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Unicellular Definition

A unicellular organism is an organism that consists of a single cell. This means all life processes, such as reproduction, feeding, digestion, and excretion, occur in one cell. Amoebas, bacteria, and plankton are just some types of unicellular organisms. They are typically microscopic and cannot be seen with the naked eye.

Functions of Unicellular Organisms

Cells are called the building blocks of life because they often come together to form multicellular organisms, such as plants or animals. However, some organisms consist of only one cell. These are called unicellular organisms. Although much smaller, unicellular organisms can perform some of the same complex activities as multicellular organisms.

Many unicellular organisms live in extreme environments, such as hot springs, thermal ocean vents, polar ice, and frozen tundra. These unicellular organisms are called extremophiles. Extremophiles are resistant to extremes of temperature or pH, and are specially adapted to live in places where multicellular organisms cannot survive. This unique feature allows scientists to use unicellular organisms in ways previously only imagined. When Thermus aquaticus was discovered in the boiling water of a Yellowstone Park hot spring, scientists used its special enzyme TAQ polymerase to replicate DNA billions of times in the span of just a few hours. Without this discovery, forensic science and genetic testing as we know them wouldn’t exist. Other extremophiles have been used for treating arthritis and autoimmune diseases, making paper, treating waste, and radiation resistance.

However, not all unicellular organisms are extremophiles. Many types live under the same narrow range of living conditions as multicellular organisms, but still produce things necessary to all life forms on Earth. For example, phytoplankton is a type of unicellular organism that lives in the ocean. Not only are they the very foundation of the ocean’s food chain, but phytoplankton also provide most of the oxygen in Earth’s atmosphere. Without them, humans could not breathe, plants could not thrive, and life would cease to exist completely.

Types of Unicellular Organisms

Scientists use taxonomy to categorize all living organisms into groups based on certain characteristics. A domain is the highest rank of taxonomy, which can then be broken down into more specific kingdoms. There are six kingdoms in all, and four of those focus solely on unicellular organisms. Those four types can then be divided into two groups: eukaryotic or prokaryotic organisms.

Eukaryotes are unique because they can be unicellular or multicellular; however, a cell must have membrane-bound organelles to fit in this category. These cells have a nucleus where the DNA is stored, mitochondria for energy, and other organelles to carry out cell functions. Conversely, prokaryotes consist of a single cell with no membrane-bound organelles. DNA floats freely in the cytoplasm since there is no nucleus. The organism also has to adapt to other ways of carrying out reproduction, feeding, and waste excretion, since it does not have any specialized organelles.


Archaeabacteria: This kingdom was initially categorized as bacteria, but scientists later realized that these organisms are really unicellular microbes. What makes archaeabacteria unique is that these organisms thrive in conditions that few others can, such as deserts and tundra. For this reason, archaebacteria are considered extremophiles. However, they can also survive in normal environments, including soils, oceans, and the human colon.

This diversity has allowed breakthroughs in both medicine and technology. Pyrococcus, a species that can function in temperatures over 100°C, allows for food processing at extremely high temperatures, such as with whey and other dairy products. Other archaeabacteria potentially hold the key to a new strain of antibiotics. They differ from bacterial antibiotics in structure, so they will be able to treat patients differently than the antibiotics typically prescribed now.

Eubacteria: Most organisms in this kingdom are unicellular bacteria. Although they aren’t typically considered extremophiles, bacteria can be found nearly everywhere on Earth. People often think about diseases or germs when they think about bacteria, but most eubacteria are helpful. They can be found in yogurt, cheese, and other foods that help with digestion.

Bacteria are also the basis of many antibiotics available today. If these antibiotics did not exist, the mortality rate would skyrocket for even the smallest ailment. Erythromycin, a medication made from good bacteria, is often prescribed to fight off bad bacteria through shutting down protein production and replication.

Beyond medicinal uses, bacteria decompose dead and decaying matter for nutrients. Every organism on Earth benefits from this, especially as industrial advances (and disasters) occur. For example, Pseudomona is a bacteria that degrades oil spills in the ocean and on soil. Other bacteria break down heavy metal contamination and treat harmful substances in the waste-water treatment process.


Protozoa: This kingdom only consists of unicellular organisms. They come in many shapes and sizes, live in various environments, and have several different purposes. Some are completely harmless, but others can be parasitic and cause disease.

One type of harmless protozoa is the amoeba. An amoeba is a unicellular predator that lives in wet environments, including decaying vegetation, wet soil, or inside humans. Although they are unicellular, they can be carnivores, herbivores, or omnivores. They obtain food by preying on smaller organisms, such as bacteria living on rotting vegetation. Amoebae have such great hunting skills because of their jellyfish-like tentacles called pseudopodia. They use these tentacles to move around, touch, and grab their prey. Once the prey is engulfed, enzymes inside the amoeba digest it and then eliminate the waste by pushing it back through the membrane.

Sporozoans are types of protozoa that most people try to avoid. These protozoa are very parasitic, causing malaria in both birds and mammals. Host mosquitos already infected with malaria are responsible for injecting sporozoans into the bloodstream, causing malarial infection. More people today suffer from malaria than any other disease, especially in Africa where the naturally warm conditions are ideal for growth and contagion.

Protista: The Protista kingdom recently split into five supergroups that classify protists based on how they move and how they obtain nutrition. Some are animal-like, plant-like, or fungus-like, based on characteristics they exhibit. This kingdom holds some of the most important unicellular organisms on the planet, such as phytoplankton and euglena.

Phytoplankton are unicellular protista that live in aquatic environments, either salty or fresh. Some are bacteria, but most are single-celled, plant-like organisms. Diatoms and green algae are two great examples of phytoplankton. They are similar to terrestrial plants because they use photosynthesis for chemical energy, a process that uses carbon dioxide and releases oxygen. When conditions are ideal, phytoplankton populations explode into what is known as a bloom. These blooms are so large and last so long that they actually provide most of the oxygen present on Earth.

Euglenas are unique because they are like a unicellular plant-animal hybrid. They can make their own food like a plant, but can also eat things like an animal. Most euglena are green because they eat green algae when there is not enough light for photosynthesis. When photosynthesis can occur, the resulting oxygen emission is equal to that of phytoplankton.

Examples of Unicellular Organisms


Diatoms are the most common type of phytoplankton. Scientists believe they are the most significant group of organisms on the entire planet because they produce a quarter of all the oxygen available. They make a glass-like transparent shell that varies in shape, size, and pattern depending on the species of diatom. Although they are unicellular, they can form colonies by joining together, and produce even more oxygen together. The colonies can be in the shape of ribbons, zigzags, or even stars.


Yeast is one of the few unicellular organisms that fall into the Kingdom Fungi. It is mostly found in sugary areas, like on flower nectar and fruits. There are several different types of yeast, and many are used to make bread, beer, and wine. Yeast has recently been manipulated to produce ethanol as well, leading to new ideas and improvements in environmental fuel sources.

Slime Molds

Slime molds are one of the most unique types of unicellular organisms. They were considered fungi for many years, but scientists recently realized they were completely unrelated. Although unicellular, they swarm together and form one giant cell-like structure with several nuclei. Since this “cell” is so large, it has enabled scientists to understand cell interactions more easily.

  • Taxonomy – Branch of science concerned with the classification of organisms based on certain characteristics.
  • Enzyme – A protein molecule produced by living organisms to catalyze (speed up) reactions.


1. How do unicellular and multicellular organisms differ?
A. Unicellular organisms don’t have organelles, but multicellular ones do.
B. Unicellular organisms use photosynthesis to make food, but multicellular ones do not.
C. Unicellular organisms consist of prokaryotes and multicellular organisms are eukaryotes.
D. Unicellular organisms are made up of one cell, and multicellular organisms are made up of more than one cell.

Answer to Question #1
D is correct. Unicellular organisms can have organelles, eat through predation, and be eukaryotic or prokaryotic. However, they are always made up of just a single cell.

2. Why are extremophiles important?
A. They can survive environments no other organisms can.
B. They provide oxygen to the environment.
C. They can treat diseases like malaria.
D. They provide food sources such as dairy and bread.

Answer to Question #2
A is correct. Extremophiles survive in extreme conditions. Some have been key to scientific discoveries, including in the fields of forensic science and genetic testing.

3. What are the two categories of unicellular organisms?
A. Extremophiles and non-extremophiles.
B. Protista and Eubacteria.
C. Prokaryotes and Eukaryotes.
D. Photosynthesis based and non-photosynthesis based.

Answer to Question #3
C is correct. Unicellular organisms have a variety of functions and characteristics, but the one distinguishing factor is that some have membrane-bound organelles (eukaryotes) while others do not (prokaryotes).

Cite This Article

Biologydictionary.net Editors. "Unicellular." Biology Dictionary, Biologydictionary.net, 22 Nov. 2016, https://biologydictionary.net/unicellular/.
Biologydictionary.net Editors. (2016, November 22). Unicellular. Retrieved from https://biologydictionary.net/unicellular/
Biologydictionary.net Editors. "Unicellular." Biology Dictionary. Biologydictionary.net, November 22, 2016. https://biologydictionary.net/unicellular/.

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