Question 1
Paper 4 short-answer style1 markName the product of anaerobic respiration in human muscle. (1 mark)
Model answer
Lactic acid.
Why this scores
One mark for 'lactic acid'.
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Detailed notes on Respiration for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Without oxygen — glucose only partly broken down. Animals make lactic acid; yeast makes ethanol + CO₂. Useful for short bursts, brewing, breadmaking.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Glucose → lactic acid. Used by muscles during heavy exercise.
The reaction.
glucose → lactic acid (+ small amount of energy)
Symbol equation:
C6H12O6→2C3H6O3
Note: NO oxygen used. NO CO₂ produced. Just lactic acid.
When does this happen?
Why limited?
Recovery: oxygen debt.
Worked qualitative. Why do sprinters look exhausted at the END of a 100 m race, not during it?
Cambridge tip. Always link OXYGEN DEBT to LACTIC ACID. Don't say 'breathing makes up for not breathing'.
Glucose → ethanol + CO₂. Used in brewing and breadmaking.
The reaction.
glucose → ethanol + carbon dioxide (+ small amount of energy)
Symbol equation:
C6H12O6→2C2H5OH+2CO2
(C₂H₅OH = ethanol)
Yeast. Single-celled fungus that can do BOTH aerobic and anaerobic respiration. When O₂ is present, it does aerobic. When O₂ is absent, it switches to fermentation.
Industrial uses:
1. Brewing (alcohol production).
2. Bread making.
3. Biofuel.
Worked qualitative. Why does yeast switch to fermentation when sealed in a brewing tank?
Cambridge tip. Memorise the YEAST equation separately from the ANIMAL one. Cambridge often asks for both.
Aerobic = complete + O₂ + lots of ATP. Anaerobic = partial + no O₂ + little ATP + by-products.
| Feature | Aerobic | Anaerobic (animal) | Anaerobic (yeast) |
|---|---|---|---|
| Oxygen | Required | Not used | Not used |
| Glucose breakdown | Complete | Partial | Partial |
| Products | CO₂ + H₂O | Lactic acid | Ethanol + CO₂ |
| ATP per glucose | ~30 | ~2 | ~2 |
| Site | Mitochondria | Cytoplasm | Cytoplasm |
| Speed | Slower | Fast | Fast |
| When? | Normal | Heavy exercise | Brewing, breadmaking |
Worked qualitative. Why can yeast survive in a sealed jar of fruit juice but a fish can't survive in stagnant water?
Cambridge tip. Cambridge often gives a table for students to fill in. Memorise the row-by-row contrast.
Verbatim phrases and definitions Cambridge mark schemes credit.
Anaerobic appears every Paper 4 (5-7 marks: equations, oxygen debt, fermentation uses). Examiner reports flag students confusing animal and yeast equations, and saying anaerobic releases 'no energy'.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (12.3); 0610/42 May/Jun 2024 — Q12 (anaerobic respiration); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on anaerobic respiration, written exactly the way a tutor would explain them at the board.
Question
Write the word equations for anaerobic respiration in (a) human muscles and (b) yeast.
Step-by-step solution
Step 1
(a) Human muscles: glucose → lactic acid (+ a small amount of energy).
Step 2
(b) Yeast (fermentation): glucose → ethanol + carbon dioxide (+ a small amount of energy).
Step 3
Both release much less energy than aerobic respiration, because the glucose is only partly broken down.
Answer
Muscles: glucose → lactic acid. Yeast: glucose → ethanol + carbon dioxide.
Examiner tip
Cambridge wants BOTH equations — they have different products. No carbon dioxide is produced in animal anaerobic respiration.
Question
Name the product of anaerobic respiration in animal muscle, and the two products in yeast.
Step-by-step solution
Step 1
In animal muscle, the product is lactic acid.
Step 2
In yeast, the products are ethanol and carbon dioxide.
Answer
Muscle: lactic acid. Yeast: ethanol + carbon dioxide.
Examiner tip
A frequent error is adding carbon dioxide to the animal pathway — there is none; the carbons stay in the lactic acid.
Question
Explain how yeast fermentation is used in (a) brewing and (b) bread making.
Step-by-step solution
Step 1
(a) Brewing: yeast ferments the sugars in grain or fruit, producing ethanol (the alcohol) and carbon dioxide.
Step 2
The ethanol is the useful product; the carbon dioxide escapes as gas (or gives fizz).
Step 3
(b) Bread making: yeast added to the dough ferments the sugars, producing carbon dioxide.
Step 4
The carbon dioxide forms bubbles that make the dough rise; the ethanol evaporates during baking.
Answer
Brewing uses the ethanol produced by fermentation; bread making uses the carbon dioxide, which makes the dough rise (the ethanol bakes off).
Question
Explain why a sprinter's muscles respire anaerobically during a race, even though this releases less energy.
Step-by-step solution
Step 1
During vigorous exercise the muscles need energy very quickly.
Step 2
Breathing and the blood cannot deliver oxygen fast enough for the muscles to release all the energy aerobically.
Step 3
So the muscles also respire anaerobically, which does not need oxygen and releases energy quickly, producing lactic acid.
Step 4
This provides extra energy for a short time, although it releases less energy per glucose and causes lactic acid to build up.
Answer
In hard exercise, oxygen cannot be supplied fast enough, so muscles respire anaerobically (without oxygen) to release extra energy quickly, producing lactic acid.
Question
Explain why an athlete continues to breathe deeply and quickly for several minutes after a hard race has finished.
Step-by-step solution
Step 1
During the race the muscles respired anaerobically and produced lactic acid, which built up in the muscles and blood.
Step 2
After the race, this lactic acid must be broken down, and that requires extra oxygen — the oxygen debt.
Step 3
The athlete keeps breathing deeply and quickly to take in this extra oxygen.
Step 4
The oxygen is carried in the blood to the liver/muscles, where the lactic acid is broken down, and breathing returns to normal once the debt is repaid.
Answer
During exercise lactic acid built up from anaerobic respiration. After exercise, extra oxygen (the oxygen debt) is needed to break down this lactic acid, so the athlete keeps breathing deeply until the debt is repaid.
Examiner tip
The chain: anaerobic respiration → lactic acid → extra oxygen needed afterwards to remove it = oxygen debt.
Question
A graph shows the lactic acid concentration in an athlete's blood is low at rest, rises sharply during a 400 m sprint, and slowly falls back to normal over the 30 minutes after the sprint. Explain the shape of the graph.
Step-by-step solution
Step 1
At rest, the muscles respire aerobically, so almost no lactic acid is produced and the level is low.
Step 2
During the sprint, oxygen cannot be supplied fast enough, so the muscles respire anaerobically, producing lactic acid — so its concentration rises sharply.
Step 3
After the sprint, the athlete takes in extra oxygen (the oxygen debt) and uses it to break down the lactic acid.
Step 4
Because the lactic acid is removed gradually, its concentration falls slowly back to the resting level.
Answer
Low at rest (aerobic respiration); rises sharply during the sprint (anaerobic respiration makes lactic acid); falls slowly afterwards as extra oxygen (the oxygen debt) breaks the lactic acid down.
Examiner tip
Link each part of the graph to the type of respiration and to the build-up/removal of lactic acid.
High-scoring sample answers for anaerobic respiration on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Name the product of anaerobic respiration in human muscle. (1 mark)
Model answer
Lactic acid.
Why this scores
One mark for 'lactic acid'.
Write the word equation for anaerobic respiration in yeast. (2 marks)
Model answer
glucose → ethanol + carbon dioxide (+ energy).
Why this scores
One mark for glucose as the reactant, one for both products (ethanol AND carbon dioxide).
Explain why bread dough rises when yeast is added to it. (3 marks)
Model answer
The yeast respires anaerobically, fermenting the sugars in the dough. This produces carbon dioxide gas. The bubbles of carbon dioxide become trapped in the dough, making it expand and rise. (The ethanol also produced evaporates during baking.)
Why this scores
Three marks: yeast respires anaerobically/ferments sugar; produces carbon dioxide; the gas makes the dough rise.
Explain why the muscles respire anaerobically during vigorous exercise, and name the substance that is produced. (4 marks)
Model answer
During vigorous exercise the muscles need energy very quickly, but the lungs and blood cannot deliver oxygen fast enough for all the energy to be released by aerobic respiration. So the muscles also respire anaerobically, which does not need oxygen and releases extra energy quickly. The substance produced is lactic acid.
Why this scores
Four marks: muscles need energy fast; oxygen cannot be supplied quickly enough; muscles respire anaerobically (no oxygen) for extra energy; product = lactic acid.
Explain what is meant by the oxygen debt, and why an athlete breathes deeply after exercise. (5 marks)
Model answer
During hard exercise the muscles respired anaerobically and produced lactic acid, which built up in the muscles and blood. The oxygen debt is the extra oxygen that the body needs after the exercise to break down this lactic acid. After exercise the athlete therefore continues to breathe deeply and quickly to take in this extra oxygen, which is carried in the blood to break down the lactic acid. Once all the lactic acid has been removed, the oxygen debt has been repaid and breathing returns to normal.
Why this scores
Five marks: anaerobic respiration produced lactic acid; oxygen debt = extra oxygen needed; to break down the lactic acid; deep breathing supplies this oxygen; debt repaid when lactic acid removed.
Compare anaerobic respiration in human muscle with anaerobic respiration in yeast, and explain why both release less energy than aerobic respiration. (6 marks)
Model answer
In both cases, glucose is broken down without oxygen. In human muscle, the glucose is converted to lactic acid, and no carbon dioxide is produced. In yeast, the glucose is converted to ethanol and carbon dioxide — so the products are different. In both, only a small amount of energy is released. This is because the glucose is only partly broken down: in muscle, much of the energy remains locked in the lactic acid, and in yeast, much of it remains in the ethanol. In aerobic respiration, by contrast, the glucose is broken down completely into carbon dioxide and water, releasing far more of the energy it contains. So both forms of anaerobic respiration release much less energy per glucose than aerobic respiration, although they are useful when oxygen is in short supply.
Why this scores
Up to 6 marks: both without oxygen; muscle → lactic acid (no CO₂); yeast → ethanol + CO₂; both release little energy; because glucose only partly broken down / energy left in the products; aerobic breaks glucose down fully so releases more.
Definitions to memorise and the exact keywords mark schemes credit for anaerobic respiration answers — sharpened from recent examiner reports for the 2026 0610 sitting.
The release of a relatively small amount of energy from glucose without oxygen. The glucose is only partly broken down.
The product of anaerobic respiration in animal muscle (especially during vigorous exercise). It builds up and causes fatigue.
Anaerobic respiration in yeast: glucose → ethanol + carbon dioxide. Used in brewing, bread making and biofuel production.
The extra oxygen needed after exercise to break down the lactic acid that built up during anaerobic respiration.
The traps other students keep falling into on anaerobic respiration questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
It releases much less than aerobic.
How to avoid it
Anaerobic releases SOME energy — less than aerobic, but enough for short bursts of activity. Not zero.
Why it happens
Both are anaerobic.
How to avoid it
ANIMALS → lactic acid (no CO₂). YEAST → ethanol + CO₂. Learn them separately.
Why it happens
Aerobic respiration does, so students assume.
How to avoid it
Anaerobic respiration in muscle produces ONLY lactic acid — no carbon dioxide. The carbons stay in the lactic acid.
The things students keep getting wrong in this sub-topic, answered.