This is the video for D2.3 on water potential, and we'll cover the standard level content related to the movement of water. Water is an excellent solvent, and it's water's polarity that really gives it that ability to be a great solvent, to be able to dissolve solutes. When water is dissolving ionic compounds like salt here, The water forms shells around ions to prevent them from rejoining. So you'll notice they're here in red, they're teeny tiny. These partially positively charged hydrogens that are part of the water molecule surround negative ions, and the partially negative oxygens in the water molecule surround positive ions.
So it's able to take apart these ionic compounds and prevent them from getting back together. With polar molecules or polar compounds, water forms hydrogen bonds around them to dissolve them. So here we have glucose, and hydrogen bonds will actually form between the oxygen of the water and the hydrogens sticking off here of glucose. So I'm not going to draw all of them, but you can see that water is partially charged and therefore attracted to the partially charged or polar.
glucose molecule. Now, if we have two solutions that have different concentrations and they are separated by a semipermeable membrane that does not allow that solute to cross, then osmosis will take place. Osmosis is the passive movement of water across the semipermeable membrane towards areas of higher concentration.
So water always follows Water always follows the higher solute concentration. So here, let's say these red things are solutes and they cannot cross this membrane. Some of these water molecules in here will cross the membrane and will come over to this side until these are of equal concentration. So again, the solute numbers or the solute amounts aren't equal, but the concentrations are equal.
And that's what osmosis is. And depending on the differences in the solute concentrations, we actually have descriptive words to talk about these solutions. So an isotonic solution has the same concentration of whatever you're comparing it to.
So here in each of these beakers, I have like, let's just say this is a dialysis tubing or a cell filled with 0.1 molar solution. If it is sitting in a solution that has the same concentration... We consider this solution to be isotonic. The word iso or this root word iso means same.
If the solution has a lower concentration than what you're comparing it to, it is said to be hypotonic. So it's important to remember that these are comparative words. You can't just say something is hypotonic. Well, hypotonic compared to what? So I would say the solution is hypotonic.
to the cell. This prefix hyper means above, and this refers to solutions that have a higher concentration compared to the cell or whatever it is that you're comparing that to. So these are very important terms to have moving forward. Now, we said that water is always going to follow areas of higher solute concentrations.
So in this case, water would flow into this cell. Water would flow towards areas of higher concentration. In this scenario, in a hypertonic solutions, water would exit the cell and flow towards areas of higher concentrations.
So I'm using this arrow to mean water. In an isotonic solution, you get a small amount of movement, but it's the same in both directions. So we say no net movement of water. So I'll erase those to avoid confusion. And here's a look at how that actually happens.
So remember, the semi-permeable membranes that surround cells allow water to pass through. Water is permeable, but it's much less permeable to solute. So that's why osmosis is occurring if there's a concentration gradient and that solute cannot pass. Remember that this is passive.
No energy is required. And cells can actually control the rate of osmosis sometimes. There's a few ways that they might do that. They might change their solute concentration, right? So we think about like contractile vacuoles or salt pumps, or they can change their permeability to water in regards to their membranes.
This is so cool. Embedded within membranes are sometimes we'll find these channel proteins called aquaporins. These are channel proteins that allow water to pass through. Now, water can go through the phospholipid bilayers, but it can much easier, like faster, go through these aquaporins. So the more aquaporins a cell produces and embeds in its membranes, the more permeable that cell will be to water and the faster you'll have osmosis happening.
So it's a really cool adaptation that cells use. And if you've already studied things like the kidney or ADH, You probably know a little bit about this, and if you haven't, that will be coming soon. But we want to be thinking about controlling solute concentrations or controlling permeability if cells want to control their rates of osmosis. Now, when we looked at this earlier, we already knew the molarity of, like, let's say this cell that I was looking at here. Well, what happens if I don't know the concentration of solutes of that cell and I want to find out?
That's something called the osmolarity. It's the total solute concentration in a cell. And let's say I'm looking at something like a carrot, and I want to know the osmolarity of the cells in that carrot.
How would I do that? Well, you can actually put that plant tissue in solutes or solutions that have different solute concentrations, and you can measure the percent change in mass. If that plant tissue is gaining mass, That must be because it is in a hypotonic solution.
So that's telling me whatever the osmolarity of the carrot is, it must be greater than 0.1 because when I put it in a 0 molar or a 0.1 molar solution, it gains mass, which likely indicates, well, definitely indicates that it was in a hypotonic solution. We see that movement of water into the cell. On the other hand, when I'm noticing...
that that plant tissue is losing mass, that means that it is in a hypertonic solution. So remember that cells that are sitting in hypertonic solutions are going to lose water and therefore lose mass. What I'm looking for in order to find the osmolarity of that plant tissue is the isotonic solution. So in an isotonic solution, there's no net movement of water and therefore no net gain or net loss of mass. So that means I have found where the solution and the plant tissue or the carrot are equal in concentration.
And I can say that that carrot has an osmolarity of 0.2 molar. If you don't have an exact dot on here, you can see where it crosses your horizontal axis. That's where there's zero net gain in mass.
There are some really cool things that you can do here. You can see that we've calculated and visually represented variation in our data. So you can have a look at either standard deviation or standard errors.
And so you should be able to put error bars on your graph. You should be able to talk about controlled variables. So things like getting the plant tissue sample from the same carrot, from the inside of the carrot, not including the skin.
having the same surface area to volume ratio, allowing the same amount of time, controlling the temperature of the solution, lots of things like that that I recommend that you investigate. Now speaking of plants, plants have a cell wall and that cell wall does a lot of things, but one of the things that it can do is prevent excess water from entering. Even when a plant is placed in a very hypotonic solution and lots of water wants to enter the cell via osmosis, there's a limit to how much water can actually enter due to the rigidity of that cell wall. So it prevents excess water entry and it prevents the cell from bursting. Animal cells do not have a cell wall.
So when they are placed in a hypotonic solution, water enters the cell, and if enough water enters the cell, that cell can actually burst, and then that cell would die. On the other hand, if placed in a hypertonic solution, then these animal cells will lose water and will shrivel up, okay? So they will shrink, and you'll see them actually get smaller.
Some freshwater eukaryotes, like this Paramecium here, have adaptations that prevent this. So this does not have a cell wall, so if it's placed in a hypotonic environment, lots of water could enter here and then that cell could burst. But they have these really cool adaptations called contractile vacuoles that can kind of pump some of that excess water out. It requires energy, but it's a great example of homeostasis and an adaptation that this organism has to prevent things like bursting.
So let's focus for a minute on just plants, okay, or things with the cell wall. We'll talk about plants specifically. Plant cell walls are made of cellulose. They're very strong.
They can handle lots of pressure, and it is that pressure that helps plants stand up and be erect. So plants don't have a skeleton, let's say. They rely on turgidity inside of their cells to remain upright. So when we say turgid, that means a cell that has a lot of internal pressure, and that internal pressure is coming from water.
This will only be the case when you put a plant cell in a hypotonic medium. That's going to drive water to enter the cell. it kind of blows up this cell and creates a lot of water pressure on the inside of the cell.
Again, the cell wall prevents it from bursting. So this is great. This is when plants are actually very happy, we'll say. If the pressure of the inside of the cell drops due to the exit of water, the plant becomes flaccid and you get a plant that looks like this.
It looks like wilting. That's if it's in an isotonic environment. So even when the solute concentrations are equal, the plant is flaccid. We really need it to be hypotonic in order to remain upright. If you put a plant in a hypertonic solution, then the cell membrane can actually shrink away from the cell wall due to the excessive amount of water that is lost.
And we call that plasmolysis. And this is going to result in plant death. Now, whereas plants really need to be in a hypotonic environment, animal cells need to be bathed in isotonic solutions.
So we talked a little bit about if you put an animal cell in a hypotonic solution, it could burst. In a hypertonic solution, it could shrink. So we need our, this is a blood cell, our blood cells and other cells to be bathed in isotonic solutions.
So often if you're getting an IV like this, it's going to be connected to a bag with saline solution. So saline solution is a salt solution, and we use this to rehydrate someone because it is isotonic to human cells. So we want the blood plasma to be isotonic when compared to the blood cells or other tissues. So great application there. Also, if you are preparing an organ for transplant.
Between the patient that it is harvested from and the patient that it will go into, you want to make sure that that organ is bathed in an isotonic solution. Again, to keep those cells in an isotonic solution means to prevent them from bursting or prevent them from shrinking.