OCR GCSE Biology 2026 – 400 Free Practice Questions to Pass the Exam

Maintaining a stable pH level is crucial for controlling metabolic reactions in cells because enzymes, which facilitate these reactions, are sensitive to pH changes. Each enzyme has an optimal pH range in which it functions most effectively. Deviations from this optimal pH can lead to changes in the enzyme's shape, affecting its ability to bind to substrates and catalyze reactions efficiently. If the pH is too acidic or too alkaline, it can result in denaturation of the enzyme, rendering it inactive, which can disrupt metabolic processes.

In the context of the other options, changing substrate concentration frequently can influence reaction rates temporarily, but it does not serve as a long-term method for controlling metabolic pathways. Modifying enzyme shapes is an essential aspect of enzyme function but is not a common method implemented by cells to regulate metabolic reactions—rather, it is a consequence of environmental factors including pH and temperature. Adjusting temperature regularly can impact enzyme activity, but significant temperature fluctuations can also lead to denaturation and loss of function. Therefore, maintaining stable pH levels stands out as a consistent and vital method for ensuring metabolic reactions proceed optimally.