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Tonicity - comparing 2 solutions

Find out how tonicity is determined by ions that don't move across membranes and how it affects the movement of water. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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Video transcript

So I'm going to draw a tube here. And this tube has a bit of a curve at the bottom and comes back up. And let's say that it's exactly the same diameter all the way across, so same shape on both sides. And we'll label them side A and side B. And at the bottom of my tube I put a membrane. This is my purple membrane. And I let some stuff through, but not everything. So I begin by putting in water. And the water goes in on one side and fills up to, let's say, about that level. And that's because water passes through my membrane very easily. No difficulty passing through my purple membrane right here. So there's no trouble crossing. And so then I decide to take it a step further. I get a little green solute. We'll call it Solute A. It can be anything you can think of, some solute. And I pour it in on this side. And Solute A, just like the water, can easily cross over and get to the other side. So Solute A now has also very easily passed through the membrane. So Solute A passes. And this whole passes no passes thing is important, because now Solute B comes in, and Solute B does not pass through my membrane. Solute B, let's say, is a bigger molecule, something like that. And it just gets stuck on this side. Not enough-- or it doesn't have the ability to get across, so there's going to be very little Solute B on the other side. So Solute B cannot pass. And because it cannot pass, what happens is that if you actually were to check this-- let's say you come back after letting this sit on the table for a little while-- the level of water will rise on this side of the tube, and will fall on this side of the tube. And there becomes a real difference here between the two sides. And so if you were to name these things, you would call this side, this side A, hypertonic relative to side B. And this side you'd call hypotonic relative to side A. So you basically call it hyper or hypotonic relative to something else in that when you say relative, there has to be a difference, and that difference is going to be the membrane. So the other side of the membrane becomes the thing that you compare it to. And you can also see another interesting thing, which is that the only reason that side A became hypertonic was because of the fact that we have this Solute B that couldn't pass. Though it's because of something not being able to pass the membrane that it offered a chance for side A to become hypertonic. So in a way, this inability to pass is what led to tonicity. So the fact that you have a difference in tonicity, specifically more tonicity on side A, is a direct result of the fact that Solute B couldn't pass through the membrane. So just keep that in mind, because that's a really important point. It's also important to note that this did not contribute to the tonicity. And so if you were to calculate osmolarity or something like that, you would say, yeah sure, Solute A contributes to osmolarity, and it contributes to osmolality, but it does not contribute to tonicity. So that is one key difference between things that do and don't contribute to tonicity, is how well do they cross membranes. So let me redraw this now. So now I'm going to draw for you a cup. Let's draw a nice large cup. And inside of this cup I'm going to draw basically half of the volume of this cup. Half of it is going to be taken up by this cell. So in your mind, just remember half of the volume of this cup is inside of this cell, and half is on the outside. So we've got, let's say, a water level here, and it's exactly 50/50 between what's on the inside of the cell and what's on the outside of the cell. So this is our water level. And let's do a couple scenarios. So let me actually cut and paste this a few times, and we'll see how you can actually have a few different things happening if you change what is on the outside of that cell. So we have three scenarios here, and I want to prove to you that they start out looking the same. So that's why I wanted to just cut and paste it, so it looks identical. Now in the first scenario I want to remind you-- actually in all three scenarios-- that these cells make proteins, and they have DNA, and they have, basically, solutes that are going to not be able to get on the outside of that membrane. So they start out with some solutes that really can't get outside of the membrane. And let's say that, for the argument-- for the moment rather-- that there are four solutes on the inside that really can't make their way outside. Now I'm gonna go ahead and sprinkle in some Solute A and B. So remember we had Solute A and B. And Solute A passes through the membrane, and Solute B does not. And that was the key difference, we said. So we said Solute A does not really contribute to tonicity, but Solute B does. So Solute A-- let's sprinkle in, let's say, six molecules here-- three, four, five, six. And actually, it gets a total of 12 molecules, and 6 make their way inside of the cell. And here I'll sprinkle in just three molecules on the outside and three on the inside. A total of six. And here, let's do 10 molecules on the outside and 10 on the inside. And again, I'm saying 10 and 10 because anything that goes on the inside, the exact same amount will go on the outside, because we know the two volumes are the same. So we have 3, 6, 7, 8, 9, 10, and on the outside we have 10. So in all three scenarios I put different amounts of Solute A, but because it passes through the membrane easily, it distributes evenly. Now Solute B. Let's say that we have one, two molecules here, and here let's put four molecules of Solute B-- one, two, three, four. And we know that, again, Solute B cannot pass through the membrane. And here, let's put six molecules of B. None of them can actually get to the membrane, of course. So if you were to add up what's on the inside versus what's on the outside in scenario one, you actually have a total of, let's see, 10 molecules over here. And on the outside you only have six molecules. So here, we would call the solution hypotonic, because there's less solute on the outside relative to the inside. And so from the solution's perspective, it's hypotonic. That's this part. So if this is hypotonic, what will happen to our cell? So our cell is going to attract water, all right? Water is going to want to basically gush into this cell. And if it wants to gush into the cell, it's actually going to make the cell get bigger. So actually, let me draw that for you. Let's draw a bigger cell. Actually, I'll just keep half of it the same, but you'll get the idea that this cell is going to get really big. So compared to what it did look like, it looks much, much bigger. So the cells swell up. And so I just think of them as fat cells, fatter than usual-- fatter cells. And in the middle scenario, going to that one, we have, actually, isotonic solution. Because in this case, we have the same amount of volume, or same number of solutes, on the inside as outside. We have a total of seven here, and we have seven on the outside. So because it's equal-- the number of solutes is equal, we call it isotonic. And the cells stay the same. They don't change. And on the last example, we have what we call hypertonic solution, because from the solution's perspective, it's got way more solute then what was on the inside of the cell. So here we have more on the outside than the inside. We have-- let's see. Let's count up. We have 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 out here, and on the inside we have 14. So it has more solute on the outside. And so what will happen in this case is that the water will want to rush out. It'll rush out, because there's more solute on the outside. So if water rushes out, then I'd have to redraw this cell, redraw it to reflect what it will look like. And it'll look like this. Something like this. Actually, I didn't lose any solute, let's do that. And maybe even to make it more obvious I can erase this bit over here, and show you that basically what's happening is that this cell is shriveling down. It's becoming skinny and shriveling down. And so these cells become very skinny cells. So if you're in a hypertonic solution, the cell will shrink down or become skinny. So this is how a solute that cannot pass-- in this case Solute B, the red ones-- those are the ones that are going to affect whether the cells get fat or skinny, because they're the ones that affect tonicity.