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

What occurs when enzyme concentration is increased?

1. The rate of reaction decreases regardless of substrate availability

2. The rate of reaction increases until there is an excess of enzyme

3. The rate of reaction becomes independent of substrate concentration

4. The rate of reaction remains constant

The correct answer is: The rate of reaction increases until there is an excess of enzyme

Increasing enzyme concentration generally leads to an increase in the rate of reaction, provided that substrate availability is not a limiting factor. Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy. More enzymes in a reaction mixture mean that there are more active sites available for substrate molecules to bind to, leading to an increased likelihood of substrate-enzyme collisions. As long as there is sufficient substrate present, each enzyme can work on different substrate molecules, facilitating more simultaneous reactions. This trend continues until the substrate becomes the limiting factor—meaning there are no more substrate molecules available for the excess enzymes to act upon. At that point, adding even more enzymes would not further increase the rate of reaction, resulting in a situation where there could be an "excess of enzyme." In contexts where substrate is in limited supply, the reaction rate will plateau when the active sites of the enzymes are fully occupied, indicating that other factors—such as the amount of substrate—become limiting. Thus, the increase in enzyme concentration leads to increased reaction rates up to a certain point, after which adding more enzymes does not provide additional benefits in terms of reaction speed.