receptor protein on the outside of the plasma membrane and this causes a

Receptor protein on the outside of the plasma

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receptor protein on the outside of the plasma membrane, and this causes a channel to open in the protein, allowing ions into or out of a cell. An ion channel consists of a protein embedded in the plasma membrane, which when opened lets ions (e.g., Ca + and Na + ) flow through by diffusion. These kinds of receptors are particularly important to the nervous system, because the signaling ligands are often released by one nerve, travel across the synapse between nerve cells, and open channels on the next nerve to allow ions in. These ions, in turn, cause the next nerve to “fire” (we’ll see that in another chapter). There are other kinds of ion channels besides those in nerve cells.
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Intracellular Receptors Most extracellular (from outside the cell) signal molecules are water soluble (hydrophilic) and too big or polar to pass through the plasma membrane. Some very small (e.g., nitric oxide, NO) signal molecules or hydrophobic (non- polar) signal molecules (e.g., steroid hormones) can pass through the membrane and reach intracellular protein receptors in the cytoplasm or nucleus. When they reach the receptors, hormone- receptor complexes form, and these act as transcription factors , causing the expression (transcription and translation) of particular genes. E.g. testosterone from testes activates a transcription factor , causing the expression of genes for male sexual characters. E.g., Aldosterone is another steroid, this time secreted by the adrenal gland. It joins a receptor protein to form a hormone-protein complex that acts as a transcription factor causing the expression of genes influencing blood
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Why is there Signal Transduction? The purpose of signal transduction is to link a mechanical or chemical signal from outside a cell to a response within the cell Advantages: 1. Transduction amplifies the signal : One signal molecule can yield millions of response molecules – hence a large response 2. Transduction allows greater control over cellular response : The greater the number of transduction steps, the greater the control over the response
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Transduction often involves a domino effect Each “domino” is a relay molecule, usually a protein. Each interaction changes the next protein’s conformation (shape), causing the information to be passed on Relay molecules
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Transduction: Phosphorylation Conformational (shape) change is often caused by phosphorylation Protein kinases are enzymes that transfer phosphate groups from ATP to other proteins , thereby activating them. Often, protein kinases are thesubstrates in this process (i.e., kinases cause the phosphorylation of other kinases, to activate them). Usually, phosphate groups are added to the amino acids serine or threonine . These are highly polar amino acids, and their phosphorylation causes a major change in their proteins overall structure.
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