Aerobic and anaerobic respiration

Bioenergetics · Respiration · note 1 of 3

Spec 4.4.2.1Download all Respiration notes (PDF)Download all notes (PDF)6 pages

Aerobic and anaerobic respirationSpec 4.4.2.1

In short

Respiration is an exothermic reaction in living cells that transfers energy for living processes. Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water. Anaerobic respiration happens without oxygen and transfers much less energy, because glucose is only partly oxidised. In muscles it gives lactic acid; in plant and yeast cells it gives ethanol and carbon dioxide.

Cellular respiration is an exothermic reaction which is continuously occurring in living cells. In an exothermic reaction, energy is transferred to the surroundings. The energy transferred supplies all the energy needed for living processes.

Organisms need energy for:

chemical reactions to build larger molecules
movement
keeping warm.

Respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen), to transfer energy.

Aerobic respiration

glucose + oxygen → carbon dioxide + water

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic respiration in muscles

glucose → lactic acid

As the oxidation of glucose is incomplete in anaerobic respiration, much less energy is transferred than in aerobic respiration.

Anaerobic respiration in plant and yeast cells

glucose → ethanol + carbon dioxide

Anaerobic respiration in yeast cells is called fermentation. It has economic importance in the manufacture of bread and alcoholic drinks.

Aerobic and anaerobic respiration compared
	Aerobic	Anaerobic (muscles)	Anaerobic (plant and yeast cells)
Oxygen needed?	Yes	No	No
Products	Carbon dioxide and water	Lactic acid	Ethanol and carbon dioxide
Energy transferred	A large amount, as glucose is completely oxidised	Much less, as glucose is only partly oxidised	Much less, as glucose is only partly oxidised

Common mistake:

Plants respire all the time, not only at night, and so do all other living cells. Respiration is not breathing: it happens inside cells.

Exam tip:

In a comparison, give both sides: aerobic needs oxygen, and anaerobic does not. Say anaerobic respiration transfers much less energy (never 'makes energy') because the oxidation of glucose is incomplete.

Aerobic respiration transfers much more energy than either type of anaerobic respiration.

Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026

Frequently asked questions

Why is respiration an exothermic reaction?

Respiration is exothermic because energy is transferred from the reaction to the surroundings. It happens continuously in living cells, and the energy transferred supplies all the energy needed for living processes, such as chemical reactions to build larger molecules, movement and keeping warm. Both aerobic and anaerobic respiration transfer energy in this way.

How is respiration different from breathing?

Respiration is a chemical reaction that happens inside living cells to transfer energy from glucose. Breathing moves air into and out of the lungs, which supplies the oxygen used in aerobic respiration. Respiration happens all the time in every living cell, including plant cells, not only at night, and it can also happen anaerobically without oxygen.

What is anaerobic respiration in yeast?

Anaerobic respiration in yeast is respiration without oxygen, in which glucose is converted into ethanol and carbon dioxide. It is called fermentation, and it has economic importance in the manufacture of bread and alcoholic drinks. Much less energy is transferred than in aerobic respiration, because the oxidation of glucose is incomplete.

All 5 questions on Respiration