Lesson video

- Hi everyone.

Mrs. Buckle here and I'm really excited to get started with our science lesson today.

All you're going to need is somewhere to write down your ideas, so a book and a pen or a digital device whichever works best for you.

And once you're ready, let's get started.

The title of today's lesson is "Osmosis", and this fits into the cell biology unit within science.

Today we're gonna look at what osmosis is and give a definition, and then we're going to see how this affects cells in both plants and animals.

Before we get started with today's learning, I just want to go over a couple of keywords that are going to pop up.

The first is concentration, which tells us about the number of particles in a given volume.

The next is partially permeable, particularly, we're going to be talking today about partially permeable membranes.

Partially permeable membranes are membranes that only allow certain substances to pass through them normally due to the size of the molecules.

And lastly, one word that I want to make is aware of is solution.

A solution is a mixture, but it is a mixture that contains a liquid solvent with a solute dissolved in it.

So this could be something like salt water where the solvent would be the water and the solute would be the salt.

The solution is a mixture of a solvent and a dissolved solute.

I've already mentioned this, but we are gonna do three things today.

We're going to start by defining osmosis, and then we're going to describe osmosis and its process within animal cells and implant cells too.

Let's get started with our definition then.

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

For us to really understand osmosis, we first need to know what is meant by a dilute solution and a concentrated solution.

A dilute solution is a solution which has a high concentration of water molecules compared to the concentration of solute molecules.

Solution on your screen is an example of a dilute solution because it has a large number of water molecules for every given solute molecule.

The opposite of a dilute solution is a concentrated solution.

In a concentrated solution, there is a low concentration of water molecules compared to the number of solute molecules.

We can see in both of these solutions, there is the same number of solute molecules, but the concentrated solution has much fewer water molecules.

Osmosis also takes place across a partially permeable membrane.

Remember, a partially permeable membrane is one which only allows some substances to pass through.

In this scenario, we can see the partially permeable membrane dividing this box.

I know that it is partially permeable because it has some gaps moving down the partially permeable membrane.

These gaps are small enough that water molecules would be able to pass through but too small for the solute molecules to be able to move across.

Therefore, this membrane is partially permeable because it is only permeable to the water molecules but not permeable to the solute molecules.

So to summarise, osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

Let's take a look at this taking place now.

In our example, we have a box separated by a partially permeable membrane.

On the left of the membrane is a much more dilute solution than on the right.

I know this because there is a higher number of water molecules for every solute molecule present in the solution.

Because there is a difference in concentration, osmosis would take place.

The water molecules would move from the left where there is a dilute solution to the right where there is a more concentrated solution.

This would happen as the water molecules move through the partially permeable membrane.

The water molecules would continue to move until the concentration of water became equal on either side of the membrane.

We would say that these solutions are now isotonic because there is the same concentration of water on each side of the membrane.

If we were to look at this in a beaker, we would actually be able to visualise the osmosis taking place by looking at the level of solution in the beaker.

For example, this beaker is separated by a partially permeable membrane.

The left of the membrane is much more dilute than the right of the membrane because there are more water molecules for every solvent molecule.

In this scenario, osmosis would take place, and water molecules would move from the dilute solution on the left to the concentrated solution on the right.

This would change the level of solution in the beaker.

Where water had been lost, the volume of solution would decrease, and where water had been gained the volume of the solution would increase.

We would see this by the solutions appearing to rise and fall as water were moving between the partially permeable membrane.

This can be modelled in our lab by using things like Visking tubing or membranes that have very, very fine mesh.

So we've had a look at the definition of osmosis.

Let's do some quick check for understanding questions.

Now I'm gonna read the question.

I then want you to pause the video and choose your answer.

And once you're happy with it, resume the video, and I will give you the answer.

First question, which molecule moves by osmosis? Well done if you said water.

Only water molecules can move by osmosis.

Next question, which type of membrane is required for osmosis to take place? Well done if you said partially permeable.

That takes us nicely onto task one for today.

There are three tasks for you to do in task one.

Task A is to give a definition of osmosis.

Task B is to look at the diagram showing water and solute molecules on either side of a partially permeable membrane.

I would like you to tell me what will happen to the water level on each side of the beaker.

And for task C, I would like you to draw particles in the beaker to represent a dilute solution and a concentrated solution.

If you are drawing circles for both your solute and your solvent, remember to include a key so that whoever is marking your work knows which particles are solute and which particles are solvents.

This should take you five to 10 minutes.

Pause the video here, and once you're ready to self-assess your work, come back and we will go through the answers.

Brilliant, welcome back.

Question A was asking us to define osmosis.

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

So two really key ideas here.

We've got water, and we've got dilute to concentrated.

Don't forget as well that we need to reference that partially permeable membrane to make sure we get full marks on those questions if it were to come up again.

Question B, what will happen to the water level on each side? The water this time will rise on the left and fall on the right.

This is because water molecules will move to the left from the dilute solution to the concentrated solution.

Draw particles in the beaker to represent dilute solution.

So there's a concentrated solution there.

I can see I've got four solute molecules, and I've got 1, 2, 3, 4, 5, 6, 7, 8, 9 water molecules.

For a dilute solution.

I've got significantly more water molecules within my beaker than I did solute molecules.

Remember, a dilute solution has a higher concentration of water molecules compared to the number of solute molecules.

So we've already defined osmosis.

We are now going to look at this taking place in animal cells.

So this is an animal cell.

I can see that because it's got an irregular shaped membrane, and it also doesn't have a cell wall.

Osmosis can happen in animal cells because the cell membrane that surrounds it is partially permeable.

There are spaces in an animal cell membrane that allow substances to move into and out of the cell.

The water concentration of cells is also constantly changing depending on dissolved substances in the cell.

For example, glucose, amino acids, or other ions such as sodium and potassium.

This means that the cytoplasm within the cell can be dilute or it can be concentrated depending on the dissolved substances that are present.

Let's take a look at this cell here.

So we're using the same model.

We've got red circles for our solute molecules and blue circles for our water molecules.

In the diagram, the concentration of water is higher on the outside of the cell.

We would say the outside of the cell is more dilute.

We would then, therefore, have osmosis taking place.

The water molecules would move into the cell by osmosis because the water molecules are moving from a diluted solution outside of the cell to a more concentrated solution inside of the cell.

In this scenario we have the opposite.

The solution inside of the cell is more dilute.

Therefore osmosis would still happen, but this time the water molecules would move from the dilute solution in the cell to the more concentrated solution outside of the cell.

Again, these water molecules would be moving through the partially permeable cell membrane.

Let's take a look at a more specific example.

This is an animal cell.

In fact, it's a red blood cell.

Red blood cells are contained within a solution called blood plasma.

That's how they're carried around the body and the solution blood plasma has water and other minerals and ions and substances dissolved in it.

The concentration of blood plasma, so whether it is a concentrated solution or a dilute solution, is constantly changing.

And it can change depending on how much water a person consumes through their food or through drinking, how much a person is sweating, and how much a urine a person is producing.

For example, if you were to consume a lot of water, your blood plasma would become more dilute.

However, if you ran a marathon and you were sweating lots, the concentration of your blood plasma would become more concentrated because you would be losing water from your body.

Because the concentration of plasma changes, that means that this can have an effect on the red blood cells that are within them.

So if the blood plasma is concentrated, the cytoplasm in the red blood cell will be more diluted than the solution it is held in.

Water would leave the red blood cell via the membrane by osmosis.

As the water leaves, this red blood cell would shrink.

The opposite can also happen in a dilute solution.

If the blood plasma would dilute, the cytoplasm in the red blood cell will be more concentrated than the solution it is held in.

Water will, therefore, enter the red blood cell through the cell membrane by osmosis.

The red blood cell will fill with water, and eventually, it will burst.

The pressure caused by the water inside of the cell cannot be withstanded by the cell membrane.

The cell will burst, and it will die.

There are also scenarios where a red blood cell would be in an isotonic solution.

In this scenario, the cytoplasm of the red blood cell will have the same concentration of water as the solution.

Here, water will enter and leave the red blood cell at the same rate.

The red blood cell does not appear to change its appearance because there is no net movement, overall movement of water.

There is no more water moving in.

The net is moving out.

The cell will not shrink, and it will not burst.

Let's just check we've understood what we've just talked about there then.

Question one, osmosis rarely happens in animal cells, true or false? Well done.

If you said false, can you now justify your answer? Well done, if you went for air.

B is not correct because if water was constantly leaving ourselves by osmosis, our cells would constantly be shrinking, and that wouldn't be very good for us at all.

So osmosis rarely happens in animal cells.

That's false.

It does happen quite a lot, and that's because our cell membranes are partially permeable, and therefore, provide the perfect opportunity for osmosis to take place.

Next question.

What will happen to a red blood cell in a dilute solution? Well done, if you said it will burst.

Remember, in a dilute solution, there'll be more water outside of the cell than inside of the cell.

Water will move from the dilute solution outside of the cell to the concentrated solution in the cytoplasm.

The water will fill up the cell and eventually the cell will burst.

That takes us nicely onto task two for today's lesson.

I've got a table here showing the structure of red blood cells in different solutions.

I want you to tell me what has happened to each red blood cell, and how has this happened? This should take you around five minutes.

Once you're happy with your answers and ready to check them, resume the video.

But for now, pause this and get started with your task.

Hi, welcome back.

Let's go through the answers.

So if the first cell has burst and this is because water has moved into the cell by osmosis.

The second cell, nothing appears to have happened to the cell.

So that means there's been no net movement of water into or out of the cell.

And lastly, the cell has shrunk.

This is because water has moved out of the cell by osmosis.

So far we have defined osmosis, and we've talked about osmosis taking place in animal cells.

The last thing for us to do today is to describe osmosis in plant cells.

Okay, before we get into looking at osmosis in plant cells, we just need to have a look at these two cells.

We've just talked about osmosis taking place in animal cells, but now we're going to look at plant cells.

The structure of a plant cell is much different to the structure of an animal cell, and therefore, when osmosis is happening, it will have a different impact.

Spend two minutes now to compare the structure of a plant cell to that of an animal cell.

There are lots of differences between a plant and an animal cell.

The main ones that we need to look at today, and the fact that although animal and plant cells both have a cell membrane, only plant cells have a cell wall, and the presence of the cell wall means that when osmosis makes changes to a plant cell, the results that we see are much different to when osmosis is taking place in an animal cell.

If we were to place a plant cell in a concentrated solution, the cytoplasm of the cell will be more dilute than the solution.

Water will leave the cell via the cell membrane just like it is in an animal cell.

This is osmosis taking place.

The water is moving from a dilute solution in the cytoplasm to a more concentrated solution out of the cell.

The cell wall becomes flaccid.

There is less pressure being placed upon it from inside of the cell.

This means it can start to lose its shape very slightly.

If osmosis continues to happen, then the cell membrane actually comes away from the cell wall, and we say that the cell is plasmolysed.

In a dilute solution, the cytoplasm in a plant cell is more concentrated than the solution.

Water will enter the cell by the cell membrane.

This is just osmosis taking place.

Water is moving from a dilute solution outside of the cell to a more concentrated solution in the cytoplasm.

This will fill up the cell cytoplasm with more water and the cell contents begin to push against the cell wall.

The pressure inside of the cell becomes much, much greater.

We say now that the cell has become turgid.

In an isotonic solution, the cytoplasm in a plant cell will have the same concentration as the solution.

There won't be a solution which is more dilute so water will enter and leave the cell at the same rate.

The cell looks to appear the same.

It appears that there are no changes taking place within the cell.

Let's do some quick check for understanding questions then.

First one, true or false osmosis has different effects on plant cells than animal cells.

Well done if you said true.

Now, can you justify your answer? Fantastic.

If you said B, plant cells have a cell wall the animal cells don't.

Remember, both cells have a cell membrane and that membrane is partially permeable which means that osmosis can take place.

Next one.

What happens to a plant cell in a concentrated solution? Well done if you said the cell becomes flaccid, this is because the cytoplasm will be more dilute than the solution.

So water will move out of the plant cell and the cell will become flaccid.

If that was to continue, the cell will become plasmolysed.

For task three, the final task of today's lesson, what I would like you to do is complete the table similar to what we did for the red blood cell.

Describe what has happened to each of the cells and how this has happened.

Again, this should take you roughly five minutes.

Once you're happy with your answers, come back to me, and I will go through them.

The first cell, the cell has become turgid.

I know this because I can see that the vacuole is much, much bigger.

This is because water has moved into the cell by osmosis.

The second cell, nothing appears to have happened to the cell which means that there has been no net movement of water into or out of the cell.

And the last one, the cell has become flaccid or plasomolysed, and this is because water has moved out of the cell by osmosis.

Now I want us to apply what we've learned about osmosis implant cells to an investigation in the lab.

Let's look at this together.

The student decides to investigate the effect of osmosis on a potato chip.

They take a potato and create a potato chip.

They place a piece of potato in the solution of salt water.

They leave the potato for around 20 minutes, and we can see the results below.

The first thing I would like you to do is describe and explain the results of the experiment in the image above.

Write your answer as a full sentence.

Once you're happy, come back.

Okay, so your first task was to describe and explain the results of the experiment.

I can first describe the results by saying that the potato has increased in length.

I would then explain this by saying that water has entered the potato cells, and the potato cells have become turgid.

I could further explain this by saying that this must mean that the concentration of water inside the cells is lower than the concentration of water in the solution.

Or the idea that the solution is more concentrated than the cytoplasm of the potato cells.

Imagine now that that person has repeated this experiment, and we've got the results below.

We can see that they've repeated it at different concentrations of water from a 100% to 20%.

We've been given the length of the potato before and the length after.

What I would like you to do is complete the table.

I've done the first and last result for you.

You should use these as a model when completing the rest of the results.

Pause the video here, and come back to check your answers.

Okay, here are the answers for this.

Make sure you've got the positive and negative symbol to show that we have either got an increase in length of the potato or a decrease in length in potato.

This is really important when we're presenting our results in this kind of experiment.

Okay, now we have the results.

I want you to tell me, what was the concentration of water inside the potato cells? You need to use the results that you have just completed to tell me how you know.

Again, pause the video here, and come back when you're ready to check your answer.

What was the concentration of water inside the cells, and how do we know? The concentration of water in the cells is 60%.

This is because the potato chip has not changed in length.

Therefore, there must have been no osmosis or net movement of water molecules taking place.

This can only happen if the solution inside and outside of the cell has the same concentration of water.

Some really tricky questions there.

It's possibly one of my favourite experiments to do within the lab as well.

So if you do get chance, I would definitely suggest having a go at that experiment.

What we're gonna do now is just summarise what we've learned today about osmosis.

We started by looking at the definition.

So the definition of osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

If water moves into an animal cell by osmosis we've seen that the cell can burst.

But if water moves out of the animal cell by osmosis, the cell will shrink.

However, the changes in plant cell is much different, and this is because of the presence of a cell wall.

So if water moves into a plant cell by osmosis, the cell becomes turgid.

If water moves out of a plant cell by osmosis, the cell becomes flaccid, and the membrane shrinks away from the cell wall.

Really well done today.

I hope you've learned a lot about osmosis.

You've done some fantastic work on lots of different scenarios, looking at how water's moving into and out of cells.

I hope you have a lovely rest of your day, and I look forward to seeing you for some more science in the future.