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Hypotonic, isotonic, and hypertonic solutions (tonicity) | Khan Academy

Hypotonic, isotonic, and hypertonic solutions (tonicity) | Khan Academy


– [Voiceover] I have three different scenarios here of a cell being immersed in a solution, and the cell is this magenta circle, that’s the cellular membrane. I have the water molecules depicted by these blue circles, and then, I have the solute inside of the solution, inside of the water solution that we depict with these yellow circles. I’ve clearly exaggerated the size of the water molecules and the solute particles relative to the size of the cell, but I did that so that we can visualize what’s actually going on. We’re going to assume that the cellular membrane, this phospholipid bilayer, is semipermeable, that it will allow water molecules to pass in and out, so a water molecule could go from the inside to the outside, or from the outside to the inside, but we’re gonna assume that it does not allow the passage of the solute particles, so that’s why it’s semipermeable. It’s permeable to certain things, or we could say, selectively permeable. Now, what do we think is going to happen? Well, the first thing that you might observe is we have a lower concentration of solute on the outside than we have on the inside, so at any given moment of time, you will have some water molecules moving in just the right direction to go from the outside to the inside, and you will also have some water molecules that might be in just the right place to go from the inside to the outside, but what’s more likely to happen, and what’s going to happen more over a certain period of time? The water molecules that are on the outside, and we talk about this in the osmosis video, they’re going to be less obstructed by solute particles. If this one happens to be moving in that direction, well, it’s gonna make its way to the membrane, and then, maybe get through the membrane, while something, maybe, if this water molecule was moving in this direction, well, gee, it’s gonna be obstructed now, maybe this is bouncing back, and it’s gonna ricochet off of it, so the water molecules on the inside are more obstructed. They’re less likely to be able to fully interact with the membrane or move in the right direction. They’re being obstructed by these solute particles. Even though you’re going to have water molecules going back and forth, in a given period of time, you have a higher probability of more going in, than going out, so you’re going to have a net inflow. Net inflow of H2O, of water molecules. Now, a situation like this, where we’re talking about a cell and it’s in a solution that has a lower concentration of solute, it’s important that we’re talking about a solute that is not allowed to go to the membrane, the membrane is not permeable to that solute. We call this type of situation, this type of solution that the cell is immersed in, we call this a hypotonic solution. Hypotonic solution. Anytime we’re talking about hypotonic, or as we’ll see, isotonic and hypertonic, we’re talking about relative concentrations of solute that cannot get through some type of a membrane. The word hypo, you might’ve seen it in other things. It’s a prefix that means less of something, so in this case, we have a lower concentration of solute in the solution than we have inside of the cell, and because of that, you’re going to have osmosis, you’re gonna have water molecules going from the outside, I should say, to the inside. That’s actually going to put pressure on the cell. The cell itself might expand, or it could even, if there’s enough pressure, it might even explode. Now, let’s go to the next scenario. In this scenario, we have roughly equal concentrations of solute on the outside and on the inside, at least, I tried to draw them that way. In this situation, the probability of a water molecule, in a given period of time, going from the outside to the inside, or from the inside to the outside, is going to be the same, so you’re not going to have any net inflow or net outflow. You’re always gonna have water molecules going back and forth, but there’s not gonna be any net inflow or outflow. Let’s see, let me write no net, no net flow. In this type of solution, where you have the same concentration of solute in the solution, as you do inside the cell, we would call this an isotonic. This is an isotonic solution. Isotonic solution. The prefix, iso, refers to things that are the same. It has the same concentration of solute, and so you have no net inflow. Hypotonic solution, you have water molecules going into the cell, the cell expanding, kind of like a filling balloon. Isotonic solution, no net flow. Of course, you could imagine in this last scenario, I have a higher concentration of solute on the outside than I have on the inside. We can guess what’s going to happen. First, what would I call this? Well, I have more of something in the solution, so I would use the prefix hyper. I have more of it, more, hypertonic. This is a hypertonic solution. Once again, the solute can’t go across the membrane, but the water molecules can, and you’re gonna have water molecules going from the outside to the inside, and from the inside to the outside, but the probability that the ones on the inside are gonna be less obstructed to go out, than the ones on the outside to go in, so you’re going to have a net outflow. You have a higher probability of things going from the inside to the outside, than you do from things going from the outside to the inside because they’re gonna be more obstructed, so they’re gonna be held back, I guess, in different ways. In this situation, you’re gonna have the water escape the cell, and the cell actually might shrivel up. Since it’s gonna lose that pressure from the water, the cell itself might shrivel up in some way. You could actually see this in actual living systems. If you were to put a red blood cell into a hypotonic solution, the water’s gonna rush into it, and it’s gonna blow up. It’s going to expand, so it’s gonna look like a overinflated red blood cell, and an isotonic solution is gonna look the way that we’re used to seeing a red blood cell, actually, having kind of that little divot in the middle area, while over here, it’s all going to expand. Then, in the hypertonic solution, the water’s going to escape the red blood cell, then you would actually see it kind of shrivel up, shrivel up a little bit like this because we have a net outflow of water molecules.

100 comments on “Hypotonic, isotonic, and hypertonic solutions (tonicity) | Khan Academy

  1. bless khan academy , taught this in 6 minutes while my prof took 40 minutes and just confused me, even though i already knew the material

  2. Thank u very much!u hv saved my days!!!!but may i kindly ask if water molecule is larger or smaller than electrolyes?

  3. Is salt good for our health in general?i remember salt is important to our body, but i dont remember how……😢

  4. You should have made the items on the inside of the cell, the yellow solute particles, a different shape than the yellow solute particles outside of the cell. The reason I say that is because the yellow particles on the outside must have at some point crossed the semi-permeable cell membrane in order to get inside the cell, but as you say, this is in fact, not the the case. They cannot pass through the cell membrane at all. Therefore, the yellow particles on the outside are not the same kinds of particles as the yellow particles on the inside. Therefore, they should have been drawn a different shape and color, say, a triangle in red for example to show that they are different from the particles on the outside of the cell (if you originally place them on the inside of the cell). If you don't, surely at some point, someone is going to ask why you say the yellow particles cannot pass through the cell membrane but in fact, they appear to have, since they are both inside and outside the cell, meaning that at some point, they must have crossed the cell membrane.

  5. So i would like to know
    In both asmosis and diffusion all of them represents the movement of water malecular or diffusion represents the movement of water and osmosis represents the movement of particles
    Please i need your answer any scientist can answer me!

  6. who else is here from a teacher suggesting this video to the class as a good video to watch in preparation for a test?

  7. I have a teacher who does not teach 😂 he leaves everything to us and expects us to succeed in his course. This really helped me in understanding this concept that took me a long time to try to figure out on my own. I have a test tomorrow and I think I'm screwed but this helped me at least a bit to boost up my mark 😂👏🏼

  8. In other words, water follows solutes.
    Hypotonic: there is a higher concentration of solutes outside of the cell (eg. rbc) and the water wants to follows the solutes. As a result, the cell will crenate or burst.
    Hypertonic: there is a higher concentration of solutes INSIDE the cell and the water will follow inside. As a result the cell is so full of water and solutes that the cell will burst like a ball. :p

  9. If there is a lower concentration of the yellow things then why did he draw more of them in the circle? The other cells have less yellow things so don't they have the lesser concentration of the yellow things.

  10. I do not know why they have a black board to write on. I barley can see it. Weird. Content is nice, I can not even see it.

  11. this is awesome!!! Khan academy is a way better method of learning than actually goin to school. I have my biology exam tomorrow and just a few minutes before i had no clue what isotonic and hypertonic and everything meant because my teacher confused me. After watching this vid everything is clear and i'm pretty sure i'm going to nail the tonicity questions in the exam.

  12. Say, Khan, in the diagrams, i think it's better to draw the same amount of solute inside the cell for every scenario, hypo, iso, and hyper. generally, only the concentration of solute in the environment changes.

  13. are these normal processes for healthy cell?? or the cell dies if inside hypotonic or hypertonic solution?? how to make solution in my body isotonic?? does eating specific type of food causes these abnormal behavior?

  14. this is all clear. My only difficulty to approach Khan Academy videos is that they use black background. It makes it hard for me to watch.

  15. I started using Kahn when I was in 7th grade math before anyone really knew what it was. Now I’m starting my sophomore year of college with a rigorous 6 week biology course. I love these videos and how they’re made so simply

  16. I get it… It helps when I draw out the pictures and I understand it… But isn't the definition of osmosis the movement of water from a high concentrated area to a lower area? Therefore, shouldn't the first image on the left be water moving from the inside to the outside?…. Similarly, for the hypotonic solution you described (3rd image on the right), shouldn't water from the inside should be moving towards the outside (higher concentration solute to lower concentration solute)?…. I know I'm wrong but I don't understand why

  17. youre awesome dude! what took 40mins of confusion, took you 6mins of well explained things! thank you very much with what all u do!!

  18. Sat through a whole class lecture for this and it took 1:59 of this video to make fully grasp the whole topic!!

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