Lesson video

Hello and welcome to this lesson from the unit plant nutrition and photosynthesis.

This lesson is called adaptations of plants for photosynthesis: absorbing light.

So in this lesson we're gonna be thinking about how the structures of plants help them to carry out photosynthesis so that they can make their own food.

My name is Mr. Moore and I'm gonna be taking you through the lesson today.

So if you're ready, let's make a start.

So by the end of today's lesson you should be able to describe how plants are adapted to absorb light for photosynthesis.

There are some key words to look out for in this lesson.

They are chloroplasts, adaptation, chlorophyll and surface area.

Now, some of those words might be familiar to you already.

Some of them might be new, but don't worry, we will go through all of them as we go through the lesson today.

The lesson is in two parts.

The first part is about how the structures of plant cells help them to carry out photosynthesis.

And we'll also start to think about what that word adaptation actually means.

And in the second part we'll be thinking about how the structures of leaves help them to absorb the light that they need for photosynthesis.

So let's get straight into the first part.

Adaptations of plant cells for photosynthesis.

You should already have learned a bit about this thing called photosynthesis.

So you should know that plants are producers and that means they make their own food and they do that by this process called photosynthesis.

Now you should remember that photosynthesis is a series of chemical reactions and it takes place in the leaves and other parts of the plant that are above the ground.

Now why is that important? Why is it taking place in the parts of the plant including the leaves that are above the ground? Can you remember? Well, there's a big old shining clue in the photograph there.

It's because photosynthesis requires light and specifically light provides the energy that is needed for photosynthesis to take place.

You should also remember that all living organisms are made up of cells and that includes plants and the leaves of plants.

So if you were to look at a leaf under a microscope you would see something like this.

You would see that there were lots of cells there like living building blocks that made up the leaf of the plant.

Now one important thing to remember about photosynthesis is that it takes place inside the cells that make up the leaf.

So what do we already know about cells? Well, this diagram shows one cell from a leaf of a plant and it shows some of the parts that the cell is made up of.

Now think back to when you learned about plant cell structure and hopefully you should recognise some of these parts.

Some of these parts are present in both plant cells and in animal cells.

So do you recognise these ones? Can you think what they are? Well, one of these parts that's present in both plant cells and animal cells is the nucleus.

And can you remember what the nucleus does? What is its job? What is its function in the cell? I'll give you a few moments just to have a think and then we'll go through the answer.

So the function of the nucleus is to store the cell's DNA and it also controls the cell's activities.

Now what about the cytoplasm? Again, this is present in both plant cells and animal cells.

What is the cytoplasm? What is it for? Well, the cytoplasm is a sort of jelly-like substance and it's where most of the chemical reactions that support life in the cell take place.

What about the cell membrane? What does that do? Well, the cell membrane is a barrier, so it holds everything in the cell and also it controls the movement of substances into and out of the cell.

And finally, mitochondria.

These are present in both plant and animal cells.

What is the job of the mitochondria? What happens there? Well you probably remember mitochondria from your lessons on cellular respiration.

So aerobic cellular respiration, the type that needs oxygen, that takes place in mitochondria.

And I'm sure you'll remember that cellular respiration is the process of using food as a fuel, as a source of energy to provide the energy that the cell needs to stay alive.

There are some parts of a plant cell that are only present in plant cells.

They're not present in animal cells.

So this includes the permanent vacuole.

Now can you remember what that's for? Well the permanent vacuole is essentially a storage compartment.

So the cell fills up the vacuole with cell sap and that is a mixture of water and other substances that the cell wants to store.

What about the cell wall? This is only present in plant cells.

What does the cell wall do? Well, the cell wall is on the outside of the cell membrane and it provides strength to the cell.

It helps to maintain the structure and it also protects the cell from pathogens, from things that might harm the cell.

And finally, what about the chloroplasts? What do they do? Well, chloroplasts are the structures in the cell that are most relevant to our lesson today because they are where photosynthesis actually takes place in a plant cell.

So a quick check.

Can you add the correct names to the parts of the cell? I'll stay quiet for about five seconds, but you might want to pause the video to give you yourself time to add all of the labels on and then press play again when you're ready to go through the answers.

Did you manage to add all the labels to the parts? Let's go through the answers and see which ones you got right.

So we have the nucleus, we have mitochondria, we have the vacuole or permanent vacuole, the cell wall on the outside, just inside that we have the cell membrane.

And finally, most important for our lesson today, we have chloroplasts.

And another quick check.

Can you match each part of the cell on the top row to its correct function on the bottom row? So draw lines to match the boxes.

I'll give you five seconds.

You might want to pause the video and play again when you're ready.

All done? Let's go through the answers.

So you should have matched chloroplasts to the box that says where photosynthesis takes place.

You should have matched mitochondria to the box directly below which says where cellular respiration takes place.

And that leaves the nucleus, which you should have matched to the box that says it stores the cell's DNA.

So let's start exploring that word adaptation because that's actually a really important bit of vocabulary in science and in biology.

In biology, an adaptation is a feature that helps an organism to function better and to stay alive.

And adaptations are amazing for that reason.

We would actually all be dead without adaptations, we wouldn't be able to function, we wouldn't be able to stay alive.

You've probably learned about some adaptations already in plants and in animals.

So our own eyes are an amazing adaptation that enables us to see.

The digestive system is an adaptation that enables us to take in food and break it down and absorb it.

There are so many examples of adaptations.

Some animals have wings that enable them to fly.

Some predators have sharp teeth and sharp claws that mean they're better able to catch their prey.

And some animals that are prey have fur or particular colours that provide camouflage and that helps them to hide from predators.

It helps them to survive.

And plants have adaptations as well.

So some plants have flowers that enables them to reproduce.

Some plants have spikes and thorns that protect them from being attacked.

So you can probably think of many other examples of adaptations And maybe just pause the video and take a few minutes to think of some examples of adaptations for yourself.

Try to include some really amazing ones like bats that use echo location to find their way around or plants that are adapted to eat insects.

Try and include a couple of really amazing inspiring adaptations.

So pause the video for a few seconds and I'll let you have a think.

Did you think of some good ones? I hope you did.

So adaptations are going to come up a lot in your biology lessons.

And that be that's because really adaptations are everywhere.

One of the most important things to take away from your biology lessons is that all living organisms are usually well adapted to function and to stay alive.

And every part of every organism is adapted to help it function and to help it stay alive.

So the biological molecules that make up the cells, that make up the tissues, that make up the organs, and the organ systems that all make up the organism every single bit of it is adapted to help the organism to function better and to stay alive.

So let's come back to plant cells and to photosynthesis.

So we know that plant cells have chloroplasts and chloroplasts are an adaptation.

They're an adaptation that enables the cell to carry out photosynthesis.

And they're an absolutely amazing adaptation because without it, plants would not be able to photosynthesize.

They would not be able to make their own food and they would not be able to stay alive.

And remember that producers, plants and other organisms that can make their own food, they are at the start of every food chain on earth.

So all the food that we eat comes originally from plants and is made by photosynthesis in chloroplasts.

And that's why chloroplasts are an absolutely amazing adaptation and they are vital to support all life on earth.

But having said that, not all plant cells have chloroplasts.

So if we were to zoom in on a single cell from a root, so the roots that grow down into the soil if we took a single cell from there it might look a bit like this.

So this root cell has mitochondria, it has a cell wall, it has a cell membrane, it has a nucleus and it has a permanent vacuole.

But do you notice it doesn't have chloroplasts? Now why might that be? Well, we know the chloroplasts are where photosynthesis takes place.

We know that photosynthesis can only take place when it's light, and, of course, it's usually quite dark underground where the roots are.

So there's no need for root cells to have chloroplasts.

But as we know, cells in the leaves and other parts of the plants that are above the ground they do have chloroplasts because that's where there's light for photosynthesis to take place.

So let's do a quick check of what we've just been thinking about.

So what is an adaptation? Do you think it's a feature that helps an organism to function and survive? Do you think an adaptation is part of a plant cell or do you think an adaptation is the structure where photosynthesis takes place? I'll give you a few seconds to decide.

The answer is A, that an adaptation is a feature that helps an organism to function and to survive.

Okay, let's do a little activity now.

So here we have a diagram of a cell from a root of a plant.

And what we want you to do is to write your own explanation to explain why this root cell does not need to have chloroplasts.

So think about all the things that we've just been learning about and try to write your own explanation for why the cell does not need to have chloroplasts.

So you'll want to pause the video for this one to give you time to write your explanation.

Press play again when you're ready to talk through the answer.

Did you manage to write your own explanation? Well, here are some examples of what you might have included in a good explanation for why the root cell does not need to have chloroplasts.

Chloroplasts are where photosynthesis takes place.

Photosynthesis needs energy from light and the plant's roots are underground.

And of course there's no light underground.

That's why a root cell does not need to have chloroplasts.

So we've just been thinking about plant cells and how cells are adapted to help them carry out photosynthesis.

But as we move into the second part of the lesson let's zoom out a bit from cells and think about whole leaves and how leaves are adapted to help them to absorb plenty of light for photosynthesis.

We know that leaves are made up of cells and that these cells contain chloroplasts.

So if we looked at a leaf through a microscope this is the kind of thing we might see.

We see cells like living building blocks and each of these cells contains lots of chloroplasts.

But what do you notice about the chloroplasts? They're all very green, aren't they? Now that is because chloroplasts contain a green substance and that substance is called chlorophyll.

We know that photosynthesis can only happen when it's light and it's actually the chlorophyll that absorbs the light.

So the chlorophyll that is inside the chloroplasts that are inside the leaf cells, it's the chlorophyll that's absorbing the light that is needed for photosynthesis.

And it's the light that provides the energy that is needed for the chemical reactions of photosynthesis to take place.

So let's do a quick check on that before we go any further.

So what we want you to do is to use some of the bold words from the list to fill in the gaps and complete these sentences.

So pause the video and decide which of the bold words you want to use to put into the gap for each sentence.

Let's take a look at the answers.

Photosynthesis takes place in structures called chloroplasts inside the cells of plant leaves.

These structures contain a green substance called chlorophyll.

This substance absorbs light for photosynthesis.

Now have you ever picked up a leaf and turned it onto its side? What did you notice? Well, if you do that, you'll see that leaves are usually very thin.

Now we know that leaves are made up of cells.

So if we took a a small chunk from a leaf and we zoomed right in close enough to see the cells we would see that that chunk of leaf is made up of several layers of cells.

And we know that light has to move through the leaf so it can be absorbed by the chlorophyll inside the chloroplasts that are inside the cells.

But light doesn't just need to be absorbed by the cells near the top of the leaf.

It needs to pass all the way through to the cells in the middle and all the way through down to the cells at the bottom so that they can all carry out photosynthesis.

And that is much more likely to happen, light is much more likely to be able to pass all the way through to those cells at the bottom if the leaf is thin.

So it makes a lot of good sense that leaves are adapted to be thin so that light can pass all the way through to reach all of those cells so that all of those cells can be photosynthesizing and producing food for the plant.

So another quick check before we go any further.

Again, what you have to do is to pick bold words from the list to fill in the gaps in the sentences.

Pause the video while you do it and then we'll go through the answers.

Let's have a look at the answers in that first sentence.

Leaves are adapted to be thin.

Second sentence.

This allows light to pass through the leaf and reach all the cells that contain chloroplasts.

As well as being thin, leaves are adapted in another way to maximise the amount of light they can absorb for photosynthesis.

So think about the way that a leaf has held on a plant.

Most of the light that they absorb for photosynthesis is absorbed through the top side of the leaf.

Now that top side has a surface area.

That's the area that we've highlighted in pink on the diagram, and that's the area of the leaf through which light is absorbed for photosynthesis.

So it makes sense that leaves are adapted to have a large surface area because that means they can absorb as much light as possible for photosynthesis.

And if they're absorbing more light that means more photosynthesis can take place.

And if more photosynthesis is taking place then more food is made, and that is what supports the growth and survival of the plant.

So let's think a bit more about area, the concept of area.

And this should be familiar to you from your maths lessons.

So if we want to calculate the area of a square or a rectangle, we multiply its length by its width.

So if the length and the width of the rectangle are measured in centimetres then the area in centimetre squared is equal to the length multiplied by the width.

So let's try doing that for these pieces of leaf.

So I'll have a go first and then we'll let you have a go.

So this one we can see on the screen.

This piece of leaf measures three centimetres by three centimetres.

So to calculate the surface area, we multiply the length by the width, which is three times three and that's equal to nine centimetres squared.

So that is the surface area of the leaf.

Now you have a go with this piece of leaf that measures three centimetres by six centimetres.

Pause the video and then we'll come back and check the answer.

Are you ready? So to calculate the surface area of this piece of leaf that measures three centimetres by six, we multiply three by six and we get 18 centimetres squared as the surface area of this piece of leaf.

But of course, in real life, leaves are not usually square or rectangular in shape.

In fact, most leaves have a very irregular shape.

So we can't just easily calculate the surface area of a leaf like this by multiplying its length by its width because it has this really irregular shape.

So what we can do instead is estimate the surface area of this leaf.

Now an estimate is like a well-informed guess of what the surface area is.

So to do that we can place a grid of squares over the leaf and each square has an area of one centimetre squared.

So to estimate the total surface area of the leaf we can simply count up the number of squares that are at least half filled with leaf.

Now what does that mean? So squares like these two we would include in the count because they are at least half filled with leaf But these other two squares that are indicated, they're not at least half filled with leaf.

So we're not going to include those in the count.

So let's have a go at that.

I'll do it first and then we'll bring up another leaf that you can have a try at yourself.

So for this one on the left, let's put our grid of squares over the leaf.

Each square is one centimetre by one centimetre.

So we'll highlight all of the squares that are at least half filled with leaf and count those up and that comes to 30.

Now we know that each of those squares is one centimetre by one centimetre.

That means it has a surface area of one centimetre squared.

So we can estimate the surface area of the leaf to be 30 centimetres squared.

Now you have a go, another leaf, another grid.

Again, each square measures one centimetre by one centimetre.

So use the method that we've just explored to estimate the surface area of this leaf.

Pause the video and then we'll check the answer.

Have you finished? Let's take a look at the answer.

So first of all, we'll highlight all of the squares that are at least half filled with leaf.

We'll count those up, and in this case it's 13.

So we know that each square has an area of one centimetre squared so we can estimate the surface area of the leaf to be 13 centimetres squared.

So here's a problem for you to solve.

So which of these two leaves has the largest surface area? So use the method that we've just practised on each of these leaves and then decide which of them has the largest surface area.

Again, you'll want to pause the video while you do it and then we'll go through the answer.

If you're ready, let's have a look at the answer.

So for leaf one, the first thing we can do is highlight all of the squares that are at least half filled with leaf and then count them up.

And in this case it's 13.

Now, unlike before, notice that our grid of squares is a bit different.

So each square measures 0.

5 centimetres by 0.

5 centimetres.

So to work to estimate the surface area of this leaf we will have to multiply 0.

5 by 0.

5.

That gives us the surface area of one square and then we multiply that by 13 and the answer is 3.

25 centimetres squared.

So let's do the same method for leaf two.

So we'll highlight all of the squares that are at least half filled with leaf, count them up and it comes to 11.

So again, each square is 0.

5 by 0.

5.

So we times that by 11 and we get the answer of 2.

75 centimetres squared as our estimate of the surface area of this leaf.

So the answer to the question is leaf one has the largest surface area.

And to finish off this lesson, one more problem for you to have a go at.

And to answer this one, you'll have to think about all the things we've been talking about in the lesson today.

So the problem is this.

Imagine 16 plant cells arranged in two different ways.

This is arrangement one, this is arrangement two.

Each of those green cubes represents one plant cell and they're being lit from above.

So light is shining on the cells from above.

Now, you can imagine that each one of those cubes, each one of those cells contains chloroplasts with chlorophyll and that they can absorb the light and carry out photosynthesis.

So the question is, which arrangement of cells is the best adaptation for photosynthesis? And what we'd like you to do is write your own explanation for your answer.

So decide which arrangement you think is best and then explain why you think it is best.

Pause the video and then we'll go through the answer.

Are you ready to go through the answer? So the question was, which arrangement of cells is the best adaptation for photosynthesis? The answer is arrangement two is the best adaptation for photosynthesis.

And in your explanation, you should have explained that arrangement two is the best because arrangement one is thicker.

So light may not be able to pass through all the layers of cells to the ones at the bottom.

But of course, arrangement two is much thinner.

And we know that being thin is a good adaptation for photosynthesis because light can reach all of the cells and they can all be photosynthesizing.

But also, arrangement two has a larger surface area, so more of the cells can be absorbing light at the same time.

To finish off this lesson, let's run through a quick summary of what we learned today.

The leaves of plants are made up of cells, and these cells contain small structures called chloroplasts.

And chloroplasts are where photosynthesis takes place.

Chloroplast contain a green substance called chlorophyll and it's chlorophyll that absorbs the light that is needed to provide the energy for photosynthesis.

So chloroplasts are an adaptation of plant cells that enables the cells to photosynthesize and therefore to make food for the plant.

Plant leaves are adapted to be thin and that allows light to pass all the way through the leaf to reach all of the cells containing chloroplasts that contain chlorophyll to absorb the light.

And plant leaves are also adapted to have a large surface area so that they can absorb as much light as possible to enable photosynthesis to take place and therefore to enable as much food as possible to be made for the plant.

We've come to the end of this lesson.

I hope you enjoyed it.

Don't forget to have a go at the exit quiz and we'll see you in the next lesson.