Lesson video

This lesson is called "The importance of Exchange Surfaces and Transport Systems in Humans", and is from the unit "Transport and Exchange Surfaces in Humans".

Hi there, my name's Mrs. McCready, and I'm here to guide you through today's lesson, so thank you very much for joining me today.

In our lesson today, we're going to explain how exchange surfaces and transport systems enable cells in multicellular organisms to be quickly supplied with substances that they need to stay alive.

Now in our lesson today, we're going to cover a good number of keywords, and they're shown up there on the screen now for you.

You may wish to pause the video to make a note of them, but I will introduce them to you as we come across them.

So in our lesson today, we're going to firstly consider the problem of size, before considering the solutions of transport systems and then the solution of exchange surfaces.

So are you ready to start? I am.

Let's go.

Now, organisms have a surface area and a volume, and we can calculate the values of these by using equations.

So the surface area of a cube can be calculated by timesing the width by the height by the number of faces of the cube.

And the volume can be calculated by multiplying the width times the height times the depth.

So we can compare the surface area and the volume as a ratio, and in a cube of one micrometre wide, as in this bacterium cube, we can see that the surface area is six micrometres squared.

The volume is one micrometre cube, so the surface area to volume ratio is six to one.

Now all organisms require nutrients and make waste products, and these substances need to be moved into or out of the organism in order for the organism to survive.

Now these substances move by the process of diffusion, down concentration gradients, from areas of higher concentration to lower concentration.

So in this case, the particles of glucose and oxygen will move from outside the cell into the cell, down the concentration gradient.

And this is a passive movement.

It doesn't require energy in order for this to happen.

It will just occur.

Now we can see how the surface area and volume of a cell impacts diffusion rate.

So the surface area impacts the rate of the diffusion, whereas the volume affects the distance over which diffusion has to occur.

So both surface area and volume impact the rate of diffusion.

So let's just pause, and decide which of these statements about diffusion are correct.

A, the rate of diffusion is affected by the surface area.

B, diffusion is a passive process.

Or C, diffusion involves particles moving from low to high concentration.

I'll give you five seconds to decide.

Okay, so let's see which statements are correct.

So, "the rate of diffusion is affected by the surface area" is correct, and "diffusion is a passive process" is correct.

But the final statement is incorrect, because in diffusion, particles move from high to low concentration areas, and not low to high.

Well done if you got both of those correct.

So let's apply that to a real world situation.

Why don't we see elephant-sized butterflies? Can you explain that? Because we don't, do we? So let's look at that from a perspective of diffusion, surface area, and volume.

Now organisms, including elephants and butterflies, are irregularly shaped, and that makes it really really difficult to calculate surface area to volume.

So we're going to simplify this by using cubes to represent the different animals, rather than trying to work out the intricacies of their actual surface area to volume ratio.

So you'll just have to go with me on this, but we're just dealing with the cubes for now.

Now we can calculate the surface area to volume ratio of a butterfly cube with a height of four millimetres or 0.

4 centimetres, by timesing the width times the height times the number of faces.

So in the butterfly cube of four millimetres wide, that's 4 × 4 × 6, which is 96 millimetres squared.

Whereas the volume is 4 × 4 × 4, and that gives us a volume of 64 millimetres cubed.

So we can see that for a butterfly cube with a height of four millimetres, its surface area to volume ratio is 96 to 64, and that we can simplify down to 1.

5 to one.

Now in an experiment with agar cubes, it takes about 15 minutes for a substance to diffuse into the centre of a one centimetre wide cube, and it takes about 30 minutes for the same substance to diffuse into the centre of a two centimetre wide cube.

So as we're doubling the width, we're also doubling the time.

So if it takes about 15 minutes for a substance to diffuse into the centre of a one centimetre wide agar jelly cube, how long will it take for substances to diffuse into the centre of a butterfly cube, which is 0.

4 centimetres wide? I'll let you think about that for a moment.

So if it takes 15 minutes to go one centimetre, it's gonna take about six minutes to go 0.

4 centimetres.

Did you work that out correctly? Well done if you did, 'cause that was a bit of quick maths in your head.

So let's compare this to the elephant, 'cause that's what we were talking about, weren't we? Why don't we get elephant-sized butterflies? So let's do the same calculations, but for an elephant.

Now an elephant cube, let's make that four metres wide.

So that's 4,000 millimetres wide.

Let's use those values to calculate surface area firstly.

So that's 4,000 × 4,000 × 6.

So surface area of a four metre wide cubed elephant is 96 million millimetres squared.

What about volume then? So volume will be 4,000 × 4,000 × 4,000.

Goodness me, that's going to make 64 billion millimetres cubed.

Crikey.

So let's simplify that into a ratio for surface area to volume.

So let's knock off a few zeros, shall we? Well if we put the 96 million down to just 96, and lose six zeros off of that, and then take another six zeros off of the volume, we get a ratio of 96 to 64,000.

And if we put that in a something to one ratio, we'll get 0.

0015 to one.

So this is starting to explain why we don't get butterflies the size of elephants.

But let's look at that in a bit more detail.

How long would it take for substances to diffuse into the centre of an elephant cube, which is four metres or 400 centimetres wide? Bearing in mind it took 15 minutes for a substance to diffuse into a one centimetre wide cube.

This elephant is 400 centimetres wide.

So give yourself a bit of chance to work that out if you can.

Okay, well that's gonna take about 6,000 minutes.

6,000 minutes is the equivalent of 100 hours, which is the same as four days and four hours.

Goodness me.

Now that's a heck of a long time, isn't it? For substances to diffuse from the outside of the elephant into its centre, if the elephant is a four metre wide cube.

So again, we're adding a bit more information into our explanation, as to why we don't get elephant-sized butterflies.

But let's just check our work before we go on further.

So as the width of a cube increases, the time taken to diffuse into the centre of the cube decreases, stays the same, or increases.

I'll give you five seconds to decide.

So as the width of the cube increases, the time taken to diffuse into the centre of the cube also increases.

I hope you got that correct.

So let's consider some scenarios here then.

We've seen that, as width doubles, the time taken to diffuse into the centre of a cube also doubles, and a 0.

4 centimetre cube butterfly takes about six minutes, and a four metre wide cube elephant takes about 100 hours.

So what I'd like you to do is to use this rule and the timings there to estimate the approximate length of time for diffusion in an ant cube of 0.

2 centimetres wide, and a human cube of 1.

5 metres in length.

Then I'd like you to consider this real world example.

So amoeba are really interesting single celled organisms, and there's one pictured here on the screen.

Now that one pictured on the screen is about 80 micrometres wide and about 120 micrometres long.

So it might take up to 11 seconds for substances to diffuse into its centre.

So can you suggest why amoeba have so many pseudopodia, that's those sticky outfits, those projections, rather than it being purely spherical or purely cuboidal? So pause the video to have a go at those two tasks, and come back to me when you're ready.

Okay, let's see how you got on with those two questions then.

So we were considering how long it would take for a substance to diffuse into the centre of an ant cube 0.

2 centimetres wide and a human cube 1.

5 metres long.

So we know that for the butterfly cube of 0.

4 centimetres wide, it took about six minutes.

So if the time taken doubles when the width doubles, if we're halving the width then we're halving the time, and therefore a 0.

2 centimetre wide ant cube should take about three minutes for diffusion.

What about the human cube? Well, we know that an elephant cube of four metres wide took about 100 hours.

So a human cube of 1.

5 metres in length is going to take about 37 and a half hours for diffusion to take place into the centre.

Did you get both of those answers correct? Well done if you were able to calculate those correctly.

Then I asked you to consider the amoeba, and to suggest why it has so many pseudopodia sticking out of its body, rather than it being purely spherical or cuboidal.

So your answer might include that pseudopodia increase the surface area of the amoeba, and this will increase the rate of diffusion, either into or out of the amoeba, you might have added.

And therefore you might also have said that this will increase its chances of survival.

So just check your answer over.

Did you say that? Did you come across with that correct message? Obviously you might have worded it differently, but well done if you've said anything along those lines.

So let's move on to some solutions now, because we've seen the problem of size, but obviously large animals do exist, so there must be ways of getting around this problem of size.

So let's consider transport systems, firstly.

Now, the poor elephant would never survive.

It would be impossible to survive if it had to wait more than four days for nutrients to diffuse into, and waste products to diffuse out of, its four metre wide cell.

That would just be unviable.

Even the butterfly is likely to struggle, having to wait six minutes for substances to diffuse into or out of its cell.

So to get around this, what we need to do is to increase the surface area that is exposed, without reducing the volume.

In other words, cut up the big cell into lots of small cells.

And we know that is what happens, because many organisms are multicellular.

Their total volume is divided up into many, many small cells put together.

Now the sizes of the cells vary within multicellular organisms, but they tend to be between 10 and 100 micrometres wide.

So at the rate of diffusion in an agar cube, it would take between 0.

9 and nine seconds for substances to diffuse into the centre of cubes this size.

So let's just review that, firstly.

I'd like you to use either the word "increases" or "decreases" to complete the sentences.

As width increases, surface area to volume ratio, "what"? Multicellular organisms are made of many small cells.

This "something" the distance for diffusion.

This "something" the time taken for substances to diffuse.

I'll give you five seconds to think about it.

So let's review our answers.

As width increases, surface area to volume ratio decreases.

Multicellular organisms are made of many small cells.

This decreases the distance for diffusion, and this decreases the time taken for substances to diffuse.

Did you get those correct? Well done if you did.

So to maximise the benefit of having so many cells making up an organism's body, multicellular organisms have a way of transporting substances up to the cell membrane, to reduce the distance that diffusion has to occur over, the diffusion distance.

So in humans, we use blood, and that transports nutrients and waste products all around our body.

Blood flows through blood vessels.

So blood vessels are transporting that blood around our body, and nutrients and waste products are exchanged with the tissues and the blood through the capillaries, which are present at the surface of cells.

So blood and blood vessels form part of the circulatory system, along with the heart.

And this is the transport system in our body.

It moves nutrients and waste products around the body, all of the time.

Now all organs and tissues have many capillaries.

So the distance that substances have to diffuse between the blood and the tissues in either direction is very, very small.

Sometimes those capillaries are right up to the membrane of the cell, and if they're not, it's really only one or two cells width that the substances have to diffuse over.

So each cell is very small, and each cell is very close to a capillary, and therefore the diffusion distance is reduced to as short a distance as possible, and no more than the width of a cell or two.

Really really, very small distance.

And because of that, that makes diffusion extremely quick and very efficient.

Okay, so these three boys are trying to give a correct explanation of why diffusion into tissues is efficient, but who has given the most correct explanation? So Lucas has said, "Diffusion is quick and efficient because it only takes a few seconds per molecule." Andeep says that, "All organs are very small, so the distance to diffuse is very short." Whereas Jun explains that, "Cells are really small, so diffusion can happen rapidly across a short distance." But whose explanation is correct? I'll give you five seconds to think about it.

Okay, so hopefully you have said that Jun is correct, with his explanation of why diffusion is efficient.

Well done if you got that correct.

So what I'd like you to do now is to consolidate your understanding about transport systems, by firstly describing how the circulatory system reduces the distance for substances to diffuse.

Then I'd like you to ponder this scenario.

So people who have been exposed to extreme cold develop a condition called gangrene.

So this is where the tissues die because there is a sustained lack of blood supply, and it commonly affects the fingers and toes, they turn black.

It's really quite awful.

So what I would like you to do is to use your understanding and the importance of the circulatory system to explain why gangrene develops.

So pause the video to consider your responses to these two questions, and come back to me when you're ready.

Okay, let's see what you said for these questions.

So firstly, I asked you to describe how the circulator system reduces the distance for substances to diffuse.

So you might have included ideas that firstly, the circulatory system is a transport system, and it moves nutrients and waste products around the body.

Capillaries are blood vessels, and they bring blood near to the surface of the cells, and this reduces the distance and also the time taken for substances to diffuse into or out of the blood, or cells, you might have said as well.

So we're talking about essentially reducing the distance between the blood where the nutrients are, and the cells where the nutrients have to get to, or the cells where the waste products are, and the blood where the waste products are trying to get to.

Reducing that distance.

So check your work over.

Have you got that idea? Well done if you have.

Then I asked you to consider the condition of gangrene, and explain the importance of the circulatory system, and why gangrene develops in tissues where there is a sustained lack of blood supply.

So you might have said that gangrene occurs because tissues lose their blood supply, and this means that the distance and time taken for substances to diffuse between the cells and the blood increases significantly, because essentially, the blood vessels are further away from the tissues.

This means that the exchange of substances just can't happen fast enough, and because of this, you might have added, the tissues die because essentially they run out of nutrients, and waste products build up to toxic levels within the cells.

And this is how gangrene occurs.

So just review your answer, check you've got the right message there in your notes, and well done.

That was perhaps a bit gruesome, and a very tricky scenario to consider.

So well done.

Let's move on to our last section of the lesson then.

And this is looking at another type of solution.

This time, exchange surfaces.

So we know that the circulatory system is transporting nutrients and waste products around the body, and nutrients include glucose, amino acids, water, and oxygen.

Whereas waste products include urea, lactic acid, and carbon dioxide.

As shown in the not-to-scale diagram.

Now, oxygen diffuses into our body via specialised exchange surfaces in our lungs, called alveoli.

And alveoli have multiple specific adaptations to enable them to be highly efficient exchange surfaces, where oxygen can diffuse into the blood, and carbon dioxide can diffuse out of the blood.

Now those adaptations include very thin cells which are only one layer thick.

As you can see in the diagram, the alveoli are only made of one cell thick layers, and the capillaries are one layer thick as well.

They also have a very close blood supply.

The blood supply is touching the surface of the alveoli, so it's very, very nearby.

And these two adaptations reduce the diffusion distance, so the diffusion distance is extremely short.

Furthermore, the alveoli create an enormous surface area.

So if we took our lungs and we spread them out completely, so that they were only the one layer thick, they would cover half the size of a tennis court.

That is how much space, how much surface area, is squidged up and folded into our lung space.

That's really quite incredible, isn't it? To think about it, half the size of a tennis court.

But effectively, what this is doing is creating a high diffusion rate, because there are many many surfaces over which diffusion can take place.

So the alveoli are specialised exchange surfaces by having a very short diffusion distance and a very high diffusion rate.

Another example is the exchange of nutrients in the digestive system.

So glucose and amino acids diffuse into our body across specialised exchange surfaces in the digestive system called the villi.

And the villi have many adaptations as well.

So firstly, they are made of very thin cells, which are one layer thick.

Sounding familiar? They also have a very nearby blood supply.

So there really isn't very far to go for the nutrients to be absorbed from the digestive system into the bloodstream.

And these very thin cells, which are only one layer thick, and the very close blood supply, both cause a short diffusion distance.

Furthermore, there are many many many of them, and the surface of each villi is covered in even more minute folds called microvilli.

So those two hillocks there in the picture is a cell, and you can see how the surface of the cell is incredibly closely folded with these microvilli.

So they have a phenomenally large surface area as well.

And the large surface area means a high diffusion rate.

So again, the villi have fantastic adaptations for being an extremely efficient exchange surface.

So all exchange surfaces, such as the lungs and the digestive system, have certain characteristics.

They have very thin cells which are one layer thick, and they have a very close nearby blood supply.

They also have an incredibly large surface area.

And when you put all of those adaptations together, you have a very short diffusion distance and a very high diffusion rate.

And it is this that makes these exchange surfaces so excellent at exchanging substances.

So what I'd like you to do is to just consolidate that by considering this question.

Both the alveoli in the lungs and the villi in the small intestines have a nearby blood supply.

True or false? So you should have said that that is true, but can you justify your answer? Is it because this creates a short diffusion distance, or is it because this creates a large surface area? What do you think? So you should have said that the nearby blood supply creates a short diffusion distance.

Well done if you got both of those correct.

So what I'd like you to do now is just to bring the whole lesson together.

So you have been asked to write an article for StudentNews, the student newspaper, about the adaptations in large animals to increase the rate of diffusion.

So in your article, you need to include an explanation as to why larger organisms require specialised exchange systems to survive.

You need to give examples of exchange surfaces and transport systems that do this, and explain how they are adapted to their function.

And you need to include an eye-catching title to grab your reader's attention.

So pause the video and take your time to write a good quality article, which explains this problem and the solutions in detail and accurately, and come back to me when you're ready.

Okay, let's see how you got on then.

So in your article, you might have included that large animals require specialised exchange surfaces to allow the efficient exchange of nutrients and waste products between cells and the outside environment.

Otherwise, diffusion will take too long, and the organism will die.

Now you might have said that the alveoli of the lungs or the villi of the digestive system are examples of exchange surfaces, and you should have included the circulatory system as an example of a transport system.

Then you should have said that these parts, these exchange surfaces and transport systems, are highly specialised, because they have very thin cells and a nearby blood supply to decrease the diffusion distance, and they have a large surface area to ensure a high diffusion rate.

So have you included all those important points within your article? You might have put some more detail down about the exchange surfaces or the transport system, which is excellent.

Well done.

So just check over your work, make sure you haven't missed anything or got anything incorrect, and good job for writing that.

That was quite a long, complicated article they commissioned from you.

So in our lesson today, we have seen that, as the width increases, surface area and volume increase, but their ratio decreases.

And that's the critical part, because decreasing the surface area to volume ratio decreases the diffusion rate, and that means that specialised exchange surfaces such as the villi in the digestive system or the alveoli in the lungs are required in order to increase diffusion rate.

And they can do that by having a high surface area.

However, the substances, once they've been absorbed in via the digestive system or exchanged at the lungs, need to then be transported to the rest of the body where they are required, and so the circulatory system provides that transport system, and enables substances to be moved quickly around the body, therefore minimising the diffusion distance as well.

So I hope you found that interesting, and quite fascinating in places.

Thank you very much for joining me.

Well done for making such a great job of your work today, and I hope to see you again soon.

Bye!.