 Hello, hello, everybody out there. It's time to talk about how we can get stuff in and out of the cell membrane. We've already dealt with the structure of the cell membrane. We've dealt with the cell, stuff we've got inside the cell. Now we've dealt with the cell membrane, which keeps that stuff inside the cell. And now, hopefully at the end of the last lecture, we started thinking, you know, if we're packaging things inside the cell, we probably are going to need to export them, or if you prefer, barf them out of the cell and into the extracellular space. And so how are we going to do that? Well, we're going to start out by looking at the process of diffusion because diffusion fuels an unbelievable amount of transport. And it's based on a few key principles that are pretty important to make sure that we understand. Then we're going to look at the kinds of molecules that can simply diffuse across the cell membrane. Some things can, not very many things, but some things can. And then we're going to look at, okay, if you want something to diffuse across your cell membrane, if we can't go directly through the cell membrane, how can we get it across? And we can actually use proteins to facilitate the diffusion. And that's another whole process that we'll look at. And then, every now and then, you want to pump things basically against their concentration gradient, and that's going to be active transport. We can do that, too. Sometimes things are too huge to go through the cell membrane in the ways that we're talking about, and so we're going to deal with bulk transport, and we're going to end the whole conversation with the idea of osmosis, which might seem to be pretty straightforward, but if you think it's straightforward, it's because you don't understand it. Okay, so let's start with the concept of diffusion. First, diffusion is based on the fact that all molecules have kinetic energy. Remember, they bounce around, all of them. If they stopped moving, it would be because we reached the temperature of absolute zero, which we never have done, and someone hypothesized that if we did reach absolute zero, we'd all turn into a neutron protoplasm, which, of course, I know exactly what that is. Molecules, because of random molecular motion, are totally random. They tend to spread out until there's equal concentration everywhere. It's random. I like to think of, here's a whole bunch of molecules that are on one side of the cell membrane. I like to think of them being like, dude, it's crowded over here. This is a party on this side, and there's too many people here, so some of us go over here, and so some of us go over here to the other side. They slowly start passing through the cell membrane, and eventually, when the concentrations on both sides are about equal, they stop. They're like, oh, it's cool now. That is not so. They are constantly moving. They are constantly moving like, yes, it looks like there is a lot of them over here and not very many over here, so they are going to be moving this direction. Yes, there will be a net movement to spread out, mostly because they're running into each other when they're randomly moving around, and as they run into each other, the act of running into someone is going to push you farther away from them, right? And so that random movement is going to cause them to spread out, but in this state where now it's equal, there is still random movement that brings this guy over and brings this guy back, but the net change is now zero. Now the concentration on both sides is equal, and we've reached what's called equilibrium. Let's write down the concept of equilibrium because it's something that, I think it's one of those things that I am hoping someday when you take chemistry that you'll be like, boom, we got equilibrium down. Equilibrium is this idea that forward motion, and by motion that's totally a windy-ism. I mean the forward direction of the reaction, the forward movement of the molecules like across the cell membrane in one direction, is equal to backward motion. Keep in mind, motion, like what the hell, and backward, all of that is just really, those are words that I'm using to help us visualize this concept. Forward motion, the movement of the molecules in one direction is equal to the movement of the molecules in the other direction. The net change, right, so the net change equals zero. The molecules are equal on both sides. They're moving forward across the membrane at the same rate that they're moving back across the membrane, and so the net change is zero. That's equilibrium. Things can reach equilibrium and not be in equal concentrations. That totally can happen, and this concept is worth spending some time kind of getting your head around. So if we go back to our visual here, you can see that, okay, I want to draw on this, I don't think I can, but this guy might come across, but every time one comes across, another one is going to go back. And it isn't like, I mean, you could randomly end up with five of them across before one of them goes back, but eventually, in the big picture, they're going to even out. There's going to be a net equality of movement. Whoa, did you follow that? Diffusion is actually really slow, and it's all random. Okay, let's look at the kinds of things that can directly diffuse across the cell membrane.