Each cell has the ability to receive and respond to extracellular signaling which ususally follows four distinct steps. First of all, the signaling cell synthesizes and releases the signal molecule which is then transported to the target cell. For activation of the proper response, the signal molecule binds to the correspinding receptor on the surface of the target cell which in turn initiates a very specific signal transduction pathway.
A great variety of agents can work as signaling molecule, for example pathogens, carbon dioxid levels or biosynthezised signal molecules such as hormones. There is a wide variation in their fundamental properties (e.g. solubilty, state of aggregation). Signal molecules that are released by membrane-diffusion, exocytosis or cell damage from the signaling cell are specifically synthezised through a large variety of biosynthetic pathways. Some of these signal rely on activation by proteolytic cleavage or other modifications and others remain at the surface of the signaling cell (juxtacrine signaling).
The recognition of the signaling molecule (ligand) usually occurs by binding to a corresponding receptor. However, some receptor can also respond to physical signals (e.g. light). The initial binding of the ligand/activation of the receptor by a physical signal leads to the tramsmission of the signalinduced by a conformational change of the receptor molecule. While enzymatic receptors have enzymatic activity upon activation, channel receptors show opening or closing activity. Some receptors do not harbor enzymatic or channel activity themselves but are linked to specific enzymes or transporters to transmit the signal. In any case, the activation of the receptor leads to the transmission of the signal and the activation of the signal transduction cascade allowing the propagation of the signal through the cell.