Ligands: These are substances that form complexes with biomolecules to serve biological purposes. In this context, they're like "keys" for receptor "locks."
Signal Transduction:This is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases.
G Protein-Coupled Receptors (GPCRs): These are large family of signal-receiving molecules found in eukaryotes.