Cell Membrane: Structure and Selective Permeability

The cell membrane, made of lipids and proteins, regulates selective permeability and is dissolved by soap due to its lipid composition.

### The Cell Membrane: Selective Permeability and Structural Components
* 🍌 "One of my favorite demonstrations is extracting DNA from a banana, which involves **dissolving the cell membrane with soap**."
* The cell membrane is a key component in selective permeability, allowing **only certain substances to pass through**.
* The fluid mosaic model describes the cell membrane as a **dynamic structure** composed of phospholipids and proteins.
* Cholesterol helps stabilize the cell membrane by preventing phospholipids from **drifting apart or getting too close**.
* 🦠 **Glycoproteins** play roles in **immunity** and **glycolipids** play roles in **cell signaling**.
* Phospholipids are amphipathic molecules with a **polar head and nonpolar tail**, allowing small, uncharged particles to **pass through**.
* 🚪 Proteins in the cell membrane facilitate the transport of **larger or charged molecules**.
* 🌿 In addition to cell membranes, some organisms have cell walls that provide **structural support and additional protection**.

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 that the nuclear membrane. And then the cell itself is 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. Glycolipid 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, 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.

Cell Membranes

Bozeman Science

Understanding the Cell Membrane and DNA Extraction from a Banana

The cell membrane is a key player in selectively allowing substances to enter and exit the cell.

It is primarily composed of lipids and proteins, which contribute to its selective permeability.

The process of extracting DNA from a banana involves using soap to dissolve the cell and nuclear membranes, highlighting the role of lipids in the membrane structure.

The Structure and Function of the Cell Membrane

The cell membrane is described by the fluid mosaic model, consisting of phospholipids, proteins, cholesterol, and glycoproteins.

Cholesterol plays a crucial role in connecting and spacing out phospholipids, maintaining the membrane's integrity in fluctuating temperatures.

Glycoproteins, made of proteins and sugar chains, include important molecules like antibodies, which are vital for the immune response.

The Role of Glycolipids and Phospholipids in Cell Signaling and Membrane Structure

Glycolipids are fats with sugars attached, playing a crucial role in cell signaling and the attachment of molecules to the cell membrane.

Phospholipids, with a charged polar head and a nonpolar tail, contribute to the structure of the cell membrane, creating a barrier that allows the passage of small, uncharged particles like oxygen and carbon dioxide.

This unique structure of the phospholipid bilayer enables the selective permeability of the cell membrane, regulating the movement of substances in and out of the cell.

The Role of Proteins in Cellular Membranes

Proteins play a crucial role in cellular membranes by allowing the movement of large or charged molecules through facilitated diffusion.

They also facilitate the movement of water using aquaporins and are involved in active transport, as seen in the sodium-potassium pump.

Additionally, in organisms like plants, the cell wall provides additional selectivity and rigidity, primarily composed of cellulose.

The Importance of Cell Walls in Plant, Bacterial, and Fungal Cells

Cell walls provide structural integrity to plant cells, allowing them to hold water without bursting, while also offering protection to bacterial cells.

The cell wall is the target of antibiotics for bacteria and is made of peptidoglycan, providing durability.

Fungal cells also have a cell wall made of chitin, adding an extra layer of protection.