What you'll learn
Respiration is the process by which living organisms release energy from glucose. This guide covers both aerobic and anaerobic respiration as specified in the Pearson Edexcel International IGCSE Biology syllabus. You'll understand the word equations, practical investigations, and how respiration differs from breathing and photosynthesis.
Key terms and definitions
Respiration — the chemical process that releases energy from glucose in all living cells
Aerobic respiration — respiration that uses oxygen to completely break down glucose, releasing maximum energy
Anaerobic respiration — respiration without oxygen, producing less energy and different waste products
Mitochondria — cellular organelles where aerobic respiration takes place
Lactic acid — the waste product of anaerobic respiration in animals and some bacteria
Ethanol — the alcohol waste product of anaerobic respiration in yeast and plants
Oxygen debt — the extra oxygen needed after anaerobic respiration to break down lactic acid
ATP — adenosine triphosphate, the molecule that stores and transfers energy in cells
Core concepts
The purpose of respiration
All living organisms require energy for essential life processes. Respiration releases this energy from glucose in a controlled, step-by-step manner.
Energy released through respiration is used for:
- Muscle contraction — movement and locomotion
- Protein synthesis — building new proteins from amino acids
- Cell division — growth and repair of tissues
- Active transport — moving substances against concentration gradients
- Maintaining constant body temperature — particularly in mammals and birds
- Transmission of nerve impulses — sending signals through the nervous system
- Building larger molecules from smaller ones — such as making starch from glucose in plants
The energy released during respiration is transferred to ATP molecules, which then release energy when and where it is needed in the cell.
Aerobic respiration
Aerobic respiration occurs in the presence of oxygen and releases the maximum amount of energy from each glucose molecule.
Word equation: glucose + oxygen → carbon dioxide + water (+ energy)
Chemical equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Aerobic respiration takes place primarily in the mitochondria. Cells with high energy demands, such as muscle cells and liver cells, contain many mitochondria.
The process involves multiple enzyme-controlled reactions that gradually release energy. Approximately 32 ATP molecules are produced from one glucose molecule during aerobic respiration.
Key features of aerobic respiration:
- Requires oxygen as the final electron acceptor
- Produces carbon dioxide and water as waste products
- Releases a large amount of energy (approximately 2880 kJ per mole of glucose)
- Occurs continuously in all living cells
- More efficient than anaerobic respiration
Anaerobic respiration
Anaerobic respiration occurs when oxygen is not available or in insufficient supply. It releases much less energy than aerobic respiration because glucose is only partially broken down.
In animals and some bacteria:
glucose → lactic acid (+ energy)
C₆H₁₂O₆ → 2C₃H₆O₃
In yeast and plants:
glucose → ethanol + carbon dioxide (+ energy)
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Anaerobic respiration in animals occurs during:
- Vigorous exercise — when muscles cannot receive oxygen quickly enough from the blood
- High altitude activities — where oxygen availability is reduced
- Any situation where oxygen demand exceeds supply
Only approximately 2 ATP molecules are produced per glucose molecule during anaerobic respiration, making it much less efficient than aerobic respiration.
Anaerobic respiration in yeast (fermentation)
Anaerobic respiration in yeast is called fermentation and has important economic applications:
Bread making:
- Yeast is mixed with flour and water
- Sugar in the dough is broken down anaerobically
- Carbon dioxide gas makes the bread rise
- Ethanol evaporates during baking
Brewing and wine making:
- Yeast ferments sugars in grape juice or malted barley
- Ethanol accumulates to produce alcoholic drinks
- Carbon dioxide is released (gives fizz to champagne and beer)
- Fermentation continues until ethanol concentration becomes toxic to yeast (typically 12-15%)
Oxygen debt and recovery
During vigorous exercise, muscles respire anaerobically when oxygen supply cannot meet demand. Lactic acid accumulates in muscle tissue, causing:
- Muscle fatigue and pain
- Reduced muscle efficiency
- Cramps (in extreme cases)
After exercise stops, the body must repay the oxygen debt:
- Breathing rate and depth remain elevated
- Heart rate stays high to deliver oxygen to muscles
- Lactic acid is transported to the liver via the blood
- Oxygen is used to break down lactic acid into carbon dioxide and water
- Some lactic acid is converted back to glucose
The oxygen debt is the amount of oxygen needed to remove lactic acid from the body. This explains why you continue breathing heavily after exercise stops.
Comparing respiration and breathing
Students often confuse respiration with breathing. These are different processes:
Breathing (ventilation):
- Physical process of moving air in and out of lungs
- Involves muscular movements of diaphragm and intercostal muscles
- Exchanges oxygen and carbon dioxide between air and blood
- Occurs only in organisms with lungs
Respiration:
- Chemical process releasing energy from glucose
- Occurs in all living cells
- Involves enzyme-controlled reactions
- Occurs in all living organisms (animals, plants, microorganisms)
Investigating respiration practically
Demonstrating carbon dioxide production:
Using hydrogen-carbonate indicator or limewater:
- Hydrogen-carbonate indicator turns from orange/red to yellow in the presence of carbon dioxide
- Limewater turns milky/cloudy when carbon dioxide is bubbled through it
- Can be used to show respiration in germinating seeds, small animals, or yeast
Investigating respiration in yeast:
Variables to control and measure:
- Temperature — affects enzyme activity and rate of respiration
- Sugar concentration — substrate availability affects respiration rate
- pH — affects enzyme function
- Oxygen availability — determines aerobic vs anaerobic respiration
Measuring respiration rate:
- Count bubbles of carbon dioxide produced per minute
- Measure volume of gas collected in a given time
- Monitor temperature change (respiration releases heat energy)
- Measure decrease in oxygen concentration using oxygen sensors
Investigating heat production:
Germinating seeds respire rapidly and release heat:
- Use vacuum flasks to insulate germinating seeds
- Compare temperature change with dead seeds (control)
- Use small seeds like peas or beans
- Ensure equal masses for fair comparison
Worked examples
Example 1: Interpreting experimental data
Question: Students investigated the effect of temperature on yeast respiration. They measured the volume of carbon dioxide produced in 5 minutes at different temperatures. Their results are shown:
| Temperature (°C) | Volume of CO₂ (cm³) |
|---|---|
| 20 | 15 |
| 30 | 32 |
| 40 | 48 |
| 50 | 22 |
| 60 | 0 |
(a) Explain the results at 40°C. [2 marks]
(b) Explain why no carbon dioxide was produced at 60°C. [2 marks]
Model answer:
(a) At 40°C, the rate of respiration is highest [1 mark] because enzymes controlling respiration work at or near their optimum temperature [1 mark].
(b) At 60°C, the enzymes that control respiration have been denatured [1 mark], so they can no longer catalyse the reactions of respiration / the active site has changed shape [1 mark].
Example 2: Comparing respiration types
Question: A student runs 400 metres as fast as possible.
(a) Name the type of respiration occurring in the leg muscles during the race. [1 mark]
(b) Write the word equation for this type of respiration. [2 marks]
(c) Explain why the student continues to breathe heavily for several minutes after finishing the race. [3 marks]
Model answer:
(a) Anaerobic respiration [1 mark]
(b) glucose → lactic acid (+ energy) [2 marks] (1 mark for glucose, 1 mark for lactic acid)
(c) Lactic acid has built up in the muscles [1 mark]. Extra oxygen is needed to break down the lactic acid [1 mark] in the liver / to convert it to carbon dioxide and water / to repay the oxygen debt [1 mark].
Example 3: Application to bread making
Question: Explain the role of yeast in bread making, including:
- The type of respiration occurring
- Why the bread rises
- Why bread does not contain alcohol [5 marks]
Model answer:
Yeast respires anaerobically / ferments [1 mark] breaking down sugar/glucose in the dough [1 mark]. This produces carbon dioxide gas [1 mark] which forms bubbles that make the bread rise [1 mark]. The ethanol/alcohol produced evaporates when the bread is baked at high temperature [1 mark].
Common mistakes and how to avoid them
Confusing respiration with breathing — Remember respiration is a chemical process in cells; breathing is physical gas exchange in lungs. Always use "respiration" when discussing energy release from glucose.
Writing incomplete word equations — Don't forget to include all products. Aerobic respiration produces BOTH carbon dioxide AND water. Anaerobic respiration in yeast produces BOTH ethanol AND carbon dioxide.
Stating that anaerobic respiration only occurs during exercise — Anaerobic respiration occurs whenever oxygen is unavailable, not just during exercise. Yeast respires anaerobically in bread dough at rest.
Claiming lactic acid is broken down in muscles — Lactic acid is transported to the liver where it is broken down or converted back to glucose. This is a common mistake worth avoiding.
Forgetting that respiration occurs in plants — Plants respire 24 hours a day in all living cells. They photosynthesize only in light, in cells containing chloroplasts.
Saying oxygen debt is the oxygen needed during exercise — Oxygen debt is the EXTRA oxygen needed AFTER exercise to break down lactic acid that accumulated during exercise.
Exam technique for "Respiration"
Command word "explain" requires you to give reasons or mechanisms. For 2 marks, give two separate points. Example: "Explain why respiration rate increases during exercise" needs both increased energy demand AND how this is met.
Word equations must be complete and accurate — Check you've included all reactants and products. Use arrows (→) not equals signs. Write "+ energy" in brackets at the end if asked.
Use precise terminology — "Glucose is broken down" is better than "food is used up". "Lactic acid accumulates" is better than "lactic acid is made". Avoid vague phrases like "gives energy" — use "releases energy" instead.
Link structure to function in mitochondria questions — If asked why muscle cells have many mitochondria, link this to high energy demand for contraction and that mitochondria are the site of aerobic respiration.
Quick revision summary
Respiration releases energy from glucose in all living cells. Aerobic respiration requires oxygen and produces carbon dioxide and water, releasing maximum energy in mitochondria. Anaerobic respiration occurs without oxygen: in animals producing lactic acid, in yeast producing ethanol and carbon dioxide. During vigorous exercise, oxygen debt builds up as lactic acid accumulates and must be broken down afterwards. Respiration differs from breathing, which is physical ventilation. Temperature, substrate concentration and oxygen availability affect respiration rate, which can be investigated by measuring carbon dioxide production or heat release.