Students should be able to relate knowledge of enzymes to Metabolism.

The idea

This statement asks you to connect digestive enzymes to metabolism as a whole. Metabolism is the sum of all the chemical reactions in the body, and almost all of them are controlled by enzymes. Digestive enzymes are extracellular: carbohydrases, proteases and lipases are released into the gut, where they hydrolyse large food molecules into small soluble products. Those products are then absorbed and used in metabolism. Some are built into new molecules in anabolic reactions, such as amino acids joining to form proteins or glucose joining to form glycogen. Others are broken down in catabolic reactions, such as glucose being oxidised in respiration to release energy. So digestion supplies the substrates for metabolism, and enzymes control both the digestive reactions and the metabolic reactions that follow.

What earns the marks

• Metabolism is the sum of all the chemical reactions in a cell or the body.
• Enzymes are biological catalysts that control the rate of metabolic reactions.
• Digestive enzymes break down large food molecules into small soluble products that can be absorbed.
• Absorbed products are used in anabolic reactions, such as building proteins from amino acids or glycogen from glucose.
• Absorbed products are also used in catabolic reactions, such as respiration breaking down glucose to release energy.
• Enzyme action is specific because each enzyme has an active site complementary to its substrate.

Common mistakes and how to put them right

• Defining metabolism as only the breakdown of food.

  Put it right by stating that metabolism includes both breakdown and building reactions.

• Thinking enzymes are only found in the digestive system.

  Put it right by noting that enzymes control reactions throughout cells and the body.

• Saying an enzyme is complementary to any substrate.

  Put it right by stating that each enzyme's active site is complementary to one specific substrate.

Exam tips

• Define metabolism clearly before giving examples of reactions it includes.
• Link each digestive enzyme to the metabolic use of its product, such as amino acids for protein synthesis.
• Use the terms anabolic and catabolic correctly when classifying example reactions.