Second messenger

In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay signals within a cell. They are synthesized or released by specific enzymatic reactions, usually as a result of an external signal that was received by a transmembrane receptor and pre-processed by other membrane-associated proteins. There are three basic types of second messenger molecules: These intracellular messengers have some properties in common:
 * Hydrophobic molecules like diacylglycerol, InsP3 and phosphatidylinositols are membrane-associated and diffuse from the plasma membrane into the juxtamembrane space where they can reach and regulate membrane-associated effector proteins.
 * Hydrophilic molecules are water-soluble molecules, like cAMP, cGMP, and Ca2+, that are located within the cytosol.
 * gases, nitric oxide (NO) and carbon monoxide (CO), that can diffuse both through cytosol and across cellular membranes.
 * They can be synthesized/released and broken down again in specific reactions by enzymes.
 * Some (like Ca2+) can be stored in special organelles and quickly released when needed.
 * Their production/release and destruction can be localized, enabling the cell to limit space and time of signal activity.