Comparing prokaryotic and eukaryotic cells | High school biology | Khan Academy

In other videos, we talk about how cells are the basic building block of life. In this video, we're now going to talk about the two main categories of cells: prokaryotic cells and eukaryotic cells.

So, what I'm going to do here is I'm going to diagram out a prokaryotic cell and a eukaryotic one, and we're going to see both the similarities and differences. Let's just start with the similarities. They both have a plasma membrane. So this is a plasma membrane for the prokaryotic cell, and this is for the eukaryotic cell.

Now, this is a layer, an outer layer that separates the cell's interior from its surrounding environment. In some ways, it defines the outside, or it defines the boundaries of the cell. I'm drawing it in two dimensions; you can think of this as a cross-section, but in reality, it will have three dimensions to it. You could almost imagine it as kind of like a bubble, although it doesn't have to be a perfect sphere; it can be distorted in certain ways.

Now, the reason why I made this one bigger is because eukaryotic cells tend to be larger than prokaryotic cells, depending on which ones you look at, but roughly about 10 times bigger. There is variation on either side.

Now, another commonality is that they both have genetic material in the form of DNA. So I will draw that here; that's the genetic material. Let me label all these things. So we have our membrane, and this is a plasma membrane. Plasma membrane, it's in both of them. Then, we have genetic material, and I'm going to talk a little bit about how the genetic material might be a little bit different.

There's going to be a lot more stuff that I'm going to be drawing in this eukaryotic cell, but we also have DNA; that is the genetic material in both of these. I'm focusing on the similarities first. Now, they also will both have ribosomes. These are structures that build the cell's proteins. You'll learn more about that in the future. So they tend to be very small compared to the cell itself. So these are the ribosomes, and let me label those. That's where the genetic information is turned into, or it's used to construct proteins—ribosomes, right over there.

Now, these things aren't just floating in space; they're floating inside a jelly-like substance, which we call cytosol, which is also common to both. So this stuff here, cytosol, that is also common to both. That's where everything is floating inside of. When you consider everything inside the plasma membrane—the cytosol plus the DNA plus everything else that's in here, the ribosomes, and we're going to see a lot more in eukaryotic cells—you can call that the cytoplasm.

So, cytosol plus everything else in the cell, that is the cytoplasm. Now, let's think about how these things are going to be different. Generally speaking, eukaryotic cells are going to be more complex than prokaryotic cells.

So, prokaryotic cells, they tend to have a single strand of DNA. Oftentimes, it is in a loop and it's bundled in there, and we call that, and I'll do this in a color that I haven't used yet, maybe for the difference, we call that a nucleoid.

When we're talking about eukaryotic cells, you're talking about multiple strands, and they are inside what's called an organelle that itself has a membrane, and that organelle is called a nucleus. Let me write also multiple strands—typically multiple strands of DNA.

This idea of having a membrane-bound organelle, you could view an organelle as a substructure of the cell. That's actually not just when we're talking about the nucleus; there are also other membrane-bound organelles. Like, you have things like mitochondria. I'll just draw mitochondria right over here, but there are other membrane-bound organelles which you are not going to see in prokaryotic cells.

Prokaryotes are generally unicellular organisms, which means the whole organism is just a cell. Eukaryotic cells can be an organism on their own, so they can be unicellular organisms, but they can also be the building blocks of multicellular organisms, like ourselves or, say, like a tree.

Now, you'll also sometimes hear people talk about cell walls in the context of prokaryotic cells. Prokaryotic cells generally have cell walls that give them a little bit more rigidity, a little bit more strength on the outside. Eukaryotic cells can have cell walls; many plant cells have cell walls, but in many cases, they don't have cell walls. For example, animal cells don't have cell walls, so it's a little bit here and there with eukaryotes.

But it's generally true in prokaryotes. Then, prokaryotic cells are generally going to reproduce asexually, not using sexual reproduction. We'll talk more about that in other videos. While eukaryotic cells, depending on which type you're talking about and where they are in, say, a larger organism, some can reproduce sexually, while some will reproduce asexually.

Now, the last thing you might be wondering is, "All right, you kind of touched on examples of organisms that might have eukaryotic cells. I didn't talk a lot about prokaryotic cells. What are examples?" Well, generally speaking, if we're talking about prokaryotic cells, we are talking about bacteria, which you might be familiar with. We have good and bad bacteria that are constantly interacting with our bodies, and you also have this other type of organism called archaea. And I'm not going to go into the details of archaea, but they are also prokaryotes.

Now, what about eukaryotic cells? Well, here we're talking about animals like ourselves. We're talking about plants. We are talking about fungi—think mushrooms—and we are talking about protists. So I will leave you there. Hopefully, you now see both the similarities and the general differences between prokaryotic and eukaryotic cells.