 We're back. This time in this lecture, we're going to focus entirely on the cell membrane. And I hinted in the last lecture that we were also going to look in this lecture at organelles or cellular structures that are made out of cell membrane. We've already talked about, well, we know that the cell membrane exists and we know that it separates the intracellular fluid from the cytoplasm from the extracellular fluid that surrounds it. We also know we learned that the nucleus was surrounded by a cell membrane. And then if the nucleus can be surrounded by cell membrane, then we can imagine that we could build other things out of cell membrane. So we're going to look at everything cell membrane related in this lecture so that we understand the structure and function. And in the next lecture, we're going to talk about how stuff gets in and out of the cell membrane. I love this visual of the cell membrane. You can see that it actually is a double-layered structure. We're going to talk in detail about what these things are. The other thing that I think is important for you to notice is, holy stuff going on. I mean, there's stuff all over embedded in the cell membrane. The more active the cell is, the more stuff you're going to find embedded in it. And we'll talk about what are these things that are embedded in the cell membrane. So first of all, we know some of these facts already. The cell membrane is semi-permeable. Some stuff can get in and out and some stuff can't. The cell membrane mediates or controls the movement of stuff in and out. So the cell membrane and it's all of that stuff that's embedded in it will help determine what can go in and out of the cell. And the characteristics of the cell membrane are going to determine what the function, what functions that cell can carry out. If it's super permeable to some things, then it will be able to carry out certain functions. Whereas if it wasn't permeable to those things, it couldn't carry out those functions at all. Makes sense? The other thing that you need to know is that all cells have a cell membrane. Even cells that have a cell wall, they still have a cell membrane. I'm going to draw you a picture. Tell me what kind of a cell has a cell wall. We learned that plant cells have cell walls. And the cell wall, I've just drawn in green just so you can remember. Oh yeah, plant cells have that. And many, many, many prokaryotes have a cell wall. In fact, more critters have a cell wall than don't have a cell wall. We just go, oh my gosh. Animals don't have a cell wall. And so surely we are cooler than anything with a cell wall. Surely. So here's my cell wall in green always. Even if you have a cell wall, you also have a cell membrane. And that would be an awesome question to ask. I'm going to undo that because I want to point just to the yellow. This is my cell membrane. You know, when we're talking about the anatomy of anything biological, anything living, directional terminology can get a little bit tricky. One of the words that we can use to describe this is that the cell membrane is deep to the cell wall. It's farther in. The cell wall is superficial to the cell membrane. Your skin is superficial to your muscles. Your muscles are deep to your skin. So this is my cell wall, which is superficial. And of course, that's only compared to the cell membrane, which I just brought up over here. The cell membrane is deep. I want you to be sure to understand that every cell has a cell membrane. And all of my cell membranes have the same structure. So your cell membrane is made out of a phospholipid bilayer. Can't help it. Got to write it down. Phospholipid bilayer. That's the structure that makes up the cell membrane. And hopefully you're like, dude, what's that? What's that? Well, we're going to talk about it. We're going to look at what is a lipid in the first place. The lipids that we think of are fats, but there are multiple kinds of lipids, including phospholipids. So let's go look closer at what lipids are before we come back to the cell membrane itself.