What is the role of G proteins in GPCR signaling?

Hormones can bring about substantial physiological changes at very low concentrations. How does this relate to the concept of second messenger systems?

You are studying how a hormone affects a cell and find that when oxytocin binds the receptor, the intracellular $Ca$$^2$$^+$ concentration increases. Based on this information, what could you logically conclude about this cell and hormone?

cAMP, $IP_3$, and DAG are all molecules that are used as secondary messengers. Which statement below correctly identifies a difference between the pathways in which they are found?