The molecules that convert extracellular signals into intracellular ones are called membrane receptors. Membrane receptors are proteins that span the cell membrane and have a binding site for a specific extracellular signaling molecule, or ligand. When a ligand binds to its receptor, it causes a conformational change in the receptor that triggers a cascade of intracellular events, ultimately leading to a cellular response. Different types of membrane receptors include G protein-coupled receptors, ligand-gated ion channels, and receptor tyrosine kinases.