Metabolism: Metabolism includes:

The idea

This statement introduces the range of reactions that count as metabolism. Metabolism is not one reaction but the total of all chemical reactions in a cell or body. It includes synthesis reactions that build larger molecules from smaller ones, such as joining glucose molecules to form starch or glycogen, joining amino acids to form proteins, and joining fatty acids and glycerol to form lipids. It also includes breakdown reactions, such as the breakdown of excess protein into urea in the liver, and the breakdown of glucose during respiration to release energy. Some metabolic reactions use energy transferred by respiration, while others release energy. Because all these reactions happen in living cells, they are enzyme controlled and continual. A useful way to remember the range is to think of building up and breaking down, with named examples for each.

What earns the marks

• Metabolism includes synthesis reactions that build larger molecules from smaller ones, for example glucose to starch or glycogen.
• Metabolism includes the formation of proteins from amino acids and lipids from fatty acids and glycerol.
• Metabolism includes breakdown reactions, such as the breakdown of excess protein to urea in the liver.
• Metabolism includes respiration, in which glucose is broken down to release energy for the cell.
• All metabolic reactions are controlled by enzymes, so temperature and pH affect their rate.
• Metabolism is the sum of all reactions in a cell or body, not a single process or organ.

Common mistakes and how to put them right

• Listing only synthesis reactions.

  Put it right: Also giving a breakdown example such as the formation of urea in the liver.

• Treating metabolism as a single reaction.

  Put it right by defining it as the sum of all chemical reactions in a cell or body.

• Confusing the breakdown of glucose in respiration with digestion.

  Put it right by stating that respiration is a series of enzyme controlled reactions inside cells.

Exam tips

• Give at least one build-up example and one breakdown example when asked what metabolism includes.
• Name the molecules involved, such as amino acids and proteins, rather than saying big molecules and small molecules.
• Use the term enzyme controlled when describing metabolic reactions to show how they are regulated.