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Learn0610Papers 2, 4 · ExtendedRespirationAerobic and anaerobic respiration
Respiration · Subtopic 1 of 1

Aerobic and Anaerobic Respiration

Respiration releases energy from glucose in every living cell. Aerobic respiration uses oxygen, happens in mitochondria and releases lots of energy: C6H12O6+6O2→6CO2+6H2O\mathrm{C_6H_{12}O_6+6O_2\rightarrow6CO_2+6H_2O}. Anaerobic respiration happens without oxygen and releases much less energy. In muscles: glucose → lactic acid; this builds up during vigorous exercise, causing fatigue and an oxygen debt repaid afterwards by fast breathing. In yeast: glucose → ethanol + carbon dioxide (used in brewing and baking). Energy is used for muscle contraction, protein synthesis, cell division, active transport, growth, nerve impulses and keeping warm.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Write equations for aerobic and anaerobic respiration.
  • Compare aerobic and anaerobic respiration.
  • Explain oxygen debt.
  • List uses of energy in organisms.
Pause and think

Why do sprinters keep breathing hard for minutes after a 100 m race lasting 10 seconds?

Section 2 of 14

Core idea

Start hereBurning without flames

Glucose burned in a flame releases its energy in a flash. Your cells release the same energy from glucose step by step, with enzymes, at 37 °C.

The big idea

Respiration releases energy from glucose in every cell. With oxygen it's efficient; without oxygen it releases far less, making lactic acid in muscles or ethanol and CO₂ in yeast.

1. Two types

Aerobic vs anaerobic

Same fuel, different products.

Worked exampleOxygen debt

After a hard sprint, an athlete breathes fast for 10 minutes.

Goal
Explain.
Strategy
Link anaerobic respiration to lactic acid.
  1. O2 supply too slow\text{O}_2\ \text{supply too slow}
    Muscles respire anaerobically.
  2. lactic acid builds up\text{lactic acid builds up}
    Causes fatigue.
  3. extra O2 afterwards\text{extra O}_2\ \text{afterwards}
    Breaks down lactic acid: repaying the oxygen debt.
AnswerFast breathing supplies oxygen to remove lactic acid.
✓ CheckHeart rate also stays high to carry lactic acid to the liver ✓.
Common misconceptionRespiration vs breathing Respiration is the chemical reaction in cells. Breathing (ventilation) just moves air in and out of the lungs.
Pause and think

Why do yeast stop making bread rise if the dough is put in a hot oven?

Summary

  • Aerobic: CO₂ + water.
  • Anaerobic: lactic acid (muscle), ethanol + CO₂ (yeast).
  • Oxygen debt.
Key words
Aerobic respiration
Respiration using oxygen.
Anaerobic respiration
Respiration without oxygen.
Oxygen debt
Extra oxygen needed after exercise to break down lactic acid.
HistoryLavoisier and the guinea pig. Around 1780, Lavoisier put a guinea pig in a box surrounded by ice and measured how much melted, showing that respiration is a slow form of combustion that releases heat.
Fun factBread bubbles. The holes in bread are bubbles of carbon dioxide made by yeast respiring anaerobically; the ethanol evaporates during baking.
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