A molecule used to respond to a chemical signal; the signaling molecule binds to it is called?

• A. Receptor
• B. Ligand
• C. Enzyme
• D. Transporter

Answer: (A)

Cells respond to chemical signals through receptors—proteins that detect specific signaling molecules and trigger a response inside the cell. The molecule that receives the signal and initiates the cellular response is the receptor. When a signaling molecule (the ligand) binds to the receptor, the receptor often changes shape and starts a cascade of events that can change enzyme activity, gene expression, or transport in the cell.

Remember that the signaling molecule itself is the ligand, not the receptor, and enzymes or transporters have other roles in the cell and aren’t the primary receptors for most signaling signals.