Organelles in eukaryotic cells | The cellular basis of life | High school biology | Khan Academy

What we're going to do in this video is give ourselves a little bit of a tour of eukaryotic cells. The first place to start is just to remind ourselves what it means for a cell to be eukaryotic. It means that the inside of the cell there are membrane-bound organelles. Now, what does that mean? Well, you could view it as subcompartments within the cell's membrane-bound organelles. In this video, in particular, we're going to highlight some of these membrane-bound organelles that make the cells eukaryotic.

So, let's just start with some of the ingredients that we know are true of all cells. You'll have your cellular membrane here, a little big, so that we have a lot of space to draw things in. So, this is our cellular membrane. I'm doing that little nice shading so you appreciate that it'll actually be three-dimensional. We see so many slices of cells that sometimes we forget that they are more spherical or that they have a three-dimensional shape to them; they're not all spherical; they can have different shapes.

Now, all cells—and there are some exceptions that we've talked about in previous videos—I should say most cells will have some genetic information in them in the form of DNA. So, that is our DNA right over there. Now, one of the key characteristics of a eukaryotic cell is that that genetic information is going to be inside a membrane-bound organelle. That membrane-bound organelle, or the membrane that binds or surrounds the DNA here, that is the nuclear membrane.

So, let me draw the nuclear membrane right over here, and I'll put some shading in to appreciate that that also is going to be in three dimensions around the DNA. And so, that is the first membrane-bound organelle that we're going to discuss: the nucleus. Now, the nucleus, it turns out, is connected to another membrane-bound organelle, and we're going to study this in future videos. But right here, I'm drawing holes or pores in the nuclear membrane, and those pores connect to something—it's a very fancy word called the endoplasmic reticulum.

The endoplasmic reticulum is essentially these layers of membranes. So, I'm going to do my best job at trying to draw an endoplasmic reticulum. Imagine extending from these pores, going into a space that has these layered membranes that have a lot of surface area. I'm not going to go all the way around this nucleus, but in many cells, it will go all the way around the nucleus. And this right over here—and this is just a rough diagram—that is our endoplasmic reticulum.

As I've mentioned in previous videos, it would be an excellent name for a band. What goes on in the endoplasmic reticulum is when you are in the process of taking that genetic information from DNA, and as we talk about in other videos, it gets transcribed into mRNA. So, that mRNA is now containing that information. That mRNA will make its way out of that nuclear membrane through one of these pores and then make its way to a ribosome that is attached to the membrane of the endoplasmic reticulum.

And so that's a ribosome there. I'm going to do a bunch of ribosomes. As we've talked about in previous videos, the ribosomes are really where you take that genetic information from that mRNA, and then you translate it into a protein. So, the ribosomes are the protein synthesis. So, let me label that. So, this right over here is a ribosome.

Some ribosomes might be attached to the endoplasmic reticulum; some of them might just be floating out here in the cytoplasm. So that would be a free ribosome. Free ribosome. And even from the point of view of the endoplasmic reticulum, the parts of the endoplasmic reticulum where you have ribosomes attached, this is known as rough endoplasmic reticulum. It's the ribosomes that are making them rough—it looks that way in a microscope. So, I'll say rough ER for endoplasmic reticulum for short.

Then, you also have parts of the endoplasmic reticulum where you do not have ribosomes attached, and because that looks smooth through our microscope, it has been called—you can imagine—smooth endoplasmic reticulum. There are things known as Golgi bodies. Once again, another fascinating name. You got to love these names in biology that look kind of like an endoplasmic reticulum but detached from the nuclear membrane.

So, let's say something like that. That's my best drawing there. That's a Golgi body, and these are really good at packaging molecules, even proteins that might have just been produced, and packaging them so that they can be used outside of the cell, for example. So, we'll go into detail in other videos where a protein might go to the Golgi body, get a little envelope around it, get some little processing going on, and then make its way outside of a cell.

Now, another—and this is maybe one of the most famous membrane-bound organelles outside of the nucleus—is what's known as the powerhouse of the cell, and that is the mitochondria. So, I'll do this in magenta because that's a nice powerful color. So, mitochondria. I love mitochondria because it's fascinating how they even came to be. Mitochondria actually have their own DNA, and all of your mitochondrial DNA comes from your mother.

So, that's actually very interesting for tracing maternal lineage, but mitochondria—this is where your, I'm going to say, let's see—what could you see inside of this? This is where your ATP is produced. This is your mitochondria; it's really the powerhouse of the cell. What's interesting about mitochondria is evolutionary biologists believe that the ancestors of mitochondria—because mitochondria have their own DNA—might have been independent organisms, independent cells. At some point in our evolutionary past, they started living in symbiosis inside of what would be the ancestors of our cells, and over time, they became so codependent that they started to replicate together.

Mitochondria, in fact, became part of these eukaryotic cells. Now if this eukaryotic cell was a plant cell or maybe an algae cell, you would have something called chloroplasts. There we don't have them because we don't have photosynthesis, but this is a chloroplast. If you could see inside, you could see the little thylakoid stacks right over here; you could see the little thylakoids if you could see inside. So this right over here is a chloroplast—chlorophyll—and this would be plants and algae; animals do not have these, and these are where you have your photosynthesis take place.

Photosynthesis. Now, there's also some other membrane-bound organelles that are maybe less famous than the mitochondria or the chloroplast—or for sure the nucleus—and that might be something like a vacuole. In plants, vacuoles tend to be very big. I could draw it—you know, this is three-dimensional, so I'll draw it on top of some of what I've drawn before. So, if a vacuole right over here—this is a—and a plant could be a fairly significant compartment inside. In fact, it can even give structure to the plant itself because it is so big and it contains water and enzymes.

It's viewed as a kind of a storage compartment, but it can also contain enzymes that help digest things that help break things down so that they can be used in some way. So, that is a vacuole. They don't just exist in plants; they can also exist in animal cells, but in plant cells, they tend to be very, very, very visible.

Now, something that is somewhat related to some of the functions that a vacuole plays that are most associated with animal cells—but now there's evidence that they also exist in plant cells—is the idea of a lysosome. So, a lysosome right over here that also is a compartment, and it's going to contain a whole series of enzymes in it that are useful for lysing—you could say—that is useful for breaking down either waste products as the cell lives or even foreign substances that might not be helpful for the cells.

So, it's going to contain a bunch of enzymes, and it helps break down things. Now, I'll leave you there. These aren't all of the structures in eukaryotic cells, but these are enough of the structures so that you can appreciate that there are a lot of membrane-bound organelles in eukaryotic cells.

To be clear, even if I were to show all of the membrane-bound structures, that's not all the complexity of a cell. The big thing to appreciate is cells are incredibly complex. There are all sorts of structures in here that help transport things and move things around. If you could shrink yourself down and look inside of a cell, it would look more complex than the most complex cities. There's all sorts of activities—things being moved around, shuttled around. The cell itself is replicating and copying things.

So, this is just the beginning. We're just starting to scratch the surface at the complexity of the most basic unit of life.