Any mechanism by which binding of an extracellular signal molecule to a cell-surface receptor triggers a response inside the cell. The mechanism depends on the type of signal molecule (e.g. hormone, paracrine, or autocrine signals), but it often involves changes in concentration of a second messenger (e.g. cyclic AMP, inositol trisphosphate, calcium ions) within the cell, which in turn can amplify the initiating signal and create a cascade of interactions that affect numerous cell activities. Many receptors are associated with G proteins, which act to turn signal transduction pathways on and off. Other important components of signal transduction include protein kinases, which activate enzymes by transferring a phosphate group from ATP. See also ionotropic receptor; metabotropic receptor.
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