Move the lamp, wait, count the bubbles. Keep everything else the same.
Pondweed giving off bubbles of oxygen is the classic IGCSE Biology experiment on light and photosynthesis, and Paper 6 asks you to plan it, improve it and draw its apparatus. Here an 8 cm piece of Elodea sits in a boiling tube of 1% sodium hydrogencarbonate solution, an LED lamp is moved from 10 to 50 cm along a metre rule with a heat shield in between, and the bubbles are counted for 1 minute after 5 minutes to adjust. Watch the set-up built step by step, then build it yourself and check it against the points the examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge IGCSE Biology 0610 Paper 6 (Alternative to Practical). It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 0610/61 May/June 2024, (planning: the effect of light intensity on photosynthesis in an aquatic plant) · official past papers
Practise next: 0610/63 May/June 2025, (planning: light intensity and the oxygen given off by aquatic plants) · official past papers
Practise next: 0610/62 October/November 2024, (DCPIP and chloroplasts at two lamp distances; leaf discs and a heat shield) · official past papers
Examiner reports used: 0610 June 2023 (Paper 61), June 2024 (Paper 61), November 2024 (Paper 62), June 2025 (Papers 62 and 63) and November 2025 (Paper 63).
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What a full-mark diagram shows
The independent variable
Light intensity, changed by moving an LED lamp to at least three (here six) distances from the pondweed, measured with a metre rule.
The dependent variable
The number of oxygen bubbles given off from the cut end in 1 minute, counted three times at each distance (or the volume of gas collected in a set time).
The method
Pondweed weighted under 1% sodium hydrogencarbonate solution; a heat shield (tank of water) between lamp and tube; a darkened room; 5 minutes to adjust after every move of the lamp.
Control variables and repeats
The same plant, the same concentration and volume of sodium hydrogencarbonate solution, the same temperature (checked with a thermometer), the same counting time; three counts and a mean at each distance.
Habits the examiner reports keep asking for
The gas is oxygen, not carbon dioxide. Count bubbles or collect the gas; a starch test or limewater does not measure the rate.
A heat shield goes between the lamp and the plant, and must not block the light. Label it on the diagram.
“Time” on its own is not a control variable: say the 5 minute wait at each distance, or the 1 minute count.
Bubbles are different sizes and easy to miscount. Collecting the gas and measuring its volume is the improvement; “human error” alone scores nothing.
The conclusion links light intensity to the rate of photosynthesis, not just the distance to the number of bubbles.