On the membrane of liver cells is a receptor, a seven transmembrane receptor, seven TM receptor known as the glucagon receptor. And glucagon binds onto this side of this glucagon receptor, shown in orange. And once the binding takes place, that initiates a process on the other side of this seven TM receptor protein. So on the other side, we have this g protein that contains a GDP bound to it. When the GDP is bound to it, it is inactive. But upon the binding of glucagon, the GDP is expelled and a GTP enters this pocket.