Identify the secondary messengers generated by hormones which interact with membrane-bound receptors.

(a) Cyclic AMP
(b) Ca²⁺
(c) IP₃
(d) G protein

Hormones which interact with membrane-bound receptors normally do not enter the target cell, but generate second messengers which in turn regulate cellular metabolism.

cAMP, ${\mathrm{Ca}}^{2+}$, ${\mathrm{IP}}_3$ and DAG are examples of second messengers. 

G proteins, by themselves, are not second messengers. They activate enzymes (e.g., adenylate cyclase) that produce second messengers.