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

What is indicated by the plateau on an enzyme activity graph?

1. The reaction rate has increased significantly

2. The reaction has stopped increasing

3. Enzymes have denatured completely

4. Substrate concentration is at its peak

The correct answer is: The reaction has stopped increasing

A plateau on an enzyme activity graph indicates that the reaction rate has reached a maximum and is no longer increasing despite any further additions of substrate or other changes in conditions. This state reflects that the enzyme has reached its saturation point, meaning that the active sites of the enzymes are fully occupied by substrate molecules. As a result, no more substrate can be converted into product, leading to a stabilization in the reaction rate. At this stage, adding more substrate will not enhance the reaction velocity because the enzyme cannot process any additional substrate until some product is released. Understanding this concept is crucial for grasping how enzymes function and interact with substrates, as it highlights the efficiencies and limits of enzymatic reactions in biological systems.