Study Guide for Exam 4 Chapter 11 Cell Communication Cells can signal to each other and interpret the signals they receive from other cells and the environment the signals may include light and touch but MOST often they are chemicals o A chemical produced in one cell that can affect the function of another cell Cells in a multicellular organism usually communicate via signaling molecules targeted for cells that may or may not be immediately adjacent Two general types of cellular communication o Local Signaling animal cells may communicate by direct contact or by releasing molecules that contact nearby cells o Long distance signaling plants and animals use chemicals called hormones Types of Local Signaling o Direct Contact Cell Junction both animals and plants have cell junctions that allow molecules to pass readily between adjacent cells without crossing plasma membranes Cell Cell Recognition two cells in an animal may communicate by interactions between molecules protruding from their surfaces Important in embryonic development and the immune response o Paracrine Signaling a secreting cell acts on nearby target cells by secreting molecules of a local regulator i e Growth factors Growth factors are compounds that stimulate nearby target cells to grow and divide o Synaptic Signaling a nerve cell releases neurotransmitter molecules into a synapse stimulating the target cell such as a muscle or nerve cell Occurs in animal nervous system Synapse the narrow space between nerve cells and its target cell o Hormonal Signaling aka Endocrine Signaling in Animals specialized cells release hormone molecules which travel via the circulatory system to other parts of the body where they reach target cells that can recognize and respond to the hormones Plant hormones sometimes travel in vessels but more often reach their targets by moving through cells or by diffusing through the air as gas Types of Long distance Signaling Earl W Sutherland is responsible for our current understanding of how chemical messengers act via signal transduction pathways He discovered how the hormone epinephrine acts on cells Three Stages of Cell Signaling o 1st Stage Reception the target cell s detection of a signaling molecule coming from outside the cell A chemical signal is detected when the signaling molecule binds to a receptor protein located at the cell s surface or inside the cell o 2nd Stage Transduction the binding of the signaling molecule alters the receptor and initiates a signal transduction pathway transduction often occurs in a series of steps o 3rd Stage Response the transduced signal finally triggers a specific cellular Reception A signaling molecule binds to a receptor protein causing it to change response shape o A receptor protein on or in the target cell allows the cell to hear the signal and respond to it The signaling molecule is complementary in shape to a specific site on the receptor and attaches there o The signal molecule acts as a ligand the terms for a molecule that specifically binds to another molecule often a larger one o Ligand binding generally causes a receptor protein to undergo a change in shape the shape change directly activates the receptor enabling it to interact with other cellular molecules A shape change in a receptor is often the initial transduction of the signal o Most signal receptors are plasma membrane proteins but others are located inside the cell 3 types of Receptors in the Plasma Membrane o Most water soluble signaling molecules bind to specific sites on transmembrane receptor proteins that transmit info from the extracellular environment to the inside of the cell o G protein coupled receptors GPCR s a cell surface transmembrane receptor that works with the help of a G protein G protein a GTP binding protein that relays signals from a GPCR to other signal transduction proteins inside the cell These receptors vary in the binding sites for their ligands and also for different types of G proteins inside the cell When GDP is bound to the G protein the G protein is inactive When the appropriate signaling molecule binds to the receptor it is activated and changes shape Its cytoplasmic side then binds an inactive G protein causing a GTP to displace the GDP activating the G protein The activated G protein dissociates from the receptor diffuses along the membrane and then binds to an enzyme altering its shape and activity Once the enzyme is activated it can trigger the next step to a cellular response Binding of ligands is reversible The changes in the enzyme and G protein are temporary because the G protein also functions as a GTPase enzyme in other words it hydrolyzes its bound GTP to GDP and Pi Intracellular Receptor o Receptor Tyrosine Kinases RTK The cytoplasmic part of which can catalyze the transfer of a phosphate group from ATP to a tyrosine on another protein RTKs often respond to the binding of a signaling molecule by dimerizing and then phosphorylating a tyrosine on the cytoplasmic portion of the other receptor in the dimer o Ion Channel Receptors Contains a pore that opens or closes as it changes shape in response to a ligand allowing or blocking the flow of specific ions o Intracellular proteins are found in either the cytoplasm or nucleus of target cells To reach such a receptor a signaling molecule passes through the target cell s plasma membrane o Only signaling molecules that are hydrophobic enough or small enough to cross the hydrophobic interior of the membrane reach the intracellular receptors Steroid hormones and Thyroid hormones of animals are hydrophobic Nitric oxide NO has small enough molecules to pass through the chemical messengers membrane o Once a hormone has entered a cell it may bind to an intracellular receptor The binding changes the receptor into a hormone receptor complex that is able to cause a response in many cases turning on or off of particular genes o How does the activated hormone receptor complex turn on genes By acting as a transcription factor o By acting as a transcription factor the hormone receptor complex carries out the transduction part of the signaling pathway Signal Transduction Pathways The binding of a signaling molecule to a receptor triggers the first step in a chain of molecular interactions that leads to a particular Transduction Cascades of molecular interactions relay signals from receptors to response within the cell target molecules in the cell o When receptors for signaling molecules are plasma membrane
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