Diffusion and osmosis | Membranes and transport | Biology | Khan Academy
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Diffusion and osmosis | Membranes and transport | Biology | Khan Academy


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 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. Now in this type of situation,
we call the thing that there’s 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. 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’s more of, that’s what we’re going
to call the solvent. So in this case, water
is the solvent. And whatever there is less of–
in this case, that is the sugar– that is considered
the solute. It doesn’t have to be sugar. It can be any molecule that
there’s less of, in the water, in this case. And we say that the sugar has
been dissolved 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 the 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 the idea of a diffusion. And the idea is actually
pretty straightforward. If I have, let’s say,
the same container. Let me do it 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 and let’s say it just has a bunch of– let’s say it just
has some air particles in it. It could be anything– oxygen
or carbon dioxide. So let me just draw a couple
of air molecules here. So 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? And let’s say that this is the
current configuration, that all of this is a vacuum here
and that there’s some temperatures. So these water molecules,
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 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 up-right 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 up-right
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, you’re
more likely to bump into something. So it’s almost common sense. Over time, if you just let this
system come to some type of equilibrium– 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 spread 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 a 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 could imagine other
experiments like this. You could imagine a solution
like– let’s do something like this. Let’s say I have
two containers. Let’s go back to the
solution situation. This was a gas, but I started
off with that example so let’s stay with that example. So 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. 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 odds of a water molecule 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 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 if for whatever reason, a
bunch more water molecules were 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 just two containers
of water. Now let’s put some
solute in it. Let’s dissolve some solute in it
and 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 around. Well, over time, the water’s
going back and forth. This water molecule
might go that way. That water molecule might go
that way, but they net each other out, but over time one of
these big sugar molecules will be going in just the
right direction to go through– maybe this guy’s,
instead of going that direction, he starts off going
in that direction. He goes just through this tunnel
connecting the two containers and he’ll end
up there, right? And this guy will still
be bouncing around. There’s some probability he goes
back, but there’s still more sugar particles
here than there. So there’s still more
probability that one of these guys will go to that side
than 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 maybe you’ll have
two here over time. But when you’re only dealing
with three or four or five particles, there’s some
probability it doesn’t happen, but when you’re doing it with 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. 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. It’s all relative, right? It’s higher than this guy. And this right here had
a 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, in general, meaning
having a lot of something, having too much of something. And this lower concentration
is hypotonic. You might have heard maybe one
of your relatives, if they haven’t had a meal in awhile
say, I’m hypoglycemic. That means that they have
not– they’re feeling lightheaded. There’s not enough sugar in
their bloodstream and they want to pass out so
they want a meal. If you just had a candy bar,
maybe you’re hyperglycemic– or maybe you’re just
hyper in general. So these are just good
prefixes to know, but hypertonic– you have
a lot of the solute. You have a high concentration. And then in hypotonic, not too
much of the solute so you have a low concentration. These are good words to know. So in general, diffusion– 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– 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 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? Let’s think about
that situation. In order to think about it,
I’m going to do something interesting. Let’s say we have a
container here. 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. Water can go in and out
of this membrane. So it’s semi-permeable. 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 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’m just picking on sugar. It could have been anything. So we have some sugar molecules
here that are just a little bit bigger– or they
could be a lot bigger. Actually, they’re a lot bigger
than water molecules. You have a bunch of– and I only
draw four, but you have a gazillion of them, right? You have that much more
water molecules. I’m just trying to show you have
more water molecules than sugar molecules. And this membrane is
semi-permeable. Permeable means it allows
things to pass. Semi-permeables means it’s
not completely permeable. So semi-permeable– in this
context, I’m saying I allow water to pass through
the membrane. So water can pass,
but sugar cannot. Sugar is too large. 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 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, but the sugar molecules
are about that big. So 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, sugar is our solute. Water is our solvent. Semi-permeable membrane. Which side of the membrane
has a higher or lower concentration of solute? Well, the inside does. The inside is hypertonic. The outside has a lower concentration so it’s hypotonic. Now, if these openings were big
enough, based on what we just talked about– these guys
are bouncing around, water is travelling in either direction,
and equal probability or– 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’d probably end up with equal concentrations
eventually. And so you would have your
traditional diffusion, where high concentration
of solute to low concentrations of solute. But in this case, these
guys– they can’t fit through the hole. Only water can go
back and forth. 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 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– it’s less likely because these guys
might be in the approach position of the holes– that’s
slightly less likely for water to be in the approach position
for the holes so it’s actually more probable that water could
enter than 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 direction. 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 it is blocking the
approach to the hole. They’ll never be able to go
through the hole themselves and might not even be blocking
the hole, but they’re going in some random direction. So if a water molecule was
approaching– 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 going to have a flow of water inside. So in this situation, with a
semi-permeable membrane, you’re going to have water. You’re going to have a net
inward flow of water. And so 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. But water– if you flip it the
other way– if you’ve used 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 in what situation,
the solution is going to do what it can to try to
equilibriate the concentration. To make the concentrations
on both sides as close as possible. And 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 stretchable, more water will keep flowing
in and this membrane will stretch out. I won’t go to too much detail
here, but this idea of water– of the solvent– if in this
case, water is the solvent– of water as a solvent
diffusing through a semi-permeable membrane,
this is called osmosis. You’ve probably heard learning
by osmosis– 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, I have high
concentration here, low concentration here. If there was no membrane here,
these big molecules would exit, but because there’s this
semi-permeable membrane here, they can’t. So the system just
probabilistically– no magic here– more water will enter
to try to equilibriate concentration. Eventually– if maybe there’s a
few molecules out here– not as high concentration here–
eventually 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 any particle going from
higher concentration and spreading into a region that has
a lower concentration and just spreading out. Osmosis is the diffusion
of water. And usually you’re talking about
the diffusion of water as a solvent and usually it’s
in the context of a semi-permeable membrane, where
the actual solute cannot travel through the membrane. Anyway, hopefully you’ve
found that useful and not completely confusing.

About James Carlton

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100 thoughts on “Diffusion and osmosis | Membranes and transport | Biology | Khan Academy

  1. I am so confused, can someone help me out? My textbook is telling me that within the same medium, a difference in concentration affects the rate of diffusion, but between two media such as air and blood, its is the difference in partial pressure of oxygen and carbon dioxide that determines the rate of diffusion.
    I simply cannot understand why – the book gives no specific reason whatsoever. Is it because the pressure of these gases are different in the different media, or that the temperature is different? And if so, how does diffusion increase the partial pressure of a gas to create an equilibrium situation?

  2. God bless the Whole Khan Academy and its lecturers/providers. You are making this world a better place. Planting hope into the youth by educating them <3

  3. hi i got a little bit confuse but may u answer he some of the questions? All about osmosis
    this is what i understand from this video , inside the molecule have sugar (salute).So the water outside the cell will go into the molecule so the water outside the molecule and inside the molecule become the same.
    Am I right?
    pls answer me, I am going to have a exam.pls thx

  4. i swear,you are the best teach ever! my biology teacher is good,but in these cases where i wasnt in class and i have no idea hor to explain these stuff to myself,youre a life savior. god bless your soul!!!

  5. i have been very sick for almost two weeks, and the day i get back I have to take a test on this. My class has been going over it for two weeks, so I had to learn it all in one day. I watched this video and got an A on the test, while everyone else got a b-c on it. Thank you so much.

  6. take a shot every time this man stutters or repeats himself. Coulda shortened this video by like 5 mins smh

  7. The video was 19 minutes long and I didn't even realise. But thanks, this was so useful. Love the colours too.

  8. 0:00 solvent review
    2:28 diffusion introduction
    6:29 diffusion with solutes
    8:16 diffusion vocabulary
    10:55 diffusion with membrane
    17:12 osmosis

  9. you said that a hypertonic solution has a lot o something and a hypotonic solution does not have enough, I think its the opposite way I believe. Unless i'm wrong

  10. thanks a lot sal!!
    i never understood this topic with my bio teacher and i understood it fully by this 19 minute video
    Thanks!!!

  11. Thank you so much for making this video. it was good and I understand everything what is the difference between diffusion and osmosis. I like it when you put english subtitle on the video because it's helped me a lots. 😀

  12. I´m really starting to doubt about education in the future. I mean, I´ve learned WAY more in this 20 min video than in 2/3 biology classes. For real, this is the future.

  13. I love you. I watched this 45 min before taking my bio quiz. Please keep doing this. Is there anyway I can support? This is free and I learn more from you than I do in College.

  14. If the sugar molecules are blocking water from leaving, doesn't it also block the water from going in, because they enter and leave through a opening? I am only talking about one hole, so doesn't water not flow in or out?

  15. If there are too many molecules on one side and the semi-permeable membrane is not stretchable, will the membrane explode? What will happen?

  16. so basically diffusion goes from high concentration to low concentration. Osmosis is the diffusion in water but this time from low concentration to high concentration. Am I correct?? coz this is really confusing me out…

  17. plz don't say your not good…your awesome ….and my teacher ……can't explain …even I don't want to 👍👍👍👍👍👍👍 keep up the awesome work

  18. Why did the low concentration solution want to go into the high concentration solution to create equilibrium? Shouldn't it be the other way around?

  19. Thank you so much im in college right now and you explain this way better then my professor

  20. I'm studing at home and was ment to go on a break, but i was actully enjoying your teaching style so much i watched into my break, so clear, thankyou so much!!!! You rock.

  21. You know what makes sense too he said that what we have more of is solvent which vent also means that someone who is venting says the most

  22. In the diagram on the left at 2:34, if you also put salt in, for example, at a concentration that is in between the one of the water and the sugar, would it have a specific name?

  23. Your video helped me soooo much to finally understand diffusion and Osmosis!! I'm studying for my CCHT test and these are the related questions I had the hardest time with…..until you explained it in a way I could understand. Thank you so much!!!

  24. Thank you so much for this video👍👍, and can anyone help me to know what is the different between :

    1- Net osmosis
    2- net diffusion

    I got confused 🤷🏻‍♀️ 😭

  25. I have a test on cells, heredity(etc)tomorrow couldnt remover anything about this. Super helpful. You explained ThisWAY better than my science teacher ever could

  26. the solvent is the solution, the solute is substance in the solution and aqueous is if the solute is dissolved in water.

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