B. Signal Transduction Pathway (cell signaling) 1. Process by which a signal on a cell surface is converted into a specific cellular response Multistep process Relies on plasma membrane proteins as receptors 2. Three stages: Reception Transduction response

1. Reception- Target cell’s detection of a signal coming from outside the cell “Detection “– binding of a signal (ligand) to a cell protein (receptor) at the cell’s surface Binding causes “conformational change” in receptor Receptor becomes activated to work with another molecule 3 types of receptors (G-protein receptors, ion channel receptors, kinase receptors

2. Transduction – converts the signal to a form that can bring about a specific cellular response Usually involves a series of changes in a sequence of molecules relay molecules - protein kinases that “phosphorylate” the next molecule “cascade of phosphorylation” activates proteins in a step-by-step manner Secondary messengers – small, non-protein, water soluble molecules that can readily spread through the cell by diffusion- protein kinases (Ex. cAMP, cGMP, Ca+ ions)

3. Response – two ways 1. Can regulate activities in the cytoplasm i.e. enzyme activation 2. Can activate specific genes by altering transcription factors