OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What occurs in stage two of the lock and key model?

1. The enzyme changes shape

2. The substrate is released into the cell

3. The substrate fits into the enzyme's active site

4. The enzyme is denatured

The correct answer is: The substrate fits into the enzyme's active site

In stage two of the lock and key model, the substrate fits into the enzyme's active site. This model illustrates how enzymes and substrates interact, drawing an analogy to a key fitting into a lock. The shape of the substrate is complementary to the shape of the enzyme's active site, allowing the substrate to bind effectively. This binding is crucial because it positions the substrate in a way that will facilitate the chemical reaction, leading to product formation. The specificity of this interaction is central to the function of enzymes, ensuring that they catalyze only particular reactions with specific substrates. The other options discuss different aspects of enzyme behavior. The enzyme changing shape pertains more to the induced fit model, where the enzyme adjusts to better fit the substrate once they interact. Releasing the substrate is related to the completion of the reaction but is not part of the binding process in stage two. Denaturation of the enzyme refers to a loss of functional shape due to factors like heat or pH changes, which would prevent any substrate from binding effectively.