# Interpret the Limiting Factors of Photosynthesis

In this worksheet, students will interpret graphs about the limiting factors of photosynthesis.

Key stage:  KS 4

Difficulty level:

### QUESTION 1 of 10

Photosynthesis is the process plants use to produce glucose.

Any factor that slows down the rate of photosynthesis is called a limiting factor. These factors include:

Temperature

Amount of carbon dioxide

Light intensity

Amount of chlorophyll

They are called limiting factors because they “limit” the reaction that is taking place.

Water isn't generally considered a limiting factor because even a small amount would allow photosynthesis to occur.

Light Intensity

As you might already know, light is needed for photosynthesis. At night, there's no light for plants to photosynthesise with, so plants respire at this time. During the day, plants photosynthesise and respire.

The more light there is, the greater the rate of photosynthesis. If the rate of photosynthesis stops increasing even though there's still lots of light, something must be limiting photosynthesis. Carbon dioxide, the temperature or the amount of chlorophyll could be the limiting factors. Graphs are useful to demonstrate this:

The graph above shows that as the light intensity increases, so does the rate of photosynthesis. The rate of reaction begins to level off because another factor is limiting the reaction e.g. carbon dioxide or temperature.
This means that increasing the light intensity will have no further effect because another factor is preventing the reaction from happening any faster.

Carbon dioxide concentration

Carbon dioxide is also essential for photosynthesis. If there's plenty of carbon dioxide, photosynthesis can occur. But if there isn't much light, or the temperature is too high, or there's not enough chlorophyll, the rate of photosynthesis will decrease.

The graph above shows that as the carbon dioxide concentration increases, so does the rate of photosynthesis. The rate of reaction begins to level off because another factor is limiting the reaction e.g. light or temperature.
This means that further increases in carbon dioxide have no further effect because another factor is preventing the reaction from happening any faster.

Temperature

As the temperature increases, so does the rate of reaction. This is because more collisions occur between particles and enzymes when the temperature is warmer. The highest point on the graph is the optimum temperature - this is the best temperature for enzymes to work at, so photosynthesis is at its fastest (usually around 30 - 40ºC). Any further increase in temperature causes enzymes to change shape or denature, so the rate of reaction quickly falls.

Chlorophyll

Chlorophyll found in chloroplasts absorbs light energy in order to power photosynthesis. The amount of chlorophyll affects the rate of photosynthesis. Plants in darker conditions will produce more chlorophyll, so  that the plant can absorb the required amount of light in order to photosynthesise.

In the following activity, you will interpret graphs about the limiting factors of photosynthesis.

Any factor that slows down the rate of photosynthesis is called a limiting factor. These include the concentration of carbon dioxide and light intensity.

Which other two factors affect photosynthesis?

Temperature

pH

Enzyme

Amount of chlorophyll

The concentration of carbon dioxide affects the rate of photosynthesis -  it's a limiting factor.

The graph below shows the effect of carbon dioxide on the rate of photosynthesis.

Describe what's happening at 'X' by filling in the blanks.

Temperature

pH

Enzyme

Amount of chlorophyll

A farmer in the UK wants to grow carrots in a greenhouse. He wants to sell the carrots he produces.

He decides to heat the greenhouse to 35°C.

Explain why he should not heat the greenhouse to a temperature higher than 35°C.

He will produce too many carrots

Enzymes in the carrots will denature and won’t photosynthesise

He will have high energy bills

The above graph shows how temperature affects the rate of photosynthesis.

What is happening at X?

He will produce too many carrots

Enzymes in the carrots will denature and won’t photosynthesise

He will have high energy bills

What's the role of chlorophyll and how does the amount of chlorophyll affect photosynthesis?

The role of chlorophyll is to allow enzymes to work

The role of chlorophyll is to absorb light

More chlorophyll allows more carbon dioxide to be absorbed

More chlorophyll allows better absorption of light

The graph shows the effect of temperature on the rate of photosynthesis in pondweed.

Which factor is limiting the rate of photosynthesis between points A and B?

Light

Temperature

Carbon dioxide

Which factor might be limiting the rate of photosynthesis between points C and D?

Light

Temperature

Carbon dioxide

Between which temperatures is the rate of photosynthesis fastest?

0 - 10°C

10 - 20°C

20 - 30°C

A farmer wants to grow peppers in a greenhouse.

The concentration of carbon dioxide in the atmosphere is 0.02%. The farmer adds carbon dioxide to the greenhouse, so that its concentration is 0.10%.

Why does the farmer use a concentration of  0.10% carbon dioxide?

To increase the rate of growth of the pepper plants

To increase water uptake by the pepper plants

To increase the rate of respiration of the pepper plants

A farmer plans to grow peppers in a large greenhouse.

The concentration of carbon dioxide in the atmosphere is 0.02%. The farmer adds carbon dioxide to the greenhouse, so that its concentration is 0.10%.

Why doesn't the farmer use a concentration of carbon dioxide that is higher than 0.10%?

Because it would increase water loss from the pepper plants

Because it would decrease the temperature of the greenhouse

Because it wouldn't increase the rate of photosynthesis of the pepper plants any further

• Question 1

Any factor that slows down the rate of photosynthesis is called a limiting factor. These include the concentration of carbon dioxide and light intensity.

Which other two factors affect photosynthesis?

Temperature
Amount of chlorophyll
EDDIE SAYS
This was quite a tricky question because of the choices on offer. Did you hesitate over the option of enzymes? If so, you are correct that enzymes do play a part in photosynthesis. They are catalysts that speed up the process, and it is also true that enzymes need an optimum pH as well as temperature to work. However, these two are not actually limiting factors. The two you needed here were temperature and chlorophyll levels. Try to memorise all the four limiting factors - you will often be asked about them in your exams and they will help you get easy marks!
• Question 2

The concentration of carbon dioxide affects the rate of photosynthesis -  it's a limiting factor.

The graph below shows the effect of carbon dioxide on the rate of photosynthesis.

Describe what's happening at 'X' by filling in the blanks.

EDDIE SAYS
There are four limiting factors, including the concentration of carbon dioxide. If photosynthesis has levelled off despite there being a high concentration of carbon dioxide, then something else must be limiting the reaction. This could be light intensity, temperature or the amount of chlorophyll.
• Question 3

A farmer in the UK wants to grow carrots in a greenhouse. He wants to sell the carrots he produces.

He decides to heat the greenhouse to 35°C.

Explain why he should not heat the greenhouse to a temperature higher than 35°C.

Enzymes in the carrots will denature and won’t photosynthesise
EDDIE SAYS
It is true that his energy bills could be high if he increases the temperature of his greenhouse, but this is not the reason why he shouldn't do it! Generally, temperatures above 35°C will cause enzymes in plants to break down or become denatured. If this were to happen, the carrots wouldn't be able to photosynthesise or grow.
• Question 4

The above graph shows how temperature affects the rate of photosynthesis.

What is happening at X?

EDDIE SAYS
As the temperature increases, so does the rate of photosynthesis because enzymes work best in warmer temperatures. However, if the temperature becomes too high, bonds in the enzymes will break and they will become denatured. Once this has happened, the enzymes cannot work any more and the process of photosynthesis will stop.
• Question 5

What's the role of chlorophyll and how does the amount of chlorophyll affect photosynthesis?

The role of chlorophyll is to absorb light
More chlorophyll allows better absorption of light
EDDIE SAYS
Chlorophyll is a substance that is vital for the process of photosynthesis. It is produced in the chloroplasts, in the leaves of a green plant. More chlorophyll will allow plants to absorb the required amount of light to photosynthesise. It doesn't necessarily mean that more photosynthesis will occur, however, there could still be other limiting factors at work, such as light intensity, the concentration of carbon dioxide or too high a temperature.
• Question 6

The graph shows the effect of temperature on the rate of photosynthesis in pondweed.

Which factor is limiting the rate of photosynthesis between points A and B?

Temperature
EDDIE SAYS
How did you get on with this one? This is a graph showing the rate of photosynthesis as the temperature is increased. At first, the rate of photosynthesis is increasing as the temperature goes up. This shows that the limiting factor is the temperature - the rate continues to rise until the temperature reaches 20°C. Once that temperature has been reached, the rate of photosynthesis levels off, meaning that the temperature is no longer the limiting factor - no amount of increasing the temperature is going to increase the rate of photosynthesis because there is some other limiting factor at work.
• Question 7

Which factor might be limiting the rate of photosynthesis between points C and D?

Light
Carbon dioxide
EDDIE SAYS
How did you get on? Between C and D the temperature is high enough for photosynthesis to occur, so other factors such as light or carbon dioxide levels must be limiting photosynthesis. We don't have enough information to know which one of these could be the cause, but we do know that it must be one or more of these factors that is limiting photosynthesis.
• Question 8

Between which temperatures is the rate of photosynthesis fastest?

20 - 30°C
EDDIE SAYS
The highest points on the graph are between 20 - 30°C suggesting that this is where photosynthesis is happening fastest. We can also see from the graph that the rate has levelled off at this temperature, which tells us that this is the optimum temperature for the enzymes to allow photosynthesis to happen.
• Question 9

A farmer wants to grow peppers in a greenhouse.

The concentration of carbon dioxide in the atmosphere is 0.02%. The farmer adds carbon dioxide to the greenhouse, so that its concentration is 0.10%.

Why does the farmer use a concentration of  0.10% carbon dioxide?

To increase the rate of growth of the pepper plants
EDDIE SAYS
Did you get this one correct? Increasing carbon dioxide concentration will mean that the plants have more of the gas they need to photosynthesise with. The peppers' growth might still be limited by other factors, however, such as light intensity and temperature.
• Question 10

A farmer plans to grow peppers in a large greenhouse.

The concentration of carbon dioxide in the atmosphere is 0.02%. The farmer adds carbon dioxide to the greenhouse, so that its concentration is 0.10%.

Why doesn't the farmer use a concentration of carbon dioxide that is higher than 0.10%?

Because it wouldn't increase the rate of photosynthesis of the pepper plants any further
EDDIE SAYS
Using a concentration above 0.10% would have no further effect on photosynthesis. If he wanted to increase the rate of photosynthesis further, he would need to consider other factors such as increasing light intensity or raising the temperature. Another activity completed. Well done!
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