Cellular respirationSpec 8.9
In short
Cellular respiration is an exothermic reaction that happens continuously in living cells to release energy for metabolic processes. Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water. Anaerobic respiration happens without oxygen and produces lactic acid in animal cells, or ethanol and carbon dioxide in plant cells and yeast. Respiration is not breathing.
Cellular respiration is an exothermic reaction. It transfers energy to the surroundings, and some of this energy is released as heat. It happens continuously in living cells (in plants as well as animals) to release energy for metabolic processes.
Metabolic processes include building larger molecules from smaller ones, muscle contraction, and keeping the body warm in mammals and birds.
Aerobic respiration
Aerobic respiration uses oxygen to break down glucose.
glucose + oxygen → carbon dioxide + water
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Anaerobic respiration
Anaerobic respiration happens without oxygen. In animal cells, such as muscle cells working hard, glucose is broken down to lactic acid. In plant cells and yeast, glucose is broken down to ethanol and carbon dioxide.
glucose → lactic acid
glucose → ethanol + carbon dioxide
Common mistake:Respiration is not breathing, and it is not the same as gas exchange. Respiration is a chemical reaction in cells that releases energy.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026
Frequently asked questions
Why is respiration an exothermic reaction?
Respiration is exothermic because it transfers energy to the surroundings, and some of this energy is released as heat. It happens continuously in the living cells of plants and animals to release energy for metabolic processes, such as building larger molecules, muscle contraction and keeping mammals and birds warm. Aerobic and anaerobic respiration are both exothermic.
How is respiration different from breathing?
Respiration is a chemical reaction in cells that releases energy from glucose, while breathing is the movement of air into and out of the lungs. Respiration happens continuously in all living cells, in plants as well as animals. It is also not the same as gas exchange, which is the diffusion of gases between the alveoli and the blood.
What is the difference between aerobic and anaerobic respiration?
Aerobic respiration uses oxygen and completely breaks down glucose to carbon dioxide and water, releasing a large amount of energy. Anaerobic respiration happens without oxygen and breaks glucose down only partly, so it releases much less energy. In animals it produces lactic acid, and in plants and yeast it produces ethanol and carbon dioxide.
All 5 questions on Respiration
