Final answer:
A second messenger is a substance within a cell that propagates a signal initiated when a first messenger, such as a hormone, binds to a receptor on the cell surface.
Step-by-step explanation:
A second messenger is a small, non-protein molecule that propagates a signal within a cell after activation of a receptor causes its release. It is not the ligand itself but a substance that is involved in the signaling pathway once the first messenger (usually a hormone) binds to a receptor. One well-known second messenger is cyclic AMP (cAMP), which is synthesized from ATP and plays a crucial role in activating enzymes like cAMP-dependent kinase (A-kinase) that alter cellular functions.
A second messenger is a small molecule that propagates a signal within the cell after the activation of a receptor causes its release. These molecules help spread the signal through the cytoplasm by altering the behavior of certain cellular proteins. They are typically produced in response to the binding of a signaling molecule to a receptor protein on a target cell.