Why do lines flatten out once you reach a high substrate concentration?

Lines flatten out at high substrate concentrations due to the saturation of enzyme active sites. When the substrate concentration is sufficiently high, all available enzyme active sites are occupied, leading to a maximum reaction rate (Vmax) that cannot be exceeded. This phenomenon is described by the Michaelis-Menten kinetics model, where the reaction rate approaches Vmax as substrate concentration increases, resulting in a plateau in the graph.