Source: Video and Images Created by Amanda Soderlind
In this lesson today we are going to discuss the plasma membrane and its structure and function in a cell. So the two main characteristics I want to talk about the plasma membrane are, one, that is selectively permeable, and two, that is composed of a lipid bilayer.
So let's go back to number one, selective permeability. So plasma membranes are selectively permeable. What this means is that they can control what goes into and what comes out of a cell. They're able to control what's going in and what's going out of the cell.
And number two, they're composed of a lipid bilayer. So the prefix bi- means two. So this lipid bilayer is a double layer of lipids that is composing the plasma membrane. And the specific lipid is a phospholipid. So I have a drawing of a phospholipid right here.
So phospholipids are the main lipid that compose this lipid bilayer of plasma membranes. And they are made of a hydrophilic head and two hydrophobic tails. So the heads are attracted to water. The word hydrophilic means that they're attracted to water. And hydrophobic means that they're repelled by water. So the heads will face out towards the inside of the cell or towards the cytoplasm. And the tails will face inward, towards each other, away from where that water is.
So let's take a look at the structure of a plasma membrane right here. So I've just drawn a very simple structure of a plasma membrane. And plasma membranes have a very fluid quality. They're not rigid at all.
And they're actually very, very thin. So in order to equal the thickness of one page, one piece of paper, you would have to stack about 1,000 plasma membranes on top of each other. So that just kind of gives you an idea of about how thin they actually are.
And the plasma membrane, as I mentioned, is composed of phospholipids. But it also includes other lipids, like glycolipids and cholesterol.
In addition, the plasma membrane is embedded with several different proteins. And these proteins might be enzymes. They could be channels. They might be transporter proteins. They could be receptors for signalling molecules, such as hormones. Or they could be recognition proteins that help identify what type of cell it is.
So there's various, various different types of proteins that are embedded in the cell membrane. So I just drew an example of just one protein. So this is just a very simplified version of a plasma membrane. Actually a real one would have various types of proteins embedded in it.
But here's a protein right here that could be, for example, a transporter protein that would transport a molecule from the outside of the cell to the inside of the cell. So like I said, plasma membranes are selectively permeable. Some substances, such as oxygen, carbon dioxide, water, very small, nonpolar molecules, can just cross through the plasma membrane on their own.
But some molecules that are larger or that are polar are not able to cross through this membrane on their own. So they need to take advantage of these proteins that are embedded in the membrane, such as the transporter protein. So a transporter protein would be able to allow a molecule for example like sodium, which wouldn't be able to cross through the plasma membrane on its own, to get across the plasma membrane either into or out of the membrane by use of this type of protein. So again there's various proteins that are embedded in the plasma membrane that all have very different roles.
So this has been an example of what a plasma membrane looks like. And again, let's refer back to it being a lipid bilayer. So we discussed phospholipids. You'll see a phospholipid right here. And we have another phospholipid right here. So we actually have these two layers of phospholipids.
And you'll notice-- let's say that this is the inside of the cell. And this is the outside of the cell. So we have the heads, the hydrophilic heads, facing the inside and the outside of the cell. And remember the heads are hydrophilic, meaning that they are attracted to water.
So obviously we know that the inside of the cell, the cytoplasm, is made up of mostly water. And the outside of the cell is also going to contain a good amount of water. So these heads are facing out or in to where the water as, while the tails, we discussed that the tails are hydrophobic, meaning that they are repelled by water. So those tails are facing inward towards each other, away from water. So this is the basic structure of a plasma membrane.
So this lesson has been an overview on the structure and function of the plasma membrane in a cell.