Protists: Definition & Location

Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Protists are a diverse group of organisms belonging to the kingdom Protista. Having traits in common with plants, animals, and fungi, these organisms are unique in their own ways. Read on to learn more about protists.

A Microscopic World

Have you ever examined a drop of pond water under a microscope? This seemingly quiet and innocent body of water may look quite shocking when you zoom in. Unseen by the naked eye is an enormous population of tiny microscopic organisms floating, spinning, and whipping about. A look at these protozoa, which are single-celled organisms closely related to animals, gives one small glimpse into the astoundingly varied group of organisms that belong to the kingdom Protista.

An example of protozoa under a microscope
Protozoa

Protists (members of the kingdom Protista) make up a group of organisms so diverse, it can be hard to imagine that they belong together. Protista includes creatures as small as protozoa, as large as giant kelp, and as unsavory as slime mold and some algae. Some protists are animal-like, but many resemble plants. With such extreme diversity, how do we know what organisms are protists? Let's explore the characteristics that make up protists.

What Is a Protist?

In general, the kingdom Protista is made up of all unicellular, or single-celled, organisms that do not fit into any of the other kingdoms (Animalia, Plantae, Fungi, Archaea, and Bacteria). In essence, protists simply do not have enough in common with organisms of these kingdoms to join their groups. Protozoa such as amoeba and paramecium are great examples of minuscule single-celled organisms that are protists. It's also important to note that there are select multi-cellular organisms within the Protista kingdom, and we'll discuss these later in the lesson.

A second common factor among members of the kingdom Protista is that all protists are eukaryotic, which means their cells contain a nucleus and other organelles that are enclosed within membranes. This is in comparison to prokaryotes, which do not have a nucleus or membrane-bound organelles.

Protista Sub-Categories

The kingdom Protista is divided into several sub-categories according to similar behaviors: animal-like, plant-like, and slime mold. Let's explore these sub-categories below.

Animal-Like Protists

Animal-like protists are protozoa. When we say the're animal-like, we don't mean they're fur-covered creatures running around on four legs. Rather, we mean that they can move around in their environments to find food, and they're heterotrophic, which means they feed on other organisms. The amoeba, for instance, simply oozes out part of its body over its prey, engulfs it, and digests the nutrients.

It should be noted that not all protozoa look like the shapeless amoeba. Under a microscope, some look more like holiday ornaments than living things. Marine radiolarians, for example, have elaborate and delicate glass-like shells.

Radiolarian under magnification
Radiolarian

Plant-Like Protists

As you may have guessed, plant-like protists behave like plants in that they are autotrophic, which means they create their own food. Just like plants, they use use sunlight energy, carbon dioxide, and water to undergo photosynthesis.

This group of protists is made largely of various forms of algae and seaweed. If you've ever been panicked by the feeling of seaweed touching your foot in a lake, then you've had a close encounter with a plant-like protist.

Green algae, a type of plant-like protist, under a microscope.
Green algae

As we mentioned earlier, not all protists are unicellular. In fact, if you're looking for multicellular protists, look no further than plant-like protists, many of which have more than one cell. Seaweed and kelp, for instance, are multicellular. In addition, other types of unicellular algae often form an aggregate and colonize into a larger, multicellular entity. This would explain the large clump of slimy seaweed brushing past your foot.

Giant kelp, an example of a multicellular protist
Giant kelp forest

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