So let's begin that exploration by taking a look
at the fluid mosaic model,
which is the current model of
what we understand of membrane function.
Previously, we've covered that the membrane
is primarily composed of a phospholipid bilayer.
Also we're going to add to it, cholesterol,
which has a big impact on the fluidity of the cell membrane.
And then we will look at some of the
embedded proteins and carbohydrate molecules
that impact the communication of the cell
with the external environment.
So let's begin by looking at cholesterol. Cholesterol is
a molecule involved in membrane fluidity as I mentioned before.
In fact if you recall the kinky tails
that we see in some of our phospholipids,
those steroid molecules will fit in very nicely between
those kinky tails to actually stabilize the membrane further.
And then in addition to those, we'll see that
there are glycoproteins on the cell surface.
These glycoproteins mostly act as sort of nametags for the cell.
It says "Hey, I'm a hair cell" or "I'm a liver cell".
So these are self labels.
We also have multiple membrance associated proteins.
Some of these proteins are peripheral as in
they're attached to the hydrophilic edges of the membrane.
And others of them spend the whole membrane.
These are transmembrane proteins. They might be channels.
They might be receptors. So we'll see a number of those
as we continue throughout the section of the course.
Another label that we might see on the cell are glycolipids.
Glyco meaning sugar, lipid meaning lipid.
So these glycolipids are glucose associated with
a phospholipid in the membrane.
And they also act in communication as we'll see
in a future lecture on cell communications.
So let's just recall the structure of phospholipids
in a number of different ways, we have to express them.
We have the full chemical structure that you can see.
And next to that, a space filling model.
And then we have our sort of icon version,
the dot with the legs.
And that's sort of where we'll go from now on
with our phospholipids.
This always represents a phospholipid from now on.
Again, you can see that they associate in a bilayer.
We have the phospholipid head, it's hydrophilic,
it likes water, and so it's to the outside.
In addition to that, we have the hydrophobic tails which stay away
from water and thus they are on the inside of that molecule.
So, when we have these phospholipid bilayers coming together, there's
all sorts of other stuff that you can see that we need to put into it.
Again, cholesterol is going to maintain the fluidity
by stopping the kinky tails of the phospholipid
from swinging back and forth. So the more cholesterol we
put in there, the more stability we see in the membrane.
So you can see that because cholesterol
is really important in cell membranes,
our consumption of cholesterol is actually
a very important component in our diet.
Without cholesterol, we cannot have stable membranes
and we also will not be able to make many of the hormones
and vitamins. So they're so important
to keeping everything running smoothly.
So again we take a look at these
saturated versus non saturated fatty acid tails.
These are also involved in how much a membrane moves.
These proteins and such that are embedded in the membrane
are not static. They don't stay in one place.
They essentially float around,
hence the name, the fluid mosaic model.
With the kinky tails, we saw that oils
were more liquid at room temperature.
And with straight tails, we saw fats
were solid at room temperature.
It's much the same when we consider our phospholipids.
They are associated in the membrane with the steroid molecules
in order to change membrane fluidity.
Now in human condition, we see that
the membrane fluidity stays relatively stable.
However, in plants and some other animals
that are not generating heat from inside,
they need to change the composition of their membrane
in order to say not crack their membrane like it's an egg shell.
So some of them are saturated, some of them are unsaturated.
Saturated tails, more solid.
More cholesterol, more solid, less fluid.
And the kinky tails lend themselves to having much more fluidity.