Cell respiration: aerobic and anaerobicSpec C1.2.4, C1.2.5
In short
Cell respiration is a system for producing ATP within the cell using energy released from carbon compounds, mainly glucose and fatty acids. Aerobic respiration needs oxygen and mitochondria, gives a large ATP yield and produces carbon dioxide and water. Anaerobic respiration in humans happens in the cytoplasm without oxygen, uses only glucose, gives a small yield and produces lactate.
Cell respiration is a system for producing ATP within the cell using energy released from carbon compounds. Glucose and fatty acids are the principal substrates, but a wide range of carbon (organic) compounds can be used, including amino acids.
Do not confuse it with gas exchange. Gas exchange is the diffusion of oxygen and carbon dioxide between an organism and its environment, for example across the alveoli. Cell respiration is a set of enzyme-catalysed reactions inside every living cell. Gas exchange supplies the oxygen that aerobic respiration uses and removes the carbon dioxide it produces.
Word equations (glucose as substrate)
| Aerobic | Anaerobic | |
|---|---|---|
| Respiratory substrates | Carbohydrates, lipids (fatty acids) and amino acids | Glucose (carbohydrate) only |
| Oxygen required | Yes | No |
| Relative ATP yield per glucose | Large (about 30 ATP; older textbooks give 36–38) | Small (2 ATP) |
| Waste products | Carbon dioxide and water | Lactate |
| Where in the cell | Starts in the cytoplasm, mostly in mitochondria | Cytoplasm only |
| Mitochondria needed | Yes | No |
Humans use anaerobic respiration when oxygen cannot be supplied fast enough, for example in muscles during very intense exercise. It gives ATP quickly but in small amounts, and the lactate must be removed later.
In 'compare' questions, write paired statements: 'aerobic respiration needs oxygen whereas anaerobic does not'. Lactate is the IB term; do not write lactic acid unless the question does.
Written and checked against the IB Biology HL specification · Updated October 2026