Cell membrane overview and fluid mosaic model | Cells | MCAT | Khan Academy

In this video, we're going
to explore a little bit

about the cell membrane.

So just as a little
refresher, let's say

this is a picture of our cell
with a little tiny nucleus

in the middle.

Our cell membrane is what's
on the outside of our cell,

so our cell membrane is
what protects our cell

from a really harsh
outside environment.

If it weren't for
the cell membrane,

we wouldn't be alive
today, because there

would be nothing to protect
us from the outside world.

So we're going to talk
about the main three things

that make up the cell membrane--
the first, phospholipids,

the second, cholesterol,
and the third, proteins.

So the first one we're
going to talk about--

and this makes up the
majority of what's

in our cell membrane--
are phospholipids.

And just for the
sake of time, I've

predrawn a picture of
the cell membrane here.

And you'll notice that all
of these individual pieces

are phospholipids, and a
phospholipid looks like this.

It has that polar head group,
that polar phosphate group.

And it has two fatty acid tails.

And so this is the
way that we normally

represent what a
phospholipid looks like.

And in the cell
membrane, you can

see that these phospholipids are
packed pretty closely, pretty

tightly together, all
throughout the entire membrane.

And we're looking
at this membrane.

This is kind of like
a cross-section.

You can imagine that we
cut the membrane in half.

So what we have here
is actually what

we call our
phospholipid bilayer,

and sometimes it's also
called the lipid bilayer.

The second thing that we
can find in our membrane

is cholesterol.

Now, we often hear cholesterol
in foods and cholesterol

in our blood, and we
think it's a bad thing.

But in this case
cholesterol is actually

very important for
our cell membrane.

And cholesterol looks like this.

And again, just for
the sake of time,

I've predrawn what
cholesterol looks like.

And you'll notice that
cholesterol has a lot of rings,

and this gives cholesterol
a pretty stable structure.

And what cholesterol
does is cholesterol kind

of inserts itself
between phospholipids,

kind of like that.

And the way I think
about it is cholesterol

is kind of like a buffer.

It maintains the fluidity
of our cell membranes.

So as temperatures
become lower, cholesterol

will help increase the fluidity.

And as temperatures
become higher,

cholesterol will help reduce the
fluidity of the cell membrane.

So cholesterol keeps
our cell membrane

in kind of a happy middle
ground of fluidity.

And the third thing that
makes up our cell membrane

are proteins, and proteins
are actually a big one.

And depending on
the cell, some cells

will actually have
a significant amount

of protein in the membrane.

And so proteins can
take two major forms.

The first is you can
have a protein that

crosses the entire membrane.

We call this an
integral protein.

We also can call this a
transmembrane protein.

And this can occur throughout
different areas of the cell,

like that.

And some proteins actually kind
of sit on top of the membrane,

like this.

Or they might sit on
another protein, like that.

And these are what we
call peripheral proteins.

There are some
very rare proteins

that actually can go halfway
through the membrane.

And even rarer, there
are occasionally

a few proteins that actually
can be found inside the cell

membrane, like this,
between the two

phospholipids
inside our bilayer.

Now, proteins are
a very big player

in the function
of cell membranes.

They actually carry
out nearly all

of the membrane processes
that we can think of.

And the two biggest things
that proteins do is, the first,

they can actually
act as receptors.

So the proteins
can actually tell

the cell what's going
on in the outside world.

They act as communication.

And the second thing
that proteins can do,

which generally occur in
transmembrane proteins,

is that proteins can actually
help transport molecules

in and out of the cell.

So now that we know the
function of proteins,

why do you think proteins
that are lipid-bound or bound

within our lipid bilayer, like
this one here, is so rare?

Well, it's because if
the role of proteins

is primarily to act as
receptors-- to communicate

with our outside world-- or
to act as transport-- to allow

things to go from the
inside to the outside

or the outside to the inside--
the proteins that are kind

of stuck in between don't really
have a big role in our cell

membrane.

And lastly, there's one very
important type of molecule

that actually binds to our
lipids or our proteins,

and these are carbohydrates.

And we call these
glyco for short.

So they would be glycoproteins,
or they might be glycolipids.

And what these do is they play
a big role in communication.

So for example, it allows
a cell to recognize

another cell in our body.

If they play a role
in communication,

in cells recognizing
other cells,

where do you think
these sugars would go?

Well, these sugars
would mainly occur

on the outside of our membrane.

So they would kind of
stick out on proteins--

these would be
glycoproteins-- and they

can be on peripheral
or integral proteins.

Or they might stick out
on lipids, like this.

And these would be glycolipids.

Now, this a little
confusing to look at it.

What we've just drawn
is a cross-section

of our cell membrane.

But what if we were looking
at the cell membrane

from the outside, kind
of like a top view?

What would that look like?

Well, again for
the sake of time,

I've predrawn our phospholipids.

So if we were looking
at the cell membrane

from the outside-- looking onto
the top of the cell membrane--

all we would see are these head
groups of our phospholipids.

We might see some cholesterol
in between our cell membranes,

like this.

And we might see
some larger proteins

that are on top of
our cell membrane,

like this, scattered
throughout our cell.

And lastly, we might actually
see some glycoproteins

and glycolipids on the outside.

And these would
attach themselves

to our proteins and our
phospholipids, like that.

So from the top, this is
what our cell membrane

would look like.

And you know something really
special about this-- this kind

of looks like a piece of art.

So if we think back to
elementary school, where

we had the project where
we would put a lot of beans

or different macaroni together
to create a piece of art,

this kind of reminds me of that.

So this is actually
what we call a mosaic.

So scientists kind of
thought the same thing.

So scientists actually
named this model

of the cell the
fluid mosaic model,

and so the mosaic portion of
our cell can be described here.

Again, you can see that there
are a lot of different pieces--

different colorful
types of pieces--

put together to create this
beautiful cell membrane.

But why did we call it fluid?

Well, the reason we call
the cell membrane fluid

is because these pieces
in our cell membrane

can actually move around.

They're not set in stone.

So the proteins
and phospholipids

in our cell membrane can
move around, like that.

This is why we call it fluid.

What would that look like if
we look at the cell membrane

from the top?

Well, the movement
is actually not

two-dimensional-- just up and
down, or just left and right.

It can actually go in a lot
of different directions.

So our proteins can move all
around the cell membrane,

and so can our phospholipids.

So again, this is what we
call the fluid mosaic model.

And just as a little
bit of a fun fact,

this was only really
discovered in 1972.

So it was only 40 years ago
that we really figured out

that our cell
membrane was actually

the fluid mosaic model.

So in summary, our cell
membrane is made up

of three major things.

The first are phospholipids.

These make up the most
of the cell membrane,

and they're kind of like a basic
building block for our cell

membrane to exist.

The second are cholesterol.

Cholesterol is
scattered randomly

through our cell
membrane, and it

helps maintain the fluidity
of the cell membrane.

And the third are protein, and
proteins carry out nearly all

of the essential cell
membrane functions.

And together we call this
our fluid mosaic model,

because our cell
membrane is made up

of so many different things,
and all of these things

are always moving
around like a fluid.