Aerobic and anaerobic respiration equationsSpec 2.37, 2.38
In short
Aerobic respiration uses oxygen to break down glucose and releases a large amount of energy: glucose + oxygen → carbon dioxide + water, or C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Anaerobic respiration happens without oxygen. In animals it makes lactic acid, and in plants and yeast it makes ethanol and carbon dioxide.
Aerobic respiration
Aerobic respiration uses oxygen to break down glucose. It releases a large amount of energy to make ATP. You need to know both the word equation and the balanced symbol equation.
glucose + oxygen → carbon dioxide + water
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Anaerobic respiration
Anaerobic respiration happens without oxygen. The products depend on the type of organism. You need to know the word equations only.
Anaerobic respiration in animals and in plants| Organism | Word equation |
|---|
| Animals (for example muscle cells during hard exercise) | glucose → lactic acid |
|---|
| Plants (and yeast) | glucose → ethanol + carbon dioxide |
|---|
Exam tip:Learn the symbol equation as C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. To check it is balanced, count the atoms on each side: 6 carbon, 12 hydrogen and 18 oxygen.
Common mistake:Animals make lactic acid, and plants make ethanol and carbon dioxide. Mixing these up is a very common way to lose marks.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026
Frequently asked questions
How is respiration different from breathing?
Respiration is a chemical reaction in every living cell that releases energy from glucose to make ATP, while breathing is the movement of air into and out of the lungs. Breathing, or ventilation, supplies the oxygen needed for aerobic respiration and removes the carbon dioxide it produces, but the two are separate processes.
What is ATP used for in cells?
ATP provides energy for cells. When ATP is broken down to ADP and phosphate, energy is released for muscle contraction, active transport, making large molecules such as proteins from amino acids, cell division and growth, and sending nerve impulses. The ADP and phosphate are then reused in respiration to make more ATP.
What is anaerobic respiration in yeast?
Anaerobic respiration in yeast is the breakdown of glucose without oxygen to make ethanol and carbon dioxide: glucose → ethanol + carbon dioxide. Plants respire anaerobically in the same way. Because glucose is only partly broken down, much less energy is released per glucose molecule than in aerobic respiration.
All 5 questions on Respiration
