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.