Today I want to introduce you

to the wonderful world of cell membranes.

Okay, so if you ask people to draw a cell,

generally what you'll get is a circle,

and most people don't really think too much

about the circle itself, but it's actually really important.

The circle really defines, you know, you versus everything else

in the universe that's not you, but it's more complicated

than that because inside the cell you're going

to produce wastes that need to get out, and there's going

to be food and good stuff that needs to get in.

How are you going to do it?

What happens if you puncture this barrier and goodness

from the inside leaks out and bad stuff

from the outside leaks in?

How are you going to manage all this?

So the circle itself is actually worthy of our investigation.

So what the heck are these membranes like?

Are they sort of like a container or, you know, a bag?

I mean, that separates, you know, inside from outside.

I mean, oh my God.

That's so cute.

Is it like a barrier?

I mean, if you get a hole in your barrier, that's not good.

The truth of the matter is that membranes really aren't

like any container or barrier that we're familiar

with in our everyday life,

so that makes them a little bit harder

to imagine and talk about.

Membranes have to control what goes in and out of the cell.

They have to be free to move and signal and communicate

and repair and do all kinds of other things.

I mean, so much of what the cells do happens

at the cell membrane, but you always have this balance

of control versus freedom.

So how are you going to build a barrier

that can do all of those things?

One of the more amazing things I think that membranes do is

that they form all by themselves,

and if you puncture them, they reseal immediately.

It's kind of like magic.

How the heck are we going to do that?

The magic of cell membranes comes from the fact

that they're built from molecules called amphiphiles.

Now, that may not be a term that you're familiar with,

but it comes from the Greek, and there's a word

that you probably do know, and that's amphibian.

Amphis means both, so amphibians live both on land

and in the water, so that will help you to remember it.

In chemistry, we're talking about molecules

that have both a hydrophilic -- that is, a polar end --

and a hydrophobic -- that is, a nonpolar end.

So these are really cool molecules

because they kind of play both sides.

There are actually amphiphiles that are present

in our regular life in the kitchen and so forth.

We use amphiphiles for detergents as well as for soaps.

We use them for surfactants.

They help to break up fat.

They make fats dissolvable.

This is a really cool little mnemonic device

that you can remember.

Amphibians hold amphiphiles.

So this little image can help you to remember that word.

The secret then is not in the sauce;

it's in the amphipathic lipids that build cell membranes.

So we have on every molecule of a phospholipid a hydrophilic --

that means water loving -- polar head.

This polar head is comprised of a glycerol molecule,

a phosphate, and then there's the side chain in our group.

We'll see what those are here in a second.

And then there is a nonpolar end that's made

up of two fatty acids tails, and they are hydrophobic.

They are water fearing.

So you have this amazing, amphipathic molecule

that has one side that is polar and likes water and one side

that is nonpolar and fears water.

Oh my gosh.

We're going to see why that's such a great thing,

but before we do that, I said I would show you some

of the actual particulars

about phospholipids and also glycolipids.

If you need to know a little bit more detail about them,

probably one of the more common varieties

in animal cell membranes is this guy, phosphatidylcholine.

So these are the different options, and then the rest

of it, the, these are the fatty acids that are all trailing.

These are the hydrophobic parts of the beast, okay,

if you needed a little bit more detail there.

So so what?

What you got to remember is that in chemistry, like likes like.

So polar molecules stick together

and nonpolar molecules stick together, but polar

and nonpolar molecules do not mix.

You have seen this if you have poured oil in water.

Everybody knows that oil and water don't mix.

The reason is that water is polar and oil is nonpolar.

They hate each other.

So you'll get this film of oil on top of water.

So what if you have a molecule that plays both sides?

And that's the key.

We've got these amazing phospholipids

that will spontaneously form a bilayer all by themselves.

Oh my God.

That's so cool.

They actually will form other shapes too, but for the purposes

of cell membranes, this is the shape

that we are most concerned with.

The most favorable shape for phospholipids to make is a shape

where all the polar ends face the water because they

like water and all the nonpolar ends face each other.

The fatty acid tails point inward.

Isn't that cool?

If you really want to see this, I guess the easiest way

to see it is if you play with your soup

and you have some oil drizzled on the top of your soup

and get yourself a fork, okay.

Don't let anybody tell you not to play with your soup.

Get yourself a fork and experiment

with different amounts of pressure and speed

and drag the fork through the oil and see

if you can get the big globs of oil to break

up into smaller globs, and then see

if you can get the little globs to join together

and congeal into big globs.

If you watch the behavior of the oil, that's really kind

of the most similar thing that you can see with the naked eye

that is similar to what phospholipids do in solution.

So in 1972 after a lot of study --

membranes are actually very delicate,

so it turns out they're very hard to study --

they were able to freeze and then split a membrane

and expose the center of the phospholipid tails,

and that's what you're seeing right here.

And then they, it's called freeze fracture

electron microscopy.

They were able to actually verify

that this is what cell membranes look like

and these are the different components of cell membranes.

So what you've got, about 75% -- this is for animal cells,

by the way -- about 75% of the membrane is made

of these phospholipids, the molecules

that we just looked at, which the polar end shown here

and the fatty acid, the nonpolar end, shown there.

And they insert themselves all by themselves

into a bilayer like this.

In addition to that, we have glycolipids.

They make up about five percent of the membrane.

And cholesterol.

Check out the cholesterol.

Cholesterol, about 20% of most animal cell membranes can

be cholesterol.

That can be as high as 50%, by the way.

Depends on the species of animal.

The cholesterol will actually stabilize the structure

of the membrane, and by stabilizing the phospholipids,

animal cells actually get away with not having

to have a cell wall, which is pretty cool.

The other thing that you notice throughout are proteins,

and some proteins span the cell membrane like these.

We call them integral proteins.

And some of them are only on one side

of the cell membrane or the other.

Could be the inside or the outside.

We call those peripheral membrane proteins.

You might think for a minute

about how a cell could anchor proteins in a cell membrane,

and how you do it is of course you recall

that proteins are made of chains of amino acids,

so how you anchor something into a structure

that is not a solid is you have

to make the membrane-spanning portions nonpolar,

so they like to hang out with the fatty acid tails.

And then on either side of these integral proteins, so here

and here, these amino acids would be polar.

So they don't like to hang out with the fatty acid tails.

They prefer the polar heads

and the water that's surrounding this bilayer on both sides.

So that's pretty amazing.

All of this allows for the amazing variety

of membrane functions that we're going to see as we go

through biology and physiology.

Membranes really do account for a lot

of what cells are able to do.

One thing that's really important is membranes allow

for compartmentalization, and that's going

to allow us to create gradients.

Something you're going to hear me say all the time is,

you know, this is a gradient-driven process.

Well, how do you have a gradient?

You have to have a separation.

You have to have one side which is different

from the other side -- different in concentration, in pH,

or whatever the variable is --

but membranes allow for that to be possible.

Membranes are going to allow for cells to recognize each other.

They're essential for cell-cell recognition, communication,

for one neuron to talk to another, for hormones

to be received by receptors, and so much more.

So you're going to see as you go through your studies of biology,

membranes are essential.

I hope that that introduction

to cell membrane structure was helpful.

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Good luck.