Question 1 (2 marks)
Variables
State the independent and dependent variables.
Show model answer / marking points
Light intensity/distance from lamp (1); oxygen production per minute (1).
Required Practical 05: Photosynthesis | AQA GCSE Science | The Science Department
About this practical and its aim
Aim
Investigate how light intensity affects the rate of photosynthesis using pondweed or another aquatic plant.
Introduction
Provided you complete this at the right time of year, this is an excellent practical in which you will clearly see the link between light intensity and the rate of photosynthesis. Lots of links to good scientific method, such as control variables (ambient light level so complete on the same day etc) and using the same plant. The usually "invisible" process of photosynthesis comes to life. Don't forget to test the gas produced.
The science behind the practical
Light supplies energy for photosynthesis. Oxygen produced by aquatic plants can be counted as bubbles or measured as a gas volume per unit time.
Key things to remember
Vary lamp distance; allow the plant to adjust; control temperature and carbon dioxide supply; repeat measurements.
Equipment and apparatus
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Fresh aquatic pondweed
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beaker
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water
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sodium hydrogencarbonate solution
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lamp
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ruler
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stopwatch
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thermometer
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funnel and gas collection apparatus if available
Method: How to carry out the experiment
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Set up the apparatus as shown by your teacher. You should have your plant specimen in a glass vessel, completely submerged under water.
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Place your light source 1 m away from your plant and turn on the light.
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Time 1 minute and count how many bubbles of oxygen leave your plant and float to the surface. Record the number of bubbles for that distance.
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Move the lamp to be 80 cm away and repeat the above. Record how many bubbles are released in 1 minute.
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Continue this until the lamp is 20 cm away.
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If there is time, repeat each reading.
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Use your completed table to calculate a mean then plot a graph of distance from the light source (x axis) against the mean number of bubbles per minute (y axis).
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Any results that you need must be collected during this lesson. If you repeat this on another day, the plant will be different (deteriorated, grown or replaced with a different one) and the ambient light will also be different.
Variables and fair testing
| Type of variable | What is measured or controlled? |
|---|---|
| Independent variable | Light intensity, varied by lamp distance |
| Dependent variable | Oxygen bubbles per minute or oxygen volume per minute |
| Control variables | Plant species and length; timing period; carbon dioxide availability; water temperature; background light |
Safety and managing risks
Follow your teacher's instructions and your school's risk assessment. This page is not a substitute for a formal risk assessment.
| Hazard | Risk | Precaution |
|---|---|---|
Electric lamp near water |
Electric shock if electrical equipment gets wet |
Use low-voltage equipment and keep connections dry. |
Warm lamp |
Burns from hot lamp housing |
Allow lamp to cool and use heat shielding where required. |
Results, calculations and analysis
Recording, presenting and interpreting results
Table: distance (cm), repeat bubble counts or gas volume, mean rate. Graph the rate against distance or estimated relative light intensity.
Calculations
Rate = oxygen bubbles or volume ÷ time; relative light intensity ∝ 1/distance² when suitable.
Conclusions, accuracy and evaluation
Drawing a conclusion
As light intensity rises, rate usually rises until another factor becomes limiting.
Evaluating the investigation
Measure oxygen volume for greater accuracy than bubble count; use heat shield/water bath; repeat and control ambient lighting.
More to learn
Interpreting limiting factors
If extra light no longer increases oxygen production, another factor such as carbon dioxide concentration or temperature may limit photosynthesis.
Common exam mistake
Bubble counts are an estimate of gas production; bubbles are not all the same volume.
GCSE examination practice
Answer each question in full before opening the model answer. Check your work against the marking points and look for scientific key words.
Question 2 (2 marks)
Evaluation
Why is gas volume more reliable than counting bubbles?
Show model answer / marking points
Bubbles vary in size (1), so bubble count does not directly measure a fixed volume of oxygen (1).
Question 3 (1 marks)
Analysis
Why might photosynthesis stop increasing at very high light intensity?
Show model answer / marking points
Another factor becomes limiting, such as carbon dioxide availability or temperature (1).
Question 4 (2 marks)
Evaluation
Suggest one improvement to make the investigation more reliable and explain why it helps.
Show model answer / marking points
Repeat measurements and calculate a mean (1), reducing the influence of random variation/anomalies (1). Alternative valid improvement specific to the experiment with explanation can gain both marks.
This page was updated on: 12th June 2026