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
Anaerobic respiration in muscles
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
Anaerobic respiration in yeast cells is called fermentation. It has economic importance in the manufacture of bread and alcoholic drinks.
| 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 |
Plants respire all the time, not only at night, and so do all other living cells. Respiration is not breathing: it happens inside cells.
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.
(opens full size in a new tab)Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026