Wikipedia: Second messenger

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In biology, second messenger 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

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* Hydrophobic molecules like diacylglycerole and phosphatidylinositole?s 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 located within the cytosol.

* Hydrophobic molecules like diacylglycerol? and phosphatidylinositol?s 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.

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 membrane-associated proteins. There are two basic types of second messenger molecules:

Hydrophobic molecules like diacylglycerol? and phosphatidylinositol?s 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.

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.