Hi. It's Mr. Andersen and welcome to Biology Essentials video number 15. This
is on the cell membrane. One of my favorite demonstrations to do for people who don't
know a lot about biology is to extract the DNA from a banana. It's really simple. If
you were to google it you could find a quick recipe for taking DNA out of a banana. The
first step though is to add shampoo to it. And that's confusing to people. But if you
have an understanding of the DNA, where it's found and then how a cell is organized, it
shouldn't be that confusing. So the DNA sits here in the inside of a nucleus in a eukaryotic
cell. It's surrounded by a cell membrane. We call the that nuclear membrane. And then
the cell itself if surrounded by another cell membrane. Now if it's a plant cell it's going
to be cell wall around the outside. But on the inside of the DNA it's pretty mushy. And
you can get through that really quickly with just a blender. So if you add soap to that,
what does that do? Well the soap is going to dissolve the membrane. It's going to dissolve
the nuclear membrane and it's going to release the DNA, which you can eventually add to alcohol
and get it to come out of solution. Now why is a membrane really easily dissolved by soap?
It's because it's made of lipids. It's made up of fats. So it tells you a little bit about
the structure of cell membranes. And so in this video I'm going to talk about selective
permeability. What does that mean? It's that cells only allow certain things in and certain
things out. And it can regulate what gets in and what gets out of a cell. And it does
that using the cell membrane. Now our understanding of cell membrane is what's called the fluid
mosaic model. And I'll talk more specifically about that in just a second. Now there are
a few things, cholesterol, glycoproteins and glycolipids that are found within the cell
membrane that are important. But the majority of the important things in a cell membrane
are the phospholipids. Those are going to be the fats. And then the proteins. And so
the phospholipids are amphipathic. That means they have a part that likes water and a part
that hates water. Or they have dual nature. And what they do is they allow small and uncharged
particles to move through it. And proteins are going to be found within the membrane.
What they mostly do is regulate transport. What can get through and what can't. And they
allow things that are bigger and charged to actually get through. And so selective permeability
in all cells is formed through the cell membrane. And all living things have cell membranes.
Now not all living things have cell walls. That gives them more structure. And so I'll
show you a picture of a plant cell and the cell wall in that. Also pictures of bacterial
and fungal cell walls. But animals don't have cell walls. We just have that cell membrane
around the outside. Okay. So let's start with the parts of the cell membrane. Again I said
that the cell membrane is, our understanding of it is that it's a fluid mosaic model. Let's
first start with the mosaic. What does mosaic mean? It's made up of a number of different
things. And if you look at this, the first time you actually look at a picture of a cell
membrane or a diagram of it, you should be blown away by how complex it actually is.
It's made up of a number of different things. And it's also fluid. What that means is that
all of these things are moving. And so these things are called phospholipids. These are
those fats. And so they will actually migrate. They'll float around. And so all of these
things are influx. If we were to have a movie of it, all of these things would be floating.
The proteins would be floating. Everything would be floating. And so it would be moving.
And if it's not floating, if it's not fluid, then material can't actually get through.
So that's the fluid mosaic model. It's made up of a bunch of different things and then
it's also constantly influx or in movement. There's a few things on here that I would
like to point out before I actually talk just about phospholipids and proteins. And those
would be, let's start with cholesterol. Let's see if I can find cholesterol. Here's a cholesterol
molecule right here. Cholesterol molecule we like to, most people think of cholesterol
as bad, but it actually has a huge role inside our membranes. What it does is it'll actually
connect phospholipids together. And so what that does is it keeps the phospholipids from
drifting apart too quickly. And so when you get hot or when the temperature increases,
our cell membranes would start to fall apart if it weren't for cholesterol kind of grabbing
those phospholipids and holding them together. The other role that cholesterol does, it actually
keeps them apart so that they can't get too close. So as the cell membrane gets too cold,
cholesterol keeps those things apart. So cholesterol actually has a really important role inside
the cell membrane. Another thing here, we've got the glycoprotein. Glycoprotein is made
up of two things. Protein, which is going to be inside the membrane and then this glyco
means sugar. So it's going to have these strings of sugars on the outside. Glycoproteins, probably
the most famous one that you're familiar with are what are called antibodies. Antibodies,
which are important in the immune response are actually a form of a glycoprotein. And
then another thing would be glycolipids. Let's see if I can find a glycolipid. Oh here it
is. Glycolipid right here. Glyoclipid is going to be a fat, but it's also going to have sugars
attached on the outside. And glycolipids are important when we have signaling. So if I
have a molecule that's coming in and it's going to attach to this and maybe I want to
take some of that molecule in, there are going to be an attachment, almost like a key in
a lock between that molecule and the glycolipid. And so there's a number of different things
on a cell membrane. I'll talk more about those later when we talk about cells. But right
now I want to talk about two, phospholipids and then proteins. So phospholipid, lipids
are one of those major macromolecules that we have in all living things. But what's interesting
about phospholipid is that it has two parts to it. Okay, phospholipids are fats. And so
like any fat, most of the phospholipid is actually made up of carbon and hydrogen or
it's a hydrocarbon. And so that's half of the phospholipid. But what makes it different
from other lipids is that it has a head that has a charge to it. In other words, the head
has a charge. And that has a result of a phosphate group that it has on the inside. So this is
charged. And then there's no charge on the inside. Or another way to think about that,
is this head is going to be polar and the tail is going to be nonpolar. So what does
that mean? All of the heads will line up next to each other. All the polar parts of the
phospholipids will line up. And so the surface of a cell membrane is going to have a charge.
And then on the inside there's going to be no charge. And all the tails will face the
inside. Well, things like water, which is polar, is going to be on the outside of the
cell and on the inside of a cell. And it's going to be attracted to that membrane. But
it's not going to be able to move through the middle. And the reason why is that there's
no charge. Or it's nonpolar on the inside. So as a result of phospholipids, we get certain
things that can easily come across. And certain things that can't. And so let me talk about
two things that can easily come across. If it's really really small, as a particle then
you can sometimes just scoot across. And if you have no charge you can move across as
well. You can move through this kind of no zone, no fly zone here. And so an example
would be carbon dioxide and oxygen. So carbon dioxide and oxygen can freely move back and
forth and the reason why is that they have no charge and they're also really small. And
so example, when you breathe in, oxygen, how does that oxygen eventually get into the cell
itself where we need it for cellular respiration? Well it's just going to move through diffusion.
And it can move easily across that membrane because it's very small and it has no charge.
Likewise, when I breathe out, I breathe out carbon dioxide. How does that move? Well it
moves easily across because there's no charge. And so if it were just phospholipids that
made up membranes then we'd be out of luck. Because we couldn't move things that are large
or things that have any amount of charge. And that's where proteins come in. Proteins,
there's a number of different proteins. This would be a channel protein. And so it goes
all the way across. We'll sometimes have like peripheral proteins. So proteins come in a
bunch of different shapes and sizes. But what they essentially do is allow big things and
things that have a charge to move across. And so this process I'll talk about in the
next podcast is called Facilitated Diffusion. But what you can do with a protein is you
can actually move molecules across. So right here we're moving glucose it looks like across,
or a sugar. And here we're moving some particles through facilitated diffusion. So these are
things that maybe are too large or have a charge and couldn't move through this no fly
zone. One thing we used to think about was H2O and how does water move across the membrane?
Well it's small but it has a charge. It's polar. So it can't really move across this
middle. And so what scientists discovered was something called an aquaporin or a protein
that allows water to move through. And it actually can control the amount of water that
moves through. So actually water is moving through a tight little bind in this aquaporin.
And then we can also use proteins to actually do active transport. So this is the famous
sodium-potassium pump. And what it's doing is it's cashing in ATP to move sodium to the
outside and then potassium to the inside of a cell. And so proteins are important because
they allow big things and things with charge to actually make it across the membrane. So
those are cell membranes. But other organisms are actually going to have one other layer
outside of that. And that's called the cell wall. Now the cell wall gives them additional
selection of what gets in and what doesn't. And it also gives them rigidity. So for example
in a plant, the plant is going to have a membrane, that's this yellow portion. But it's going
to also have a cell wall around the outside. If we actually look at what that cell wall
is made up of, most of the durable part of the cell wall is actually cellulose. And that's
why it's hard to eat a tree for example. But what that cell wall gives them is structural
integrity. And so water, as water flows into a plant, if it was a human cell, it would
actually explode or lyse the cell. But as a result of this cell wall it can actually
hold that water in. Bacteria have a cell wall as well. And so bacteria have a cell membrane
on the inside but they have a cell wall outside of that. And some of them will have a capsule
on the outside of that. But this cell wall gives them protection as well. It's also what
we usually attack when we formulate antibiotics. It's what's actually killing the bacteria.
Now their cell wall is made up of a different chemical. It's not cellulose, but it serves
the same purpose. It's something called peptidoglycan that mostly makes up the durability. And then
this is a fungal cell. So a fungus will have a, fungi have a cell wall as well. But it's
actually made up of another chemical called chitin. And so cell wall just adds to the
protection that some cells have. But again, animal cells don't have that. And so that's
cell membranes, that's cell walls, selective permeability and I hope that's helpful.