hey everybody it's your ap bio teacher mr poser we are continuing topic 2.8 today tonicity and osmo regulation we are picking up where we left off in our last video um so where we left off here was talking about the difference between having a cell an animal cell in a plant a cell in isotonic solution hypotonic solution and a hypertonic solution um and we were talking about how the where water moves what relative to the inside of the outside of a cell is really really important for its functioning right because it could blow up it could shrink it could shrivel it could plasmalize if that water well if the water motion or osmosis is not regulated hence osmoregulation so what we're gonna talk about today um is some additional factors that um influence osmo osmosis and osmoregulation um so as i put over here and we're gonna involve some math here so get your calculators ready uh solute concentration and physical pressure both play a role in osmosis so in our last video we focused on okay if there's more solute on the inside then on the outside then water comes in if there's more solute on the outside then inside the water goes out right so that has to do with the concentration of solutes like say sugar or salt right those little green dots that we had but that's not the only thing that plays a role in osmosis physical pressure like pressure also plays a role in osmosis um so a way to both incorporate solute concentration and physical pressure when we're talking about the motion of water is through water potential which is as i put down here a property that predicts the direction in which water will flow and of course it includes solute concentration and pressure so you will be provided on the ap exam with this formula here so i suggest writing it down for now you're gonna need it um so we have psi equals psi s plus psi p so psi is a greek letter um psi represents what we call water potential which is this property that predicts the direction of which water will flow and that constitutes both solute potential and pressure potential so psi s s is for solute psi p p is for pressure all right so that is just kind of illustrating that both solute concentration and pressure play a role in where water is going to go when it comes to cells all right so something to absolutely remember here and i have it on like every page coming up and it's underlined is that water is going to move from a region of high water potential to regions of low water potential all right so to illustrate this we have a plant cell down here i know more illustrations by me right so check it out this is the outside of the plant cell and this is the inside of the plant cell uh let's practice here if the water potential outside of the plant cell is 0.5 what we call megapascals megapascals um and the inside of the cell has a water potential of zero megapascals which is you know a measure of pressure it's a measure of water potential which way will water move well if water moves from region of higher water potential to regions of lower water potential then water will move in to the cell okay so this is a good thing for a plant so a cell a plant cell wants more water wants a net inflow of water um in comparison to say plants or excuse me animal cells where they want it to be you know no net inflow or outflow of water so in a in a plant cell if this would become turgid okay so that means a plant cell wants to maintain a higher water potential on the outside than relative to the inside it wants to maintain a low water potential on the inside okay because it wants water to keep coming in all right um so how about now if we have water potential that's negative 0.5 megapascals on the outside and the inside is still zero then water will flow out okay and that's not good for a plant cell that's not good for an animal cell either if it's flowing out then the plant cell might become plasmalized where the plasma membrane kind of shrinks relative to the cell wall and it gets all wilting so that's actually what happens when a plant wilts by the way those cells become plasmalized all right so water your plants um all right so there's another added factor into this though okay now that we know that you know water moves from high water potential to low water potential and water potential has to do with solute and pressure okay let's get a little bit more into what solute potential is all about okay solid potential is always going to have a zero or negative value and as we had before water potential is both solute and pressure potential right but here's what constitutes uh what we call solute potential okay i represents an ionization constant or so it's psi s equals negative i crt okay and we're going to dissect each of these variables here in just a second um so i as i said before represents what we call an ionization constant and it's basically a measure of how many ions a substance dissolves into okay so for example if you dissolve salt nacl sodium chloride into water it'll dissociate into two ions so that means the ionization constant is two but if you dissolve sugar in water okay like glucose or sucrose all right sucrose only forms one ion when it's dissolved in water so the ionization constant of sucrose would be one all right c is a measure of molar concentration so you're going to see those units as m versus molarity okay so molarity is a measure just how many moles and how many liters of volume okay so it's just a measure of concentration right so concentration r represents what we call the pressure constant and there's some kind of complicated math that goes into calculating pressure constant but it's a constant which is cool right so it's going to be the same thing every single time and that's just going to be a value of 0.038 so r is always going to be 0.038 or excuse me 0.0831 i don't know why i said three eight but eight three one all right so negative i see r and then t t is representing temperature um in degrees kelvin in degrees kelvin okay so if i give you a question where it involves uh oh the solution is you know 0.5 molar concentration of salt with a temperature of 25 degrees celsius okay you got to take 25 degrees celsius and add 273 to make it into kelvin all right so both of these uh formulas here you are absolutely going to have to have these somewhere and i will give them to you on a test okay and they will be available to you on the ap exam as well okay but you have to know what these mean and you don't have have to know how to use them capisce all right let's keep going um another few things to remember okay as we before we head into the example problems of calculating water potential and solute potential okay the solute potential of pure water okay if there's no solutes at all dissolved in that water okay then the solute potential is zero it doesn't matter how much you know it doesn't matter the temperature it doesn't matter uh pressure okay the solute concentration of pure water is zero because it doesn't have any solutes excuse me the cell did i say the solute potential solute potential of what pure water is zero okay because it doesn't have any solutes all right the pressure potential of a solution in open air which most of our examples are going to be an open air um is zero all right so unless say we were to put a plant cell in like a pressurized chamber or in like a syringe or something and we squeezed the plant cells together i mean applied more pressure then the pressure potential would change but if it's just in an open container it's or it's like open to atmospheric pressure uh then our pressure potential is what is zero all right um so as i alluded to before the ionization constant of salt since it dissociates into two ions when it's dissolved in water is two and the dissociation or the ionization constant of sugar is one okay because it just forms one ion all right let's do this let's do some example problems here all right example if a plant cell's pressure potential equals two megapascals and it's solid potential equals negative three megapascals what is the water potential all right so this one's not too bad uh this is your water potential this is your solute potential this is your pressure potential if you sub in those variables maybe you can calculate this on your own or maybe in your head if you put in those variables you get negative three plus two okay so that means the water potential of this plant cell is negative one megapascals not bad right gotta know that equation though all right um here's another example the plant cell from the previous question is placed in a beaker that should say in a beaker with no added pressure with a water potential or excuse me a solute potential of negative five megapascals in which direction will water flow so think about this water always moves from an area of high water potential to an area of low water potential okay so the solution in the beaker outside of the plant cell is negative five and the plant cell itself has a water potential of negative one okay so think about that it goes from high to low so where's water going to go it'll it's going to go out of the cell all right because well we have a high water potential on the inside low water potential on the outside so that means water will flow out which is not good for this plant cell all right it's not good not good it's going to plasmalize or something all right let's get to our third example here now this is going to you know it's gonna i i skipped a few steps here kinda ish uh you'll see um a plant cell's water potential was found to be at negative 3.5 megapascals if you put the plant cell in an open beaker with 0.2 molar salt solution as 20 degrees celsius what is the water potential of the solution and where would water flow if you want to try this on your own be my guest go for it but if not then you don't have to pause the video but if you are going to try i encourage you to try it and pause the video try it for yourself and then we can get going but all right let's just say you're not going to pause let's do this so we have the water potential is at negative 3.5 uh of that plant cell okay so we know the plant cell's water potential we got to find out the water potential of the solution outside of the plant cell in the beaker okay and so we got to use our other equation solute potential equals negative i c rt so negative icrt is our uh solute potential now we got to plug in our variables here okay so if we want to find out the solute potential we need to a first determine the ionization constant concentration the pressure constant and then the temperature right okay so uh if this is since this is salt our ionization constant is two um then our concentration of the salt solution is 0.2 here's our pressure constant okay and then of course we have to add 273 because our water solution is at 20 degrees celsius so in order to measure it in kelvin we got to add 273 okay so if we plug those in hopefully you did that in your calculator we get a solid concentration of negative 9.7 all right i got to finish up this video here um so if the beaker water potential is negative 9.7 because there's no pressure potential it's an open beaker and the cell has negative 3.5 where is water going to go well it's always going to go from high to low that means we're going to lose water to the outside again um so poor cell that's losing water again all right that'll be it for this video okay we're going to be practicing uh water potential calculations a lot in class so if you are intimidated by the like the psi s and psi p and stuff okay don't be we're going to be practicing this a lot maybe go through the examples one more time and you know practice a little more if you need be let me know if you have any questions see you later