Osmosis: Study with Video Lessons, Practice Problems & Examples

In this video, we're going to begin our lesson on osmosis. Osmosis is really just a type of passive diffusion, which means that osmosis requires absolutely no energy. Osmosis can be defined as the passive diffusion of a solvent across a semipermeable membrane such as a biological membrane or cell membrane. Recall that the term solvent refers to a substance that is doing the dissolving, and in biology, the solvent is almost always going to be water. We can define osmosis as the passive diffusion of water across a semipermeable membrane, and usually, this is what osmosis is referring to. The direction of net water flow across the membrane actually is going to depend on tonicity. Tonicity is a term that refers to the relative concentration of solute, or in other words, the relative concentration of substances being dissolved in the solution. It is critical not to confuse the solvent, which does the dissolving, with the solute, which is being dissolved by the solvent.

When it comes to tonicity, there are three critical terms listed below that you should be aware of. Notice that these three terms all end with the root "tonic," which refers to tenacity. These terms differ by their prefixes, and that's why we have the prefix interactive. The first important term that you should know here is the term "hypotonic." The root "hypo" means low, and it actually rhymes with low as well. This can be really helpful to remember that hypotonic solutions are going to have lower solute concentrations. The next term we have here is "isotonic," where "iso" is a root that means equal. Isotonic solutions are going to have equal solute concentrations. Lastly, the final term here is "hypertonic." "Hyper" is the opposite of "hypo," and whereas "hypo" rhymes with low, "hyper" does not rhyme with low. Hypertonic solutions are going to have a higher solute concentration. These three terms—lower, equal, and higher—are words of comparison, which means we cannot use these words unless we're comparing at least two different things. Something can't be lower, equal, or higher unless that something is being compared to something else. Similarly, the terms hypotonic, isotonic, and hypertonic are words of comparison that can only be used when comparing two different regions, usually the region on the inside of the cell and the region on the outside of the cell.

If we take a look at our example below, we can label the tonicity of the outside solution with respect to the solution inside of the cell. Notice that in each of these three images, we have this biological membrane, a semipermeable membrane, and we have two regions that we're going to be comparing: the outside of the cell with the inside of the cell. In the left image, notice that the outside of the cell has a relatively low concentration of these green solutes whereas the inside of the cell has a relatively high concentration of the green solutes. Because the outside has a lower solute concentration, it is going to be hypotonic. We can label the outside as a hypotonic solution. This means the inside solution, which has a higher solute concentration, is going to be hypertonic. We can label the inside as "hyper" for short to represent hypertonic. In the next image, notice that the outside of the cell this time has a somewhat equal solute concentration in comparison to the inside of the cell. When there are equal solute concentrations, we refer to them as isotonic solutions. This means that the outside can be labeled as an isotonic solution, and also the inside can also be labeled as isotonic. You can write "ISO" on the inside for short.

In the last scenario on the far right-hand side, notice that the outside solution has a higher solute concentration in comparison to the inside of the cell, which has a lower solute concentration. The outside, which has a higher solute concentration, is hypertonic, so we can label the outside as hypertonic. This means the inside solution is going to be hypotonic since it has a lower solute concentration. We'll write "hypo" here for short. This concludes our brief introduction to osmosis and, now that we've introduced tonicity and these three terms, we can discuss more about the direction of the net water flow across the membrane. We'll talk more about that as we move forward in our course. So, I'll see you all in our next video.

So here we have an example problem that's asking what is the tenacity of the outside solution in comparison to the inside of the cell, and we've got these 4 potential answer options down below. Now for sure we know we can eliminate answer option d since "electrotonic" is not one of the terms that we introduced in our previous lesson video, and so this is simply not going to be correct. However, hypotonic, isotonic, and hypertonic are all terms that we did discuss in our previous lesson video.

And so when we take a look at this image right here, notice that we've got this beaker of water, and what we have in the middle is a cell, a red blood cell. Notice that this image shows that the outside of the cell has a 10% solute concentration whereas the inside of the red blood cell has a 0.1% solute concentration.

Thus, we need to denote that this 10%soluteconcentrationontheoutsideofthecell is going to be a higher solute concentration. And of course, the inside of the cell is going to have a lower solute concentration. This problem is specifically asking us about the outside solution. So, we're going to focus in on the outside solution, and we want the term that corresponds with higher solute concentration.

You might recall that the term that corresponds with higher solute concentration is going to be hypertonic. Thus, hypertonic is going to correspond with answer option c. And so we can go ahead and indicate that c here is the correct answer for this example problem.

Now of course, the inside of the cell, which has a lower solute concentration, low rhymes with hypo, so this is going to be a hypotonic solution on the inside of the cell. The inside is hypotonic and the outside is hypertonic. This concludes this example, so I'll see you all in our next video.

So now that we've introduced tonicity in our last lesson video, in this video we're going to talk about how tonicity affects the direction of osmosis. What we need to recall from our previous lesson videos is that biological membranes are semi-permeable, meaning that some things can cross the membrane, but others cannot. If the solutes in a solution cannot diffuse across the membrane from areas of high concentration to areas of low concentration, then osmosis will occur. Recall osmosis is the diffusion of water across the membrane. Water will always move from hypotonic solutions towards hypertonic solutions, allowing water to move towards the more concentrated solution of solute to dilute that solution until it becomes isotonic.

Essentially, water is moving to help create equal solute concentrations in both solutions. Now you might be thinking, "Wait a second, doesn't that go against the natural tendency of diffusion? Don't substances always diffuse from high concentration to low concentration?" Here, it's important to keep in mind that hypotonic and hypertonic refer to solute concentration. However, water is not the solute; it's the solvent, and it moves from areas of higher concentration of water towards lower concentrations of water. Hypotonic solutions have lower solute concentrations but higher water concentrations, whereas hypertonic solutions, despite having higher solute concentrations, have lower water concentrations. Water is the solvent, and these terms hypo and hyper refer to the concentrations of solute.

In areas that are hypotonic, they have lower solute concentrations, but they have higher water concentrations. Hypertonic solutions have higher solute concentrations which means they have lower water concentrations. The brackets you see just mean the concentration of. So when you see something in brackets like H₂O, it means concentration of water, and solute in brackets means concentration of solute. Water will always move from hypotonic towards hypertonic solutions. If you remember that water flows from hypo to hyper, you'll be good on most of your osmosis questions. We'll be able to get some practice applying the concepts that we've learned here. Water always flows from hypo towards hypertonic solutions. That being said, I'll see you all in our next video.

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.