Final answer:
The two types of second messengers discussed are cyclic adenosine monophosphate (cAMP) and inositol triphosphate (IP3), which are involved in the signal transduction pathways following the binding of a signal molecule to a plasma membrane receptor.
Step-by-step explanation:
Two types of second messengers that can exist downstream of plasma membrane receptors are cyclic adenosine monophosphate (cAMP) and inositol triphosphate (IP3).
cAMP is a common second messenger involved in signal transduction, which propagates the signal initiated by a hormone or neurotransmitter binding to a plasma membrane receptor. It is synthesized from ATP by adenylyl cyclase. Its main role is to activate cAMP-dependent kinase (A-kinase or PKA), which phosphorylates serine and threonine residues on target proteins.
Another important second messenger is IP3, which is generated along with diacylglycerol (DAG) when phospholipase C (PLC) hydrolyzes phosphatidylinositol bisphosphate. IP3 plays a role in the release of calcium ions (Ca2+) from intracellular stores, thus further propagating the cell signal.