Hi, I'm Mrs. Hudson, and today I'm going to be teaching you a lesson called Diffusion through a selectively-permeable membrane: practical.

This is a biology lesson and it comes under the unit titled Diffusion.

The outcome of today's lesson is I can explain observations of diffusion through a selectively-permeable membrane.

There will be some key words in today's lesson, and those words are selectively-permeable membrane, particles, diffusion, concentration gradient, and net movement.

Let's have a look at what those words mean.

A membrane is selectively-permeable if some substances can move through it, but others cannot.

Particles, all substances are made up of particles that are too small for us to see.

Diffusion.

Diffusion is the net movement of particles of a substance down a concentration gradient.

A concentration gradient is a difference in the concentration of particles from one area to another.

And net movement is the overall movement of particles from one area to another.

If you need to pause the video to make a note of those keywords, please do and then press play ready for me to keep going.

Today's lesson is going to be split up into two different parts.

In the first part of the lesson, we're going to be looking at diffusion through a membrane, and then in the second part we'll be using that knowledge to predict, observe, and explain.

Let's get going with the first part of the lesson, diffusion through a membrane.

So on this slide we can see the model of an animal cell, and on the very outside of that animal cell is the cell membrane.

The cell membrane is a selectively-permeable membrane.

What this means is it has holes in it that allow particles of substances to move into and out of the cell by diffusion.

So if you look at that animal cell, the cell membrane is that sort of dotted line around the outside, and we can see that it's selectively-permeable.

It's got little holes inside of it that allow some substances to diffuse in and out, but others can't.

So those arrows here are representing the movement and the diffusion of particles into and out of the animal cell.

The holes in the cell membrane are just big enough to let particles of some, but not all substances through, but the holes are also not so big that everything in the cell can escape.

So some examples of substances that might diffuse in and out are carbon dioxide and oxygen, whereas substances like larger protein molecules will be too big to be able to diffuse through the selectively-permeable membrane.

And we can see a little bit more detail, some examples in this diagram here.

So again, we've got the animal cell with the selectively-permeable membrane, and we've kind of magnified that onto this diagram here.

So we've got a key showing that the orange brown dash lines are the cell membrane, and then we've got three different molecules inside and outside of the cell.

Particles of some substances are too large to diffuse through the holes in that selectively-permeable membrane.

So in this diagram here, we can see that the protein molecule is very large and therefore the protein molecules inside of the animal cell will be unable to diffuse through the holes of the selectively-permeable membrane.

So there we can see it just is too big to get through that cell membrane.

Particles of substances that are small enough to move through the holes move through in both directions.

So here you can see that the oxygen molecules are small enough to fit through the cell membrane, and they will move in both directions, both out of the cell and into the cell.

And so looking here, we've got a similar diagram where we have some oxygen molecules and a cell membrane inside of that rectangular box.

And when there is a concentration gradient across the membrane, two things will happen.

A few particles diffuse through the holes from low to high concentration, which is what we can see with that arrow there.

So there's a low concentration on the right hand side of that rectangle and a high concentration on the left hand side.

But because particles are always moving randomly in all directions,.

Some particles, only a few, will travel from low to high concentration, whereas many more particles diffuse from high to low concentration, which is what we can see now with those arrows on the diagram.

The net movement, however, is from high to low concentration.

What net movement means is the overall movement of particles.

So if you take into consideration movement from low to high and from high to low, overall more particles are moving from high concentration to low concentration, and that is called the net movement.

Let's check our understanding so far.

So we've got a diagram here, which is showing you a cell membrane and then some particles either side of that cell membrane.

In which direction would carbohydrate particles diffuse through this cell membrane? A, in both directions.

B, in neither direction.

C, only from side A to side B.

Or D, only from side B to side A.

So for this one, we should have B in neither direction.

The carbohydrate particles are those very large particles, and they are just too big to pass through the selectively-permeable membrane, so they will not move through the cell membrane.

Let's try another one.

We've got the same diagram here, but the question is different.

In which direction would salt particles diffuse through this cell membrane? A, in both directions? B, in neither direction.

C, only from side A to side B.

Or D, only from side B to side A.

And the answer to this one is A, in both directions.

We're not talking here about the net movement, we're just talking about diffusion in general.

And remember that there will be movement from low to high concentration as well as from high to low concentration.

It's just that the net movement is from high to low concentration.

We can observe diffusion through a manmade, selectively-permeable membrane.

A really good example of this is a tea bag in cold water without stirring.

So the reason we're doing it in cold water without stirring is to show that diffusion is a passive process.

You don't need to have hot water and you don't need to stir in order for diffusion to take place.

So we can see here we've got a cup of cold water with a teabag in it.

And what will happen after several hours is that the tea will end up looking like this.

So let's have a think about what might be happening.

So the teabag itself is a selectively-permeable membrane.

That means it's got small holes in it that allow some substances to pass through.

The bag itself contains tea leaves.

The tea leaves contain particles of tea.

The leaves are too big to fit through the holes, but the tea particles are small enough to diffuse through.

The net movement of tea particles is down the concentration from inside of the bag through the membrane into the water.

So what we're saying is that the tea particles inside the teabag are at a high concentration, and there's a low concentration of tea particles in the water.

So the net movement of tea particles will be from inside the teabag to the outside of the teabag where the water is.

Now over time, the tea particle concentration outside of the teabag will increase, and so there will still be movement of tea particles from low to high concentration as well as from high to low.

Diffusion will never stop happening.

Let's do another check for understanding.

So this is a true or false question.

Diffusion through a membrane only happens when the solution is heated and stirred.

True or false.

And then justify this answer.

A, diffusion requires energy to be provided from heating or stirring.

Or B, diffusion happens due to the constant random movement of particles without extra energy being provided.

This is false.

Diffusion doesn't have to have heat or be stirred in order to occur, and the justification is B.

And this is because diffusion happens due to that constant random movement of particles without extra energy being provided.

It's a passive process.

Well done if you got those right.

We're now ready to move on to the first task of the lesson.

So Aisha and Izzy try to explain the experiment involving a teabag in cold water.

Aisha says the bag is fully permeable, so tea leaves can diffuse through it into the water.

And Izzy says tea particles are more concentrated outside the teabag, so there is a net gradient.

So this is the task.

Each person has made mistakes.

You need to rewrite each person's statement so that it is correct.

I'm sure you're gonna do a great job of this.

Pause the video and then press play when you're ready for me to go through the answers.

Let's have a look at how we did.

So we're going to start with what Aisha said.

Aisha said the bag is fully permeable, so tea leaves can diffuse through it into the water.

The mistakes that she's made is that she said the bag is fully permeable and that the tea leaves can diffuse through it.

So if we were going to correct Aisha's statement, we would say the bag is selectively-permeable so the tea particles can diffuse through it into the water.

Remember, selectively-permeable means only some substances can pass through, and the tea leaves were just too big to pass through the selectively-permeable membrane.

Let's look now what Izzy said.

Tea particles are more concentrated outside the bag, so there is a net gradient.

Well, Izzy has said that the tea particles are more concentrated outside, which is incorrect, and that there's a net gradient.

So if we wanted to correct Izzy's answer, we would say tea particles are more concentrated inside the bag, so there is a concentration gradient.

Now let's move on to the second part of task A.

So Lucas and Laura try to explain what happens as time passes during the experiment.

Lucas says, after a few minutes, tea particles are only moving in one direction, out of the bag, down the concentration gradient.

Laura says, eventually when tea particles have moved all the way through the water diffusion will stop.

Each person has made a mistake again.

Can you rewrite each person's statement so that it is correct? Give this a go and then press play when you're ready for me to feed back on the answers.

Right, let's look at what Lucas said first.

So Lucas has got wrong the fact that the tea particles are only moving in one direction.

So if we wanted to rewrite what Lucas had said to make it correct, we would say after a few minutes, tea particles move in both directions into and out of the bag.

Net movement is down the concentration gradient.

And then Laura, eventually when the tea particles have moved all the way through the water diffusion will stop.

It's the idea that diffusion will stop that is incorrect.

So if we wanted to correct that, we would say eventually when tea particles have moved all the way through the water diffusion continues into and out of the bag.

An amazing job if you manage to get all four of those correct.

Well done.

Let's move on to the next part of our lesson.

So now we've learned about diffusion through a membrane.

Let's use that knowledge to predict, observe, and explain.

Let's get going.

We can make a scientific prediction about what we might observe when particles diffuse through a man-made, selectively-permeable membrane.

Now remember, a scientific prediction is a testable statement about a possible outcome of an experiment.

For example, particles of a substance will diffuse through the membrane from side A to B down the concentration gradient.

That is a testable scientific prediction.

So we are going to observe an experiment that looks a little bit like this picture here.

And the apparatus that we are going to use is a beaker, tubing.

The tubing is a selectively-permeable membrane.

It has holes in it that only small particles can move through.

And the materials we're going to use, we've got iodine in the water.

So the the yellow colour is the iodine, and that has been put inside some water.

And iodine is made of very small particles.

So if the iodine particles are very small, they should be able to pass through the selectively-permeable membrane.

And then inside of the tubing, we've got starch in water.

Starch is made of very large particles, so we might assume that they're too large to pass through the selectively-permeable membrane.

Iodine is an indicator used to test for the presence of starch.

So iodine is usually an orange or brown colour.

So if there's no starch present, iodine will have this orangey brown colour.

Whereas if iodine is present, then it will turn a blue black colour.

So in our experiment, if the iodine is orange, what that means is there's no starch present.

Where as if the iodine turns blue or black, that means that there is starch present.

Let's check our understanding.

So here we've got an image of our apparatus.

We have got our tubing, which has got starch in water, and then the beaker, which contains iodine and water and our selectively-permeable membrane.

Predict what will happen to the liquid inside the tubing.

A, it will stay colourless.

B, it will change to orange brown.

Or C, it will change to blue black.

The answer is it will change to blue black.

And the reason this will happen is because the iodine particles in the water will be able to diffuse through the selectively-permeable membrane into the tubing and in that tubing there is starch.

So therefore the iodine will turn a bluey black colour because of the presence of starch inside the tubing.

Let's look now, this is the same apparatus.

Predict what will happen to the liquid outside of the tubing.

A, it will stay orange brown.

B, it will change to blue black.

Or C, it will change to colourless.

The answer to this one is it will stay orange brown.

The reason for this is that the starch molecules are too big to pass through the selectively-permeable membrane.

So therefore, even though the iodine is moving into and out of the tubing, there won't be any starch present outside of the tubing to turn that beaker blue black.

Well done with those questions.

Now we've got our scientific prediction, which is the liquid inside the tubing will change to blue black and the liquid outside the tubing or stay orange brown.

We're going to watch a demonstration now to observe what happens.

Remember, our scientific prediction has to be testable.

We need to be asking ourselves, did the observations from the experiment support our prediction? Let's watch the video.

Observing diffusion through a man-made, selectively-permeable membrane.

So what we can see here is we've got our beaker, which has got water and iodine in the water, and then we've got our tubing, which has starch and water in it, and that tubing is been placed inside of the water.

Now, the tubing has got a selectively-permeable membrane, which means that it only allows small particles to pass through and not large.

The particles of iodine are small and therefore they are able to diffuse into and out of the tubing.

The starch inside of the tubing is very large, and therefore starch particles cannot diffuse out of the tubing into the water.

So what will happen over time is the random passive movement of particles in all directions will mean the particles of iodine are diffusing from a high concentration in the water to a low concentration in the bag.

The net movement will go from high to low concentration.

Remember though, there will still be some movement from low concentration to high concentration.

And because the iodine has diffused into the tubing where there's starch, we can see now that the tubing is starting to go a blue black colour.

And this is because iodine turns blue black colour in the presence of starch.

You will also notice that the iodine in the water is still remaining the orangey brown colour and this is because no starch molecules are able to diffuse outta the tubing.

Now that we've watched that video, let's check our understanding.

Who correctly describes the observations from the experiment? A, Aisha, the liquid outside the tube changed to blue black and the liquid inside stayed orange brown, B, Izzy, the liquid inside the tube changed to blue black, and the liquid outside stayed orange brown.

Or C, Lucas, the liquid inside and outside the tube both changed to blue black.

Who is correct? Well, this is B Izzy and Izzy's observations match our prediction.

So our prediction was correct.

Let's think about that prediction now in terms of what is actually happening in the experiment.

So who correctly explains the observations from the experiment? Aisha, A, particles of starch were small enough to diffuse through the tubing into the water with iodine.

B, Izzy, particles of water diffuse through the tubing in both directions and mixed it all up.

Or C, Lucas, particles of iodine were small enough to diffuse through the tubing into the water with starch.

Who is correct? This is C, Lucas.

It was the fact that the iodine particles were small enough to diffuse through the tubing into the water with starch, and therefore, because there was a presence of starch, the inside of the tubing turned a blue black colour.

We're now ready to move on to the second task of our lesson.

So some students have set up an experiment, and this experiment looks very similar to what we have just been doing.

And there's some tubing.

The tubing is a selectively-permeable membrane.

It has holes in it that's only small particles can move through.

There's a beaker with some water in.

And then inside the tubing you've got starch in water.

And what the students have done is that they have added iodine into the tube.

So this is slightly different to what we did in our last experiment where there was iodine in the water.

In this example, they've added iodine into the tube.

So what you need to do is look at these pictures here and say, what is the most likely outcome of the experiment A, B, or C? And then for number two, write a prediction in words for the outcome of the experiment.

And then three, explain the scientific ideas behind your prediction.

So for number three, we need to talk about what particles are moving, where are they moving from and to, and why.

I'm sure you're gonna do a really great job of this.

Pause the video and then press play ready for me to go through the answers.

Okay, let's have a look at how we did.

So which one is the most likely outcome of the experiment? You should have had B, but the inside of the tubing is the blue black colour, and the water has this orangy brown colour.

Write a prediction in words for the outcome of the experiment.

The liquid inside the tubing will change blue black and the liquid outside the tubing will change to orange brown.

And then finally, explain the scientific ideas behind your prediction.

Iodine turned blue black inside the tube because starch is present.

Particles of iodine, were small enough to diffuse through the holes in the tubing into the water outside the tube, but particles of starch were too large to diffuse through.

Therefore, the liquid in the beaker will be orange brown.

You might not have that exactly the same word for word, but maybe pause the video and just check that you've got the main points in your answer.

It's the idea that the starch is too big to escape the tubing, so therefore the iodine is small enough to pass through the selectively-permeable membrane so the water turns orangey brown.

But because of the presence of starch inside the tubing, then that will turn blue black.

That's the end of today's lesson.

You've done an absolutely fantastic job, well done.

Let's summarise everything that we have learned.

So today we've been looking at Diffusion through a selectively-permeable membrane: practical.

And we said particles of some substances are small enough to move through holes in a selectively-permeable membrane by diffusion.

Particles of these substances move through the selectively-permeable membrane in both directions.

Remember, the movement of particles is passive and random and all directions.

The net movement of these particles is down a concentration gradient.

So the net movement is the overall movement of particles and overall they move down a concentration gradient from high to low concentration.

A scientific prediction is a testable statement about a possible outcome of an experiment.

We can make a scientific prediction about what we might observe when particles diffuse through a man-made, selectively-permeable membrane.

And we looked at that today with tubing in a beaker of water and using iodine to test for starch.

You've done a brilliant job this lesson.

Well done, I'm looking forward to seeing you next time.