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Video transcript
In this video, we're going to explore membrane proteins. Did you know that the cell membrane can be composed of up to 75% protein? So most cell membranes have about 50% or less protein, and the proteins are there because the cell membrane uses proteins for pretty much everything that it does-- all of these cell membrane processes that it performs. So just to remind us what a cell membrane actually is, a cell membrane is made up of little things that look like this, which are called phospholipids. And they come together and form what we call a lipid bilayer. So over here, I've pre-drawn a lipid bilayer. And it'll look something like this. It'll be made up a lot of these small phospholipids that we've drawn above, and it'll make up our bilayer. So you can see that there are two layers of these phospholipids. Now, there's two major types of proteins in the cell membrane. The first can look something like this. And this can appear anywhere in the cell membrane, and there are usually quite a few of these throughout the entire cell. So this is what we call an integral protein. You'll notice that it's called an integral protein, because you can think of it like it's integrated throughout the entire membrane. Another type of protein that we might encounter might appear on top of the membrane. Occasionally, it might be slightly into the membrane, and it can also rest on top of integral proteins. And this we call peripheral proteins. And the reason why we call it a peripheral protein is because it's on the peripheral, or the outside, of the cell membrane. The difference between peripheral and integral proteins is that integral proteins are really stuck inside the cell membrane. As you can see in this picture, the integral protein is really inside the membrane, and as a result, it will be very difficult to remove. Peripheral proteins kind of attach and remove themselves from the cell membrane or from other proteins. They generally are there for different cell processes, so for example, a hormone might be a peripheral protein, and it might attach to the cell, make the cell do something, and then leave. Peripheral proteins can also exist inside the cell on the cell membrane. Another type of protein is extremely rare, and it can appear inside the cell membrane like that. And we call this a lipid-bound protein. Why might you think a lipid bound protein is so difficult to find, so rare? Well, the reason why is because proteins are there to interact with the outside environment, and lipid bound proteins are stuck on the interior of the cell membrane itself. So it can really interact with the outside of the cell or the inside the cell, so it doesn't really serve a big function in terms of the cell membrane performing its duties. We're going to spend a little bit of time talking about two types of integral proteins that are extremely important, because these two proteins are found all over the cell, and they help the cell maintain homeostasis, or balance. The first type can look something like this. Again, this is an integral protein. What do you think this protein might be used for? This isn't two proteins. It's actually one protein with a hole through it. Well, this protein is actually used to allow things to pass through the cell. We call this a channel protein, and like the name kind of implies, there's a channel, or hole, inside the protein that lets things pass through. So for example, if there is some sort of ion-- let's say this is an Na+ ion, a sodium ion, this is outside the cell. And the cell at this point really needs these sodium ions to perform a really important process. So what the channel proteins do is they'll allow these outside extracellular ions into the cell. And normally, these sodium ions wouldn't be able to pass through the cell membrane just by themselves. These channel proteins allow our bodies to take in different materials from the outside environment into our cells. What they can also do is they can also do the reverse. So let's say your cell has way too much sodium, and it needs to get rid of it. So channel proteins can start pumping these out. Channel proteins generally don't require energy, so there's no energy needed. Sometimes we call energy ATP. And another thing that's special about channel proteins is you'll notice that it will go with the concentration gradient. So out here, there's a lot, and inside, there's very little. So it'll pump from where there's a lot of sodium into where there's very little. So it'll go what we call down a concentration gradient. The second type of very important integral protein is called a carrier protein. And like the name implies, it carries substances into the cell. I kind of picture it like a baseball glove, like this. So if there's a molecule that's outside the cell and the cell actually needs this molecule-- so what the carrier protein will do is it'll actually protect this substance so that it can enter the cell safely. It can also do this in reverse. It can take something inside the cell and pump it outside the cell. And this type of protein is really important, because unlike channel proteins, carrier proteins can go against the concentration gradient. And this is really important, because say your cell has a lot of chloride ions, and your body needs more to perform a certain process. So what your body can do is it can bring more chloride ions into your cell, even though your cell already has a lot of chloride ions. So carrier proteins can sometimes use energy or ATP. Finally, there is a type of protein that can exist on any of these that we've drawn here, and this is what we call a glycoprotein. So what a glycoprotein would look like is there'll be a chain of sugars attached to a protein, and it can be on integral proteins, peripheral proteins, channel proteins. Glycoproteins, you'll notice, have the prefix glyco, which means sugar. And basically, it's just sugar plus protein. And the purpose of glycoproteins is that it's used in signaling. So it allows a cell to recognize another cell. So in summary, in this picture that we have drawn out of a cell membrane and several different proteins, we have two main classes of proteins. We have peripheral proteins, which are on the outside of the cell, and they're really easy to remove. We have our integral proteins, which are stuck inside the cell and really tough to remove. We have our lipid bound proteins. We have channel proteins, which allow things to move through the cell by its concentration gradient, and it doesn't require energy, and it doesn't require ATP. We have our carrier proteins, which are kind of like a baseball glove. It can take in a particular molecule and let it out inside the cell, or it can do it in reverse. And these can sometimes use ATP, and what's special is they can go against the concentration gradient. And finally, we have glycoproteins, which really can be any of the proteins that we've drawn out. It's a sugar plus a protein, and it participates in signaling, so cells can recognize each other.