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MICHAEL CIMA: So first we're going to remind you about something called a
fatty acid. And we talked about this numerous times, but
this becomes important. A fatty acid is just something that's got
a lot of hydrocarbons. It goes on like this.
And then finally it ends with a carboxylic acid group.
And so this one here in this picture-- oh I can't see how many-- that's palmitate.
So, what is that? How many carbons?
12? I think that's 12 carbons or something like
that in this. And you can see, it's totally aliphatic.
Just the CH2 groups. The other thing that can happen in these fatty
acids, they can be unsaturated, right?
So, remember, saturated and unsaturated fats? This is unsaturated.
It's got a double bond in there. You can see what it does to the molecule,
it puts a kink in it. Now, this is important, because these things
make up a lot of the tails in surfactants.
So I can choose a surfactant with a tail that looks like that.
Or I can change, it'll look like that. Now, let's go to some surfactants that have
relatively small polar groups. When I say relatively small, it means compared
with the non-polar group, right?
It's this ratio that you have to worry about. And the most important are these--
and I drew one last time, so you have it in your notes--
is what are called phospholipids. So here's one here.
It's got this non-polar head group. It's got what's called glycerol here.
It's a glycerol group. And then it's got--
in this case-- it's got a very water-soluble phosphate quaternary
ammonium combination.
Remember we talked about this? And this guy has no net charge, but it's definitely
charging in water. And, in fact, if you were to crystallize this,
it would be like a salt itself.
And we'll talk about some other examples of that.
But, clearly, what we've got here is-- these are not shown to scale.
This is relatively small compared with this thing.
And so these type of phospholipids, when you put them in water make--
go back a slide-- they make these, right?
They're all the way over on that side of the scale.
And why they're so important is because these things, these
phospholipid bilayers make up the membranes that are not only the cell
wall, but all the membranes inside the cell, the myelin membrane that
surrounds your nerves. All these membranes that make up your body
are composed of these surfactant molecules.
And here's some other ones. There's all sorts of them.
Here's another one. This one is negatively charged at the end.
So this one has zero net charge. This one is negatively charged at the end.
It's a phosphate group with, basically, is one unit of a sugar that
we'll talk about. And then there's some that have no charge.
And the interesting thing about this is when I mix these to make this
phospholipid bilayer, by adjusting the composition, I can actually make the
surface charged or uncharged. Or I can make one side of it charged, and
the other one not charged. And, so in fact, it's very interesting, the
interior surface of the cell, the cytosol side of the cell, has
a different phospholipid composition on the outside.
OK, here's just another picture. What this is supposed to show--
I found this in a book. I thought it was a sort of the space filling
model type. You'll notice that some of these have kinks
in them. So, in other words, this is a fatty acid group
that has a double bond in it.
And so that affects the structure of this bilayer, depending on what
composition is in it. Oh, this just shows you typical--
this is just a chemical composition of some purified membranes in your--
some of these are in your body, some of these are in plants.
But you can see the membranes of your body have protein that we're going to
talk about and then lipid. Some of them, like myelin, is mostly lipid.
Myelin is the protective sheath that goes around the nerves in your body.
Now, this is solid state chemistry class. Why are we talking about this gunky stuff?
Well, it turns out, these things are solids. They have a yield stress.
Remember, we talked about yield stress? Here's strain, and here's stress.
If you push on this membrane and let go, it comes right back.
And this is its elastic region, and if you pushed above the
yield stress, if floats. Just like a wall of gel.
OK, so gel is a material, soft material but a gel.
And they exhibit melting points. So, if you take it purified--
a purified phospholipid layer. One that's just one phospholipid it actually
exhibits a melting point. In other words, these things are kind of regularly
packed. And then you go above a certain temperature,
and they start to jiggle around.
And there's literally a change in volume. Just like the whole thing.
And still, it's still a lipid bilayer, but now these things are random in
there, not packed. And, not too surprisingly, you start messing
with the kinks in here, and guess what?
You changed the melting point. So if I mix two phospholipids--
oh I have a phase diagram, right? Purified one on this side, melts at one temperature,
purified one on this side melts at another temperature.
And I start to mix them, I could have a eutectic. It's possible I could have a higher melting
compound. A two dimensional solid is what we're talking
about here. It's exactly the same as what we learned before.