Lesson video

Hello and welcome to today's lesson.

The lesson today comes from the unit plant nutrition and photosynthesis, and this it's the last lesson in the unit.

And today's lesson is called adaptations of plants for photosynthesis: gas exchange and stomata.

I'm Dr.

Mason from the Oak National Academy.

Let's have a look at the outcome from today's lesson.

So by the end of the lesson today you'll be able to describe how plants, and specifically leaves, are adapted to take in and release gases to support photosynthesis, and the gases we're talking about here are oxygen and carbon dioxide.

During the lesson today, there are a number of key words that you need to look out for and make sure you understand what they mean and how to use them.

The first is gas exchange, then we have stomata, guard cells, diffusion and net movement.

So make sure you keep an eye out for those words and you know what they mean.

Today's lesson is in three parts.

We're gonna start off by looking at photosynthesis and gas exchange.

And then in the second part we'll look at how leaves are adapted for gas exchange.

And in the third part we'll look in more detail at diffusion.

So let's get going with the first part, photosynthesis and gas exchange.

We know that plants make their own food using the process of photosynthesis, and we know that photosynthesis takes place in parts of plants where there's light and most of it takes place in the leaves, and in particular it's inside the cells.

And you can see here on the screen we've got a cross section, a diagram of what a leaf would look like if you cut through it and looked at it under a microscope.

So you can see that we've got tightly packed layer of cells across the top that are full of chloroplasts and also some chloroplasts in the cells in the second layer towards the bottom of the diagram, but it's in these cells that photosynthesis takes place.

And the process of photosynthesis makes glucose, and glucose is a sugar, so a carbohydrate, and it's this that we call the plants food.

We can summarise photosynthesis and the chemical reactions that form part of that process using a word summary.

And so what we've got here is water and carbon dioxide are the reactants.

So the plant gets the water from the soil, which goes in via the roots, and carbon dioxide, which is in the air around us, and that goes into the plant through the leaves.

So those are your reactants and it produces glucose which is the product that the plant wants, that's its food.

And then you've got oxygen gases produced and that's a waste product.

So we can summarise all the chemical reactions using this word summary.

So we're gonna start by having a look at one of the reactants.

So the first one we'll look at is carbon dioxide.

So carbon dioxide is a colourless odourless gas and it forms a very small percentage, much less than 1% of the air around us, and it's this that the plant needs for photosynthesis and it takes it in from the air via its leaves.

One of the products of photosynthesis is a gas called oxygen and oxygen is not required by the plants, and so it's a waste product and it leaves through the leaves.

So just like the carbon dioxide went in through the leaves, the oxygen gas leaves through the leaves and goes back into the air.

So we've got two gases here, we've got carbon dioxide going into the leaves and we've got oxygen gas coming out of the leaves, and we call this gas exchange.

You might have come across the word gas exchange before for example, thinking about the human breathing system, and this is another example of gas exchange.

Right, we're gonna check our understanding now.

So what I'd like you to do is to join the boxes to describe the process of gas exchange.

So we've got here three sets of boxes, you should draw lines between the boxes to create correct statements.

You might want to pause the video while you do this and then we'll have a look at our answer together.

Let's have a look at what you put.

So carbon dioxide gas moves into the leaves and we've got oxygen gas moves out of the leaves.

So that's right.

So this gas exchange is taking place in the leaves.

The roots are not involved at all.

Well done if you got that correct.

If you didn't, it might be a good idea if you want to note down what the correct answer is so that you know for next time.

Now we're gonna have a go at a practise question.

So you can see in the picture we have a commercial farm growing, what looks like salad crops.

So a farmer grows plants in a greenhouse and the farmer decides to add some extra carbon dioxide gas to the air inside the greenhouse.

What I'd like you to do is to write down an answer to explain, first of all, how the plants will make use of this carbon dioxide gas and how this will benefit the farmer.

You'll need to pause the video while you do this and then we'll come back and have a look at the answer.

Let's have a look at the answer.

So, first of all, we asked you to explain how the plants will make use of the carbon dioxide gas that the farmer's been putting into the greenhouse.

Well, first of all, the plants are gonna take in the carbon dioxide through their leaves and this carbon dioxide can be used in photosynthesis, remembering it's a reactant in photosynthesis so it's taken in via the leaves and it's used photosynthesis to make glucose and that's the plant's food, and if the plant's got more food that's going to support its growth and survival.

And how will this benefit the farmer? Why would a farmer pay to put more carbon dioxide into a greenhouse? Well, it gonna benefit the farmer because more gas, more carbon dioxide gas means the plants will photosynthesize more which means they'll produce more glucose means they'll grow bigger and the farmer will get a bigger crop, and the more crop you've got to sell the more money the farmer will make.

Well done if you got those answers correct.

If you didn't, don't worry, just add in some of the points here to your answer so that you've got a complete answer for next time.

So we've had a look at photosynthesis and gas exchange and we know what two gases are being exchanged.

Now we're gonna move on to the second part of the lesson where we're going to have a look at how the leaves of plants are actually adapted to exchange these two gases.

So we're gonna start by looking at the surface of a leaf.

And if you look at it under a microscope you'll see that there are lots of tiny holes in the surface and these holes are called stomata, and there are lots of stomata on every leaf.

And this is what it looks like if you look at them under a powerful microscope.

There are also some types of plant where you can actually just put the leaf under the microscope and you'll be able to see the stomata, because they're quite big.

Let's take a closer look at the structure of the leaves and where the stomata are.

So you can see here we've got a cross-section diagram of a leaf, and you can see there that the stomata is shown as a hole in the bottom of the leaf.

You do get stomata in the top surface of a leaf, but they're much less common.

The majority of the stomata of a leaf are on the underside, so the bottom surface of the leaf.

And you can see there that the stomata just lead to an airspace where the air is able to circulate.

The stomata are an adaptation for photosynthesis and they allow the gases of carbon dioxide and oxygen to move in and out of the leaf.

So you can see there we've got our gas exchange taking place there, carbon dioxide's able to move in because the plant needs it for photosynthesis and the waste gas, the oxygen is able to leave and go back into the air via the stomata.

Now let's check our understanding.

I want you to have a go at completing the sentences about how plant leaves are adapted for gas exchange.

You might want to pause the video while you complete this activity and then we'll look at the answer together.

Right, let's have a look at the answer.

So there are holes called stomata in the surface of plant leaves.

Carbon dioxide gas for photosynthesis moves in through the holes.

And oxygen gas made by photosynthesis moves out through the holes.

So we know that carbon dioxide is a reactant and oxygen is a product of the reaction.

Well done if you got those correct, if you didn't just correct your answer so that you know for next time.

So the word stomata refers to the holes in the leaf surface, and as I've said the majority of the stomata are on the underside of a leaf.

And you can see we've got their nice diagram of a stomata.

And you can see that there are two cells, in particular, that surround the stomata and they are called guard cells, and so you have two guard cells around each stomata.

Guard cells change shape, and when they change shape it controls the opening and the closing of the stomata.

So you can see here on the left hand side as the stomata, oh, as the guard cells, sorry, look, take on sort of more arched appearance it opens the stomata, and on the right hand side you can see that the stomata are closed and it's the changing in shape of the guard cells that controls the opening and the closing of the stomata.

So you might want to have a think about why might a plant want to open and close the stomata? What's the advantage to the plant of being able to do that? Well, water vapour can move in and out through the open stomata.

And if you think about it, a plant doesn't want to lose too much water, and therefore when a plant's not undergoing photosynthesis it wants to not lose too much water, so it'll close the stomata.

Now let's check your understanding, and I want you to write down which of the labelled structures are guard cells.

Is it A, B, or C? You can pause the video while you have a think about it.

Let's have a look at the answer.

The answer is of course A.

B refers to the stomata, so the hole, and C are a different type of cell.

A is the two, or shows, it points to one of the two guard cells that surround B, which is the stomata, the opening hole in the middle.

Well done if you got that correct.

Now we're gonna have a look at a practise question.

So sometimes the stomata are open and sometimes they are closed, and you can see the diagrams there of them being open at the top and closed at the bottom.

So Lucas thinks that the stomata are more likely to be open during the night and Izzy thinks they're more likely to be open during the daytime.

So we've got two different points of view there.

Have a talk with your partner, or if you're by yourself just have a think about who do you agree with and why.

So try and explain your answers to why you agree with Lucas or Izzy.

You'll probably want to pause the video while you complete this activity.

Let's have a look at the answer.

So who did you agree with? Well, Izzy has the correct idea.

They're more likely to be open during the daytime and that's because that's when it's light, and of course we know you need light for photosynthesis to take place, and we also know that you need carbon dioxide gas for photosynthesis to take place.

Therefore, the stomata are more likely to be open during the day in order to allow carbon dioxide to move in through the stomata into the leaf and support the process of photosynthesis.

Photosynthesis doesn't take place at night therefore the stomata close at night and it prevents the plant from losing too much water.

Well done if you've got those correct.

If you didn't get all of those points, make sure that you note those down so you know for next time.

So we've had a look at photosynthesis and gas exchange, so we know which gases are being exchanged.

And we've had a look at how leaves, in particular, are adapted for this gas exchange.

And now we're gonna move on and we're gonna have a look at diffusion.

We're gonna start by taking a more detailed look at carbon dioxide and oxygen.

So carbon dioxide and oxygen are both gases and they're made up of particles.

So you can see on the left hand side of the screen we've got carbon dioxide gas which is made up of a carbon atom and two oxygen atoms. And on the right hand side of the screen we've got particles of oxygen gas and they're made up of two oxygen atoms. And these gases move around randomly, so the arrows you can see show movement and you can see there's no particular direction, they're moving in all different random directions, and it's this random movement that causes diffusion.

So diffusion causes the carbon dioxide and the oxygen to move in and out of the airspace through the open stomata on the bottom of the leaf.

And you can see there the red and the pink arrow showing the carbon dioxide moving in through the stomata and into the airspace inside the leaf and the pink arrow showing the oxygen particles leaving the leaf through the stomata, and this movement is completely random and it's caused by diffusion.

So we're gonna start by taking a look at carbon dioxide.

So carbon dioxide goes in through the stomata and it's needed for photosynthesis because it's a reactant in that process.

And you can see there the red arrows show the carbon dioxide gas going in and it will go into the airspace and it'll be taken into the cells so it can take part in photosynthesis, which takes place inside the chloroplasts, inside the cells.

If we have a look at oxygen, so oxygen is made, it is a product of photosynthesis and so it'll diffuse out of the cells into the airspace and then out through the open stomata on the bottom of the leaf.

Leaves are really thin which keeps the diffusion distance short and that allows the cells to be supplied with carbon dioxide gas very quickly so that they can keep photosynthesizing because diffusion takes time, and so if the leaves were too thick it would take a long time for some of the cells to get enough carbon dioxide.

So by having what we call a short diffusion pathway, which basically means a short distance that the gas has to diffuse over, and it's the same for the waste oxygen that's made it can also be removed quickly because it's got a short distance in which to diffuse over.

Let's check our understanding.

So which process moves particles of gases through open stomata? Is it A adaptation, B diffusion, or C photosynthesis? You might want to pause the video while you choose your answer and then we'll have a look at the correct answer together.

Let's have a look at the answer.

So the answer is B it's diffusion.

Well done if you got that correct.

If particles of gas are more concentrated in one area, so we have what's called an area of higher concentration, which you can see on the screen, and then we have an area of lower concentration, so where there's less particles in an area we have what we call a concentration gradient.

And a concentration gradient is where you have more in one area than you have in another.

So this means more of the particles will be moving from the area of higher concentration to the area of lower concentration going down a concentration gradient, than are moving in the opposite direction, i.

e.

from the lower to the higher concentration.

Again, it's important to remember that the particles are still moving randomly.

They're not choosing to move from a higher to a lower area, they are simply moving randomly, but because you've got more in one area than another they will even out by moving from a higher to a lower concentration, and we call this the net movement.

So you've got particles moving from higher to lower, lower to higher, but overall we will get more particles going from the higher concentration area to the lower concentration area, and so we have a net movement in the direction of the lower concentration area.

Now let's have a look in more detail at how diffusion affects the movement of carbon dioxide.

So inside the leaf in the airspace you've got carbon dioxide particles because these have been taken into the cells and used in photosynthesis, we actually have fewer particles of carbon dioxide in the airspace inside the leaf compared to the air outside the leaf, and therefore the concentration gradient is from the outside to the inside, and you can see that quite clearly on the diagram.

We've got lots of carbon dioxide molecules outside of the leaf and inside in the airspace we've got fewer particles.

So we've got a concentration gradient.

And so the net movement of carbon dioxide is going to be from a higher to a lower concentration so from the outside to the inside.

And so that's how carbon dioxide moves into the leaves.

Now let's have a look at oxygen and take a closer look here.

So you've got lots of particles of oxygen inside the airspace inside the leaf and that's because oxygen was made in photosynthesis, and you can see it's filling the air space.

And so we've got a higher concentration of oxygen inside than we have outside the leaf, and therefore the concentration gradient is gonna be the opposite of carbon dioxide.

This time it's gonna be from the inside to the outside.

So the net movement of oxygen particles is gonna be from inside the leaf in the airspace to the outside of the leaf.

Let's check our understanding of diffusion of these gas particles.

Use these words or some of these words from the list to fill in the gaps.

You might need to pause the video while you do this.

Let's have a look at the answer.

So if particles of a gas are more concentrated in one area than another, this creates a concentration gradient.

More of the particles will be diffusing from the area of higher concentration to the area of lower concentration.

And this is called the net movement.

Well done if you got those correct, and if you didn't remember to note down the right answer for next time.

Now let's have a go at a practise activity.

So what we'd like you to do here is to complete the two diagrams below to show the diffusion of carbon dioxide and oxygen particles through open stomata when the leaf cells are undergoing photosynthesis.

So you'll need to draw particles to show the concentration gradient and add arrows to show the direction of the net movement of the particles.

You'll need to pause the video while you complete this activity and then we'll have a look at the answer together.

Let's have a look at the answer.

So for carbon dioxide, you're asked to draw particles.

And you can see here that we've got more particles of carbon dioxide outside the leaf and much fewer inside the leaf, and then if we add an arrow, it shows the net movement of particles from the high to the low concentration going down a concentration gradient.

And for oxygen, we've got the opposite, we've got lots of oxygen particles inside the leaf because that's where photosynthesis has been taking place and the gas has moved into the airspace and we've got much fewer oxygen particles on the outside and therefore the direction of movement is gonna be from the higher concentration to the lower concentration going from inside to outside the leaf.

Well done if you got that correct.

We've almost reached the end of today's lesson.

Let's go through a summary of the key points to make sure that you've got those down and that you understand those.

So first of all, plants take in carbon dioxide gas they take it in through their leaves and it's a reactant in photosynthesis and they get the carbon dioxide from the air around us.

The process of photosynthesis releases oxygen and that's released as a gas from the leaves back into the air, and we call this process gas exchange.

So we've got carbon dioxide gas going into the leaves and we've got oxygen gas leaving the leaves.

Then we looked at stomata, so stomata are the very small holes that are in the surface of leaves, most of them are on the underside of the leaf.

And the stomata, these holes can open and close, and it's the guard cells, so there are two guard cells around each stomata and they control the opening and the closing of the stomata.

And when the stomata are closed the gases are not able to diffuse, but when they're open, the particles of carbon dioxide gas and oxygen move through the open stomata and they do that by process of diffusion.

Well, that's it for today, we've reached the end of today's lesson.

You can head on over and complete the exit quiz and I think that'd be a really good idea to have a go at that to make sure that you've definitely understood the key points.

That's all for today, see you next time.