 Signaling molecules that function within an organism to control metabolic processes within cells, the growth and differentiation of tissues, the synthesis and secretion of proteins, and the composition of intracellular and extracellular fluids.

 Extracellular signaling molecules are synthesized and released by signaling cells and produce a specific response only in target cells that have receptors for the signaling molecules.

 The overall process of converting signals into cellular responses, as well as the individual steps in this process, is termed signal transduction.  Signal transduction pathways may involve relatively few or many components.

 Communication by extracellular signals usually involves the following steps:  (1) synthesis and  (2) release of the signaling molecule by the signaling cell;  (3) transport of the signal to the target cell;  (4) binding of the signal by a specific receptor protein leading to its activation;  (5) initiation of one or more intracellular signal- transduction pathways by the activated receptor;  (6) specific changes in cellular function, metabolism, or development;  (7) removal of the signal, which often terminates the cellular response

 Endocrine, Hormones  Paracrine, Growth factors  Autocrine,  Receptor Proteins Exhibit Ligand-Binding and Effector Specificity.

 Intracellular Signal Transduction

 Many Conserved Intracellular Proteins Function in Signal Transduction

 GTPase Switch Proteins

In mammals, twenty distinct genes for RGS proteins have been identified.

 At last count the human genome encodes 500 protein kinases and 100 different phosphatases.  In some signaling pathways, the receptor itself possesses intrinsic kinase or phosphatase activity; in other pathways, the receptor interacts with cytosolic or membrane associated kinases.

 Hormone binding to receptors coupled to either a Go or a Gq protein (see Table 13-1) induces activation of the isoform of phospholipase C (PLC).

 The IP3-mediated rise in the cytosolic Ca 2+ level is only transient because Ca 2+ ATPases located in the plasma membrane and ER membrane actively pump Ca 2+ from the cytosol to the cell exterior and ER lumen, respectively. Furthermore, within a second of its generation, one specific phosphate on IP3 is hydrolyzed, yielding inositol 1,4-bisphosphate, which does not stimulate Ca 2+ release from the ER.

 The activation of protein kinase C in different cells results in a varied array of cellular responses, indicating that it plays a key role in many aspects of cellular growth and metabolism.  In liver cells, for instance, protein kinase C helps regulate glycogen metabolism by phosphorylating and thus inhibiting glycogen synthase.  Protein kinase C also phosphorylates various transcription factors; depending on the cell type; these induce synthesis of mRNAs that trigger cell proliferation.