Osmosis: Study with Video Lessons, Practice Problems & Examples

Osmosis is the passive diffusion of water across a semipermeable membrane, influenced by tonicity, which refers to solute concentration. Solutions can be hypotonic (lower solute concentration), isotonic (equal solute concentration), or hypertonic (higher solute concentration). Water moves from hypotonic to hypertonic solutions to achieve equilibrium. In hypotonic environments, animal cells may lyse, while plant cells thrive due to turgor pressure. Isotonic conditions maintain cell size, and hypertonic environments cause dehydration. Understanding these concepts is crucial for grasping cellular behavior in varying environments.

Osmosis

In this video, we're going to introduce osmosis. Osmosis is a type of passive diffusion, which means that absolutely no energy is required for osmosis to occur. Osmosis is defined as the passive diffusion of a solvent across a semipermeable membrane. Recall that solvents are substances that dissolve other substances, and usually in biology, the solvent is going to be water.

We can define osmosis as the passive diffusion of water across a semipermeable membrane such as a biological membrane or a cell membrane. The direction that water will flow across that semipermeable membrane will depend on the tonicity of the solution. Tonicity is defined as the relative concentration of solute dissolved in the solutions. When it comes to tonicity, there are three terms you should know, and they all end in 'tonic', referring to tonicity.

The difference between these three terms is the prefix that comes before. The first term is 'hypotonic'. 'Hypo' is a prefix that means low. Hypotonic solutions have lower solute concentrations. The second term is 'isotonic', where 'iso' means equal, indicating isotonic solutions have an equal solute concentration. The third term is 'hypertonic'. While 'hyper' sounds like 'hypo', it actually means higher. Hypertonic solutions have higher solute concentrations. 'Lower', 'equal', and 'higher' are words of comparison, requiring at least two regions for a meaningful comparison.

Typically, we are comparing the inside of the cell, which is region one, and the outside of the cell, or the surrounding solution, which is region two. For example, in our scenario, notice that in the first case, there are only two green solutes outside the cell while a higher concentration is inside. The outside solution is hypotonic because it has a lower solute concentration compared to the inside which is higher. Moving to the second scenario, the outside solution has a concentration nearly equal to the inside. Therefore, the outside is isotonic with respect to the inside.

In the final scenario, the outside solution has a much higher concentration than the inside. Thus, the outside is hypertonic. Consequently, the inside in the first image is hypertonic compared to the outside's hypotonic state. In the middle scenario, the inside is isotonic, matching the outside's solute concentration. Finally, in the last image, the inside has a lower concentration than the outside, making it hypotonic.

This concludes our introduction to osmosis, and as we move forward, we'll be able to continue talking more about tonicity and the direction of water flow. I'll see you guys in our next video.

Osmosis Example 1

Alright. 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, of course, when we look at option d, it says electrotonic, which is not a word that we described in our last lesson video. So for sure, we know that we can eliminate answer option d.

But hypotonic, isotonic, and hypertonic are all terms that we talked about in our last lesson video, so it's going to be between one of these three answers. And so when we take a look at this image down below, what we need to notice is that there is a beaker, and the beaker has an outside solution, here, and then it has a cell, a red blood cell right here in the middle. And so this image is indicating that the outside solution is 10% solute and it indicates that inside of the red blood cell is 0.1% solute. And so what we need to realize is that 10% and 0.1%, when we compare these two numbers, 10% is higher. It is a higher solute concentration and 0.1% solute is lower.

And so recall from our last lesson video that isotonic is a word that means equal solute concentrations. But because 10% solute is higher than 0.1% solute, they are not equal to each other. 10% is not equal to 0.1%. So we know that we can eliminate answer option b, isotonic. It would be isotonic if they both had the exact same percentage of solute, the same concentration of solute.

And so what we need to realize here is that this problem is specifically asking us to label the tonicity of the outside solution. So we need to focus on the outside solution. The outside solution is 10% solute. It is higher solute concentration. And so recall the word that means higher solute concentration is hypertonic. Hypertonic means higher solute concentration, and so that means that the outside solution, the one that we're trying to label here, is going to be hypertonic with respect to the inside of the cell. So the correct answer here is going to be answer option c. And so this, of course, means that the inside of the red blood cell is going to be hypotonic. So inside here, we can say it is hypotonic, and the outside here, we can say is hypertonic. And so, again, we're labeling the outside solution, so it will be hypertonic and option a is not going to be correct for the outside solution.

So, option c, hypertonic, is the correct answer for this example, and I'll see you guys in our next video.

Direction of Osmosis

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. And so what we need to recall from our previous lesson videos is that biological membranes are semi-permeable, which means that some things can get across the membrane, but other things cannot get across the membrane. And so if the solutes that are in a solution cannot diffuse across the membrane from areas of high concentration to areas of low concentration, then osmosis is going to occur. And recall, osmosis is the diffusion of water across the membrane. And so here's what you guys need to know.

Water will always move from hypotonic solutions towards hypertonic solutions. This is the direction of water movement, from hypotonic towards hypertonic solutions. This movement allows water to move towards the more concentrated solution of solute in order to dilute that solution and it will continue to move towards that solution until it becomes isotonic. Essentially, the 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? But here you're telling me that it's diffusing from low concentration to high concentration. So how does that make sense? It's important to keep in mind that hypotonic and hypertonic are terms that refer to the solute concentration. However, water is not the solute, water is the solvent. And so it's important to keep in mind that water is still moving from areas of higher concentration of water towards lower concentrations of water. Let's take a look at this in a little bit more depth here to clear this up. Hypotonic solutions do have lower solute concentrations as described in our last lesson video. However, hypotonic solutions, even though they have lower solute concentrations, they actually have higher water concentrations.

Hypertonic solutions do have higher solute concentration as described in our last lesson video. However, hypertonic solutions, although they have higher solute concentrations, they have lower water concentrations. And so what's really important to just realize here is that water is the solvent, whereas solutes are not the solvent. And so, these terms hypo and hyper are referring to the concentrations of solute. However, water is not the concentration of solute and water is still going to be moving from areas of higher concentration of water towards lower concentrations of water.

So once again, let's take a look at our image down below to try to clear this up even further. And so notice what we have here down the middle is a biological membrane, which is semipermeable, meaning that some things can cross the membrane but other things cannot cross the membrane. And so in a scenario where the solutes cannot cross the membrane, although the solutes would love to diffuse from higher concentration to lower concentration across the membrane, they can't because they're being blocked. And so in some scenarios, if the solutes cannot diffuse across the membrane, instead of the solutes diffusing from high to low concentration, because they can't, what's going to happen is water is going to move instead. And in areas that are hypotonic, yes, they have lower concentration because hypotonic is referring to the lower solute concentration, but it turns out that they actually have a higher water concentration.

They have higher water. And hypertonic solutions, yes, they have a higher solute concentration because remember hypertonic is referring to the solute concentration being higher. However, higher solute concentration means that it's going to have lower water concentration. And by the way, the brackets that you see here, just mean the concentration of. And so when you see something in brackets like H₂O in brackets, it means the concentration of water, and solute in brackets means the concentration of solute.

And so remember what we said, what you need to know is right here. Water will always move from hypotonic towards hypertonic solutions. And so over here on the left, we have hypotonic, and over here on the right, we have hypertonic. So water is always going to flow in this direction. And so if you're able to remember, water flows from hypotonic towards hypertonic, then you'll be good on most of your osmosis questions.

And so we'll be able to get some practice applying the concepts that we've learned here. But once again, keep in mind, water always flows from hypotonic towards hypertonic solutions. So that being said, I'll see you all in our next video.

Osmosis is best defined as the movement of:

• A. Water molecules across a semi-permeable membrane into a region of low solute concentration
• B. Solute molecules from an area of high concentration to an area of lower concentration
• C. Water molecules across a semi-permeable membrane into a region of high solute concentration
• D. Water molecules inside a cell that can't be transported out
• E. Solute molecules from an area of low concentration to an area of higher concentration

Which direction would you expect water to move across the cell membrane?

• A. Into the cell
• B. Out of the cell
• C. Into the cell and out of the cell at equal rates
• D. Water will not move across the cell membrane

Environmental Tonicity Affects Cells

So in our previous lesson videos, we introduced tonicity and the direction of osmosis across a membrane. How water flows from hypotonic solutions towards hypertonic solutions. In this video, we're going to talk about how the environmental tonicity affects cells. We're mainly going to focus on the outside environment that surrounds a cell. If the outside environment surrounding a cell is hypotonic, then in this scenario, water is going to enter the cells and cause the cells to swell up or enlarge like a hippo. Hypotonic environments cause cells to swell up or enlarge like a hippo. For animal cells that do not have a cell wall, they could potentially lyse or burst under these conditions. Think of it like a balloon. If the cell was a balloon, if you blow too much air into a balloon then it's going to expand, enlarge, or swell like a hippo. But if you blow too much air into it, then it could potentially pop open. With cells, if too much water enters, they could swell up too much, and if they swell up too much, they could potentially lyse or burst. For animal cells that do not have cell walls, hypotonic environments are harmful as they could lead to cell lysis or bursting, which could kill them. However, for plant cells that do have cell walls, the cell walls prevent the expansion of the cell membrane, protecting it from lysing or bursting open. Plant cells actually prefer hypotonic environments because they do not lyse or burst open. When water enters plant cells it increases what is known as turgor pressure, defined as the pressure of the water on the cell membrane, which allows plants to maintain their upright and healthy structure. Let's clear up some of these ideas by focusing mainly on the left-hand side of the image below. Notice in the top part of the image, we show an animal cell in a hypotonic environment where water moves inside, causing expansion and potential lysis, killing the cell. Here we are showing animal cells do not prefer hypotonic environments. Below, observe a plant cell where the cell wall surrounds the cell membrane, preventing lysis. Plant cells prefer hypotonic environments, which leads to high turgor pressure allowing plants to maintain an upright healthy structure. Now, let's look at the second type of environment. If the outside environment surrounding a cell is isotonic, then, in this scenario, water will enter and exit the cell at equal rates, so the cell will not change in size. This is preferred by animal cells, like our blood cells shown here in the middle, where the outside environment is isotonic. Water entering and exiting keep the cell size stable. For plant cells, in an isotonic environment, water will also enter and leave at equal rates, and the cell will not change in size. However, there are gaps between the cell membrane and the cell wall, indicating a low amount of turgor pressure. Plants do not prefer isotonic environments as it does not allow for the high turgor pressure needed for their upright healthy formation. Finally, let's look at hypertonic environments. Here, water exits cells, akin to deflating a balloon, causing the cell to get smaller and shrink as they dehydrate, similar to how a hyperactive kid gets dehydrated. Observe the image on the right hand side, where the outside environment surrounding the cell is hypertonic, causing water to leave, leading to shrinking and dehydration of the cells shown. This means animal cells do not prefer hypertonic environments due to the risk of lethal dehydration. Below with the plant cell, water leaving causes a drop in turgor pressure, leading plants to lose their structure and wilt, being harmful to both animal and plant cells. Neither prefer hypertonic environments. This concludes our lesson on how environmental tonicity affects cells, providing a basis to practice applying these concepts as we move forward in our course. See you all in our next video.

Plants become turgid when placed in this type of solution:

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

What would you expect to happen to the cell under the following conditions?

• A. The cell will swell
• B. The cell will lyse
• C. The cell will shrivel
• D. The cell will stay the same

Here’s what students ask on this topic:

What is osmosis and how does it differ from diffusion?

Osmosis is the passive diffusion of water across a semipermeable membrane, requiring no energy. It specifically involves the movement of water from an area of lower solute concentration (hypotonic) to an area of higher solute concentration (hypertonic) to achieve equilibrium. Diffusion, on the other hand, is the movement of solutes from an area of higher concentration to an area of lower concentration. While osmosis focuses on water movement, diffusion can involve any type of molecule moving down its concentration gradient.

How does tonicity affect the direction of water movement in osmosis?

Tonicity refers to the relative concentration of solutes in solutions. Water moves from hypotonic solutions (lower solute concentration) to hypertonic solutions (higher solute concentration) during osmosis. This movement aims to equalize solute concentrations on both sides of the semipermeable membrane. For example, if the outside of a cell is hypertonic, water will exit the cell, causing it to shrink. Conversely, if the outside is hypotonic, water will enter the cell, causing it to swell.

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

In a hypotonic environment, animal cells may lyse (burst) due to excessive water intake, while plant cells thrive due to increased turgor pressure. In an isotonic environment, both animal and plant cells maintain their size as water enters and exits at equal rates. In a hypertonic environment, animal cells shrink and dehydrate, and plant cells lose turgor pressure, causing them to wilt. Thus, animal cells prefer isotonic environments, while plant cells prefer hypotonic environments.

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 pressure helps maintain the plant's upright and healthy structure. Unlike animal cells, plant cells have rigid cell walls that prevent them from lysing (bursting) when they take in excess water. This structural advantage allows them to thrive in hypotonic conditions.

How does osmosis contribute to maintaining cell homeostasis?

Osmosis helps maintain cell homeostasis by regulating the water balance within cells and their surrounding environment. By moving water from areas of low solute concentration to high solute concentration, osmosis ensures that cells do not become too swollen or too shriveled. This balance is crucial for proper cell function, nutrient absorption, and waste removal. In plant cells, osmosis also maintains turgor pressure, which is essential for structural integrity and growth.