Hi. It's Mr. Andersen and in this video I'm going to talk about water potential,
which is really what it sounds like. It's the potential energy of water per unit area
compared to pure water. And so it allows us to figure out where water is going to flow
due to osmosis, gravity, pressure. Even surface tension. And so it allows us to figure out
if water is going to flow into the cell or not. And so we measure it using something
called psi or p s i. And a quick way to remember that is that Poseidon was this Greek god of
the ocean. Carried a trident. And it looks a lot like the trident that we use to represent
water potential or psi. Now psi is going to be equal to the psi S with is solute potential
and pressure potential. But before I scare you off with a bunch of formulas, let's get
started and talking about how water potential works. Let's first talk about osmosis. And
if you don't know this you may want to watch the video on osmosis. But if you wanted to
do something really cruel you could pour salt on a slug. Don't do it. It will kill it. But
what it would do is it would shrivel up that slug. And so what would happen is it would
pull water out of the slug. Now why does that occur? Let's zoom in to the surface of the
slug. So let's say this represents a cell membrane on the outside of the cells of the
slug. We've got water on the outside, water on the inside. And let's say we add just one
crystal of sodium chloride, or salt. Sodium chloride is going to be made up of two ions
that are bonded together using an ionic bond. And when we add that to the water, something
weird happens. They'll break apart into their two ions. We've now got the chlorine ion and
the sodium ion. The negative and the positive charge. And the negative charge is immediately
going to be surrounded by the positive parts of the water. And the negative sides of the
water are going to surround the positive sodium. But look what it did. It opened up all these
areas. So it decreased the water potential above the slug or on the surface of the slug.
And so now we have areas where the water inside the slug can move into that. And it's more
radical than I have in this simple kind of a diagram. So what it's going to do is it's
going to move water outside the slug. And so we measure water potential on either side
of that membrane. On the outside it's going to be negative 40 bars. And on the inside
it's going to be -5 bars. Now know this. Pure water is going to be right at zero bars of
water potential. And so the water is going to flow from here into here. So the water
is going to flow from an area of high water concentration to low concentration. Or it's
going to flow from an area of high water potential now to low water potential. And that's what
you want to remember. Water's always going to glow from high to low water potential.
And so this drives water even up a tree. And so if you were to pour some distilled water
below a tree, that's going to have a water potential of 0 bars. But the roots are going
to be around -2. And that's because they have a lot of solutes or salts inside them. And
so the water is going to flow in through osmosis. But the stems are going to have even a greater,
excuse me, a lower water potential. And the leaves as well and even the atmosphere. And
so the water is moving up a tree along this water potential gradient. Now what's driving
that? We're evaporating all the water up at the top. So there's not much water there at
all. Really, really low water potential if we're to look at the leaves of the plant.
And so now let's get to those equations. So water potential is built on two things. It's
built on the solute potential. And so think of that as like water flowing through osmosis.
And then the pressure potential. And that's like physical squeezing of the cell. And so
solute potential is going to drop as we increase the number of solutes in that area. And so
if I were to add just two little bits of sodium chloride or salt to it, what would that do
to the solute potential? It's going to drop that. It's going to get a lower value? Why
is that? Remember we're opening up spaces in here for water. So we're going to have
less water. Let's say we add a whole bunch of solutes to it. That's really going to decrease
that solute potential. And so maybe it's going to be around -5 bars. So that's due to osmosis
or that push of osmosis. What about the pressure potential? Well that's a physical pressure.
And so imagine that water keeps flowing into this cell. And let's make this a plant cell.
So water is going to keep flowing in. That's going to push out on that cell. But it doesn't
explode. Our cells would explode. But that has a cell wall around the outside of it.
And so that wall is now going to start exerting a pressure to the inside. And so what that's
going to do is create what's called a pressure potential. And so we measure that in bars
as well. So let's say that's 2 bars. Why is it a positive value? Remember that's going
to be pushing in. It's going to want to push water out of that kind of an area. And so
those two things, if we add those together, are going to be our water potential. What
would be the water potential in this case? It would be -5 bars plus 2 bars. So it's going
to be -3 bars. That's the overall water potential. And those two things are going to determine
if water flows into a cell or if it doesn't. Sometimes we'll be asked to do a little bit
more detail here on the solute potential. And there's an equation for that, which in
my class I would not want you to memorize. But let's throw that up here right now. So
solute potential is equal to negative iCRT. So we've got to go through each of those part.
The i, the C, the R and the T. Let's start with the ionization constant. Ionization constant
is not going to have the units associated with it. It's just a factor. And it's always
going to be somewhere from 1 to 2. Sometimes including 1. And so if we were to look at
sodium chloride, remember sodium chloride is one molecule when it's outside of the water.
But when you add it to the water it's going to break apart into two ions. And so the reason
we're multiplying it times 2 is if you add one mole of sodium chloride, it's really like
adding one mole of chloride ion and one mole of sodium ion. And so we have to multiply
that times two. Now it's really easy if we're dealing with something like sucrose which
is just table sugar. That's going to have an ionization constant of 1. Because when
you add sugar to water it just stays as sugar. So we don't have to multiply anything. So
again, if we increase the ions were increasing the i and that's going to give us a lower
solute potential. Okay. What about concentration? Obviously the more of the stuff that we add
to the water, that's going to increase or decrease rather the solute potential. And
so moles per liter in concentration is going to be what we measure for C. And so if you
add there the molarity, so let's say something is a one molar solution, that means there's
one mole per liter. The next thing we have in our equation if the pressure constant.
Pressure constant's just that. It's always going to be the exact some thing. And it's
always going to be 0.0831. I wouldn't memorize it. These units at the end are going to be
important as we solve a quick problem. And then the next one is going to be the temperature.
Obviously it's important that if we increase the concentration that that's going to decrease
solute potential. But if we increase temperature then the molecules are going to be bouncing
around more readily and so that's also going to decrease our water potential. And so when
we measure that in this equation we use Kelvin. And so what you're going to do is take the
celsius degrees and add 273. If you don't do that you're simply going to get the wrong
answer. And so knowing that, let's throw you a quick problem. So let's say we have a molar
concentration of sugar solution in an open beaker, that will become important in just
a second. It's a 0.2 molar concentration and what they're asking you to do is calculate
the solute potential at 22 degrees celsius. And so on the AP exam you're going to get
these two things. They're going to give you water potential, which we already went over.
That's equal to the pressure potential plus the solute potential. They're going to explain
that here. And then this is even the equation for solute potential, which is -iCRT. And
so how do you solve that? Let me show you how I would solve it. First thing I would
do is I would plug everything in. What's my i? My i is going to be 1. That's just because
we're dealing with sugar. And since sugar remember doesn't ionize, we're just going
to put in 1 because it stays as sucrose or stays as sugar. Where did I get this one?
This is my concentration. That 0.2 moles per liter. Because they gave me 0.2 molarity as
the concentration. Next thing is going to be my pressure constant. I'm simply copying
that off the sheet. We've got it right here. And then I'm going to have my temperature.
Since they told me it was 22 degrees celsius, I'm adding that to 273, so I get 295 K. And
so first thing to do is to get rid of all of these units. So for example we have Kelvin
here on the bottom and we have it on the top. Likewise we've got liters on the top, liters
on the bottom. First thing I would do is I would cancel out all of those units. What
am I left with? It's not surprisingly bars. That's going to be what we measure solute
potential in. Next thing I would do is I'd put the bars on the end and then I would multiply
those values. And so what I get is -4.9029 bars. Now that's way too many significant
digits. If I go back to my question, this one only has one significant digit, 0.2. And
so my answer should really be -5 bars. And so I've quickly figured out the solute potential.
But they could also ask you this question. What's the overall water potential? Okay.
So then we're going to have to think about this a little bit. We've got the solute potential
and again that's going to be half of this water potential. What's the other half? It's
on pressure. And so how much pressure are we going to have on a beaker that's open?
We're going to have zero pressure on it. And so if I want to figure out my overall pressure
I'm just going to add those together, so it's also going to be -5 bars. And so that's water
potential. Again it measures where water is high, as far as potential energy of water.
And it allows us to figure out where they go. And if you can remember that, then remember
our friend Poseidon. You can do well on all of these problems. And I hope that was helpful.