cell membrane proteins

/n this video, we're going to explore membrane proteins.

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Did you know that the cell membrane

0:07

can be composed of up to 75% protein?

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So most cell membranes have about 50% or less protein,

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and the proteins are there because the cell membrane uses

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proteins for pretty much everything

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that it does-- all of these cell membrane

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processes that it performs.

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So just to remind us what a cell membrane actually is,

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a cell membrane is made up of little things that

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look like this, which are called phospholipids.

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And they come together and form what we call a lipid bilayer.

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So over here, I've pre-drawn a lipid bilayer.

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And it'll look something like this.

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It'll be made up a lot of these small phospholipids

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that we've drawn above, and it'll make up our bilayer.

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So you can see that there are two

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layers of these phospholipids.

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Now, there's two major types of proteins in the cell membrane.

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The first can look something like this.

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And this can appear anywhere in the cell membrane,

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and there are usually quite a few

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of these throughout the entire cell.

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So this is what we call an integral protein.

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You'll notice that it's called an integral protein,

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because you can think of it like it's integrated

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throughout the entire membrane.

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Another type of protein that we might encounter

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might appear on top of the membrane.

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Occasionally, it might be slightly into the membrane,

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and it can also rest on top of integral proteins.

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And this we call peripheral proteins.

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And the reason why we call it a peripheral protein

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is because it's on the peripheral, or the outside,

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of the cell membrane.

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The difference between peripheral and integral

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proteins is that integral proteins are really

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stuck inside the cell membrane.

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As you can see in this picture, the integral protein

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is really inside the membrane, and as a result,

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it will be very difficult to remove.

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Peripheral proteins kind of attach and remove themselves

2:15

from the cell membrane or from other proteins.

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They generally are there for different cell processes,

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so for example, a hormone might be a peripheral protein,

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and it might attach to the cell, make the cell do something,

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and then leave.

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Peripheral proteins can also exist

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inside the cell on the cell membrane.

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Another type of protein is extremely rare,

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and it can appear inside the cell membrane like that.

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And we call this a lipid-bound protein.

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Why might you think a lipid bound

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protein is so difficult to find, so rare?

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Well, the reason why is because proteins are there

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to interact with the outside environment, and lipid

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bound proteins are stuck on the interior

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of the cell membrane itself.

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So it can really interact with the outside

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of the cell or the inside the cell, so it doesn't really

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serve a big function in terms of the cell membrane

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performing its duties.

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We're going to spend a little bit of time talking

3:16

about two types of integral proteins

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that are extremely important, because these two proteins are

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found all over the cell, and they

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help the cell maintain homeostasis, or balance.

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The first type can look something like this.

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Again, this is an integral protein.

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What do you think this protein might be used for?

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This isn't two proteins.

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It's actually one protein with a hole through it.

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Well, this protein is actually used

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to allow things to pass through the cell.

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We call this a channel protein, and like the name kind

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of implies, there's a channel, or hole,

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inside the protein that lets things pass through.

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So for example, if there is some sort of ion--

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let's say this is an Na+ ion, a sodium ion,

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this is outside the cell.

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And the cell at this point really

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needs these sodium ions to perform

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a really important process.

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So what the channel proteins do is

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they'll allow these outside extracellular

4:20

ions into the cell.

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And normally, these sodium ions wouldn't

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be able to pass through the cell membrane just by themselves.

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These channel proteins allow our bodies

4:30

to take in different materials from the outside environment

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into our cells.

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What they can also do is they can also do the reverse.

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So let's say your cell has way too much sodium,

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and it needs to get rid of it.

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So channel proteins can start pumping these out.

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Channel proteins generally don't require energy,

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so there's no energy needed.

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Sometimes we call energy ATP.

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And another thing that's special about channel proteins

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is you'll notice that it will go with the concentration

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gradient.

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So out here, there's a lot, and inside, there's very little.

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So it'll pump from where there's a lot of sodium

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into where there's very little.

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So it'll go what we call down a concentration gradient.

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The second type of very important integral protein

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is called a carrier protein.

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And like the name implies, it carries substances

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into the cell.

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I kind of picture it like a baseball glove, like this.

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So if there's a molecule that's outside the cell

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and the cell actually needs this molecule-- so what

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the carrier protein will do is it'll actually

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protect this substance so that it can enter the cell safely.

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It can also do this in reverse.

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It can take something inside the cell

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and pump it outside the cell.

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And this type of protein is really important,

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because unlike channel proteins, carrier proteins

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can go against the concentration gradient.

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And this is really important, because say your cell

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has a lot of chloride ions, and your body needs more

6:09

to perform a certain process.

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So what your body can do is it can bring more chloride ions

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into your cell, even though your cell already

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has a lot of chloride ions.

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So carrier proteins can sometimes use energy or ATP.

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Finally, there is a type of protein

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that can exist on any of these that we've drawn here,

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and this is what we call a glycoprotein.

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So what a glycoprotein would look like is

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there'll be a chain of sugars attached to a protein,

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and it can be on integral proteins, peripheral proteins,

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channel proteins.

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Glycoproteins, you'll notice, have the prefix glyco,

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which means sugar.

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And basically, it's just sugar plus protein.

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And the purpose of glycoproteins is that it's used in signaling.

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So it allows a cell to recognize another cell.

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So in summary, in this picture that we have drawn out

7:08

of a cell membrane and several different proteins,

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we have two main classes of proteins.

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We have peripheral proteins, which

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are on the outside of the cell, and they're really

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easy to remove.

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We have our integral proteins, which are stuck inside the cell

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and really tough to remove.

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We have our lipid bound proteins.

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We have channel proteins, which allow things

7:32

to move through the cell by its concentration gradient,

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and it doesn't require energy, and it doesn't require ATP.

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We have our carrier proteins, which

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are kind of like a baseball glove.

7:43

It can take in a particular molecule

7:46

and let it out inside the cell, or it can do it in reverse.

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And these can sometimes use ATP, and what's special

7:53

is they can go against the concentration gradient.

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And finally, we have glycoproteins,

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which really can be any of the proteins that we've drawn out.

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It's a sugar plus a protein, and it participates in signaling,

8:07

so cells can recognize each other.