The specification says: Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.
Aim
To find how the distance of a lamp from a piece of pondweed affects the rate at which it photosynthesises.
Background
Photosynthesis is the process that uses light energy to make glucose from carbon dioxide and water. Oxygen is released as a by-product: carbon dioxide + water → glucose + oxygen.
The rate of photosynthesis depends on the light intensity, the carbon dioxide concentration and the temperature. If any one of these is in short supply, it limits the rate. It is called a limiting factor.
Pondweed is an aquatic plant. Its oxygen escapes as bubbles from the cut end of the stem, so the number of bubbles (or the volume of gas) in a set time is a measure of the rate of photosynthesis.
Light intensity gets lower as you move away from a lamp. So the distance from the lamp is a way of changing the light intensity: a larger distance means a lower light intensity.
Hypothesis
The closer the lamp is to the pondweed, the higher the light intensity, so the faster the rate of photosynthesis and the more oxygen bubbles are released per minute.
Variables
| Independent | Distance of the lamp from the pondweed (cm), which changes the light intensity |
|---|---|
| Dependent | Number of oxygen bubbles released in one minute (or the volume of oxygen collected in a set time) |
| Control |
|
Equipment
- Fresh pondweed (for example Elodea or Cabomba), a piece about 10 cm long
- Boiling tube or 250 cm³ beaker
- Sodium hydrogencarbonate solution (about 1 %) to supply carbon dioxide
- LED lamp (or a bench lamp with a heat shield)
- Metre rule or 30 cm ruler and a clamp stand
- Large beaker of water to act as a water bath and heat shield
- Thermometer
- Stopwatch
- Scissors or scalpel (for cutting the stem under water)
- Optional: an inverted funnel and a measuring tube or gas syringe to collect the oxygen
Risk assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Electrical lamp near water | Electric shock if water gets on the lamp or its cable | Keep the lamp and cable well away from the water. Wipe up spills at once and do not touch plugs with wet hands. |
| Hot bench lamp | Burns from touching the lamp | Use a cool LED lamp where possible. Do not touch the lamp and let it cool before moving it. |
| Glassware | Cuts if the glass breaks | Support the boiling tube in a clamp or beaker. Report any breakages and do not pick up broken glass. |
| Cutting the pondweed stem | Cut from the blade | Cut on a tile, cutting away from the body, using a sharp blade carefully. Wash hands after handling pondweed and water. |
Method
- Set up a large beaker of water as a water bath at room temperature. This absorbs heat from the lamp and keeps the temperature steady.
- Pour sodium hydrogencarbonate solution into a boiling tube until it is about three-quarters full. This supplies carbon dioxide so it does not limit the rate.
- Cut about 10 cm of pondweed. Cut the end of the stem at an angle, under water, so that bubbles come out of the cut end.
- Place the pondweed in the boiling tube with the cut end at the top, then stand the tube in the water bath.
- Darken the room as far as you can so the lamp is the main light source.
- Place the lamp 10 cm from the pondweed. Use the ruler to measure from the lamp to the middle of the pondweed.
- Switch the lamp on and leave the pondweed for 2 minutes to adjust to the new light intensity.
- Check the temperature of the water bath with a thermometer and record it. Replace some of the water if it has warmed by more than 1–2 °C.
- Count the number of bubbles released from the cut end of the stem in one minute. Record the number.
- Repeat the count two more times at the same distance, and calculate the mean number of bubbles per minute.
- Move the lamp to 20 cm, wait 2 minutes, and repeat steps 8–10.
- Repeat for lamp distances of 30, 40 and 50 cm.
- Calculate the mean number of bubbles per minute for each distance and plot a graph of your results.
Results
Fill this table in as you go. Print the PDF for a copy to write on.
| Distance of lamp (cm) | Test 1 (bubbles per min) | Test 2 (bubbles per min) | Test 3 (bubbles per min) | Mean (bubbles per min) |
|---|---|---|---|---|
| 10 | ||||
| 20 | ||||
| 30 | ||||
| 40 | ||||
| 50 |
Drawing the graph
Line graph of mean number of bubbles per minute (y-axis, with the unit) against distance of lamp from pondweed in cm (x-axis). The independent variable goes on the x-axis. Plot each mean as a small cross and draw a smooth curve or line of best fit. The curve should fall as distance increases. Use more than half of the grid for each axis, with equal-sized scale steps. Higher tier: you can also plot rate against 1 ÷ distance² (a measure of light intensity), which gives a straight line through the origin while light is limiting.
Example results and answersPractice data, conclusion, errors and 10 exam questions (27 marks) with mark schemes
Example results
| Distance of lamp (cm) | Test 1 (bubbles per min) | Test 2 (bubbles per min) | Test 3 (bubbles per min) | Mean (bubbles per min) |
|---|---|---|---|---|
| 10 | 42 | 45 | 45 | 44 |
| 20 | 24 | 26 | 25 | 25 |
| 30 | 14 | 12 | 13 | 13 |
| 40 | 8 | 9 | 7 | 8 |
| 50 | 5 | 4 | 6 | 5 |
Conclusion
As the lamp moved further from the pondweed, the number of bubbles per minute fell from a mean of 44 at 10 cm to 5 at 50 cm. So the rate of photosynthesis decreases as the distance increases, that is, as the light intensity decreases. This is because light provides the energy for photosynthesis. With less light energy, less glucose and oxygen are made each minute, so fewer bubbles are released. The fall is steepest close to the lamp. In these results light was the limiting factor over the whole range. (Higher tier) At very high light intensity the rate would level off, because another factor, such as carbon dioxide concentration or temperature, would then be limiting.
Errors and improvements
| Error | Effect on the results | Improvement |
|---|---|---|
| Counting bubbles: bubbles are different sizes and several can be released together, so counts are not accurate (random error and a measurement limitation). | Counts are uncertain, so the means are not a precise measure of the volume of oxygen made. | Collect the gas in a gas syringe or an inverted measuring tube and measure its volume in cm³ over a longer time, such as 5 minutes. |
| The lamp warms the water, so temperature rises as the lamp gets closer (systematic error from an uncontrolled variable). | Enzymes may work faster at higher temperature, so the rate at short distances looks too high. | Use a cool LED lamp, keep the tube in a water bath and check the temperature before each count. |
| Light from the room or windows adds to the light from the lamp. | Light intensity is higher than expected, especially at long distances, so the rate is too high. | Carry out the practical in a dim room, or shield the apparatus with a box or black card. |
| The pondweed does not settle straight away after the lamp is moved. | Counts taken too soon give a rate that belongs to the previous distance. | Wait at least 2 minutes after each move before counting, and keep this time the same. |
| Carbon dioxide is used up during the experiment. | Carbon dioxide may become a limiting factor, so the rate is too low at high light intensity. | Use fresh sodium hydrogencarbonate solution and do not leave the pondweed for a long time. |
Exam questions
10 questions, 27 marks. Write your answers on paper, then open each mark scheme.
Question 1
State the independent variable in this investigation.
Show mark scheme for question 1
- distance of the lamp from the pondweed (1) allow light intensity
Question 2
A beaker of water was placed between the lamp and the pondweed in some versions of this practical. Explain why.
Show mark scheme for question 2
- the water absorbs heat from the lamp (1)
- so the temperature of the pondweed stays the same / temperature is controlled (1) allow so temperature does not affect the rate of photosynthesis
Question 3
Sodium hydrogencarbonate solution was added to the water. Explain why.
Show mark scheme for question 3
- it releases / provides carbon dioxide (1)
- so carbon dioxide is not a limiting factor / the rate of photosynthesis is not limited by carbon dioxide (1)
Question 4
Light intensity is inversely proportional to the square of the distance from the lamp. A student moves the lamp from 10 cm to 20 cm from the pondweed. Calculate how many times lower the light intensity is at 20 cm than at 10 cm.
Show mark scheme for question 4
- light intensity ∝ 1 ÷ distance² (1)
- 1 ÷ 10² = 0.01 and 1 ÷ 20² = 0.0025 (1) allow (20 ÷ 10)² = 2²
- 4 times lower / one quarter of the light intensity (1)
Question 5
Use the results in the table to describe the effect of distance on the rate of photosynthesis.
| Distance of lamp (cm) | Mean number of bubbles per minute |
|---|---|
| 10 | 44 |
| 20 | 25 |
| 30 | 13 |
| 40 | 8 |
| 50 | 5 |
Show mark scheme for question 5
- as the distance increases, the number of bubbles per minute decreases / rate of photosynthesis decreases (1)
- the decrease is fastest / steepest at short distances, and slows down at longer distances (1)
- data quote, for example 44 bubbles at 10 cm and 5 bubbles at 50 cm (1)
Question 6
Calculate the percentage decrease in the mean number of bubbles per minute when the lamp is moved from 10 cm to 20 cm. Use the table in question 5. Give your answer to the nearest whole number.
Show mark scheme for question 6
- change = 44 − 25 = 19 (1)
- 19 ÷ 44 × 100 (1)
- = 43 % (1) allow 43.2 %
Question 7
Counting bubbles is not a very accurate way of measuring the rate of photosynthesis. Explain why, and suggest a better method.
Show mark scheme for question 7
- bubbles can be different sizes (1) allow bubbles can be missed or two released together
- so the number does not match the volume of oxygen (1)
- collect the gas and measure its volume (in a gas syringe / measuring tube) (1)
Question 8
At very high light intensity the rate of photosynthesis stops increasing. Name two other factors that could be limiting the rate at this point.
Show mark scheme for question 8
- carbon dioxide concentration (1)
- temperature (1) allow chlorophyll / number of chloroplasts
Question 9
Explain why the student repeated the count three times at each distance and calculated a mean.
Show mark scheme for question 9
- to identify anomalous results / reduce the effect of random error (1)
- so the results are more repeatable / the mean is closer to the true value (1)
Question 10
A student wants to find how light intensity affects the rate of photosynthesis in pondweed. Describe a method the student could use. Include how the student would make the results valid and repeatable.
Show mark scheme for question 10
| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A clear, logical method with all key steps: pondweed in water with a source of carbon dioxide, lamp at measured distances from the pondweed with a ruler, a measurable outcome (bubbles or gas volume per set time), and several repeats with a mean. At least two variables are controlled with reasons. The method could be followed by another person. |
| 2 | 3–4 | A mostly clear method with the main steps. Some details missing, for example only one control variable given, or repeats not mentioned. |
| 1 | 1–2 | A few simple points, for example ‘move the lamp and count bubbles’, with little detail or no control of variables. |
Indicative content
- place pondweed in a tube of water with sodium hydrogencarbonate
- place a lamp at a measured distance (for example 10 cm) from the pondweed
- leave 2 minutes to adjust, then count bubbles / collect gas for one minute
- repeat at the same distance and calculate a mean
- repeat at other distances, such as 20, 30, 40 and 50 cm
- control temperature (water bath / heat shield), carbon dioxide concentration, the pondweed, time and room light
- plot a graph of mean rate against distance
Exam tips
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026