ABSTRACT
Cell signaling In multicellular organisms, there is a need for the cells to communicate with one another in order to coordinate their growth and metabolism. The principal way by which cells communicate with each other is by means of extracellular signaling molecules or hormones. These molecules are synthesized and secreted by signaling cells and produce a specific response in target cells that have specific receptors for the signaling molecule. Different cells can respond differently to the same signaling molecule depending on the type of receptor and the intracellular reactions initiated (see below). Cell signaling can be classified into three distinct types based on the distance over which the signaling molecule acts. In endocrine signaling, the signaling molecule (e.g. insulin) acts on target
cells distant from its site of synthesis in cells of an endocrine organ (e.g. the pancreas; Figure 1a). The endocrine cells secrete the signaling molecule into the bloodstream (if an animal) or the sap (if a plant), which carries it to the target cells elsewhere in the organism. In paracrine signaling, the signaling molecule affects only target cells close to the cell from which it was secreted (Figure 1b). The communication from one nerve cell to another by chemical neurotransmitters is an example of paracrine signaling (Section E6). The third type of cell signaling is autocrine signaling, where a cell responds to a molecule that it has produced itself (Figure 1c).
