Lesson video

Hello, my name's Mr. Jarvis, and I'm taking you through this lesson from the unit Photosynthesis: factors affecting the rate.

Today's lesson is all about factors affecting the rate of photosynthesis.

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

There are five key words to today's lesson.

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

You can pause the video at this point if you'd like to read through those definitions for yourself, but we will come to them as we go through today's lesson.

Today's lesson is broken down into three parts.

First of all, we're going to look at the different factors that affect photosynthesis.

Then, we're gonna move on to temperature and the rate of photosynthesis.

And finally, we're gonna look at carbon dioxide and the rate of photosynthesis.

So if you're ready, let's get started with our first section, which is all about factors affecting photosynthesis.

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

Plants and other producers make their own food using a process called photosynthesis.

Photo means light, synthesis means to make, and so plants and producers make their own food using light.

Photosynthesis occurs in chlorophyll, which is found in chloroplasts, in the cells of leaves and other parts of the plant that are above the ground, and it's that chlorophyll that gives plants a green colour.

Reactants and products of photosynthesis are as follows.

Water and carbon dioxide gives us glucose and oxygen.

Water and carbon dioxide are the reactants, the things that we put into the reaction, and glucose and oxygen are the products, the things that come out of the reaction, and it's glucose which the plants use as food.

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

Here's a quick check.

What are the two reactants 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.

So the reactants of photosynthesis are carbon dioxide and water, A and E.

Well done if you got those correct.

The rate of photosynthesis is how quick it happens, and this is affected by various factors, and these include the amount of light or the light intensity, the amount of chlorophyll that's found within the leaves, water availability, how hot or cold it is, the temperature, and the amount of carbon dioxide that there is.

Increasing any of these factors can speed up the rate of photosynthesis, but insufficient levels of these factors can slow down or limit the rate of photosynthesis, and so they're called limiting factors because when they're not in enough supply, then they slow the reaction of photosynthesis down.

Here's another check.

Who's correct? Andeep says the rate of photosynthesis is affected by the amount of light and chlorophyll.

Izzy says the concentration of glucose and oxygen limit the rate of photosynthesis.

And Laura says the rate is limited by how much water and carbon dioxide is available.

I'll pause for a few seconds, and I'll leave you to work out which of these three have correct statements.

Good luck.

The correct answers here are Andeep, the rate of photosynthesis is affected by the amount of light and the amount of chlorophyll that's found within the plant, and Laura, the rate is limited by how much water and carbon dioxide that is available.

Well done if you got both of those correct.

So let's look at the different factors that affect the rate of photosynthesis.

Let's look at light, first of all, and we know that light is required for photosynthesis.

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

At really low light levels, the rate of photosynthesis will be really slow because there's insufficient energy to be transferred to the chlorophyll in leaf cells.

And so when we get late at night or early in the morning, when the light intensity is really low, then there'll be no or very little photosynthesis taking place.

In brighter light, for example, in the middle of the day, the rate of photosynthesis will increase as more energy is transferred to the leaf cells.

Chlorophyll is the second limiting factor.

The energy required for photosynthesis to take place is absorbed by the green pigment chlorophyll that's found in the chloroplasts of plant cells.

And you can see on the right-hand side of the screen, zooming into a leaf, we can see some cells, and we can see some chlorophyll within the chloroplasts of this leaf.

If the leaves are pale green and contain little chlorophyll, then the rate of photosynthesis will be slow, and in darker green leaves with more chlorophyll, the rate of photosynthesis will be much faster.

Water is our third factor, and water is a reactant in photosynthesis.

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

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

So for example, in a drought, a plant will not be able to photosynthesize because there won't be enough water available for it to do so.

When there's plenty of water, the rate of photosynthesis will no longer be limited by the lack of water, and so water will no longer be a limiting factor, and most of the time, there's enough water in the soil to allow a plant to photosynthesize.

Here's a check.

True or false? The rate of photosynthesis will be faster in part A.

Is that true or is it false? And then just have a think about why you've given that answer.

I'll pause for a few seconds, and then we'll check to see how well you've done.

So the rate of photosynthesis will be faster in part A.

That's true.

And why is that the answer? Part A is darker green, and that tells us that there's more chlorophyll there, and that will enable the plant to absorb more energy, so enabling it to photosynthesize more quickly.

Well done if you got that right, and you managed to work out why that was the correct answer.

Here's our first task of the lesson.

First of all, I'd like you to complete the word summary for photosynthesis.

You can see that I've left two blanks in the equation.

So something and something gives us glucose and oxygen.

Write down what those first two blanks are.

The second thing I'd like you to do is to explain what's meant by a limiting factor, and give two examples of a limiting factor in your answer.

And then, finally, all of the plants in the photograph are the same age.

Explain why the plants close to the window have grown larger than the plant further away on the table.

You'll need to pause the video at this point to write down your answers.

When you've done that, you can press play, and we will check to see how well you've done.

Good luck.

How did you do? Let's see whether you got the right answers.

First of all, I asked you to write down a completed word summary equation for photosynthesis.

You should have written water and carbon dioxide gives us glucose and oxygen.

It doesn't matter which way round you got water and carbon dioxide, as long as you got both of those answers.

Secondly, explain what's meant by a limiting factor.

Give two examples of limiting factors in your answer.

And here's what you should have written.

When a limiting factor is in short supply, it slows down or limits the rate of photosynthesis.

That's what we mean by a limiting factor, something that slows down the rate of photosynthesis.

And examples of limiting factors are light, chlorophyll, water, temperature, and carbon dioxide concentration.

Well done if you got those.

Thirdly, I asked you to explain why the plants close to the window have grown larger than the plant further away on the table.

Well, you might have suggested something like this.

The plants close to the window receive more light, and this means more energy is transferred to the leaf cells for photosynthesis to take place, so the rate of photosynthesis is faster, and faster photosynthesis means more glucose, which is the food of the plant, is produced, which is used for growth, and so those plants closer to the window are able to grow more quickly than the plant that's further away on the table.

Well done if you got that.

That brings us to the second part of today's lesson.

Now, we're going to look at the effect of temperature on the rate of photosynthesis.

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

A rate is a measure of how much change occurs per unit of time, and the rate of a chemical process, such as photosynthesis, can be determined by measuring the amount of change.

For example, the amount of reactant used per unit of time, or the amount of product formed per unit of time.

Remember, the reactants of photosynthesis are water and carbon dioxide, and the products are glucose and oxygen.

For any chemical reaction to take place, the reactants must collide with one another.

Or if it's an enzyme catalysed reaction, the reactant must collide with the enzyme's active site.

The chemical reactions of photosynthesis do involve enzymes, and an enzyme is a protein that acts as a biological catalyst, and that means that it speeds up the reaction.

The active site of an enzyme has a specific shape, so that only specific substrates, reactants, will fit into it.

So here is our substrate, and you can see that it fits snugly into the active site, the reaction is catalysed, and then the products are released, and that then leaves the active site free for the next substrate to come along.

Here's a check.

The part of the enzyme with which the substrate must collide for a reaction to occur is called the.

A, catalyst, B, active site, C, protein, D, product.

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

The correct answer is B, active site.

The active site is the part of the enzyme with which the substrate must collide for a reaction to occur.

Well done if you got that.

Temperature affects the rate of all chemical reactions, and at lower temperatures, as we can see in this animation, the particles move really slowly and with much less energy.

That means that there's a lower frequency of successful collisions, and that means that the reactions don't take place very quickly.

And you can see the number of collisions that are taking place in this animation are really small, and so this reaction would be taking place very slowly.

It would have a slow rate.

In comparison, at higher temperatures, the particles move much more quickly and with more energy, and this means that there's a higher frequency of collisions that result in a chemical reaction taking place.

And so at higher temperatures, the rate of chemical reactions is much quicker.

In enzyme catalysed reactions, such as photosynthesis, there's an optimum temperature, and at the optimum temperature, the collisions happen often enough that all enzymes active sites are full with substrate molecules almost all of the time.

At the optimum temperature, the reaction takes place at its fastest rate, and the substrate will be used up, and the product will be made at the fastest rate in each unit of time.

For example, every second.

Here's a check.

What's the name given to the condition, for example, temperature, at which the rate of an enzyme catalysed reaction is fastest? Is it A, the optimum, B, the active site, C, the substrate, or D, the limiting factor? I'll pause for a few seconds, and we'll check your answer.

The correct answer is A, optimum.

The optimum temperature is the temperature at which the rate of the enzyme catalysed reaction is the fastest.

Well done if you got that.

When we increase the temperature up to the optimum, the substrate and enzyme particles move faster and with more energy.

There's a higher frequency of successful collisions, and so as a result, the rate of reaction increases, as we can see with the graph.

Enzymes are formed of a chain of amino acids, which are folded into a specific shape.

If we zoom into the enzyme, we can see that amino acid chain looping backwards and forwards within the enzyme molecule.

And if we look closer still, we can see how the chain is held together by bonds between the amino acid.

You can see labelled in the diagram are the bond between the green and the blue amino acid within that chain.

When the temperature gets too high, the bonds between these amino acids break, and the amino acid chain then starts to unravel.

And when it starts to unravel, the active site shape changes, and the substrate no longer fits, and we can see that the bonds have now broken, the chain is unravelled, and the active site is now a different shape, and so the substrate no longer fits in.

Increasing the temperature above the optimum causes the shape of the enzyme's active site to change, and that means that the substrate no longer fits.

The enzyme is, as I've said, denatured, and we saw that in the previous slide.

What that means is that the rate of reaction will decrease, and eventually reach zero, when all of the enzymes active sites are denatured, and there's no longer an enzyme to catalyse the reaction.

Here's a quick check.

Which graph shows how temperature affects the rate of photosynthesis? Is it A, B, or C? I'll pause for a few seconds, and then we'll check your answer.

The correct answer is B.

The rate of photosynthesis increases until it reaches the optimum, and at the optimum, the enzymes start to denature because those amino acid chains start to get their bonds broken, and the amino acid starts to unravel, which changes the shape of the active site, and the enzyme becomes denatured, and at that point, the rate of the reaction starts to decrease.

Well done if you got that.

Here's a task.

A farmer grows crops in a greenhouse, and he decides to heat the greenhouse to 35 degrees C.

Explain why the farmer should not heat the greenhouse to temperatures higher than 35 degrees.

Use information from the graph in your answer.

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

Good luck.

So how did you find that? I hope that it wasn't too tricky for you.

I asked you to explain why the farmer should not heat the greenhouse to temperatures higher than 35 degrees C using information from the graph in your answer.

So you might have suggested that 35 degrees is the optimum temperature, which the rate of photosynthesis, and therefore the rate of growth, is the fastest.

And above this temperature, there's no further increase in the rate of photosynthesis.

In fact, it starts to decrease in the rate, as the enzymes start to become denatured.

Heating above 35 degrees will also cost the farmer more money, and produce less crop to sell as the rate of photosynthesis decreases.

So financially, it is not a good idea, because he'll be spending lots of money on heating, and not getting as much crop in return.

Well done if you got those points in your answer.

Now, we're going to move on to our final part of the lesson, which is all about carbon dioxide and the rate of photosynthesis.

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

Carbon dioxide is a reactant to photosynthesis.

We've seen before in the lesson today that water and carbon dioxide gives us glucose and oxygen, and we've got molecular diagrams of what these molecules look like.

Only approximately 0.

04% of the atmosphere is carbon dioxide.

Imagine a summer, sunny afternoon in which it rained in the morning.

It's now really sunny, and the temperature is warm, but not too hot.

Which factor is likely to limit the rate of photosynthesis, do you think? In these conditions, the low concentration of carbon dioxide in the atmosphere may well be the limiting factor.

Many commercial crops are grown in a greenhouse with higher concentrations of carbon dioxide in the air, such as these herbs that are being grown in a greenhouse.

Can you explain why this increases the rate of photosynthesis? I'll pause for a second or two.

Well, it's warmer in the greenhouse, so that will increase the rate of photosynthesis, and there's more carbon dioxide, and that means that there's more reactant for photosynthesis as a reaction to take place.

Here's a check.

Which factor is likely to limit the rate of photosynthesis in lettuces growing in a field on a warm sunny day? Is it A, light intensity, B, temperature, or C, carbon dioxide concentration? I'll pause for a few seconds, and then we'll check your answer.

The correct answer is carbon dioxide concentration.

That's likely to be the factor that limits the rate of photosynthesis because there's a low concentration of CO2 in the atmosphere, and it's a warm day, so there's going to be a nice temperature for photosynthesis to take place, and it's a sunny day, so the light intensity will be relatively high.

Well done if you got that answer.

When we increase carbon dioxide concentration, there's more substrate molecules to collide with enzymes active sites, and this increases the rate of reaction, as you can see on the graph.

At the optimum concentration, all of the enzymes active sites are full with substrate molecules, and increasing the concentration above the optimum doesn't increase the rate of reaction because there's no more enzymes active sites available, and so what happens is we reach a plateau in the rate of reaction.

Here's a check.

Look at the reactants and products of photosynthesis.

We've got water and carbon dioxide giving us glucose and oxygen.

What will happen to the rate of photosynthesis if you increase the concentration of carbon dioxide available to plants? Will it A, decrease the rate of photosynthesis, B, the rate of photosynthesis will stay the same, or C, it will increase the rate of photosynthesis? I'll pause for a few seconds, and then we'll check your answer.

The correct answer is C, increasing the concentration of carbon dioxide available to plants will increase the rate of photosynthesis up to the point that we reach the optimum.

And remember, the optimum is the point at which carbon dioxide as a substrate is filling all of the enzymes active sites, so the reaction can't physically go any faster than it is.

Well done if you got that answer.

That brings us to our final task of today's lesson.

There are two tasks here, and I'm gonna do them one after the other.

So first of all, the two photographs show the same trees in the summer and the autumn.

I'd like you to explain why the rate of photosynthesis is much higher in the trees in the summer than it is in the autumn, and I'd like you to include some ideas about limiting factors in your answer.

You'll need to pause the video at this point, so that you can answer this first question, and then we'll check this first question before moving on to our second part of this task.

Good luck.

So I asked you to explain why the rate of photosynthesis is much higher in the trees in the summer than in the autumn, and I asked you also to include some ideas about limiting factors in your answer.

So you might have suggested that the temperature is higher in the summer, and so the rate of reaction is also faster because there are more collisions between the substrate and the enzymes active sites.

The days are also longer in the summer, and so there's more light to transfer energy for photosynthesis.

And in the summer, the trees have more leaves, and they're much greener, and this is because there's more chlorophyll in the leaves, so there's more photosynthesis being able to take place.

And temperature, light, and the amount of chlorophyll become limiting factors in the autumn.

Temperature decreases, and so that will slow the rate of reaction.

The amount of light is less in the autumn because the days are shorter, and the light intensity in the middle of the day is less, and the chlorophyll starts to be retracted back into the tree in the autumn, and that's why the leaves lose their green colour.

Well done if you manageD to get any or all of those answers written down for yourself.

Now, let's move on to our second question.

The concentration of carbon dioxide around plants increases slightly at night.

First of all, explain why plants do not photosynthesize at night, even though levels of carbon dioxide are usually that little bit higher.

And for part B, I'd like you to suggest why the carbon dioxide concentration around plants increases at night.

So I'll leave you now to pause the video, write down your answer, and then when you're ready, press play, and we'll check question number two.

So I asked you to explain why plants don't photosynthesize at night, even though levels of carbon dioxide are usually that little bit higher.

You might have answered that there's not enough light at night for photosynthesis to take place.

So light is the limiting factor, even though there's plenty of carbon dioxide for photosynthesis to happen.

And second, I asked you to suggest why carbon dioxide concentration around plants increases at night.

Well, photosynthesis does not take place because there's not enough light.

So plants are not using carbon dioxide from the atmosphere, and cellular respiration continues within the plant, and this does happen at night, so they produce carbon dioxide as a waste product of respiration, which they release into the atmosphere.

Well done if you got that answer.

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

We've seen that the rate of photosynthesis depends on factors including light, chlorophyll, water, temperature, and carbon dioxide, and a factor that when in short supply slows down or limits the rate of photosynthesis is called a limiting factor.

Increasing the temperature increases the rate of photosynthesis up to the optimum because it increases the frequency of collisions between the substrate molecules and the enzymes active sites.

Above the optimum, enzymes are denatured, and the rate of photosynthesis decreases.

Increasing the concentration of carbon dioxide increases the rate of photosynthesis up to the optimum, at the point that all active science are full with substrate molecules.

I hope you've enjoyed today's lesson all about factors affecting the rate of photosynthesis.

It's been great learning with you, as always, and I look forward to seeing you all again soon.

Bye-bye for now.