Cell signaling involves communication between cells through extracellular signaling molecules that bind to receptor proteins on the cell surface or inside the cell. These receptors activate intracellular signaling pathways that lead to changes in target proteins and cell behavior. The major types of cellular signaling pathways are those using membrane receptors, including ion channel-linked receptors, enzyme-linked receptors, and G protein-coupled receptors, which activate intracellular signaling proteins upon ligand binding.

43. G Protein-Linked Receptors

48. Three Stages of Signal Transduction