 So think about what the cell membrane is made out of. I know you already know this, but I'm going to draw you a picture anyway, and we can play a little bit of Pictionary, because I'm drawing you, what are these things? They are not two-tailed sperm. I know that was your best guess. Oh, and there's two layers of these bad boys. What is this thing? What's the cell membrane made out of? It's a phospholipid bilayer. Phosphos are the round circular molecules, the phosphate part of the phospholipid, which means these little tails are fatty acid tails. A phosphate attached to two fatty acid tails is a phospholipid. That's what makes up that structure. And when you put a whole bunch of phospholipids in water, they actually will arrange themselves in a little bubble like this. And imagine, like I just took a little tiny chunk out of the cell membrane, imagine a whole giant cell that is surrounded. The walls are made of phospholipid bilayer. Why? Well, this fatty acid portion is hydrophobic. It doesn't like being around water. It's like oil in your salad dressing. It separates, and it tries to get away from the watery hydrophilic or water-loving molecules. The phosphates are hydrophilic. They're like, dude, water, let's hang out. But water can't get through the lipid portion. So you end up with this bubble of cell that some stuff can get through and some stuff can't. Most cell membranes. Okay, all cell membranes have stuff embedded in them. Your cell membrane isn't just one little line of phospholipid bilayer. It's a bubble of phospha, a three-dimensional balloon of phospholipid bilayer. And embedded in the phospholipids are often proteins or other molecules. You have things like little, this is knee molecules. So this is what my, this is, oh, that one got crazy. That's my little, like, hey, this is Wendy's cell. And structures like that are used to identify self. And they're like embedded in the phospholipids. And your immune cells will come along and you'll be like, oh, yeah, that's that crazy lady's cell. That's actually us, we won't kill it. Okay, you also have, there are some proteins that actually span the entire width of the cell membrane. So check this out. You have proteins like channels. They're like little tunnels. And you could have like a little fun, like you could be out here and then crawl through the channel and be in here now. Wouldn't that be cool? You know that'd be cool. In fact, let's label this extracellular fluid out here, intracellular fluid in here. Channels can allow you to go in and out. Channels can be embedded in the cell membrane. We got to get the right shade of orange going on here. It's just going to be a deal breaker. Well, that's a nicer shade. Can you guys even tell the difference between that? It looks darker to me. Is that just me? I'm going to use it because it's late. What? I'm recording a lecture late? I never do things like that. You can have carriers. Little guys that I think of them as looking like little Pac-Man dudes. They're kind of like channels in that they allow stuff in and out, but they open only to one side at a time. So they're a little bit different than channels, but they still, they allow stuff in and out. I'm adding more cell membrane because those two molecules are transporters. That molecule is a signaling molecule or like a communication, like a identification molecule. It's like a driver's license. You cannot get through here unless you have that molecule. But then there's also, okay, signaling molecules. So you can have proteins that are embedded in your cell membrane. Okay, watch this crazy scene. It could be like Joe protein with maybe a little receptor associated with it, and if the correct molecule comes along that can bind to that receptor, that protein embedded in your cell membrane. I'm just going to draw some more cell membrane in here so you can see that, yeah, it totally is embedded in here because you know you needed all these. Oh, that was cool. Did you see that? You really just drew a phospho for me. I love that trick. Okay, when this molecule binds to this protein, holy, if this is a receptor, we can actually have other proteins that are associated with this guy and maybe the last one's like a little Pac-Man guy and you get a little cascade of reactions. So cool. So you can actually have communication through the cell membrane by binding to some kind of receptor that stimulates, that carries the messenger, passes it on to molecules that are inside the cell membrane. You can go the other direction as well. Our next entire lecture is on how cells communicate with each other or how communication happens in the body. So we'll start looking at this in a little bit more detail. Once you get the idea that we have a whole bunch of other things embedded in the cell membrane, the more stuff you have embedded in the cell membrane, the more active your cell is going to be. If it has lots of molecules like this that are waiting for a message from someone that can respond to lots of different messages, they have to be able to do lots of different things. If you have lots of transporters, you're taking lots of stuff in and out. If you have lots of channels, you're going to be moving lots of items in and out. Why? Well, because your cell is doing something specific in that area. These, we're trying at this point just to give you kind of an overview, kind of broad. We're going to be sort of vague, which might be irritating to you, but we'll slowly fill in details all semester long and say, oh, you know, we were talking about the receptor protein. This is a specific receptor that's involved in skeletal muscle contraction. This is a specific receptor that's involved in neural transmission. This is a specific receptor that's involved in the heart beating. All of those things are found in the cell membrane. Now, some things can get through and some things can't. Let's make a list of... There are two factors that basically come into play if you're going to decide whether or not a substance can get through. First of all, you've got to look at size. If you think about this, do you think big things can get through easily or small things? Small things are more likely to be able to get through the cell membrane. But the other factor is also very important when you're going into play and that's like charge. Charge molecules or not charge molecules have a different ability to get through the cell membrane. Substances that are lipophilic or hydrophobic. So substances that don't like water but do like lipid, they can often float right through, they can lose right through the cell membrane. Substances that are charged like water are usually hydrophilic, they like water, they don't like oil, and they're actually going to be repelled by this lipid core. So these are some things that will affect whether or not a molecule can just go directly through the cell membrane. You can imagine not very many molecules can. Really important molecules can. Hydrogen, carbon dioxide, urea, these are important molecules that actually can go directly through the cell membrane. Water, water actually goes better through a channel but it can go through, it's small enough to where it actually can squeeze through, it's just really slow through the cell membrane itself. The guess who's going to come in handy, it's going to be super handy to have some transporters in our pockets. Let's look at some different ways that stuff can get in and out of the cell membrane as long as I can find the off button.