Cambridge O Level Biology · Syllabus 5090 · Respiration
Anaerobic Respiration
What is Anaerobic Respiration?
Anaerobic respiration is the chemical reactions in cells that release energy from glucose without using oxygen. Because the glucose is only partly broken down, a relatively small amount of energy is released compared with aerobic respiration of the same amount of glucose. The products depend on the organism: human muscle cells produce lactic acid, while yeast produces ethanol and carbon dioxide. The word anaerobic means without using oxygen; it does not mean without carbon dioxide.
This definition is part of the Respiration chapter in Cambridge O Level Biology.
Common mistakes with Anaerobic Respiration
- 9. “Anaerobic respiration releases no energy.” Then it would be pointless. It releases relatively less energy than aerobic respiration, because glucose is only partly broken down. Write Anaerobic respiration releases a relatively small amount of energy from each glucose molecule.
- 10. “Anaerobic means without carbon dioxide.” It means without using oxygen. Yeast respiring anaerobically produces carbon dioxide as one of its two products. Write Anaerobic respiration releases energy from glucose without using oxygen.
- 13. “Anaerobic respiration starts when all the oxygen has gone.” It starts when oxygen cannot be delivered fast enough for the rate of aerobic respiration required. Aerobic respiration carries on alongside it. Write When oxygen delivery is insufficient for the rate required, some cells respire anaerobically as well.
Examiner tips on Anaerobic Respiration
- Writing a matched comparison. Weak: “Aerobic respiration releases a lot of energy. Anaerobic respiration is used in exercise.” Those two sentences are about different features, so they do not compare anything. Strong: “Aerobic respiration releases a relatively large amount of energy from each glucose molecule, whereas anaerobic respiration releases a relatively small amount.” One feature, both processes, one sentence.
Questions students ask about Anaerobic Respiration
What are the products of anaerobic respiration in humans and in yeast?
In human muscle, glucose → lactic acid. In yeast, glucose → ethanol + carbon dioxide. The two must never be swapped: yeast does not make lactic acid, and humans do not make ethanol.
Why does anaerobic respiration release less energy?
Because the glucose is only partly broken down. The products — lactic acid, or ethanol — still contain a great deal of chemical energy, so less of the energy in the original glucose molecule is released to the cell.