Lesson video

Welcome to this lesson from the Oak National Academy.

Today's lesson is called Models of Photosynthesis, and it is taken from the unit Photosynthesis: requirements and products.

Hi here, I'm Mrs. Wheate and I'm gonna be your teacher for today's lesson.

By the end of today's lesson, you'll be able to use simple models to explain what happens during photosynthesis, including a model of the two main stages.

Let's have a look at our keywords.

We have five keywords for today's lesson, and I'm gonna read them out now.

Model, a simpler representation of a complex thing or idea.

Photosynthesis, the process used by producers to make their own food via a series of chemical reactions.

Chloroplast, a subcellular structure in which photosynthesis takes place.

Prediction.

A scientific prediction is a testable statement about a possible outcome which can be tested in an experiment.

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

So if that wasn't enough time, you wanna read them through again, I'll be quiet for five seconds, or if you wanna read through them a couple of times or maybe write 'em down, you can pause the video and then click play when you're ready to move on with the lesson.

Today's lesson is in two parts.

In the first part of today's lesson, we will talk about simple models of photosynthesis.

We'll talk about what a scientific model is and look at different examples of models we use to help us understand photosynthesis.

Then in the second part of today's lesson, we will look in detail at the two-stage model of photosynthesis.

But first, a simple model of photosynthesis.

A model is a simpler representation of a complex thing or idea, and we use models a lot in science.

It's common to see models in everyday life.

So this is a mannequin or a model of a person used to display clothes in a shop.

This isn't a real person, obviously.

They're made of like plastic or whatever, but they help you see what the clothes might look like if you were gonna wear them.

Models are really commonly used in science, as I said, and so this is a model of the human heart.

A real human heart is made out of muscle.

It's filled with blood.

This model, again, is probably made of plastic silicone.

It's not made of muscle, it's not filled with blood, but it's really, really useful to help us to learn and explain the structure of the heart.

So it's not the real thing, but it helps us understand the real thing better.

Models are used in science to explain things that can be difficult to see or understand.

Let's take cells for example.

You can only see a cell with a microscope.

So what do you do if you don't have a microscope right in front of you? We can use models to help us understand them.

We can have a plant cell modelled in 2D, two dimensions.

So this picture here is a model of a plant cell.

It's not really a plant cell.

It's a representation of one.

And by looking at this diagram, we can learn the main structures of a plant cell.

So it's useful.

We can look at more detailed models in 3D like this one.

And advantage of this model is that it is more similar to what a real plant cell is like because cells are three-dimensional.

I can't easily draw a three-dimensional model of this on a whiteboard to teach my class what a cell is like.

So all models have advantages and disadvantages.

Different models of the same thing can include different amounts of details.

Let's take a car as an example.

We can have a very, very simple model of the car.

Here we have a really simple toy that's made in wood.

It's just showing that a car has got wheels, it's showing the basic shape of a car and it's showing that, well, the window is just a hole.

It's not very accurate.

And here's a much more detailed example of a model of a car.

So this is way more detailed.

The shape is much more realistic, and there's a material in the windows, probably like a transparent plastic, which is showing you what the windows of a car are like, the wheels much more proportional, it's got hubcaps, it's got lights, it's got a bumper, it's got a fender.

This is much, much closer to the real thing.

Neither model is incorrect.

They just contain different levels of detail.

If I showed you the wooden car with no context and asked you what is this or what is this representing, you would know that, oh, it's representing a car.

You know it's not the real thing, but you know that, oh yeah, I can recognise that as a car.

It's not wrong.

It's just doesn't have anywhere near as much detail as the more detailed model.

But neither model contains all the details of a real car.

As good as that final model is, it almost definitely doesn't have a working internal combustion engine.

So it's not really the car.

Sometimes in science, in science lessons, maybe you heard it when you're in year five or year six from a secondary school student who's like, oh, you are not learning real science.

You'll learn the real science when you're in secondary school, they're lying to you at the moment.

And that's not true.

Your teachers haven't lied to you intentionally.

Just at different levels of your education, we'll use models with different levels of complexity to explain certain things.

And so the model we're learning about today in photosynthesis is telling you part of the story.

If you study biology beyond DCSC, you'll learn about photosynthesis in greater detail and at degree level, at master's level, at PhD level.

You're just adding more and more and more detail to our understanding of photosynthesis.

Our model can be a physical model, like a 3D model that we can actually touch.

It can be a descriptive model, which uses words, diagrams and numbers, or it can be a mathematical model, which shows how measurements of things are related.

All three of these models are telling us about the car in some way, but they're each telling us something different about the car.

The physical model is telling us, what does it look like.

The descriptive model is telling us, what are the different parts of it called.

The mathematical model we can use to calculate its average speed.

So we can use different models depending on what it is that we want to understand about the real thing.

Let's check to see if you're following on so far.

So this is the sentence.

A model.

a, includes every part and detail of the real thing.

b, is a simpler version of the real thing.

c, is the real thing.

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

Okay, let's look at the answer.

It is b.

It is a simpler version of the real thing.

Great job if you got that right.

Photosynthesis is a very important process.

The growth and survival of producers and consumers depends on the food and oxygen made by photosynthesis.

Producers use photosynthesis in order to make glucose, which is a food, which they can use to grow and repair themselves, or they can use it as a fuel in cellular respiration.

Whenever we eat a plant, we're also gaining that glucose so we can use it for growth and repair and for cellular respiration.

The oxygen that's made by producers like plants is incredibly important in aerobic cellular respiration - a process that living organisms, producers, consumers alike need to survive.

But photosynthesis happens on a tiny, tiny, tiny scale - molecules reacting together inside the chloroplasts, which are subcellular structures inside certain plant cells.

So it's far beyond our ability to observe the process happening.

It's too tiny.

But it's very useful for us to know what happens during photosynthesis, and that's where models come in.

So photosynthesis is not just one chemical reaction.

It's a series of reactions catalysed by enzymes.

But it's useful to summarise the reactants and products of photosynthesis using a word equation.

The reactants are water and carbon dioxide, and they react to make glucose and oxygen - the products.

This is a very simple model of the process of photosynthesis.

It's not a lie, it's just not very detailed.

We can add another detail to this model: the idea that light transfers the energy needed for photosynthesis to take place.

Even a very simple model like this can be used to explain aspects of what happens.

For example, from this model, we can see that the reactants of photosynthesis are water and carbon dioxide, and the products are glucose and oxygen.

So we can use that to make a prediction.

For example, from this model, we can predict that if more water and more carbon dioxide are available, photosynthesis could make more glucose and oxygen.

So again, even though it's a very, very simple model, it still tells us something very important about photosynthesis.

We can use diagrams as models of the atoms and molecules in photosynthesis.

So this is our key.

Black circles are gonna be carbon, red circles are oxygen and white circles are gonna be hydrogen.

So here's our water, here's our carbon dioxide, this is our glucose, and this is our oxygen.

We can use a balanced symbol equation as a model to see how these different molecules react.

So here are our reactants, the water and the carbon oxide, and here are our products, the glucose and the oxygen.

This model shows us how many atoms and molecules are involved in making one molecule of glucose produced by photosynthesis.

So the first model here on the slide is showing us the structure of these different molecules, and that's very useful for us to know.

But the second model, the balanced symbol equation, that's telling us how much water and carbon dioxide do we need to make one molecule of glucose.

That's also really important information for us to know.

So again, two different models telling us different aspects of photosynthesis.

Because photosynthesis happens on a tiny scale, we can't see the substances, atoms and molecules involved.

The simple models we've explored help us to understand the roles of these substances in photosynthesis.

Photosynthesis is also important on much larger scales.

For example, it affects the population of producers and consumers that depend on the food it makes.

We can use models such as a food web diagram to help us understand these relationships.

This food web tells us that the chicken and the grasshopper and the mouse are consumers of the corn.

The fox eats the grasshopper and the mouse, as does the owl.

The human eats the chicken and the corn.

This food web also shows us how every single organism in the food web is reliant on the corn.

If there's no producer, this whole thing falls apart.

Let's check to see if you've understood what we've been learning so far.

Which diagram is a simple model of a molecule of glucose? Is it a, b, or c? Take five seconds or if that's not enough time, click pause and click play when you're ready to see the answer.

It is a.

This is the only model that is showing six carbon atoms, 12 hydrogen atoms, and six oxygen atoms all bonded together.

b is representing a molecule of hydrogen, one atom of oxygen bonded to two hydrogen atoms. And c is showing us carbon dioxide, one carbon atom bonded to two oxygen atoms. Well done if you got that right.

Let's try another.

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

The correct answer is d.

This is the only balanced symbol equation which shows the correct reactants and the correct products.

Well done if you got that right.

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

Number one, complete the word summary model of photosynthesis.

Number two, complete the balanced symbol equation model of photosynthesis.

Number three, explain why the symbol equation is a more detailed model of photosynthesis.

And number four, predict what would happen to the growth of lettuces in the greenhouse when more water, carbon oxide and light are provided.

Use the word summary model of photosynthesis to help you.

So 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 and move on with the lesson.

Good luck.

Let's go through the answers.

So number one, complete the word summary model of photosynthesis.

Water and carbon dioxide react to make glucose and oxygen.

Here we have that represented as a balanced symbol equation.

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

Question three, explain why a balanced symbol equation is a more detailed model of photosynthesis.

It shows how many atoms and molecules are involved in making one molecule of glucose by photosynthesis.

Question four, predict what would happen to the growth of lettuces in the greenhouse when more water, carbon dioxide and light are provided.

More glucose would be made by photosynthesis.

Some of these glucose would be used as a source of materials for growth, so the lettuces would grow larger.

Great job if you got those right.

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

We've defined what a scientific model is, and we looked at many different simple models of photosynthesis.

Now I'm gonna look at a slightly more detailed model, a two-stage model of photosynthesis.

A more detailed model of photosynthesis summarises two main stages in the process.

Both stages take place in chloroplasts in plant cells.

Here we go.

Plant cell, chloroplast.

Inside chloroplasts, there are many layers of membranes containing chlorophyll, which is that green pigment where that's necessary for photosynthesis to take place.

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

Chloroplasts also contain enzymes to catalyse the reactions of photosynthesis.

So all this information is gonna be summarised in our new model, the two-stage model.

Let's take a look at the first stage of this model of photosynthesis.

This stage is taking place inside those membranes within the chloroplasts, and these membranes are where chlorophyll is.

In this stage, water enters the membranes and is split apart using energy from light.

The water is split into oxygen, which diffuses out of the chloroplast, and hydrogen ions, which are used in the second stage of photosynthesis.

The second stage of photosynthesis doesn't require light, so it doesn't require chlorophyll, either.

It takes place a different part of the chloroplast where there are special enzymes necessary for this part to take place.

Carbon dioxide diffuses into a leaf and eventually makes its way into the chloroplast, and there are a series of chemical reactions takes place using the hydrogen ions made from the first stage and from the carbon dioxide that diffused into the chloroplast to create a molecule of glucose.

We just added a lot of detail to our understanding of photosynthesis.

Let's run through that again to help us to remember it all.

Right, okay.

So can you summarise what happens in the first stage of photosynthesis? Take five seconds, but that's probably not gonna be enough time.

So I'd suggest pausing and then clicking play when you're ready to move on with the lesson.

In the first stage of photosynthesis, water enters the chloroplast where there are membranes that contain chlorophyll.

Energy from light is used to split the water into hydrogen ions, which are used in the second stage, and oxygen, which diffuses out of the chloroplast, out of the leaf.

Great job if you got that right.

But if it's still a bit confusing, don't be afraid to go back and watch this section of the video again to help you understand it.

Now have a go at summarising the second stage of the two-stage model of photosynthesis.

I will be quiet for five seconds, but that probably won't be enough time.

So you should pause the video to give yourself enough thinking time to think about the answer, and then click play when you're ready to see the answers and move on with the lesson.

Good luck.

Okay, let's have a look at the answers.

In this stage, carbon dioxide is involved.

There's no light involved and there's no chlorophyll involved.

Carbon dioxide enters the chloroplasts and there's a series of chemical reactions, which is the carbon dioxide and hydrogen ions created in the first stage in order to make a molecule of glucose.

Really great job if you understood that and you got that right.

So this model adds even more detail than the models we learned about in the first part of the lesson.

The two-stage model of photosynthesis helps us to understand that: light is only needed for the first stage, which scientists call the light-dependent stage, the reactions of the first stage take place in the chloroplast membranes, which is where the chlorophyll is located, and the reactions of the second stage take place elsewhere in the chloroplast, where these special enzymes that catalyse these reactions are.

Which stage of photosynthesis requires energy from light? Is it a, both stages, b, only the first stage, c, only the second stage, or d, neither stage? Take five seconds or if you want some more time, click pause and click play when you're ready to see the answer.

Let's take a look at the answer.

It is b, only the first stage.

It can also be called the light-dependent stage.

Well done if you got that right.

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

Add labels and arrows to the diagram to complete the model of the two main stages of photosynthesis.

Okay, you'll need to pause the video now to give yourself enough time to do that and click play when you're ready to see the answers.

Good luck.

Let's have a look at what you should have written.

In the first stage, water enters the chloroplast where the membranes containing chlorophyll are, and the energy from light is used to split that water into hydrogen ions and oxygen gas.

That oxygen gas diffuses out the chloroplast out of the leaf into the atmosphere.

The hydrogen ions are used in the second stage.

In the second stage, carbon dioxide gas diffuses into the chloroplast, but is used in a different section of the chloroplast 'cause chlorophyll doesn't need to be there, so we don't have those membranes that contain chlorophyll involved in the second stage, and we don't have any light involved in the second stage.

A series of chemical reactions uses the carbon dioxide and the hydrogen ions produced by the first stage to make glucose.

Amazing work if you got that right.

Great work on today's lesson.

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

A model is a simpler representation of a complex thing or idea.

Models are used in science to explain things that can be difficult to see or understand and can be used to make predictions.

The word summary and balanced symbol equation for photosynthesis are simple models of the process.

Photosynthesis can also be summarised using a more detailed model of the two main stages, which both take place in chloroplasts.

In the first stage, energy from light is absorbed by chlorophyll and used to split water molecules into hydrogen and oxygen.

The oxygen is released as waste.

In the second stage, the hydrogen is combined with carbon dioxide to make glucose.

Again, amazing work on today's lesson.

I hope you really enjoyed the lesson, and hopefully I'll see you again soon for our next lesson.