Water Potential


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.

100 thoughts on “Water Potential

  1. Please please please could anyone elaborate on pressure potential for me???? I don't understand it and it's the only thing letting me down in bio

  2. hi sir,
    i am quiet confused that why solute potential shows only negative value and pressure potential shows positive
    is there any condition where solute potential shows positive value and pressure potential shows negative value?

  3. Can someone help me with determining if my calculation for the final Pressure Potential/Turgor Pressure in the following example is correct? If I have 14.64 Grams of Potassium Acetate in .1737 ml of water at 300K in a sealed cell with a Pressure Potential of the cell being 0 then the Solute Potential would be 42.75 Bars based on the following. The molecular mass of Potassium Acetate is 98.15 gram/mol. Therefore, the Mol/Liter would be 14.64 gram/(98.15 gram/mol) = .1492 mol. Mol/liter = .1492/.1737 liter = .8587 mol/liter. Therefore the Solute Potential would be 2 * (.8587 mol/liter) * (.083 liter/bar/mol/k) * 300 k = 42.75 Bars. Now if I were to plunge the cell into pure water where the cell would absorb .1977 liters of water, then there would be a reduction of of Pressure potential to 20 Bars by the following equation [2*(14.64 gram/98.15g ram/mol) * (.083) (300k) = 20 Bar. Now, if I want to determine the pressure exerted on the cell way, would I simply subtract the 20 Bar from the 42.75 Bar resulting in a Turgor Pressure of 22.75 Bar?

  4. SO GOOD! THANK YOU! I UNDERSTAND IT NOW!!! my textbook literally had one paragraph on it and it was crazy

  5. Thank you for this! I missed this lecture in my ecohydrology class, and you filled in the gap perfectly!

  6. I found your videos so helpful while taking AP Biology! Thank you so much. I hope I can use your videos again while majoring in biology at UC Davis

  7. im using this for a 200 level Biology class at a top 10 university and its made for high school students wow

  8. Isn't 'i' called the vant hoff's factor. And also sucrose instead of being non electrolyte dissociates into glucose and fructose due to hydrolysis on water, hence i=2.

  9. Love how you break everything down and explain everything systematically. Thank you

  10. Hey isn't 'i' the Vant Hoff Factor? As far as I know ionisation constant is Ka (for acid) or Kb(for base) or Kw(for water)?

  11. Professor Anderson, why did you choose to utilize bars over MPa megapascals. Bars is not approved by the international system of units (SI). I would also assume since psi pressure is a physical force relatable to measurements conducted in physics studies and other STEM science studies. It is also labeled in my pearson textbook as such. I ask because the only place I have ever seen bar measurement is weather stations, and on pneumatic technology and compressors frequently.

  12. Question – if the ionization constant of sugar (no ions) is 1, NaCl (two ions) is 2, is the ionization constant of MgCl2 (three ions) 3?

  13. If I can like this video a trillion times, I would. This was extremely helpful. Thank you!!

  14. my name is Sai and every time I hear you refer to psi, i chuckled, thanks for explaining well, also our AP Bio teacher told Us to take notes on your videos so that is what Im doing right now

  15. Could you please explain to me as to why the pressure potential is zero on an open beaker?

  16. This is something that confuses me sooo much I’ve had tutors try to explain it my teacher tried to explain it to me, only some of it has sticked.

  17. Really ur teaching is appreciable, it really helps to understand water potential. Actually I have doubt in question given in Ncert book, can help me?? Send ur reply I waiting…

  18. Can someone help me with this problem:
    A closed, saclike membrane, completely filled by a solution with an osmotic potencial of -27 bars, is immersed in a solution with an osmotic potencial of -21 bars. Assume that the membrane is permeable to water only and that the osmotic potencials will not change with osmosis. What will be the water potencial of the internal solution at equilibrium? The osmotic potencial? The turgor pressure? Answer the same question for an external solution with an osmotic potencial of -16 bars and for one with an osmotic potencial of -20 bars. thanks

  19. Question: what would you multiply -5 by in the example problem if the coontainer was closed? How would you find out the pressure

  20. heyyyyyyyyyyy, why is the pressure on an open beaker zero? Shouldn't it be one atmosphere?

  21. Paul, there is another convention for significant figures. The rule I use does not use only figures in the answer equal to those given. it uses the all the, “certain” digits and then the first uncertain digit, but no more. So, at 8:47, I would provide -4.9 bars as the answer.
    In engineering, this is common practice.

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