Lesson video

Hello, I hope that your day's going well.

I'm Mr. Jarvis and I'm going to be taking you through today's lesson from the Unit, Photosynthesis factors affecting the rate.

Today, we are looking at photosynthesis and limiting factors.

By the end of today's lesson, you should be able to explain the effects of limiting factors on the rate of photosynthesis.

There are five keywords to today's lesson.

They are photosynthesis, rate, limiting factor, enzyme and optimum.

The definitions of those words are on the screen and we will go through them as we go through the lesson, but if you want to pause to read through those definitions in a bit more detail, then please pause the video now and then when you're ready, press play and we'll carry on.

Today's lesson is broken down into three parts.

First of all, we're going to look at what limiting factors are.

Then we're going to talk about the effect of limiting factors and photosynthesis.

And finally, we're going to look at how the limiting factors interact with each other.

So if you're ready, let's get started with our first section, looking at limiting factors.

Like all living organisms, plants need food to stay alive and to grow.

Plants and other producers make their own food using the process of photosynthesis.

Photosynthesis as a word, can be broken down into photo which means light, and synthesis which means to make, and so plants make their food using light.

Photosynthesis occurs in the chlorophyll that's found in chloroplasts, in the cells of leaves and other parts of the plant that are above the ground.

The reactants and products of photosynthesis can be summarised by the following equation.

Water and carbon dioxide gives us glucose, the food for the plant and oxygen.

Water and carbon dioxide are reactants.

Glucose and oxygen are products.

The light transfers the energy that's needed for the chemical reactions of photosynthesis to take place.

Here's a check.

What are the two products of photosynthesis? A, carbon dioxide, B, glucose, C, light, D, oxygen, or E, water.

I'll pause for a few seconds and then we'll check your answer.

The correct answer is B, glucose and D oxygen.

Glucose is the food that the plant produces and oxygen is a waste product to photosynthesis.

It's important to note that light is not a reactant or a product to photosynthesis and it's not a substance either.

Light provides the energy that's needed for the reactions of photosynthesis to take place.

The rate of photosynthesis is how quick photosynthesis happens and it's affected by various factors.

These include light, chlorophyll, water, temperature and carbon dioxide.

Increasing these factors can speed up the rate of photosynthesis and insufficient levels of these factors can slow down or limit the rate of photosynthesis, so they're known as limiting factors.

Here's a check.

What's meant by the term limiting factor? Is it A, a factor which, when present, slows down the rate of photosynthesis? Is it B, a factor which, when present, speeds up the rate of photosynthesis? Is it C, a factor which, when in short supply, slows down the rates of photosynthesis.

Or is it D, a factor which when, in short supply, speeds up the rate of photosynthesis? I'll pause for a few seconds and then we'll check your answer to see if you got it right.

The correct answer is C.

Limiting factors are factors which when in short supply, slow down the rate of photosynthesis.

Well done if you got that right.

So let's look at some limiting factors.

Let's start with light intensity.

Light transfers the energy needed for the chemical reactions of photosynthesis to take place.

Light intensity is the amount of light that reaches a given surface in a particular period of time.

At low light intensity, the rate of photosynthesis is limited because there's insufficient energy to be transferred to the chlorophyll in leaf cells.

At high light intensity, the rate of photosynthesis won't be limited by the lack of light, so light will no longer be a limiting factor.

The rate of photosynthesis can increase as more energy is transferred to the chlorophyll.

Water is another limiting factor.

It's a reactant in the process of photosynthesis.

We've seen that water and carbon dioxide gives us glucose and oxygen.

If there's not enough water, the rate of photosynthesis will be limited until more water becomes available.

When there's plenty of water, the rate of photosynthesis is no longer limited by the lack of water, so water will no longer be a limiting factor.

Carbon dioxide concentration is another limiting factor.

Carbon dioxide is also a reactant in photosynthesis.

Remember, water plus carbon dioxide gives us glucose and oxygen.

Carbon dioxide enters the plant through the stomata in the leaves, and we can see a picture of some of the stomata on the underside of this plant in the picture.

The atmosphere only contains 0.

04% of carbon dioxide, and this means that carbon dioxide often limits the rate of photosynthesis because there's a very low concentration in the atmosphere.

On a warm sunny day, carbon dioxide concentration is the most common limiting factor.

Chlorophyll is another limiting factor.

The energy required for photosynthesis to take place is transferred by light to the green pigment chlorophyll in chloroplasts found in plant cells.

And we can see some chloroplasts containing chlorophyll within the image on the screen.

If the leaves are pale green in colour, they often contain little chlorophyll, and this means that the rate of photosynthesis is limited in these cells.

In darker green leaves, with more chlorophyll, the rate of photosynthesis can increase.

Our fifth limiting factor is temperature.

Temperature affects all chemical reactions.

As temperature increases, the rate of photosynthesis speeds up, The photosynthesis is controlled by enzymes, and remember, enzymes are biological catalysts.

They speed up reactions.

The active sites of enzymes work by accepting a substrate of a particular shape and then convert it into a product which is then released and the active site is available for more reactant to go into the active site and the reaction to take place.

However, if the optimum temperature increases above the optimum, they denature and the rate of photosynthesis is then limited.

The optimum is the rate at which the reaction takes place as quickly as it possibly can.

So temperatures above the optimum temperature may well be a limiting factor.

Here's a check.

Which of the following are not limiting factors of photosynthesis? A, carbon dioxide.

B, oxygen.

C, light.

D, chlorophyll.

Or E, glucose.

I'll pause for a few seconds and then we'll check your answer.

The factor that's not a limiting factor is glucose and oxygen, B and E are the correct answers.

They're both products of photosynthesis.

Here's our first task of the lesson.

I'd like you to name five different limiting factors that can affect the rate of photosynthesis, and then for each limiting factor, explain how and why the factor limits photosynthesis.

You'll need to pause the video while you write down your answers, and then when you're ready, press play, and we can check them to see how well you've done.

Good luck.

So how did you get on with that? I asked you first of all to name five different limiting factors that can affect the rate of photosynthesis.

The answers that you should have had are light intensity, carbon dioxide, concentration, temperature, chlorophyll, and water availability.

Well done.

If you've got all five answers.

Then I asked you for each limiting factor to explain how and why the factor limits photosynthesis.

Let's take light intensity first.

Light is required to transfer the energy needed for the reactions of photosynthesis to take place.

So without light, photosynthesis cannot take place.

Carbon dioxide is a reactant to photosynthesis.

It's only found in small concentrations in the atmosphere, 0.

04% and this often limits how quickly photosynthesis can happen.

Temperature affects all reactions.

As temperature increases, the rate of reactions include photosynthesis increase too.

When the temperature is too high, the enzymes that catalyse photosynthesis become denatured.

They change shape, and that means the active site then no longer accepts the substrate.

This limits the rate of photosynthesis.

Chlorophyll.

The energy required for photosynthesis is transferred to chlorophyll.

The amount of chlorophyll therefore limits how much energy can be absorbed.

And finally, water is a reactant to photosynthesis.

If water is scarce, it can become a limiting factor and slow the rate of photosynthesis down.

Well done if you've got those answers.

That brings us to the second part of today's lesson, which is all about limiting factors and photosynthesis.

So if you're ready, let's move on.

How do limiting factors affect the rate of photosynthesis? Well, let's look at temperature.

As the temperature increases towards the optimum temperature, that's when the rate is fastest.

The substrate and the enzyme particles move faster and with more energy.

There's a higher frequency of successful collisions, and this increases the rate of the reaction to its fastest rate, which is the optimum level, the optimum temperature.

At the optimum temperature, the collisions happen often enough that all enzyme active sites are full with substrate molecules almost all the time.

However, as temperature increases beyond and above the optimum, enzymes start to denature.

The active sites change shape and the substrates no longer fit in, and there's a lower frequency of successful collisions as a result.

This decreases the rate of photosynthesis and eventually it reaches zero.

Temperature has become a limiting factor.

Increasing the light intensity, or the amount of water, or carbon dioxide, or chlorophyll will all increase the rate of photosynthesis as shown in the graph on the screen.

At the optimum concentration, or the optimum light intensity, or the enzymes active sites are full with substrate molecules.

Increasing one individual factor above the optimum does not increase the rate of reaction because there's no more enzyme active sites available to be used.

Here's a check which graph or graphs show how a limiting factor affects the rate of photosynthesis A, B, or C.

I'll pause for a few seconds so that you can look at the graphs.

You might want to even pause the video so you can study the graphs in a bit more detail.

And then when you're ready, press play and we'll check your answer.

The correct answer is A and B.

A shows the correct graph for light intensity, and B shows the correct graph for temperature.

Well done if you got both of those.

Here's another check.

Lucas drew a graph showing how light intensity affects the rate of photosynthesis.

What's wrong with the graph? Is it A, the axis labels are the wrong way round.

B, the line should be a straight line.

Or C, the line is plotted incorrectly.

I'll pause for a few seconds and then we'll check your answer.

The correct answer here is C.

The line is plotted incorrectly.

This is what the line should have looked like.

Why? Again, I'll pause for a few seconds while you think about that answer.

Well, the reason that the graph was plotted incorrectly is that when there's no light, so zero light, there's no photosynthesis, zero photosynthesis, so the line has to pass through the origin of the graph.

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

When there's no light, there's no photosynthesis.

Well done if you worked that answer out yourself.

All limiting factors interact with each other.

In the natural environment there are daily fluctuations in the limiting factors.

Which of the limiting factors are most likely to prevent the rate of photosynthesis from increasing? Well, carbon dioxide concentration because the levels are low, only not 0.

04% in the atmosphere, and that's likely to be the most common limiting factor.

Light intensity can also be a limiting factor during the day.

For example, early in the evening, or on cloudy days where light intensity is lower, then limiting factor could be light.

Temperature could also be a limiting factor if the day is particularly cold or if the day is really hot, so really hot sunny days, because the temperature might reach a level where enzymes in the plant become denatured.

The rate of photosynthesis will be limited by the factor that's in the shortest supply.

If we consider light intensity, increasing in light intensity increases the rate of photosynthesis, but at the point where the rate no longer increases, other factors may well be limiting the rate.

Because of this, the rate cannot increase any further, even if light intensity increases.

If the carbon dioxide concentration increases, the rate of photosynthesis increases too.

Carbon dioxide or another factor limits any increase and the rate becomes constant.

If carbon dioxide concentration is still the limiting factor, then increasing the concentration again will increase the rate even further, but another limiting factor may well then cause the rate to become constant again.

If we consider light intensity and temperature, we get a similar picture.

If the temperature is increased at different light intensities, then the rate increases until a limiting factor stops the rate from increasing further.

At each of the points, where the rate is constant temperature or another limiting factor is at play.

Here's a check.

A farmer grows cucumbers in a greenhouse.

He adds carbon dioxide to the greenhouse until it reaches a concentration of 0.

08%.

Why does he do this? Is it A, he needs to add carbon dioxide to the greenhouse for photosynthesis to take place.

Is it B, adding carbon dioxide to the greenhouse will help to increase the rate of cellular respiration.

Or is it C, carbon dioxide is often the limiting factor, so increasing it will increase the plant's rate of photosynthesis and they will grow faster.

I'll pause for a few seconds and then we'll check to see if you've got the answer right.

The correct answer is C.

Carbon dioxide is often the limiting factor, so increasing it will increase the plant's rate of photosynthesis and it will grow faster.

Well done if you got that.

That brings us to our next task.

The graph shows the effect of concentration of carbon dioxide on the rate of photosynthesis in lettuces in a greenhouse.

What's the maximum rate of photosynthesis? And then secondly, at point x carbon dioxide is not a limiting factor.

Suggest what could be limiting photosynthesis at this point.

And explain your answer.

You'll need to pause the video at this point, answer your questions, and then when you're ready, press play and we'll check to see if you've got them right.

First of all, I asked you to tell me what is the maximum rate of photosynthesis.

So if we look at the scale, we can see 15 arbitrary units is the maximum rate of photosynthesis.

Then I ask you to suggest what would be limiting at X.

And to explain your answer, you might have suggested temperature, because it could be cold in the greenhouse and this would reduce the number of collisions between the reactants of photosynthesis.

Or you might have said light intensity.

It could be a cloudy day, and light provides the energy that's needed for the reactions to take place.

Well done if you got those answers.

That brings us to the third and final part of the lesson today.

And we're going to now look at the interaction of many limiting factors.

So if you're ready, let's move on.

This graph shows the effect of a number of limiting factors on the rate of photosynthesis.

It shows how both carbon dioxide, concentration, and temperature affect the rate as light intensity increases.

What does the graph tell you? You might need to pause the video at this point, to have a look at the graph and try and work out what it tells you.

And what I'll do on the next slide is explain what the graph's showing.

So what does the graph tell us? So as the light intensity increases at 0.

04% of carbon dioxide, the rate is faster at the higher temperature.

And we can see this, we've got two lines, one at 10 degrees, that's the blue line, one at 25 degrees, that's the pink line.

And we can see at the same carbon dioxide concentration 0.

04%, then the rate has increased at 25 degrees.

At 4% carbon dioxide, the rate is also faster at the higher temperature.

And again, we've got two lines that we've added to this graph.

The blue line at the top is at 10 degrees.

The pink line is at 25 degrees, and the two upper lines are 4% carbon dioxide, and we can see that at 25 degrees, 4% carbon dioxide rate is higher than it is at 10 degrees.

The increase in carbon dioxide percentage also increases the rate, and the highest rate is when the temperature, that carbon dioxide percentage and the light intensity is high, but as we can see, there's still a limiting factor.

And how do we know that there's a limiting factor? It's because the rate of photosynthesis has become constant.

Farmers might use this information to help them to grow crops in a greenhouse.

By providing the optimum conditions for photosynthesis in the crops, they are able to grow high yields of crops at a fast rate, and this increases the profit and the quantity of food available to humans, and this can help to improve and increase food security.

Here's a check.

Assuming there are no other factors, in which of the following scenarios would the rate of photosynthesis be fastest? A, high light intensity, 0.

04% carbon dioxide and a temperature of 20 degrees C.

B, low light intensity, 4% carbon dioxide, temperature of 10 degrees C.

C, high light intensity, 4%, carbon dioxide concentration, and a temperature of 20 degrees C.

Or D, high light intensity, 0.

04% carbon dioxide concentration and a temperature of 20 degrees C.

I'll pause for a few seconds and then we'll check to see what the right answer is.

So the scenario in which the rate of photosynthesis would be fastest is C.

High light intensity, 4% carbon dioxide concentration, and a temperature of 20 degrees C.

Well done if you've got that.

Let's move to our final task.

The graph shows how different factors affect the rate of photosynthesis, which factors are limiting the rate at points A, B, and C.

And I'd like you to explain your answers.

You'll need to pause the video at this point, write down your answer, and then when you're ready, press play and we'll check them to see how well you've done.

Good luck.

So which factors are limiting the rate of photosynthesis at points A, B, and C? And explain your answers.

Let's go through them.

At A, light intensity is the limiting factor.

As the light intensity continues to increase, the rate of photosynthesis is continuing to increase too.

B, it's temperature.

The pink line shows that by increasing temperature, the rate increases further.

And C, carbon dioxide the temperature is high.

So it could be another limiting factor which is stopping the rate of photosynthesis from increasing.

The most likely limiting factor is going to be carbon dioxide.

However, it could be temperature.

We don't know if increasing the temperature will have any effect on increasing the rate of photosynthesis, but we do know that if we start to increase the temperature beyond 30 degrees, we may start to denature the enzymes.

Well done if you got that answer correct.

That brings us to the summary of today's lesson.

We've seen that the rate of photosynthesis is controlled by limiting factors in including carbon dioxide, concentration, light intensity, temperature, the availability of water and availability of chlorophyll.

Some of these factors affect the rate of photosynthesis because they affect how many successful collisions occur between enzymes and their substrates.

As the factors increase in their availability, the rate of photosynthesis increases until one or more of the factors becomes limiting.

And farmers grow crops in conditions that are as close as possible to the optimum conditions for photosynthesis.

This helps to produce more food at a faster rate and helps with human food security.

Thanks for learning with me today.

I hope you've enjoyed today's lesson, and I look forward to seeing you sometime soon.

Bye-bye for now.