Diffusion and Osmosis – For Teachers
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Diffusion and Osmosis – For Teachers


Hi. My name is Dana Murchison. I’m a scientist here
at Science North. And today I’m going to do
three experiments that are going to demonstrate diffusion and osmosis in the
context of cell biology. When we think about
cell biology and we’re looking at
diffusion and osmosis, it’s really helpful to zoom down
to the level of the cell, which really is
the molecular level. Cells are made of molecules and
they’re surrounded by molecules. They are a part of their
molecular environment. And what that means is that
the dynamics in the forces that effect molecules
also have a big affect on cells
and on cell biology. So to get started, we’re going to talk a little bit
about diffusion. Diffusion is
the movement of molecules from an area
of high concentration to an area of low concentration. So to demonstrate
how that works, we’re going to do
an experiment in which I’m going to drop
a drop of food colouring into a beaker of hot water and another drop
of food colouring into a beaker of cold water
at the same time. So I’m using red food colouring
to demonstrate the hot beaker and blue food colouring to
demonstrate the cold beaker. So, we’re going
to be looking to see how quickly the food colouring
spreads out in the two different beakers. So here we go. Now, as you can see,
the red food colouring is moving through the beaker of
hot water much more rapidly than the blue food colouring is moving through
the beaker of cold water. And that’s because diffusion
is based on molecular movement. Molecules will tend to speed up
at a higher temperature which means that diffusion
will happen more rapidly. Now, how is this important
in the context of the cell? Now, in the cell, diffusion can happen across
the cell membrane. The cell membrane is what we
call a semi-permeable membrane, and what that means
is that some molecules can flow freely across it
or diffuse freely across it while others will be blocked
or stopped from crossing. One of the most important
criteria that we use to determine whether a molecule
will be able to cross is size. So small molecules can cross
freely across the cell membrane whereas large molecules
may be blocked from diffusing across
the cell membrane. So we’re going
to do an experiment that’s going to demonstrate
the movement of molecules, the diffusion of molecules across
a semi-permeable membrane. What I have right here
is dialysis tubing. Dialysis tubing,
just like the cell membrane, is a semi-permeable membrane
that allows some molecules to move through
and excludes others. What I’m going to do is that
I’m actually going to pour some liquid into the tube
and a different solution into the beaker for each of
these two experiments and we’re going to see whether
the solutions are able to cross the membrane
or diffuse across the membrane in order
to join each other. So the first thing
I’m going to do is I’m going to add a solution
of potassium thiocyanate into the tube, pour it in carefully,
and hang it on the edge. So the potassium thiocyanate
is a colourless liquid. The next thing that I’m going
to be adding is a solution
of iron III chloride which I’m going
to pour into the beaker. So here the iron III chloride
goes into the beaker, and as you can see,
it’s a light orange in colour. So how are we going to know
that diffusion has taken place? We’re going to know that
diffusion has taken place if there is a colour change
in the beaker, in the tube,
or in both. So while we wait for something
to happen there, I’m going to set up
our second experiment. Here I’m going to be adding
distilled water, which is going to be diluting
an iodine solution, into the tube,
the dialysis tubing here. And then I’m going to add
to the beaker, a concentrated
starch solution. Now the iodine solution is
a light yellow in colour. The starch solution, which you
can see here, is a cloudy white. And once again, we’ll know
if diffusion has taken place because we’re going
to see a colour change in either the beaker
or the tube or both. So, here I’m adding the starch
solution to the beaker. Now, if we go back
to our first experiment, we can see that there’s a marked colour change
that has already happened. So here the liquid inside
the tube has turned a dark red and there’s
a dark red colour that’s also forming in the
liquid inside the beaker. Now what that means is that
the two substances have been able to diffuse
across the membrane and react with each other. And what they’ve done is they’ve
actually formed that reaction in both the tube
and in the beaker. What that tells us is that
2-way diffusion took place
in this example, whereas, here,
in our second experiment, we can see that there is a colour change
that’s starting to happen but that colour change is only
happening inside the beaker. Our tube still has the same
yellow-y clear colour. Inside the beaker, we’ve got our
starchy cloudy white solution but we’re seeing
a dark precipitate which is a purple-ly brown
which is starting to form. Now that purple-ly brown colour
indicates that the molecules, the solution in the tube, were able to diffuse
into the beaker and cause
a colour change. But there’s no colour change
happening in the tube. The reason for that is that,
as I mentioned, the dialysis tubing is
a semi-permeable membrane. It allows some molecules
to cross but not others. The iodine solution
that we have in the tube is a very small molecule. It’s easily able to pass through
the pores in the tube and into the beaker where it causes a
colour change with the starch. The starch, on the other hand,
is a very large molecule. It isn’t able to penetrate
through the pores of the dialysis membrane. And what that means is
that it is– it demonstrates that this is
a semi-permeable membrane. Even though the starch would
tend to want to diffuse into the tube, it can’t because of the
size exclusion of the pores in that semi-permeable membrane. This is similar to what
happens in a cell membrane. The next thing that we’re going
to demonstrate is osmosis. So, osmosis is similar
to diffusion but osmosis is the movement of
water molecules from an area of low
concentration to an area
of high solute concentration. We’re going to demonstrate that
using an experiment with what we call
a thistle funnel tube. Now,
the thistle funnel tube, just like the tubes
that we looked at previously, has a length of dialysis
membrane stretched across the bottom. So, once again, this is
a semi-permeable membrane. What I’m actually going to do is
that I’m going to fill the thistle funnel tube with a very concentrated
sucrose solution so we’re actually
going to just add that sucrose solution directly
to the thistle funnel tube. And once we have added it to
a high enough degree, we’re actually going to suspend
this thistle funnel tube in a beaker
of distilled water. Now, that would mean that
the concentration of solute in the thistle funnel tube
is much higher than the concentration that we
would see in the beaker of water and so the tendency would be
for the water to osmose across the
semi-permeable membrane entering into
the thistle funnel tube. So, we’ve added enough of our
concentrated sucrose solution, we’re going to suspend this in a beaker of
double distilled water and we’re going to leave it because osmosis
in this context takes a little bit of time
in order to occur. So, what we’re going to see is
that water is going to cross that semi-permeable membrane
through osmosis, moving from an area
of low solute concentration to an area of
higher solute concentration. In this case,
the solute is sucrose. And we’re actually going to see the level of fluid
in the tube rising over time. Now, there you have it, three experiments
that demonstrate diffusion and osmosis in the context
of cell biology. Hope you enjoyed this video.

About James Carlton

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33 thoughts on “Diffusion and Osmosis – For Teachers

  1. what if you swap the sucrose with glucose solution? Glucose will diffuse out of the tubing while water molecules will just diffuses into the tubing until an equilibrium is reached , and so no change in the level of the solution in tubing and beaker, is that right?

  2. Do you know …that was awesome .for so many years a was perplexing In osmosis and diffusion .but today it is crystel clear ..
    Thanks a lot .God bless you..

  3. Dr.S.S.Ahmed, from
    Hyderabad, India
    I really appreciate your efforts you made for this video..Its awsome.
    Will you please let me know the preperation on Semi Permeable Membrane too……..

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