Required Practical: Photosynthesis

This lesson covers the AQA required practical on the effect of light intensity on the rate of photosynthesis. This practical uses an aquatic plant (usually pondweed such as Elodea or Cabomba) and measures the volume of oxygen produced at different distances from a light source. You must be able to describe the method, explain the science behind it, analyse results and evaluate the procedure.

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The Investigation: Effect of Light Intensity on the Rate of Photosynthesis

Aim

To investigate the effect of light intensity on the rate of photosynthesis using an aquatic plant.

Hypothesis

As light intensity increases (by moving the lamp closer to the plant), the rate of photosynthesis will increase, producing more oxygen per minute. This is because light provides the energy needed for photosynthesis, so more light means more energy is available for the reaction.

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Apparatus

Item	Purpose
Beaker of water	Contains the pondweed and acts as a heat shield
Aquatic plant (e.g. Elodea)	The photosynthesising organism
Lamp	Light source with adjustable distance
Ruler or metre stick	To measure distance from lamp to plant
Stopwatch	To time the collection period (e.g. 5 minutes per distance)
Gas syringe or inverted measuring cylinder	To collect and measure the volume of oxygen produced
Sodium hydrogen carbonate (NaHCO3) solution	Added to the water to provide excess CO2 so it is not a limiting factor
Thermometer	To monitor water temperature
Paper clip or weight	To hold the pondweed in place underwater

graph TD
    A[Set up apparatus] --> B[Place pondweed in beaker of NaHCO3 solution]
    B --> C[Position lamp at set distance]
    C --> D[Wait 2 minutes for plant to acclimatise]
    D --> E[Count oxygen bubbles or collect gas for 5 minutes]
    E --> F[Record results]
    F --> G[Move lamp to new distance]
    G --> C

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Method — Step by Step

1. Fill a beaker with water and add a measured amount of sodium hydrogen carbonate (NaHCO3) to ensure CO2 is not a limiting factor.
2. Cut a fresh piece of pondweed (about 5-8 cm) and place it in the beaker, weighted down with a paper clip if needed.
3. Set up a gas collection method — either an inverted measuring cylinder filled with water (to collect oxygen by displacement), or a gas syringe connected to the pondweed via tubing.
4. Position the lamp at a measured distance from the plant (e.g. 10 cm).
5. Wait for 2 minutes to allow the plant to acclimatise to the new light intensity.
6. Start the stopwatch and count the number of oxygen bubbles produced in 5 minutes, or measure the volume of gas collected.
7. Record the results in a table.
8. Move the lamp to a new distance (e.g. 15 cm, 20 cm, 25 cm, 30 cm, 40 cm, 50 cm).
9. Repeat the measurement at each distance.
10. Repeat the entire experiment three times at each distance to calculate a mean and identify anomalies.

Exam Tip: The question may ask why sodium hydrogen carbonate is added to the water. The answer is: it dissolves to release CO2, ensuring that carbon dioxide concentration is not a limiting factor. This means any change in the rate of photosynthesis can be attributed to changes in light intensity alone.

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Variables

Variable Type	Variable	Details
Independent (the one you change)	Distance of lamp from plant	Measured in cm; used to calculate light intensity
Dependent (the one you measure)	Rate of photosynthesis	Measured as volume of O2 produced per minute or number of bubbles per minute
Control (kept the same)	Temperature of water	Use a large beaker of water between the lamp and the plant as a heat shield
Control	CO2 concentration	Use the same concentration of NaHCO3 solution throughout
Control	Type and length of pondweed	Same species and similar-sized piece for each repeat
Control	Duration of each measurement	Same time period (e.g. 5 minutes) for each distance
Control	Colour and power of lamp	Same lamp throughout
Control	Background lighting	Ideally perform in a darkened room or with blackout blinds

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Calculating Light Intensity

You do not measure light intensity directly. Instead, you calculate relative light intensity using the inverse square law:

Light intensity is proportional to 1 / d squared

Where d = distance from the lamp to the plant (in cm).

Distance (cm)	1 / d squared (relative light intensity)
10	1 / 100 = 0.0100
15	1 / 225 = 0.0044
20	1 / 400 = 0.0025
25	1 / 625 = 0.0016
30	1 / 900 = 0.0011
40	1 / 1600 = 0.000625
50	1 / 2500 = 0.000400

When plotting your graph, plot 1/d squared on the x-axis (not distance) because this represents relative light intensity.

Exam Tip: Examiners may give you a table of distances and ask you to calculate light intensity. Always use 1/d squared. If you plot distance on the x-axis instead of 1/d squared, you will not get a straight-line relationship and may lose marks on graph interpretation questions.

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Recording and Processing Results

Sample Results Table

Distance (cm)	Trial 1: Bubbles/min	Trial 2: Bubbles/min	Trial 3: Bubbles/min	Mean Bubbles/min
10	42	45	40	42.3
15	28	30	27	28.3
20	18	20	19	19.0
25	12	13	11	12.0
30	8	9	8	8.3
40	5	4	5	4.7
50	3	3	2	2.7

Calculating the Mean

Add the three values and divide by 3. For example, at 10 cm: (42 + 45 + 40) / 3 = 127 / 3 = 42.3 bubbles per minute (to 1 decimal place).

Identifying Anomalies

An anomalous result is one that does not fit the expected pattern. If one trial is very different from the others at the same distance, it may be anomalous and can be excluded from the mean — but you must note this in your evaluation.

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Expected Results and Graph

When you plot 1/d squared (light intensity) on the x-axis and mean bubbles per minute (rate of photosynthesis) on the y-axis, you should see:

• A positive correlation — as light intensity increases, the rate of photosynthesis increases
• At low light intensities, the relationship is approximately linear (directly proportional)
• At high light intensities, the line begins to level off (plateau) because another factor (temperature or CO2) becomes limiting

graph LR
    A[Low light intensity] --> B[Rate increases linearly]
    B --> C[Light is the limiting factor]
    C --> D[Rate begins to plateau]
    D --> E[Another factor is now limiting]
    E --> F[CO2 concentration or temperature]

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Sources of Error and Improvements