Lesson video

- Hi everyone, Mrs. Buckle back again to do some biology with you today.

Today, we're gonna be looking at diffusion and exchange surfaces within the human body in particular, but also a couple of example in plants.

For today's lesson, to make sure you're super-prepared and ready to learn, you are gonna need somewhere to write down your ideas, so a notebook and a pen or a digital device.

And you are also going to need a calculator for one of our tasks later on.

Once you've got all the equipment and you're ready to get started, let's get straight into it then.

Today's title is Diffusion and Exchange Surfaces.

This form's part of the cell biology unit within science, and what we're going to do today is describing and explaining, so giving reasons why diffusion takes place within organisms within living things.

Before we get straight into the content, I just want go over a few keywords that you might not have come across before and these will help us as we move through today's learning.

The first one is concentration.

This is the idea of the number of particles in a given volume of space.

The more particles there are within a given space, the higher the concentration.

So if we had two containers and they took up the exact same volume, the exact same amount of space, but one container had a higher number of particles, we would say that the concentration of particles in that container would be higher than the other.

The second word I want to make us aware of is equilibrium.

An equilibrium is a state where everything is equal and it gives us a bit of a clue, because at the start of that word equilibrium, we do have a prefix that sounds similar to the word equal.

And last one is this idea of partially permeable.

Lots of things can be partially permeable, but we're going to be talking about cell membranes today.

A cell membrane is partially permeable, because it only lets some substances pass through it.

It could be to do with the charge of these substances or the size of the substances or the actual structure of the membrane itself.

So quick overview of what we're going to be going through today.

We're gonna start by defining diffusion.

Then we're gonna look at some factors that affect how quickly diffusion takes place.

We're then gonna have a go at calculating one of these factors, which is called surface area to volume ratio.

And finally, we are going to look at some adaptations in organisms for exchange or diffusion to take place.

Without much further ado, let's get started then by defining diffusion.

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.

So if we had these particles at the top, I would say that they have a higher concentration than the particles at the bottom.

There are more particles in the same volume.

If diffusion were to take place, particles would move from an area of high concentration at the top of the screen down to an area of lower concentration at the bottom of the screen.

Diffusion will happen until an equilibrium is reached, until there is an equal concentration of particles in both of those places.

Diffusion can only take place within fluids, so liquids and gases, and this is because for diffusion to take place particles need to be free to move.

We know that particles in a solid are in a fixed position so they wouldn't be able to carry out diffusion, because those particles wouldn't be free to move around.

Let's have a look at this in a model.

In this model, we have a high concentration of particles in the bottom corner of the room.

If diffusion were to take place, the particles will begin to move from where they are at a higher concentration to where they are at a lower concentration.

Particles are always moving, so this would just happen as time progressed.

We can see here, the particles are already starting to spread out from a high concentration that bottom corner to anywhere else within this space.

The particles will move from a higher concentration at this side of the room to a lower concentration at the other side of the room.

And this will keep happening.

And we can see here the particles are even more spread out than they were before.

Diffusion will continue until eventually an equilibrium will be reached and that's where we've got the same concentration of particles at all places within the room.

The particles will continue to move, because they are in a fluid state, but they would move around randomly making sure that the concentration was equal at all parts in the room.

Diffusion explains how we smell things.

Let's take a look at this example of perfume.

Once spread, there would be a high concentration of perfume particles in the air surrounding the bottle.

It would look something like this.

Because those particles were at a high concentration, they would want to move by diffusion to a lower concentration elsewhere in the space.

Over time, the particles will move to a lower concentration closer to the nose, it would look something like this.

Eventually, the particles will move really close to the nose of the person and those particles will be inhaled and the person will be able to smell the perfume as the chemicals activated receptors in the nose.

We smell substances, because of the diffusion of chemicals from an area of high concentration near that object, in this case, the perfume bottle, to an area of low concentration in this case, near this person's nose.

Diffusion can also happen in cells.

This occurs across a partially permeable membrane.

Membranes are partially permeable as they allow some substances to pass through and other substances cannot.

In the examples of diffusion, we are referencing the size of the particles.

This diagram shows a partially permeable membrane, because there are small gaps between areas of the membrane.

We can see that the particles in this example are very small and would be able to fit through the membrane, so diffusion would be able to take place.

In this example, we can see that the concentration of particles is greater on the left side of the membrane, than the right.

This means that diffusion would take place, the particles would move from the left of the membrane through the gaps in the partially permeable membrane to the right side of the membrane.

This would take place until an equilibrium had been reached.

We can see that this has happened here, because there is an equal concentration of particles on both sides of the membrane.

Let's have a look at this in terms of cells.

So in this model we have an animal cell and I can see that due to its irregular shape and absence of a cell membrane.

In this scenario, there is a higher concentration of particles outside of the cell, compared to inside of the cell.

Diffusion would take place and if particles were small enough, they would move through the cell membrane into the cell until the concentration inside and outside was at an equilibrium.

To put it more simply, we would have a higher concentration of particles outside compared to inside the cell.

Diffusion would take place, particles would move into the cell until we reached an equilibrium.

In this example, we have the opposite.

There is a higher concentration of particles inside the cell compared to outside of the cell.

Diffusion would take place if the particles are small enough in this case they are, particles would move through the cell membrane until there was an equal concentration inside the cell compared to outside of the cell.

So we would have a higher concentration of particles inside the cell.

Those particles would move to an area of lower concentration until there was an equal concentration of particles inside the cell as there is outside of the cell.

Okay, let's have a go at some check for understanding questions.

I'm gonna read the question.

You will pause the video and choose your answer and resume when you are ready to check if it is correct.

The first one is fill in the gaps.

So I want you to figure out which keyword is missing.

Diffusion is the movement of particles from an area of high to an area of low.

Well done if you said concentration.

The next one, true or false, diffusion would happen to the left between these boxes.

Don't forget to justify your answer.

Well done, if you said false, diffusion wouldn't happen to the left, because the concentration on both sides of the membrane is the same, there would be no net movement of particles in this scenario.

Okay, for task 1 of today's lesson, there are two short questions for you to have a go at.

The first one is a definition of diffusion.

And question b, I would like you to explain how we can smell food being cooked from other rooms in a house.

For that question, I want you to use your ideas about diffusion.

You might also want to refer back to the model we looked at when we were talking about perfume.

This should take you roughly five minutes.

Once you are happy with your answers, come back to me and we will go through my model answers.

Okay, welcome back.

Let's go through these answers then.

So the first one, defining diffusion.

Diffusion is the net or overall movement of particles from an area of high concentration to an area of low concentration.

And for question b, we can smell food, because particles from the food enter the air around it where they would be at a high concentration.

Those particles would then move from an area of high concentration near the food to an area of low concentration near our noses.

And that happens by diffusion.

So key idea is there we've got a high concentration of particles around the food, a low concentration of particles around our noses, and the idea that diffusion happens when those particles move from a high concentration to a low concentration.

Make sure you've self-assessed.

Double check the spelling of that word concentration, 'cause it is a tricky one.

If you need to make any changes, do that now and pause the video.

And once you're ready to move on, resume and we will head into the second part of our lesson.

Okay, so we've described diffusion.

We're now gonna look at factors that the rate of diffusion.

The rate of diffusion is just how quickly diffusion happens and we are gonna look at ways to maximise this rate.

So try and get diffusion to happen much quicker in certain scenarios.

There are several factors that affect the rates of diffusion temperature, surface area to volume ratio and concentration gradient.

In all of those scenarios, the higher the factor, the higher the rate of diffusion.

So the higher the temperature, the higher the rate of diffusion, the greater the surface area to volume ratio, the higher the rate of diffusion.

And the steeper or larger the concentration gradient, the higher the rate of diffusion.

We're gonna look at each of these in turn and talk about them in a bit more detail.

Let's start with temperature.

The higher the temperature, the higher the rate of diffusion and this is to do with the energy that the particles have at different temperatures.

At higher temperatures, particles have much more energy.

We can see this, because on our particle diagram the arrows are much larger, whereas at lower temperatures, particles don't have as much energy, they are moving much more slowly.

The more energy particles have, the quicker that they can move.

We know that diffusion is all about the movement of particles.

So if particles are moving quicker, diffusion is taking place quicker.

The particles that are higher temperature have more energy, therefore, they move quicker and therefore, the rate of diffusion increases.

Our next factor is surface area to volume ratio.

The higher the surface area to volume ratio, the higher the rate of diffusion.

We've got two cells here.

We have got a root hair cell and a palisade cell.

The root hair cell has a larger surface area.

This is because the extension on the root hair cell means that there is more cell membrane.

A cell with a larger surface area has more space for particles to move through the membrane.

If there are more opportunities for particles to move into or out of the cell, the rate of diffusion will be higher.

So a cell with a higher surface area to volume ratio has a higher rate of diffusion.

And finally, the steeper the concentration gradient, the higher the rate of diffusion.

In this scenario, we have a stepper concentration gradient than in this scenario the shallower concentration gradient.

We can see in both scenarios there is a higher concentration of particles outside of the cell compared to inside of the cell.

But where we have a steeper concentration gradient, there is a greater difference in concentration outside compared to inside of the cell.

The greater the difference, the greater the steepness of the concentration gradient.

The steeper the concentration gradient, the more particles will move in the direction towards the lower concentration.

The more particles that are moving, the higher the rate of diffusion.

Okay, so more check for understanding questions then.

Number 1, why does an increase in temperature increase the rate of diffusion? Well done if you remembered that this is, because particles have more energy.

Remember they have more energy, they're moving around much, much more.

Next one.

Why does an increase in surface area increase the rate of diffusion? Well done if you said it's because there is more space for the particle to diffuse.

In which scenario is where diffusion will happen the quickest? Well done if you said A.

A has the steepest concentration gradient, because there are more of a difference in concentration outside compared to inside of the cell, therefore, in scenario A, diffusion would happen the quickest.

Okay, for task 2 of today's lesson, I just want you to summarise what we've learned and explain why each of these three factors increases the rate of diffusion.

This should take you 3 to 5 minutes.

Once you're happy with your answers, resume the video and we will go through what I would be expecting.

Pause video now and have a good go at that.

Hello and welcome back.

Let's go through these answers then.

So increasing temperature, this because particles have more energy, specifically kinetic energy to move around.

Increasing the surface area, particles have more places to move through cell membranes and concentration gradient, there will be a higher difference in concentration, meaning the particle's gonna move much, much quicker.

Make sure you happy with your ideas.

If you need some more time, pause the video here, because we're going to move on to part 3 of our lesson.

So we've looked at what diffusion is.

We have looked at how we can increase the rate of diffusion.

We are now going to have a good at calculating surface area to volume ratio.

Surface area to volume ratio is a ratio where we compare the size of the outside of an object to the amount of space inside of an object.

The easiest way I can think to show you this is by using a cube.

So the surface area can be represented by the net of a cube is all the space that would make up the outside area of the cube.

The volume of the cube is how much space that takes up as a 3D object.

So this volume of a cube could be represented by an actual fully-fledged, fully-built cube.

We can compare the surface area to the volume and that will give us a surface area to volume ratio.

This is expressed as a number.

We're going to have at calculating the surface area to volume ratio together now.

The first thing that we would need to do is calculate the surface area.

Remember that surface area is an indication of the size of the outside of an object or the net of an object.

To calculate the surface area of a cube or a cuboid, we would do the area of a face multiplied by the number of faces on that shape.

For this to be able to take place, I need to know the length of one of the sides of the faces.

Because this is a cube, all of the lengths are the same.

In my example, the length of a side is two centimetres.

If I substitute that value two centimetres, I can find the area of a face by doing 2 multiplied by 2 and I know that there are six faces on a cube so I would multiply that answer by six.

If I solve the first part of my equation, I know that I'm going to actually need to do 4 multiplied by 6.

If I put that on my calculator, I can say the surface area of this cube is 24 centimetres squared.

Remember the unit for a surface area is centimetres squared.

The next thing I'm going to do is calculate the volume.

The volume remember, is an indication of how much space there is inside of a 3D object.

So for example, a cube.

To calculate the volume of a cube or a cuboid, we do the length multiplied by base, multiplied by the height.

In a cube, all of these values are the same.

It's the same as the length of the side of one of the cube faces.

In our cube, remember the length was two centimetres.

I'm gonna substitute that value into my equation and I know that to calculate volume, I'm going to need to do 2 multiplied by 2 multiplied by 2.

If I pop that onto my calculator, I can tell you that the volume of this cube is eight centimetres cubed.

The units for volume are centimetres cubed.

Now I've got the value of surface area and volume.

I can calculate the surface area to volume ratio by comparing these values.

The surface area of my cube was 24 centimetres squared and the volume of my cube was eight centimetres cubed.

The surface area to volume ratio can therefore be written as 24 as a ratio to 8.

I then need to simplify that into the smallest value.

To do that, I divide both numbers by the given number for volume.

In this case, this is eight.

I know that 24 divided by 8, 'cause I'm going to do it on my calculator is three.

And 8 divided by 8 is 1.

So the simplified ratio of the surface area to volume ratio of this cube would be 3:1.

Let's have another go at this example.

In this example I have a cube again, but this time the length of the side is four centimetres.

The first thing I'm going to do is calculate the surface area.

I know that the surface area is found by doing the area of the face multiplied by the number of faces.

To do this, I'm going to do 4 multiplied by 4 to get the area of one face and I'm going to multiply that number by six, because I know that there are six faces on a cube.

If I solve 4 times 4, I know that that is 16.

16 multiplied by 6 would give me 96 and my units are centimetres squared.

I then need to calculate the volume.

I know that to calculate volume, I need to do length multiplied by base multiplied by height and this case would be 4 multiplied by 4 multiplied by 4, and that would give me 64 centimetres cubed.

The surface area to volume ratio would be 96:64.

To simplify that, I'm going to divide both numbers by 64 and that would give me a simplified surface area to volume ratio of 1:5.

You're going to have a go now.

In this scenario we have a cube that has a side length of five centimetres.

Using my model, I want you to calculate the simplified surface area to volume ratio.

Make sure you follow each step, pause the video and once you're ready to check your working, come back and I'll reveal how this would be calculated.

Okay, the first thing you would do is calculate the surface area of the cube.

To do this, you need to find the area of the face and multiply it by the number of faces.

To do this, you would do 5 multiplied by 5.

You would take that number and multiply it by six, because there are six faces.

You would get 150 centimetres squared.

Then you would calculate the volume of the cube by doing length multiplied by base, multiplied by height.

To do this, you would do 5 multiplied by 5 multiplied by 5, and you would get an answer of 125 centimetres cubed.

The surface area to volume ratio could be written as 150:125.

To simplify this, you would divide both numbers by 125 and you would get a simplified surface area to volume ratio of 1.

2:1.

Well done if you got that right, that's a really long process and I hope that you were successful.

If not, don't worry, we're gonna have a couple more opportunities to practise.

For task 3 of today's lesson, I have given you five cubes and I want you to complete the table.

And the table has given you the side length and the picture of the cube.

What you have to do is calculate the surface area and then the volume and then the surface area to volume ratio.

On your worksheet, there should be space for you to show y'all working out at each step and don't forget for that last part you need to simplify your surface area to volume ratio.

Don't forget to check your working out on your calculator.

Pause video here and once you're ready come back to me and I will go through the answers.

Okay, welcome back.

Here are the answers then for that task, for the 1.

4-centimeter side length cube, I have rounded my surface area to volume ratio, just a one decimal list just to simplify it a little bit for you.

So you might need to simplify yours to match my answer.

Make sure you've gone through each one and self-assessed.

Pause the video if you need a bit more time and then I will explain what I want you to do for task b.

So for task b, we've got this table that's showing how the surface area to volume ratio changes as the size of the cube changes.

What I would like you to is describe that trend.

So describe the trend in surface area to volume ratio as the cube gets bigger.

Just one sentence is enough to summarise this.

Pause video and come back when you're done and I will go through my answer.

Welcome back.

So hopefully we found that as a size of the cube increases the surface area to volume ratio decreases.

This isn't good for diffusion, because the higher the surface area to follow ratio, the quicker diffusion takes place.

So the bigger the SL is the higher its surface area to volume ratio would be and the slower diffusion would take place.

Okay, well done some great calculations there and some fabulous practise.

This is going to take us now into the last part of our lesson, but we're going to have a look at diffusion in different living things and explain how living things are adapted to maximise the rate of diffusion.

So we know that there are two types of organisms. We have unicellular organisms such as bacteria which are made of one cell.

We also have multicellular organisms such as animals, plants including humans, and there are organisms that are made of more than one cell.

What do you think diffusion would be like though if this wasn't the case? If humans were just made of one giant human-sized cell? Pause the video and have a think.

Okay, diffusion would happen really, really slowly.

We've just seen that as the size of a cube gets bigger, the surface area to volume ratio decreases and we know that the higher the surface area to volume ratio, the quicker diffusion would take place.

So if we were just made of one giant cell, the surface area to volume ratio would be super-small and therefore, diffusion would happen really slowly.

So instead of being made of one cell, multicellular organisms are made of lots of different cells working together and the cells make structures called tissues and tissues make organs.

And within these tissues and organs we have adaptations to maximise diffusion.

Multicellular organisms such as plants and animals have adapted surfaces that allow this diffusion to happen quick enough.

To maximise the rate of diffusion, these surfaces have short diffusion distances, so a short pathway for a substance to get from one place to another, large surface areas and the ability to maximise concentration gradient.

This is because diffusion is an incredibly important process, especially in humans.

It allows for substances to move into and outta our cells.

Substances that move in our body include oxygen, carbon dioxide, glucose, and other nutrients from our digestive system.

It's really important that all of these substances are able to move freely around the body and to get to all cells where they are needed.

One example of this is diffusion in the gas exchange system.

In the gas exchange system there is an organ called the lungs and within the lungs we have these folded structures called alveoli.

Surrounding each alveoli there are capillaries and the capillaries have this rich blood network that is constantly running alongside this folded structure.

Which substances do you think are going to move by diffusion in this system? Hopefully you realise that this is going to be carbon dioxide and oxygen.

This is the gas exchange system where oxygen is taken out of the air that we inhale and carbon dioxide is taken out of our blood as a waste product, but it's to be exhaled out of the body.

This happens in the alveoli.

The first type of diffusion that happens in the alveoli is the diffusion of oxygen from the alveoli into the bloodstream.

The alveoli contains air which has a high concentration of oxygen.

The oxygen moves from a high concentration in the air to a lower concentration into the blood.

This allows oxygen to be found in the blood and therefore, pumped all the way around the body.

The second diffusion that takes place in the alveoli is the diffusion of carbon dioxide.

Carbon dioxide diffuses from the blood and into the alveoli.

Blood has a higher concentration of carbon dioxide compared to the air in the alveoli.

Carbon dioxide moves from a high concentration in the blood to a low concentration in the alveoli.

This carbon dioxide can then be exhaled and removed from the body.

The alveoli have features that allow the maximum rate of diffusion to take place.

One of those features is that they are only one cell thick.

This means that diffusion only has to happen through one cell.

The shorter the diffusion distance, the quicker that this happens, being one cell thick means that diffusion happens quickly.

The alveoli also have this irregular folded shape.

This maximises the surface area and means that more gases can pass through at each point, at any given time.

And lastly, there is a rich large blood supply surrounding the alveoli.

We've already talked about this capillary network.

This means that as soon as a gas moves into or out of the blood, that blood cell is moved along very, very quickly in the bloodstream.

This constantly maintains a concentration gradient whereby there is a high concentration of oxygen in the alveoli compared to the concentration of oxygen in the blood.

Diffusion also happens in the intestines.

The intestine walls are covered in these tiny projections or extrusions called villi, and each villi contains a large network of blood vessels, we can see here on the diagram.

Water and dissolved ions, which are moving through the intestine after diffusion are be able to then move through the villi and into the blood vessels to be carried around the body.

The villi are kind of like finger-like projections all along the inside wall of the intestine.

Again, the villi are adapted so that diffusion can take place very quickly within the intestines.

This is because we want the products from digestion to be able to diffuse into the blood so that they can be used elsewhere in the body, especially things like glucose and amino acids.

The first adaptation is again that they are only one cell thick.

This minimises the diffusion distance and means that diffusion can happen quickly through the villi.

They also have a folded shape which increases the surface area of the intestine and because there are so many villi, this also increases this surface area on the internal side of the intestines.

And finally, they've got this rich blood supply, which again maximises concentration gradient.

As soon as a substance is diffused into the villi and into the blood, that substance is quickly carried away to another part of the body, maintaining that concentration gradient, there is always a higher concentration of glucose and amino acids and water inside of the intestine compared to inside the blood.

That means that diffusion will happen off those substances into the blood where it is needed.

Plants also require diffusion to take place so that they can get vital substances into their tissues.

Here we've got a diagram of a leaf.

A leaf is in organ in a plant, and gases such as carbon dioxide move into and out of the leaf through holes at the bottom called stomata.

When carbon dioxide is used by photosynthesis, it creates concentration gradient.

This allows for more carbon dioxide to diffuse into the leaf through these spaces.

So it's not just animals and humans that need diffusion, but plants also rely on diffusion to be able to carry out photosynthesis.

Okay, let's do some quick check for understanding questions then.

The first one explain why multicellular organisms need exchange surfaces.

Well done, it's to maximise the rate of diffusion.

Remember, these exchange surfaces are adapted to allow diffusion to happen as quickly as possible.

Okay, next one.

Choose three adaptations of gas exchange surfaces.

Remember to choose three.

Okay, hopefully you got short diffusion distance, large surface area, and high concentration gradient.

Next one, how are villi adapted to maximise concentration gradient? Well done if you said they've got a lot of blood vessels, remember the presence of those blood vessels means that substances moved by diffusion are immediately moved to another part of the body.

This maximises the concentration gradient.

Next one, how are alveoli adapted to have a short diffusion distance? Yeah, well done if you said that they are only one cell thick, this means that the substances don't have very far to travel at all and therefore, diffusion happens much quicker.

Okay, for task 4, we've got a couple of questions to do.

The first two are on the screen here.

Question a, explain how alveoli in the lungs are adapted for efficient gas exchange.

And question b, smoking can cause a disease called emphysema.

So emphysema is just a disease in the lungs.

This damages the alveoli resulting in fewer, large air sacs instead of the many small ones that are there in a person without emphysema.

Explain, give reasons why, why this often leaves sufferers of emphysema breathless.

So constantly trying to get more oxygen into their bodies.

Write down your ideas wherever you're working and once you're ready for the answers, come back and I'll go through them.

Okay, question 8.

How are alveoli in the lungs adapted for efficient gas exchange? So they've got small alveoli with a large surface area, they've got a good blood supply, they're really thin, they're one cell thick, and they are moist surfaces, which also helps diffusion.

That's not something that we've been through today, but just to make you aware, moisture does impact the rate of diffusion.

The more moisture there is, the quicker diffusion can take place.

So question b, we were asked to explain why people with emphysema are breathless.

This is because the surface area of the lungs is reduced, therefore, there's less oxygen diffusing into the blood or oxygen isn't diffusing into the blood as quickly.

Therefore, our body reacts to that by trying to get us to get more oxygen into our blood by increasing our breathing rate and sort of gasping in as much air as possible.

Okay, the next one, describe and explain how the villi are adapted to maximise the rate of absorption of the products of digestion.

Don't be put off by how that sentence is written.

Read it again and think about how it applies to what we've been learning.

Pause the video and have a go.

Once you're ready to go through the answers, come back to me and I'll share my ideas.

Okay, let's have a look at the answer then.

They've got a large surface area.

They have got capillaries which maintains the concentration gradient and they have really, really thin or one cell thick and that gives a short diffusion distance as well.

Let's just quickly go through a summary of everything that we've learned today.

So first of all, we defined diffusion as the net movement of particles from an area of high concentration to an area of low concentration.

We learned that the rate of diffusion can be increased by increasing temperature, concentration gradient and surface area to volume ratio.

We also learned by looking at the calculations of surface area to volume ratio, that the smaller cell is the larger its surface area to volume ratio.

We then looked at exchange surfaces in multicellular organisms and we said that these are needed to ensure that diffusion happens quickly enough within organisms. And we looked at the adaptations of exchange surfaces such as the villi in the intestine and the alveoli in the lungs, and we said that they are adapted to have a short diffusion distance, a large surface area, and a high concentration gradient.

And this maximises the rate of diffusion and makes sure that it can happen quickly enough within these organisms. So we've learned an incredible amount today.

You've had done a really good job.

I hope you're as proud of you as I am of you and I hope see you very soon for some more science.

Have a lovely rest of your day.