 Hey everybody, we're back as promised to talk about the cell membrane. In the last lecture we totally spent an entire lecture talking about cells and we looked at all the various cell structures and we looked at the cell membrane, but then I promised you a sequel to the last lecture where we did nothing but talk about the cell membrane. I personally find the cell membrane fascinating and I think probably as we work through this lecture today you probably will see why I like it so much. It's an interesting structure because just by its existence we separate, we establish an extracellular space and an intracellular space and you can imagine that if you didn't have a structure separating those two spaces you actually would not have a cell. Like there's no reason to have a bubble of fluid, a bubble of cytoplasm unless somehow you can control what goes in and out from the environment. So the cell membrane does that work. If you remember, well maybe I should ask you, do you remember what the cell membrane is made out of? I have a hint here because I have an illustration of a cell membrane, but I'm going to go ahead and draw you just a couple, not very many. What am I drawing? I hope you all answered loud and proud. Those are phospholipids. Am I done? Is that all I need for my cell membrane to illustrate my cell membrane for you? No, what is true? The cell membrane is composed of a phospholipid bilayer. So write that down even if I don't write that down for you. Can't remember to save my life if I said it already in the last lecture, and I'm certainly not going back to watch those things. But this right here, cell membranes, anything that's made of cell membrane is made of phospholipid bilayer. So remember we talked about the whole endomembrane system and we had Golgi bodies and endoplasmic reticulum and what else, smooth and rough versions of the endoplasmic reticulum, that all is made of a phospholipid bilayer. It's all made of cell membrane. The cell membrane, this phospholipid bilayer that's effectively establishing an inside and outside of the cell, let's name inside and outside of the cell just because it will help us with direction and it'll help us when we start talking about what's going in and out. Convention has the upper part of a picture is usually the extracellular fluid and extracellular outside of the cell and we can abbreviate that as ECF and I do abbreviate it frequently. If extracellular fluid is outside of the cell, what's inside the cell? Well hopefully some of you might have even said, dude the cytoplasm, did your voice sound like that when you said it? This cytoplasm is inside the cell and let's give it a name that's similar to extracellular fluid but not extracellular intracellular fluid or ICF. Separating, distinguishing between extracellular fluid and intracellular fluid is the job of the cell membrane and not just separating it, like if you think about a balloon, the wall of the balloon, the latex, whatever balloons are made out of, that does nothing more than separate the inside of the balloon from the outside of the balloon. It doesn't do any kind of, well it's not supposed to do any kind of exchange of material. Cell membrane is a very active structure and physically, actively, proactively mediates what comes in and out, controls what comes in and out of a cell and the cell can change what a cell membrane is capable of doing. So in this lecture we're going to talk about how, how does the cell membrane get stuff in and stuff out because we've already established that the cell membrane is semi-permeable which just alone means some stuff can get in and out and some stuff can't and so figuring out what stuff can get in and what stuff can't is is part of the importance of this lecture. I also wanted to say there was one more thing that, oh, I wanted to throw out there these words. If you look at our cell membrane, it isn't just phospholipid bilayer, it has a bunch of stuff floating in it and the cell membrane has been described in the past as a fluid mosaic, mosaic, that is not spelled correctly, mosaic, we'll try that. And I want to give you this visual to stick with. The phospholipids actually float and the stuff or, or they move, they're, they're fluid. I think of them like balls in those like ball pits, like you have, you know, do you know what I'm talking about where you can jump in a ball pit and the balls move all around you? Phospholipids float and move around in the cell membrane and then all that stuff, all those channels and proteins and things hanging off float in there, almost like a boat, so they can move. So you have like this fluid mosaic of lots of different things that can move around in the cell membrane and that's important. And like I said just a moment ago, I think it changes over time depending on the needs of the cell. All right. The first thing, our whole lecture is on strategies that the cell uses. How does a cell get stuff in and out? And the first thing we're going to talk about is the easiest and that's diffusion.