Tonicity – comparing 2 solutions | Lab values and concentrations | Heatlh & Medicine | Khan Academy

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.


  1. You didn't count the red molecules in the first cup.. you said there are 6 not 8? but in the second one you did count them..7..? why?

  2. Hi! Great video – cleared things up really nicely. Just one question: can the terms hypertonic and hypotonic be used to describe the cell as well? I know we use these terms for the solution; but what about the cell? Is a cell in a hypotonic solution called a hypertonic cell? And is a cell in a hypertonic solution called a hypotonic cell? Or are these terms just used to describe the solution? Thanks!

  3. I have a point of contention about claims that only Non permeable solutes affect tonicity. I think of tonicity as an extension of osmolarity. Basically, sometimes the fact that there are permeable solutes does affect the volume change of a cell. For example, we have a red blood cell of 300 milliosmoles of sucrose (which is impermeable to the membrane). If we place this cell into a solution of 300 milliosmoles of sucrose plus 50 milliosmoles urea, then urea crosses the membrane. If urea crosses, then water will follow and cause the cell to swell. So it is essentially the concentration difference of the permeable solute that is causing tonicity change. You said permeable solutes do not affect tonicity? Please correct me if I am wrong, I am a TA and would like to know if telling students this is incorrect.

  4. Doesn't the fact that solute A is permeable to the membrane make this example not work? Wouldn't solute A in the first cup move back out of the cell to form a balance in the amount of solute and thus the cell would not swell? And wouldn't solute A in the last cup move back into the cell to form the balance of solute and thus the cell wouldn't shrink?

  5. Brilliant video
    Question is, when talking about tonicity should one count the molecules that distribute equally between compartment plus those that do not penetrate 
    Or count those that dont penetrate only?
    arent there isOSMOTIC, hypoTONIC solutions?

  6. Isn't there a contradictory thing?First you tell us the actual mediator of tonicity are those that are non permeable solutes but later you explain those three solutions with different tonicity by using just the sums of solutes on either sides, which is basically osmolarity, not stating that which side having more non permeable solutes…would you please be kind enough to clarify that??

  7. Thank you so much! i didn't understand it from my teacher as well as i understood it from you! Really appreciate your time.

  8. So in sample 1) of the water and cell. What if it had no solute on the outside instead of the 2 molecules of solute B. would it not burst as it would always be hypotonic?

  9. This video quite helpful, and I liked it. The only thing that bugged me is that the first of the 3 water filled beakers should be 10 solutes on the inside and 8 on the outside of cell.  Not 10 and 6.

  10. Second part of the video: does it make all 3 solutions hypertonic if only solute B is accountable for the solution tonicity?

  11. You see, counting the molecules that distribute equally across the whole thing is the same as not counting them. They don't really matter. If you have 10 molecules inside of the cell, and 15 outside of it, and say, 8 of these molecules distribute equally, that means you could substitute 8 from both sides, and you will end up with 2 molecules on the inside, and 7 on the outside, which doesn't really make the tonicity any different than before.

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