Cell signaling is a general term referring to the many and varied processes by which communications controlling cell-level activities are generated, maintained, used, and terminated. A substantial part of an organism's genome generally codes for proteins involved in signaling processes, as cell signaling is involved in everything from the development of an organism to response to various stimuli inside and outside of the organism. To illustrate the importance of proper signaling, defects in a single signaling pathway can, in many organisms, cause developmental problems that lead to infant mortality. Most signaling occurs between the cells of an individual organism, but it can sometimes occur between organisms of different species.
Cell signaling can occur in many different ways. Some takes place within an individual cell with no regard to outside cells. Intercellular communication can occur through juxtacrine signaling, by which two cells in direct contact exchange signals; paracrine signaling, by which cells communicate over relatively short cellular distances; and endocrine signaling, by which cells that are relatively distant from each other communicate. Cells communicate with non-adjacent cells by releasing signaling elements into the nearby cellular environment or into the blood. Signals released into the blood are referred to as hormones and are able to cause significant changes in distant cells.
It is necessary for the generation and proliferation of cell signals to be closely regulated, so signaling pathways are seldom as simple as "cellular signal A causes effect B." Additionally, many signaling processes overlap significantly and many different signaling components may be necessary to generate an effect. Cell signaling processes are, consequently, commonly organized into "signaling pathways" that describe the many different signaling and regulatory components that exist between the initial signal or signals and the final effect or effects. Differences in cell signaling pathways can affect the speed, efficiency, and precision of the signal. This is important, as sometimes a stimulus or signal must be able to induce a powerful effect very rapidly while in other cases it is more important for a stimulus to induce a response that corresponds to the strength of the signal.
There are several different methods by which researchers study cell signaling. Traditional methods are generally focused on identifying and describing particular parts of a cell signaling pathway. One possible method for doing so involves preventing the expression of a gene suspected of involvement in a signaling pathway and observing the effects. Another approach to studying cell signaling pathways involves a biological approach known as systems biology. In systems biology, broad, branching signaling pathways are analyzed, their emergent properties are noted, and the effects of small alterations across the entire system are analyzed.