Metabolism is the sum of all the reactions in a cell or the body.

The idea

Metabolism means every chemical reaction occurring in a cell or in the whole body, added together. It includes synthesis reactions that build larger molecules, such as joining glucose to form glycogen or amino acids to form proteins, and breakdown reactions that split larger molecules into smaller ones, such as hydrolysis of starch to glucose. It also includes respiration, which transfers energy from glucose and other substrates. Because metabolism is a sum, it is not one single reaction or one organ; it is the total activity of all cells.

For example: A liver cell simultaneously synthesises glycogen, breaks down excess amino acids and respires glucose, and all these reactions together are part of that cell's metabolism.

What earns the marks

• Metabolism is defined as the sum of all the reactions in a cell or the body, not a single reaction or a single organ.
• Respiration is part of metabolism because it transfers energy from substrates such as glucose for other cellular processes.
• Examples include the synthesis of glycogen from glucose, the synthesis of proteins from amino acids, and the breakdown of excess amino acids.
• Metabolism occurs in all living cells, so the metabolism of the body is the combined metabolism of its cells.
• The idea of a sum means individual reactions are linked; the products of one reaction can be substrates for another.

Common mistakes and how to put them right

• Defining metabolism as only breakdown or only respiration.

  Put it right: Metabolism is the sum of all reactions, including both synthesis and breakdown.

• Thinking metabolism occurs only in the liver or only during exercise.

  Put it right: Metabolism occurs in all living cells all the time.

• Confusing metabolism with digestion.

  Put it right: Digestion is only one set of breakdown reactions, whereas metabolism includes all reactions in cells and the body.

Exam tips

• Use the phrase sum of all reactions when defining metabolism, then give one synthesis and one breakdown example to show the range.
• Link metabolism to respiration by explaining that respiration supplies energy for synthesis reactions.
• Avoid treating metabolism as a single process; refer to reactions in cells or the body collectively.