Cambridge IGCSE • Year 11 • Biology
Respiration
Aerobic and anaerobic respiration and energy release.
Chapter 12
Verified Curriculum Topic
What is Respiration?
Aerobic and anaerobic respiration and energy release.
Respiration matters because it helps students explain living systems with precise vocabulary and clear cause-and-effect reasoning. At Year 11 level, strong performance usually depends on understanding processes, structures, functions, and diagram-based explanations.
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Summary
The One Thing
Respiration is a set of enzyme-controlled chemical reactions that releases energy from glucose for cellular activities. Aerobic respiration uses oxygen and releases more energy, whereas anaerobic respiration occurs without oxygen and releases less energy while producing different waste products.
Reactions, Processes and Experiments
| What happens | Equation or process | What you observe | Type |
|---|---|---|---|
| Glucose is broken down using oxygen, producing carbon dioxide and water. | glucose + oxygen → carbon dioxide + water | A large amount of energy is released. | Aerobic respiration |
| Glucose is completely broken down in the presence of oxygen. | C6H12O6 + 6O2 → 6CO2 + 6H2O | A high energy yield is produced. | Aerobic respiration; balanced symbol equation |
| Animal muscle cells break down glucose without oxygen, producing lactic acid. | glucose → lactic acid | Lactic acid can accumulate in muscles and contribute to fatigue. | Anaerobic respiration in animals |
| Yeast and some plant cells break down glucose without oxygen, producing ethanol and carbon dioxide. | glucose → ethanol + carbon dioxide | Ethanol and carbon dioxide are produced; in yeast, this process is useful in bread-making and alcoholic drink production. | Anaerobic respiration in yeast and plants; fermentation |
| Glucose is split into smaller molecules, releasing a small amount of energy. | Glycolysis occurs in the cytoplasm. | A small amount of energy is released at the first stage of respiration. | First stage of respiration |
| Most aerobic respiration takes place inside the mitochondria. | Aerobic respiration occurs mainly in mitochondria, while its first stage occurs in the cytoplasm. | — | Cellular respiration process |
| Muscles increase anaerobic respiration when oxygen supply does not meet demand during vigorous exercise. | During vigorous exercise, insufficient oxygen causes increased anaerobic respiration. | Lactic acid builds up and contributes to fatigue. | Anaerobic respiration during exercise |
| Lactic acid is removed after exercise using additional oxygen, mainly in the liver. | Oxygen debt is the additional oxygen needed after exercise to break down lactic acid and restore normal conditions. | Breathing remains elevated after exercise while the additional oxygen is taken in. | Oxygen debt |
| Yeast carries out anaerobic respiration to produce ethanol and carbon dioxide. | Fermentation is used in bread-making and alcoholic drink production. | Carbon dioxide is useful in bread-making, while ethanol is produced in alcoholic drinks. | Fermentation |
| Oxygen uptake during respiration is measured. | A respirometer can measure oxygen uptake, often with carbon dioxide absorbed by a chemical such as potassium hydroxide. | A change indicating oxygen uptake can be measured by the apparatus. | Respiration investigation using a respirometer |
| Respiration is investigated under controlled conditions. | Temperature, organism size or mass, and time should be controlled so that results are fair and reliable. | — | Experimental control in a respiration investigation |
| Energy release is estimated from a temperature change. | Temperature change is measured in a food sample or organism. | A temperature increase may indicate energy release, although heat loss can make the method inaccurate. | Energy-release investigation |
Key Terms
- Respiration: Chemical reactions in cells that break down glucose or other fuels and release energy.
- Aerobic respiration: Respiration that uses oxygen to release energy from glucose, producing carbon dioxide and water.
- Anaerobic respiration: Respiration that releases energy from glucose without using oxygen.
- Glucose: A soluble sugar commonly used by cells as a fuel for respiration.
- ATP: A small energy-transfer molecule made during respiration and used to power cellular processes.
- Mitochondrion: A cell organelle where most aerobic respiration takes place.
- Cytoplasm: The cell region where glycolysis and anaerobic respiration begin.
- Glycolysis: The first stage of respiration, in which glucose is split into smaller molecules and a small amount of energy is released.
- Lactic acid: The product formed when animal muscle cells respire anaerobically.
- Ethanol: An alcohol produced during anaerobic respiration in yeast and some plant cells.
- Oxygen debt: The additional oxygen needed after exercise to remove lactic acid and restore normal conditions.
- Limiting factor: A condition that restricts the rate of respiration, such as temperature, oxygen supply, glucose concentration, or enzyme activity.
Easily Confused
- Respiration and breathing: Respiration is a chemical process inside cells, whereas breathing is the movement of air into and out of gas-exchange organs.
- Aerobic and anaerobic respiration: Aerobic respiration uses oxygen and produces much more energy; anaerobic respiration does not use oxygen and produces less energy.
- Aerobic respiration and glycolysis: Most aerobic respiration occurs in mitochondria, whereas glycolysis is the first stage and occurs in the cytoplasm.
- Anaerobic respiration in animals and in yeast or plants: Animal cells produce lactic acid, whereas yeast and some plant cells produce ethanol and carbon dioxide.
- Energy release and ATP production: Respiration releases energy from glucose, and this energy is transferred mainly to ATP for use in cellular processes.
- Oxygen debt and oxygen supply during exercise: Oxygen debt refers to additional oxygen required after exercise, whereas oxygen supply during exercise refers to the oxygen available to muscles at that time.
What Gets Asked
- Writing aerobic respiration equations: Questions may require both the word equation, glucose + oxygen → carbon dioxide + water, and the balanced symbol equation, C6H12O6 + 6O2 → 6CO2 + 6H2O. A common mark-losing error is omitting oxygen or giving an unbalanced symbol equation.
- Comparing aerobic and anaerobic respiration: Answers must state that aerobic respiration releases more energy because glucose is broken down more completely. The specific anaerobic products must also be identified.
- Identifying anaerobic products: In animals, the product is lactic acid; in yeast and plants, the products are ethanol and carbon dioxide. Substituting one set of products for the other loses accuracy.
- Explaining fatigue and oxygen debt: Vigorous exercise can make oxygen supply insufficient, increasing anaerobic respiration and causing lactic acid to accumulate. Additional oxygen after exercise is needed to break it down, mainly in the liver.
- Interpreting respiration investigations: A respirometer measures oxygen uptake, often with carbon dioxide absorbed by potassium hydroxide. Temperature, organism size or mass, and time must be controlled for a fair and reliable investigation.
- Explaining factors affecting respiration: Respiration is enzyme-controlled, so its rate is affected by temperature and pH; very high temperatures can denature respiratory enzymes. Other limiting factors include oxygen supply and glucose concentration.
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- 12.1Define respiration as the chemical reactions that break down nutrient molecules to release energy for metabolic processes.
- 12.2State the word equation for aerobic respiration and the balanced chemical equation.
- 12.3State the word equation for anaerobic respiration in muscles (producing lactic acid) and in yeast (producing ethanol and carbon dioxide).
- 12.4Compare the amount of energy released by aerobic and anaerobic respiration.extended
- 12.5Explain the term oxygen debt and describe how it is repaid after vigorous exercise.extended
- 12.6Describe an experiment to investigate the effect of temperature on the rate of respiration in yeast.
Practice questions
Q1. What is the product of anaerobic respiration in human muscle cells?1 mark · core
- A. Ethanol
- B. Lactic acid
- C. Carbon dioxide only
- D. Water only
Answer: B
- • 1 mark for selecting B
Human muscle cells produce lactic acid during anaerobic respiration; yeast produces ethanol and carbon dioxide instead.
Q2. Explain what is meant by "oxygen debt" and how the body repays it after exercise.3 marks · extended
Answer: During vigorous exercise, the body cannot supply oxygen fast enough, so lactic acid builds up (oxygen debt); after exercise, faster/deeper breathing supplies extra oxygen to break down the lactic acid.
- • 1 mark: oxygen debt is the extra oxygen needed after exercise to break down lactic acid built up during anaerobic respiration
- • 1 mark: breathing rate and depth stay elevated after exercise
- • 1 mark: this supplies the extra oxygen needed to oxidise/break down the lactic acid
Q3. State which type of respiration releases more energy per glucose molecule, and why.2 marks · extended
Answer: Aerobic respiration releases more energy, because glucose is broken down completely into carbon dioxide and water, whereas anaerobic respiration only partially breaks it down.
- • 1 mark: aerobic respiration releases more energy
- • 1 mark: because it breaks glucose down completely (fully oxidises it), unlike anaerobic respiration
Q4. Write the word equation for aerobic respiration.2 marks · core
Answer: Glucose + oxygen → carbon dioxide + water (+ energy).
- • 1 mark: correct reactants (glucose + oxygen)
- • 1 mark: correct products (carbon dioxide + water)
Key ideas to master
- Master the important terms, labelled structures, and process sequences in Respiration.
- Explain how the system works step by step using accurate biological vocabulary.
- Practise diagram-based recall, comparisons, and function-based questions.
- Focus on causes, effects, and interactions rather than memorising isolated points.
Common exam prompts
- Describe the process or structure in Respiration in the correct sequence.
- Label or explain a likely diagram-based question from this topic.
- Compare related systems, tissues, organs, or processes where the chapter requires it.
- Summarise the functional importance of Respiration in concise exam language.
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What is Respiration in Cambridge IGCSE Year 11 Biology?
Aerobic and anaerobic respiration and energy release.
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