PATHOLOGY

Detailed explanation-1: -In the case of peptide hormones, the first messenger is the hormone itself and the second is a signal molecule within the cell. The hormone binds to a specialized receptor spanning the plasma membrane (transmembrane protein), changing its conformation as to release a compound on the opposite side.

Detailed explanation-2: -G-protein-coupled receptors (GPCRs) are cell surface receptors and are generally assumed to signal to second messengers such as cyclic AMP (cAMP) exclusively from the plasma membrane. However, recent studies indicate that GPCRs can continue signaling to cAMP after internalization together with their agonists.

Detailed explanation-3: -Phospholipase C splits the PIP2 into two smaller molecules that each act as second messengers. One of these messengers is diacylglycerol (DAG), a molecule that remains within the membrane and activates protein kinase C, which phosphorylates substrate proteins in both the plasma membrane and elsewhere.