The lac operon is present in the genome of the bacterium Escherichia coli. When glucose is not available, the presence of lactose in the extracellular environment leads to the expression of the genes of the lac operon. This leads to an increase in the uptake and metabolism of lactose. When glucose and lactose are available in the extracellular medium, lactose is prevented from entering the bacterial cell. Fig. 3.1 shows what happens when glucose enters a bacterial cell. • Glucose enters the bacterial cell using a transport protein, A. • B is an enzyme found in the cytoplasm of the bacterial cell. • B catalyses the phosphorylation of glucose to produce glucose 6-phosphate.