Water Potential

by Jason Amores Sumpter
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Hi. In this lesson, we're going to talk about how plants transport water and sugar through their asylum and flow him to begin. We're going to talk about the concept of water potential represented by the Greek letter Sigh, which, if you're curious, is spelled like this pronounced like this ha side. Now, before we get into the nitty gritty here, I highly recommend that if you feel a little fuzzy on the concepts we talked about when we discussed osmosis and diffusion to go back and re watch those videos because everything we're going to deal with now directly builds on the builds on those ideas. So if you're kind of going there thinking kind of remember that stuff not 100% I'd say go back and re watch those videos now, water potential is the potential energy of water to move between two environments basically, and the differences in water potential between two environments will determine the direction of flow. So water potential is actually based on two concepts. We're going to go over and you can see that in the equation. Here we have this sy s that stands for salute potential and also saipi that is the pressure potential, So water potential is going to be due to. Both of those concepts will review both of them in just a moment before we get there. A za general rule or as a rule of thumb, I should say water is always going to move from areas of high potential. Two areas of lower potential and the way I like to think of this is that water wants to lose its potential, right. It's gonna skip class, get high behind the gym and just throw everything away. Wants to lose its potential. Water wants to be a deadbeat. Now. Water potential is a pressure. It is a form, or it is a type of pressure. It's measured in units of pressure, and those units are often mega Pascal's, which is basically just a million Pascal's uh, Pascal's. If you've taken physics, you might recall, are actually chemistry to it comes up. You might recall that Pascal's are an S I unit of pressure. If you have no idea what I just said, don't worry about it. It's just a unit of pressure. That's all you need to worry about now. The water potential Grady int in plants is actually what causes water to move up from the soil, through the plant and against gravity and in a little shrub or bush or something that might not seem to incredible. But think about the fact that redwoods, which can be upwards of 300 ft tall, are able to transport water from the bottom of their roots all the way up to the tippy tops of those trees. And when I say that redwoods are over 300 ft tall, sometimes that's counting from the ground up. If we talk about how far water travels in, those trees were actually talking about a greater distance because those roots go underground. So water is moving amazing distance through some plants, and it's doing that against gravity and believe it or not, what we're gonna find out is that this process actually is very, very energy efficient. It's almost you could think of solar powered, but I'm getting ahead of myself. Let's talk about salute potential, so salute potential. Which again, is that gonna be psy s? That Greek letter Psy s solid potential is the solute concentration relative to pure water. Here's where things get confusing. High concentrations of solute mean low salute potential. So I know this is a little weird, so let's give a visual aid. So here we have low salute potential because we have a high concentration of salutes. Over here we have hi solitude potential because we have a low concentration of solids. So again, this is a little counterintuitive. I get that. But the reason for this is remember we said that water always wants to move from high salute potential toe low, solid potential, right. Recall from our discussion of osmosis, that water will go from an area of high solute concentration toe low solute concentration, right water. I'm sorry I said that backwards. I mean, water wants to go from an area of low solute concentration toe high solute concentration. Right. So if water is going to go from low solute concentration to high solute concentration, that means it's going to go from high salute, potential toe low salute potential, meaning it's gonna lose its potential. Now it's very important to remember here that if we don't have a semi permeable membrane like, for example, this uh you know this cell cell membrane right here behind me. Then what's gonna happen the solid particles, they're gonna move right. But if the solid particles air prevented from moving, then the water is gonna flow. So I want to do a little example using our semi impermeable membrane here. Right, This is our plasma membrane. Mhm, not plan za plasma membrane. Okay, so our plasma membrane, semi permeable membrane. And let's say we're gonna put a hi concentration of salutes on this side and a low concentration of salutes on this side. Well, obviously the water is going to flow from the low concentration to the high concentration. So it's going to go this way. Well, let me make things a little more confusing. It's not my fault. I promise I didn't invent this stuff. Solitude potential is actually a negative pressure. The reason is, pure water has a solid potential of zero mega Pascal's or just zero right zero mega Pascal's is the same. A zero Pascal. Since you know, whatever it zero solid potentials air going to be negative pressures, meaning that their measurement of pressure is going to be a negative number. Now the concept of negative pressure can be a little weird. So let me give you, um example that you probably are very intimately familiar with if you've ever used a straw. Sure, you've used a straw before. So what happens when you use a straw? You put it in your drink, right, and you suck on it. What happens when you suck on the straws? You actually remove the air or some of the air from the straw. You create a vacuum in there. That vacuum asserts negative pressure on the liquid below it. Right. Uh, you know, the opposite would be like a positive pressure, right? Like when you turn on the faucet full blast and there's just like tons of water gushing right. That water has, ah, high positive pressure. There's, like a lot of stuff, their theocracies it. When you kind of remove stuff from an environment, you can create negative pressure, which, instead of pushing things, is pulling things. So getting back to our straw, when you create that negative pressure in the straw by sucking some of the air out of it, what happens? It that vacuum, or that partial vacuum pulls up that negative pressure pulls the liquid up the straw. That's why straws work, right? That's why you can drink out of them. They're creating negative pressure, which pulls the liquid up the straw. Okay, Getting back to our example here. This is the reason I went through all that explanation because I want to just throw some numbers into this so that you get a sense of how these negative pressures work. So we said low salute potential means high concentration. Right. So let's say are low salute potential here is negative one Mega Pascal's Our high salute potential. Could be something like Oops, solid potential, not water potential. Our high salute potential might be in this case zero. Okay, this is where things were confusing. Right? Uh, low low means like mawr Negative in this case. Right. So to put other numbers to this, let's say we have negative five. This on here. This over here on the low concentration side might be something like negative one. All right, so this might be a lower numeral, right? One is lower than five, but negative one is actually greater than negative. Five, right. Negative. Five ISMM or negative. So it's a lower salute. Potential again. I don't want you to stress too much about the math in any of this. I just want you to get, like, a qualitative understanding of what I mean by you know, uh, things like the solid potential is negative or something. So the last thing I want to say is that sells always, always have dissolved salutes inside of them, right? So they're always going to have some salute potential with that. Let's flip the page and talk about what happens inside the cell with all these water potentials.