The human digestive system: Students should be able to describe the nature of enzyme molecules and relate their activity to temperature and pH changes.

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Yes. This guide is linked to the The human digestive system: Students should be able to describe the nature of enzyme molecules and relate their activity to temperature and pH changes. topic in the AQA GCSE specification.

Enzymes are globular proteins with a specific three-dimensional shape.

Study notes

From the checked teaching notes for this specification point.

The idea

Enzymes are globular proteins with a specific three-dimensional shape. Part of the molecule forms an active site, which is complementary in shape to its substrate, so the substrate binds and the reaction is catalysed. Enzyme activity is affected by temperature and pH. As temperature rises, molecules and substrates gain kinetic energy and collide more frequently, so the rate of reaction increases up to an optimum temperature. Above the optimum, bonds holding the tertiary structure break, the active site changes shape and the enzyme is denatured, so substrate can no longer bind and activity falls sharply. Each enzyme also has an optimum pH. Away from this pH, ionic and hydrogen bonds in the enzyme break, altering the active site shape and reducing activity; extreme pH denatures the enzyme.

For example: Pepsin works best at acidic pH in the stomach, while amylase works best near neutral pH.

What earns the marks

Enzymes are proteins with a specific tertiary structure that includes an active site.
The active site is complementary in shape to the substrate, allowing enzyme-substrate binding and catalysis.
Increasing temperature increases kinetic energy and collision frequency, raising reaction rate up to the optimum.
Above the optimum temperature, bonds break, the active site changes shape and the enzyme is denatured, so activity decreases.
Each enzyme has an optimum pH; deviation from this pH alters bonds in the enzyme and reduces activity.
Extreme pH changes can denature the enzyme by permanently altering the active site shape.

Common mistakes and how to put them right

Saying enzymes are 'killed' by high temperature.
Put it right: Enzymes are molecules and are denatured, not killed.
Stating that denaturation is reversible.
Put it right: Denaturation changes the active site shape permanently, so activity does not return on cooling.
Claiming all enzymes have the same optimum pH.
Put it right: Optimum pH varies, for example pepsin is adapted to acidic conditions in the stomach.

Exam tips

Describe the graph shape: rising rate to an optimum, then a steep fall after denaturation.
Use the terms 'denatured' and 'active site changed shape' rather than 'destroyed' or 'broken'.
For pH questions, name a specific enzyme and its location, such as pepsin in the stomach or amylase in the small intestine.

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