Aerobic and anaerobic respiration: Students should be able to compare the processes of aerobic and anaerobic respiration with regard to the need for oxygen, the differing products and the relative amounts of energy transferred.

The idea

Respiration transfers energy from glucose in cells. Aerobic respiration needs oxygen and occurs mainly in mitochondria; glucose and oxygen react to form carbon dioxide and water, transferring a relatively large amount of energy. Anaerobic respiration occurs without oxygen. In animals and some other organisms, glucose is converted to lactic acid, transferring less energy. In plants and yeast, anaerobic respiration produces ethanol and carbon dioxide. Both processes begin with glucose breakdown in the cytoplasm, but only aerobic respiration uses oxygen and completes glucose breakdown. For comparison, state the oxygen requirement, name the products, and say which transfers more energy per glucose molecule.

What earns the marks

• Aerobic respiration requires oxygen, whereas anaerobic respiration does not require oxygen.
• Aerobic respiration produces carbon dioxide and water from glucose and oxygen; anaerobic respiration in animals produces lactic acid, while in plants and yeast it produces ethanol and carbon dioxide.
• Aerobic respiration transfers a relatively larger amount of energy per glucose molecule than anaerobic respiration.
• A valid comparison must address all three criteria: oxygen need, products and relative energy transferred.

Common mistakes and how to put them right

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

  Put it right: In animals it produces lactic acid, while carbon dioxide and water are aerobic products.

• Stating that anaerobic respiration transfers more energy because it is faster.

  Put it right: Aerobic respiration transfers more energy per glucose molecule, even though anaerobic respiration can supply energy rapidly for short periods.

• Confusing respiration with breathing.

  Put it right: Respiration is a series of enzyme-controlled reactions in cells, whereas breathing is ventilation of the lungs.

Exam tips

• Use a three-row comparison table with columns for oxygen, products and energy transferred to ensure every clause is covered.
• Name the organism when giving anaerobic products, because lactic acid applies to animals while ethanol and carbon dioxide apply to plants and yeast.
• Use comparative wording such as 'more energy than' rather than vague phrases like 'lots of energy'.