Wind, a shoot and a bubble. Measure the rate, not the loss.
Transpiration planning questions in 9700 Paper 5 ask how a rate is measured, how the independent variable is changed and measured, and how every other condition is kept the same. Here a leafy shoot in a bubble potometer meets an electric fan at a fixed distance. 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 AS & A Level Biology 9700 Paper 5, Question 1. 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: 9700/51 May/June 2024, Question 1 (effect of wind speed on the rate of transpiration) · official past papers
Practise next: 9700/52 October/November 2025, Question 1 (width of stomata of plantain lily, with transpiration data in Question 2) · official past papers
Examiner reports used: 9700 June 2024 (Paper 51) and November 2025 (Paper 52).
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What a full-mark diagram shows
The independent variable
At least five wind speeds: one fan at a fixed, measured distance on different settings, with each wind speed read on an anemometer at the leaves.
The dependent variable
Bubble position on the scale at the start and after a fixed time (5 min); rate = distance moved ÷ time. A “decrease” cannot be measured directly.
Standardised variables, with a method
Thermostatically controlled room for temperature, lamp at a fixed distance for light, same shoot throughout, the same acclimatisation time at each wind speed.
Replicates and safety
Three or more readings at each wind speed and a mean for each; a named hazard with its precaution (plant allergen: gloves; scalpel: cut away from the hand onto a tile; fan: keep hair and clothing clear).
Habits the examiner reports keep asking for
Do not repeat the given set-up. If the question shows the apparatus and says not to describe setting it up, that earns nothing.
Give the method, not just the variable: “a thermostatically controlled room”, not “keep the temperature constant”. A water-bath does not work for a shoot in air.
Acclimatise the plant at each new wind speed before you start timing, and reset the bubble before each reading.
Write “mean”, not “average”, and make clear there is a mean for each wind speed.
Predict a rate. “As wind speed increases, the rate of transpiration increases.” Saying the water or mass goes down is not enough.