cells respond to chemical messages produced by other cells or from the environment cell signaling begins when the chemical message or lag end is received by a target cell ligands can be peptides proteins steroids or small chemicals ligands bind to specific cell structures called receptors some receptors are found in the cell on the nuclear membrane while most receptors are embedded in the cell membrane [Music] receptors have several key domains or regions the lag and binding domain is extracellular and has a chemical structure that is specific to the ligand the transmembrane domain anchors the protein into the plasma membrane some transmembrane domains actually have channels for transport in a gated ion channel the binding of the ligand regulates the opening or closing of a transmembrane channel finally membrane receptors have a very important intracellular domain when a ligand binds to a receptor it causes a conformational change in the intracellular domain in other words a shape change which alters the function of the domain proteins one important example of a membrane receptor in eukaryotes are g-protein-coupled receptors g-protein-coupled receptors have an intracellular domain associated with a group of molecules called g-proteins when the lag end binds a conformational change in the receptor causes the g proteins to break off or dissociate this is the start of a signal transduction pathway signal transduction describes the series of chemical events that take place from lag and binding to an actual response by the cell let's stick with this g-protein example to follow a typical series of events in a single transduction pathway the dissociated g proteins can interact with and activate other molecules such as channels or enzymes here the activated enzyme begins the mass production of molecules that will be involved with activating the cellular response while the ligand is the original message the molecules mass produced by the enzyme are called second messengers because the binding of a single lag end message results in the production of many second messengers this step is called signal amplification one of the most common second messengers is a molecule called cyclic amp [Music] the next step in signal transduction is the phosphorylation cascade phosphorylation describes the addition of phosphate in biology it's really important to understand that adding or removing phosphate results in shape change this shape change can activate or deactivate a molecule cyclic amp activates molecules called protein kinases which literally have the job of transferring phosphate groups in the cascade kinases transfer phosphate groups from one molecule to the next to the next activating and deactivating proteins along the way like a relay race in fact kinases are often called relay molecules in the signal transduction pathway eventually the phosphorylation cascade reaches a target protein [Music] when this protein is phosphorylated a specific cellular response is achieved examples of target proteins include enzymes that control important metabolic processes and transcription factors that regulate gene expression [Music] interpreting the final response of a signal transduction pathway can be tricky but it's all about understanding how the final target protein is affected and what the function of that target protein is [Music] from ligand binding to cellular response signal transduction is a sequential series of cellular events disruption at any point in the pathway can prevent downstream events and alter cellular response [Music]