Hi everybody, your AP Bio teacher here, Mr. Poser. Today we are continuing our second unit on cell structure and function by continuing with topic 2.4 which is on plasma membranes and the actions of the plasma membrane and what happens in it and along it is going to be a very very big topic for us going forward in this unit. It's going to be very important. for us to discuss.
And today is going to be our introduction to it. It's going to be like what is it made of, what kind of molecules make it up, and what's the structure of the plasma membrane. And we already kind of know a little bit about it.
We talked about the function of it in our first topic, 2.1. So we know that the plasma membrane separates a cell from its surroundings and allows cells to maintain a separate and stable internal environment. Now, your cells are full of membranes, right?
Your tissues are full of membranes. Your body's body is full of membranes. However, when we're talking about the plasma membrane, we are talking about the membrane that surrounds the outside of the cell. It keeps the cells insides intact, doesn't allow it to like float out everywhere.
And it keeps the cell in a stable, or it keeps the cell in a stable internal environment, right? So whatever's going on outside of the cell shouldn't play too much of a factor on what's going on inside of the cell. It allows the cell to maintain homeostasis in a way.
So it separates it from its surroundings. And it's kind of taboo to compare the cell structures to, say, like buildings or, you know, have catchy analogies for it. But the cell is kind of like a gatekeeper. Excuse me, the plasma membrane is a gatekeeper for the cell in that it allows certain things into the cell and keeps other things out.
And it allows certain things out, but it keeps other things in. So that's what we call selectively permeable. These membranes are selectively permeable in that they allow some substances to cross it more easily than others. So if you're being selective, you're choosing, right?
You're choosing what comes in, what goes out. And if you're permeable, that means things can pass through, right? If you're impermeable, things can't pass through. So if you're selectively permeable, you're allowing, and you're a cell membrane, you are allowing some things to pass through. You're choosing what's able to pass through and what's not able to pass through.
Okay? So let's keep going here. Let's talk about the structure of the cell membrane. The current model for the animal cell membrane consists of proteins, that should say, proteins, glycoproteins, and sterols. that can flow in a phospholipid bilayer.
This model that we have is called the fluid mosaic model. So the cell membrane, you know, as simple as we can distill its function down into, it's a pretty complex structure. It has a wide variety of membrane proteins, channel proteins, carrier proteins, receptors, signaling proteins, glycoproteins.
It has all sorts of different types of molecules embedded all within this cell. what we call the phospholipid bilayer. And we're going to take a closer look at that in just a minute. But we call it the fluid mosaic model because the cell membrane, think of a bubble, okay?
Like an actual soap bubble. Hopefully we'll be using those in class. But a bubble isn't rigid, right? You can pop a bubble pretty easily. You know, if you have a bubble in a frame, you can kind of make it move around and go, right?
It's fluid. That's pretty much the same consistency of a cell membrane. It is a...
fluid structure. A lot of times we think of a cell membrane as rigid, it's like a force field around the cell, but it's not like that. And we call it a mosaic because it's kind of peppered with these proteins and various other molecules that are all embedded in what we call this phospholipid bilayer.
So most of the cell membrane is made up of these phospholipids, but these proteins are going to play a huge role in the other functions of the cell membrane, or the plasma membrane, I should say. All right, so So let's talk about the structure of the plasma membrane here. As I said just now, phospholipids constitute most of the membrane.
So as the name suggests, phospholipids are lipids. And what separates out phospholipids from, say, like triglycerides or sterols or other types of lipids that we've talked about is that phospholipids have a phosphate group. And they have, well, that means they have phosphorus in them. Phospholipids. Phospholipids, here's a fancy word for you, are amphipathic.
Amphipathic, say that 10 times fast, I dare you. They have both a hydrophilic and hydrophobic region. So let's take a look at this picture over here.
This is your typical phospholipid. phospholipid and you have millions of these in a cell membrane. The top portion here is what we call the polar head, okay, and the bottom portion is what we call the nonpolar tail.
So we talked about polar versus nonpolar in our very first topic, 1.1, when we talked about the nonpolar about how water is a polar molecule, right? So a phospholipid has one polar side and one non-polar side. Okay, sorry, I'm getting texts here. But the polar head, okay, it's you know, polar like water. And over here we have two fatty acid tails, which are nonpolar.
All right. So as I said, a polar head and nonpolar tails. All right. So this is my phospholipid bilater that I drew.
I'm going to to get more into these drawings as we make the videos. So phospholipids form what we call a bilayer, bi meaning two. So two layers of hydrophilic heads facing out and hydrophobic tails facing in. So most of the plasma membrane is constituted of these phospholipids that are kind of facing each other like this, or they're facing away from each other, I guess. But the tails are towards the inside of the membrane and the hydrophobic, the hydrophilic heads are by the outside.
Okay, so the cell is in always an aqueous environment, meaning there's lots of water around. Okay, so there's water on the outside of the cell and there's water on the inside of the cell. However, you know, just take a look at this word, hydrophobic.
There's not going to be any water that sits in between. these fatty acid tails. Hydrophobic means water hating.
It doesn't like water. It's going to repel it away. All right. So the structure of this phospholipid bilayer really, really plays a huge role in what the phospholipid bilayer in terms of molecules, what it's going to let pass through and what it won't let pass through. All right.
So if we take a look at here a little more detail when it comes to phospholipids, fatty acid tails. As we know from our first unit, fatty acid tails can be unsaturated or saturated. When we built our triglyceride models in class on paper, what we did is we had... unsaturated fatty acids and we had saturated fatty acids.
And unsaturated fatty acid tails increase membrane fluidity. So this one with the bend in it, with the double bond, is going to be more fluid than, say, the saturated fatty acid tails. They're going to be able to pack more in together. One last thing, a lipid called cholesterol, which you may have heard of before in the bilayer, also increases membrane fluidity. So a membrane can be more fluid with some added lipids.
Okay, I believe that is it for this. video on 2.4. Please let me know if you have any questions, and we'll get to membrane permeability in our next video. Bye, everybody.