Cambridge IGCSE Biology
Cambridge IGCSE Biology 12 Respiration Exam Questions
Free Cambridge IGCSE Biology revision for 12 Respiration: what you need to know, common mistakes and exam-style questions with answers. Covers uses of energy, aerobic and anaerobic respiration equations, yeast, lactic acid, oxygen debt and the effect of temperature on respiration.
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About Cambridge IGCSE Biology 12 Respiration
Respiration is the set of chemical reactions in cells that break down nutrient molecules and release energy for metabolism. In Cambridge IGCSE Biology you need to list the uses of energy in the body, give the word equations for aerobic respiration and for anaerobic respiration in yeast and in muscles, and compare the energy released. Extended candidates also give the balanced symbol equations and explain oxygen debt and how lactic acid is removed. Practical questions investigate respiration in germinating seeds or yeast at different temperatures. A common trap is mixing respiration up with breathing; respiration happens in every living cell, all the time.
What you need to know
- 12.1 Uses of energy Energy from respiration is used for muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses and maintaining a constant body temperature. Respiration involves enzymes, so temperature affects its rate.
- 12.2 Aerobic respiration Chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. Word equation: glucose + oxygen → carbon dioxide + water. Balanced: C6H12O6 + 6O2 → 6CO2 + 6H2O.
- 12.3 Anaerobic respiration 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.
- 12.3 Yeast and muscles Yeast: glucose → alcohol (ethanol) + carbon dioxide; Extended balanced equation C6H12O6 → 2C2H5OH + 2CO2. Muscles during vigorous exercise: glucose → lactic acid.
- 12.3 Oxygen debt (Extended) Lactic acid builds up in muscles and blood during vigorous exercise, causing an oxygen debt. It is removed by continued fast heart rate (transporting lactic acid to the liver) and continued deep and fast breathing, supplying oxygen for aerobic respiration of lactic acid in the liver.
- Investigations Use respirometers or measure carbon dioxide production with hydrogencarbonate indicator or limewater. Germinating seeds or yeast can be used at different temperatures, with boiled seeds as a control.
Common mistakes in 12 Respiration
- Confusing respiration with breathing. Respiration is a chemical process in cells; breathing is ventilation of the lungs.
- Saying respiration produces energy. Energy is released, not produced.
- Saying anaerobic respiration in muscles produces carbon dioxide. In human muscle the only product is lactic acid.
- Saying plants do not respire because they photosynthesise. Plants respire all the time, day and night.
- Writing the balanced equation as C6H12O6 + O2 → CO2 + H2O. It must be C6H12O6 + 6O2 → 6CO2 + 6H2O.
Try three 12 Respiration exam-style questions
1. State the word equation for anaerobic respiration in yeast. (1 mark)
Show the mark scheme answer
Glucose → alcohol (ethanol) + carbon dioxide.
2. Give three uses of energy in the human body. (3 marks)
Show the mark scheme answer
Muscle contraction; protein synthesis; cell division; active transport; growth; passage of nerve impulses; maintaining constant body temperature; any three.
3. Explain why an athlete continues to breathe deeply and quickly after a sprint. (3 marks)
Show the mark scheme answer
Anaerobic respiration produced lactic acid (oxygen debt); extra oxygen is needed; for aerobic respiration of lactic acid in the liver (lactic acid carried to the liver by the blood).
12 Respiration FAQs
What does Cambridge IGCSE Biology 12 Respiration cover?
It covers the uses of energy in the body, aerobic respiration, anaerobic respiration in yeast and muscles, the equations for each and, for Extended candidates, oxygen debt.
Which paper is 12 Respiration examined in?
It is examined in the theory and multiple choice papers: Papers 1 and 3 for Core candidates, or Papers 2 and 4 for Extended candidates. Practical skills linked to it can also appear in Paper 5 (Practical Test) or Paper 6 (Alternative to Practical).
Which equations do I need for respiration?
All candidates need the word equations for aerobic respiration, anaerobic respiration in yeast and anaerobic respiration in muscles. Extended candidates also need the balanced equations for aerobic respiration and for anaerobic respiration in yeast.
