this is a video for d2.3 On Water potential and it is higher level content related specifically to water potential so what is water potential exactly well it's a measure of relative potential energy and it is symbolized by the Greek letter Sai it's measured in either kilop pascals or megap pascals depending on the relative abundance of that potential and pure water at 20° CI at sea level is going to have a potential of zero and that's because water potential is mainly affected by two things one of which is pressure so hydrostatic pressure this is the pressure caused by water the more pressure you have the higher the potential is it's also affected by solute concentration so this has the opposite relationship the higher the solute concentration the lower the water potential so you can see in this diagram there are different conditions on either side of a semi-permeable membrane it's important to note that water will always move from a high water potential to a low water potential in order to minimize potential energy so let's think about what that might mean I know just by looking at this um diagram here that water is going to move from the left side to the right side so we know from the standard level content that in osmosis water will always move from areas of low solute concentration to areas of high solute concentration let's say let's see if we can put that in the context of water potential so over here on the left side okay we have pure water and over here on the right side we have um a high solute concentration so that means means that we are going to have a very negative number for the water potential here and a zero um for the water potential over here so it will always move towards areas of lower water potential so it'll move from zero to the negative so from zero to the negative water potential in addition to being affected by solute concentrations it's also affected by pressure so water will move from areas of high pressure to areas of less pressure and that'll be uh become evident here in a little bit when we talk about water moving into and out of cells I can actually mathematically quantify the contributions of solute concentration and pressure to water potential water potential is going to be the sum of the solute potential and the pressure potential remember solute potential can either be Z zero if it's pure water or negative so anytime I add solutes into the water it is going to turn that solute potential negative and the more solutes you add the more negative that potential becomes pressure can either be positive or negative so this could be due to atmospheric pressure or it could be due to pressure within a cell and we'll look at the application of this in cells with walls particularly in plants so let's say I put this plant tissue in a hypotonic solution well that means that the water potential of this plant is more negative than the water potential of this solution because we know that solutes drive down the water potential so I I'll just like simplify this this might be have a water potential of zero and this is going to have a negative water potential and we know that water moves in the direction of lower potentials so water will move from the solution into that plant cell now the more dilute this solution is the greater the difference in uh will be between the water potentials and the more water will move what's interesting about this is that as water moves into this plant's cell it creates an area of high pressure it causes that plant to become turgid and so because water potential is influenced by both solute concentration and pressure as the pressure starts to increase here that is going to raise the overall water potential so it won't be so negative anymore it might even it all the way out to zero once we get that water to move in and then the movement of water will cease because the water potential of of the plant is equal to the water potential of the solution so you can see that this water movement is at first driven by the differences caused by the solutes but eventually the increase in pressure causes them to equalize in hypertonic Solutions we see the opposite so a hypertonic solution is going to have a a more negative value than the inside of the plant so they're both negative right they both both have solutes dissolved but the solution is way more negative because the solute concentration here is higher now water is going to move okay and water will move towards areas of high concentration but also if we're thinking about this in terms of water potential water will move towards areas of lower water potential and this because it's very negative is lower so again water moves out of the plant that is going to decrease the pressure inside of the plant so if the plant already has a negative value of solutes and the pressure is going down that will eventually cause the water potential of this plant to become more and more negative again because losing water means you're losing pressure eventually the water potential of the plant will equal the water potential of this solution again because we're losing pressure and also because this solute con concentration will go down due to the influx of water So eventually they will be the same and the movement of water will cease