Core practical: rate of respiration

Core practical 8.11 uses a simple respirometer to measure how fast small organisms such as woodlice or germinating seeds take in oxygen.

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Respiration in every cell

Respiration is an exothermic reaction that happens continuously in every living cell, releasing energy from glucose.
Aerobic respiration uses oxygen, so measuring oxygen uptake shows the rate of respiration.

The respirometer

1. Put soda lime in a boiling tube to absorb carbon dioxide, with a gauze platform above it.
2. Place the organisms (for example woodlice or germinating peas) on the gauze.
3. Connect a capillary tube with a drop of coloured liquid.
4. As the organisms use oxygen, the CO2 they give out is absorbed, so the gas volume falls. The coloured liquid moves towards the organisms.
5. Measure the distance moved in a set time. Repeat at different temperatures using a water bath.
A control tube has glass beads of the same mass instead of organisms.

Calculating and working ethically

rate = distance moved ÷ time, for example in mm per minute.
Volume of oxygen used = π r2 × distance (r = radius of the capillary).
Handle animals carefully, do not let soda lime touch them, do not use temperatures that would harm them, and return them to their habitat afterwards.
Worked example

The liquid moves 18 mm in 6 minutes at 20 °C and 30 mm in 6 minutes at 30 °C. Calculate both rates and describe the effect of temperature.

  1. 20 °C: 18 ÷ 6 = 3 mm/min
  2. 30 °C: 30 ÷ 6 = 5 mm/min
  3. The rate is higher at 30 °C.

Answer: 3 mm/min at 20 °C and 5 mm/min at 30 °C: a higher temperature increases the rate of respiration.

Key idea

Soda lime absorbs CO2, so as organisms use oxygen the coloured liquid moves towards them. Rate = distance ÷ time. Use a water bath to vary temperature and glass beads as a control. Treat animals ethically.

Key words
Respiration
An exothermic reaction in all living cells that releases energy from glucose.
Exothermic
A reaction that releases energy to the surroundings, such as respiration.
Aerobic respiration
Respiration using oxygen: glucose + oxygen gives carbon dioxide + water, releasing a lot of energy. It happens in mitochondria.
Capillary
The smallest blood vessel. Its wall is one cell thick so substances can be exchanged with tissues.

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Extra practice

2 more questions

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  1. The coloured liquid moves 30 mm in 12 minutes. Calculate the rate in mm per minute.

    Show the answer

    Answer: 2.5 mm/min

    30 ÷ 12 = 2.5 mm/min

  2. What would happen if no soda lime was used in a respirometer?

    1. The liquid would hardly move because CO2 released replaces the O2 used
    2. The liquid would move faster
    3. The organisms would die immediately
    4. The organisms would stop respiring
    Show the answer

    Answer: The liquid would hardly move because CO2 released replaces the O2 used

    Soda lime absorbs carbon dioxide.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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