Final answer:
When a G-protein-coupled receptor activates a G protein, the α subunit exchanges its bound GDP for GTP, leading to activation and subsequent signaling events within the cell.
Step-by-step explanation:
The question pertains to what happens when a G-protein-coupled receptor activates a G protein. In this process, the correct event is that the α subunit exchanges its bound GDP for GTP (Option C). When a signaling molecule binds to the G-protein-coupled receptor, the α subunit releases guanosine diphosphate (GDP) and acquires guanosine 5'-triphosphate (GTP), resulting in the activation of the G protein.
Upon GTP binding, the G-protein subunits dissociate into the α subunit and the βγ complex. The α subunit or the dissociated βγ pair may then activate other proteins within the cell. After some time, the GTP on the active α subunit is hydrolyzed back to GDP, deactivating both the α subunit and the βγ complex, and the subunits reassociate to form the inactive G-protein, ready to start the cycle anew.