Water Potential - Online Tutor, Practice Problems & Exam Prep

Plants transport water and sugars through xylem and phloem, driven by water potential, which is influenced by solute potential and pressure potential. Water moves from areas of high potential to low potential, utilizing processes like osmosis and transpiration. The cohesion-tension theory explains how evaporation creates negative pressure, pulling water upward. In phloem, sugars are transported from sources to sinks via bulk flow, facilitated by pressure differences. Understanding these mechanisms is crucial for grasping plant physiology and adaptations to their environments.

Water Potential

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 discuss 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 delve deeper, I highly recommend that if you feel a little fuzzy on the concepts we discussed when we talked about osmosis and diffusion, to go back and rewatch those videos because everything we're going to deal with now directly builds on those ideas. So, if you're kind of going there thinking, I kind of remember that stuff, not 100%, I’d say go back and rewatch those videos. Water potential is the potential energy of water to move between two environments. The differences in water potential between two environments will determine the direction of flow. 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 Ψs, that stands for solute potential, and also Ψp, that is the pressure potential. Water potential is influenced by both of these concepts. We'll review both of them in just a moment before we continue. As a general rule, water always moves from areas of high potential to areas of lower potential. The way I like to think of this is that water wants to lose its potential. Right? It's going to skip class, get high behind the gym, and just throw everything away. It wants to lose its potential. Water wants to be a deadbeat. Water potential is a form of pressure; it is measured in units of pressure, and those units are often megapascals, which is basically just a million Pascals. Pascals, as you might recall from physics or chemistry, are an SI unit of pressure.

Now, the water potential gradient in plants is what causes water to move up from the soil through the plant, and against gravity. And in a little shrub, or a bush, that might not seem too incredible, but consider 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. When I mention that redwoods are 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 referring to a greater distance because those roots go underground. Water is moving an amazing distance through some plants, and it's doing that against gravity. Believe it or not, this process is very energy efficient, almost solar-powered, but I'm getting ahead of myself. Let’s discuss solute potential. Solute potential, which again, is represented by Ψs, is the solute concentration relative to pure water. Here's where things get confusing: High concentrations of solute mean low solute potential. This is a little counterintuitive, but remember, we said that water always wants to move from high solute potential to low solute potential. Recall from our discussion of osmosis, that water moves from an area of low solute concentration to high solute concentration. So, if water is going to move from low solute concentration to high solute concentration, that means it's going to move from high solute potential to low solute potential, meaning it's going to lose its potential.

It's very important to remember that if we do not have a semi-permeable membrane, like this cell membrane behind me, the solute particles are going to move. But if the solute particles are prevented from moving, then the water is going to flow. Let’s consider an example using our semi-permeable membrane here. This is our plasma membrane, a semi-permeable membrane. Let’s say we're going to put a high concentration of solutes on this side, and a low concentration of solutes on this side. Obviously, the water is going to flow from the low concentration to the high concentration side. Solute potential is actually a negative pressure. The reason is pure water has a solute potential of 0 megapascals, or just 0. Solute potentials are negative pressures, meaning their measurement of pressure is going to be a negative number. The concept of negative pressure can be illustrated by using a straw. When you use a straw, you create a vacuum by sucking some of the air out, which exerts negative pressure on the liquid below, pulling it up the straw. So, to put numbers to this, our low solute potential here might be -1 megapascals, and our high solute potential could be something like 0. This is where it gets confusing: lower numerals, such as 1 is lower than 5, but -1 is actually greater than -5 in terms of solute potential. Again, I don’t want you to stress about the math, just get a qualitative understanding of what I mean by solute potential being negative. Cells always have dissolved solutes inside them, always having some solute potential. With that, let's flip the page and talk about what happens inside the cell with all these water potentials.

Pressure Potential and Turgidity

Pressure potential is physical pressure on water. And whereas solute was a negative type of pressure, pressure potential can be positive or negative. 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, they usually die. We'll get to that in just a moment. Now, when membranes are present, we're usually going to see water move in response to solute potentials. Right? From high to low solute potential. When membranes are absent, we're really going to see 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 going to diffuse. Meaning, there is going to be no potential difference in solute potential. There is going to 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 of examples here, just to 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 going to 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 going to add some numbers in here. So let's say that our low solute potential is going to be 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 going to 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 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 difference in the heights, as you can see, of the water levels on the two sides, but the concentrations will now be the same. Right? Even though there are more molecules of solute on this side, there's more water. So the concentrations balance. Right? This is going to be like our equilibrium point.

So this might all seem 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 going to make my pressure potential equal to 1 mega Pascal. What do you think is going to happen if I do that? What's going to happen if I add a pressure potential of 1 mega Pascal pushing down on this side of the tube? What actually is going to happen is we're going to 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 1 mega Pascal 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 mega Pascals, and we also have a pressure potential of 1 mega Pascal. On this side, we have a solute potential of negative 1 mega Pascals, and no pressure potential. So our pressure potential is just equal to 0 mega Pascals. 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 mega Pascals, and our water potential on this side is also negative 1 mega Pascals. Meaning, we don't have any net flow of water, and I say net flow there because in actuality, there's going to 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 going to 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 seen wilted plants start to droop over like this sad plant here. Well, this is wilting, and this is due to a drop in turgor pressure.

Water Potential in Soil and Air

Water potential in soil will vary depending on the conditions. Generally speaking, dry soil will have a lower water potential than the water potential in plant roots. Conversely, damp soils will have a 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 a 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 the water potential is low enough, water can actually flow from the plant into the soil, which would be devastating to the plant. I mean, the root's 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 a water potential. Warm 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 going to be the 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 going to 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. These stomata control gas exchange, which is 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's going to evaporate. Now, how do these stomata 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 charged ions, they will cause depolarization. Basically, they're going to 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 going to 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. 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 stomata, 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 sag; they're going to 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 stomata, or the guard cells of the stomata, I should say.

Now, stomata 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 stomata according to the day-night cycle. 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 absorbing water as effectively, which means it's going to 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. It's the compromise between conserving water and maximizing photosynthesis. The plant wants those stomata open for photosynthesis. That's how it's going to 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.

How Water Enters Xylem

Now that we've talked about the conditions that lead water to flow, that is, water potential, let's discuss how water gets from the soil into the xylem. Water flows from the soil into the root hairs, and from there it moves into the xylem, which is represented as this orange structure in the image. Water moves into root hairs via osmosis, pressured by factors related to water potential. Solute potential is the major factor in moving water into the root hairs.

Once inside the plant, water can travel through one of three routes to reach the xylem. The first route we'll talk about is the transmembrane route. Aquaporins, which are channels that allow water to pass through, play a crucial role here since they increase the efficiency of water movement through the membrane. A smaller amount also passes directly through the membrane, but the majority goes through the aquaporins.

The second route is the apoplastic route, involving flow outside of the plasma membranes of cells, specifically in the spaces between cells and their porous cell walls. This region outside the plasma membranes is called the apoplast. This route is interrupted by the Casparian strip, a waxy layer consisting of suberin that is secreted by the endodermis to block off unauthorized access to the xylem. The apoplast is often forced to direct water into the endodermal cells, which act as filters regulating ion flow and maintaining concentration gradients.

The third route is the symplastic route, where water flows through the cytosol of cells, interconnected by plasmodesmata. This network of cytoplasm is called the symplast. As water flows through the symplastic route, it bypasses the Casparian strip, allowing cells to manage solute concentration and ion flow effectively.

When water moves through the xylem, it does so without crossing membranes, propelled by differences in pressure potential. This movement of molecules along a pressure gradient is known as bulk flow. Water enters the xylem where pressure potential is higher and moves to regions of lower pressure potential, allowing the efficient transport of xylem sap, which includes dissolved minerals, nutrients, and hormones in addition to water.

Xylem's primary function is to move water upward through the plant, while phloem transports sugars. However, xylem sap also carries other substances as it travels. Consequently, the concept of bulk flow includes the movement of all these molecules influenced by the pressure potential difference. This understanding is crucial for grasping how water and nutrients are circulated within plants.

With this explanation, let's flip the page.

Root Pressure Theory

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 up. 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 the idea of transpiration was going to 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 going to 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 going to 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, the greater the positive pressure.

Right? So the basic idea is that more water goes into the root xylem here, then leaves the leaves through transpiration. And there is some evidence for this, for example, stomata close at night, but roots continue to absorb 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 this sight is most common in mornings when pressure is the highest, and like right before the plants open their stomata and start transpiring again. You know, and they've had the whole night to suck up water and ions basically. So there are going to be 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 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 three factors. We see capillary action as a product of three factors. One of those is adhesion, and this is the attraction between unlike molecules. So in the case of capillary action, 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 terms of capillary action when we're talking about water, it's going to be the attraction between water and itself. And you can see a nice example of cohesion here. 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, you might have to look closely to see this, the water is actually forming orbs. Instead of the droplet having a flat bottom, 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 "ST" 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 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 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 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.

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 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 parenchyma cells. Right? It's going to enter that apoplastic region. Now, what's going to essentially happen is the evaporating water is going to pull water in from the parenchyma 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 menisci. 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 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.

Pressure Flow Hypothesis

Now that we've talked about how water moves through xylem, let's look at how sugar is transported through phloem. The movement of sugars happens via bulk flow, and the sugars move from what's called the source to the sink. The source is basically tissue where sugar enters the phloem, and a sink is tissue where sugar exits the phloem. This whole process, this bulk flow of sugars from source to sink, is called translocation. When sugars are loaded into the phloem, or enter the phloem, we call that phloem loading. They enter via secondary active transport. What's going to be used is a proton pump to create a proton gradient. 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. Out here, we are 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. 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 some other dissolved sugars, a little bit of water, and some hormones and minerals, but mainly, very sugary, mostly sucrose, like sappy, sticky material.

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. I put it in a leaf because, a leaf is going to be one of those tissues that is going to produce sugars, and from there they will enter the phloem. Here we have our symporter. This is our proton sucrose symporter. Here's our proton pump. So we're going to load sugar into the phloem, and this is going to be our phloem. Over here, we have the xylem. Now, what's going to happen if a lot of solutes, in this case, sucrose, wind up in the phloem over here? What's that going to do to the solute potential compared to the xylem that's mostly water. Right? This is going to be our low solute potential. We're going to have higher solute potential over here. So water is going to 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. This is going to cause an increase in turgor pressure in the phloem over here.

Now, down by the sink, 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 going to 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 going to be higher, and our solute potential in the xylem over here is going to be lower compared to the phloem. So over here, water is going to go back to the xylem, and this means we're going to have lower turgor 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 going to have a positive pressure potential compared to 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 xylem sap moves through xylem, and phloem sap moves through phloem. I'll see you guys next time.

Here’s what students ask on this topic:

What is water potential and how is it calculated?

Water potential (Ψ) is the potential energy of water to move between two environments. It determines the direction of water flow, moving from areas of high potential to low potential. Water potential is calculated using the equation:

$\Psi =\Psi s_{}+\Psi p_{}$

where Ψ_s is the solute potential and Ψ_p is the pressure potential. Solute potential (Ψ_s) is influenced by the concentration of solutes, with higher solute concentrations resulting in lower (more negative) solute potential. Pressure potential (Ψ_p) can be positive or negative, depending on the physical pressure exerted on the water. Together, these factors determine the overall water potential in a given environment.

How does the cohesion-tension theory explain water movement in plants?

The cohesion-tension theory explains how water moves upward through the xylem in plants. According to this theory, evaporation of water from the leaves (transpiration) creates negative pressure (tension) in the leaf mesophyll. This negative pressure pulls water from the roots to the leaves. Cohesion (attraction between water molecules) and adhesion (attraction between water molecules and xylem walls) facilitate this upward movement. The process is essentially solar-powered, as the sun's energy drives transpiration, creating the necessary tension to pull water up through the plant, even against gravity.

What is the difference between solute potential and pressure potential?

Solute potential (Ψ_s) and pressure potential (Ψ_p) are components of water potential. Solute potential is the effect of solute concentration on water potential. It is always negative, as adding solutes lowers the water potential. High solute concentration means low (more negative) solute potential. Pressure potential, on the other hand, is the physical pressure exerted on water. It can be positive (e.g., turgor pressure in plant cells) or negative (e.g., tension in xylem during transpiration). Together, these potentials determine the overall water potential in a system.

How do plants adapt to low water potential in soil?

Plants adapt to low water potential in soil by developing mechanisms to maintain water uptake. One adaptation is increasing the concentration of solutes in root cells, which lowers the root's water potential, ensuring water moves from the soil into the roots. Additionally, plants may close their stomata to reduce water loss through transpiration. Some plants also develop deep root systems to access water from deeper soil layers. These adaptations help plants survive in environments with low water availability.

What role do aquaporins play in water transport in plants?

Aquaporins are specialized protein channels in cell membranes that facilitate the efficient movement of water molecules. In plants, aquaporins are crucial for water transport, especially in the transmembrane route. They allow water to move quickly through cell membranes, enhancing the plant's ability to absorb and distribute water. This is essential for maintaining cell turgor, nutrient transport, and overall plant health, particularly under varying environmental conditions.