So prokaryotic cells have genes that respond to changes in concentration of certain types of biological molecules such as glucose. So basically when glucose concentration is high the glucose can basically or the cell can metabolize the glucose and use the ATP produced by the process of glycolysis to power the different types of processes that take place within our cell. However, what happens when the glucose concentration is low? When the glucose concentration is low the cell must actually metabolize some other type of biological molecule. And one other common type of biological molecule that the cell metabolizes is lactose. So lactose is a sugar molecule.