The human digestive system: Students should be able to relate knowledge of enzymes to Metabolism (page 42).

The idea

Metabolism is the sum of all the chemical reactions occurring in the cells of an organism. Enzymes are biological catalysts: globular proteins that lower the activation energy of a reaction and speed it up without being used up. Each enzyme has a specific active site complementary in shape to its substrate, so it catalyses only one reaction. Anabolic reactions build larger molecules from smaller ones, for example joining glucose molecules to form glycogen in liver and muscle cells, or amino acids to form proteins. Catabolic reactions break larger molecules into smaller ones, for example respiration breaking down glucose to release energy, or digestion hydrolysing starch to maltose. Because every metabolic reaction is controlled by a particular enzyme, the total rate of metabolism depends on enzyme activity, which in turn depends on temperature and pH.

What earns the marks

• Metabolism is defined as the sum of all chemical reactions in the body or in cells, not a single reaction.
• Enzymes are proteins that act as biological catalysts, speeding up metabolic reactions by lowering activation energy.
• Enzyme specificity arises from the active site being complementary in shape to a particular substrate.
• Anabolic reactions build larger molecules from smaller ones, such as glucose to glycogen or amino acids to proteins.
• Catabolic reactions break larger molecules into smaller ones, such as glucose breakdown in respiration or starch hydrolysis in digestion.
• Enzyme activity controls the rate of metabolic reactions, so temperature and pH changes affect metabolism.

Common mistakes and how to put them right

• Saying enzymes are 'used up' in reactions.

  Put it right: Enzymes are catalysts and are not consumed, so they can be reused.

• Describing metabolism as only digestion.

  Put it right: Metabolism includes all reactions in cells, including respiration, protein synthesis and glycogen formation.

• Confusing anabolic with catabolic.

  Put it right: Anabolic builds larger molecules, catabolic breaks larger molecules into smaller ones.

Exam tips

• Link each named metabolic reaction to a named enzyme, for example starch to maltose by amylase or glucose to glycogen by glycogen synthase.
• Use the phrase 'complementary active site' rather than 'same shape' when explaining specificity.
• When asked to relate enzymes to metabolism, give one anabolic and one catabolic example to show breadth.