Water Potential

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.

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.

water potential in soil will vary depending upon the conditions. Generally speaking, dry soil will have a lower water potential. Then the water potential in plant roots and conversely, damp soils will have higher water potential than the water potential found in plant roots. This is because the water in damp soil usually has few dissolved salutes compared to plant roots, which of course are made of cells. And, as we know, cells are filled with salutes. Now soil near the ocean will have much lower water potential than roots, and that's because of all the salt in the water, which, of course, will end up in the soil around coastal areas. And if water potential is low enough, water can actually flow from the plant into the soil, which would be devastating to the plant, right? I mean, the roots job is to absorb water from the soil. You don't want it going the other way. So plants have actually evolved adaptations that allow their roots to store high concentrations of salutes and therefore ensure that water is going to go from the soil into the roots. Now, just a soil can have water potential here can have water potential and warm air and dry air have low water potentials. So air that is both warm and dry will actually have a very low water potential. It's gonna be perfect conditions for evaporation. Now, you might recall that plants will evaporate water through their leaves in a process known as transporation. What you might not have realized is transpiration actually helps pull water up from the roots. We're gonna talk about this in greater detail momentarily, but for now I just want to focus on transpiration. So how does that happen? Well, plants have these pores on their leaves that are called stone mata, or the singular is just stow MMA. So these stone mata control gas exchange, which is there another one of their purposes. They control gas exchange by opening and closing. But that opening and closing also has an effect on the amount of water that will evaporate from the plant. And if the air outside is dry, which literally just means less than 100% humidity, you bet water is gonna evaporate. Now, how do these stone open and close? Well, one mechanism is based on these proton pumps, these proton pumps. When the plant wants to open its toma will actually pump protons outside of the cell by concentrating. So these proton pumps will pump protons outside the cell. This leads to a high concentration of protons outside the cell. And because these are charged particles are charged ions, uh, they will cause a deep polarization. Basically, they're gonna affect the charge balance between the outside and inside of the cell. And this allows potassium ions to enter the cell, and water is going to follow those potassium ions. So I know that's a little confusing, so let's just go through it once more. So, uh, Thio open the stone mata or to open us toma proton pumps. They're gonna concentrate protons outside the cell. This is going thio mess with the electrical balance between the inside and outside of the cell, which results in potassium ions entering the cell so potassium is going to enter. The cell is a result of this, and water will follow. The potassium in water follows potassium in plants, unlike in humans, where we usually see water following sodium one of those differences between plant human cells. So to close the stone A, the plant is going Thio. Get all of those potassium ions out of the cells and the water is going to follow them. And that means that the cells are going thio shrink and slides. They're gonna lose their tiger pressure, and that's going to allow the stomach to close. So closing over here, opening over here. And it is due to the movement of water in and out of that stone A or the guard cells of the stomach, I should say now, stone mama open in response to a variety of factors. One of those is circadian rhythms, which are just natural rhythms that organisms experience. So in general, plants will open and close their stone mama according to the day night cycle, and additionally, they will also respond to hormonal signals like that of a BSI ZIC acid, which is often just abbreviated A B A. This stuff Eva is actually produced in roots, and it's produced in response to low soil water potential. And what a BA does is cause the stone mata too close. It induces the stone model to close, and that reduces transporation. Why is this important? Well, if soil water potential is low, that means that the plant is not going to be as absorbing water is effectively, which means it's gonna want to reduce its transpiration so it doesn't lose a bunch of water, right. It wants to keep its water levels balanced. And so we often talk about this idea that we call the photosynthesis transpiration compromise. And it's the compromise between conserving water and maximizing photosynthesis. We want the plant wants those D'Amato open for photosynthesis. That's how it's going to get the gas is it needs to carry out photosynthesis and, uh, you know it, Czar. It's essential that the plant gets that carbon dioxide in order to carry out the Calvin cycle. However, you know it can lose water in the process by doing that. So plants need to find the perfect balance to maximize photosynthesis and maximize water conservation. And again, that idea is known as the photosynthesis transpiration compromise. But plants have also come up with a bunch of other adaptations for water loss. We've talked about some of these, including the cuticle. Occasionally, um, you'll see something like Samata in deep pit surrounded by try combs. Remember, we said, try. Combs could be involved in preventing water loss and also, uh, in the section where we talked about photosynthesis. We talked about adaptations in what are called cam plants and C four plants, uh, where plants that carry out camp and C four photosynthesis. Um, so if you want Thio, review those particular concepts, go back and check out the video on photo respiration at the end of the photosynthesis section. With that, let's flip the page.

Now that we've talked about the conditions that lead water to flow, that is water potential. Let's talk about how water gets from the soil into the asylum right, whereas ultimately it's going to be transported through the plant, so water flows from the soil so soil into the root hairs. So here we have a root hair, and from there it's going to move into the xylem, which is represented as this orange structure in the image. Water is going to move into root hairs via osmosis, but it's going thio thebe. Pressures on it are going to be things we talked about when we talked about water potential, right? So salute potential is going to be the major factor in moving water into, uh, into the root hairs. Now, once it's in the plant, it can actually go through a few different routes to get to this island. There's actually three routes that water can travel. The first we're gonna talk about is the trans membrane route. Basically, remember that all of our you know, cell membranes here are going to have these things called aqua porn's right Those air those channels that allow water to pass through because you might recall, Even though water is small enough to go through the plasma membrane, it doesn't pass through the plasma membrane efficiently enough. Thio have living organisms rely on that alone. They need these aqua porn's to increase the efficiency of water moving through the membrane so the trans membrane route is going to be flow through the aqua porn's and, of course, a little bit directly through the membrane. Uh, however, the majority is going through the aqua por ins, so that is pretty straightforward. Now the other two are a little, um, less straightforward. Shall we say we have the April plastic route, which is going to be flow outside of the plasma membranes of cells, and this is going to be basically, uh, in the spaces between cells and other porous cell walls. We call this region this space outside of the plasma membranes, the a pope last, and you might recall that this area is interrupted by what's known as the cast Sperry. In strip, the Caspian Strip is a waxy layer made of a material called Subaru in, and that is secreted by the end. Oh Durmus. And this is done in order to block off the asylum. So in this image, me jump out of the way here you can see that the a pope last is colored in this light blue color. So everything you see in our sort of diagram showing you the different layers of cells that the water is going to have to travel through to reach this island. All of these light blue regions are the A pope. Last. So let's talk about the cast Berrien Strip for a second, you can see that the cast very in strip blocks off the April, plastered some point. And it does this because it wants to force water, as you can see happening here and here. It wants to force the water into the end of dermal cells. And the reason it wants to do this is that allows the end of dermal cells to act as filters, which means they can control ion flow and concentration. Grady INTs. This is super important for a number of reasons. I mean, for one, you want to make sure that you're not letting in any bad stuff, and also those concentration Grady INTs are going to be literally a matter of life and death. So it's super important that the concentrations of salutes and ions are super super tightly controlled. Now water has thio flow. Uhh or water can also flow through what's known as the Sim plastic route, and this is flow through the site is all of cells. You might remember that plant cells we'll have their site is ALS linked by what are known as plasma Dez Mata. Uh, in fact, you can see plasma. Does mata in these channels between the cells in the image this area, this continuous network of plant cells that are linked by these plasma does Mata. This continuous network of cytoplasm is called the simp last, and it's labeled in this sort of orange color. So we have the April plast in the simp. Last. Those were the two regions that represent two of the routes which water can take. And, of course, if water is flowing through the Sim plastic route, it doesn't need to worry about the cast Berrien Strip. And that's all good because it's moving through cells, which basically means those cells they're going to be able to regulate, uh, solute concentration ion flow, All that good stuff they're going to be in control? No. When water water flows through the asylum, as you can see here without crossing membranes, and it moves due to differences in pressure potential, and we will go over the mechanism of that in just a little bit. But first I want to talk about the water moving through the asylum. First of all, it's not just water. It's what we call xylem sap because it's water with dissolved minerals, nutrients and hormones. I mean, there's, you know, there's a bunch of other junk in the water. Of course, Asylums main job is to transport water up the plant and the flow. Um, is there to transport sugar? Um uh, but other stuff hitches arrived is kind of the basic idea. So the movement of molecules along a pressure ingredients the movement of this xylem sap is known as bulk flow. So if I use the term bulk flow, you know that I'm just talking about the movement of all these molecules in here due to the pressure potential difference. So let me actually go ahead and mark that. Let's say pressure potential is high up here and lo here, get my head out of the way so you can see that. That's why Oh, my gosh, She did it backwards. Sorry. Water wants to lose its potential. So it's gonna be high down there, low up their dirt. And that is our bulk flow of water or xylem sap through the asylum. And with that, let's flip the page.

cohesion. Tension theory is the most widely accepted theory as to why water flows up through asylum. However, there are some other theories that try to explain this phenomenon, and they're not mutually exclusive with cohesion, tension. It's important to point that out. However, given all of the evidence, uh, it is most likely that cohesion. Tension is the dominant reason that water is flowing up. Silom thes uh, other things might contribute, but they're probably not. The main factor is the point. So I want to talk about one of these other theories route pressure theory, which basically says that positive pressure builds up in the root xylem due to increased absorption of water relative to transpiration. So remember I said that that idea of transpiration was gonna come back in terms of water moving up through the asylum. Well, here's where it comes back to haunt us, or it's just the start of it, really. Now, the reason all of this water is gonna build up in the root Silom is that ions are pumped into root silom, and this creates a negative water potential relative to soil. So it's gonna drive water into those roots asylum. Azzawi said before water enters via osmosis and the more water that builds up in the roots. I'll, um, you know, like the more water that builds up in our roots island, this little blue line that you can see traveling through the root there, the greater the positive pressure. Right? So the basic idea is that mawr water goes into the root Silom here than leaves the leaves, leaves the leaves, get it through transpiration. And there you know, there is some evidence for this. For example, stomach close at night. But roots continue to absorb waters of that water and ions from soil. And in fact, if you look at root pressure, it is highest in the morning due to this. And in fact, you can even sometimes see this phenomenon known as rotation, where water is forced out of leaves due to all of this pressure, and you can actually see a picture of dictation happening here. These water droplets are being squeezed out of the leaves due to this super high pressure. And, um, you know, this is the most common. This site is most common in mornings when pressure is the highest and like right before you know, the plants open, They're still mata and start transpiring again, you know? And they've had the whole night to suck up water and ions. Basically. So they're gonna be Cem. Let's say, Cem, factors that we need to cover in order to understand cohesion, tension theory and the movement of water through zeile. Um, in general, water has something some ability known as cap Hillary action or capital charity. This is the ability of liquid in this case, water what we're talking about to move through narrow spaces. And it's basically due to three three factors. We see capital a reaction as a product of three factors. One of those is adhesion. When this is the attraction between unlike molecules. So in the case of capital a reaction it's going to be the attraction between water and the different molecules that make up the tube. You can see an example of adhesion here, where these water droplets are clinging to the spider. Web cohesion is the attraction between, like molecules. So, in our, uh, you know, in terms of capital reaction, when we're talking about water, it's gonna be the attraction between water and itself. And you can see a nice example of cohesion. Here, let me get my head out of the way. The water beating up on the surface of these leaves is due to the fact that, uh, the leaves surfaces hydrophobic, and so water is going to want to cling to itself. Here, Uh, this is an example of cohesion because and, you know, you might have toe, you know, kind of look closely to see this. The water is actually forming orbs, right? So instead of the droplet having a flat bottom like that, the water is actually beated up in an orb like that. So it's lifting off the surface because the molecules are being attracted to each other in that droplet. Now, this cohesion will sometimes lead to what's known as a meniscus, which is a, uh, con cave surface boundary due to cohesion and adhesion. You can see an example of a meniscus here. That's the type of meniscus that water is going to form where it comes up on the sides like that. There are some liquids that will form a convex meniscus like that. Those tend to be heavier liquids than water. For example, mercury forms a convex a meniscus like that. Now, the last force that I want to talk about in terms of capital reaction is surface tension, and you can see an example of surface tension and straighten S t on this image for surface tension right here with this paper clip that is seemingly floating, though it's actually being held up on the surface of the water. It's not actually floating because it hasn't broken the surface. And basically, surface tension is the force between the water molecules at the air water interface. So the water is going to be attracted to itself that that air water interface and it's going to create, um, a tense like a tension across the surface, which is why this paperclip can just sit there on the surface without breaking it. So how does this all come into play in terms of capital? A reaction that you can see here with water has moved up the tube against gravity and now is higher than the surface level of the water it's in. Basically, adhesion pulls up from the container wall, right? So the adhesion between the water molecules and theme the tube wall is going to pull up surface. Tension is going. Thio pull up from the very surface, and then cohesion basically transmits the pole between all the water molecules. So as surface tension pulls up from the surface, that meniscus adhesion is going. Thio allow, you know, pull from the walls and cohesion is going to transmit that pulled all the water molecules in the tube. That is how we get capital a reaction. And with all of that in mind, let's actually flip the page and talk about cohesion, tension theory.

cohesion. Tension Theory says that evaporation from the leaves creates negative pressure or tension, and this tension pulls water up from the roots. Now leaves contain a humid air, and that will evaporate when the stone mata are open and the external humidity is less than 100%. So the conditions needed for that evaporation or transpiration is I'm gonna label it here are that this external humidity is less than 100% which it's almost certain to be now. The evaporation lowers the humidity in the mezza feel or that inner tissue in the leaves. So inside these leaves, the evaporation is going to result in lower humidity, and this causes water. Thio enter into the space in the Mazza Phil from the from the parent came a cells, right. It's going to enter that a plastic region. Now what's gonna essentially happen is the evaporating water is going to pull water, and from the parent came a cells, and this is going to create a steep menace. Guy menace, guys, the plural of meniscus. So many meniscus is, ah, steep menace. Sky will form at the air water interface in the leaf cell walls, and this is going to create that tension now each meniscus is small, right? But there's many of them and all their forces added together become significant. The tension from all these menace guy in in the leaf cells is going to pull water up from the roots, which is going to be assisted by cohesion and adhesion. So, looking at our diagram here, essentially, what's gonna happen is this water leaving is going to create a negative pressure potential on that negative pressure potential is going Thio cause water to be drawn up from the roots to the leaves. And this is because that force from the negative pressure potential, right that tension is going to be transmitted all the way along through the asylum all the way along through that water due to cohesion and adhesion. Right? So that's how those factors we talked about previously are going to influence this process. You might be wondering now, wait, you didn't mention surface tension Guys, those meniscus is That's the surface tension that is pulling up on the water that is in that surface. Tension is in those many menace guy, So this process is basically solar powered and the reason I say that is plants don't expend energy to create the upward force on the water. Uh, the sun heats up the atmosphere and that's going thio, uh, you know, in part facilitate this transpiration happening. It's that transpiration happening that's going to create this negative pressure and that negative pressure is going to pull water up from the roots. To be fair, it should be noted that plants do expend energy to take ions up into the roots. And this is what allows water to enter the root hairs via osmosis. So, you know, technically there is some energy expenditure coming into play that is, you know, going to have some impact on this process. But the main point is that this cohesion tension aspect is totally energy free for the plant. The you know, they don't have to expend any energy for that part of the process. And, you know, just to kind of illustrate the amazing force that this congenital rate, you know, keep talking about redwood trees. There's super super tall, this process, this cohesion tension can create a negative pressure significant enough to draw water up over 300 ft vertically. That is a massive amount of pressure. In fact, it's so much pressure that if the secondary cell walls of the vascular tissue were not lignin fied, they wouldn't be able to withstand it. You know, the plant would actually, like break itself trying to do this. It's just another reason that lignin fication or that the, uh that including lignin in the secondary cell walls, is so important to the vascular tissue of plants. All right with that, let's flip the page.

Now that we've talked about how water moves through asylum, let's look at how sugar is transported through flow. Um, now the movement of sugars happens via bulk flow, and the sugars move from what's called the source to the sink. And the sources basically, uh, tissue where sugar enters the flow. Um, and a sink is tissue where sugar exits the flow. Um, and this whole process this bulk flow of sugars from source to sink is called trans location. Now, when sugars are loaded into the flow, um, or enter the flow and we call that flow um, loading and they enter via secondary active transport. What's what's gonna be used is a proton pump to create a proton Grady int. So here we have the outside of the cell. Here we have the inside. This is our membrane, and this proton pump is going thio pump protons out here and out here, we're going to have a high concentration of protons. This concentration radiant is going to allow this proton sucrose sim porter to bring sugar into the flow. Um, so it's going to move a proton down, its concentration radiant and at the same time, move a sucrose into the flow of against its concentration. Radiant, We call the uhh stuff inside the flow him Flow him, Sapp. And basically, it's mostly suit gross with, you know, some other dissolved sugars. And, you know, uh, a little bit of water and some hormones and minerals, But mainly, you know, what we have here is, uh, very sugary, mostly sucrose, like sappy, sticky material. Now, how this flow, um, sap is going to move through the flow. Um, is explained by the pressure flow hypothesis. This is the most commonly agreed upon theory for the movement of sap through flow. Um, essentially, the idea is this sugar is going to be more concentrated at the source, right. So here we have our source, which I'm representing with this little faucet icon. Their rights are source of sugar. It's our sugar tap. And I put it in a leaf because you know, leaf is gonna be one of those tissues that's going to produce sugars, and from there they will enter the flow. Um, here we have our Sim Porter. This is our proton super gross suit. Gross. Sim Porter. Here's our proton pump. So we're gonna load sugar into the flow. Um, and this is going to be our flow him over here. We have a Zeile. Um, now, what's gonna happen if, ah, lot of salutes in this case, sucrose wind up in the flow. Um, over here. What's that going to do? So what's that going to do to the salute potential compared to the asylum, that's mostly water, right? This is gonna be our low salute potential. We're gonna have higher salute potential over here. So water is gonna move from the asylum to its neighbor. The flow, um, due to the high concentration of sugar in the areas near the tissues that produce sugars. So this is going to cause an increase in ter ger pressure in the flow. Um, over here. So because of this movement of water, we're going to see ter ger pressure go up now down by the sink. Right, The sink. That's like your bum ass roommate. You know, they never do the dishes. They don't pull their weight. They don't do anything. They just take your stuff. The sink is going to just suck up all that sugar. So sugar is going to be way less concentrated down the sink, and that's actually going to cause water to leave the flow, um, and enter the asylum. So here our salute potential compared to the asylum is going to be higher. Are solid potential in the asylum over here is going to be lower compared to the flow. Um so over here water is going to go back to these asylum, and this means we're gonna have lower ter ger pressure there. So putting this all together, we have hi Turker pressure up here and low turker pressure down there due to water entering the flow him up here, leaving the flowing down here. And that's going to cause a positive pressure difference to build between the sink and the source. So we're gonna have a positive pressure potential compared down here, which means we're going to get bulk flow in this direction. So that's all I have for this lesson. Hopefully Now you have a good understanding of how water or I should say xylem. Sap moves through asylum and flow him. Sapp moves through, flow him. I'll see you guys next time