Final answer:
Bacterial cells will use glucose first due to preference regulated by the lac operon. The lac operon is activated in the absence of glucose and presence of lactose, producing enzymes for lactose digestion. High glucose levels suppress this system.
Step-by-step explanation:
If both lactose and glucose are available to a bacterial cell, the cell will preferentially utilize glucose as the carbon source due to the organism's regulatory system, particularly the lac operon. The lac operon, which enables the digestion of lactose, is tightly regulated by two main mechanisms: the presence of a repressor protein and the action of the catabolite activator protein (CAP).
The repressor protein is coded by the I gene of the lac operon, which binds to the operator site to prevent transcription when lactose is not available. In the presence of lactose, its metabolite allolactose binds to the repressor and induces a conformational change, preventing it from binding to the operator and allowing transcription of the genes needed for lactose metabolism.
However, for the lac operon to be effectively activated, glucose levels must be low or absent, as high glucose levels suppress cAMP formation, which is necessary for CAP to bind to the promoter region and induce transcription. Therefore, only when glucose is depleted will the cells turn to lactose for energy, activating the lac operon and producing the enzymes needed for lactose digestion.