Aerobic and anaerobic respiration: As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration.

The idea

Respiration transfers energy from glucose. In aerobic respiration, glucose is fully oxidised using oxygen, so all its chemical potential energy can be released: about 2 880 kJ per mole of glucose in cells. In anaerobic respiration, oxygen is absent, so glucose is only partially broken down. In animals, glucose is converted to lactic acid; in plants and yeast, to ethanol and carbon dioxide. Because carbon–oxygen bonds remain in these products, the oxidation of glucose is incomplete and only a small fraction of the energy is transferred, roughly 120 kJ per mole in muscle. The rest stays locked in the products.

What earns the marks

• Aerobic respiration uses oxygen and oxidises glucose completely to carbon dioxide and water.
• Anaerobic respiration occurs without oxygen and breaks glucose down only partially.
• The products of anaerobic respiration still contain stored chemical energy, so oxidation is incomplete.
• Less energy is transferred per glucose molecule in anaerobic respiration than in aerobic respiration.
• Energy transferred is used for muscle contraction, active transport and maintaining body temperature.
• The difference in energy transfer is due to the extent of oxidation, not the speed of the reaction.

Common mistakes and how to put them right

• Saying anaerobic respiration releases no energy.

  Put it right: It transfers some energy, just much less than aerobic respiration.

• Confusing 'less energy transferred' with 'less glucose used'.

  Put it right: The same amount of glucose can be used but less energy is released per molecule.

• Writing that anaerobic respiration produces carbon dioxide and water in human muscle.

  Put it right: In animals it produces lactic acid, while plants and yeast produce ethanol and carbon dioxide.

Exam tips

• Use the phrase 'incomplete oxidation of glucose' when explaining why less energy is transferred.
• Compare the two processes directly: state that aerobic respiration transfers much more energy per glucose molecule.
• If asked to explain fatigue, link lactic acid build-up and reduced energy transfer to anaerobic respiration during vigorous exercise.