 All right, water. Let's start out with a definition of osmosis. Osmosis is the movement of water, but it has to be across a semipermeable membrane. Now think about this for a second. You don't have any semipermeable membranes to move water across. That's a definition of a cell membrane. A cell membrane is semipermeable. So yes, osmosis is going to happen in cells. We're going to be able to move water across cell membranes in and out of a cell. Now just take a deep breath and imagine, okay, in fact I'm going to take a deep breath and I'm going to take you over here, and I want to say what happens? What's going to happen if water goes into a cell? What's going to happen to the cell if water goes into it? Are you like, dude, that cell is going to swell. Do you agree with that? Like water is going into the cell, and of course the cell is going to swell. What happens if water goes out of the cell? Now think about this. Water, here's my little round cell, and water leaves the cell. What's going to happen to the cell? It's going to shrivel. How cool is that word? It's going to shrivel, shrivel up. Have you ever thought of that word? What happens if water goes in and out at the same rate? Do you agree that if water goes into the cell and water goes out of the cell at the same rate, such that it is in equilibrium, what's going to happen to the cell? Nothing, there's going to be no change in the cell. It's not going to shrink, it's not going to swell. It's just going to stay the same because water is moving in and out at the same rate. Now watch this. I'm going back to this little beaker that I drew, and I'm going to throw some water in it, and I'm going to throw fresh water in it. I'm just going to say this is just pure fresh water. There's no solutes in there, so it's not concentrated with anything. It's a very low concentration. And now I'm going to throw in my friend the yellow cell. And I'm going to tell you, you know what? In the yellow cell, there's particles in there because the cell, the cytoplasm, isn't pure water. It's got proteins in there, and it's got ions, it's got potassium ions, it's got all sorts of great stuff for doing its stuffy stuff. Okay, if, if the particles can move, what's going to happen to the system? If the particles can pass through the cell membrane, what's going to happen? If the particles can move, they're going to move down their concentration gradient into the fluid until equilibrium is reached. That's not osmosis. Water didn't move, the particles moved. That's just diffusion, and we totally just did it and fine, that's cool. Nothing's going to happen to my cell. My cell isn't going to swell, it's not going to shrink, it's just going to, like, stuff's going to go out and whatever. But what if those particles, they want out, right? There's a billion of them in there. What if they go and try to get out and they can't get out? This is the particle coming in and it bounces off, it gets rejected. It can't get out, none of these particles can get out. Now, are the concentrations different? Totally. And can you imagine what is water going to do to try and, like, to try? It's not really trying. Water is going to move across this semi-permeable membrane until equilibrium, equal concentrations, are reached inside and outside. Is water going to move out to make the inside have a smaller volume and be more concentrated? Or is water going to move in to try and decrease the concentration inside? I think of it as a wholly concentrated pile of concentration outside. The particles can't move, water is going to go in. If water goes in, what's going to happen to my cell? You know it's true. It's going to fill with water. It's going to swell. If the cell swells in a solution, solution, I love this, is hypotonic. And we're going to be able to see this. We're actually going to cause cells to swell, and we're going to cause them to shrink. If the particles, if it was way more concentrated outside than inside, I just feel like, dude, I can't do this without drawing you a picture. Here's the beaker. Here's the fluid. I'm changing the scenario. Here's my yellow cell, and it still has purple particles in it, okay? But now I'm not going to repeat this and the whole thing because I just am not going to. But do you agree that it's more concentrated than purple particles outside than inside? Please say yes. Please say yes right now. You can see that, huh, there's no comparison. It's way more concentrated outside. Now, talk to me. Water. Remember, my particles try to move. My particles are actually going to try to get in, but oh, they're going to get rejected. If they try to get in, it's not happening because the cell membrane is impermeable to these particles. Now, water is going to want to equal out the concentrations and water is going to move out of the cell into, I should have done it this direction, from the area where there's lots of extra water out here to almost try and dilute this and what's going to happen to my cell itself? Here goes little shrinky shrink as the water leaves. Do you see that? How cool is that? Guess what that's called? The cell just shriveled. When it shrivels, it is hypertonic. The solution is hypertonic to the cell. Do you have that? Does that make sense? You're going to have to mess with it because I'm telling you this concept is not easy. If, I just can't help it, this is how much I love you, if the concentrations inside and outside are the same. So here's my solution. Here's my little yellow cell and I'm telling you that do you agree that my concentrations inside and outside are basically the same? What's water going to do? Water's going to move out and water's going to move in and is there going to be any change in my cell? No, no change. This solution, if the cell does not change, the solution is isotonic. True story. You got it? Of course you do. Now, you're going to have to evaluate, is a solution hypotonic, hypertonic or isotonic to a cell. Cell concentrations don't change. All cells, and whenever you say all in any kind of biology class, you know that's not always the case, but most of the time, all cells are a consistent concentration inside their intracellular fluid. So it's not like they're just going to go around changing all the time. So if you can assume that, okay, these cells are probably the same concentration and then all you do is change the solution that they're sitting in and then see what happens. Did the cells shrink? Did it explode? Did it stay the same? And then you can make an assessment about osmosis and figure out what's happening there. Oh my gosh, we just transported the Holy Living, Hootily Hoot out of this thing. And now you've transported and cell membrane and celled and chemistry and science did and life did. And now it's time for an exam. Let's do it. Bye-bye.