 Okay, I love osmosis. I do not think it's easy. I think it's very challenging, but there's something really satisfying about the osmosis puzzle. Let's start with a definition of osmosis, and I would love, I wish we were all sharing space so I could ask you to define osmosis and I could hear what you have to say. I bet most of you have a definition and I would bet also most of those of you who have a definition have one that I do not agree with. So we're gonna be really careful about how we describe osmosis. We're gonna say osmosis is the movement of water across a semi permeable. I'm gonna abbreviate that. Membrane, okay. Water is the osmosis is the movement of water across a semi permeable membrane. Give me an example of a semi permeable membrane like the cell membrane. Water will move in order, like we have the movement of water. This will happen, don't like that, will happen in order to establish, okay, and I have to go down here so I have room to write this and I'm writing it in bright green. Water will move across a semi permeable membrane only to establish osmotic equilibrium. Okay, how did that feel? We've never heard the word osmotic equilibrium before. What would you guess? Osmotic equilibrium is when the concentrations concentration of particles is equal on both sides of a semi permeable membrane. Okay, think about that one. If the concentration of the particles on one side or the other side of your semi permeable membrane, if the concentrations are different, then you have an osmotic gradient. I'm gonna write that down too. Osmotic gradient. What does that make you think of? It makes you think of an area of high osmolarity or high concentration and an area of low osmolarity or low concentration and water will move across a semi permeable membrane if there is an osmotic gradient, if there's a difference in concentration on both sides. Semi permeable membranes matter. Take a look at this top scenario right here. I have two scenarios that are going on. This is scenario two down here and on top is scenario one. I want you to look at scenario one. First of all, is there a semi permeable membrane in that first situation? I see a beaker of water. Do you agree with that? We'll just assume this is water. In fact, we'll assume it's water everywhere. Everywhere that we see that there's water. These are watery solutions. And then I see a bunch of solutes, a bunch of particles. And in scenario one, let's call this one A and one B. In one A, all those particles are glumped up in one side. Do we have a concentration gradient? Yes, we have a concentration gradient, right? We have high concentration to low concentration. Do we have an osmotic gradient? Totally, we also have, I mean, a osmotic gradient means we have a high concentration somewhere and a low concentration somewhere else. But in the top scenario, is osmosis going to occur? The answer is no. Why not? I'm about to tell you. What's missing? In the first scenario, we have no semi permeable membrane. In that first scenario, water's moving. Water's moving like everything else. It's totally moving in that top scenario. But so are the particles. And the particles are going to move just like the water. And eventually we're going to establish, we're going to remove our concentration gradient. I tried to make it so now our concentration is the same all the way around. We had a hill and now it's flat because the concentration is the same everywhere. Does that work for you? We had no osmosis. We had diffusion, diffusion of particles, but we didn't have the movement of water. Scenario two, what do you think the line down the middle is? What do you think this line right here is? That's my semi permeable membrane. Okay, I'll even highlight it with the same color that I highlighted with, okay? If we have a semi permeable membrane, the next question is in scenario one, do we have a concentration gradient? I'm looking at it going, yeah. There's a high concentration of these particles over here and a low concentration of these particles over here. Do you agree with that? Is there an osmotic gradient? Well, yeah, we definitely have different concentrations. We have a low concentration here. Do you agree with that? And a high concentration here on the other side. Possible, what will the particles do? If possible, the particles will just move and then will water move? No, but this is a semi permeable membrane. And I'll tell you right now, these particles are gonna, what direction are they gonna try to go? Don't you agree that the particles want to go down their concentration gradient and they're gonna get rejected. They can't get through. This is a semi permeable membrane. These particles are non-penetrating. That means they can't get through that semi permeable membrane. If they could get through, they would diffuse down their concentration gradient and you would no longer have an osmotic gradient. Are you following with me here? Particles can't move, but we are not in osmotic equilibrium. We do not have osmotic equilibrium between the two sides now. In our semi permeable membrane, water can pass through. So you tell me, what direction is water gonna move in order to establish osmotic equilibrium? Well, those particles, they wanna move left. But water, water wants to move right. Do you agree with that? Water wants to dilute the concentration on the other side so that we can have the concentrations equal. Water isn't going, well, would it work if water moved the other direction? If water moved to the left, the left would become more dilute. If water moves to the right, the right becomes more dilute, less concentrated. Which one, if we want osmotic equilibrium, which one needs to become less concentrated? That's where water's gonna go. Water's gonna go to where it can make things less concentrated to establish osmotic equilibrium. And in fact, water did indeed cross this semi permeable membrane. And water actually goes into that space and increases the volume on that side. This can happen to cells. You can have cells, if they are in an environment where inside the cell has more particles, water will rush in. A cell can actually swell. It's one of the reasons why, don't laugh at me. You don't give someone an IV of fresh water. Someone who comes into the ER totally dehydrated. You don't give them an IV of fresh water to rehydrate them because that fresh water, all that water is gonna rush in to establish osmotic equilibrium. It's gonna rush into those cells and the cells are gonna swell. I want to, I ran out of room. Oh no, I didn't. I saved a page for myself. I was like, why is there nothing on that page? Because I saved it for my notes. There are a couple of words that we need to use to describe. You now know why water moves. I don't know why, but this is hard. And you're gonna wanna come back to this. You're probably gonna have to listen to it a couple of times and you're gonna wanna test yourself on your understanding because there's something that is counterintuitive about this. I don't know what it is. If anybody figures out why this is counterintuitive, tell me because then we can call it out. But there are words that we can use to describe solutions. If a cell swells in a solution. Okay, did you follow that? So if I drop a cell into a solution and the cell swells, why would it swell? Because water moved into it. The solution is hypotonic. Did you follow, did you go with me? Tenacity is a set of words that describe what happens to a cell when we drop it in a solution. If the cell swells, the solution is hypotonic. Guess what it is if the cell shrinks. If the cell, I'm not gonna write any more words. You write this all out, but if the cell shrinks or shrivels, why would it shrivel? Here's a cell with all of its juicy cytoplasm and all of a sudden the cell shrivels, why would it shrivel? One reason. Water, left, water, the cytoplasm, the watery cytoplasm, the water part left. Probably left all those particles in there. If it shrinks, it is hypertonic. The solution is hypertonic if it causes a cell to shrink. And if the cell stays the same, so the cell does not change, the solution is isotonic, iso-same. If someone comes in to the ER dehydrated, you probably wanna give them an isotonic solution, maybe slightly hypotonic, but not freshwater hypotonic, because you do not want their cells to burst. Okay, how about if we just sit with just that? Let me just check really fast to see if there's anything. The only other thing that I have on my list here that I just don't wanna, I think I'm done. I hope I do not regret this. I will re-record this osmosis lecture part if I do regret it, but I don't know if I will. Okay, what do you think? Okay, we'll call it good. I think there's some ice cream waiting for me. I hope there's ice cream waiting for you because I think we tied on all the bets that we had tonight. Okay, night night.