How does the cell recognize that lactose needs to be metabolized?

Lactose metabolism is regulated by lac operon in prokaryotic cells. When in the environment lactose is present, the cell signals the presence if lactose and synthesizes the enzyme required for the degradation of lactose. Removal of lactose from the environment leads to the termination of the enzyme synthesis. Thus, the metabolism of lactose is environment dependent.

The metabolism of lactose is also dependent on the presence of glucose. Glucose is the favorite food for bacteria. If glucose and lactose both are present in the environment, the bacterial cells prefer glucose over lactose and lactose is not metabolized. In the absence of glucose, the cells will metabolize lactose.