What is the difference between aerobic and anaerobic respiration in the context of the Cambridge IGCSE Coordinated Science curriculum?

Aerobic respiration is the process of producing cellular energy in the presence of oxygen. It involves the complete breakdown of glucose into carbon dioxide and water, releasing a significant amount of energy. Anaerobic respiration, on the other hand, occurs without oxygen and results in the partial breakdown of glucose, producing lactic acid in animals or ethanol and carbon dioxide in plants and yeast, with less energy released. Understanding these processes is crucial for the Cambridge IGCSE Coordinated Science curriculum.

How does the process of respiration differ from breathing in the context of Gas Exchange and Respiration?

Respiration is a biochemical process that occurs in cells, where glucose is broken down to release energy. Breathing, however, is a physical process involving the inhalation and exhalation of air to facilitate gas exchange in the lungs. While respiration provides energy for cellular activities, breathing ensures that oxygen is supplied to the bloodstream and carbon dioxide is expelled, supporting the overall process of gas exchange and respiration.

Why is respiration considered an exothermic reaction in the Cambridge IGCSE curriculum?

Respiration is considered an exothermic reaction because it releases energy. During the breakdown of glucose in cells, energy is released in the form of ATP (adenosine triphosphate), which is used for various cellular processes. This release of energy is a key concept in the Cambridge IGCSE Coordinated Science curriculum, highlighting the importance of respiration in maintaining life processes.

What role do mitochondria play in cellular respiration according to the Cambridge IGCSE Coordinated Science syllabus?

Mitochondria are known as the powerhouses of the cell because they are the primary site of aerobic respiration. Within the mitochondria, glucose is broken down in the presence of oxygen to produce ATP, carbon dioxide, and water. This process is essential for energy production in cells, and understanding the role of mitochondria is a fundamental aspect of the Cambridge IGCSE Coordinated Science syllabus.

How does the body compensate for oxygen debt during vigorous exercise as explained in the Cambridge IGCSE Coordinated Science course?

During vigorous exercise, the body may experience oxygen debt due to the high demand for energy, leading to anaerobic respiration and the accumulation of lactic acid. To compensate, the body increases breathing and heart rates to supply more oxygen to the muscles and remove carbon dioxide. Post-exercise, the body continues to consume oxygen at an elevated rate to metabolize the lactic acid and restore normal conditions, a concept covered in the Cambridge IGCSE Coordinated Science course.

Why is "Confusing respiration with breathing" a common mistake in Respiration ?

Why it happens: The term 'respiration' is used in everyday language to mean breathing. How to avoid it: **Respiration** is the intracellular chemical process of releasing energy from glucose. **Breathing** (ventilation) is the mechanical movement of air in and out of the lungs. All living cells respire; only organisms with lungs breathe. Source: 0654 Examiner Report 2022

Why is "Stating that anaerobic respiration produces no energy" a common mistake in Respiration ?

Why it happens: Students know anaerobic produces 'less' energy and over-generalise to 'no' energy. How to avoid it: Anaerobic respiration does release energy (ATP), just far **less** than aerobic. E.g. aerobic: ~38 ATP per glucose; anaerobic: ~2 ATP per glucose.

Why is "Saying yeast produces lactic acid during anaerobic respiration" a common mistake in Respiration ?

Why it happens: Students confuse the two types of anaerobic respiration. How to avoid it: **Animals** produce lactic acid. **Yeast** produces ethanol and CO_2. These are completely different products.

Why is "Stating that mitochondria are the site of photosynthesis" a common mistake in Respiration ?

Why it happens: Students confuse mitochondria (respiration) with chloroplasts (photosynthesis). How to avoid it: Mitochondria = site of **aerobic respiration**. Chloroplasts = site of **photosynthesis**. Remember: both release/capture energy but are different organelles.

Gas Exchange and RespirationCambridge IGCSE Coordinated Sciences 0654

Respiration

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Respiration — Cambridge IGCSE Coordinated Science 0654

Respiration is the chemical process in cells that releases energy from glucose. It is NOT breathing. Cambridge tests both aerobic and anaerobic equations, uses of energy, and the practical evidence for aerobic respiration.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

B12.1 — Define respiration and state the equation for aerobic respiration.
B12.2 — Describe anaerobic respiration in animals and in yeast.
B12.3 — Describe the uses of energy released by respiration.
B12.4 — Explain the concept of oxygen debt.
B12.5 — Investigate the rate of respiration experimentally.

Aerobic Respiration

Aerobic respiration uses oxygen to completely oxidise glucose, releasing a large amount of energy as ATP. Occurs in the mitochondria.

Aerobic respiration takes place in the mitochondria and requires oxygen.

Word equation:

glucose + oxygen → carbon dioxide + water (+ energy released as ATP)

Symbol equation:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ ~2870 kJ/mol energy, yielding ~38 ATP)

Why mitochondria? The inner membrane of mitochondria contains the enzymes for the final stages of aerobic respiration (the electron transport chain). Cells with high energy demands (muscle cells, liver cells) contain more mitochondria.

Evidence aerobic respiration occurs:

O₂ is consumed → tested with a glowing splint (rekindles) or oxygen sensor
CO₂ is produced → tested with limewater (turns milky) or hydrogen carbonate indicator (turns yellow)
Heat is released → temperature rises in a sealed container of respiring organisms
Dry mass decreases over time as organic molecules are oxidised

Aerobic: glucose + O₂ → CO₂ + H₂O + energy (ATP).
Occurs in mitochondria; requires oxygen.
CO₂ detected by limewater (milky) or hydrogen carbonate indicator (yellow).

Anaerobic Respiration

When oxygen is absent or insufficient, cells respire anaerobically, producing far less energy and different end products depending on the organism.

Anaerobic respiration occurs WITHOUT oxygen, in the cytoplasm.

In animals (including humans):

glucose → lactic acid (+ small amount of energy)

Lactic acid builds up in muscles → cramp, fatigue, acidic pH inhibits enzymes
After exercise, lactic acid is transported to the liver → oxidised or converted to glucose (requires extra O₂ → oxygen debt)

In yeast and plant cells:

glucose → ethanol + carbon dioxide (+ small amount of energy)

This is fermentation — used in bread-making (CO₂ makes dough rise) and brewing (ethanol produced)
Yeast die when ethanol concentration exceeds ~15%

Comparison of aerobic vs anaerobic:

Feature	Aerobic	Anaerobic
O₂ required?	Yes	No
ATP yield	High (~38 per glucose)	Low (~2 per glucose)
End products	CO₂ + H₂O	Lactic acid (animals) OR ethanol + CO₂ (yeast)
Location	Mitochondria	Cytoplasm
Duration	Sustained exercise	Short bursts of intense exercise

Animals: glucose → lactic acid (no O₂, low ATP, causes oxygen debt).
Yeast/plants: glucose → ethanol + CO₂ (fermentation).
Anaerobic gives ~2 ATP vs aerobic ~38 ATP per glucose.

Uses of Energy from Respiration

Energy released by respiration (as ATP) powers all metabolic processes in living cells.

Energy released by respiration is used for:

Use	Example
Muscle contraction	Movement, heartbeat, breathing
Active transport	Pumping ions/molecules against concentration gradients across membranes
Protein synthesis	Building enzymes, structural proteins, antibodies
Cell division	Mitosis and meiosis for growth and reproduction
Maintenance of body temperature	In endotherms (mammals and birds) — heat released by metabolism warms the body
Activation of reactions	Many metabolic reactions require an energy input (activation energy)

ATP (adenosine triphosphate) is the universal energy currency. It is made during respiration and broken down to release energy wherever it is needed in the cell.

Energy from respiration → muscle contraction, active transport, protein synthesis, cell division, temperature maintenance.
ATP is the immediate energy carrier used by cells.

Oxygen Debt

Oxygen debt is the extra oxygen needed after anaerobic exercise to oxidise accumulated lactic acid back to CO₂ and water (or glucose).

During intense exercise, muscles use anaerobic respiration → lactic acid accumulates.

After exercise:

Breathing rate and heart rate stay elevated
Extra O₂ is used to:
- Oxidise lactic acid → CO₂ + H₂O (in muscles)
- Convert lactic acid → glucose (in liver, then stored as glycogen)
The total extra O₂ needed = oxygen debt

Why you keep panting after exercise: The respiratory centre in the brain detects elevated CO₂ and low O₂ in the blood and maintains high ventilation rate until oxygen debt is repaid.

Signs of oxygen debt: Continued rapid breathing and elevated heart rate after exercise stops, flushed skin (more blood to skin to lose heat from elevated metabolism).

Oxygen debt = extra O₂ needed to oxidise lactic acid built up during anaerobic respiration.
Breathing rate stays elevated after exercise until O₂ debt is repaid.
Liver converts lactic acid to glucose (requires O₂).

How it’s examined

Paper 4: 'Write the equation for aerobic respiration' (1 mark — both word and symbol equations should be known). 'State TWO differences between aerobic and anaerobic respiration in animals' (2 marks). 'Explain why a runner's breathing rate remains elevated after a race' (3 marks — lactic acid, oxygen debt, oxidation). Paper 2 MCQ commonly tests: which product of anaerobic respiration in yeast? (ethanol and CO₂); what is the site of aerobic respiration? (mitochondria). Practical questions ask about experimental design to measure respiration rate using a respirometer.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B12); 0654/42 May/Jun 2024 — Q4 (respiration); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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Step-by-step worked examples — Respiration

Step-by-step solutions to past-paper-style questions on respiration , written exactly the way a tutor would explain them at the board.

1Write the word equation for aerobic respiration
Core• aerobic respiration, word equation
Question
Write the word equation for aerobic respiration.
Step-by-step solution
1. Step 1
  Aerobic respiration uses oxygen to oxidise glucose completely, releasing carbon dioxide, water, and energy (as ATP).
Answer
Glucose + oxygen → carbon dioxide + water (+ energy released as ATP)
2Explain why lactic acid builds up in muscles during intense exercise
Extended• anaerobic respiration, lactic acid, oxygen debt
Question
A student runs a 400 m race. During the last 100 m, lactic acid builds up in their muscles. Explain why.
Step-by-step solution
1. Step 1
  During intense exercise, muscles require energy at a rate that exceeds the rate of oxygen delivery to the muscles.
2. Step 2
  When insufficient oxygen is available, muscles switch to anaerobic respiration: glucose is broken down to lactic acid (without oxygen), releasing a small amount of ATP.
3. Step 3
  Lactic acid accumulates in muscle cells → causes muscle fatigue and pain. The lactic acid must be broken down (oxidised) after exercise.
Answer
During intense exercise, oxygen supply to muscles is insufficient for aerobic respiration. Muscles switch to anaerobic respiration: glucose → lactic acid + small amount of ATP. Lactic acid accumulates, causing muscle fatigue.
3Explain what is meant by oxygen debt
Extended• oxygen debt, lactic acid, anaerobic
Question
Explain what is meant by oxygen debt.
Step-by-step solution
1. Step 1
  During anaerobic exercise, lactic acid accumulates in muscles and blood.
2. Step 2
  After exercise, extra oxygen is needed to break down (oxidise) the accumulated lactic acid — transporting it to the liver where it is converted back to glucose using oxygen.
3. Step 3
  This extra oxygen that must be consumed after exercise to restore normal conditions is called the oxygen debt.
4. Step 4
  Oxygen debt explains why breathing and heart rate remain elevated for some time after exercise stops — the body is 'repaying' the oxygen debt.
Answer
Oxygen debt is the extra oxygen consumed after anaerobic exercise to oxidise accumulated lactic acid. Lactic acid is transported to the liver and converted to glucose. Continued elevated breathing after exercise repays the oxygen debt.
4Explain why carbon dioxide is produced during yeast fermentation
Extended• yeast, fermentation, anaerobic, CO2
Question
A student investigates fermentation in yeast. Explain why carbon dioxide is produced.
Step-by-step solution
1. Step 1
  In the absence of oxygen, yeast carries out anaerobic respiration (fermentation).
2. Step 2
  The equation is: glucose → ethanol + carbon dioxide (+ small amount of energy).
3. Step 3
  $\text{CO}_2$ is a by-product of the fermentation reaction. It is produced as glucose is broken down into ethanol and $\text{CO}_2$ .
Answer
Yeast undergoes anaerobic respiration (fermentation) in the absence of oxygen: glucose → ethanol + CO₂ + small amount of energy. CO₂ is a by-product of this reaction.
5Compare aerobic and anaerobic respiration in animals
Extended• aerobic, anaerobic, comparison
Question
Compare aerobic and anaerobic respiration in animals.
Step-by-step solution
1. Step 1
  Aerobic: requires oxygen; products are $\text{CO}_2$ and water; releases a large amount of ATP per glucose molecule; occurs in mitochondria; sustainable over long periods.
2. Step 2
  Anaerobic (in animals): no oxygen required; product is lactic acid; releases much less ATP per glucose; occurs in cytoplasm; causes fatigue; not sustainable — lactic acid must be removed.
Answer
Aerobic: requires O₂, produces CO₂ + H₂O, large ATP yield, occurs in mitochondria, sustainable. Anaerobic: no O₂, produces lactic acid, much less ATP, occurs in cytoplasm, causes fatigue and unsustainable over long periods.

Key Formulae — Respiration

The formulae you need to memorise for respiration on the Cambridge IGCSE 0654 paper, with every variable defined in plain English and a note on when to use it.

Aerobic respiration (symbol equation)
$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (ATP)}$
$\text{C}_6\text{H}_{12}\text{O}_6$
glucose (substrate)
$\text{O}_2$
oxygen (required)
$\text{CO}_2$
carbon dioxide (waste product)
$\text{H}_2\text{O}$
water (waste product)
When to use
Any question requiring the equation for aerobic respiration or asking you to identify reactants and products.
Anaerobic respiration in animals
$\text{glucose} \rightarrow \text{lactic acid} + \text{small amount of energy (ATP)}$
$\text{glucose}$
substrate (only partially oxidised)
$\text{lactic acid}$
product; accumulates in muscles
When to use
Questions about intense exercise, muscle fatigue, oxygen debt, or anaerobic respiration in animal cells.
Anaerobic respiration in yeast (fermentation)
$\text{glucose} \rightarrow \text{ethanol} + \text{CO}_2 + \text{small amount of energy (ATP)}$
$\text{ethanol}$
product; used in brewing and baking
$\text{CO}_2$
product; causes bread to rise / detected in respirometers
When to use
Questions about yeast fermentation, brewing, bread-making, or comparing yeast and animal anaerobic respiration.

Key Definitions and Keywords — Respiration

Definitions to memorise and the exact keywords mark schemes credit for respiration answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Aerobic respiration
Examiner keyword
The process by which glucose is completely oxidised using oxygen to release energy (ATP), producing carbon dioxide and water. Occurs in the mitochondria.
Anaerobic respiration
Examiner keyword
The partial breakdown of glucose without oxygen to release a small amount of energy (ATP). Products: lactic acid (in animals); ethanol and CO₂ (in yeast).
ATP (adenosine triphosphate)
Examiner keyword
The energy currency of cells; chemical energy released by respiration is stored in ATP and used to drive cell processes such as muscle contraction and active transport.
Oxygen debt
Examiner keyword
The extra oxygen required after anaerobic exercise to oxidise accumulated lactic acid. It causes continued elevated breathing and heart rate after exercise stops.
Fermentation
Examiner keyword
Anaerobic respiration in yeast and some bacteria: glucose → ethanol + CO₂ + energy. Used in brewing, wine-making, and bread-making.
Mitochondria
Examiner keyword
Organelles that are the site of aerobic respiration in cells. Cells with high energy demands (e.g. muscle cells, sperm) have many mitochondria.

Common Mistakes and Misconceptions — Respiration

The traps other students keep falling into on respiration questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Confusing respiration with breathing
0654 Examiner Report 2022
Why it happens
The term 'respiration' is used in everyday language to mean breathing.
How to avoid it
Respiration is the intracellular chemical process of releasing energy from glucose. Breathing (ventilation) is the mechanical movement of air in and out of the lungs. All living cells respire; only organisms with lungs breathe.
✕Stating that anaerobic respiration produces no energy
Why it happens
Students know anaerobic produces 'less' energy and over-generalise to 'no' energy.
How to avoid it
Anaerobic respiration does release energy (ATP), just far less than aerobic. E.g. aerobic: ~38 ATP per glucose; anaerobic: ~2 ATP per glucose.
✕Saying yeast produces lactic acid during anaerobic respiration
Why it happens
Students confuse the two types of anaerobic respiration.
How to avoid it
Animals produce lactic acid. Yeast produces ethanol and $\text{CO}_2$ . These are completely different products.
✕Stating that mitochondria are the site of photosynthesis
Why it happens
Students confuse mitochondria (respiration) with chloroplasts (photosynthesis).
How to avoid it
Mitochondria = site of aerobic respiration. Chloroplasts = site of photosynthesis. Remember: both release/capture energy but are different organelles.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Respiration — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Respiration

Short Study Notes in the form of Slides

Short Study Notes — Respiration

Detailed Notes

What you’ll learn

How it’s examined

1Write the word equation for aerobic respiration

2Explain why lactic acid builds up in muscles during intense exercise

3Explain what is meant by oxygen debt

4Explain why carbon dioxide is produced during yeast fermentation

5Compare aerobic and anaerobic respiration in animals

Aerobic respiration (symbol equation)

Anaerobic respiration in animals

Anaerobic respiration in yeast (fermentation)

Aerobic respiration

Anaerobic respiration

ATP (adenosine triphosphate)

Oxygen debt

Fermentation

Mitochondria

✕Confusing respiration with breathing

✕Stating that anaerobic respiration produces no energy

✕Saying yeast produces lactic acid during anaerobic respiration

✕Stating that mitochondria are the site of photosynthesis

Practice questions

Past paper style quiz

Assess your understanding

Respiration — frequently asked questions