Some others are bioluminescent, basically meaning they

Some organisms are very familiar. However, there are some that play vital roles in our lives that we don’t even think about. We will look at a group of one of these unfamiliar kingdoms – protists.

Characteristics of Protists

The cellular structure of protists
Protist Cell Structure

You know the basics of what plants, animals, and fungi are, but what about those eukaryotic organisms that don’t quite fit within these groups? Within our system of classification, these organisms fall into the Protista kingdom. Protists are eukaryotic, mostly unicellular, and mostly aquatic. ‘Eukaryotic’ means that they have cells with a nucleus and membrane-bound organelles and are similar to the cells that make up our bodies.

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Because they are eukaryotic, protists fall within the domain Eukarya. Most protists are unicellular, meaning they are only made of one cell. However, some protists are multicellular and are made of more than one cell. Most are aquatic, meaning that they live in the water.

Types of Protists

Protists are broken up into three main groups based on how similar they are to other eukaryotic kingdoms. The three groups are animal-like, plant-like, and fungus-like protists.Animal-like protists are heterotrophs and get their nutrients by ingesting food, which is how animals, who are also heterotrophs, get their nutrients and energy.

Animal-like protists are also called ‘protozoa,’ which means ‘first animals.’Plant-like protists are autotrophs and get their nutrients by performing photosynthesis, which is a process that uses sunlight to make sugars and oxygen and is also performed by plants to make nutrients and energy. Plant-like protists are also called ‘protophyta,’ which means ‘first plants.’The third group, the fungus-like protists, are also heterotrophs and get their nutrients by absorbing food. Now, this may seem a bit unusual, but this is how fungi get their nutrients and energy.

Rather than eating food and then digesting it like animals and protozoa, fungus and fungus-like protists externally digest food and then absorb the nutrients.

Endosymbiosis in Protists

You may remember that endosymbiosis is basically the idea that one cell engulfs another cell. The endosymbiosis theory used this idea to explain the evolution of eukaryotic cells – and specifically, the presence of chloroplasts and mitochondria in cells.

Within the Protista kingdom, there is evidence to help support this theory. Remember that the endosymbiosis theory proposed that the mitochondria was engulfed first. Later, a cell that was capable of photosynthesis, most likely cyanobacteria, was then engulfed and became the chloroplast. Red and green algae, while you may think of them as plants, are actually protists. These simple plant-like protists have DNA that is very similar to the genetic material of cyanobacteria. This similarity helps show the evolutionary connection between cyanobacteria and algae – the more simple cyanobacteria is an evolutionary ancestor of both red and green algae.

Examples and Importance of Protists

Unicellular diatoms are used in nanotechnology
Some dinoflagellates release toxins that kill aquatic organisms.

You may already be familiar with the impact of our next example. Dinoflagellates can live in both marine and freshwater environments, and some species cause red tide. Red tide occurs when these small dinoflagellates produce toxins that kill fish and other organisms.

Not all dinoflagellates are harmful. In fact, many have mutualistic relationships – those in which both organisms benefit – with coral, while others are bioluminescent, basically meaning they glow in the dark.

Euglena are able to produce their own food.

Euglena Image

Our last example is the euglena. These protists can be either autotrophic or heterotrophic – or even both at the same time – depending on what is available at the time. Imagine being able to just make your own food when hungry! Euglena are able to use photosynthesis when there is enough sunlight, but they can consume and process nutrients when sunlight is not readily available.

Lesson Summary

Protists are a unique group of organisms that share some similarities with each other but are mostly grouped together because they don’t fit with any other kingdom.

All protists are eukaryotic, most are unicellular, and most are aquatic. However, they have drastically different modes of nutrition. Animal-like protists, or protozoa, get their nutrients by ingesting food. The plant-like protists, or protophyta, get their energy by performing photosynthesis. Fungus-like protists get their nutrients by absorbing food.

We looked at a few examples of protists, including diatoms, dinoflagellates, and euglenas. All three of these groups differ not only from each other, but also within their groups. Remember that protists have a variety of roles to play in our environment and in our lives.

Learning Outcome

Following this lesson, you should be able to:

  • Describe some characteristics of protists
  • List the types of protists
  • Provide examples of protists

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