AP Biology Lab 1: Diffusion and Osmosis


Hi. It’s Mr. Andersen. And welcome
to the AP Biology Lab 1 walkthrough. This lab is on two things. It’s on diffusion and
osmosis. And the diffusion portion of this lab we’re actually just going to do a demonstration.
And the osmosis lab we’ll do the potato lab. So we’re going to put potatoes in different
concentrations of sugar water. But let me quickly define what diffusion and osmosis
are. Diffusion is going to be movement of molecules from an area of high concentration
to low concentration. So if I were to remove this lid on the top of the perfume, the perfume
molecules are going to move from an area of high concentration inside the bottle to low
concentration, in the air. And so pretty soon you’d be able to smell that perfume just due
to diffusion. Now osmosis is a specific type of diffusion. It’s movement of water. And
so we’re going to study that in this lab using potatoes and sugar water. And so let me go
into a little more depth on those two processes. First one is diffusion. Diffusion is, best
example would be right here. Let’s say we have this jar. And this jar is separated in
the middle using a membrane. And that membrane is porous. In other words it allows material
to move back and forth. Well if I were to let it sit here at time 0 and come back at
time 15 minutes what we’re going to find is that on this side we have sugar water. On
this side we could have less sugar water. Or almost distilled water on this side. If
we let it sit for 15 minutes however those sugar molecules are going to start to migrate
over here on to this side. So when we come back it’s going to be in equal concentration
of sugar on either side. Now the water would have also been flowing. But the water and
the sugar are going to move back and forth and it’s essentially going to be isotonic
on either side of that membrane. That’s diffusion. It’s simply movement of molecules from an
area of high to an area of low concentration. Now osmosis is a specific type of diffusion.
It’s the movement of only water across a semi-permeable membrane. So how does that work? Well let’s
say we have the same set up. Different container. But it’s got semi-permeable membrane down
the middle. This only allows the movement of water but it doesn’t allow the movement
of those solute molecules. Well you can see right here that this side over here is going
to be hypertonic. In other words it has more solute on that side. And less water. This
side over here is going to be hypotonic. And so if we let it sit now for another 15 minutes,
it’s going to magically raise on this side. And the reason why is that water is going
to flow from an area of high water concentration to low water concentration. Now what aren’t
the sugar molecules moving? It’s because they can’t move through that membrane. And so this
would be osmosis. It would be the flow of water across a semi-permeable membrane. And
so we’ll see how that plays out in just a second. And so the first part of this lab
is the diffusion lab. In this diffusion lab we’re going to do essentially a quick demonstration.
And so to do this you’re going to use dialysis tubing. So we’re going to have dialysis tubing
on this side. Dialysis tubing is a little bit of tubing that’s used in kidney dialysis.
It’s essentially, it looks just like plastic but it’s going to have tiny little holes in
it. Tiny little pores in it that you can’t see. And then we’re going to have a beaker.
Now in the beaker we’re going to put water or H2O. And then we’re going to put a chemical
called IKI. IKI essentially has iodine inside of it. So in the beaker all we’ve got is water
and IKI. Inside the dialysis tubing we’re going to have water. We’re going to have glucose.
Glucose remember is a simple sugar. It’s a monosaccharide. So it’s going to just have
one sugar. We could test for the presence of glucose using something called testate.
We put it into the solution. We’re going to find that there’s going to be glucose inside
there. And then we’re going to have starch. Now starch is a polysaccharide. So instead
of just one sugar molecule it’s going to be sometimes hundreds if not thousands of sugar
molecules that are attached together. Starch is going to be that sugar that you find in
stuff like pasta. And so inside here we’ve got water, starch and glucose. Outside no
glucose. Now we’re going to let it sit for, I do this during class. So we let it sit for
about 40 minutes. Before I show you what happens I should also remind you this. We can test
for the presence of glucose using test tape. Which is simply a little bit, it almost looks
like litmus paper. But to test for the presence of starch, if IKI and starch are ever in the
same place at the same time it’s going to turn this bluish color. So let’s see what
happens. An hour later it’s going to look like this. And so in class I ask students
to figure out, you know, what’s going on? Why do we get this blue color on the inside?
What has moved back and forth? And so basically when we test it when it’s done we’ll find
that there’s glucose again inside the dialysis tubing. Makes sense because it was there before.
There’s starch. There’s water. But we can infer that IKI has moved inside. And how do
we know that? Well starch and IKI if they’re ever in the same place at the same time we’re
going to get this blue color. On the outside we could test and we’ll find that there’s
a presence of glucose inside here. There’s IKI like there was before. There’s water.
But there’s no starch on the outside. How do we know there’s no starch? Well there’s
no blue color on the outside. And so we could figure out the size of all these molecules
in relation to the sizes in the holes in the dialysis tubing. In other words are the holes
inside the dialysis tubing bigger than the IKI? For sure. Because the IKI was able to
move on the outside in. Are they bigger than the starch? No. Because the starch is not
able to make it’s way out. And so what we’re showing here is diffusion. What’s the one
thing that we didn’t talk about the movement of? That would be water. In order to figure
out if water is moving or not we’d have to mass it before and mass it after and see if
there’s a change in mass. Alright. Let’s get to the actual potato lab. In the potato lab
what you’re going to do is you’re going to core potatoes. So you’re going to basically
cut out a little core of a potato like that. We usually use about 3 of those inside there.
And then you’re going to put them into a beaker that’s containing different amounts of sugar
concentration. And so we’re going to put it in distilled water. 0 molar. 0.2, 0.4, 0.6,
0.8 and 1.0 molar. So this is a ton of sugar inside it. We’re going to get the mass of
them on day 1. And then we’re going to get the mass of them on day 2. So this would be
like the mass on day 1. This is the mass on day 2. And so if we think about osmosis and
what’s going on, well in the distilled water you can see here that on day 1 the mass of
those two potato cores was 4.74 grams. And then on day two it’s 6.14 grams. And so what’s
happening? Well if you put the potatoes inside distilled water, they’re gaining mass. Why
are they gaining mass? It’s due to osmosis. In other words water is flowing into the potatoes.
Because it’s moving from an area of high water concentration outside the potato to low water
concentration on the inside. Or from hypotonic to hypertonic. If we look at the 0.2, the
0.2 is also increasing mass. If we graph this out. But as we look at the 0.4, 0.6, 0.8 and
1.0 we find that it’s actually losing mass. In other words it’s shriveling out. Why is
that? Well if we put it in real sugary water, high molarity water, what we’re finding is
that now the water instead of moving into the potato is actually moving from the potato
out. And so you can see here that there’s like a best fit line. If I were to draw it
I’d put the best first line somewhere like that. And so basically what you can figure
out is that point at which this line crosses the axis, that would be 0% change in mass.
And so we can really figure out the molarity of potato. Because if you took a potato with
the molarity of, I don’t know what this is, like right around 0.3 molar and we were to
put in 0.3 molar, it’s going to have movement of water back and forth but it’s not going
to relatively change its mass. And so that’s the potato lab. That’s diffusion. And I hope
that’s helpful.

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