Transcription
All right, there are um two things that we, two terms that are important, uh, when we talk about cell membranes. The first one is that membranes are what we call selectively permeable. So, selectively permeable. All right, what that means, uh, pretty straightforward. Selectively means that they get to choose or select to pick, um, and then the word permeable means what can get through. You when you permeate something, you kind of get through it. So, selectively permeable together, this means that the cell membranes get to choose what substances can pass through them. It's not just an open door.
So, the membrane gets to pick what can go from the, this is the outside of the cell, and this is the inside of the cell, and this is the phospholipid bilayer that's making up the cell membrane, separating the out from the in. The cell membrane gets to choose what molecules can move in, and it also gets to choose what molecules can move out, and it can control those things. So, it is, it isn't just like an open door policy. There is, uh, lots and lots of checks and balances and like, um, uh, ticket holders to make sure that, ticket checkers to make sure the right things are getting in and the right stuff's getting out. So, that's selectively permeable.
The other term that we use is that we say the membrane is a fluid mosaic. Fluid mosaic. Now, um, what does that mean? Well, a mosaic is a picture that's made up of lots of little pieces all stuck together to build a bigger, a bigger design, right? And fluid, uh, means in science, it means kind of flexible or movable. So, a fluid mosaic, um, describes something that's made up of lots of individual pieces, um, but there, but it's not fixed in place. All those pieces are kind of moving around.
And so, if you look at this, the overall diagram of a membrane, you can see it's a lot more complicated than just a simple phospholipid bilayer. Here's the bilayer, these are the, these are the phospholipids with the gray heads and the yellow tails, but there's lots of other stuff going on in here as well. And I'm going to get to what all those things are in some later, um, videos. But, um, you can see that there's, there's the phospholipids, but there's also these purple blobs and then these green hexagons. There's a whole bunch of stuff that all goes together to make this structure.
So, I like to think of this a little bit like, if you imagine a bathtub or a, um, a swimming pool all filled up with ping pong balls, and the ping pong balls were representing the heads of the phospholipids, like you can kind of see on this diagram stretching off into the distance. Um, and then if I was to throw like a couple different other balls in there, like a volleyball and a beach ball and a couple of footballs and a soccer ball, they would fit down in between the ping pong balls, but they could move around, right? The beach ball could move over here, and it would still be surrounded by ping pong balls, but everything could move around. And, and if I press down on one side of the bathtub, it would all kind of go boom and everything would move and jiggle around. So, I've got a mosaic of different types of balls all kind of fitting together, but they're also moving around relative to each other. And that's what we mean by a fluid mosaic. And that's kind of the, sort of the best mental image to have of a membrane. All these phospholipids lined up in this bilayer, and then other things inserted in the layer, and it's all kind of moving around. So, it's kind of an interesting mental picture to make.