 Okay, so remind yourself. What was the salmon brain made out of? Again, somebody tell me true. A phospholipid bilayer. Let's write the words down. It's a phospho... I can't help it. Lipid, and it's a bilayer. Bi-2 layer. It's like a double layer. Okay, and now I'm going to draw you a picture of it. Here's one phospholipid. Do you agree? And what was the lipid part of it? Remember, that's just a fatty acid. And what I'm going to tell you, I told you it was a bilayer. So look at this. This is my other layer. My phospholipid, there's one half, and here's the other half. Okay, so now I'm going to do hyperspeed, and I'm going to draw all of my phosphos. Whoa. My phosphos are a little bit far, but that's okay. I can embed stuff in that salmon brain right there. So look at how... I've got all these phosphos, and now let's add my lipids. Two fatty acid tails on each one, and you see my two layers? My phospholipid bilayer? Okay, here's my question for you. Label the intracellular fluid and the extracellular fluid. Oh my gosh, I wish that I was making clicker questions right now, because this would be a phenomenal clicker question too, to have this and then ask you where everything is. Where is the intracellular fluid? Here, here, or here? First of all, the one thing that you do know is it is not here. That is not intracellular fluid. This is like the wall of the cell. If I were to draw a tiny little cell up here in the corner, and here's a nucleus, and here's a nucleolus, this line, that cell membrane, is actually two layers thick. If I were a rock star, I could totally draw out a bilayer right in there. I am not a rock star, I'm almost a rock star, though. All right, so let's just arbitrarily label this as my extracellular fluid and this as my intracellular fluid. I want to know where is the hydrophobic part of the cell membrane? Remember, hydrophobic, phobic fear, hydro water, fear of water is going to be in the lipid portion. So this area, the whole green area from here to here, this whole thing is hydrophobic. So water, can water get through my little cell membrane? It's going to be kind of repelled because we have a little water molecule we're in here, H-H-O. This little water guy was in here. You'd be like, oh, everybody hates me in here. They're all afraid of me, boo! And then all the little phospholipids, the lipid part of the phospholipids would be like, hey, we're scared, it's water. But what about the phospho part? This is what's cool, this is what's unique about the phospholipid bilayer. The phospho part is hydrophilic, hydrophilic. Now, go ahead and just think for a second about the consequences of this. If the phospho part was not hydrophilic, then imagine it would be hydrophobic and it would like curl up in a ball. It would be like a blob of oil in water. It would just, all of it would come together. It's all so afraid of the water that it's going to form a giant ball of fat so that the fewest number of fat molecules are touching the water. Can you imagine that? Like if you smeared out a whole bunch of people along a wall and each one of the people are touching the wall, but then if you turn them into a ball, same number of people, but you just smush them up into a ball, you could actually make it to where like only two or three of them are touching the wall because the rest of them are like all crammed together in a little ball. Does anybody know what I'm talking about? I'm doing a windy like analogy in my brain. The point is that the shape, the phospholipid bilayer, the very nature of it allows a separation between inside and outside and that is really important. In addition, this is cool. Inside here are, I'm going to have to draw it like that and that's exactly what it looks like. Okay, whatever, nobody knows what that is. I'm trying to draw you cholesterol. Now, cholesterol is super hydrophobic because it's a lipid also and so if water tries to come through and there's a whole bunch of cholesterol in the cell membrane, the water is going to not be able to come through at all. So if you want a waterproof cell that won't let water in and out, there's a whole bunch of cholesterol in the cell membrane and you will totally get what you wanted. If you want a more permeable cell to water, then throw in less cholesterol. Cholesterol also, in addition to making the cell more waterproof, it also increases stability. So it makes it harder for the cell to pop basically because the cell membrane is a little more solid. So the other thing that you're going to find are various proteins. This is a transporter and then you also have little signaling molecules and we'll see those all embedded in. You can see that this is a cholesterol, these little yellow things are cholesterol, but there's lots of stuff embedded in the cell membrane. Go back to the fantastic X vivo video on the cell and get a sense of this kind of fluid nature of the cell membrane. It's almost like if you covered a swimming pool with all, you know, those little balls, those plastic balls that you jump into the pit of them at like Great America or something and I always wanted to do it when I was a kid and never got to. It's kind of like that and like you could jump into them and you'll go through all those balls and then they'll just like settle back together again at the top of the surface. It's like a fluid thing and they refer to it as a fluid mosaic because the stuff embedded in the cell membrane itself like here actually floats around. Like these are little islands. Go back and watch the X vivo cell video again and you can totally get a sense of that kind of watery but you know, all those phosphos of the phospholipids you can totally see them. All right, I think that's everything for the structure of the cell membrane. Now we're going to look at what kinds of things can we build using cell membrane. We think of the cell membrane as surrounding the entire cell and that's true but we can use cell membrane, this stuff, the phospholipid bilayer with stuff embedded in it. We can use that to build structures inside our cell. So let's look at some of those structures we can build.