Pressure Potential and Turgidity

by Jason Amores Sumpter
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pressure potential is physical pressure on water. And whereas salute potential was a negative type of pressure, pressure potential can be positive and negative. And generally that negative pressure is referred to as tension because it's kind of like a pulling force as opposed to a pushing force. Remember our straw example? We're pulling liquid up with a negative pressure. It should be noted, though, that living cells have positive pressure and that's because living cells, they're going to have salutes inside of them. They're also going thio be filled with water. And that's important. If cells shrivel up, usually they're gonna die. We'll get to that in just a moment. No, When membranes air present, we're usually gonna seawater move in response to solid potentials right from high to low, solid potential When membranes air absent, we're really going to be seeing water move from high to low pressure potential. Why do you think a solid potential on Lee has an effect, Really, when their membranes present well, why do we need membranes at all? To concentrate those salutes right? Without a membrane present those salutes air just going to defuse, meaning there is going to be no potential difference in solid potential. There is going to be no difference in solid concentration. So we need the membrane to have the difference in solid concentration, which is basically the same thing as saying we need the memory and have the difference in salute potentials. So I want to do a couple examples here, just show you a few things. So over on, uh, all the way over on the left. Here we have this U shaped tube. It's filled with water. Onda, those red dots are representing dissolved salutes. Those are, um, salutes in the water, and there is a concentration difference. Hopefully, you can see there's more dots on one side than the other. Meaning there is a concentration difference between the two sides and this dotted line right here. That is our semi permeable membrane. Right? So it's gonna allow water to pass through, but not the salutes. So, on this side we have a high concentration of salutes. Meaning we have a low water potential. I'm sorry. Low, solid potential. So are solid. Potential is low. Here we have a low saw you concentration, Meaning our solid potential is high. Now, just for I don't know just for giggles. I'm gonna add some numbers in here, So let's say that our low salute potential is going to be negative. One. I'm sorry. Ah, negative to mega Pascal's right, Gotta have units. Otherwise that's meaningless. And let's call this negative one Mega Pascal's Alright, So what's gonna happen? Water wants to lose its potential, right? So we're going to go from high potential to low potential, which is the same as saying we're going to go from a low concentration of salutes to a high concentration of salutes, meaning the water is going to move over to this side like that. So over time you are going to wind up with a U shaped tube that looks like this right. There's going to be a in fact, a difference in the heights, as you can see of the difference in the water levels on the two sides. But the concentrations will now be the same, right? Even though there's more molecules of solid it. On this side, there's more water, so the concentrations balance right. This is gonna be like our equilibrium point. So this might all seem like, very familiar from our example uh, that we talked about when we talked about us. Moses. Here's where e want to spice things up a little. Let's pretend that now in this U shaped tube, I'm going to add a pressure potential on this side. And I'm gonna make my pressure potential equal to one mega pass cow. What things gonna happen if I do that? What's gonna happen if I add a pressure potential of one mega Pascal pushing down on this side of the tube? What actually is gonna happen is gonna end up with something like you see over here, let me jump out of the way the water levels are going to become even again. Why is that? Because by adding a pressure potential of one mega Pascal over on the less left side of the U shaped tube, I've actually balanced out the water potentials between the two sides. So to recap on the left side, we have a solid potential of negative two mega Pascal's and we also have a pressure potential of one mega Pascal. On this side, we have a solid potential of negative one mega Pascal's and no pressure potential. So our pressure potential is just equal. Thio zero mega Pascal's. If we use our formula right, that water potential equals solid potential plus potential pressure. We'll see that our water potential on this side. So this is plain old water potential, his negative one mega Pascal's and our water potential on this side. It's also negative one mega Paschal's meaning. We don't have any net flow of water, and I say net flow there because in actuality, you know there's gonna be water kind of going back and forth between the both sides, but the net amount on each side is going to remain the same. So hopefully all of that makes sense now. Have a good understanding of what all these types of potentials are. Rate water potential, solid potential pressure potential. Now let's take these ideas and actually apply them to a living cell. So hopefully you remember there was that idea we talked about before. Turgay er pressure. That's the pressure inside the cell due to the um, usually it's mostly the vacuole swelling, Um, but generally speaking, it's the contents of the cell pushing against the cell wall and usually, uh, turker. Pressure is experienced because the HVAC you'll in the plant cell will swell up and cause the cell contents to push against the cell wall. We call those cell contents, by the way, proto plast. That's the all the living stuff inside the cell, plus the plasma membrane, and it does not include the cell wall. So, uh, you may remember Einstein's famous words right? That every action has an equal and opposite reaction, right? Well, if ter GERD pressure is pushing against the cell wall, it's equal. And opposite reaction is wall pressure, which is the force exerted by the cell wall on the cell contents. So it's equal and opposite to Turker pressure. Now, actually, you have to turn ter giggity up. You have thio increased trigger pressure to induce wall pressure, right? So if the cell is what we call flaccid meaning there's no turker pressure or no pressure potential. Like, you know, we see over here, we're not actually gonna have wall pressure because we don't have triggered pressure. So you have to increase turbidity to induce wall pressure, right? You have to swell up the cell contents so that you can start experiencing those two pressures. Now, in some cases, cells will become placid, right? They'll have no Turker pressure. In fact, sometimes they can shrivel up. We call this plasma license and you can kind of see that happening right here. The cell is all shriveled up due toa water loss. I mean, look how much smaller that vacuole is as compared to this vac, you'll or this vacuole over here now, in non woody plants, when this happens when targeted E is lost, sometimes what we'll see is wilting. And, you know, maybe you've gotten some flowers before something you left them out for a couple days. At first they're really nice and pretty, And then after a while they start to droop over, like this sad plant here. Well, this is wilting, and this is due to a drop in ter ger pressure. And as you can see, the cells inside this sad wilt e plant have shrunken vacuums. Right? Says empty. You know, it's probably not gonna be totally empty, but it's much smaller when those HVAC you'll zehr all nice and full like you can see over here. Then our plant will stand upright. It'll be direct right? The reason that we only see this in non woody plants is because Woody plants have lignin fide cells. Right? You might remember that those leg defied cells are actually going to contribute to the structural integrity of the plant. That's why Woody Plants don't will like this. Where the Woody regions don't will like this anyways. All right with that, let's flip the page.