BIO 211 Lecture Notes - Lecture 20: Codocyte, Cardiac Muscle, Cell Membrane
1 Dec 2020
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Document Summary
Hormones stimulate only those cells that have receptors for them. On cell plasma membrane, in the cytoplasm, or in the nucleus. Receptors act like switches turning on metabolic pathways when hormone binds to them. Usually each target cell has a few thousand receptors for a given hormone. Saturated when all receptor molecules are occupied by hormone molecules. Thyroid hormone enters target cell by diffusion mostly as t4 with little metabolic effect. Within target cell, t4 is converted to more potent t3. T3 enters target cells and binds to receptors in chromatin activating gene transcription. Make a muscle protein (myosin) enhancing cardiac muscle response to sympathetic stimulation. One hormone molecule can trigger the synthesis of many enzyme molecules. Very small stimulus can produce very large effect. Target cell sensitivity adjusted by changing the number of receptors. Up-regulation means number of receptors is increased: Sensitivity is increased e. g. uterus produces many receptors for oxytocin.
