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

What is the effect of increasing substrate concentration on enzyme-catalyzed reactions up to a certain point?

1. It decreases the overall efficacy of the reaction

2. It has no effect at all

3. It initially increases the reaction rate

4. It eventually leads to a decrease in reaction rate

The correct answer is: It initially increases the reaction rate

Increasing substrate concentration has a significant effect on enzyme-catalyzed reactions, particularly at lower concentrations. Initially, as the substrate concentration rises, there are more substrate molecules available for the enzyme to catalyze. This leads to a higher frequency of enzyme-substrate collisions, which increases the rate of reaction. The enzymes can work more efficiently as they are saturated with substrate, allowing for more products to be formed over time. This trend continues up to a certain point, where the enzymes become saturated. At saturation, all active sites of the enzyme molecules are occupied, and adding more substrate will no longer increase the rate of reaction. This explains why, while increasing substrate concentration enhances the reaction rate initially, there is a limit to this effect. Understanding this dynamic is crucial for grasping the basic principles of enzymatic activity and how enzymes behave under varying conditions.