Water Potential - Video Tutorials & Practice Problems
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1
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Water Potential
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Hi. In this lesson, we're going to talk about how plants transport water and sugar through their xylem and phloem. To begin we're going to talk about the concept of water potential represented by the Greek letter, psi. Which if you're curious is spelled like this, pronounced like this, sigh. 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 rewatch those videos because everything we're gonna deal with now directly builds on the builds on those ideas. So if you're kind of going there thinking, I kinda remember that stuff, not a 100%, I'd say go back and re watch those videos. Now water potential is the potential energy of water to move between 2 environments, basically. And the differences in water potential between 2 environments will determine the direction of flow. So water potential is actually based on, 2 concepts we're gonna go over, and you can see that in the equation here. We have this psi s, that stands for solute potential, and also psi p, that is the, pressure potential. So water potential is going to be due to both of those concepts. We'll review both of them in just a moment before we get there. As a general rule, or as a rule of thumb I should say, water is always going to move from areas of high potential to 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. It 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 megapascals, which is basically just a 1000000 Pascals. Pascals, if you've taken physics, you might recall are, or actually chemistry 2 comes up, you might recall that Pascals are an SI 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 gradient 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 a bush, or something that might not seem too incredible, but think about the fact that redwoods, which can be upwards of 300 feet 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, you know, over 300 feet tall sometimes, that's counting from the ground up. If we talk about how far water travels in those trees, we're actually talking about a greater distance because those roots go underground. So water is moving an 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 solute potential. So solute potential, which again, is that gonna be psi s, that Greek letter psi s, solute potential is the solute concentration relative to pure water. Here's where things get confusing. High concentrations of solute mean low solute potential. So I know this is a little weird, so let's give a visual aid. So here we have low solute potential, because we have a high concentration of solutes. Over here, we have high solute potential, because we have a low concentration of solutes. 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 solute potential to low solute potential. Right? Recall from our discussion of osmosis, that water will go from an area of high solute concentration to 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 to 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 solute potential to low solute potential. Meaning it's going to 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, you know, this cell, cell membrane right here behind me, then what's gonna happen? The solute particles are gonna move. Right? But if the solute particles are prevented from moving, then the water is gonna flow. So I want to do a little example using our semipermeable membrane here. Right? This is our plasma membrane, not planza, plasma membrane. Okay. So our plasma membrane, semipermeable membrane, and let's say we're gonna put a high concentration of solutes on this side, and a low concentration of solutes on this side. Well, obviously, the water is going to flow from the low concentration to the high concentration. So it's gonna 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. Solute potential is actually a negative pressure. The reason is pure water has a solute potential of 0 megapascals, or just 0. Right? 0 megapascals is the same as 0 pascals, so it's, you know, whatever it's it's 0. Solute potentials are 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 an 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 straw is 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? 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 a high positive pressure. There's, like, a lot of stuff there. The opposite, 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 solute potential means high concentration. Right? So let's say our low solute potential here is negative 1 megapascals. Our high solute potential could be something like, oops, solute potential not water potential. Our high solute potential might be, in this case, 0. Okay. This is where things are confusing. Right? Low low means, like, more negative in this case. Right? So to put other numbers to this, let's say we have negative 5, this on here, this over here on the low concentration side might be something like negative 1. Alright. So, this might be a lower numeral. Right? 1 is lower than 5, but negative one is actually greater than negative 5. Right? Negative 5 is more negative, so it's a lower solute 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, things like the solute potential is negative or something. So, the last thing I want to say is that cells always always have dissolved solutes inside of them. Right? So they're always going to have some solute potential. With that, let's flip the page and talk about what happens inside the cell with all these, water potentials.
2
concept
Pressure Potential and Turgidity
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Pressure potential is physical pressure on water. And whereas solute 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 the liquid up with a negative pressure. It should be noted though, that living cells have positive pressure, and that's because living cells are going to have solutes inside of them, they're also going to 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. Now, when membranes are present, we're usually gonna see water move in response to solute potentials. Right? From high to low solute potential. When membranes are absent, we're really gonna be seeing water move from high to low pressure potential. Why do you think, solute potential only has an effect really when there are membranes present? Well, why do we need membranes at all to concentrate those solutes. Right? Without a membrane present, those solutes are just gonna diffuse. Meaning, there is gonna be no potential difference in, solute potential. There is gonna be no difference in solute concentration. So we need the membrane to have the difference in solute concentration, which is basically the same thing as saying we need the membrane to have the difference in solute potentials. So I want to do a couple examples here, just show you a few things. So over on, all the way over on the left here, we have this u shaped tube, it's filled with water, and those red dots are representing, dissolved solutes. Those are solutes 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's a concentration difference between the two sides. And this dotted line right here, that is our semipermeable membrane. Right? So it's gonna allow water to pass through, but not the solutes. So on this side, we have a high concentration of solutes. Meaning, we have a low water potential, I'm sorry, low solute potential. So our solute potential is low. Here we have a low solute concentration, meaning our solute 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 solute potential is going to be negative 1, I'm sorry, negative 2 mega Pascals. Right? Gotta have units, otherwise, that's meaningless. And let's call this negative one mega pascals. Alright. So what's gonna happen? Water wants to lose its potential. Right? So we're gonna go from high potential to low potential, which is the same as saying we're gonna go from a low concentration of solutes to a high concentration of solutes, 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, or 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 solute 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 that we talked about when we talked about osmosis, here's where I 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 1 mega pascal, what do you think is gonna happen if I do that? What's gonna happen if I add a pressure potential of 1 megapascal pushing down on this side of the tube? What actually is gonna happen is we're gonna end up with something like you see over here, let me jump out of the way. The water levels are gonna become even again. Why is that? Because by adding a pressure potential of 1 megapascal over on the 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 solute potential of negative 2 megapascals, and we also have a pressure potential of 1 megapascal. On this side, we have a solute potential of negative 1 Megapascals, and no pressure potential. So our pressure potential is just equal to 0 megapascals. If we use our formula, right, that water potential equals solute potential plus potential pressure, we'll see that our water potential on this side, so this is plain old water potential, is negative 1 megapascals, and our water potential on this side is also negative 1 megapascals. 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 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. Right? Water potential, solute 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, turgor pressure. That's the pressure inside the cell, due to the, usually, it's mostly the vacuole swelling, but generally speaking, it's the contents of the cell pushing against the cell wall. And usually, turgor pressure is experienced because the vacuole 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, protoplasts. That's the, all the living stuff inside the cell plus the plasma membrane, and it does not include the cell wall. So, you may remember Einstein's famous words. Right? That every action has an equal and opposite reaction. Right? Well, if turgor pressure is pushing against the cell wall, its 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 turgor pressure. Now, actually, you have to turn turgidity up. You have to increase turgor pressure to induce wall pressure. Right? So if the cell is what we call flaccid, meaning there's no turgor 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 turgor pressure. So you have to increase turgidity 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 flaccid. Right? They'll have no turgor pressure. In fact, sometimes they can shrivel up. We call this plasmolysis, and you can kinda see that happening right here. The cell is all shriveled up due to water loss. I mean, look how much smaller that vacuole is as compared to this vacuole, or this vacuole over here. Now, in non woody plants, when this happens, when turgidity is lost, sometimes what we'll see is wilting. And you know, maybe you've gotten while they while they start to droop over like this sad plant here. Well, this is wilting, and this is due to a drop in turgor pressure. And as you can see, the cells inside this sad, wilty plant have shrunken vacuoles. Right? It says empty, you know, it's probably not gonna be totally empty, but it's much smaller. When those vacuoles are all nice and full, like you can see over here, then our plant will stand upright, it'll be erect. Right? The reason that we only see this in non woody plants is because woody plants have lignified cells. Right? You might remember that those lignified cells are actually going to contribute to the structural integrity of the plant. That's why woody plants don't wilt like this, or the woody regions don't wilt like this anyways. Alright, with that let's flip the page.
3
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Water Potential in Soil and Air
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Water potential in soil will vary depending upon the conditions. Generally speaking, dry soil will have a lower water potential than 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 solutes. Compared to plant roots, which of course are made of cells and as we know, cells are filled with solutes. 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 solutes, and therefore ensure that water is going to go from the soil into the roots. Now, just as soil can have water potential, air 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 transpiration. 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 stomata, or the singular is just stoma. So these stomata control gas exchange, which is their, you know, 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 a 100% humidity, you bet water's gonna evaporate. Now, how do these stoma open and close? Well, one mechanism is based on these proton pumps. These proton pumps, when the plant wants to open its stoma, 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, or or charged ions, they will cause a depolarization. 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. I know that's a little confusing, so let's just go through it once more. So, to open the stomata, or to open a stoma, proton pumps are gonna concentrate protons outside the cell. This is going to 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 as 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 and human cells. So to close the stoma, the plant is going to 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 to, shrink and slice, they're gonna lose their turgor pressure, and that's going to allow the stoma to close. So closing over here, opening over here, and it is due to the movement of water in and out of the stoma, or the, the guard cells of the stoma, I should say. Now, stoma 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 stoma according to the day night cycle. And additionally, they will also respond to hormonal signals, like that of abscisic acid, which is often just abbreviated ABA. This stuff, ABA, is actually produced in roots, and it's produced in response to low soil water potential. And what ABA does is cause the stomata to close. It induces the stomata to close, and that reduces transpiration. Why is this important? Well, if soil water potential is low, that means that the plant is not going to be as absorbing water as 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 stomata open for photosynthesis. That's how it's gonna get the gases it needs, to carry out photosynthesis. And, you know, 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, you'll see something like stomata in deep pits surrounded by trichomes. Remember, we said trichomes can be involved in preventing water loss. And also, in the section where we talked about photosynthesis, we talked about adaptations in what are called CAM plants and C4 plants, or plants that carry out CAM and C4 photosynthesis. So if you want to review those particular concepts, go back and check out the video on photorespiration at the end of the photosynthesis section. With that, let's flip the page.
4
concept
How Water Enters Xylem
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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 xylem. 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 this orange structure in the image. Water is going to move into root hairs via osmosis, but it's going to, the pressures on it are going to be things we talked about when we talked about water potential. Right? So solute potential is going to be the major factor in moving water into the root hairs. Now, once it's in the plant, it can actually go through a few different routes to get to the xylem. There's actually 3 routes that water can travel. The first we're gonna talk about is the transmembrane route. Basically, remember that all of our, you know, cell membranes, here are going to have these things called aquaporins. Right? Those are 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 to, have living organisms rely on that alone. They need these aquaporins to increase the efficiency of water moving through the membrane. So the transmembrane route is going to be flow through the aquaporins, and of course a little bit directly through the membrane. However, the majority is going through the aquaporins. So that is pretty straightforward. Now, the other 2 are a little, less straightforward, shall we say. We have the apoplastic route, which is gonna be flow outside of the plasma membranes of cells, and this is going to be, basically, in the spaces between cells and, their porous cell walls. We call this region, this space outside of the plasma membranes, the apoplast. And you might recall that this area is interrupted by what's known as the Caspian strip. The casparean strip is a waxy layer made of a material called suberin, and that is secreted by the endodermis, and this is done in order to block off the xylem. So, in this image, let me jump out of the way here, you can see that the apoplast 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 the xylem, all of these light blue regions are the apoplast. So, let's talk about the Casparean strip for a second. You can see that the Casparean strip blocks off the apoplast at 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 endodermal cells. And the reason it wants to do this is that allows the endodermal cells to act as filters, which means they can control ion flow and concentration gradients. This is super important for a number of reasons. I mean, for 1, you want to make sure that you're not letting in any bad stuff. And also, those concentration gradients are going to be literally a matter of life and death. So it's super important that the concentrations of solutes and ions are super super tightly controlled. Now, water has to flow, or water can also flow through what's known as the symplastic route. And this is flow through the cytosol of cells. You might remember that plant cells, will have their cytosols linked by what are known as plasmodesmata. In fact, you can see plasmodesmata in these channels between the cells in the image. This area, this continuous network of plant cells that are linked by these plasma desmata, this continuous network of cytoplasm, is called the symplast, and it's labeled in this sort of orange color. So we have the apoplast and the simplest. Those are the two regions, that represent 2 of the routes which water can take. And of course, if water is flowing through the symplastic route, it doesn't need to worry about the caspian strip, and that's all good because it's moving through cells, which basically means those cells are going to be able to regulate, solute concentration, ion flow, all that good stuff, they're gonna be in control. Now when water, water flows through the xylem, 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 xylem. 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, xylem's main job is to transport water up the, plant, and the phloem is there to transport sugar, but other stuff hitches a ride is kind of the basic idea. So the movement of molecules along a pressure gradient, so 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 low here. Get my head out of the way so you can see that. That's why Oh my gosh, I did it backwards, sorry. Water wants to lose its potential, so it's going to be high down there, low up there, derp, and that is our bulk flow of water or xylem sap through the xylem. And with that, let's flip the page.
5
concept
Root Pressure Theory
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Cohesion tension theory is the most widely accepted theory as to why water flows up through xylem. 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, it is most likely that cohesion tension is the dominant reason that water is flowing upsilon. These 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, root 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 xylem. 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 xylem is that ions are pumped into root xylem. And this creates a negative water potential relative to soil, so it's gonna drive water into those root xylem. As we said before, water enters via osmosis, And the more water that builds up in the root xylem, you know, like the more water that builds up in our root xylem, 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 more water goes into the root xylem here, then leaves the leaves, leaves the leaves, get it, through transpiration. And there, you know, there is some evidence for this, for example, stomata close at night, but roots continue to absorb waters, no, that water and ions from the 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 guttation, where water is forced out of leaves due to all of this pressure. And you can actually see a picture of guttation happening here. These water droplets are being squeezed out of the leaves due to this super high, pressure. And, you know, this is most common this site is most common in mornings when pressure is the highest, and like right before, you know, the plants open their stomata and start trans transpiring again. You know, and they've had the whole night to suck up water and ions basically. So there are gonna be some, let's say some, factors that we need to cover in order to understand cohesion tension theory, and the movement of water through xylem in general. Water has something, some ability known as capillary action or capillarity. This is the ability of a liquid, in this case water, what we're talking about, to move through narrow spaces. And it's basically due to 3, three factors. We see capillary action as a product of 3 factors. One of those is adhesion, and this is the attraction between unlike molecules. So in the case of capillary action, it's gonna 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, you know, in terms of capillary action 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 beading up on the surface of these, leaves is due to the fact that, the leaves' surface is hydrophobic, and so water is going to want to cling to itself here. This is an example of cohesion because, and, you know, you might have to, 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 beaded 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, concave 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, meniscus like that. Now, the last force that I want to talk about in terms of capillary action is surface tension, and you can see an example of surface tension, I'm just writing 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 at that air water interface, and it's going to create, a tense, like a tension across the surface, which is why this paper clip can just sit there on the surface without breaking it. So how does this all come into play in terms of capillary action, that you can see here, where the water has moved up the tube against gravity pulls up from the container wall. Right? So the, it pulls up from the container wall. Right? So the, adhesion between the water molecules and the tube wall is going to pull up. Surface tension is going to pull up from the very surface, and then cohesion basically transmits the pull between all the water molecules. So as surface tension pulls up from the surface, that meniscus, Adhesion is going to allow, you know, pull from the walls, and cohesion is going to transmit that pull to all the water molecules in the tube. And that is how we get capillary action. And, with all of that in mind, let's actually flip the page and talk about cohesion tension theory.
6
concept
Cohesion-Tension Theory
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6m
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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 stomata are open and the external humidity is less than 100%. So the conditions needed for that evaporation, or transpiration, as I'm going to 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 mesophyll, or that, inner tissue in the leaves. So, inside these leaves the evaporation is going to result in, lower humidity. And this causes water to enter into the space in the mesophyll from the, from the parenchyma cells. Right? It's going to enter that apoplastic region. Now, what's gonna essentially happen is the evaporating water is going to pull water in from the parenchymus cells, and this is going to create steep menisci. Menisci is the plural of meniscus, so many meniscuses. Steep menisci 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 menisci 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 going to happen is this water leaving is going to create a negative pressure potential, and that negative pressure potential is going to 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 xylem, 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 meniscuses. That's, the surface tension that is pulling up on the water. That is, in, that surface tension is in those many menisci. 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. The sun heats up the atmosphere and that's going to, 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 can generate, You know, I keep talking about redwood trees. They're super super tall. This process, this cohesion tension, can create a negative pressure significant enough to draw water up over 300 feet 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 lignified, 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 lignification, or that the, that including lignin in the secondary cell walls is so important to the vascular tissue of plants. Alright. With that, let's flip the page.
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Pressure Flow Hypothesis
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Now that we've talked about how water moves through xylem, let's look at how sugar is transported through phloem. Now, the movement of sugars happens via bulk flow, and the sugars move from what's called the source to the sink. And the source is basically, tissue where sugar enters the phloem, and a sink is tissue where sugar exits the phloem. And this whole process, this bulk flow of sugars from source to sink, is called translocation. Now, when sugars are loaded into the phloem, or enter the phloem, we call that phloem loading. And they enter via secondary active transport. What's what's gonna be used is a proton pump to create a proton gradient. So here we have the outside of the cell, here we have the inside, this is our membrane, and this proton pump is going to pump protons out here. And out here, we're going to have a high concentration of protons. This concentration gradient is going to allow this proton sucrose symporter to bring sugar into the phloem. So it's going to move a proton down its concentration gradient, and at the same time, move a sucrose into the phloem against its concentration gradient. We call the stuff inside the phloem, phloem sap, and basically it's mostly sucrose with, you know, some other dissolved sugars, and you know, a little bit of water, and some hormones and minerals, but mainly, you know, what we have here is, you know, very sugary, mostly sucrose, like sappy, sticky material. Now, how this phloem sap is going to move through the phloem is explained by the Pressure Flow Hypothesis. This is the most commonly agreed upon theory for the movement of sap through phloem. 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 there. Right? It's our source of sugar, it's our sugar tap. And I put it in a leaf because, you know, a leaf is gonna be one of those tissues that's going to produce sugars, and from there they will enter the phloem. Here we have our symporter. This is our proton sucrose, oops, sucrose symporter. Here's our proton pump. So we're gonna load sugar into the phloem, and this is going to be our phloem. Over here, we have a xylem. Now, what's gonna happen if a lot of solutes, in this case, sucrose, wind up in the phloem over here? What's that gonna do? Oops. What's that gonna do to the solute potential compared to the xylem that's mostly water. Right? This is gonna be our low solute potential. We're gonna have higher solute potential over here. So water is gonna move from the xylem to its neighbor, the phloem, 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 turgor pressure in the phloem over here. So because of this movement of water, we're gonna see turgor pressure go up. Now, down by the sink, right, the sink, that's like your bummer mate, 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 gonna just suck up all that sugar. So sugar is going to be way less concentrated down at the sink, and that's actually going to cause water to leave the phloem and enter the xylem. So here, our solute potential compared to the xylem is gonna be higher, and our solute potential in the xylem over here is gonna be lower compared to the phloem. So over here, water is going to go back to the xylem, and this means we're gonna have lower kurger pressure there. So, putting this all together, we have high turgor pressure up here, and low turgor pressure down there, due to water entering the phloem up here, leaving the phloem 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 to down here, which means we're gonna 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 xylem, and phloem sap moves through phloem. I'll see you guys next time.