- [Voiceover] Let's talk about the cytoskeleton, and when we look at the word, we see skeleton, and the prefix cyto means cell. So, cytoskeleton simply means the skeleton of the cell, and although pretty much all cells have some form or another of a cytoskeleton, we're going to focus mainly on the cytoskeleton that's found in animal cells. So the cytoskeleton in cells serves purposes that are actually very similar to the purposes that our skeleton serves in our body. So the first thing is the cytoskeleton provides structural support. Number two, the cytoskeleton helps with movement, and number three, it helps with the transport of substances within the cell. These are the three basic functions of the cytoskeleton, but as we go along there are a couple of others that are going to come up as well. So now let's take a look at a cross section of a cell, and let's orient ourselves and label this diagram. So right over here we have the cell membrane. Then we have some mitochondria. We have the endoplasmic reticulum, and you can see it's dotted with these blue things. Those represent ribosomes. And finally let's get to the structures of the cytoskeleton. So we're going to go through the various structures, explain briefly what each of them do, and then afterwards we're going to go into each one separately and in more detail. The first structure we're going to mention are these green tubes that you see. These are the microtubules, and they have a diameter of approximately 25 nanometers, and the microtubules are involved in a number of things. They're involved in the mitotic spindle. If you're not sure what that is, we're going to discuss it in more detail later. The microtubules also make up cilia. Cilia are these hairlike projections outside of a cell that will help to sweep substances across the outside of the cell. They also make up flagella. Flagella are these tail-like structures that are outside of a cell and help move it. And again, if you're not really familiar with this, don't worry. We're going to go into it in more detail soon. And microtubules also help with transport of substances within a cell. The next structure we're going to talk about are these blue fiber-like things. They are the intermediate filaments, and they have a diameter of approximately 10 nanometers. And I want you to take a look at the diagram that's a cross section of a cell and see if you can maybe figure out what the purpose of the intermediate filaments is. So, the intermediate filaments basically provide structural support to the cell. And maybe a way to help you think about this is, we can compare this picture that we're looking at to a mattress. So we'll say that the cell membrane on either side is kind of like the top and bottom of a mattress, and let's say that mattress was filled only with air, maybe some other stuff that were really not hard, and if you sat on the mattress, maybe there's a hole somewhere in the mattress, some of the air might come out, and then when you got up, the mattress might be in a very flattened position. However, most mattresses have these metal springs inside. So, you sit on the mattress, it gets a little bit flatter, but then when you stand up, the mattress goes back to its original position. So those springs inside are providing structural support. So the intermediate filaments provide structural support in a very similar way. We can see that they help resist mechanical stress and help the cell retain the shape that they're supposed to have, in a way that's very similar to the springs in the inside of a mattress. And the last structure we're going to mention are these purple tubes. They are the microfilaments, and they have a diameter of approximately seven nanometers. And just keep in mind that microfilaments, intermediate filaments, and microtubules are all made out of protein. And microfilaments are involved in movement of the cell, and this is a movement that's different than the movement that would be provided by cilia and flagella. Cilia and flagella are both outside of the cell, and microfilaments help movement of the entire cell from within. So let's give two examples of this. One example is during cell division this cell will turn into something that looks like this. So it's going to be kind of pinched in the middle, and then eventually it'll separate into two separate cells. So how does the cell kind of move in that way and form the interesting shape? Well, with the help of the microfilaments. Another example of this would be amoeba, and amoeba's not exactly an animal, but we'll mention it anyway. So an amoeba has these projections known as pseudopods, and it will engulf a piece of food. The pseudopods will kind of reach out, engulf a piece of food, and that's how it ingests food. But how do those pseudopods move? With the help of microfilaments. And this applies to other cells that are not necessarily amoeba but move like amoeba and have pseudopods. They also move with the help of microfilaments.