Final answer:
The G-protein acts as the intermediary between the hydrophilic hormone (first messenger) and the production of cAMP (second messenger), by activating adenylyl cyclase which catalyzes the transformation of ATP into cAMP.
Step-by-step explanation:
The link between the hydrophilic hormone serving as the first messenger and the second messenger is the G-protein. When a water-soluble hormone binds to its receptor in the cell membrane, it activates the associated G-protein. Subsequently, the activated G-protein then activates the enzyme adenylyl cyclase, which is responsible for converting adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP), the second messenger. cAMP goes on to activate protein kinases, initiating a phosphorylation cascade that alters cellular activity.