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

What limits the rate of reaction despite an increase in enzyme concentration?

1. The unavailability of substrate molecules

2. The saturation of enzymes

3. The presence of inhibitors

4. The increase in temperature

The correct answer is: The unavailability of substrate molecules

An increase in enzyme concentration generally enhances the rate of reaction, up to a certain point. However, if there is not enough substrate available for the enzymes to bind to, the reaction rate will plateau. This is because enzymes need substrate molecules to carry out their catalytic function, and when the concentration of substrate is low relative to enzyme concentration, there are free enzymes that cannot find substrate to react with. When the substrate is limited, the enzymes cannot work efficiently, no matter how many enzymes are present. Thus, the rate of reaction cannot continue to increase simply with more enzymes; instead, it is the availability of substrate that ultimately determines the reaction rate when enzyme concentration is high. This concept highlights the importance of both enzyme and substrate levels in biochemical reactions.