Protist Cells: 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.

Protist Cells 1

Hi. In this video, we're going to talk about protists, which are essentially the category of eukaryotes that include everything that isn't a plant, an animal, or a fungus. So it's kind of like a biological grab bag or potpourri. It's a real mixture, and as you'll see there aren't really a lot of unifying themes that tie protists together. So let's begin our discussion by talking about eukaryotic cells. Right? Protists are eukaryotes. So what are eukaryotes? Well, you might recall that eukaryotes have a nucleus, they have membrane-bound organelles, and these include a cytoskeleton. Now, the unifying theme of eukaryotes, if you will, is the nucleus. And let's use a little fancy terminology here. The synapomorphy, right? The derived character that is shared between the organisms in a clade, the synapomorphy that unifies eukaryotes is the nucleus. Right? Now, eukaryotes also tend to be much larger than prokaryotes. Right here we've got some little prokaryotic cells. Here we have our eukaryotic cell, I should say, just one cell. A eukaryote tends to be a lot larger, it's got this nucleus, you can see all these membrane-bound organelles, we have mitochondria here, these are Golgi apparatus, right? There are our centrioles and, you know, here we're really showing the same thing, you know, now you can see the rough ER and the smooth ER. And if you want to review these concepts, go back and check out the video on cells. Before we dive into Protists, I also want to revisit the idea of endosymbiotic theory. And that is because endosymbiotic theory is actually going to play into Protists in kind of a unique and pretty darn cool way. So if you recall, the basic idea of endosymbiotic theory is that as prokaryotic cells, which were the first form of cells, got bigger, they faced this problem, the surface area to volume ratio issue that cells and organisms in general run into as they get larger. And so, these infoldings, these membrane infoldings, appeared to help counteract, or to help maintain rather a surface area to volume ratio that was ideal for the organism. Eventually, these infoldings become the nucleus and the endomembrane system and, ultimately, now we're going to get to the endosymbiotic part, ultimately what's going to happen is these cells are going to engulf a Proteobacterium. And it's worth noting that a lot of these single-celled organisms, you know, that act as hunters will literally just engulf their prey like this but instead of digesting this bacterium, what happened is these two organisms, essentially became reliant on each other. They formed a symbiotic relationship where they were helping each other. You know, for example, the larger cell that engulfed the proteobacterium, was like offering protection, probably feeding nutrients to the proteobacterium, and in exchange, the proteobacterium is going to generate a bunch of ATP, by oxidative phosphorylation. So eventually, these proteobacterium, through what is called symbiogenesis, write that down. So through symbiogenesis, these proteobacterium that were engulfed by prokaryotic cells will eventually become a eukaryotic cell, and those proteobacterium are going to become mitochondria. There's a lot of evidence for this. We're not going to cover it now, other than saying that like prokaryotes, mitochondria have a double membrane, which is unlike other membrane-bound organelles. And the reason I'm mentioning that, will become significant later. So in addition to this, there was also another symbiogenesis event where cyanobacterium were engulfed and eventually became chloroplasts. Right? So these are both instances of symbiogenesis. We have proteobacterium becoming mitochondria and cyanobacteria becoming chloroplasts. Now with that, let's flip the page and talk about how protists experience some unique forms of endosymbiosis.

Protist Cells 2

While the organisms of the Plantae lineage have chloroplasts as a result of what we'll call primary endosymbiosis, meaning they engulfed a cyanobacterium, which then turned into the chloroplast. There are lineages of protists that actually have these quadruple membraned chloroplasts. Right? So remember that bacteria have that double membrane structure to them. Right? So, like mitochondria, chloroplasts have that double membrane. But in certain protists that have chloroplasts, they have this quadruple membrane chloroplast. People take this as evidence of what's called secondary endosymbiosis. So basically, this eukaryotic cell would engulf a photosynthetic eukaryotic cell, as we see happening here. It's going to engulf it, and then this photosynthetic eukaryotic cell will eventually become this quadruple-membraned chloroplast structure, and this will account for the origin of chloroplasts in other eukaryotic lineages outside the Plantae lineage. So within Plantae, it is what we see going on right here. Outside of Plantae, it is going to be this situation here. Right? They engulfed an organism that already had undergone endosymbiosis, and then there was another endosymbiosis, which we call secondary endosymbiosis.

Now, let's actually get to a little bit about protists. What defines them? Well, not a lot, unfortunately. Protists, again, they're a grouping of convenience. Right? They're basically everything that does not belong to fungi, Animalia, and Plantae. Right? Fungus, animals, plants. Every eukaryote that is not in one of those three groups is a protist. That means that protists are a paraphyletic group. Right? They are not monophyletic, unlike fungi, animals, and plants, which are monophyletic. Protists are a paraphyletic group. So, as a group, they do have some shared features, but many protists will display unique characteristics. In a later video, we will actually go through and look at some of those characteristics. So most protists are unicellular, but some are colonial, meaning they'll live in colonies, and others actually are multicellular. And these are, like brown algae, red algae, and slime molds. Those are some examples of multicellular protists.

It is worth noting that multicellularity actually arose independently in many eukaryotic lineages. There wasn't some original multicellular organism from which all multicellular organisms came, instead, multicellularity has cropped up various times in various lineages. Right? Right? So you might remember, when we talked about evolution, we mentioned that the eye evolved independently over 16 different times. So, just like that, multicellularity arose independently many times in eukaryotic lineages. Now, most protists undergo asexual reproduction, which will include mitosis, unlike binary fission. Right? Remember, protists are eukaryotes, so they're going to have a nucleus, which means they're going to need to perform mitosis when the cells divide. However, they generally, sorry, while protists undergo asexual reproduction, some of them have evolved various forms of sexual reproduction. Remember, sexual reproduction is a major event in evolution because it allows for the introduction of tons of genetic variation, which is super important for evolutionary purposes.

Now, it's worth looking at this little mock-up of a phylogenetic tree, and I really want to emphasize "mock-up." This is not a very nice phylogenetic tree. I'm really just trying to show one little point here, and that is how many different lineages there are outside of plants, animals, and fungi. So here, Archaeplastida: this is plants, basically. That's not to say that there aren't protists in this group, but that's where land plants are. We have fungi over here, and it's I should note that this whole branch is coming off of this one right here. And so fungi right there, and then you can see down here we have animals. So look at all these other lineages that exist outside of plants, animals, and fungi. Right? These are all the protists. Big group there. Big group. And as you can see, lots of distantly related cousins, so to speak. That's what I meant by this is not a monophyletic group, but a paraphyletic group. There are all these phyla parallel to each other in this taxonomy.

And with that, let's flip the page.

Protist Cells 3

As I keep saying, there is nothing that really unifies or defines protists as a group outside the fact that they're all eukaryotes. However, there are certain features that are shared by many protists across various lineages. So let's talk about a couple of those. One of the coolest, I think, is that protists can be multinucleated, and that is when a single eukaryotic cell will have more than one nucleus. And you'll see this actually comes in a variety of different forms. One of the craziest of which is, you know, types of slime molds, that we'll take a look at, that actually can have thousands of nuclei in a single cell. It is worth noting that there are human cells, for example, that are multinucleated, and these include types of muscle cells. However, this is just one feature of protists that you can kind of apply generally. It's like a blanket statement. Of course, there are many protists that are not multinucleated because, again, the grouping of protists is just out of convenience. There isn't really like one unifying, or more than one unifying thing that brings them all together.

So, many protists have cell walls or shells that serve as protective outer layers. Some of these shells are made of inorganic materials. It's super cool. Some of them are very beautiful. We'll actually be taking a look at them when we discuss the various lineages of protists.

Now, like all eukaryotic cells, protists can use cilia and flagella for movement as well as other things. Here we see a picture of cilia. Here we have our flagella. Now I do want to mention that prokaryotic flagella and eukaryotic flagella are actually different. There are structural differences between them and we have discussed that, previously in the video on cells. I'm not going to get into that here. I'm just pointing this out because I am using the same picture of flagella that I used when I was talking about prokaryotic flagella. I just really like the way that these two images look next to each other. So that's why I'm reusing it here. Fear not, just know that they are actually structurally different. The fact that I'm using the same image isn't going to affect your understanding at all.

So, the last kind of common feature that I want to point out is amoeboid movement. Now many protists are considered amoeba. And amoeba are basically organisms that move by reshaping their cells and forming what are called pseudopodia, which literally means 'false foot'. Pseudo is false, podia is foot. And you can see an example of these pseudopodia on this amoeba here. These little, tentacle-like things that it's stretching out, those are the pseudopodia. And these are literally cytoplasmic protrusions, and the cell will use actin and myosin to stretch various parts of the membrane out into these pseudopodia.

Now it is worth pointing out since we're talking about pseudopodia and amoeba, there is a lineage of protists called Amoebozoa, but amoeba exists in other lineages as well, because amoeba are simply organisms that reshape their cells in the form of pseudopodia to move around.

Now, many protists are actually predators. Yeah, unicellular predators. Not something we often think about, but trust me, it gets savage at the cellular level. Right? You know, they're not like stunning their prey or something and taking them out in a nice way. No. They do phagocytosis. They literally engulf the whole cell and just eat it like that. You know, almost think like a snake swallowing its prey whole. Now, what's pretty cool is when they do phagocytosis, they're actually using pseudopodia, right to stretch out the membrane of the cell around the thing they're trying to eat, which isn't necessarily another cell but it can be, you know, here in this image it just says solid particle, could be like a particle of food. It could also be another cell though. For example, like a bacteria cell or another eukaryotic cell.

It is worth noting that some protists actually do use photosynthesis. You might recall we just talked about secondary endosymbiosis, right? So, there are chloroplasts in various forms present in different protists, and they will do photosynthesis, though many protists do not rely solely on photosynthesis. They will do some photosynthesis, but then they'll also obtain nutrients in other ways.

Now, decomposers actually Oh, before I get to that, love this picture right here. These are protists and these dark dots, those are red blood cells. Yeah, so these are protists that are eating red blood cells inside an animal. As I said, it gets savage on the cellular level. They eat your blood.

Now, as I was saying, decomposers absorb nutrients directly across their cell membrane, and usually this is accomplished via transport proteins. So, many protists are decomposers and they will break down and absorb nutrients from decaying organic matter. And again, they'll take those nutrients in via transport proteins

So these are just some themes that you can apply to protists, but again, bear in mind that it's not a very well-defined group. It's really just an 'everything that isn't this stuff' group

With that, let's flip the page.

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.