Aerobic and anaerobic respiration: Students should be able to describe cellular respiration as an exothermic reaction which is continuously occurring in living cells.

The idea

Cellular respiration is an exothermic reaction that transfers energy from glucose and occurs continuously in living cells. It is exothermic because energy is transferred to the surroundings. The energy transferred supplies all the energy needed for living processes such as muscle contraction and building larger molecules. Respiration occurs continuously in all living cells. Aerobic respiration uses oxygen and transfers more energy per glucose molecule than anaerobic respiration, which occurs without oxygen.

For example: A muscle cell respires aerobically at rest, but during vigorous exercise oxygen supply may be insufficient, so anaerobic respiration also occurs, producing lactic acid and an oxygen debt.

What earns the marks

• Cellular respiration is an exothermic reaction: it transfers energy from glucose to supply all the energy needed for living processes.
• Respiration occurs continuously in all living cells, including in plants, animals, and microorganisms.
• Aerobic respiration requires oxygen and transfers more energy per glucose molecule than anaerobic respiration.
• Anaerobic respiration occurs without oxygen and transfers less energy, producing different products in different organisms.
• The energy transferred is used for processes such as muscle contraction, active transport, and building larger molecules.

Common mistakes and how to put them right

• Saying respiration is endothermic.

  Put it right: Respiration is exothermic because it transfers energy to the surroundings.

• Thinking respiration only happens during exercise or in animals.

  Put it right: It is continuous in all living cells, including plants.

• Confusing respiration with breathing.

  Put it right: Breathing is ventilation of the lungs, while respiration is a chemical process in cells.

Exam tips

• Define exothermic clearly: an exothermic reaction transfers energy to the surroundings.
• Use the word continuously when describing respiration in living cells, and give examples such as muscle cells and root hair cells.
• Compare aerobic and anaerobic respiration by stating the presence or absence of oxygen and the relative energy transferred.
• Be prepared to recognise the chemical symbols for glucose, oxygen, carbon dioxide, and water in the context of respiration.