Examples of Unicellular Protists

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  • 0:00 Definition of a Protist
  • 0:51 Examples of…
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Lesson Transcript
Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Protists are a diverse group of eukaryotic organisms that usually live in aquatic environments. This lesson describes unicellular protists - entire organisms that are just a single cell!

Definition of a Protist

A protist is a eukaryotic organism that is usually microscopic in size and lives in aquatic environments. Eukaryotic means the cells have a defined nucleus enclosed within a membrane. Most protists are unicellular, meaning the entire organism is composed of a single cell.

Protists aren't true animals, plants, or fungi, which is why they are classified in their own category, but they do share some characteristics with each of these other groups. For example, many plant-like protists are autotrophic, meaning they create their own energy through the process of photosynthesis, just like plants do. The four main groups of protists include the protozoa, the algae, the slime molds, and the water molds. Let's take a closer look at some of the protists that are unicellular (remember, that means they are made up of only one cell).

Examples of Unicellular Protists

Protozoa

Protozoa are animal-like protists believed to be some of the very first organisms capable of turning consumed food particles into usable energy; all protozoa are unicellular. Within the protozoa, there are four categories of organisms: the ciliates, the flagellates, the heliozoans, and the amoebas.

As the name suggests, ciliates are protozoa with cilia present during at least one stage of development. Cilia are hair-like structures that extend off of the cell to help it move through the water and capture food particles. Ciliates are found in all types of water bodies, including lakes, ponds, rivers, oceans, and even pores within sediments (the sand, mud, or muck on the bottom of a water body).

Flagellates are a group of protozoa that use whip-like extensions called flagella to propel the cell through the liquid environment. Flagella can also be used to guide food to the cell for consumption. You might be familiar with diseases caused by flagellate protozoa: giardiasis and African sleeping sickness are both caused by this type of protist.

Heliozoans are usually found in fresh water bodies and often have a silica-based shell surrounding the body. The shape of this shell is used to classify heliozoans into different types. The extensions protruding from the body are called pseudopodia and are used more for capturing food than for movement.

The last group of protozoa is the ameobas. Amoebas are globular cells filled with a jelly-like liquid called cytoplasm. To move and to collect food, amoebas change their body shape to form temporary pseudopods. Most amoebas reproduce through binary fission, meaning the single cell splits into two separate cells.

Algae

The second group of unicellular protists include many types of algae. Algae are plant-like protists, also found in a variety of water bodies. There are four phyla of unicellular algae: euglenophytes, chrysophytes, diatoms, and dinoflagellates.

The euglenophytes also use flagella to move around their aquatic environment and are capable of making their own food through the process of photosynthesis. They have chloroplasts for this purpose; however, if they are deprived of light, they can also act as heterotrophs and consume food around them for energy.

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