Metabolism: respiration

The idea

Respiration is the sum of enzyme-controlled reactions that release energy from glucose. Aerobic respiration uses oxygen and occurs mainly in mitochondria. It produces carbon dioxide and water and releases energy for metabolism. Anaerobic respiration occurs without oxygen and releases less energy; in animals it produces lactic acid, and in plants and yeast it produces ethanol and carbon dioxide. The energy released is used for muscle contraction, active transport, protein synthesis and maintaining body temperature. For example, muscle cells respire glucose to provide energy for contraction. Respiration is a key part of metabolism because it supplies the energy for all other metabolic reactions. It is not breathing; breathing is ventilation, which brings oxygen to the lungs and removes carbon dioxide.

What earns the marks

• Respiration is an enzyme-controlled process that releases energy from glucose.
• Aerobic respiration requires oxygen and produces carbon dioxide and water.
• Anaerobic respiration occurs without oxygen and releases less energy than aerobic respiration.
• The energy released by respiration is used for processes such as muscle contraction, active transport and protein synthesis.
• Respiration is a metabolic process and is distinct from breathing or ventilation.

Common mistakes and how to put them right

• Confusing respiration with breathing.

  Put it right: Respiration is a chemical process in cells; breathing is ventilation that moves air in and out of the lungs.

• Thinking anaerobic respiration releases more energy than aerobic respiration.

  Put it right: Anaerobic respiration releases less energy because glucose is not fully broken down.

• Believing respiration only occurs in animals.

  Put it right: Respiration occurs in all living cells, including plant cells.

Exam tips

• Write the word equation for aerobic respiration accurately: glucose + oxygen → carbon dioxide + water.
• When comparing aerobic and anaerobic respiration, state the presence or absence of oxygen and the relative energy released.
• Link the energy released by respiration to a named cellular process, such as active transport or muscle contraction, to show understanding.