OCR GCSE Biology Practice Exam 2025 – Comprehensive All-in-One Guide to Exam Success

When the pH level becomes too high for an enzyme, the enzyme denatures and loses functionality, which is represented by the correct choice. Enzymes are proteins that have a specific three-dimensional structure that is critical for their function. This structure includes an active site where substrates bind.

High pH levels can disrupt the hydrogen bonds and ionic bonds that stabilize the enzyme's structure. When the pH strays significantly from the optimal range, the shape of the active site may be altered, and this affects the enzyme's ability to bind to its substrate. As a result, the enzyme cannot carry out its catalytic function effectively, leading to a decrease or complete loss of activity.

In contrast, other options suggest scenarios that do not occur under these conditions. For example, a perfectly balanced reaction rate and an unchanged active site imply that the enzyme is functioning normally, which contradicts the impact of high pH. Additionally, implying that the enzyme starts functioning more rapidly at high pH directly opposes the concept of enzyme activity being contingent upon maintaining optimal conditions.