Final answer:
In the absence of glucose, cAMP levels are high, which activates the catabolite activator protein. The activated CAP-cAMP complex increases the transcription of the lac operon, necessary for lactose metabolism under specific conditions, such as low glucose and presence of lactose.
Step-by-step explanation:
In the absence of glucose, cAMP levels are high, leading to high expression of the lac operon. The activation of the lac operon is a result of the cellular response to differing levels of glucose and lactose. When glucose is low, cAMP accumulates, which in turn binds to the catabolite activator protein (CAP), forming a complex.
This CAP-cAMP complex then binds to the promoter region of the lac operon, increasing the affinity of RNA polymerase to the promoter, thus facilitating transcription of the genes required for lactose metabolism. However, the system also requires lactose to be present as it ensures that the cell does not waste resources in the synthesis of the enzymes if lactose is not available as a source of energy. Without glucose and with the presence of lactose, these conditions lead to the 'induction' of the lac operon and the breakdown of lactose.