Enzymes - lock and key model
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of the basic role of the digestive system.
What is digestion?
The breakdown of large, insoluble molecules into smaller, soluble molecules.
What is the function of the small intestine in digestion?
It absorbs small, soluble molecules into the bloodstream after digestion is complete.
What is the role of the pancreas in digestion?
To produce digestive enzymes including amylase, protease and lipase.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains enzymes & the lock and key model, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!
Enzymes & the Lock and Key Model
What Are Enzymes?
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Enzymes are biological catalysts made of protein.
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They speed up chemical reactions in living organisms without being used up, the enzyme can then continue to carry out more reactions.
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Each enzyme has a specific active site that fits only one type of substrate – this gives enzymes their specificity.
Lock and Key Theory
The Lock and Key Model is a simplified way to explain how enzymes work:
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The enzyme = the lock.
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The substrate (the molecule it acts on) = the key.
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The active site of the enzyme is a specific shape – only the right substrate will fit.
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We say that the substrate is complementary to the enzyme's active site.
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When the substrate binds to the active site, they form an enzyme-substrate complex.
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The reaction takes place, the products are released, and the enzyme remains unchanged, ready to be used again.
Diagram showing the lock and key mechanism of enzymes.
Key Terms
- Active site – the specifically shaped region of an enzyme that binds to the substrate.
- Substrate - the molecule that binds to the active site.
- Enzyme - a biological catalyst that speeds up a reaction.
- Enzyme-substrate complex – the temporary structure formed when an enzyme binds to its substrate.
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Exam Tips
- Use the words "specific", "active site", and "complementary" in your explanations.
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Practice Question
Explain why protease enzymes only break down proteins (4 marks)
- Protease enzymes have a specific shape with a specific active site.
- The active site is complementary to the substrate (the protein).
- The substrate binds to the active site and forms an enzyme-substrate complex.
- The enzyme catalyses the reaction and the products (amino acids) are released, leaving the enzyme unchanged.
More Practice
Try to answer the practice question from the TikTok video on your own, then watch the video to see how well you did!