Photosynthesis (AQA GCSE Biology: Combined Science)

Green plants and algae synthesise glucose. Energy is required for this process.

Where do plants get this energy from?

Plants store some of the glucose that they produce in the form of starch.

Give three other ways in which plants use glucose.

Starch is a polymer of glucose which is stored in the roots of many plants.

Explain as fully as you can, why it is advantageous to the plants to store carbohydrates as starch rather than as sugar.

Write out the full word equation for photosynthesis.

Higher Tier Only

Several environmental factors can affect the rate of photosynthesis, including temperature and the availability of water.

Describe and explain the effects of two other factors that affect the rate of photosynthesis.

You should include one or more sketch graphs in your answer.

Plants growing in soils lacking magnesium ions do not grow to their full potential.

Suggest why this is the case.

A group of students wanted to investigate how the rate of photosynthesis is affected by temperature.

They grew some tomato plants in a greenhouse on school grounds then cut disks from the leaves of the plants in their experiments.

To show the rate of photosynthesis, the students measured the rate of carbon dioxide uptake by the leaf disks.

In the dark, the leaf disks did not take up any carbon dioxide, but did produce it.

Explain this observation.

In their investigation, the scientists measure the rate of carbon dioxide taken in by the leaf discs in the light.

The scientists then measured the rate of carbon dioxide produced by the leaf discs when they were placed in the dark.

The graph in Figure 1 shows the effect of temperature on:

• Carbon dioxide uptake in the light
• Carbon dioxide uptake in the light added to carbon dioxide production in the dark

Figure 1

Use the information in Figure 1 to describe the effect of temperature on carbon dioxide uptake in the light.

Explain the effect of temperature on carbon dioxide uptake in the light when the temperature is increased from 20 °C to 35 °C and then from 45 °C to 50 °C.

Higher Tier Only

The students want to maximise the rate of growth of their tomatoes.

Suggest the best temperature that they should heat their greenhouse to, and give a reason for your suggestion.

The image in Figure 1 shows some strawberry plants growing in a greenhouse on a hot sunny day in August.

Figure 1

Identify one limiting factor that will affect the rate of photosynthesis at this time?

Explain your answer.

Higher Tier Only

The rate at which plants grow can be altered by changing the conditions in which they grow.

A propagator is a container in which the temperature, light intensity and carbon dioxide concentration can be controlled.

The graph in Figure 2 shows the effects of changing these factors on the growth of strawberry plants.

Figure 2

Describe and explain the impact of increasing carbon dioxide concentration on the mean mass of strawberry plants at bright light and 20 °C.

Higher Tier Only

Farmers wish to make as much profit as they can from their crops. 

What do farmers need to consider before making decisions about the best growing conditions for their plants?

A typical food chain in the Pacific Ocean is: 

phytoplankton → zooplankton → sardines → tuna 

Figure 1 shows how a number of factors in the ocean change across the year. 

Figure 1

Light intensity is a major factor in controlling the number of phytoplankton. 

Explain why. 

Using the information from Figure 1, suggest two reasons why the population of phytoplankton decreases between summer and autumn.

During the summer, over a 24 hour period, the mean total amount of carbon dioxide taken in by the phytoplankton is greater than the total amount of carbon dioxide released.

Explain why this is important for the phytoplankton.

Explain how autotrophs are different from heterotrophs.

A student wanted to set up an investigation to show that photosynthesis requires carbon dioxide.

The method they followed can be seen below:

1. De-starch 2 plants of the same species and size
2. Place each plant under a bell jar (bell jars A and B)
3. Put NaHCO₃ in bell jar A with the first plant
4. Put NaOH solution in bell jar B with the second plant.
5. Place a bung with delivery tubes in the top of bell jar A and B to allow an airflow
6. Add some soda lime to the inlet tube of jar B
7. Put the apparatus in full sunlight and leave for 24 hours.
8. Observe the results of the investigation.

The set up described in points 2 to 6 can be as seen in Figure 1.

The independent variable in the investigation is the presence or absence of carbon dioxide.

Suggest how stages 3, 4 and 6 allow the student to test the independent variable.

Stage 1 of the method described in part (b) refers to 'de-starching' the plants.

Describe how a plant may be de-starched and explain the purpose of de-starching in stage 1.

Before the student completed the investigation described in part (b), he made the following prediction:

'The plant in bell jar B will not photosynthesize'

After the investigation was complete, the student observed the plants but noted no fundamental differences in their visible appearance.

Describe another method that the student should use to show that the plant in bell jar A had photosynthesized whereas the plant in bell jar B had not.

An investigation was carried out to test the effect of temperature on the rate of photosynthesis of pondweed.

The volume of oxygen produced per hour was measured. The results can be seen in Table 1.

Table 1

Calculate the mean rate of photosynthesis per hour when the temperature was 45 °C.

Plot the results from Table 1 onto an appropriate graph using the graph paper in Figure 2.

Figure 2

Describe and explain the results shown in the graph.

Higher Tier Only

A farmer growing strawberries in his greenhouse used the results in Table 1 to decide on the best temperature for his greenhouse. He decided to heat the greenhouse to 30 °C.

When the farmer measured the rate of photosynthesis of his plants, he found the rate to be 11.5% slower than the data suggested.

Calculate the rate of photosynthesis that the farmer actually achieved at 30 °C.

Higher Tier Only

Suggest why the farmer may not have achieved a rate of photosynthesis as high as the data in Figure 2 predicted.

Higher Tier Only

Scientists have predicted that the weather conditions of a particular mountainous region will be influenced by global warming over the next 50 years with two potential key outcomes:

1. An increase in average summer temperatures from approximately 20 to 22 °C
2. An increase in cloud cover

What effect will this change in climate have on the growth of plants in that region.

Higher Tier Only

Figure 3 shows the effect of several different factors on the growth of tomato plants in a greenhouse.

Figure 3

Describe and explain how temperature and light intensity, as recorded in Figure 3, affect the rate of photosynthesis of the tomato plants.

Higher Tier Only

With reference to Figure 3, which factors are limiting the rate of photosynthesis between points X and Y?

Higher Tier Only

On a summer day, the tomato plants in the greenhouse will require a greater amount of water.

However, even if extra water is not provided, water is unlikely to become a limiting factor of photosynthesis.

Explain how this is possible.

Plan an investigation to show how the rate of photosynthesis in pond plants is affected by changes in light intensity.

You should include detail of the independent, dependent and control variables in your investigation.

The student carried out another investigation into the effect of soil magnesium on plant growth.

In this investigation, the student set up three environments as follows:

1. One plant was grown in sterile soil, provided with distilled water and placed in bright light.
2. One plant was grown in sterile soil, provided with magnesium dissolved in distilled water and placed in bright light.

Predict what the student will have observed after two weeks of growing in these conditions. Explain your prediction.

When selecting the plants for the investigation in part (b), the student made sure that they picked plants with the same number of leaves.

Explain why this was necessary.

One method that can be used to show that photosynthesis has occurred is to test the leaves with iodine.

Explain how this shows that photosynthesis has occurred.

Explain the benefit of storing glucose as starch in the plant.

Figure 4 shows the changes in carbon dioxide uptake and release over a 24 hour period in a tomato plant.

Figure 4

Explain what the graph shows at 7:30 and 19:30.

Calculate the percentage decrease in carbon dioxide uptake between 15:00 and 18:00 in Figure 4.

Higher Tier Only

With reference to the amount of carbon dioxide released and taken in, Figure 4 can be used to indicate how the rate of photosynthesis changes throughout the 24 hour period, assuming that the rate of respiration is constant during this time.

Explain how interactions between temperature and light intensity contribute towards the changes in photosynthesis represented in Figure 4.

Over 24 hours, the tomato plant takes in more carbon dioxide than it releases.

Explain the value of this to the tomato plant.