Receptors are proteins on the surface of cells that are specifically designed to detect and respond to signals from outside the cell. They act as 'gatekeepers,' allowing certain molecules or messages to enter the cell while blocking others.
Signal transduction refers to the process by which a receptor converts an external signal into a biochemical response inside the cell.
Ligands: Ligands are molecules that bind to receptors and activate them. They can be hormones, neurotransmitters, or other signaling molecules.
G protein-coupled receptors (GPCRs): GPCRs are a type of receptor that spans across the cell membrane and interacts with intracellular signaling molecules called G proteins.