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test_4_review - Test 4 Review Signal Transduction Ligands...

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Test 4 Review Signal Transduction: Ligands and Receptors The activities of cells in multicellular organisms are influenced by external signals. They coordinate their metabolism and development by cell-to-cell signaling Cell-cell contact signals- important during development and in the immune system Chemical signaling- includes hormone, local mediators, and neuron transmitters Extracellular signaling molecules are recognized by receptors on the surface of or inside the target cell. The signals are those receptors Ligands. Many cell surface receptors start cascades of intracellular signals when bound. The receptors transduce an extracellular signal into one or more intracellular signal. Signaling pathways amplify the original signals Receptor levels are regulated by synthesis. Each cell makes receptors to just a fraction of the available Ligands. Limits the signals a cell can receive to those appropriate to its type. A cell that makes a receptor can control the number of copies on its surface. Ligand binding can down-regulate the number of receptors (EGF). EGF stimulates proliferation of cells that have a PM EGF receptor. Proliferation is limited by ligand- induced degradation of EGF receptors. Empty receptors are spread randomly across the PM. EGF binding causes the receptor to signal to the cell to divide. Also causes a conformation change in the receptor that exposes an endocytosis signal. Both receptor and ligand are taken up in coated pits and degraded in lysosomes. Ligand levels are regulated by synthesis, secreteion, or degradation. Endocrine signaling- hormones are released by cells distant from target cells and carried by the blood (long range) Paracrine singaling- takes place between cells that are near each other (short range). Local mediators are released that do not enter general circulation. Neurotransmission- an extreme example of paracrine signaling. Neurotransmitters are released from nerve terminals and travel across the synaptic cleft to bind receptors on target cells Autocrine signaling- cells respond to their own signals. Often growth factors stimulate the proliferation of the cells that make them. Tumor cells Molecules from outside the body can bind signal receptors to produce toxic or therapeutic effects. Antipsychotic drugs that bind to neurotransmitter receptors in the brain Some signals done by cell-cell contact. Molecules on PM of some cells are ligands for receptors on adjacent cells. Cell surface ligands have receptors on the surfaces of their target cells. Hydrophobic ligands can cross the PM of a target cell and bind to intracellular receptors in the cytosol or nucleus. Often regulate the transcription of genes. Includes steroid hormones (cortisol) and dissolved NO. Hydrophillic ligands- cannot cross the PM and binds to cell surface receptors. Include insulin, EGF, and small charged molecules (adrenaline and acetylcholine) Intracellular receptors: cortisol is made in the cortex of the adrenal glands in response to stress. Diffuses through the PM of target cells and binds to the glucocorticoid
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