Biology editorial review is supported by the
named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
What this skill means
The rate of aerobic respiration can be measured using a respirometer, which tracks oxygen uptake. As organisms respire, they absorb O₂ and release CO₂. A CO₂ absorber (e.g. Soda lime or KOH) removes the CO₂, so any volume change in the gas represents oxygen consumed.
Respirometer investigations are tested in Papers 5 and 6. You need to understand how the apparatus works, why soda lime is needed, and how to control variables. This practical can be used to compare respiration rates at different temperatures or between different organisms.
How a Respirometer Works
A simple respirometer consists of two sealed tubes or syringes:
Tube 1 (experimental): contains living organisms (e.g. Germinating seeds or woodlice) on a gauze platform, with soda lime or potassium hydroxide (KOH) below to absorb CO₂
Tube 2 (control): contains dead organisms (e.g. Boiled seeds) or glass beads of equivalent volume, with the same amount of soda lime
Both tubes are connected to a manometer or a capillary tube with a coloured liquid drop. As the organisms in tube 1 respire, they absorb oxygen. The CO₂ they produce is absorbed by the soda lime. This causes a decrease in gas volume in tube 1, and the liquid in the manometer moves towards tube 1 (or the coloured drop moves towards the organisms).
The control tube accounts for any pressure or temperature changes in the environment, so the difference between the two tubes represents only the oxygen consumed by respiration.
Investigating Temperature Effects
To test how temperature affects respiration rate:
Set up the respirometer as described above
Place both tubes in a water bath at a set temperature (e.g. 20°C) and allow to equilibrate for 5 minutes
Record the distance the coloured drop moves in a set time (e.g. 10 minutes)
Repeat at different temperatures (e.g. 10, 20, 30, 40, 50°C)
Expected results: rate increases with temperature up to about 40°C (enzymes involved in respiration work faster at higher temperatures). Above about 40–45°C, the rate drops sharply because respiratory enzymes denature and lose their shape.
Why Soda Lime Is Essential
Without soda lime, the CO₂ produced by respiration would replace the volume of O₂ consumed, and there would be no overall gas volume change. The manometer would not move, even though respiration is occurring. The soda lime absorbs the CO₂, so the only gas volume change is due to oxygen uptake, making the movement of the liquid proportional to the rate of respiration.
Use this before submitting your answer
A complete answer is specific enough for another person to repeat or assess.
✓Include soda lime (or KOH) to absorb CO₂ in both tubes.
✓Use a control tube with dead organisms or glass beads of equal volume.
✓Allow at least 5 minutes for temperature equilibration before reading.
✓Measure the distance moved by the coloured drop or the change in manometer reading.
✓Use a water bath to control temperature precisely.
✓Repeat each temperature at least three times and calculate a mean.
✓Reset the apparatus between readings.
Try the skill in a question (3 marks)
In a respirometer experiment, the coloured drop moves 18 mm towards the germinating seeds in 10 minutes at 30°C, but only 5 mm in 10 minutes at 10°C. Explain this difference. [3]
Show the answer and marking guidance
At 30°C the rate of respiration is higher than at 10°C because the enzymes involved in respiration have more kinetic energy at higher temperatures
This increases the frequency of successful enzyme-substrate collisions, so reactions proceed faster
More oxygen is consumed per unit time at 30°C, causing a greater decrease in gas volume and a larger movement of the coloured drop
Original practice content written for this site. It is not an official Cambridge question.