- [Voiceover] The first of the main structures
to talk about, in the cell, probably is the plasma membrane,
because the plasma membrane's so important
because it separates the cell
from the environment around it.
It helps keep the internal environment,
the cytoplasm and the organelles, potentially,
that are in the cell, separate and safe
from what's outside and around.
The plasma membrane is going to be termed
selectively permeable.
What that means is some substances,
some particles can travel through,
while others cannot travel through.
So this selectively permeable allows
certain things through, while blocking others.
It helps the transfer of information
between environment and cell.
This way the cell can know what's going on around it,
and outside.
The plasma membrane is termed a lipid bilayer.
Now lipid, hopefully you remember
from the Chemistry chapter, are gonna be fats.
Bilayer, bi means two, so two layers.
The plasma membrane is gonna be a lipid bilayer
of phospholipids.
Phospholipids are one of the three main categories
of lipids.
Hopefully you remember that
phospholipids are gonna have a phosphate group,
then two fatty acids,
and the unique structure they had
allows for a polar head and nonpolar head.
Or, think of one head as water-loving,
one head as water-hating.
Hydrophilic, hydrophobic.
Cholesterol can also be found in the lipid bilayer.
Now, phospholipids are gonna be the highest concentration,
but cholesterol, another type of lipid,
also is found in, could be decent amounts,
could be low amounts, depends on the cell.
But it helps with the rigidity, or how firm or solid
that membrane can be.
You'll find various amounts
of proteins scattered throughout.
These proteins can help with several things,
but one is transport.
They can aid in transport by being a channel,
by being a gateway.
They can help allow substances to pass through.
Carbohydrates.
Well, carbohydrates, like proteins,
are not a type of lipid.
They're their own categories.
So, these carbohydrates can make the equivalent
of fingerprints on a cell.
So, each cell can have a special
recognition pattern
and this unique recognition pattern
can help identify it as one of your own cells,
or a cell from some place else.
Is it a cell that belongs in this part of the body,
or should be some place else?
So, these carbohydrates that make these almost like
fingerprint-type of structures,
help to identify the cells as your own,
or it's a foreign particle.
And I put a term, nonrigid.
Well, I know that sounds that it might be a little contrary
to under cholesterol where I said, "A bit more rigid."
First off, phospholipids are very fluid.
Think of a lake or an ocean
where there's just a light little ripple of waves on it.
The top of the surface kind of goes up and down
in a wave-like motion.
Not much, but a little bit.
Cholesterol will help to reduce
the wave-like movement, but will not stop it.
So, the important thing to remember,
the plasma membrane is nonrigid.
It can change shape.
It can conform to what's being put into.
It can be squished into a box, if you have one small enough.
So, it can change, or move, as necessary.
Plasma membrane, nonrigid.
Fluid mosaic.
Well, since it's nonrigid, we're allowed
to have this idea of fluid mosaic model.
What this is referring to
is that the plasma membrane, with its phospholipids,
cholesterol, proteins, carbohydrates,
is constantly in a fluid state.
It's constantly moving.
It's this mosaic pattern of having of all these components
woven together.
So, kind of think of it this way.
If you took, I don't know, 1000 rubber ducks
and put them in a bathtub, then started making some waves.
Would the rubber ducks stay flat on the surface,
or would they conform to the shape of the water
as you creates some ripples and waves?
Well, as those ducks are moving up and down,
that's the equivalent of what the plasma membrane
would be doing.
It moves up and down to accommodate the particles inside,
and particles outside.
This is an example of your phospholipid bilayer,
with cholesterol, carbohydrates, and proteins.
So, your plasma membrane.
What you can see in the far left, we'll start,
that purple oval, receptor protein.
Moving onto the right, channel protein.
Keep on moving, gated channel protein.
Transport protein, glycoprotein.
Those are all different types of proteins
with different function.
The middle three, the channel protein, gated channel,
and the transfer protein, those three are all helping
to transport things into or out of the cell.
The far left one, receptor, it's purpose is
to receive a signal and trigger a response
inside the cell.
Now the glycoprotein, look what's anchored on top.
A whole string of carbohydrates.
That's part of the fingerprints.
The four circular structures
of yellow there, that are tied together.
See one all the way on the left,
and one all the way on the right, for example.
The one on the right is labeled cholesterol.
Those are cholesterol molecules,
giving little more rigidity to the structure
of this membrane.
But the overwhelming amount of material
is that lipid bilayer, those phospholipids.
You can see the two yellow tails,
and that orangish-brown head
that's a phosphate group with the two tails coming off.
Now, they're facing opposite ways
because this allows for the polar ends,
and the nonpolar ends to stay out of contact.
Won't help anything if the polar and nonpolar,
or the water-loving and water-hating,
come into contact with each other.
So, that lipid bilayer, probably the most important part.
Keep in mind, that lipid bilayer is what really forms
the majority of the phospholipids.
Cholesterol, again, rigidity.
Don't forget, it makes it a little firmer.
All the proteins, they all have their own purposes,
from receiving to allowing passage
to anchoring a fingerprint,
and that carbohydrate group is equivalent
of the cell's fingerprint.
It's a unique identifying marker.