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Detailed notes on Gas Exchange and Respiration for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
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.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
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:
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)
In yeast and plant cells:
glucose → ethanol + carbon dioxide (+ small amount of energy)
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 |
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.
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:
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).
Verbatim phrases and definitions Cambridge mark schemes credit.
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.
Step-by-step solutions to past-paper-style questions on respiration , written exactly the way a tutor would explain them at the board.
Question
Write the word equation for aerobic respiration.
Step-by-step solution
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)
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
Step 1
During intense exercise, muscles require energy at a rate that exceeds the rate of oxygen delivery to the muscles.
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.
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.
Question
Explain what is meant by oxygen debt.
Step-by-step solution
Step 1
During anaerobic exercise, lactic acid accumulates in muscles and blood.
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.
Step 3
This extra oxygen that must be consumed after exercise to restore normal conditions is called the oxygen debt.
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.
Question
A student investigates fermentation in yeast. Explain why carbon dioxide is produced.
Step-by-step solution
Step 1
In the absence of oxygen, yeast carries out anaerobic respiration (fermentation).
Step 2
The equation is: glucose → ethanol + carbon dioxide (+ small amount of energy).
Step 3
CO2 is a by-product of the fermentation reaction. It is produced as glucose is broken down into ethanol and CO2.
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.
Question
Compare aerobic and anaerobic respiration in animals.
Step-by-step solution
Step 1
Aerobic: requires oxygen; products are CO2 and water; releases a large amount of ATP per glucose molecule; occurs in mitochondria; sustainable over long periods.
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.
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.
C6H12O6+6O2→6CO2+6H2O+energy (ATP)
When to use
Any question requiring the equation for aerobic respiration or asking you to identify reactants and products.
glucose→lactic acid+small amount of energy (ATP)
When to use
Questions about intense exercise, muscle fatigue, oxygen debt, or anaerobic respiration in animal cells.
glucose→ethanol+CO2+small amount of energy (ATP)
When to use
Questions about yeast fermentation, brewing, bread-making, or comparing yeast and animal anaerobic respiration.
Definitions to memorise and the exact keywords mark schemes credit for respiration answers — sharpened from recent examiner reports for the 2026 0654 sitting.
The process by which glucose is completely oxidised using oxygen to release energy (ATP), producing carbon dioxide and water. Occurs in the mitochondria.
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).
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.
The extra oxygen required after anaerobic exercise to oxidise accumulated lactic acid. It causes continued elevated breathing and heart rate after exercise stops.
Anaerobic respiration in yeast and some bacteria: glucose → ethanol + CO₂ + energy. Used in brewing, wine-making, and bread-making.
Organelles that are the site of aerobic respiration in cells. Cells with high energy demands (e.g. muscle cells, sperm) have many mitochondria.
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.
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.
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 it happens
Students confuse the two types of anaerobic respiration.
How to avoid it
Animals produce lactic acid. Yeast produces ethanol and CO2. These are completely different products.
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.
The things students keep getting wrong in this sub-topic, answered.