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- [Voiceover] The first
of the main structures

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to talk about, in the cell,
probably is the plasma membrane,

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because the plasma membrane's so important

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because it separates the cell

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from the environment around it.

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It helps keep the internal environment,

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the cytoplasm and the
organelles, potentially,

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that are in the cell, separate and safe

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from what's outside and around.

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The plasma membrane is going to be termed

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selectively permeable.

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What that means is some substances,

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some particles can travel through,

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while others cannot travel through.

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So this selectively permeable allows

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certain things through,
while blocking others.

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It helps the transfer of information

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between environment and cell.

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This way the cell can know
what's going on around it,

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and outside.

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The plasma membrane is
termed a lipid bilayer.

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Now lipid, hopefully you remember

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from the Chemistry
chapter, are gonna be fats.

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Bilayer, bi means two, so two layers.

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The plasma membrane is
gonna be a lipid bilayer

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of phospholipids.

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Phospholipids are one of
the three main categories

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of lipids.

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Hopefully you remember that

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phospholipids are gonna
have a phosphate group,

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then two fatty acids,

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and the unique structure they had

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allows for a polar head and nonpolar head.

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Or, think of one head as water-loving,

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one head as water-hating.

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Hydrophilic, hydrophobic.

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Cholesterol can also be
found in the lipid bilayer.

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Now, phospholipids are gonna
be the highest concentration,

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but cholesterol, another type of lipid,

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also is found in, could be decent amounts,

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could be low amounts, depends on the cell.

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But it helps with the
rigidity, or how firm or solid

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that membrane can be.

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You'll find various amounts

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of proteins scattered throughout.

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These proteins can help
with several things,

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but one is transport.

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They can aid in transport
by being a channel,

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by being a gateway.

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They can help allow
substances to pass through.

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Carbohydrates.

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Well, carbohydrates, like proteins,

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are not a type of lipid.

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They're their own categories.

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So, these carbohydrates
can make the equivalent

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of fingerprints on a cell.

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So, each cell can have a special

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recognition pattern

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and this unique recognition pattern

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can help identify it as
one of your own cells,

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or a cell from some place else.

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Is it a cell that belongs
in this part of the body,

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or should be some place else?

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So, these carbohydrates
that make these almost like

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fingerprint-type of structures,

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help to identify the cells as your own,

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or it's a foreign particle.

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And I put a term, nonrigid.

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Well, I know that sounds that
it might be a little contrary

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to under cholesterol where
I said, "A bit more rigid."

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First off, phospholipids are very fluid.

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Think of a lake or an ocean

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where there's just a light
little ripple of waves on it.

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The top of the surface
kind of goes up and down

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in a wave-like motion.

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Not much, but a little bit.

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Cholesterol will help to reduce

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the wave-like movement,
but will not stop it.

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So, the important thing to remember,

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the plasma membrane is nonrigid.

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It can change shape.

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It can conform to what's being put into.

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It can be squished into a box,
if you have one small enough.

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So, it can change, or move, as necessary.

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Plasma membrane, nonrigid.

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Fluid mosaic.

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Well, since it's nonrigid, we're allowed

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to have this idea of fluid mosaic model.

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What this is referring to

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is that the plasma membrane,
with its phospholipids,

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cholesterol, proteins, carbohydrates,

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is constantly in a fluid state.

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It's constantly moving.

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It's this mosaic pattern of
having of all these components

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woven together.

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So, kind of think of it this way.

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If you took, I don't
know, 1000 rubber ducks

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and put them in a bathtub,
then started making some waves.

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Would the rubber ducks
stay flat on the surface,

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or would they conform to
the shape of the water

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as you creates some ripples and waves?

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Well, as those ducks
are moving up and down,

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that's the equivalent of
what the plasma membrane

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would be doing.

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It moves up and down to
accommodate the particles inside,

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and particles outside.

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This is an example of
your phospholipid bilayer,

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with cholesterol,
carbohydrates, and proteins.

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So, your plasma membrane.

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What you can see in the
far left, we'll start,

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that purple oval, receptor protein.

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Moving onto the right, channel protein.

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Keep on moving, gated channel protein.

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Transport protein, glycoprotein.

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Those are all different types of proteins

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with different function.

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The middle three, the channel
protein, gated channel,

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and the transfer protein,
those three are all helping

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to transport things
into or out of the cell.

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The far left one,
receptor, it's purpose is

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to receive a signal and trigger a response

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inside the cell.

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Now the glycoprotein, look
what's anchored on top.

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A whole string of carbohydrates.

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That's part of the fingerprints.

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The four circular structures

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of yellow there, that are tied together.

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See one all the way on the left,

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and one all the way on
the right, for example.

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The one on the right
is labeled cholesterol.

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Those are cholesterol molecules,

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giving little more
rigidity to the structure

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of this membrane.

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But the overwhelming amount of material

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is that lipid bilayer,
those phospholipids.

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You can see the two yellow tails,

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and that orangish-brown head

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that's a phosphate group with
the two tails coming off.

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Now, they're facing opposite ways

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because this allows for the polar ends,

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and the nonpolar ends
to stay out of contact.

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Won't help anything if
the polar and nonpolar,

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or the water-loving and water-hating,

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come into contact with each other.

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So, that lipid bilayer, probably
the most important part.

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Keep in mind, that lipid
bilayer is what really forms

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the majority of the phospholipids.

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Cholesterol, again, rigidity.

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Don't forget, it makes it a little firmer.

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All the proteins, they all
have their own purposes,

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from receiving to allowing passage

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to anchoring a fingerprint,

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and that carbohydrate group is equivalent

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of the cell's fingerprint.

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It's a unique identifying marker.