Cambridge IGCSE Biology 0610. Topic 12: Respiration

Respiration (Aerobic & Anaerobic)

By the IGCSE Biology Team · reviewed for accuracy by our Biology specialists · last reviewed June 2026 · about us

In short

Respiration is the chemical reactions in cells that release energy from glucose. It is not breathing. Aerobic respiration (glucose + oxygen → carbon dioxide + water) releases much more energy than anaerobic respiration. In muscles, anaerobic respiration produces lactic acid; in yeast it produces alcohol and carbon dioxide.

The exam angle

The instant mark-loser in respiration is writing that it 'produces' or 'makes' energy. Respiration RELEASES energy from glucose, and examiners cross out the wrong verb. Students also confuse respiration with breathing, mix up the anaerobic products in humans versus yeast, and forget oxygen debt. Lock down the two equations, the precise comparison of aerobic and anaerobic, and the correct verb, and this topic becomes reliable marks.

What respiration is: and the verb that matters

Define respiration as the chemical reactions in cells that break down glucose (nutrient molecules) to release energy. The key word is release: energy is released from glucose, never 'produced', 'made' or 'created'. This is the most common single mark lost in the whole topic.

Distinguish respiration (a chemical reaction in cells releasing energy) from breathing/ventilation (moving air in and out of the lungs) and from gas exchange (diffusion of gases). Examiners regularly test whether you confuse them. The released energy is used for muscle contraction, growth, active transport, maintaining body temperature in mammals, and making large molecules from smaller ones. Respiration happens in all living cells, all the time, mainly in the mitochondria for the aerobic stage. Linking back to cell structure.

Aerobic respiration: the equation in full

Aerobic respiration is the release of a relatively large amount of energy by the breakdown of glucose using oxygen. Learn the word equation: glucose + oxygen → carbon dioxide + water (+ energy released).

(Extended only) The balanced symbol equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Note 'energy' is written as 'energy released' beside the equation, not as a product molecule. Examiners may accept it written after the arrow but never want it shown as a chemical formula. Aerobic respiration releases far more energy per glucose molecule than anaerobic because the glucose is fully broken down. Most respiration in human cells is aerobic; it occurs in the mitochondria. The carbon dioxide produced is carried in the plasma and removed at the lungs, connecting to transport in humans.

Anaerobic respiration: humans vs yeast

Anaerobic respiration releases energy from glucose without oxygen, and releases much less energy because the glucose is not fully broken down. The products differ by organism. A guaranteed exam distinction:

In humans (muscles during hard exercise): glucose → lactic acid (+ a little energy).
In yeast and plants: glucose → alcohol (ethanol) + carbon dioxide (+ a little energy). This is fermentation.

(Extended only) The balanced equation for yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The most common error is giving the yeast products (alcohol + CO₂) for human muscle, or vice versa. State clearly that anaerobic respiration releases less energy than aerobic because glucose is only partially broken down. Yeast fermentation links to bread-making (CO₂ makes dough rise) and brewing (alcohol). Frequent application contexts.

Oxygen debt and recovery (Extended)

(Extended only) During vigorous exercise, muscles cannot get enough oxygen, so they respire anaerobically, producing lactic acid. The lactic acid builds up and must be removed afterwards. This creates an oxygen debt: the extra oxygen needed after exercise to break down the accumulated lactic acid.

The mark-scheme reasoning: after exercise you continue to breathe deeply and rapidly to take in extra oxygen; this oxygen is used to oxidise/break down the lactic acid (transported in the blood to the liver) into carbon dioxide and water. This is why your heart rate and breathing stay raised after stopping. A common slip is saying the body 'removes lactic acid' without explaining that oxygen is needed to break it down, or forgetting that lactic acid is transported to the liver. Build-up of lactic acid also causes muscle fatigue and cramp.

Investigating respiration (practical marks)

Two classic practicals appear. First, showing germinating seeds or small organisms respire: they are kept in a sealed flask with an indicator. Hydrogencarbonate indicator turns from red towards yellow as carbon dioxide is produced (the CO₂ makes the solution more acidic). Limewater turns milky/cloudy with carbon dioxide. Examiners want you to include a control (e.g. boiled/dead seeds or glass beads) to show the change is due to living, respiring organisms.

Second, measuring rate using a respirometer. Soda lime/potassium hydroxide absorbs the CO₂ so any volume change reflects oxygen used. State control variables (temperature, mass of organisms, time). The temperature dependence links to enzymes, since respiration is enzyme-controlled. Forgetting the dead-organism control is a frequent loss. To practise these stepwise answers, book a free trial.

Worked exam question (5 marks)

During a 100-metre sprint, an athlete's leg muscles begin to respire anaerobically. (a) Write the word equation for anaerobic respiration in human muscles. (b) Explain why the athlete continues to breathe deeply for several minutes after the race has finished.

Show the mark-scheme answer

(a) glucose → lactic acid (allow + energy released); (b) anaerobic respiration produced lactic acid (which built up in the muscles); the athlete has an oxygen debt; extra oxygen is needed (taken in by deep breathing); to break down / oxidise the lactic acid (into carbon dioxide and water);

Each point separated by a semicolon is worth one mark. This is how real Cambridge mark schemes are written.

Common mistakes that cost marks

Writing that respiration 'produces', 'makes' or 'creates' energy instead of releasing it.
Confusing respiration with breathing/ventilation. Respiration is a chemical reaction in cells.
Giving alcohol + CO₂ as the products of anaerobic respiration in human muscle (that is yeast).
Forgetting that anaerobic respiration releases LESS energy than aerobic.
Explaining oxygen debt without mentioning that oxygen breaks down/oxidises the lactic acid.
Omitting a dead/boiled-organism control in respiration practical answers.

How to phrase the 6-mark answer on this topic

For a 6-mark 'compare aerobic and anaerobic respiration' answer, use matched pairs and consistent verbs. Use stems like 'Aerobic respiration uses oxygen and fully breaks down glucose, releasing a large amount of energy, with carbon dioxide and water as products...' versus 'Anaerobic respiration occurs without oxygen, only partially breaks down glucose, releasing much less energy, producing lactic acid in muscles or ethanol and carbon dioxide in yeast.' Always write 'releases energy', name the correct products per organism, and contrast the energy yields explicitly. Those comparisons carry the marks.

Learn the full 6-mark technique →

For students in Malaysia

Respiration appears in both May/June and Oct/Nov 0610 sittings, often paired with gas exchange or exercise data. The 'releases energy' wording is the single most common error among Malaysian students, partly because SPM Biology phrasing can be looser, so dual-syllabus candidates must be strict in the 0610 exam. Yeast fermentation is a relatable context given local bread and tapai (fermented) foods.

Frequently asked questions

What is the difference between respiration and breathing?

Respiration is the chemical reactions in cells that release energy from glucose. Breathing (ventilation) is the muscular movement of air in and out of the lungs. Using 'breathing' for 'respiration' is one of the most common dropped marks.

What is the word equation for aerobic respiration?

Glucose + oxygen → carbon dioxide + water. Energy is 'released', never 'produced' or 'made'. Energy cannot be created, and the wrong verb loses the mark in almost every respiration mark scheme.

What does anaerobic respiration produce?

In human muscles, glucose → lactic acid. In yeast, glucose → alcohol + carbon dioxide. Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration because the glucose is not fully broken down.

Exam guides for this topic

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Respiration (Aerobic & Anaerobic)

What respiration is: and the verb that matters

Aerobic respiration: the equation in full

Anaerobic respiration: humans vs yeast

Oxygen debt and recovery (Extended)

Investigating respiration (practical marks)

Worked exam question (5 marks)

Common mistakes that cost marks

How to phrase the 6-mark answer on this topic

For students in Malaysia

Frequently asked questions

Frequently asked questions

Related topics

Exam guides for this topic