Second messenger

In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay a signal 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:

Hydrophobic molecules like diacylglycerol 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.

These intracellular messengers have some properties in common:

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.