This is the video for the standard level portion of B2.1 on membranes and transport. Now all cells have a plasma membrane, or you may hear that referred to as cell membrane. Those two terms are synonymous, and it is composed of two layers of phospholipids, and that is called the lipid bilayer, bi meaning two. It controls what enters and leaves the cell.
So in order for a cell to function, it has to be compartmentalized. It has to have some kind of control over what comes in and what does not. And we call that ability semi-permeable or selectively permeable. Again, those two words mean the same thing. Now, an ability of a molecule to get through that membrane or its permeability is in general based on the size and the charge of that molecule.
Large molecules are not as permeable. permeable. So they're going to require different ways of getting into or out of the cell. And the membrane is not permeable to polar or charged molecules.
And the reason being these little hydrophobic tails of the phospholipids, they really don't like them. Okay. So they're going to repel anything with a charge, like an ion or something that's polar, like let's say glucose.
So what is able to get through that membrane either in or out? Well, things that are small and things that are nonpolar. So something like an oxygen molecule, those can easily pass through the membrane. Everything else is going to need some kind of alternative method for either getting into or out of the cell.
Now it's important to remember that molecules are always in motion, okay? And that motion is going to result in random collisions, okay? So the closer molecules are to each other, the more frequently they are going to collide with each other. And what happens when molecules collide is that they bounce off of each other and move off into random directions.
And so when that happens, we're going to get these molecules bumping into each other and colliding with each other and moving off into random directions. And that is going to result in these molecules spreading out, going from where they are really crowded to where they are less crowded, and then less likely to bump into each other. That is diffusion.
Diffusion is the passive movement of particles from an area of high concentration, where they're really crowded, to low concentration, where they are less crowded. And when we say passive, what we mean is that there's no energy required, okay? So we don't need any energy to be added to this system, this all just happens based on the movement of those molecules.
Now this can happen with or without a membrane. So I've kind of shown you this process without a membrane. Remember they were all really crowded in here and they just diffused outward. So that can happen, right?
If you think about spraying perfume in the air, those molecules just diffuse. When it comes to going across a membrane, molecules are going to move from a high concentration to a low concentration, again, without the input of energy, okay? So some things that can diffuse across the membrane, these are going to be our small, nonpolar things like oxygen, and that movement is going to continue until we reach equilibrium. So equilibrium is when the concentrations are the same on either side of the membrane.
So that movement from high to low. will continue until that equilibrium point is reached, at which time diffusion or the net diffusion of molecules will stop. Now, everything else requires help getting either into or out of the membrane.
Only those small nonpolar things can just diffuse. And in order for those molecules to either get in or out of the cell, it's probably going to require a protein. So remember, embedded within the cell membrane. we're going to have different proteins.
Some of them are going to be peripheral proteins, just sitting out here facing either outside or inside, but a lot of them are going to be integral proteins. So integral proteins span all the way across that cell. Now, I like this little mnemonic, jet rat, to remind me what these different proteins embedded in the cell membrane can do. So they can help cells join together.
They can act as enzymes. They can help with transport. More on that in a minute. They can help with cell recognition or attachment to other cells.
Or they can either be like transduction, like receptor points for hormones. hormone signaling. What we're going to really focus in on is this one here.
So this T for transport. And these proteins that we'll talk about can either act as channel proteins or as protein pumps. Both channel proteins and protein pumps are what we call integral proteins. And that means they are transmembrane proteins.
So this prefix trans meaning across. And that means it goes all the way across the membrane, all the way from outside the cell to inside the cell. And that means it's going to be coming into contact with both the polar heads and the nonpolar tails. So that means it's going to need polar regions and nonpolar regions. All right.
In addition to that. It's got to make sure that its polarity matches whatever is moving through here. So if I have something like glucose, which is polar, or an ion, which is charged, that means the amino acids lining this inner channel also need to be polar. So we really got to think about this form and function. If a protein is going to function as a transmembrane protein, its form, those amino acids, need to be aligned.
with the polarity of the different parts of the membrane. So some examples are things like ATP synthase, which is an enzyme in cell respiration, channel proteins, which we'll talk more about in a bit, and protein pumps. Now peripheral proteins are proteins that are only attached to the surface or periphery of the cell membrane.
They're not going to go all the way through like one of those channel proteins. So here's a great example of one. Okay, so it is only facing like the outside of the cell.
It doesn't go all the way through. Some of these proteins can function as glycoproteins for cell recognition. Cytochrome C is an electron transport chain protein.
There's lots of different examples here. But again, their different functions are going to require them to have different forms. Again, They need to be, their amino acids in different places need to align with the polarity of those phospholipids.
In a minute, we'll take a look at a special channel protein called an aquaporin. And an aquaporin, I love this. It's exactly what it sounds like.
It's a pore to help aqua, water, move into or out of a cell. And that process, water moving into or out of a cell or across some membrane. is called osmosis, okay? The net movement of water molecules across a semi-permeable membrane.
It is passive. That means no energy is required. And so let's say I have a scenario like this down here, and these little red dots are going to represent some kind of solute, okay? So I don't know.
Let's say glucose for fun. And you can see that I have a high concentration of glucose and a low concentration of glucose, and they are separated by a semipermeable membrane. So I'll show you that a little bit down here.
That's this semipermeable membrane. Now let's say this membrane is not permeable to this solute. Oh no, it's stuck. But equilibrium still needs to be reached.
So if the solute cannot cross the membrane, then water will cross the membrane, and it will keep moving until the solute concentrations are equal. And so water we'll find will always flow towards the areas of high solute concentration, okay? So some of this extra water in here, maybe I should label that more clearly.
So some of this extra water is going to move towards the areas of high solute concentration, and that will continue until maybe not the amounts, but the concentration is equal. Now, even though water is polar, and even though these tails really don't care for water because they're hydrophobic, water is small enough that it can still, in some small amounts, make it through the phospholipid bilayer. However, especially for cells that have a lot of water moving in and out, like in our kidneys, We need that process to be much more efficient, and that is going to require a special type of transmembrane protein called an aquaporin.
So these are special channels that make it easier for water to move via osmosis. So what these aquaporins do is you can see they kind of like form a barrier. a hole in the membrane where we don't have to worry about water coming into contact with those hydrophobic tails.
So it makes osmosis happen a lot faster and easier. Cells can actually do really cool things like controlling the amount of aquaporins to make things permeable or not permeable to water. So cool. It's just important for right now to understand that aquaporins are special channel proteins.
that allow for the movement of water, and that movement of water towards an area of high solute concentration is called osmosis. Now you've heard me mention channel proteins a couple of times. Channel proteins are associated with our third type of passive transport.
So we've talked about osmosis, the passive movement of water, diffusion, the movement of molecules from areas of high concentration to low concentration. And now we'll talk about the third type, which is facilitated diffusion. In my experience, it's really easy to get confused here and think that facilitated diffusion requires energy, because this word facilitated kind of sounds like helping, but it is not. Facilitated diffusion is passive, so the passive movement of particles from areas of high concentration to low concentration through a channel protein.
It is the same thing as diffusion, right? So molecules moving from areas of high concentration to low concentration. It's just that in diffusion, those molecules are moving in between the phospholipids and in facilitated diffusion, they are moving through a channel protein.
But since it is going from high concentration to low, still passive, no energy required. Why do we need channel proteins then? Well, remember these hydrophobic tails don't like charged or polar molecules. So things like glucose or other polar molecules or ions are going to require these channel proteins, okay, to kind of like make a hole in those phospholipids.
What's so cool about this is that channel proteins are specific to different molecules. So I need a glucose channel. And I need a sodium ion channel.
And I need a potassium ion channel. I need different channels for different molecules. And cells, when we say that their membranes are selectively permeable, well, one of the ways that they control which molecules come in or out is by either manufacturing certain channel proteins or not.
So if you're a cell that wants to let glucose in, you better be manufacturing a glucose channel. So. If you're a cell that doesn't want to let glucose in, you don't manufacture that.
Some channels can even be manufactured but open or close at different times. And so all of this relates back to this idea of form and function. The function of the cell is going to dictate the form, right? So which channels either need to be produced or opened in order to let those molecules into and out of the cell. Now let's talk about a different type of molecular transport, which is active transport.
I love this. It's exactly what it sounds like. It's active. It requires energy. And the reason that it requires energy is because unlike diffusion and facilitated diffusion going from high concentrations to low concentrations, in active transport, things are being moved from areas of low concentration to areas of high concentration.
And this is what we call against the concentration gradient. So if I was going to give molecules feelings, I would say these molecules like being spread out and having plenty of space, and you're going to need to put energy into them if you were going to force them into a crowded area. In reality, it's about those collisions.
that if you're going to force them into a place with lots more of collisions, and you're going to keep them there and keep them from bouncing away, you're going to need to add energy into that. And so how do we get these molecules to be going into really crowded spaces? Well, we're going to need something called a protein pump. It's exactly what it sounds like. It's a pump made out of proteins.
And they are specific to different molecules. So I might have a glucose pump, or I might have a sodium pump. Again, very specific.
And when you add in this energy molecule called ATP, then it's going to change the shape of this protein ever so slightly to move that particle towards that area of high concentration. And so that is really why we need energy in the form of ATP here. So to sum this up a little bit, membranes have to be selectively permeable. Now there are some things like oxygen and like water that are always going to be at least somewhat permeable.
But the cell needs to control what goes in and out. And again, to do that, it can either manufacture or not manufacture certain channel proteins, or it can open or close them. It can choose to manufacture certain kinds of protein pumps.
And just in general, the structure of the lipid bilayer and these hydrophobic tails is going to naturally cause some molecules not to be able to pass through the membrane. So there are a lot of different structural components that all relate to this function of semi-permeability of that membrane. Now the membrane...
isn't just there to control what goes in and out of cells. There are some other really cool features of the membrane that also aid in its functionality. And a lot of them have to do with recognition, right?
So glycoproteins are proteins that are embedded in the membrane and they have a carbohydrate chain always sticking to the outside. So this is going to be really important both for recognition and for joining cells together. Glycolipids are exactly what they sound like. They are a lipid. They're going to be embedded within those hydrophobic tails with also a carbohydrate sticking on the outside.
These are going to be particular to eukaryotic cells. Prokaryotes don't have these. And again, they're used for recognition. We're going to talk a lot more about these in the immune system.
One of my tips here is that if you get a little bit confused... Use your knowledge of prefixes here. So glyco means sugar. This is a complex of sugar and protein. And again, this is a complex of sugar and lipids, both embedded in the membrane, both with some recognition features here.
So if we get confused, you can just remember those. And we'll end this video by just talking about the model of the membrane in general. Now, this is something that you should know how to draw. So you may want to take some time later on to practice drawing this.
And when you're asked to draw this, you may be asked to draw what's called the fluid mosaic model. Now, mosaic means that it's made up of many parts. Fluid means that those parts can move around.
So what's really cool is that if this protein isn't needed here, but it's needed elsewhere in the membrane, it can just move over to another part. Now, this is the current model of the cell membrane. It hasn't always been the model that has been widely accepted, but it's the current model with the most evidence to support that this in fact is how cell membranes or plasma membranes are structured.
So what do we need to remember about this? The lipid bilayer of those phospholipids and the different protein components that can be moved around and definitely something to keep an eye on. both from a functional point of view and from a form point of view and make sure we can draw that.