorders. In fact, the physiology of a cell was discovered with the help of these protozoans.

5) Oil Exploration

Apart from their importance in scientific research, and they play a critical role in oil exploration. The marine species Foraminifera and Radiolaria contain oil deposits that can be explored using protozoa. This highlight shows that it is very helpful in oil exploration.

Nutrition in Protozoa

  • Protozoa are unicellular organisms that exhibit a diverse range of feeding habits. One common characteristic of protozoa is their holozoic nutrition, which means they require organic matter for sustenance. This organic matter can be in the form of a solution or particulate.
  • Protozoa have developed an interesting mechanism to obtain nutrients from their food sources. They engulf their food or droplet with a temporary mouth-like structure called cytostome, after which their bodies perform digestion and absorption of the food vacuole. Finally, any waste substances are ejected out.
  • Interestingly, some protozoa have evolved to possess permanent mouths for the ingestion of food particles. These mouths allow them to consume larger prey items than those that rely solely on temporary mouths.
  • In terms of metabolism, many protozoa share similar pathways with higher animals that require both inorganic and organic compounds for energy production and other metabolic processes.

Overall, despite being single-celled organisms, protozoa exhibit complex feeding mechanisms and metabolic pathways similar to higher animals - highlighting the diversity and complexity found within this group of microorganisms.


Despite their small size, Protozoa have ecological and industrial importance that cannot be overlooked. They play a significant role in various fields and provide benefits even after death. They play a significant role in various fields and provide benefits even after death.

Protozoa are important in the food chain as prey for larger organisms and also play a crucial role in nutrient cycling by breaking down organic matter into smaller particles that plants can absorb.

Protozoa are also essential for wastewater treatment and the production of medicines. They also assist in preserving ecosystems, leading to fertile soil, clean air and water, and habitats for wildlife. Hence, these organisms offer numerous advantages to all living beings, including humans.

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