Respiration — Edexcel GCSE Combined Science
Aerobic and anaerobic respiration, how they compare, and the core practical to investigate the rate of respiration in living organisms.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Cellular respiration is an exothermic reaction that happens continuously in living cells to release energy for metabolic processes. Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water. Anaerobic respiration happens without oxygen and produces lactic acid in animal cells, or ethanol and carbon dioxide in plant cells and yeast. Respiration is not breathing.
Aerobic respiration needs oxygen and completely breaks down glucose, releasing a large amount of energy per glucose molecule. Anaerobic respiration does not need oxygen and only partly breaks down glucose, so it releases much less energy. Both are exothermic. Anaerobic respiration happens when oxygen is not available, for example in muscles during vigorous exercise.
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.
Exam questions
5 exam-style questions (16 marks), each with its mark scheme.
Answer the questionsFlashcards
12 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of exchange and transport in animals on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
What does exothermic mean?
Show answer
Energy is transferred to the surroundings.
Write the word equation for aerobic respiration.
Show answer
Glucose + oxygen → carbon dioxide + water.
What are the products of anaerobic respiration in yeast?
Show answer
Ethanol and carbon dioxide.
Why does the coloured liquid move towards the organisms in a respirometer?
Show answer
The organisms use up oxygen and the carbon dioxide is absorbed, so the volume of gas decreases.
Frequently asked questions
Why is respiration an exothermic reaction?
Respiration is exothermic because it transfers energy to the surroundings, and some of this energy is released as heat. It happens continuously in the living cells of plants and animals to release energy for metabolic processes, such as building larger molecules, muscle contraction and keeping mammals and birds warm. Aerobic and anaerobic respiration are both exothermic.
How is respiration different from breathing?
Respiration is a chemical reaction in cells that releases energy from glucose, while breathing is the movement of air into and out of the lungs. Respiration happens continuously in all living cells, in plants as well as animals. It is also not the same as gas exchange, which is the diffusion of gases between the alveoli and the blood.
What is the difference between aerobic and anaerobic respiration?
Aerobic respiration uses oxygen and completely breaks down glucose to carbon dioxide and water, releasing a large amount of energy. Anaerobic respiration happens without oxygen and breaks glucose down only partly, so it releases much less energy. In animals it produces lactic acid, and in plants and yeast it produces ethanol and carbon dioxide.
What is anaerobic respiration in yeast?
Anaerobic respiration in yeast is respiration without oxygen, in which glucose is broken down to ethanol and carbon dioxide. It releases much less energy than aerobic respiration because glucose is only partly broken down. A suggested practical investigates how glucose concentration affects its rate by measuring how quickly carbon dioxide is produced.
How does a respirometer measure respiration?
A respirometer measures the rate at which living organisms use up oxygen. The organisms are sealed in a tube with soda lime, which absorbs the carbon dioxide they produce. As oxygen is used, the volume of gas falls and a drop of coloured liquid moves towards the organisms. The distance moved divided by the time gives the rate.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026