Diffusion and Osmosis

Diffusion and Osmosis

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.

I have many, many more water molecules in this container that we are dealing with.

Now in this type of situation,
we call, 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, so the more water is the solvent and in that case, that is the
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, in 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 a 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,the idea is actually
pretty straightforward.

If I have, let's say,let's same
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, 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 goign 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 each other, 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 me make

Let's say I have
two containers.

let's see two container.

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.

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 what 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, you know 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 you know if,it for whatever reason, a
bunch of 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,you know, this is just a solution, with or , this is just two containers of waters
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.

right, that's just 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, the water's
going back and forth.

This water molecule
might go that way.

That water molecule might go
that way, but they net out 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's,
instead of going that

direction, he starts off going
in that direction.

He goes just through this,throught this,uhm throught this tunnel
connecting this 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 particles,more sugar particles
here than there.

So there's still more
probability that one of, so these

guys will go to that side
than one of these guys

will go to that side, that 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 if, 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.

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?

It's higher than this guy,higher concentration

And this right here had
a lower concentration.

Lower concentrarion

And there are words
for these things.

This solution with a high
concentration is called a

hypertonic solution.

Let me write that in yellow.

Hyoer, Hypertonic solution

Hyper, in general, meaning
having a lot of something,

having too much of something.

And this lower concentration
is hypo, hypotonic

Hypotonic solution,lower concentration

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.

But, so, you know, 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 a...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?

Let's think about
that situation, let's think about the 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

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 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.