In previous questions you have looked at the way in which insulin interacts with receptors on the surface of the cell to change cellular function. Glucose homeostasis is dependent on having adequate amounts of insulin available, and adequate receptors to interact with the insulin. How does the cell know when insulin has combined with its receptor, and how is that information used to change cell function?

When insulin binds to receptors on target cells (such as skeletal muscle cells and adipocytes), the signal cascade begins, and finally the glucose transporter GLUT4 is transferred  to the cell membrane to allow the uptake of glucose. When insulin binds to the insulin receptor, it triggers the production of signal transduction molecules that are active in multiple intracellular signaling pathways. These molecules can also be described as second messengers (insulin is the first messenger).

"Signaling pathways are complex systems in which a single extracellular signal can elicit multiple intracellular events, some of which may also be triggered by other signaling pathways or may themselves cause other intracellular events. “Second messengers” is a term used to refer to a diverse group of small molecules or ions that transmit the extracellular signal initiated by a molecule (known specifically as a ligand) binding to a cell surface receptor (Links to an external site.) to effector proteins (Links to an external site.) (Links to an external site.) inside the cell. In the resting state, small amounts of second messengers exist in a cell; however, their production can rapidly ramp up once a signal has been received. Once released inside the cell, second messengers bind to their targets and alter their properties, causing a change in the cell’s homeostasis (Links to an external site.) (Links to an external site.) and thus transmitting the message."

A ligand is defined as a substance that forms a complex with a biomolecule to serve a biological purpose.