What are the functions of a receptor?

• A. To destroy ligands
• B. To transport drugs across membranes
• C. To generate energy for the cell
• D. Recognizing and binding to their ligand and, upon binding, propagating its regulatory signal into the target cell

Answer: (D)

Receptors are specialized proteins that detect extracellular signals by binding specific ligands and then translate that binding into an intracellular response. The essential idea is recognition and binding specificity, followed by a change in the receptor that activates a signaling process inside the cell. This signal transduction can involve second messengers, opening or closing ion channels, or triggering kinase cascades that alter metabolism, gene expression, or cell behavior. For example, a G protein–coupled receptor binds a hormone like epinephrine, undergoes a conformational shift, activates a G protein, and starts a cascade that changes cellular activity. In other receptor families, such as receptor tyrosine kinases, ligand binding leads to receptor dimerization and autophosphorylation, again driving intracellular signaling. The other options describe roles more typical of enzymes (destroying ligands), transporters (moving substances across membranes), or mitochondria (generating energy), which are not the primary functions of receptors. So the described function—recognizing and binding to their ligand and, upon binding, propagating its regulatory signal into the target cell—best captures what receptors do.