A Level Biology Revision "The Effect of Substrate Concentration and Enzyme Concentration on Enzymes"

The video introduces the learning objective: understanding how substrate and enzyme concentration affect enzyme-catalyzed reaction rates. It reiterates that the reaction rate depends on the frequency of collisions between substrate molecules and the enzyme's active site.

Initially, with low substrate concentration, the collision frequency and reaction rate are low. As substrate concentration increases, collision frequency and reaction rate increase proportionally, demonstrating a direct relationship. This linearity continues until the enzyme reaches its maximum rate (Vmax).

At a certain point, despite increasing substrate concentration, the reaction rate stops increasing. This is because all active sites on the enzyme molecules are constantly occupied (saturated) with substrate. Any additional substrate cannot find free active sites, hence the rate plateaus at Vmax.

With a low enzyme concentration and abundant substrate, many substrate molecules cannot find available active sites, leading to a low reaction rate. Doubling the enzyme concentration doubles the number of active sites, thereby doubling the collision frequency and the reaction rate. This shows a direct proportionality between enzyme concentration and reaction rate, provided there is more substrate than enzyme.

If the substrate becomes limited, further increases in enzyme concentration will not increase the reaction rate. This is because there won't be enough substrate molecules to collide with all the newly available active sites, meaning the substrate concentration becomes the limiting factor.