Respiration — Cambridge IGCSE Biology
Uses of energy, aerobic and anaerobic respiration, their equations, the yeast investigation, and removing the oxygen debt.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Respiration and the uses of energy
PracticalRespiration is how cells release energy from nutrient molecules such as glucose, and every living cell respires all the time. The energy is used for muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses and keeping a constant body temperature. Never say respiration makes energy, because energy cannot be made.
Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. Glucose is usually the nutrient used, and the word equation is glucose + oxygen → carbon dioxide + water. The balanced equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Respiration happens inside cells and is not the same as breathing.
Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. It releases much less energy per glucose molecule than aerobic respiration, because glucose is only partly broken down. Yeast makes alcohol and carbon dioxide, while muscles during vigorous exercise make only lactic acid.
Lactic acid and oxygen debt
SupplementOxygen debt is the extra oxygen needed after vigorous exercise to break down the lactic acid that built up in the muscles and blood. After exercise the heart rate stays fast to carry lactic acid to the liver, and breathing stays deeper and faster to supply oxygen. The liver breaks down the lactic acid by aerobic respiration.
Exam questions
7 exam-style questions (21 marks), each with its mark scheme.
Answer the questionsFlashcards
15 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of respiration 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
State three uses of energy in living organisms.
Show answer
Any three from: muscle contraction, protein synthesis, cell division, active transport, growth, nerve impulses, maintaining body temperature.
Write the word equation for aerobic respiration.
Show answer
glucose + oxygen → carbon dioxide + water
Write the word equation for anaerobic respiration in yeast.
Show answer
glucose → alcohol + carbon dioxide
Which product forms in muscles during anaerobic respiration?
Show answer
Lactic acid.
Supplement Why does breathing stay deep and fast after exercise?
Show answer
To supply oxygen for aerobic respiration of lactic acid in the liver.
Frequently asked questions
How is respiration different from breathing?
Respiration is a chemical reaction inside cells, while breathing is the movement of air into and out of the lungs. In aerobic respiration, cells use oxygen to break down glucose and release energy, producing carbon dioxide and water. Breathing simply supplies the oxygen and removes the carbon dioxide, so the two are linked but different.
What is anaerobic respiration in yeast?
Anaerobic respiration in yeast is the breakdown of glucose without oxygen to release energy, producing alcohol and carbon dioxide. It is also called fermentation. The word equation is glucose → alcohol + carbon dioxide. Bakers use the carbon dioxide to make bread dough rise, and the alcohol is used to make alcoholic drinks.
Why does anaerobic respiration release less energy than aerobic respiration?
Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration because the glucose is only partly broken down. Without oxygen, cells cannot break glucose down all the way to carbon dioxide and water. Instead yeast makes alcohol and carbon dioxide, and muscles make lactic acid, so less energy is released.
What is oxygen debt and how is it repaid?
Supplement Oxygen debt is the extra oxygen needed after vigorous exercise to break down lactic acid. It is repaid because the heart rate stays fast, carrying lactic acid in the blood from the muscles to the liver, and breathing stays deeper and faster to supply oxygen. The liver then breaks down the lactic acid by aerobic respiration.
How does temperature affect respiration in yeast?
The rate of respiration in yeast increases with temperature up to an optimum, then falls. Above the optimum, the enzymes involved in respiration are denatured, so there is little or no bubbling at very high temperatures such as 60 °C. The rate is measured by counting carbon dioxide bubbles or collecting gas in a fixed time.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026