It must be realized that biological systems are semi-isolated, in the sense that they do not have an endless source of energy and are, therefore, dependent on conserving the energy within the system for efficient function. This means that reactions in cells occur a billion times more rapidly than they would if enzymes were not present. In general enzymes function by providing a convenient surface for bringing together the reactants and then becoming separated from them. 1. larger molecules may be synthesized from smaller ones 2. Thus, when a great deal of substrate is altered by an enzyme every minute, the reaction is said to be proceeding at a rapid rate.
Concentration refers to amount in a given volume of solution.
In most enzyme reactions, enzyme concentration is small compared to the substrate concentration. Therefore, the rate of the reaction becomes proportional to the concentration of the enzyme. If the enzyme concentration is doubled, the reaction rate is doubled. At low substrate concentrations, the rate of the reaction is proportional to the substrate concentration, but at higher substrate concentrations the reaction rate is independent of substrate concentration. That is, further increase in the amount of substrate present per unit volume does not cause the reaction to proceed at a faster rate because all available enzymes are already saturated or tied up in reactions.