So in our previous lesson videos, we've already described tonicity and the direction of osmosis across a membrane. We already know that water will flow across a membrane from hypotonic solutions towards hypertonic solutions. This will be important for you to remember as we move forward and talk about this video where we're going to focus on the environmental tonicity and how it affects cells. When cells are placed into environments that are hypotonic, or in other words when the outside solution of a cell is hypotonic, then we already know that water is going to enter into the cells and that will cause the cells to swell up or enlarge like a hippo. You can think hypotonic environments cause cells to swell up or enlarge like a hippo and that's why we've got this little image of a hippo here as a memory tool to help you remember that interesting idea.
As an analogy, you can think of a cell like a balloon, and when water flows into the cell, you can think of air flowing into a balloon. When air flows into a balloon, we know that the balloon will begin to expand, and if the balloon over expands, then the balloon could potentially burst open. In other words, when too much water enters into the cell, it could potentially cause the cell to lyse or, in other words, burst or rupture, which could kill the cell. This is especially concerning for animal cells because animal cells do not have a cell wall, so they have nothing to prevent the overexpansion of their membrane. That could potentially leave animal cells susceptible to cell lysis or cell bursting, which can kill them. Animal cells do not prefer to be in hypotonic environments. However, plant cells, on the other hand, do have a cell wall that surrounds their cell membrane, and this cell wall can prevent the overexpansion of the cell membrane. This means that plant cells do not have to worry about lysing when placed into hypotonic environments and thus can survive in hypotonic environments. In fact, plant cells actually prefer hypotonic environments because when water is flowing into the cell, the plant cell doesn't need to worry about lysing, and when water flows into the cell, it will increase and maximize what is known as turgor pressure. Turgor pressure is really just the water pressure that is going to apply pressure to the cell membrane and allow the cell membrane to apply pressure to the cell wall. This is preferred by plants to have increased and maximum turgor pressure.
When the environment of a cell, on the outside of the cell is hypotonic, we know that it can cause cells to swell up like a hippo. For animal cells like this red blood cell here in this image that does not have a cell wall, water flows into the cell in this direction when the cell is placed into a hypotonic environment. When water flows into the cell, it's almost like air flowing into a balloon, and that will cause the cell to expand and swell up like a hippo. If the cell membrane overexpands too much, then that may lead to cell lysis where the cell will rupture or burst and that will kill the cell. However, notice down below we have a plant cell, and plant cells have a cell wall that animal cells do not have. The cell wall prevents the overexpansion of the cell membrane, and so plant cells do not have to worry about lysing in hypotonic environments. In fact, plant cells prefer to be in hypotonic environments because it maximizes turgor pressure. There's high turgor pressure when the cell is placed into a hypotonic environment. This is because water will again flow into the cell causing the cell to swell up, causing the central vacuole to swell up, and that will maximize turgor pressure where the cell membrane is applying maximum pressure to the cell wall, and that allows the plant to take an upright healthy position and allows the plant to thrive.
The second environmental tonicity that we're going to look at is the isotonic environment where the outside solution is isotonic with respect to the inside solution of the cell. In this case, we already know that water is going to enter and exit the cell at equal rates, and what this means is that the size of the cell is not going to change. It's almost like letting air into the balloon and letting air out of the balloon at equal rates, and the balloon size would not change. It turns out that the isotonic environments are preferred by animal cells. If we take a look at our image down below, specifically at this middle column, you'll notice we're focusing on an isotonic environment where the outside solution is isotonic or has an equal solute concentration in comparison to the inside of the cell. We're looking at red blood cells, and notice that the rate of water flow into the cell is exactly the same as the rate of water out of the cell, and that means that the cell is not going to change in size and so the animal cell is actually going to thrive in this, under these conditions, and so animal cells prefer isotonic environments.
Now for the plant cell down below, what you'll notice is again water will flow into the cell and out of the cell at equal rates, and the plant cell here, what you'll notice is that its central vacuole is a bit smaller than what it was when it was in a hypotonic environment, and also there are some gaps, really small gaps, between the cell membrane and the cell wall which means that there's not maximum turgor pressure. Although plants may be able to survive in isotonic environments, isotonic environments are not preferred by plant cells. Again, plant cells prefer high turgor pressure.
Now the final condition that we're going to look at is when the environment or the outside of the cell is hypertonic. Under these conditions, we know that water is going to exit the cell, and so that will be similar to air exiting a balloon. That will cause the cells to basically shrivel up and lose water. When they lose water, that can cause them to dehydrate. If we take a look at our image on the right-hand side, notice that we're focusing on a hypertonic environment where the outside is hypertonic, and again when that's the case, you can think of a hyper kid that will get dehydrated very quickly, and the cells will also get dehydrated. Notice that under these conditions, water will leave the cell and again the cells will dehydrate. When the cells dehydrate when they lose water, they will shrivel up and shrink in their size just like letting air out of a balloon will cause the balloon to shrivel up and shrink in size. The same will happen with a plant cell as you can see down below, water will leave the plant cell, and notice that the plant cell is shriveling up, there is a very low turgor pressure under these conditions, and again that is not going to be favored by these plant cells and it can cause plants to wilt and eventually die under these conditions. Now it turns out that this process that occurs under hypertonic environments for animal cells of shriveling up is referred to as a process called crenation, spelled as you see here. And then for the low turgor pressure here for plant cells, this process is referred to as plasmolysis. Plasmolysis and crenation are analogous to one another where plasmolysis is basically what happens to a plant cell under hypertonic conditions, and crenation is what happens to an animal cell under hypertonic conditions. This year concludes our brief lesson on the environmental tonicity and its effects on cells, and we'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.