- [Voiceover] Microfilaments
are found in the cytoplasm and they are composed of
a protein known as actin, and many molecules of actin will join together to
form an actin polymer. And then the actin polymers
will twist around each other to form an actin filament. Microfilaments are mainly involved in the gross movement of the cell. And microfilaments are dynamic, and what we mean by that is that they can lengthen and shorten very frequently, and in that sense, they're
similar to microtubules. So, they become longer in a process known as actin polymerization, and they become shorter in a process known as actin depolymerization. And it's the polymerization
and depolymerization that helps to move the cell. So, one example of where
microfilaments help with the gross movement of the
cell is during cell division. So the cell at some point is
going to be pinched in that way and then eventually it'll
separate into two separate cells, and the microfilaments help the cell make that kind of pinched shape, and then move to create
two separate cells. Another example of where
microfilaments help with the movement of a
cell is in an amoeba. So let's say here is our amoeba cell, and we're gonna say that
it's trying to capture this piece of food right over here. So in order for it to do that, it has to extend its
pseudopods in that direction, and move it around that food molecule. And what helps those pseudopods move? The microfilaments. Let's move on to intermediate filaments. So, intermediate filaments, in contrast to microtubules and microfilaments, are made up of many
different types of proteins that are strung together into polymers. These polymers twist together to make the intermediate filaments. And intermediate filaments
are different than microtubules and microfilaments in that they're pretty much permanent. So, once they're made by the
cell, they kind of stay put, as opposed to microtubules... I'm just gonna abbreviate like that... and microfilaments that
we explained are dynamic. They constantly change depending
on the needs of the cell. So, what do intermediate filaments do? Well, they provide structural
support for the cell. Or another way to say this is
they resist mechanical stress. And in the picture it almost looks like the intermediate filaments
are this supporting wire, and if you recall in our
introduction to cytoskeleton, we mentioned that the
intermediate filaments can be compared to the
springs inside of a mattress. So, if you were to sit down on a mattress, you might squish it a little bit, but once you stand up,
the mattress will revert to its original shape,
thanks to the springs inside. So the intermediate filaments, in blue in this picture, act in the same way.