Lesson video

Hi, I'm Mrs. Hudson, and today I'm going to be teaching you a lesson called "Diffusion Through the Cell Membrane." This is a biology lesson, and it comes under the unit titled "Diffusion." The outcome of today's lesson is I can explain why some substances can diffuse through the cell membrane and others cannot.

So we're going to be asking ourselves, "What is the cell membrane and what is its job?" There will be some key words in today's lesson, and they are cell membrane, particles, diffusion, selectively permeable, and net movement.

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

So all cells are surrounded by a membrane that controls what can enter and leave the cell, and that is the cell membrane.

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

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

Selectively permeable, a material is selectively permeable if some substances can move through it but others cannot.

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

If you need to write down these words in their definitions, please pause the video now.

If not, let's keep going.

So today's lesson is going to be split up into two parts.

In the first part, we're going to look at substances under the cell membrane, and then we're going to use that knowledge to look at in and out of a cell.

Let's get going with the first part though, substances and the cell membrane.

All organisms are made of one or more cells.

So here we've got a picture of a leaf, and on that leaf there's a little caterpillar having a munch.

And if we magnified that leaf, what we would see is variety of different plant cells because the leaf is made up of lots of different cells.

Same for the caterpillar.

That's a whole organism that we can see.

But if we magnified it, what we would see are lots of different animal cells.

So organisms are made up of one or more cells.

Cells also have some common parts.

So here we've got an animal cell, and we've got the plant cell.

So for example, the animal cell here would belong to the caterpillar, and the plant cell might be from the plant that we just saw on the previous slide.

And there are common parts.

So the first common part is the genetic material.

Now actually on this diagram, what we've got labelled is the nucleus.

And the reason why is because the genetic material is stored in the nucleus in animal and plant cells.

Let's see if you can recognise the next labels.

What do you think they're pointing towards? Well done if you said the cytoplasm.

So the cytoplasm is the jelly-like liquid that surrounds all of the subcellular structures.

It's where substances are stored and many chemical reactions take place.

And then finally, what are these lines pointing towards? This is pointing towards the cell membrane.

Now the cell membrane is the structure that surrounds the cell, and it can control what enters and leaves the cell.

You might notice on the plant cell that the cell membrane is actually the second layer on the outside because the plant cell has also got a cell wall, but the animal cell, it is the outermost layer of that animal cell.

Let's do a quick check for understanding.

"Which structure surrounds all cells? A, cell membrane; B, cytoplasm; or C, nucleus." The answer for this one is A, the cell membrane.

Well done if you got that right.

Next question.

"What is the function of the cell membrane? A, controls what can enter and leave the cell; B stores the genetic material, the DNA; or C, where substances are stored and chemical reactions take place." This is A, the cell membrane controls what can enter and leave the cell.

Great job if you got that right.

Let's look a little bit more at the cell membrane.

So the cell membrane is not hard like an eggshell.

So the outermost layer of an egg is very, very hard.

The cell membrane is not hard.

It is, in fact, flexible, which allows the cell to change size and shape if it needs to.

So the cell membrane is flexible.

The image shows a unicellular microorganism called an amoeba, and it's seen under a microscope.

So an amoeba is a single-celled organism.

It only is made up of that one cell.

We're gonna watch video now to see how flexible the amoeba cell is.

Let's watch the video.

Now, what we can see in this video is an amoeba.

An amoeba is a single-celled protist.

So it's a unicellular organism.

The whole organism is just made up of the one cell we can see.

And it almost looks like the amoeba is moving.

But actually what is happening is the cell membrane, which surrounds the whole cell, is just very flexible and it's able to change the size and the shape of that amoeba cell.

Something that's really important for organisms to stay alive is that substances must be able to be moved into and out of cells.

So let's look at this animal cell here, and let's think about some substances that might need to be moved in and out.

So living cells need oxygen from the air.

It's needed for the life process of respiration, which uses food or glucose as a fuel to provide energy.

So we breathe in oxygen gas from the air, and then that oxygen travels in the blood and is delivered to cells.

So the oxygen is moved into the cell for respiration.

Let's think about what might move out them.

Well, living cells need to get rid of something called carbon dioxide.

Carbon dioxide is a waste product of respiration, and it is toxic if it builds up inside of cells.

So therefore, carbon dioxide diffuses out of the cells, and then it travels in the blood and we breathe it out because we don't want carbon dioxide levels to increase in our body.

So from this side, what we know is oxygen moves into cells and carbon dioxide moves out of animal cells.

So you might be thinking now, "Well, how does this happen?" Well, substances can move in and out of the cell because the cell membrane has got holes in it, which you can see in this animal cell here.

We've changed the model slightly.

Rather than it being a solid line around the outside to represent the cell membrane, we've now got this dashed line to show that there are little holes in the cell membrane.

So this allows substances to move into and out of the cell, which we can see here are being represented by the arrows.

Which arrow do you think is representing what substance? So the blue arrow is representing oxygen moving into the cell, and the pink arrow would be representing carbon dioxide which is moving outta the cell.

And this can happen because the cell membrane has got these little holes in it.

And the holes are just big enough to let some substances through, but not so big that everything in the cell can escape.

And this is really important because the cell membrane keeps the conditions inside of the cell relatively constant.

Let's check our understanding so far.

"Which statement about the cell membrane is true? A, it has holes in it that all substances can move through.

B, it does not let any substances move through it.

Or C, it has holes in it that only some substances can move through." The answer to this one is C.

It does have holes in it that only some substances can move through, so only smaller substances.

The next question: "Which material is the most like a cell membrane of the three pictures below? A, hard plastic; B, flexible plastic; or C, flexible cloth." This is C, the flexible cloth.

And the reason why it's C is because the cell membrane is flexible, remember, it can move, but also it has small holes in it.

And the flexible cloth is the only option, which is both flexible and has small holes in it.

Well done if you got that right.

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

So Aisha and Sam have been asked to choose things that are like a model of a cell membrane.

Aisha says, "I've chosen a sieve." Whereas Sam says, "I've chosen a string bag." These are the questions you need to answer.

So number one, "In what ways is each model like a cell membrane, and in what ways is it different?" Number two, "If a cell membrane was like that, what effect would it have on the cell?" And number three, "How could each model be improved?" I'm sure you're gonna do a fantastic job of this.

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

Let's see how we did.

So in the first part we needed to say, "In what way was each model like a cell membrane, and in what way was it different?" The sieve is like a cell membrane because it has small holes in it, but it's different because it's rigid and inflexible, whereas a cell membrane is flexible.

The string bag is like a cell membrane because it is flexible, but it's different because it has very large holes in it, whereas the holes in the cell membrane are smaller.

The second part, "If a cell membrane was like that, what effect would it have on the cell?" So if the cell membrane was like a sieve, it would be good at letting only some substances pass through, but the cell would not be able to change size or shape if it needed to because it's inflexible.

If the cell membrane was like a string bag, it would allow the cell to be very flexible, but too many things would be able to pass in and out of the cell.

Brilliant job if you managed to get those right.

And number three, "How could each model be improved?" The sieve model could be improved by being more flexible like a cell membrane so the cell could change size and shape if needed.

So it might be that the material of the sieve would need to be changed to something that is more flexible.

And the string bag model could be improved by having smaller holes like a cell membrane so only some substances could pass through it.

Really good job finishing the first task.

If you need to pause the video to add anything into your answers and feedback to yourself, then please do.

But if not, let's get going with the rest of the lesson.

Really good job.

We know now about substances in the cell membrane, so let's look at how they move in and out of a cell.

The cell cytoplasm is a mixture of substances.

So if we look at this animal cell here, the cytoplasm is that jelly-like liquid that surrounds all of the subcellular structures.

The cytoplasm is made up of particles that are constantly moving in all directions.

So if we magnified the cytoplasm, what we would see is millions and millions of particles of different substances that were all randomly moving in all directions, which is what that simulation there is showing you.

And the constant movement of particles is what causes diffusion.

So what we need to know is that the cytoplasm is made up of particles of different substances, and those particles are constantly moving randomly in all directions which causes diffusion.

So if we magnified the animal cell cell membrane, there's a simplified diagram here to show you.

And on the key we can see the kind of orange dashed line, that's the cell membrane.

So that's showing you the cell membrane which has got little holes in it.

And then on the inside of the cell, which is to the left of those orange lines, you've got different substances, so different particles.

And the blue particles, the water molecules.

The very large kind of red particle is the protein molecule.

And then we've got some smaller red molecules, which are the oxygen molecules.

Now what will happen is that particles of some substances will be small enough to diffuse through the holes in the cell membrane.

And in this image here, those oxygen molecules are small enough to diffuse through the cell membrane, whereas the large protein molecule is much too large to pass through those small holes.

So therefore, particles of other substances will be too large to diffuse through the holes, and they are unable to leave the cell.

Because some substances can diffuse through the holes in the cell membrane and others cannot, the cell membrane is known as selectively permeable.

So if we break that word down, selectively and permeable, permeable means allows things to pass through it, and selectively means allows some things and not others.

So therefore, the cell membrane is selectively permeable because it does allow things to pass through it, because it's got those little holes in it.

But because the little holes are small, it only allows some things to pass through and not others.

It is selectively permeable.

Let's check our understanding of that.

"Which is the correct description of the cell membrane? A, fully permeable; B, impermeable; C, permeable; or D, selectively permeable." You should have got D for this one.

The cell membrane is selectively permeable.

So let's look at diffusion.

And this is how substances move in and out of cells.

So when there's a concentration gradient, more particles move from the area of higher concentration to the area of lower concentration, and this is the direction of net movement.

So if we look at this diagram here, we can see on the left-hand side there's an area of high concentration because there are more molecules, whereas there are fewer molecules on the right-hand side, so that's the area of lower concentration.

Now all of these molecules will be randomly moving in all directions.

But simply because there are more particles in the higher-concentration region, more will move from high to low concentration than particles will move from low to high.

And that net movement from high to low concentration is down the concentration gradient.

Now let's look at this specifically in an animal cell.

So we can see the animal cell with the selectively-permeable membrane on the left side.

And if we magnified in to look at the cell membrane, which is inside of that rectangle, we've got the key there where the selectively-permeable membrane is the kind of orangy-brown lines, and then we've got some oxygen molecules.

So on the left-hand side of the cell membrane, that would be inside of the animal cell.

And on the right-hand side is the outside of the animal cell.

And we can see that there are far more oxygen molecules outside of the cell than inside.

So there's a high concentration on the right-hand side outside the cell.

That means that there's a concentration gradient.

And when there's a concentration gradient across the cell membrane, a few particles diffuse through the holes from low to high.

So because the oxygen molecules are all randomly moving in all directions, some will just by chance move from low to high concentration.

However, many more particles will diffuse through from high to low concentration.

So overall, there will be a net movement from high to low concentration.

Let's check our understanding of that.

So who correctly describes how molecules of oxygen get into a cell? Andeep says, "The cell breathes in the oxygen molecules." Izzy says, "The cell needs oxygen to live, so it decides to diffuse the molecules in." Jacob says, "Molecules of oxygen diffuse through the cell membrane down a concentration gradient." And Jun says, "The oxygen molecules are sucked into the cell by diffusion." So who do you think is correctly describing how molecules of oxygen get into a cell? This is Jacob.

The reason why Andeep is wrong is because the cell doesn't breathe oxygen in.

Izzy is incorrect because she's saying that the cell decides to diffuse molecules in, but the cell isn't deciding to do something.

This is a passive process.

And Jun is incorrect because he says that the oxygen molecules are sucked into the cell, and sucked is not the correct word, whereas Jacob's definition is very, very good.

Here's another question.

"In which direction would carbon dioxide diffuse through this cell membrane?" And we have got an image here with a selectively-permeable membrane, carbon dioxide in the green colour, and water molecules in the purple colour.

So where would the carbon dioxide diffuse through this cell? "A, net movement from side A to B.

B, net movement from side B to side A.

Or C, equal movement in both directions." So we should have here B, it goes from side B to A, and this is because there's a higher concentration of carbon dioxide side B, so it will diffuse through to side A by diffusion.

Well done if you got that right.

We're now ready to do task B of the lesson.

So the liver is an organ in the digestive system.

Liver cells make a waste product called urea.

It is taken away by the blood.

And we can see here we've got an image of the human body with the digestive system, and the liver is labelled.

The first part of this task, you need to complete the diagram to show how urea molecules diffuse outta the liver cell into the blood.

And you can see there that the two sides have been labelled as cell cytoplasm and the blood, and they've given you a key as well.

And then in the second part of the task, you need to write an explanation of the direction of net movement of urea molecules across the cell membrane.

Now, key thing here is to look at the text at the top to say that liver cells make a waste product called urea and it is taken away by the blood.

So we're thinking about where it travels from and to.

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

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

Right, I hope we had fun with that.

So let's see how we did.

So in the first diagram, we needed to show how urea molecules diffuse out of a liver cell into the blood.

So we needed to add to the key a molecule of urea.

And in the cytoplasm, there will be a high concentration of urea, because remember, it's moving outta the liver into the blood.

So therefore the blood will have a much lower concentration of urea.

And therefore we should represent that overall net movement going from high concentration to low concentration through the selectively-permeable membrane.

And then for part two, "Write an explanation of the direction of net movement of urea molecules across the cell membrane.

There is a higher concentration of urea molecules in the liver cell cytoplasm, and a lower concentration in the blood.

The net movement of urea molecules is down the concentration gradient from the cytoplasm through the holes in the cell membrane into the blood." Now you might not have that exactly word for word the same, but just maybe pause the video now and check that you've got most of the key points in there.

You've done an amazing job this lesson.

Well done.

Let's summarise everything that we've learned.

So remember today we were looking at diffusion through the cell membrane.

And we said that all cells are surrounded by a cell membrane which has small holes in it.

And then we said that all substances are made up of particles.

Particles of some substances are small enough to fit through the holes in the cell membrane by diffusion.

An example of that we gave was oxygen and carbon dioxide.

Particles of other substances are too big to diffuse through the holes, and the example in our slideshow was protein molecules.

The cell membrane is therefore called selectively permeable because only some substances can diffuse through it.

The net movement of particles by diffusion through the cell membrane is down a concentration gradient.

And the examples that we gave in this lesson were the movement of oxygen from outside of the cell into the cell, and also the movement of urea from the cytoplasm in the liver into the blood.

You've done a brilliant job this lesson.

Well done.

I really look forward to seeing you next time.