Osmosis: Study with Video Lessons, Practice Problems & Examples

Osmosis is the diffusion of water across a semipermeable membrane, moving from hypotonic (low solute concentration) to hypertonic (high solute concentration) solutions. This process is crucial for maintaining cell integrity. In hypotonic environments, cells swell and may lyse, while in hypertonic environments, cells dehydrate and shrink. Isotonic solutions are ideal for animal cells, allowing equal water flow. Plant cells prefer hypotonic conditions due to turgor pressure, which supports their structure. Understanding these concepts is essential for grasping cellular behavior in different environments.

Osmosis and Tonicity

In this video, we're going to do a quick refresher on osmosis. So you guys know that osmosis is simply a type of diffusion. And diffusion is the movement of a substance from a high concentration down to lower concentrations of the same substance. And osmosis again is a specific type of diffusion. Osmosis is the diffusion of a solvent across a semipermeable membrane or a membrane that allows some substances to cross but prevents other substances from crossing. The solvent is usually going to be water, especially in biological systems. Osmotic pressure is the pressure that's required to prevent the flow of the solvent. Osmotic pressure is really a measure of the strength of osmosis. So the higher the osmotic pressure, the greater the strength of osmosis. Now, in our next video, we're going to talk about the direction of osmosis. But before we get there, it's important to note that the direction of osmosis depends on the tonicity of the solution. Recall that tonicity refers to the relative concentrations of solutes, not solvents, that are dissolved in the solutions. There are three different terms that refer to the tonicity of a solution: isotonic, hypotonic, and hypertonic.

Isotonic refers to a solution that has the same solute concentration as another solution. Solute concentration is important because tonicity refers to solute, not solvent concentration. The next term is hypotonic. Hypotonic sounds like low, hypo, low. Hypotonic solutions have lower solute concentrations than another solution. The opposite of hypotonic is going to be hypertonic. When I think of hyper, I think about a kid that's had way too much sugar and they're just running around all over the place. Because they've had too much sugar, hyper, this is a solution that has a higher solute concentration than another solution. These three terms are words of comparison, so you can only use them when comparing one solution to another solution.

In our example below, we're going to compare the outside solution of a cell to the inside solution of a cell, which is separated by a plasma membrane. In our first scenario of our example, the outside solution, which is in light blue, is separated by a semipermeable membrane and the inside solution is in a yellowish like orangeish brownish mustard color. We have these green solutes that are dissolved. Notice that the outside solution has a lower solute concentration than the inside solution because there are fewer green dots in a certain area. Because the outside solution has a lower solute concentration, it's going to be hypotonic in comparison to the inside solution. Over here in the next scenario, notice that the concentration of solutes, these green dots here, is exactly the same on the outside and the inside solution. So that means that the outside solution is isotonic with respect to the inside solution. In our final scenario over here, notice that the outside solution has a much greater solute concentration than the inside. Because the outside has a higher solute concentration, it's going to be hypertonic. The tonicity here is going to control the direction of osmosis. We'll talk about the direction of osmosis in our next video. So, I'll see you guys then.

Osmosis Direction

So now that we've reviewed tonicity, we can talk about osmosis direction. And recall that osmosis is simply the diffusion of a solvent across a semi-permeable membrane. And so if you tend to get confused on this, here's all you guys need to remember: Water will always move from hypotonic solutions towards hypertonic solutions. This is true when the solutes cannot diffuse across the membrane. Instead of the solutes diffusing, the solvent, or water, will diffuse. Water is always going to diffuse towards the more concentrated solute solution or towards the hypertonic solution in order to dilute the solutes until that solution is isotonic with respect to the solution that the water traveled from. Notice in our example down below, that the water flow or the water movement is always going to be in the same direction here. It's going to flow from hypotonic solutions towards hypertonic solutions. If you can remember "hypo to hyper," you'll be good on all of your osmosis direction questions. One thing that students tend not to realize is that water still moves from higher concentrations of water towards lower concentrations of water. So water is still diffusing in the normal way that we think of diffusion. Hypotonic solutions, we know, have lower solute concentrations, but they actually have higher water concentrations. Hypertonic solutions, we know, have higher solute concentrations, but they actually have lower water concentrations. Water is still moving in the same direction, from high concentration of water to low concentration of water, but also from low concentration of solute to high concentration of solute. It's just moving in the same direction here. If we apply these rules to our scenarios from above, from our previous video, which we already completed here, we labeled the tonicity of the outside solution. In the first scenario, the outside solution was hypotonic, so we'll put "hypo" on the outside. That means the inside is going to be hypertonic. Recall that water flows from hypo towards hyper. So the water flow is going to go across the membrane in this direction, towards the hypertonic solution, meaning water is going to flow into the cell. If water flows into the cell, that means the cell can expand in size. In our second scenario, it is isotonic. The outside is isotonic with the inside. That means that water flow is actually going to go in both directions at an equal rate. It's not that there's no water movement, it's that the water movement goes equally in both directions. Water is always moving across the membrane no matter what. In our last scenario over here, we said the outside solution is hypertonic. That means the inside of the cell has to be hypotonic. Water again is going to move from hypo towards hyper. It's going to move from the hypo here towards the hyper. That means it's moving outside of the cell. If water leaves the cell, that means the cell could dehydrate and it's going to shrink up and shrivel. We're going to talk more about the results of osmosis in our next video. So I'll see you guys in that video.

Results of Osmosis

So in our last lesson video, we said the direction of osmosis is impacted by the tenacity of the solutions, and water always flows from hypotonic solutions towards hypertonic solutions. And so the direction of osmosis will impact the results of osmosis, and that's what we're going to focus on in this video. Now, when cells are placed into hypotonic environments, that causes the cells to swell up just like a hippo. And that's because when the environment is hypotonic, that means that the inside of the cell will be hypertonic. And again, water always flows from hypotonic solutions towards hypertonic solutions. So water would flow into the cell and cause the cell to swell up just like a hippo. And so when the cells continuously swell up, that could potentially cause the cells to lyse or rupture/burst, and that's because the membrane is expanded too much. Now recall from your previous bio courses that cells with cell walls will actually not lyse in hypotonic solutions, and that's because the membrane expansion is prevented by the cell wall. Now hypotonic environments are actually preferred by plant cells which have cell walls, and that's due to the increased turgor pressure. And recall from your previous bio courses that turgor pressure is literally just the water pressure applied on the cell membrane, pushing the cell membrane up against the inside of the cell walls. And the turgor pressure is really what gives lots of plants their ability to maintain their structures. And we'll be able to see an example of turgor pressure down below in our image. Now, with hypertonic environments on the other hand, that causes cells to dehydrate just like a hyper kid would get dehydrated pretty quickly. And so over here we have an image of a crazy hyper kid that I would hate to babysit over a weekend. And so this hyper kid is going to get dehydrated pretty quickly, jumping around all over the place. And so, hopefully, that'll help you guys remember that when cells are placed into hypertonic environments, that will dehydrate the cells just like a hyper kid gets dehydrated pretty quickly. And so down below on the left-hand side of our image, what we have are animal cells in different environments with different tonicity. And on the right, what we have are plant cells with the same scenarios. And so starting with the animal cells on the left when, again, cells are placed into hypotonic environments, they will swell up like a hippo. And so that's because all of the water is going to be flown into the cell, and so you can see the direction of these arrows down below is pointing into the cell because water will be flowing in. And again, that's because the environment is hypotonic and water always flows towards hypertonic solutions, and so the inside of the cell will be hypertonic. And, again, that could potentially cause some cells to lyse or rupture, so you can see that we have some cells lysing here. Now, when cells are placed into hypertonic solutions, again, the cells are going to dehydrate just like a hyper kid gets dehydrated, and that's because water is leaving the cell. So you can see the direction of these arrows is leaving the cells. And, again, that's because the inside of the cell would be hypotonic and the outside of the cell is hypertonic, just like we're indicating up above. And, of course, that could cause the cells to shrivel up and dehydrate and potentially die. So this is not a good environment. So, essentially, what we're saying is for animal cells, the hypotonic environment is not a good environment and the hypertonic environment is also not a good environment. And so for animal cells, it's the isotonic environment that is preferred, and that's because we have an equal rate of water flow going into the cell and out of the cell. And so, again, isotonic environments preferred by animal cells. Now, over here on the right, again, we have plant cells. And we know that in hypotonic environments, cells are going to swell up like a hippo, and that's again because water is flowing into the cell just like what's being indicated. And so what you'll see is that when water is flowing into the cell here, it's actually not going to lyse because plant cells have cell walls and that prevents the membrane expansion. So here, what you'll see is that this hypotonic environment is actually preferred by the plant cells, And you'll see that the central vacuole here is filled up with lots and lots of water and the cell membrane is pushed up against the inside of the cell wall, and that creates a lot of turgor pressure which is helpful for the plant to maintain their structures. Now, over here with the isotonic environment, notice that we have an equal water flow going in and out of the cell. And so, notice that the central vacuole is not as filled up as it is over here with the hypotonic environment. And also notice that we have these gaps here in between the cell membrane and the cell wall, and that is not a preferred scenario for plant cells. So what we can do is we can place an x here to indicate that this is not really the ideal scenario for a plant cell. And then, of course, with hypertonic environments, cells are going to dehydrate just like a kid gets dehydrated. And that's, again, because the direction of water flow is leaving the cell. And so notice how shriveled up this plant cell is, and that's because it's being dehydrated, and this, again, is not going to be a good environment for the plant cell. And so moving forward, we'll be able to get some practice utilizing these concepts. So I'll see you guys in those videos.

What is the tonicity of the outside solution in comparison to the cell?

• A. Hypotonic
• B. Isotonic
• C. Hypertonic
• D. electrotonic

What direction will the water flow?

• A. Inside → Outside
• B. Outside → Inside
• C. Water flows in both directions.
• D. No flow of water.

Plants become turgor when placed in this type of solution:

• A. Hypotonic
• B. Isotonic
• C. Hypertonic
• D. Megatonic

Here’s what students ask on this topic:

What is osmosis and how does it differ from diffusion?

Osmosis is the diffusion of water across a semipermeable membrane, moving from a region of low solute concentration (hypotonic) to a region of high solute concentration (hypertonic). Diffusion, on the other hand, is the movement of any substance from an area of high concentration to an area of low concentration. While osmosis specifically involves water and a semipermeable membrane, diffusion can involve any type of molecule and does not require a membrane. Both processes aim to equalize concentrations, but osmosis is crucial for maintaining cellular integrity by regulating water balance.

How does tonicity affect the direction of osmosis?

Tonicity refers to the relative concentration of solutes in solutions separated by a semipermeable membrane. It affects the direction of osmosis by determining where water will move. Water always moves from a hypotonic solution (low solute concentration) to a hypertonic solution (high solute concentration) to dilute the solutes. For example, if the outside of a cell is hypotonic compared to the inside, water will flow into the cell, causing it to swell. Conversely, if the outside is hypertonic, water will leave the cell, causing it to shrink.

What happens to animal cells in hypotonic, isotonic, and hypertonic environments?

In a hypotonic environment, animal cells will swell as water enters the cell, potentially causing them to lyse (burst). In an isotonic environment, water moves in and out of the cell at equal rates, maintaining cell size and shape, which is the preferred state for animal cells. In a hypertonic environment, water leaves the cell, causing it to dehydrate and shrink, which can lead to cell death. Thus, isotonic conditions are ideal for animal cells to maintain homeostasis.

Why do plant cells prefer hypotonic environments?

Plant cells prefer hypotonic environments because the influx of water increases turgor pressure, which is the pressure of the cell membrane against the cell wall. This turgor pressure helps maintain the plant's structure and rigidity. Unlike animal cells, plant cells have a rigid cell wall that prevents them from lysing in hypotonic conditions. The central vacuole fills with water, pushing the cell membrane against the cell wall, which supports the plant's upright position and overall structural integrity.

What is osmotic pressure and how is it related to osmosis?

Osmotic pressure is the pressure required to prevent the flow of water across a semipermeable membrane during osmosis. It is a measure of the strength of osmosis; the higher the osmotic pressure, the greater the tendency for water to move into the hypertonic solution. Osmotic pressure depends on the solute concentration; higher solute concentrations result in higher osmotic pressure. This concept is crucial for understanding how cells regulate water balance and maintain their structural integrity in different environments.