Final answer:
Once the enzymes are saturated with substrate, further increases in substrate concentration do not increase the rate of reaction, and the reaction rate remains constant.
Step-by-step explanation:
In an enzyme-controlled reaction, the rate of reaction increases as substrate concentration increases, up until a certain point known as the saturation point. Beyond this point, the enzymes have all their active sites occupied by substrate molecules. When the substrate concentration is increased after the saturation point, additional substrate molecules have no effect on the rate of reaction because there are no more free enzyme sites available. The reaction rate now becomes constant, as the enzyme molecules are fully saturated with substrate. The excess substrate must wait for active sites to become available again after the enzyme catalyzes the reaction and the product is released.