Hey everybody, it's your AP bio teacher Mr. Poser. Today we are starting topic 2.8 which is on tonicity and osmoregulation. Now that seems like a seems like a wordful mouthful to say right but we're going to break this down hopefully into two separate videos because there are two really big concepts that we want to get into in this topic. So if you remember in our previous topics we talked about membrane transport, we talked about diffusion, active transport.
co-transport and facilitated diffusion, all the different ways that molecules are able to get across the cell membrane. Now, this will be about why it's important for both animal cells and plant cells to regulate how much water and how much solute is able to pass through the cell membrane, and it's important to regulate how much. is on either side of the cell membrane because that's really going to affect the cell's ability to exchange materials with its environment. So what we're going to get into is a really big topic today. It's called osmosis and you may have heard this before but osmosis is what the passive transport of water across the selectively permeable membrane.
It's also just simply known as the diffusion of water. And why does osmosis get its own special term? Why does it you know, why don't we just call it the diffusion of water? Well, because osmoregulation is absolutely crucial for both plant cells and animal cells for them to stay alive. So basically, you know, if we just have a high concentration of water on one side, we'll start with one kind of molecule here, we'll get to talk about solutes in a minute.
If we have a high concentration of water on one side of a semi-permeable membrane and a low concentration of water on the other side. of the semi-permeable membrane. Well, what's going to happen? Water is going to diffuse, or it's going to osmosis.
That's not a word. Although you can use it if you want. Osmosis is going to happen in such a way that there's equal concentration of water on both sides of that cell membrane.
All right. But what really plays a role in osmosis is not only the concentration of water. or I should say the concentration of free water that isn't attached to any kind of solute or any other molecule. It really really has to do with the concentration of solutes. And what are solutes?
Solutes are any kind of solid that's dissolved in water. So we're not going to get into the whole thing here, but how water is able to dissolve solutes is because of its polarity and water molecules kind of surround a molecule of the solute to dissolve it. So here's the situation.
that I want us to take a look at. I tried to cover up the other part of this picture here, but it's not really working all that great. Oh, well. Here's a scenario. Let's say we have this U-shaped tube here.
It's a glass tube. And on the left side of the tube, we have pure water. On the right side of the tube, we have some kind of solute dissolved in water. Let's just say it's sugar. All right.
And in between each side of the tube, there's a semi-permeable membrane. And let's just say it's got passages. that are large enough for something like as small as a water molecule to pass through, but not large enough for whatever solute this is to pass through.
So let's just say it's like sucrose or something. Sucrose is kind of a big molecule with respect to water. So let's just say water can pass through the semi-permeable membrane, but sucrose cannot, kind of like a cell membrane, right? So what is going to happen here in terms of osmosis? What's going to happen in terms of the of molecules.
Well, yeah, see I tried to like cut it off here, but it doesn't work that well. Okay, whatever. Here's what happens after.
Water diffuses from the area of the higher free water concentration to the area of low freer water concentration. So water, which is the pure solvent over here, is going to undergo a net movement to the right side of this tube here. Okay, so what's going to happen is that more water is going to follow where the solute goes, okay, you would maybe think that, you know, if the semi permeable membrane would allow for the solute to come through, then the solute would diffuse to the other side.
But since they can't get through, a water is going to move in such a way that there becomes an equal amount of concentration of water on each side and equal amount of water, or I should say free water. All right, so Water is going to tend to diffuse from a low solute concentration to high solute concentration. So think about that for a second. We have been drilling into your head that molecules will diffuse from a high to low concentration, right?
But water will kind of follow where solutes go. Let's put that down here. Water will follow solutes.
Alright, so since we have a lot of sugar on this side, water tends to follow it until there's an equal concentration of what we call free water on each side. Alright, so look, this side is going to be filled up and this side is not going to be, it's going to start to lower in terms of volume. Okay, so that's an interesting thing about osmosis.
Okay, and what we're studying here is a measure of tonicity and that's that first word in that term, right, or in the title, right? So tonicity is the ability of a surrounding solution to cause a cell to gain or lose water. And tonicity really depends on what we call non-penetrating solutes.
So like in the last example with the U-shaped tube, maybe sugar or salt could be non-penetrating solutes because they can't necessarily get through the lipid bilayer all that easily. So here's a scenario. I know, I bet you're getting really, really, you know, not tired at all of these. you know, drawings that I keep making with all the colored dots.
But check it out, we have a scenario over here, we have an animal cell, let's just say it's a red blood cell. So that means it doesn't have a cell wall. And we have concentrations of solute in green and water in this kind of bluish color. Alright, so check out this situation over here. It looks like we have an equal concentration of solutes on either side of the cell and an equal concentration of water to the inside of the cell.
So that means this cell is in what we call an isotonic environment, meaning there's no net movement of water. There's not enough solute on the outside of the cell to draw water out. There's not enough solute on the inside of the cell to draw water in.
So there's no net movement of water. Water in... you know other molecules are always constantly moving in and out of the cell but those are at small rates typically so there's no net movement there's not it's the as i put down here the water coming out is equal to the water coming in and the concentrations of both the solute and the water are equal right so animal cells like to be in this isotonic environment where we're not losing too much water we're not gaining too much water it's kind of like the goldilocks situation if you catch my drift. All right.
But what if, what if, that should be highlighted, what if there's more solute on the outside than there is on the inside? Okay. So look, we have a high concentration of just water on the inside and a low concentration of water on the outside.
So what will happen? Well, water is going to, or excuse me, this cell is going to experience a net outflow of water. All right.
So that means that this cell is in a hypertonic environment, a hypertonic environment. So imagine, you know, people can't stay in super salty bodies of water, like the Dead Sea for very long, right? Because their cells will lose water to the environment because that water that in the Dead Sea is so salty that it actually draws the water out of your cells. So you can't stay in there for very long. All right, so the hypertonic environment means that water is going to exit your cells and Think about that.
I'm going to reveal it to you later here, but what could happen if the cell is losing too much water? I don't know. Let's talk about it later.
Alright, so this is a hypertonic environment, but a hypotonic environment, and be careful not to mess these two words up. They sound similar, but they're different. The opposite is actually hypotonic environment.
The cell will gain water, so check it out. This time we have more solute concentration on the inside of the cell. than we do on the outside of the cell. And there's more water on the outside of the cell than there is on the inside.
So there's going to be what we call a net influx of water into the cell. So water is going to follow wherever there's the most solute, okay, and it's going to flow into the cell. And that's called a hypotonic environment.
All right. So again, hypertonic, water flows out. Hypotonic, water flows.
in. And a way to remember this, I don't know, it might help you out, but hypo blows up. So like water flows into a cell in a hypotonic environment. And what can happen, it can blow up like a balloon or a water balloon, I guess. All right.
So let's talk about the implications that these environments have for plants and animal cells. As I kind of hinted before, cells without cell walls, like animal cells, are best in isotonic. environments, meaning that the water coming in is equal to the water going out. But plants with cell walls that have this rigid structure on the outside, they are best in a hypotonic solution. And why might that be?
Well, a plant is dependent on what we call its turgor pressure, or how much water is on the inside of the cell so that the plant can stay rigid. It can stay upright. Think about it.
Plants don't have like bones like animals do. Well, vertebrates do. And they can't, you know, how so how do they stay rigid? How is a tree able to stand up? Well, it's because of turgor pressure and the the pressure water exerts on the inside of a plant cell.
So a plant cell always wants to be taking in more water so that it can exhibit that force that pressure on its plant cell wall. So when a plant is healthy and it's happy and it's got enough water and it's in a hypotonic solution, that's what we call turgid. It's turgid, which means it's firm and it's in a healthy state.
That plant will be able to stand upright without any issues. But if a plant cell is, say, in an isotonic solution, it's what we call flaccid. And another word for flaccid is that it's kind of like limp.
It's not exerting a lot of force on that cell wall. And that suggests that it lost some water. It wants to be hypotonic, but it's an isotonic. That means it's flaccid.
And if you, say, forget about your plants for a week when you're on vacation and you don't have your neighbor water them, your plant cells in the plants could become plasmalized, meaning that the membrane pulls, the cell membrane, the plasma membrane, pulls away from the cell wall, and that results the plant to wilt. So if the plant cell loses too much water, That means it's going to kind of like shrink, shrivel up within the plant cell wall, and it's going to become yucky like this. And that's when a plant cell wilts. All right. In a hypotonic solution in an animal cell though, okay, we don't have, we don't have cell walls on our cells.
So if this is one of our red blood cells and we end up in hypotonic solution or in hypotonic environment, water's going to inflow, right? And it's actually going to cause the cell to lyse. It's going to burst like a balloon. Because remember the plants are this. excuse me, the plasma membrane is kind of like the consistency of a bubble, right?
So it's able to pop if there's too much water that comes inside. All right, so isotonic is when we're good. But if we're hypertonic, meaning that there's more solute on the outside than there is on the inside, that means that this cell can kind of shrivel up.
Okay, so both of these can shrivel up. But an animal cell, you know, you don't want it to be hypotonic, you don't want it to be hypertonic, you want to stay isotonic if you're an animal cell. Alright, or if you have animal cells. So, alright, we're going to stop right there for right now because I want to spend two separate videos on Tennessee Osmo Regulation. Particularly because the next video involves a lot of math and something called water potential.
Alright, so I'm going to stop right here and we'll pick up later on. See you later. Let me know if you have any questions. Oh, shoot.