If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:18:59
ENE‑2 (EU)
ENE‑2.H (LO)
ENE‑2.H.1 (EK)
ENE‑2.I.2 (EK)
ENE‑2.J (LO)
ENE‑2.J.1 (EK)

Video transcript

in this video I want to cover several topics that are all related and on some level they're really simple but on a whole other level they tend to confuse people a lot so hopefully we can make some headway so a good place to start let's just imagine that I have some type of container here let's say that's my container and inside of that container I have a bunch of water molecules it's just got a bunch of water molecules they're all rubbing against each other it's in its liquid form this is liquid water and then inside of the water molecules I have some sugar molecules maybe I'll do sugar in this pink color so I have a bunch of sugar molecules right here I have many many more water molecules though I want to make that clear I have many many more water molecules in this container we're dealing with now in this type of situation we call we call the thing that there's more of we call the thing that there is more of the solvent so in this case there's more water molecules and you can literally just view more as the number of molecules now I'm not going to go into a whole discussion of moles and all of that because you may or may not have been exposed to that yet but just imagine whatever there is more of that's we're going to call the solvent so in this case water is the solvent and whatever there is less of so the more water is a solvent and in this case that is the sugar that is considered the solute this is the solute so the sugar it doesn't have to be sugar it could be any molecule that there's less of than the water in this case sugar is the solute and we say that the sugar has been dissolved into the water sugar has been dissolved dissolved into into the water and this whole thing right here the combination of the water and the sugar molecules we call a solution we call this whole thing a solution and a solution has solvent and the solute the solvent is water that's the thing doing the dissolving and the thing that is dissolved is the sugar that's the solute now all of this may or may not be review for you but I'm doing it for a reason because I want to talk about I want to talk about the idea of diffusion diffusion and the idea is actually pretty straightforward if I have let's say we'll say the same container let me let me do in a slightly different container here just to talk about diffusion we'll go back to water and sugar especially back to water let's say we have a container here now let's say it just has a bunch of let's say just set some air particles in it could be anything oxygen or carbon dioxide so let me just draw a couple of air molecules here so let's let's say that that is a gaseous just for the sake of argument gaseous oxygen so each of this is an o2 each of those right now let's say that this is the current configuration that all of this is a vacuum here that there's some temperature so these water molecules they have some type of they have some type of kinetic energy they're moving in some type of random directions right there so my question is what is going to happen what is going to happen in this type of container well any of these guys are going to be randomly bumping into each other they're more likely to bump into things in this down left direction than they are in the upright direction so if this guy was happening to go in this down left direction he's going to bump into something and then ricochet into the up right direction but in the upright direction there's nothing to bounce into so in general everything is moving in random directions but you're more likely to be able to move in the rightward direction when you go to the left or more like we're going to bump into each other into something so it's almost common sense over time if you just let this system come to some type of equilibrium and I'm not going to go into detail on what that means you can watch the thermodynamics videos if you'd like to see that you'll eventually see the container will look something like this I can't guarantee it there's some probability it would actually stay like this but very likely that those five particles are going to get relatively right out this is diffusion and so it's really just the spreading of particles or molecules from high concentration to low concentration areas right in this case the molecules are going to spread in that direction from a high concentration to low concentration area now you're saying Sal what is concentration and there's many ways to measure concentration and you can go into molarity and molality and all of that but the very simple idea is how much of that particle do you have per unit space so here you have a lot of those particles per unit space and here you have very few of those particles per unit space so this is a high concentration and that's a low concentration so you can imagine other experiments like this you could imagine a solution like let's let let's do something like this let me make let's say I have two containers let's say two containers let's go back to the solution situation so this was a gas but I started off with that example so let's stay with that example let's say that I have a door right there that's larger than either the water or the sugar molecules on either side I have a bunch of water molecules I have a bunch of water molecules on either side just like that on either side so I have a lot of water molecules so if I just had water molecules here they're all bouncing around in random directions and so the odds of a water molecule going this way equivalent to ads of a water molecule and going that way assuming that both sides have the same level of water molecule otherwise the pressures would be different but let's say that you know the top of this is the same as the top of this so there's no more pressure going in one direction or another so you know if for whatever reason a bunch of more water molecules we're going in the rightward direction then all of a sudden this would fill up with more water and we know that that isn't likely to occur so this is you know this is just a solution with or this is just a kind of two containers of water now let's put some solute in it let's dissolve some solute in it let's say we do all the dissolving on the left hand side so we put some sugar molecules on the left hand side and these are small enough to fit through this little pipe that's one assumption that I'm making so what's going to happen all of these things have some type of kinetic energy they're all bouncing they're all bouncing around well over time you know you you know the water is going back and forth this water molecule might go that way that water molecule might go that way but they Netta each other out but over time one of these big sugar molecules will be going in just the right direction to go through maybe you know maybe this guy instead of going in that direction he's starts off going in that direction he goes just through this through this up this tunnel continued connecting the two containers and he'll end up there right and you know this guy will still be bouncing around there's some probability he goes back but there's still more particles more sugar particles here than there so still there's more probability that one of so this guy's on that sub something to cross them out but there's still more probability that one of these guys will go to that side then one of these guys will go to that side so you can imagine if you're doing this with gazillions of particles I'm only doing it with four over time the particles will have spread out so that their concentrations are roughly equal so that you know maybe you'll have to hear over time but if you're you know when you're only dealing with three or four or five particles you know there's some probability it doesn't happen well you're doing it a gazillion and they're super small it's a very very very high likelihood but anyway this whole process we went from a container of high concentration to a container of low concentration and the particles would have spread from the low concentration container to the high concentration container so they diffused this is diffusion this is diffusion and just so that we learn some other words that tend to be used with the idea of diffusion when we started off this had a higher concentration the left-hand-side container had higher concentration higher concentration higher concentration it's all relative right so I'm higher than this guy higher concentration and this right here had a lower concentration lower concentration and there are words for these things this solution with a high concentration is called a hypertonic solution let me write that in yellow Hyper hypertonic solution hyper in general meaning a having a lot of something having too much of something and this lower constant concentration is hypo hypo tonic hypotonic solution lower concentration you might have heard maybe one of your relatives if they haven't had a meal in a while say I'm hypo hypoglycemic that means that they have not that they're feeling light-headed there's not enough sugar in their bloodstream and they want to pass out so they want a meal if you you know you just had a candy bar maybe your hyperglycemic or maybe you're just hyper in general but so these are just good prefixes to know but hypertonic you have a lot of the solute so you have high concentration and then when hypotonic not too much of the solute so you have a low concentration these are good words to know so in general diffusion if if there's no barriers to the diffusion like we had here you will have the solute go from a high concentration or hypertonic solution if they can travel to a hypotonic solution to a hypo where the concentration is lower now let's do an interesting experiment here we've talked about diffusion and so far we've been talking about the diffusion of the solute right and in general and this is not always the case the you know if you want to be as general as possible the solute is whatever you have less of the solvent is whatever you have more of and the most common solvent tends to be water but it doesn't have to be water it could be some type of alcohol it could be just you know it could be mercury it could be a whole set of molecules but water in most biological or chemical systems tends to be the most typical solvent it's what other things are dissolved into but what happens if we have a tunnel where the solute is too big to travel but water is small enough to travel so let's think about let's think about that situation let's think about say that situation in order to think about it I'm going to do something interesting let's say we have a container here let's say we have actually I won't even draw a container let's just say we have an outside environment that has a bunch of water this is the outside environment and then you have some type of membrane you have some type of membrane here that's a membrane water can go in and out of this membrane so it's semipermeable well it's permeable to water but the solute cannot go through the membrane so let's say that the solute is sugar so we have water on the outside and also in Sun I want to do and also inside the membrane so these are little small water molecules this is a membrane right here and let's say that we have some sugar molecules again I just I'm just picking on sugar it could have been anything so we have some sugar molecules here that are just a little bit a little bit bigger or there could be a lot bigger actually there are a lot bigger than water molecules so you have a bunch of and I only draw four but you have a gazillion of them right but you have a that much more water molecules I'm just trying to show you have more water molecules and sugar molecules and this membrane is semipermeable semi permeable permeable means that it allows things to pass semipermeable means it's not completely permeable so semipermeable in this context I'm saying I allow water to pass through the membrane so water can pass water can pass but sugar cannot sugar is too large sugar cannot and the reason why in this case is because the sugar molecule is too big so if we were to zoom in on the actual membrane itself maybe the membrane looks like this I'm going to zoom in on this membrane so I'm going to zoom in on this membrane so it has little holes in the membrane just like that and maybe the water molecules are about that size so they can go through those holes so the water molecules can go back and forth through the holes just like that but the sugar molecules are about that big so they cannot they cannot go through that hole they're too big for this opening right here to go back and forth between them now what do you think is going to happen in this situation so first of all let's use our terminology remember sugars are solute waters are solvent semipermeable membrane side of the membrane has a higher or lower concentration of solute well the inside does the inside is hypertonic hypertonic so I could write here hyper hypertonic the outside has a lower concentration so it's hypo hypo tonic it's hypo tonic out here lower concentration of solute now if these openings were big enough based on what we just talked about these guys you know these guys are bouncing around water is traveling in either direction and II equal probability or I'm gonna I'm actually I'm going to talk about that in a second if everything was wide open it would be equal probability but if it was wide open these guys eventually would bounce their ways over to this side and you would probably end up with equal concentrations eventually and so you would had your traditional diffusion where high concentration of solute to low concentration of solute but in this case these guys they can't get they can't fit through the hole only water can go back and forth if these guys were not here if these guys were not here water would have an equal likelihood of going in this direction as they would be going in that direction a completely equal likelihood but because because these guys are on the right-hand side of or in this case on the inside of our membrane this is our inside of our membrane zoomed up these guys it's less likely because these guys might be in the kind of the approach position of the holes it's less it's that slightly less likely for water to be in the approach position for the holes so it's actually more probable that water could enter then water exit and I want to make that very clear if these sugar molecules were not here obviously it's equally likely for water to go in either directions now that these sugar molecules are there these sugar molecules might be on the right hand side they might be blocking I guess the best way to think about is blocking the approach the hole they'll never be able to go through the hole themselves and not even be blocking the hole but they're going in some random direction so if a water molecule was approaching it's that it's all probabilistic and we're dealing with gazillions of molecules it's that much more likely to be blocked to get outside but the water molecules from the outside there's nothing blocking them to get in so you're gonna have a flow of water inside so in this situation with a semipermeable membrane you're going to have water you're gonna have a net inward flow of water and so this is this is kind of interesting we have the solvent flowing from a hypotonic situation to a hypertonic solution but it's only hypotonic in the solute that's when you talk about the solute it's only hypertonic when you talk about the solute but water if you if you flip it the other way if you view sugar as the solvent then you could say we're going from a high concentration of water to a low concentration of water I don't want to confuse you too much this is what tends to confuse people but just think about what's going to happen no matter what in what situation the solution is going to do what it can to try to equal liberate the concentration to make the concentrations on both sides as close as possible and it was it's not just some magic it's not like the solution knows it's all based on probabilities and these things bumping around but in this situation water is more likely to flow into the container so it's actually going to go from the hypotonic side when we talk about low concentration of solute to the side that has high concentrations of solute of sugar and actually if this thing is if this thing is stretchable more water will flow flow will keep flowing in and this and this membrane will stretch out will stretch out but we want to go into too much detail here but this idea of water of the solvent in this case water is the solvent of thus of water as a solvent diffusing through a semipermeable membrane this is called osmosis osmosis you've probably heard learning by osmosis you know if you put a book against your head maybe it'll just seep into your brain same idea that's where the word comes from this idea of water seeping through membranes to try to make concentrations more equal so if you say well you have high concentration here low concentration here if there was no membrane here these big molecules would exit but because there's there's this the semipermeable membrane here they can't so the system just probabilistically no you know not no magic here more water will enter to try to equal liberate the concentration and you know eventually if maybe there's maybe there's a maybe there's a few molecules out here not as high concentration here eventually if if all if everything was allowed to happen fully you'll get to the point where you have just as many you have just as high concentration on this side as you have on the right-hand side because this right-hand side is going to fill with water and also probably become a larger volume and then once again the probabilities of a water molecule going to the right and to the left will be the same and you'll get to some type of equilibrium but I want to make it very clear diffusion is the idea of a of any particle going from higher concentration to and spreading into a region has a lower concentration kind of just spreading out osmosis is the diffusion of water and usually you're talking about you diffusion of water as a solvent and usually it's in the context of a semipermeable membrane where the actual solute cannot travel through the membrane anyway hopefully you found that useful and not completely confusing
Biology is brought to you with support from the Amgen Foundation