Water Potential

Summary

This video explains water potential, the potential energy of water relative to pure water, and how it determines water movement in biological systems. The presenter discusses osmosis, solute potential, and pressure potential, using examples like salt on a slug and water flowing in trees. Key equations are introduced, such as solute potential (ψs = -iCRT), with steps to calculate it in problems. The video emphasizes how water moves from high to low potential, influenced by solute concentration, pressure, and temperature, and concludes with an example calculation.

Takeaways

• 💧 Water potential refers to the potential energy of water per unit area compared to pure water and determines the direction water will flow.
• 🔄 Water potential is measured using the Greek letter psi (Ψ), which is reminiscent of Poseidon's trident.
• 🧂 Water potential consists of two main components: solute potential (Ψs) and pressure potential (Ψp).
• 🧬 During osmosis, water moves from areas of high water potential to areas of low water potential.
• 🐌 Pouring salt on a slug creates a lower water potential outside the slug, drawing water out and causing the slug to shrivel.
• 🌿 Water moves up a tree along a water potential gradient, from the roots (higher water potential) to the leaves (lower water potential).
• 🔢 The overall water potential is the sum of solute potential (affected by the number of solutes) and pressure potential (caused by physical pressure).
• 🧪 The formula for solute potential is Ψs = -iCRT, where i is the ionization constant, C is the concentration, R is the pressure constant (0.0831), and T is temperature in Kelvin.
• 🍬 Substances like sucrose have an ionization constant of 1 since they do not break into ions when dissolved in water.
• 📏 When calculating water potential in an open beaker, the pressure potential is zero, so the water potential equals the solute potential.

Q & A

What is water potential and how is it defined?

Water potential is the potential energy of water per unit area, compared to pure water. It helps predict the direction of water movement, driven by osmosis, gravity, pressure, or surface tension.

How does osmosis affect water potential in the context of a slug?

Osmosis causes water to move from an area of higher water potential (inside the slug) to an area of lower water potential (where salt is applied on the slug's surface), leading to water being drawn out of the slug, which can cause it to shrivel.

How does the dissociation of sodium chloride in water affect water potential?

When sodium chloride dissolves in water, it separates into sodium and chloride ions. These ions are surrounded by water molecules, which decreases the water potential by creating more areas for water to move into.

How does water potential drive water movement in plants?

Water moves from areas of higher water potential, like the soil (0 bars), to areas of lower water potential, such as the roots, stems, and leaves. The gradient is driven by osmosis and evaporation, allowing water to move up through the plant.

What factors contribute to overall water potential?

Water potential is determined by two main factors: solute potential (which decreases with the addition of solutes) and pressure potential (which increases with physical pressure exerted by the cell wall in plants).

How do solutes affect solute potential and overall water potential?

Adding solutes, such as sodium chloride, decreases solute potential because it opens spaces for water to move into, resulting in lower overall water potential.

What role does pressure potential play in water movement in plant cells?

Pressure potential is the physical force exerted by the cell wall when water enters the cell. It resists further water influx by exerting pressure, which can prevent cells from bursting.

What is the equation for solute potential, and what do the variables represent?

The equation for solute potential is -iCRT, where 'i' is the ionization constant (number of particles a solute dissociates into), 'C' is the molar concentration, 'R' is the pressure constant (0.0831), and 'T' is the temperature in Kelvin.

Why is it important to convert temperature to Kelvin in the solute potential equation?

Temperature must be converted to Kelvin to ensure correct calculations in the solute potential equation. This is done by adding 273 to the Celsius temperature.

How do you calculate water potential in an open beaker scenario?

In an open beaker, the pressure potential is 0 bars, so the overall water potential is equal to the solute potential. If the solute potential is calculated to be -5 bars, the water potential will also be -5 bars.

Outlines

00:00

🌊 Understanding Water Potential

The concept of water potential is introduced as the potential energy of water per unit area compared to pure water. It is crucial for understanding water movement due to osmosis, gravity, pressure, and surface tension. The video explains how water potential helps predict whether water will flow into a cell or not. Psi (Ψ) is used to represent water potential, with a mnemonic to remember it resembling Poseidon's trident. The formula for water potential includes solute potential (Ψs) and pressure potential (Ψp).

05:03

🐌 Osmosis and Water Movement in Cells

Osmosis is described in detail, using the example of salt poured on a slug. Salt decreases the water potential on the slug's surface, causing water to flow out from inside the slug to balance the potential. The ionization of sodium chloride in water demonstrates how solutes affect water potential. Water always flows from high to low water potential, which explains water movement in nature, such as how water moves up a tree, driven by differences in water potential from roots to atmosphere.

🌳 Water Movement in Plants

Water potential is the key to understanding how water moves through plants. As water evaporates from leaves (with very low water potential), water is pulled up from roots, which have slightly higher water potential. This flow occurs along a gradient of decreasing water potential, with osmosis driving the movement of water up the plant. In addition to osmosis, physical pressure (Ψp) is also important in regulating water movement in cells, especially in plant cells with strong cell walls.

🧪 Solute Potential and Its Effects

The solute potential (Ψs) is influenced by the concentration of solutes, and adding more solutes decreases the solute potential. An example is given of adding salt, which reduces water availability by creating spaces for water molecules to move into, thereby lowering the water potential. This principle is essential for understanding osmotic processes and how water moves in response to solute concentrations. Plant cells develop pressure potential (Ψp) when water fills the cell, but the cell wall prevents bursting, creating turgor pressure.

📊 Calculating Water Potential

Water potential is determined by both solute potential (Ψs) and pressure potential (Ψp), and these values can be calculated using equations. An example calculation demonstrates how adding solutes decreases the water potential. Solute potential is represented by the equation Ψs = -iCRT, where 'i' is the ionization constant, 'C' is the concentration, 'R' is the pressure constant, and 'T' is temperature in Kelvin. Each variable is explained in the context of water potential.

🧮 Solving for Solute Potential

A specific example problem is solved to calculate the solute potential using the equation Ψs = -iCRT. In the example, a 0.2 molar concentration of sugar is used, and the temperature is converted to Kelvin. The process involves canceling units and simplifying to get the final answer in bars, which represents the solute potential. The open container is noted to have zero pressure potential, and the overall water potential is calculated as the sum of solute and pressure potentials.

🏞 Final Thoughts on Water Potential

Water potential helps in predicting where water will flow, either into or out of cells, based on solute and pressure potentials. The example of sugar in water shows how different substances affect water potential differently. The video concludes by emphasizing the usefulness of understanding water potential for various biological processes, encouraging viewers to remember the basics using the example of Poseidon and his trident.

Keywords

💡Water Potential

Water potential is the potential energy of water per unit area compared to pure water. It determines the direction in which water will flow, whether through osmosis, gravity, pressure, or even surface tension. In the video, it’s explained as the driving force behind the movement of water, such as when water moves into a plant’s roots from the soil.

💡Psi (Ψ)

Psi (Ψ) is the symbol used to represent water potential. It’s compared to the trident of the Greek god Poseidon in the video to help viewers remember its meaning. Psi is a key metric for measuring the direction and strength of water flow based on water potential.

💡Solute Potential (ΨS)

Solute potential is the component of water potential that accounts for the presence of solutes in water. The video explains how adding solutes like salt reduces the water potential, causing water to move towards areas of lower potential. An example from the video is when sodium chloride dissolves and lowers water potential outside of a slug, causing water to leave the slug.

💡Pressure Potential (ΨP)

Pressure potential refers to the physical pressure exerted on water within a system, such as inside plant cells. It is a positive force that can counterbalance solute potential. The video describes pressure potential as the force exerted by a plant cell wall when it prevents the cell from bursting due to incoming water.

💡Osmosis

Osmosis is the process by which water moves across a semipermeable membrane from an area of high water potential to low water potential. In the video, osmosis is illustrated using an example of pouring salt on a slug, where water moves out of the slug’s cells, causing it to shrivel.

💡Ionization Constant (i)

The ionization constant (i) refers to the number of particles a solute dissociates into when it dissolves in water. The video explains this in the context of sodium chloride, which splits into two ions (Na+ and Cl-) in water, making its ionization constant 2. This value affects solute potential.

💡Concentration (C)

Concentration (C) represents the molarity of a solute in a solution, affecting the solute potential. In the video, concentration is discussed as moles per liter, where higher concentrations of solutes lead to lower water potential. For example, adding more salt or sugar lowers water potential due to osmosis.

💡Pressure Constant (R)

The pressure constant (R) is a fixed value used in the solute potential equation, specifically 0.0831 liter bar/mole K. The video includes this constant as part of the equation for calculating solute potential (-iCRT), where it plays a crucial role in determining water flow.

💡Temperature (T)

Temperature (T) is another factor in the solute potential equation, measured in Kelvin. The video explains that as temperature increases, molecular activity increases, lowering water potential. This is important in biological systems where temperature influences the rate of water movement.

💡Bars

Bars are the unit of measurement for water potential, solute potential, and pressure potential. In the video, bars are used to quantify water potential in various scenarios, such as the water potential inside and outside a slug, or in different parts of a plant.

Highlights

Water potential is the potential energy of water per unit area compared to pure water, and helps determine where water will flow.

Water potential (psi or Ψ) is represented with a symbol resembling a trident, inspired by the Greek god Poseidon.

Water potential is influenced by solute potential (Ψs) and pressure potential (Ψp), and both factors determine the movement of water.

Water flows from areas of high water potential to areas of low water potential, and this principle can explain osmosis and water movement in cells.

The concept of osmosis is demonstrated with the example of adding salt to a slug, causing water to leave the slug's cells due to lower water potential outside.

In plant cells, water flows into cells through osmosis, but the cell wall exerts pressure back, creating pressure potential (Ψp).

Water potential explains how water moves up a tree from roots to leaves, driven by lower water potential in the leaves and atmosphere.

Solute potential decreases as more solutes are added, creating more space for water to move and lowering the water potential.

Pressure potential occurs when water pushes against the cell wall, creating a physical pressure that affects the overall water potential.

The equation for water potential is Ψ = Ψs + Ψp, and these values are measured in bars.

The formula for solute potential (Ψs) is -iCRT, where i is the ionization constant, C is the molarity, R is the pressure constant, and T is the temperature in Kelvin.

Ionization constant (i) is 2 for sodium chloride (NaCl) as it ionizes into two particles, while for sucrose (sugar) it remains 1 since it does not ionize.

Water potential in a solution can be calculated using the solute potential formula and measured in bars, with an example showing -5 bars for a sugar solution.

Water potential in open containers has no pressure potential (Ψp = 0) because there is no physical pressure exerted on the water surface.

Water potential plays a crucial role in determining the flow of water in biological systems, such as plants, cells, and solutions, using basic principles like Ψ = Ψs + Ψp.

Transcripts

Hi. It's Mr. Andersen and in this video I'm going to talk about water potential,

which is really what it sounds like. It's the potential energy of water per unit area

compared to pure water. And so it allows us to figure out where water is going to flow

due to osmosis, gravity, pressure. Even surface tension. And so it allows us to figure out

if water is going to flow into the cell or not. And so we measure it using something

called psi or p s i. And a quick way to remember that is that Poseidon was this Greek god of

the ocean. Carried a trident. And it looks a lot like the trident that we use to represent

water potential or psi. Now psi is going to be equal to the psi S with is solute potential

and pressure potential. But before I scare you off with a bunch of formulas, let's get

started and talking about how water potential works. Let's first talk about osmosis. And

if you don't know this you may want to watch the video on osmosis. But if you wanted to

do something really cruel you could pour salt on a slug. Don't do it. It will kill it. But

what it would do is it would shrivel up that slug. And so what would happen is it would

pull water out of the slug. Now why does that occur? Let's zoom in to the surface of the

slug. So let's say this represents a cell membrane on the outside of the cells of the

slug. We've got water on the outside, water on the inside. And let's say we add just one

crystal of sodium chloride, or salt. Sodium chloride is going to be made up of two ions

that are bonded together using an ionic bond. And when we add that to the water, something

weird happens. They'll break apart into their two ions. We've now got the chlorine ion and

the sodium ion. The negative and the positive charge. And the negative charge is immediately

going to be surrounded by the positive parts of the water. And the negative sides of the

water are going to surround the positive sodium. But look what it did. It opened up all these

areas. So it decreased the water potential above the slug or on the surface of the slug.

And so now we have areas where the water inside the slug can move into that. And it's more

radical than I have in this simple kind of a diagram. So what it's going to do is it's

going to move water outside the slug. And so we measure water potential on either side

of that membrane. On the outside it's going to be negative 40 bars. And on the inside

it's going to be -5 bars. Now know this. Pure water is going to be right at zero bars of

water potential. And so the water is going to flow from here into here. So the water

is going to flow from an area of high water concentration to low concentration. Or it's

going to flow from an area of high water potential now to low water potential. And that's what

you want to remember. Water's always going to glow from high to low water potential.

And so this drives water even up a tree. And so if you were to pour some distilled water

below a tree, that's going to have a water potential of 0 bars. But the roots are going

to be around -2. And that's because they have a lot of solutes or salts inside them. And

so the water is going to flow in through osmosis. But the stems are going to have even a greater,

excuse me, a lower water potential. And the leaves as well and even the atmosphere. And

so the water is moving up a tree along this water potential gradient. Now what's driving

that? We're evaporating all the water up at the top. So there's not much water there at

all. Really, really low water potential if we're to look at the leaves of the plant.

And so now let's get to those equations. So water potential is built on two things. It's

built on the solute potential. And so think of that as like water flowing through osmosis.

And then the pressure potential. And that's like physical squeezing of the cell. And so

solute potential is going to drop as we increase the number of solutes in that area. And so

if I were to add just two little bits of sodium chloride or salt to it, what would that do

to the solute potential? It's going to drop that. It's going to get a lower value? Why

is that? Remember we're opening up spaces in here for water. So we're going to have

less water. Let's say we add a whole bunch of solutes to it. That's really going to decrease

that solute potential. And so maybe it's going to be around -5 bars. So that's due to osmosis

or that push of osmosis. What about the pressure potential? Well that's a physical pressure.

And so imagine that water keeps flowing into this cell. And let's make this a plant cell.

So water is going to keep flowing in. That's going to push out on that cell. But it doesn't

explode. Our cells would explode. But that has a cell wall around the outside of it.

And so that wall is now going to start exerting a pressure to the inside. And so what that's

going to do is create what's called a pressure potential. And so we measure that in bars

as well. So let's say that's 2 bars. Why is it a positive value? Remember that's going

to be pushing in. It's going to want to push water out of that kind of an area. And so

those two things, if we add those together, are going to be our water potential. What

would be the water potential in this case? It would be -5 bars plus 2 bars. So it's going

to be -3 bars. That's the overall water potential. And those two things are going to determine

if water flows into a cell or if it doesn't. Sometimes we'll be asked to do a little bit

more detail here on the solute potential. And there's an equation for that, which in

my class I would not want you to memorize. But let's throw that up here right now. So

solute potential is equal to negative iCRT. So we've got to go through each of those part.

The i, the C, the R and the T. Let's start with the ionization constant. Ionization constant

is not going to have the units associated with it. It's just a factor. And it's always

going to be somewhere from 1 to 2. Sometimes including 1. And so if we were to look at

sodium chloride, remember sodium chloride is one molecule when it's outside of the water.

But when you add it to the water it's going to break apart into two ions. And so the reason

we're multiplying it times 2 is if you add one mole of sodium chloride, it's really like

adding one mole of chloride ion and one mole of sodium ion. And so we have to multiply

that times two. Now it's really easy if we're dealing with something like sucrose which

is just table sugar. That's going to have an ionization constant of 1. Because when

you add sugar to water it just stays as sugar. So we don't have to multiply anything. So

again, if we increase the ions were increasing the i and that's going to give us a lower

solute potential. Okay. What about concentration? Obviously the more of the stuff that we add

to the water, that's going to increase or decrease rather the solute potential. And

so moles per liter in concentration is going to be what we measure for C. And so if you

add there the molarity, so let's say something is a one molar solution, that means there's

one mole per liter. The next thing we have in our equation if the pressure constant.

Pressure constant's just that. It's always going to be the exact some thing. And it's

always going to be 0.0831. I wouldn't memorize it. These units at the end are going to be

important as we solve a quick problem. And then the next one is going to be the temperature.

Obviously it's important that if we increase the concentration that that's going to decrease

solute potential. But if we increase temperature then the molecules are going to be bouncing

around more readily and so that's also going to decrease our water potential. And so when

we measure that in this equation we use Kelvin. And so what you're going to do is take the

celsius degrees and add 273. If you don't do that you're simply going to get the wrong

answer. And so knowing that, let's throw you a quick problem. So let's say we have a molar

concentration of sugar solution in an open beaker, that will become important in just

a second. It's a 0.2 molar concentration and what they're asking you to do is calculate

the solute potential at 22 degrees celsius. And so on the AP exam you're going to get

these two things. They're going to give you water potential, which we already went over.

That's equal to the pressure potential plus the solute potential. They're going to explain

that here. And then this is even the equation for solute potential, which is -iCRT. And

so how do you solve that? Let me show you how I would solve it. First thing I would

do is I would plug everything in. What's my i? My i is going to be 1. That's just because

we're dealing with sugar. And since sugar remember doesn't ionize, we're just going

to put in 1 because it stays as sucrose or stays as sugar. Where did I get this one?

This is my concentration. That 0.2 moles per liter. Because they gave me 0.2 molarity as

the concentration. Next thing is going to be my pressure constant. I'm simply copying

that off the sheet. We've got it right here. And then I'm going to have my temperature.

Since they told me it was 22 degrees celsius, I'm adding that to 273, so I get 295 K. And

so first thing to do is to get rid of all of these units. So for example we have Kelvin

here on the bottom and we have it on the top. Likewise we've got liters on the top, liters

on the bottom. First thing I would do is I would cancel out all of those units. What

am I left with? It's not surprisingly bars. That's going to be what we measure solute

potential in. Next thing I would do is I'd put the bars on the end and then I would multiply

those values. And so what I get is -4.9029 bars. Now that's way too many significant

digits. If I go back to my question, this one only has one significant digit, 0.2. And

so my answer should really be -5 bars. And so I've quickly figured out the solute potential.

But they could also ask you this question. What's the overall water potential? Okay.

So then we're going to have to think about this a little bit. We've got the solute potential

and again that's going to be half of this water potential. What's the other half? It's

on pressure. And so how much pressure are we going to have on a beaker that's open?

We're going to have zero pressure on it. And so if I want to figure out my overall pressure

I'm just going to add those together, so it's also going to be -5 bars. And so that's water

potential. Again it measures where water is high, as far as potential energy of water.

And it allows us to figure out where they go. And if you can remember that, then remember

our friend Poseidon. You can do well on all of these problems. And I hope that was helpful.