Introduction to Protists: Study with Video Lessons, Practice Problems & Examples

Protists are a diverse group of eukaryotic organisms that do not fit into the categories of plants, animals, or fungi, making them a paraphyletic group. They can be unicellular, colonial, or multicellular, with some exhibiting unique features like multinucleation and protective shells. Protists utilize various modes of nutrition, including photosynthesis and phagocytosis, and reproduce asexually or sexually. Notably, secondary endosymbiosis has led to the evolution of quadruple-membraned chloroplasts in some protists, highlighting their complex evolutionary history.

What is a Protist?

This video, we're going to begin our introduction to protists by first answering the question, what is a protist? Well, protists are a very diverse group of eukaryotic organisms that do not quite meet the criteria for being plants, animals, or fungi. And so really protists are kinda like the eukaryotic junk drawer, if you will, since, again, it contains very diverse eukaryotes that are not very well organized just yet in terms of their evolutionary relationships. Now in the past, these protists were once considered one of the 4 kingdoms of the Eukarya domain alongside the plants, animals, and fungi. However, that's no longer considered the case because unlike these kingdoms here, which are considered monophyletic groups, research has shown that the protists are actually a paraphyletic group, which recall from previous lesson videos means that the protists are a group of organisms that includes their most recent common ancestor, but only some and not all of the descendants of that common ancestor, like what would be the case for the plants, animals, and fungi.

And so because protists are a paraphyletic group, it makes them extremely difficult to be able to group and organize, and their evolutionary relationships are heavily debated and actively being researched to this day. Now down below in this phylogenetic tree, we can actually see a lot of these ideas. So notice that these two left branches over here represent the prokaryotes, which are the bacteria and the archaea. And everything in this box here represents the eukaryotes. And notice that all of these red branches within the eukaryotic group are the protists.

And so once again, the protists are a paraphyletic group because it includes their most recent common ancestor and only some, but not all of the descendants of that common ancestor, and that's because you can see land plants, animals, and fungi are not considered protists. Now also notice that some of these protists are actually more closely related to plants, animals, and fungi than they actually are to each other. For example, the choanoflagellates are actually more closely related to animals like you and I than they are to other protists since we share a more recent common ancestor with them. Now most eukaryotes are actually protists, as you can see by all of these red branches here in the eukaryotic group. In fact, it's probably better to think that all eukaryotes are protists except for the plants, animals, and fungi.

And as their name implies with the root prot, which is derived from the Greek word proto meaning first, there's lots of evidence to suggest that these protists were likely the very first eukaryotes to arise on earth. Now the very last note that I'll leave you all off with here is that notice that the green algae charophytes and the green algae chlorophytes have a little asterisk next to their name. And we've got this note down below that says that the asterisk indicates that some authors actually classify these species as plants and not protists. And so this just goes to show that there's not consensus quite yet on how these organisms should be grouped, and there's still much work and research to be done on these protists to reveal their correct evolutionary relationships. And so this here concludes our introduction to the protist and moving forward, we'll be able to learn a lot more.

So I'll see you on our next video.

Introduction to Protists Example 1

So here we have an example problem that asks, which of the following statements is true? And we've got these 4 potential answer options down below. Now, option a says that protists are a distinct monophyletic group of eukaryotes. It is true that protists are a group of eukaryotes, but it's not true that protists are a monophyletic group, which, recall from previous lesson videos, would mean that protists are a group of organisms that includes their most recent common ancestor and all of that common ancestor's descendants. Again, this is not the case for the protist group.

They are not a monophyletic group, so we can eliminate answer option a. Recall from our previous lesson videos that instead, protists are a paraphyletic group, which means that protists are a group of organisms that includes their most recent common ancestor, but only some, and not all, of that common ancestor's descendants. Now moving on, option b says that all eukaryotes are protists, but not all protists are eukaryotes. Once again, option b is not a true statement, so we can eliminate answer option b. We know that not all eukaryotes are protists because animals like you and I, for example, and fungi and plants are examples of eukaryotic organisms that are not protists. Also, to say that not all protists are eukaryotes is incorrect as well because all protists are eukaryotes.

However, being eukaryotic is still not a unique feature of the protist group. Now we're between option c and d, and notice that answer option d says bacteria, archaea, and unicellular algae are all types of protists. However, recall that bacteria and archaea are actually prokaryotic organisms, so they're not going to fall under the group protists, which we know are eukaryotic. And so for that reason alone, we can eliminate answer option d. Now this leaves answer option c as the only option, and it is the correct answer, which says the term protists does not refer to an actual monophyletic group, but it is used for convenience and practical purposes.

Recall that the term "protist" really is just used to refer to eukaryotic organisms that do not quite meet the criteria for being animals, fungi, or plants. Once again, the correct answer here to this problem is answer option c. That concludes this problem, and I'll see you all in our next video.

Complete the sentence: All protists ____________.

• A. Have a membrane-bound nucleus. 
• B. Have a cell wall.
• C. Are unicellular.
• D. Are multicellular.

Diversity of Protist Structure & Function

So in this video, one of the main takeaways is really just to reinforce this idea that protists are incredibly diverse in terms of both their structure and their function. In fact, protists are so diverse that there's actually no single characteristic that's unique only to all protists. Or in other words, there's no single characteristic that's found in every single protist that is unique to the protist group and is not found in organisms outside of the protist group. Now you might be thinking, wait a second, aren't all protists eukaryotes? Well, yes.

All protists are eukaryotes, but being eukaryotic is not a feature that's unique to the protist group since animals like you and I, plants, and fungi are examples of eukaryotes that are not protists. Now this being said, there are still some generalizations that we can make that apply to most protists. And so most protists tend to have the standard eukaryotic organelles that we've talked about in previous lesson videos, which you can see over here on the left-hand side of the image. But some protists may have some unique organelles, like some of these unique organelles that we show you here on the right-hand side of the image, including pellicles, which are a strip of proteins that surrounds the cells of some protists just underneath the cell membrane that provides rigidity to the structure of the cells that contain them. Contractile vacuoles, which absorb water and, upon contracting, can eject water outside of the cell to prevent the cell from lysing in hypotonic environments.

And some protists have a structure known as an ocelloid, which is an eye-like structure that allows these protists to visually detect their surroundings, which is pretty remarkable to think about. Now the vast majority of protists are actually unicellular in nature. However, there are some multicellular protists. For example, kelp is an example of a multicellular protist. And most protists actually tend to live in wet, moist soils and or in aquatic ecosystems.

In fact, in most cases, protists are going to be the most abundant organism in aquatic ecosystems, and protists can play very important ecological roles, and they're also important medically as well since they can cause diseases in humans such as malaria, for example. Now, protists are also very diverse in terms of the way that they acquire their nutrients. So protists can either be heterotrophic, photosynthetic, or they can be what's called mixotrophic, which as its name implies with the root mix there is really just a combination of the two, a combination of heterotrophic and photosynthetic. And so this here concludes our lesson on the diversity of protist structure and function, and moving forward, we'll be able to learn more about protists and apply these concepts in problems. So I'll see you all in our next video.

Introduction to Protists Example 2

So here we have an example problem that says, a unicellular mixotrophic protist loses its chloroplasts, but is still able to survive. How is this possible? And we've got these 4 potential answer options down below. Now of course, recall from our last lesson video that a mixotrophic organism is an organism that is both a heterotroph, meaning it consumes its energy, and a phototroph, meaning it captures energy from the sun. And so if this organism loses its chloroplast, well, then it will not be able to perform photosynthesis since recall that chloroplasts are organelles specialized for photosynthesis.

And so if this protist cannot perform photosynthesis, then it must rely on its heterotrophic abilities. And so that means that it is going to be able to rely on engulfing nutrients and other organisms via phagocytosis. And so notice answer option b is going to be the correct answer to this example problem. Now option a is not correct because it says it relies on free-floating photosystems in its cytoplasm, but this is not going to be the case. The photosystems, recall, are going to be embedded in the membranes of the chloroplast.

So, that's not going to be correct. And of course, if option a is not correct, option d, all of the above is also not going to be correct. And option c is also not correct because it says it can use an endospore to survive without nutrients for a long period of time, but that is not what the term mixotrophic implies. So for that reason, we can eliminate option c. Option b is the correct answer, and I'll see you all in our next video.

Which of the following is a common feature of all protists?

• A. Contractile vacuole. 
• B. Ocelloid.
• C. Flagellum.
• D. Chloroplasts.
• E. Pellicles. 
• F. None of the above.

Here’s what students ask on this topic:

What are protists and how are they classified?

Protists are a diverse group of eukaryotic organisms that do not fit into the categories of plants, animals, or fungi. They are classified as a paraphyletic group, meaning they do not share a single common ancestor. Protists can be unicellular, colonial, or multicellular. They exhibit a wide range of characteristics, such as multinucleation, protective shells, and various modes of nutrition including photosynthesis and phagocytosis. Protists reproduce both asexually and sexually. Due to their diversity, protists are often grouped based on convenience rather than strict evolutionary relationships.

What is endosymbiotic theory and how does it relate to protists?

Endosymbiotic theory suggests that eukaryotic cells originated through a symbiotic relationship between early prokaryotic cells. According to this theory, larger prokaryotic cells engulfed smaller ones, which then became organelles like mitochondria and chloroplasts. In protists, this theory is particularly relevant as some protists have chloroplasts with quadruple membranes, a result of secondary endosymbiosis. This means a eukaryotic cell engulfed another eukaryotic cell that already had chloroplasts, leading to complex evolutionary histories in these organisms.

How do protists reproduce?

Protists primarily reproduce asexually through mitosis, which involves the division of the nucleus followed by the division of the cell. This is different from binary fission seen in prokaryotes. Some protists also reproduce sexually, which introduces genetic variation and is important for evolution. Sexual reproduction in protists can involve the fusion of gametes, leading to the formation of a zygote. The ability to reproduce both asexually and sexually allows protists to adapt to various environmental conditions.

What are some unique features of protists?

Protists exhibit several unique features. Some protists are multinucleated, meaning they have more than one nucleus within a single cell. This is seen in certain slime molds. Many protists have protective outer layers like cell walls or shells made of inorganic materials. They can also use cilia and flagella for movement, and some exhibit amoeboid movement using pseudopodia. Protists can be predators, using phagocytosis to engulf and digest their prey, or they can perform photosynthesis if they have chloroplasts.

What is secondary endosymbiosis and how does it affect protists?

Secondary endosymbiosis occurs when a eukaryotic cell engulfs another eukaryotic cell that already contains chloroplasts. This process results in the formation of chloroplasts with quadruple membranes. In protists, secondary endosymbiosis has led to the evolution of complex chloroplast structures, allowing these organisms to perform photosynthesis. This evolutionary event highlights the intricate relationships and adaptations that have occurred in the history of protists, contributing to their diversity and ecological roles.