Original subtitles

PROFESSOR: One more thing you need to know.

Whoops.

The one more thing you need to know is something that's not really a bond or

a force at all, but it is what we call hydrophobic forces.

What do I mean by hydrophobic forces?

What I mean is if I give you a long molecule like this, and all it is is a

hydrocarbon, lots of carbons with hydrogen around it, and you know that

all of those bonds are non polar, right?

Will that make any interactions, any hydrogen bonds with water?

No, because it doesn't have any polar bonds there.

So suppose I were to drop this poor, unsuspecting molecule into my highly

structured cage of water.

Bounce it around there.

When it gets in there, like a bull in a china shop, it is disrupting

hydrogen bonds with the water molecules would like to

make with each other.

It's breaking favorable bonds.

It's energetically unfavorable for this non polar molecule to be coming

into this polar liquid.

And when it occupies any space, it is breaking apart hydrogen bonds.

That's a big energetic cost.

Suppose I were to take a whole bunch of these non polar molecules and throw

them into water, and every one of them is there, disrupting the neighborhood,

breaking up the hydrogen bonds in its neighborhood amongst the water.

That's energetically very unfavorable.

The waters would all like to be communicating with each.

Other hydrogen bonding with each other, and they would like to push out

these interlopers.

These non polar molecules.

And what's going to happen?

Yep.

They're going to separate .

Oil and water.

That is oil and water.

If I try to mix together oil and water, they're going to separate.

I got oil and water.

Not mixing.

And that's because the non polar molecules can't hydrogen bond.

So again, macroscopic.

Big, observable difference comes from these hydrogen bonds.

All right.

So you now know everything.

Most everything.

You now know enough.

You know enough that we can now start applying what we've learned to try to

explain some of the amazing properties of life.

Section five.

Understanding the properties of molecules.

Let's try something.

Let's try to understand the cell membrane.

Example.

Making a membrane.

It's arguable that the distinctive critical thing about life is having a

cell membrane.

I've got to separate the inside from the outside.

If I've got a cell, an early cell, I've got to have some bag, some sack

of material separate from the whole primordial sea.

And so having a membrane is utterly critical to having life.

Why does this magical membrane get formed?

Well, it turns out simple chemistry tells us how the magical membrane gets

formed, and it's not so magical.

Let's start with our highly nonpolar molecule over there.

Let's take that guy.

I'm going to put six Cs here, but, you know, it could be different numbers.

And, oh, by the way, I'm beginning to drop my hydrogens.

And thre will be a whole deep dive in the course about how to write

molecules, and I'm just going to drop the hydrogens right now.

Tell me about the property of this molecule.

Is this going to be a molecule that likes to be in water or not?

No.

Not like to be in water we call hydrophobic.

It's hydrophobic.

It doesn't like to be in water.

It fears water, versus hydrophilic.

Liking water.

This is a hydrophobic molecule.

Now, it turns out I can modify this hydrophobic molecule in

the following way.

OH.

A group that you'll come to learn on the homeworks is

called a hydroxyl group.

OH is a polar bond.

It turns out that nearly putting an OH there converts this into from a

hydrocarbon into what we call an alcohol, and it can dissolve in water.

The ability to make that polar bond with its OH is enough to make it

dissolvable in water.

That's soluble.

This becomes hydrophilic.

Now, it's not absolutely hydrophobic, absolutely hydrophilic.

There are degrees of hydrophobicness this and hydrophobicity and

hydrophilicity.

But it becomes more hydrophilic.

Let me modify this further for you.

I'm going to put on another group here.

Gonna put on a carboxyl group.

Also hydrophilic.

All good.

All right, we're practicing our hydrophobics, hydrophilics.

Now let me carry out a chemical reaction here.

I'm going to take, boy, I'm getting bored drawing this.

So how about if I just do this?

And you'll know what I mean, OK?

Let's take that, and we're just going to represent our--

take another one of these guys.

And another one of these guys.

Three of these guys.

Three of these.

We call these fatty acids here.

These are fatty acids.

And I'm gonna do a chemical reaction with the following molecule.

The chemical reaction I'm going to do is I have those two hydroxyls pointing

at each other, and I'm going to do a reaction you're going to see again and

again and again and again and again, which is what's

called dehydration synthesis.

Two Hs and an O are kicked out to make a water molecule.

Water comes off, water comes off, water comes off.

And I now just instead have a bond here.

All right.

So I have my three long hydrocarbon tails.

I've got rid of my hydroxyls completely.

I just have that bond there.

Oxygen connected here, oxygen connected here.

This molecule, turns out, is called, let's see. it's got three of these

fatty acids.

It's sometimes called a triacylglyceride, or just amongst

friends, a triglyceride.

This molecule here is called glycerol.

But this thing here, when we combine them, is called a triacylglyceride, or

just plain triglyceride.

I bet, I don't know whether you've had it done, but your triglyceride levels

are measured by the doctor.

That's actually part of your cholesterol and fat and other-- and

they do an analyses, they'll measure your LDL cholesterol and your HDL

cholesterol, and you triglyceride levels.

Things like that.

That's what triglyceride is.

It's a hydrophobic molecule.

It's mostly very hydrophobic.

If I mix that with water, what would happen?

Separate.

OK.

So now, now, one more thing.

One more thing.

We'll get rid of this.

If I take my triglyceride, C, double O, O, CH2 here, CH3 here.

Two here, sorry.

C. Wiggle, wiggle, wiggle, wiggle, wiggle.

And I take off one of those three tails.

I just have two of these tails.

And instead, I put on here CH2.

I put on, whoops, a phosphate group, which is negatively charged.

What do you think a phosphate group is in terms of hydrophilic or

hydrophobic?

It's got these charges, philic, right?

It's got charges.

If it's got charges there, it interacts happily with water.

The other part of the molecule is hydrophobic.

This part of the molecule is hydrophilic.

Is this molecule hydrophobic, or is it hydrophilic?

Both.

It is confused.

This is a confused molecule.

Confused, technically is called amphipathic.

That's a word for confused, all right?

This molecule here has these two, I'm now going to even

simplify my picture further.

I'm just going to say wiggle, wiggle, wiggle, wiggle, wiggle, wiggle, and

I'm going to say this head here that's kind of negative, very polar, and this

is nonpolar.

And if I have that molecule, it's terribly confused.

And If I throw it into water, the head is saying, let's go hang out with the

water, and the tail is saying, you know, the water is pushing away the

rest of the tail, because the tail is interfering with all those nice

hydrogen bonds the water wants to make.

So if I mix this up, what's going to happen?

It's going to adopt a configuration that best resolves the problem.

And the way the low energy configuration that it's going to adopt

could be like this.

Point your charged hydrophilic heads out into the water solution and stick

those obnoxious hydrophobic tails in here, and can those hydrophobic tails

interact with each other?

Yeah, by van der Waals interactions.

Not very impressive, but they'll have a good little van der Waals party in

the middle.

And they've got these charges outside, and that's a structure that resolves

the problem.

That's one way to do it.

There's another structure that resolves the problem.

If I had sort of a glass, they could decide to do this.

Hydrophilic head groups out there, and then another layer here.

And what I would have is what's called a lipid bilayer.

This structure goes by the slightly funky name of micelle,

spelled like that.

This is a lipid bilayer, because very cleverly it's arranged itself to point

the hydrophilic groups out, and the hydrophobic groups in.

There's one other way to get this lipid bilayer to work, other than

having it going to the edges of my glass here, which is the following.

Water on the inside, water on the outside, lipid bilayer

all around in a sphere.

I don't need anything fancy.

If I shake it up, spontaneously, just by resolving this bonding property, I

get a membrane.

That's where membranes come from.

I don't need magic.

I just need those laws of bonding to get it to work.

So I get this very pretty structure here.

Somewhat better drawn there, of this lipid bilayer, and this is

the basis of cells.

Take a few minutes to think about what you've just heard by

answering this question.