IonChannelsFinal

Concentrations in different compartments can be

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Concentrations in different compartments can be orders of magnitude different. Proteins (ion channels, transporters) mediate these concentration gradients. Membrane proteins central to huge range of processes – cell signaling, nerve impulses, nutrient transport, etc.
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Membrane permeability Membrane susceptibility to mass transport characterized by a “material parameter” known as the permeability. Notice that the permeability ranges over 16 orders of magnitude.
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Ion channels under external driving force The probability for ion channels of being in open or closed states can be tuned by various external agents: Change of the V across the membrane A mechanical stress Binding of a ligand
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Ion channels Ion channels - transmembrane proteins that mediate the transport of ions in and out of cells Structure of a bacterial K + channel. The channel is made from 4 identical transmembrane subunits (only 2 are shown) which together form a central pore through the membrane
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How We Know: Structure Knowledge of the structures of different membrane proteins gives us a substrate to reason on about how they work. K Channel Nicotinic acetylcholine receptor (Unwin et al.) (Rees et al.) EM & X Ray structures Synchrotron MscL and MscS
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How We Know: Current-Pressure Relationship pA currents lasting several milliseconds. The idea: grab a patch of membrane and apply a potential difference to measure the currents. Fraction of time spent open depends upon magnitude of driving force. (Sukharev et al.)
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The avoidance response revisited This schematic traces the series of processes that occur when a Paramecium is subjected to a mechanical stimulus. (Ralph Greenspan, An Introduction to Nervous Systems)
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Ion channels and the implementation of decisions The particular scheme exploited by Paramecium has several facets involving both mechanosensitive and voltage-gated ion channels. (Ralph Greenspan, An Introduction to Nervous Systems)
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Which Ion species Mediates the Process? The clever experiment involves tuning the extracellular concentrations of the different ion species in turn and then examining the resulting membrane potential.
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