What best defines a receptor?

• A. A molecule that binds to a receptor and initiates a response
• B. A transporter protein that moves substances across the membrane
• C. The basic unit of a cell that carries genetic information
• D. Structure on the surface of a cell (or inside a cell) that selectively receives and binds a specific substance causing a change or modulating cell function

Answer: (D)

Receptors are proteins (or sometimes RNA) that receive chemical signals by binding a specific ligand and then translate that binding into a cellular response. They have selective binding sites, so a particular substance fits like a key and, when it binds, the receptor changes shape to trigger signaling pathways or alter cell function. This is why the best definition describes a structure on the cell surface or inside the cell that selectively binds a specific substance and causes a change in activity.

The other options describe what the binding event involves rather than what a receptor is: a ligand binding to a receptor that activates a response; a transporter that moves substances across the membrane; and the basic unit of a cell carrying genetic information. Receptors can be on the membrane or intracellular and they mediate various responses, from opening ion channels to changing gene expression.