The human digestive system — AQA GCSE Biology
Test yourself on The human digestive system with AQA GCSE practice questions.
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The human digestive system explained
Enzymes are proteins that act as biological catalysts.
Read the full explanation
Each enzyme has an active site, a region with a shape complementary to one particular substrate. Because shapes are specific, only that substrate binds, forming an enzyme–substrate complex. The enzyme lowers the activation energy needed for the reaction, so the reaction happens faster at body temperature. After the reaction, products are released and the enzyme is unchanged, ready to catalyse further reactions. This specificity explains why amylase acts on starch, protease acts on proteins and lipase acts on lipids. The lock-and-key idea helps describe how shape determines which reaction is catalysed, and it links directly to denaturation when shape is altered.
Your focus
- Describe enzymes as proteins with a specific active site.
- Explain how the shape of the active site determines which reaction is catalysed.
- Relate named digestive enzymes to their specific substrates and products.
The human digestive system exam tips
Marking Points
- Enzymes are proteins that act as biological catalysts in living organisms.
- The active site has a shape complementary to a specific substrate.
- Substrate binds to the active site to form an enzyme–substrate complex.
- The enzyme lowers activation energy, increasing the rate of the specific reaction.
- Enzymes are specific because only substrates with a complementary shape can bind.
- Products are released and the enzyme is unchanged, so it can be reused.
- Named examples include amylase acting on starch, protease on proteins and lipase on lipids.
Examiner Tips
- 💡Use the terms 'complementary shape' and 'enzyme–substrate complex' to show precise understanding.
- 💡Link each named enzyme to its substrate and product, for example amylase breaks starch into simpler sugars.
- 💡Explain specificity by referring to shape, not by saying enzymes 'know' their substrate.
Common Mistakes
- Saying enzymes are used up in the reaction; the correction is that enzymes are catalysts and remain unchanged.
- Stating that any substrate can bind to any enzyme; the correction is that the active site is complementary to one specific substrate.
- Confusing the active site with the whole enzyme; the correction is that the active site is the specific region where the substrate binds.