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

How does substrate concentration affect the rate of reaction?

1. The higher the substrate concentration, the faster the reaction up to a limit

2. Increased substrate concentration always slows down the reaction

3. Substrate concentration has no effect on the reaction rate

4. A higher substrate concentration decreases the reaction rate

The correct answer is: The higher the substrate concentration, the faster the reaction up to a limit

The correct choice highlights a fundamental principle in enzyme kinetics known as the saturation effect. As substrate concentration increases, the rate of reaction also increases because more substrate molecules are available to bind to enzyme active sites. With more enzyme-substrate complexes forming, the overall reaction proceeds more rapidly. However, this relationship is not indefinite. Once the enzyme's active sites are saturated with substrate, adding more substrate makes no difference to the reaction rate. This is because, at saturation, there are no available active sites for additional substrate molecules to bind to, resulting in a leveling off of the reaction rate despite increasing substrate concentration. Therefore, while higher substrate concentrations can enhance reaction rates to a certain point, this effect plateaus when the enzymes are fully occupied. The other options do not accurately convey this relationship. For example, stating that increased substrate concentration always slows down the reaction contradicts the established facts about enzyme activity. Similarly, claiming that substrate concentration has no effect overlooks the significant initial increases in reaction rates observed with rising substrate levels, and stating that higher substrate concentrations decrease the reaction rate misrepresents how enzyme kinetics function.