Introduction to the cell | Cells | High school biology | Khan Academy

You might already have some type of a notion of what a cell is. You might already realize that it is the most basic unit of life. Some would argue that maybe viruses are even a more basic unit of life. But the organisms that we consider living, like ourselves, are made up of cells. All living organisms that we for sure consider living are made up of at least one cell, so the most basic unit of life.

For example, me—this thing that's making a video right now—I'm made up of tens of trillions of these cells. Now, a common misconception is, well, these things must be small, and they indeed are very, very, very small. Some cells are on the order of one micrometer long, and a micrometer is one millionth of a meter, or you could say one thousandth of a millimeter. When you think of something that small, sometimes there's an assumption that it must be simple, but you could not be more wrong if you assume that a cell is simple.

This right over here is a picture of a budding yeast cell. You can see that it's budding off right over here, but this just begins to show you some of the complexity of the cell itself or of any cell. In other videos, we're going to talk about different types of cells, different types of structures you'll see in some cells versus others. This right over here is a eukaryotic cell, which we will talk more about in other videos.

Now, all cells have a membrane that separates it from the outside world. You see the membrane right over here. This is just a cross-section; you could imagine a three-dimensional version of this. So, this is the cell. The cell membrane kind of defines the cell in some way, and in general, the things inside the cell membrane are considered the cytoplasm.

Cytoplasm—sometimes you will hear the term cytosol—the cytoplasm includes not just the fluid but also all the stuff in the fluid, while the cytosol is referring to the fluid alone. Depending on the complexity of a cell, so this is right here, this yeast cell; this is a eukaryotic cell, which we will cover in more depth in other videos.

One of the features of a eukaryotic cell is that you will have a membrane-bound nucleus. Now you see it in this diagram right over here. This is not a common feature to all cells, but the only reason why I'm mentioning it in this video is officially the cytoplasm does not include the stuff inside the nucleus. In a eukaryotic cell, that is called the nucleoplasm, but we'll talk more about that in other videos.

Now, another feature that is common to all cells is the notion of a ribosome, and this picture is full of ribosomes. All these little dots right here, these little red dots—let me change my pen color—all these little red dots here, these are ribosomes. You might assume, "Wow, these are so small compared to this already small thing—this cell; surely they are simple," but they're actually fairly complex RNA and protein structures. Their main function is producing protein.

You could view these as almost the protein factories of living organisms. They can take genetic information in the form of RNA and produce proteins out of them. You can see this cell is full of ribosomes, and we're going to talk about different types of ribosomes in a future video.

Now, another thing that is typical in most cells is genetic information, and typically that genetic information is stored as DNA. Now, I say in most cells because it turns out that even in our own bodies, mature red blood cells don't have any DNA anymore, and there are other cells that do the same thing. But in general, in order for a cell to function and replicate, it needs some genetic information, and that is stored in DNA. That's true in both prokaryotic cells and eukaryotic cells.

Prokaryotes are ones that don't have a well-defined nucleus and membrane-bound, what we call organelles, which are these substructures in cells, which we will talk more about in other videos. In a prokaryotic cell, the DNA is just floating around in the cytoplasm, while in a eukaryotic cell, the DNA, for the most part, is inside of your nucleus, and it is part of the nucleoplasm.

So, I'll leave you there for now. The last thing I want you to appreciate is just the scale. As I mentioned, cells are small. This picture of a yeast cell right over here—this is a micrometer. On this scale, it would be about that. That would be one micrometer. To put that in context, the width of a human hair actually depends on your hair; whether it's soft or more like my hair, and it kind of sticks up, and you have thicker hair.

But if this is a human hair right over here, this is the width of a human hair. Its width is anywhere from 20 to 180 micrometers; my thick hair is probably closer to the 180 micrometers. So, one way to think about it is you could probably take 20 or so of these yeast cells, end to end, and these cells—these aren't even the small cells by any stretch of the imagination—and put them end to end, 20 or 30 of these across one human hair.

And that's what's mind-blowing, because even at that scale, you have this complexity. Even this picture doesn't do proper justice to the complexity. There are all sorts of structures inside of this that you can't even see that help transport things, move things around, and give the structure of the cell.

So, I'll leave you there. In future videos, we'll dig a little bit more into what eukaryotic cells are, what prokaryotic cells are, how do cells move around, and how do they work together.