[Music] hi it's Mr Anderson 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 um pressure even surface tension and so it allows us to figure out if water's going to flow into the cell or not and so we measure it using something called sigh or PSI and a quick way to remember that is that Poseidon who was this Greek god of the oceans carried a trident and it looks a lot like the Trident that we use to represent um water potential or Sai now SII is going to be equal to the s s which 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 want to do something really cruel you could pour salt on a slug don't do it it'll 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 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 -40 bars and on the inside it's going to be5 bars now know this pure water is going to be right at zero bars of water potential and so the water's going to flow from here into here the water's 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 flow 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 zero bars but the roots are going to be around -2 and that's because they have a lot of solutes in or salts inside them and so the water's going to flow in through osmosis is 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 were 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 so maybe it's going to be around neg five bars so that's due to osmosis or that push of Osmosis what about the 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's 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 two 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 is going to be our water potential what would be the water potential in this case it would be -5 bar plus two 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 you'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 I CT so we got to go through each of those parts the I the C the r and the T let's start with the ionization constant ionization constant is not going to have units associated with it it's just a factor and it's always going to somewhere from 1 to two sometimes including one 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 at times to 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 one 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 we're increasing the I and that's going to give us a lower solute potential okay what about concentration obviously the more of the stuff we add to the water that's going to increase uh 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 the the marity so let's say something is a one molar solution that means there's 1 mole per liter next thing we have in our equation is the pressure constant pressure constant is just that it's always going to be the exact same thing and it's always going to be 0831 I wouldn't memorize it these um these units at the end are going to be important as we solve a quick problem and then the next one's going to be the temperature obviously it's important that we 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 it 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'll become important in just a second it's a02 molar uh concentration and what they're asking you to do is calculate the solute potential at 22 de C 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 the solute potential which is negative I CT 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 one um that's just because we're dealing with sugar and since sugar remember doesn't ionize we're just going to put in one because it stays as sucrose or stays as sugar where did I get this one this is my concentration that's .2 moles per liter because they gave me 02 marity 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° C I'm adding that to 273 so I get 295k and so first thing to do is to get rid of all of these units so for example we have ke uh 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 those units what am I left with 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 - 4.92 n 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 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 NE five bars and so that's water potential it again it measures where water is is high as far as potential energy of water and allows us to figure out where they go and if you can remember that and remember our friend Poseidon um you can do well on all of these problems and I hope that was helpful