Required Practical: Light Intensity and Photosynthesis

This lesson covers the AQA GCSE Combined Science Trilogy (8464) required practical on investigating the effect of light intensity on the rate of photosynthesis using an aquatic plant. You will be expected to describe the method, identify variables, apply the inverse square law and evaluate the experiment.

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Overview of the Practical

You investigate how changing light intensity affects the rate of photosynthesis by counting the number of oxygen bubbles produced by an aquatic plant (e.g. Elodea / pondweed) at different distances from a lamp.

The Principle

• Oxygen is a by-product of photosynthesis.
• The more oxygen bubbles produced per minute, the faster the rate of photosynthesis.
• By moving the lamp to different distances, you change the light intensity reaching the plant.

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Method

1. Set up a beaker of water containing a piece of Elodea (pondweed) with a freshly cut end pointing upwards.
2. Add a small amount of sodium hydrogen carbonate (NaHCO₃) to the water to provide a source of dissolved CO₂ (ensuring CO₂ is not the limiting factor).
3. Place a bench lamp at a measured distance from the beaker (e.g. 10 cm).
4. Leave the plant for 2 minutes to acclimatise to the new light intensity.
5. Count the number of oxygen bubbles produced in 1 minute.
6. Repeat the count three times and calculate a mean.
7. Move the lamp to a new distance (e.g. 15 cm, 20 cm, 25 cm, 30 cm) and repeat steps 4–6.
8. Record all results in a table.

graph TD
    A["Set up Elodea in beaker with NaHCO₃ solution"] --> B["Place lamp at measured distance d"]
    B --> C["Wait 2 min for acclimatisation"]
    C --> D["Count O₂ bubbles in 1 minute"]
    D --> E["Repeat 3 times, calculate mean"]
    E --> F["Move lamp to next distance"]
    F --> C
    E --> G["Record results in table"]

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Variables

Variable	Detail
Independent variable	Distance of lamp from plant (used to vary light intensity)
Dependent variable	Number of oxygen bubbles produced per minute (rate of photosynthesis)
Control variables	Temperature of water, CO₂ concentration (NaHCO₃ amount), type and length of pondweed, volume of water, time for counting

Exam Tip: Always identify and explain control variables in your answer. State what you keep the same and why — for example, "The temperature is kept constant using a heat shield or by monitoring with a thermometer, because temperature affects enzyme activity and therefore the rate of photosynthesis."

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Using the Inverse Square Law

In this practical, you do not measure light intensity directly. Instead, you calculate relative light intensity using:

$$\text{light intensity} \propto \frac{1}{d^2}$$light intensity∝d21​

Distance (cm)	$d^2$d2	Relative Light Intensity $(1/d^2)$(1/d2)
10	100	0.0100
15	225	0.0044
20	400	0.0025
25	625	0.0016
30	900	0.0011

Worked Example

A student places the lamp at 10 cm and counts 42 bubbles per minute. At 20 cm she counts 11 bubbles per minute.

The light intensity at 20 cm is:

$$\frac{1}{20^2} = \frac{1}{400} = 0.0025$$2021​=4001​=0.0025

At 10 cm:

$$\frac{1}{10^2} = \frac{1}{100} = 0.0100$$1021​=1001​=0.0100

Light intensity is 4 times greater at 10 cm than at 20 cm. The bubble count is roughly 4 times higher (42 vs 11), suggesting a proportional relationship between light intensity and rate.

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Improving the Practical

Issue	Improvement
Bubbles are hard to count accurately	Use a gas syringe or collect gas in an inverted measuring cylinder to measure volume of O₂
Temperature may change (lamp produces heat)	Place a heat shield (glass tank of water) between the lamp and the beaker, or monitor temperature with a thermometer
Some bubbles are larger than others	Measuring gas volume is more accurate than counting bubbles
CO₂ may run out during the experiment	Add sodium hydrogen carbonate to ensure constant CO₂ supply
Background light from windows	Use a darkened room or cover windows to ensure only the lamp provides light
The plant may take time to adjust	Allow a 2-minute acclimatisation period each time the distance changes

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Results and Plotting

• Plot relative light intensity ($1/d^2$1/d2) on the x-axis and mean number of bubbles per minute on the y-axis.
• If the relationship is proportional at low light intensities, the graph will show a straight-line increase from the origin.
• At higher light intensities the line may begin to curve and plateau — meaning another factor (CO₂ or temperature) has become limiting.

Exam Tip (AQA 8464): You will be expected to plot light intensity (not distance) on the x-axis. Using $1/d^2$1/d2 shows the actual relationship between light intensity and rate. Plotting distance would give a curve that is harder to interpret.

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Evaluating the Experiment

When evaluating, consider:

• Repeatability — Did the three repeats give similar results? If not, there may be uncontrolled variables.
• Accuracy — Counting bubbles is less precise than measuring gas volume with a syringe.
• Anomalous results — Identify and exclude any results that do not fit the pattern, and suggest reasons (e.g. air bubble stuck to the plant, miscounting).
• Validity — Were all control variables genuinely controlled? Did background light affect results?

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Common Mistakes

Mistake	Correction
Plotting distance (not $1/d^2$1/d2) on the x-axis	Use relative light intensity ($1/d^2$1/d2) on the x-axis
Forgetting to add NaHCO₃	Without it, CO₂ may become the limiting factor, making the results about CO₂, not light
Not allowing acclimatisation time	The plant needs time to adjust to new light levels before you start counting
Saying "count bubbles for as long as possible"	Standardise the counting period (e.g. exactly 1 minute each time)
Not repeating and averaging	At least 3 repeats are needed to calculate a reliable mean

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Summary