Core practical: the rate of respiration in living organismsSpec 8.11
In short
The respiration core practical uses a respirometer to measure how fast living organisms, such as germinating seeds or small invertebrates, use up oxygen. Soda lime absorbs the carbon dioxide they produce, so the volume of gas falls and the coloured liquid moves towards the organisms. The rate is the distance moved divided by the time.
Respiration can be investigated by measuring how fast a living organism, such as germinating seeds or small invertebrates, uses up oxygen. This is done with a respirometer.
- Place the organisms in a boiling tube. Put a layer of soda lime (or another carbon dioxide absorber) in the tube, separated from the organisms by cotton wool or gauze.
- Connect the tube to a capillary tube containing a drop of coloured liquid, and seal the apparatus so that no air can get in or out.
- Place the tube in a water bath at a set temperature and leave it to settle.
- Measure the distance the coloured liquid moves in a set time, for example 5 minutes.
- Repeat at different temperatures, and repeat each temperature to calculate a mean.
- Set up a control tube with an equal volume of an inert material, such as glass beads, instead of organisms.
The organisms use up oxygen. The carbon dioxide they produce is absorbed, so the volume of gas in the tube falls and the coloured liquid moves towards the organisms. The further it moves in a given time, the faster the rate of respiration.
| Type of variable | Example |
|---|---|
| Independent variable | Temperature (or the type of organism, if comparing organisms) |
| Dependent variable | Distance moved by the liquid in a set time |
| Control variables | Mass of organisms, volume of soda lime, time, volume of the apparatus, start position of the liquid |
Typical result: the rate of respiration increases as temperature increases (up to a point), because enzymes in respiration work faster. Soda lime is corrosive, so wear eye protection and avoid skin contact. Handle living organisms gently and wash your hands afterwards.
A suggested practical for anaerobic respiration is to investigate the effect of glucose concentration on the rate of anaerobic respiration in yeast. The rate can be found by measuring how quickly carbon dioxide is produced, for example by counting bubbles or measuring the volume of gas collected.
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What does exothermic mean?
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Energy is transferred to the surroundings.
Write the word equation for aerobic respiration.
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Glucose + oxygen → carbon dioxide + water.
What are the products of anaerobic respiration in yeast?
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Ethanol and carbon dioxide.
Why does the coloured liquid move towards the organisms in a respirometer?
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The organisms use up oxygen and the carbon dioxide is absorbed, so the volume of gas decreases.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026