Initally, the rate of reaction is able to increase as concentration does because at this point, not all enzymes active sites are occupied. Therefore, by adding more substrate, a higher percentage of enzymes are able to be put to work. However, if the concentration is increased to the saturation point or above it, a graph showing the reaction will plateau. This is because the substrate is now in excess and 100% of the active sites are full. The enzyme at this point is therefore the limiting factor for increasing the rate of reaction