Lesson video

Hello, and welcome to this lesson from the Oak National Academy.

Today's lesson is on photosynthesis, an endothermic process that takes place in chloroplasts.

And it's taken from the unit "Photosynthesis: requirements and products." Hiya, I'm Mrs. Wheate, and I'm gonna be your teacher for today's lesson.

By the end of today's lesson, you will be able to describe photosynthesis as an endothermic chemical process, the role of light, and how leaves are adapted for photosynthesis.

Let's look at our keywords.

Today's lesson has got five keywords, and I'll read them out now.

Endothermic, a chemical process that requires energy transferred from its surroundings.

Adaptation, a feature that helps an organism to function and survive.

Chloroplast, subcellular structures in which photosynthesis takes place.

Chlorophyll, a green pigment in chloroplasts to which energy is transferred by light.

And stomata, pores, which are holes, in the surface of a leaf through which water, oxygen, and carbon dioxide can diffuse.

So if that wasn't quite enough time to absorb all of those, I'll be quiet for five seconds so you can read them through again.

But if you want a bit more time to read them through a couple of times or maybe to write them down, you can pause the video and then click play when you're ready to continue with the lesson.

Today's lesson is in two parts.

In the first part of today's lesson, we'll be talking about the process of photosynthesis, the reactants and the products, and the word equation and the balance symbol equation, and we'll talk about how it is an endothermic process.

In the second part of today's lesson, we will talk about how the leaves of a plant are adapted for photosynthesis to take place.

But first, an endothermic process.

I think plants are really amazing.

I was hungry this morning, and so I went into my kitchen and I made myself a bowl of cereal.

Plants don't do that, obviously.

When I need to eat, I need to go and I need to go eat another animal or eat a plant in order to get the things that our body needs in order to grow itself and to repair itself.

Plants don't eat, so where do they get their food from? Plants are a type of organism called a producer, which means they make their own food inside themselves.

So plants and other producers make their own food inside their cells using a process called photosynthesis.

Isn't that amazing? So imagine every time you needed to eat, instead of having to go and get food and eat it, you could do a process inside your own cells and it would just make food inside your cells.

I think that's really amazing.

So the process photosynthesis uses carbon dioxide and water, and it makes glucose, which is a type of sugar and a simple carbohydrate, that is the producer's food.

It is used to make biomass for growth and repair, and is a fuel for cellular respiration, which transfers energy for life processes.

The reactants and products of photosynthesis can be summarised as carbon dioxide and water, those are our reactants, and those react to make glucose, the food for the producer, and oxygen.

Those are the products.

So that was the word equation for photosynthesis.

Now we're gonna talk about the balance symbol equation for photosynthesis.

To do that, we need to think about the atoms and molecules in photosynthesis.

There are only three different types of atom in the products and reactants for photosynthesis: carbon, oxygen, and hydrogen.

So you've got carbon dioxide, a carbon bonded to two oxygens, water, an oxygen atom bonded to two hydrogen atoms. Those make glucose.

Six carbon atoms, 12 hydrogen atoms, and six oxygen atoms, and oxygen, two oxygen atoms bonded together to make an oxygen molecule.

So let's count up all those atoms on the reactant side and on the product side.

So for the reactants, we have one carbon, three oxygen, and two hydrogen.

For the products, we have six carbon, eight oxygen, and 12 hydrogen.

Can anyone tell me what's wrong with that? Take five seconds, or if that's not enough time, click pause, and click play when you're ready to continue with the lesson.

This equation is not balanced.

We only have one carbon on our reactant side of the equation that somehow at the moment magically transforms into six carbons.

That doesn't make sense.

Same for the oxygen, same for the hydrogen.

There are more atoms on the product side of this equation than there are on the reactant side of this equation.

This equation isn't balanced, so we need to balance it.

If we want to make one molecule of glucose to balance the equation, we will need six molecules of carbon dioxide, six molecules of water, and six molecules of oxygen.

We can summarise this using a balance symbol equation.

Take five seconds and have a go at trying to write down what you think that balance symbol equation might be.

If that's not enough time, you can pause the video, and you can hit play when you're ready to continue with the lesson.

This is what our balance symbol equations look like.

Okay, those are our reactions.

That's how you write down glucose in symbol formula, and that's our oxygen.

Those are our products.

I'll be quiet for five seconds in case you wanna write that down.

Which is the correct symbol equation for photosynthesis? Is it a, b, c, or d? Take five seconds, or if that's not enough time, click pause, and click play then you're ready to see the answer.

It is b, six carbon dioxide molecules reacting with six water molecules reacting to make one glucose molecule and six oxygen molecules.

Let's look at the other answers.

In a, it says that carbon oxide and glucose react to make water and oxygen.

That's wrong.

Glucose is a product.

c, glucose and oxygen react to make water and carbon dioxide.

Wrong again.

d, glucose and water react to make carbon dioxide and oxygen.

Also wrong.

b is the only one that's got the correct reactants and the correct products.

Well done if you got that right.

In the plant, photosynthesis happens in the leaves and other green parts of the plant that are above the ground.

And you'll know that if you've ever tried to grow a plant, it needs to be in somewhere that has lots of light.

And this is because photosynthesis requires light.

The process cannot happen without light.

There are, however, a lot of misunderstandings people have about the relationship between light and photosynthesis.

Light is not a substance.

It's not made of atoms. Light is not a chemical reactant in photosynthesis.

The chemical reactants of photosynthesis are carbon dioxide and water.

Light is not turned into a product of photosynthesis.

The products of photosynthesis are glucose, which is a sugar, and oxygen, a gas.

Light can't turn into sugar.

It can't turn into gas molecules.

What light is, it's an energy store, and light transfers the energy needed for the chemical reactions of photosynthesis to take place.

Okay, let's see if you understood that.

Who is correct? Is it Alex, "Light is a chemical reactant in photosynthesis," Aisha, "Light is turned into glucose by photosynthesis," or Lucas, "Light transfers the energy needed for photosynthesis"? Take five seconds, or if that's not enough time, click pause, and click play when you're ready see the answers.

Let's take a look at the answers.

It is Lucas.

Light transfers the energy needed for photosynthesis.

We'll have a look at the other answers.

Alex said, "Light is a chemical reactant in photosynthesis." Light is not a substance.

It's not made of atoms like carbon dioxide and water are.

It cannot be a chemical reactant.

Aisha says, "Light is turned into glucose by photosynthesis." Glucose is made of carbon atoms, hydrogen atoms, and oxygen atoms. There are no carbon, hydrogen, or oxygen atoms in light, so it's impossible for light to turn into glucose.

Lucas said, "Light transfers the energy needed for photosynthesis." This is the only answer that makes sense because light is an energy store.

Well done if you got that right.

Photosynthesis requires energy from a light source, such as the sun, or maybe if you're growing a plant inside, you might use a special light bulb that emits UV radiation.

A chemical process that requires energy transferred from its surroundings is described as an endothermic process.

So photosynthesis is an endothermic process because it needs energy from its surroundings, that's light from the sun, in order to take place.

Let's check to see if that made sense.

True or false? Photosynthesis is an exothermic process.

Is that true or is that false? Take five seconds, or that's not enough time, click pause, and click play when you're ready to see the answer.

That is false.

Okay, why is that false? Take another five seconds, or if that's still not enough time, click pause, and click play when you're ready to see the answer.

It is false because photosynthesis requires energy transferred from its surroundings.

The correct word to describe this kind of process is endothermic.

An exothermic reaction is a process that transfers energy to its surroundings.

Well done if you got that right.

This is the first practise task of today's lesson.

Number one, complete the word summary for photosynthesis.

Carbon dioxide plus, and then fill in the blanks.

Complete the balance symbol equation for photosynthesis.

And number three, photosynthesis is an endothermic process.

Explain what this means.

You'll need to pause the video now to give yourself enough time to answer those questions, and then click play when you're ready to see the answers.

Good luck.

Let's have a look at the answers.

Question one.

Complete the word summary for photosynthesis.

Carbon dioxide plus water reacts to make glucose plus oxygen.

Question two.

The balance symbol equation for photosynthesis.

Here are our reactants.

Here are our products.

I'll give you a few seconds to make sure you've copied those down correctly.

Question three.

Photosynthesis is an endothermic process.

Explain what this means.

Photosynthesis is a chemical process that requires energy to be transferred from its surroundings by light.

Well done if you got those right.

We've completed the first part of today's lesson.

We've talked about the word equation and balanced symbol equation for photosynthesis.

We've also talked about how it's an endothermic process.

Now we're gonna talk about the adaptations that leaves have to maximise photosynthesis.

Leaves are made up of cells.

If we were to take a section of this leaf and put it under a light microscope, it would look like this.

Can you recognise any subcellular structures on these cells? You can take five seconds, or if that's not enough time, you can click pause, and click play when you're ready to continue.

Okay, you may have been able to recognise these lines around the outside of the cells, those are the cell walls, and these green little circles, those are chloroplasts.

The cells contain subcellular structures called chloroplasts, where photosynthesis takes place.

In biology, an adaptation is a feature that helps an organism to function and survive.

Chloroplasts are an adaptation of a plant cell that enables plants to make their own food by photosynthesis.

Chloroplasts contain a green pigment called chlorophyll, which provides a chemical store of energy for photosynthesis.

Light transfers energy to chlorophyll, which enables photosynthesis to occur.

It's really easy to get chloroplast and chlorophyll confused.

So just once again, chloroplasts are the subcellular structure and they are filled with chlorophyll, which is the green pigment.

Let's check to see if that made sense.

Use words from the box to complete the sentences.

Carbon dioxide, chlorophyll, chloroplast, light, oxygen, water.

Cells in green parts of a plant contain subcellular structures called blank.

These structures contain a green pigment called blank.

Blank transfers energy to this pigment for photosynthesis.

Take five seconds, or if that's not enough time, click pause, and click play when you're ready to see the answers.

Okay, let's look at the answers.

Cells in green parts of a plant contain subcellular structures called chloroplasts.

These structures contain a green pigment called chlorophyll.

Light transfers energy to this pigment for photosynthesis.

Great job if you got those right.

So chloroplasts containing chlorophyll, that's one adaptation that leaves have to increase the rate of photosynthesis.

Let's look at some others.

Leaves are made of layers of cells.

So this diagram is showing in a lot more detail the layers of cells that leaves have.

Let's look at a small section in even more detail.

Okay, and we're gonna start to talk about what each of these layers are and how they're adapted to try and help photosynthesis.

So the top layer is called the waxy cuticle, and then underneath that we have the palisade mesophyll cells.

Palisade mesophyll cells contain large numbers of chloroplasts.

They form the primary tissue where photosynthesis takes place.

This is an adaptation to maximise the amount the amount of photosynthesis.

So palisade mesophyll cells, lots of chloroplasts, more chloroplasts means more photosynthesis taking place.

Let's look at another layer.

So the next layer down is called the spongy mesophyll.

Spongy mesophyll cells form a tissue with lots of airspace in it, like a sponge.

This adaptation provides a large surface area for gases to diffuse into and out of the cells during photosynthesis.

And we'll look at it in a bit more detail later in this learning cycle.

Let's look at another layer.

So on the bottom we have leaves have pores or holes called stomata, which is the plural, or in the singular, they're called stoma.

So leaves have pores called stomata in their surface.

Stomata allow gases to diffuse in and out of the air spaces in the spongy mesophyll tissue.

Each stoma, singular, is surrounded by a pair of guard cells.

Let's look at the stomata and the guard cells in more detail.

Okay, so we're looking at the underside of the leaf, and this is what that might look like on an electron microscope.

And this is what it looks like as a diagram.

So we can see the stomata are the holes and the guard cells are these two cells surrounding the hole.

And they open and close the hole, the stomata.

Guard cells change shape to open and close the stoma pore.

In the light, each guard cells filled with water and become turgid.

This opens the pore.

When it's dark, the guard cell becomes flacid and this closes the pore.

So why do they need to be open sometimes and closed other times? Let's look at the answer for that.

Now we're talking about the gases involved in photosynthesis, carbon dioxide and oxygen.

So here is our diagram of the layers of cells inside a leaf.

Guard cells open the stoma when it's light, and this allows molecules of carbon dioxide gas to diffuse into the airspace inside the leaf and into mesophyll cells for photosynthesis.

Molecules of waste oxygen gas diffuse out.

Stomata are an adaptation to maximise gas exchange for photosynthesis.

So the stomata are essential because they allow the carbon dioxide gas to diffuse into the leaf and waste oxygen gas to diffuse out of the leaf.

There's two more adaptations that leaves have for photosynthesis that I want to bring up.

So leaves are really thin to allow light to pass through to reach all the cells that contain chloroplasts.

If the leaf was really, really thick, then the light wouldn't be able to penetrate through all the way, and there'd be maybe the whole bottom of the leaf wouldn't be receiving any light, wouldn't be receiving any light in order to carry out photosynthesis.

Another adaptation leaves have is that they have a really large surface area or they are broad, and this is to absorb as much light as possible for photosynthesis to take place.

Let's check to see if you understood all those adaptations.

Use words from the box to complete the sentences.

Carbon dioxide, chloroplasts, light, oxygen, stomata.

Palisade mesophyll cells contain large numbers of structures called blank for photosynthesis.

Guard cells change shape to open blank to absorb blank for photosynthesis and release waste blank.

Leaves are thin and have a large surface area to absorb as much blank as possible to transfer energy for photosynthesis.

Okay, you need to take five seconds, or if that's not enough time, click pause, and click play when you're ready to see the answer.

Let's look at the answers.

Palisade mesophyll cells contain large numbers of structures called chloroplasts for photosynthesis.

Guard cells change shape to open stomata to absorb carbon dioxide for photosynthesis and release waste oxygen.

Leaves are thin and have a large surface area to absorb as much light as possible to transfer energy for photosynthesis.

Great job if you've got those right.

This is the final practise task for today's lesson.

The photograph shows a variegated leaf.

Number one, in which part of the leaf will the most photosynthesis occur? Explain your answer.

Question two, explain how carbon dioxide moves into the leaf for photosynthesis and how waste oxygen made by photosynthesis moves out.

You need to pause the video now to give yourself enough time to answer those questions, and click play when you're ready to see the answers.

Good luck.

Let's look at the answers.

Number one, in which part of the leaf will the most photosynthesis occur? And explain your answer.

In part A.

This part is darkest green so contains the most chloroplasts.

These contain chlorophyll to which energy is transferred from light for photosynthesis.

Number two, explain how carbon dioxide moves into the leaf for photosynthesis and how waste oxygen made by photosynthesis moves out.

There are pores in the leaf surface called stomata.

Guard cells change shape to open stomata to allow carbon dioxide and oxygen to diffuse in and out.

Great job.

Well done on today's lesson.

Let's summarise what we've learned to help us remember it.

The word summary of photosynthesis is carbon dioxide plus water reacts to make glucose plus oxygen.

The balance symbol equation for photosynthesis is this.

I'll give you a few seconds to make sure you copied it down correctly.

Photosynthesis is an endothermic chemical process that requires energy from a light source, such as the sun.

Light is not a chemical reactant, but transfers energy to chlorophyll to enable photosynthesis to take place.

Chlorophyll is found in chloroplasts in cells.

Chloroplasts are an adaptation of leaves for photosynthesis.

Stomata open to allow gas exchange.

Stomata are an adaptation of leaves for photosynthesis.

Amazing work on today's lesson.

I hope you really enjoyed it, and I hope to see you again soon for our next lesson.