Anaerobic respiration

Respiration · Respiration · note 3 of 4

Spec 12.3.1, 12.3.2, 12.3.3, 12.3.4, 12.3.5Download all Respiration notes (PDF)Download all notes (PDF)5 pages

Anaerobic respirationSpec 12.3.1, 12.3.2, 12.3.3, 12.3.4, 12.3.5

In short

Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. It releases much less energy per glucose molecule than aerobic respiration, because glucose is only partly broken down. Yeast makes alcohol and carbon dioxide, while muscles during vigorous exercise make only lactic acid.

Anaerobic respiration: the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen

Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration. This is because the glucose is only partly broken down.

In yeast

Yeast respires anaerobically when there is no oxygen. This is called fermentation. It is used to make bread (the carbon dioxide makes the dough rise) and alcoholic drinks.

glucose → alcohol + carbon dioxide

Supplement

Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

In muscles during vigorous exercise

When muscles cannot get oxygen fast enough during vigorous exercise, they also respire anaerobically. The product is lactic acid. No carbon dioxide is produced.

glucose → lactic acid

Aerobic and anaerobic respiration compared
Feature	Aerobic	Anaerobic
Oxygen needed	yes	no
Energy released per glucose	much more	much less
Products in yeast	carbon dioxide and water	alcohol and carbon dioxide
Products in muscles	carbon dioxide and water	lactic acid

Common mistake:

Yeast makes alcohol and carbon dioxide; muscles make only lactic acid. Do not mix up the two equations.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

How is respiration different from breathing?

Respiration is a chemical reaction inside cells, while breathing is the movement of air into and out of the lungs. In aerobic respiration, cells use oxygen to break down glucose and release energy, producing carbon dioxide and water. Breathing simply supplies the oxygen and removes the carbon dioxide, so the two are linked but different.

What is anaerobic respiration in yeast?

Anaerobic respiration in yeast is the breakdown of glucose without oxygen to release energy, producing alcohol and carbon dioxide. It is also called fermentation. The word equation is glucose → alcohol + carbon dioxide. Bakers use the carbon dioxide to make bread dough rise, and the alcohol is used to make alcoholic drinks.

Why does anaerobic respiration release less energy than aerobic respiration?

Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration because the glucose is only partly broken down. Without oxygen, cells cannot break glucose down all the way to carbon dioxide and water. Instead yeast makes alcohol and carbon dioxide, and muscles make lactic acid, so less energy is released.

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