Final answer:
Ligands are molecules that activate receptors on the surface of a cell.
Step-by-step explanation:
Receptors on the surface of a cell that are activated by molecules are called ligands. Ligands are specific molecules that bind to receptors and trigger a response in the cell. They can be proteins, ions, hormones, neurotransmitters, or other types of signaling molecules. When a ligand binds to its specific receptor, it initiates a series of biochemical events inside the cell, leading to various cellular responses.