PCB3134 - Lecture 12 Transmembrane Substance Movement

PCB3134 - Lecture 12 Transmembrane Substance Movement - C...

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Cell membrane has dual function - it retains dissolved materials of cell so they do not leak out into the environment; - it must allow the necessary exchange of materials into and out of the cell.
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The Movement of Substances across Membranes A: simple diffusion through the lipid bilayer B: simple diffusion through an aqueous, protein-lined channel C: diffusion that is facilitated by a protein transporter D: active transport Influx: movement of substance into the cell Efflux: movement of substance out of the cell Net flux: result of influx and efflux
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The Energetics of Solute Movement G = RT ln [C i ]/[C o ] = 2.303 RT log 10 [C i ]/[C o ] Diffusion is a spontaneous process in which a substance moves from a region of high concentration to a region of low concentration, eventually eliminating the concentration difference between the two regions. For uncharged solute (nonelectrolyte): At 25 ° C, G = 1.4 kcal/mol log 10 [C i ]/[C o ] If [C i ]/[C o ] < 1, G is negative, influx is thermodynamically favored; If [C i ]/[C o ] = 1, G is zero, at equilibrium, no net flux; If [C i ]/[C o ] 1, G is positive, efflux is thermodynamically favored.
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The tendency of a charged solute (electrolyte) to diffuse between two compartments depends on two gradients: 1. The chemical gradient, determined by the concentration difference 2. The electric potential gradient, determined by the difference in charge These difference are combined to form an electrochemical gradient. For an electrolyte: G = RT ln [C i ]/[C o ] + zF E m Where z is the charge of the solute, F is the Faraday constant, E m is the potential difference between two compartments. At 25 ° C, G = 1.4 kcal/mol log 10 [C i ]/[C o ] + zF E m
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+ + - - - - - - - - - - - - - - - - - - - + + + + + K + K + K + K + K + K + K + K + K + Na + K + K + K + K + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + Na + If the external [Na + ] is 10 times the internal, the voltage across the membrane of a cell is typically –70 mV, the free-energy change for the movement of a mole of Na + into the cell under these conditions would be G = 1.4 kcal/mol log 10 [C i ]/[C o ] + zF E m = -1.4kcal/mol + (1)(23.06kcal/V mol)(-0.07V) = -3.1kcal/mol PM
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Diffusion of Substances through Membranes For a substance to diffuse passively across a membrane, the membrane must be permeable to the substance. The membrane permeability of the substance depends on the substance’s partition coefficient in a nonpolar solution and water. The greater the lipid solubility a substance has, the
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PCB3134 - Lecture 12 Transmembrane Substance Movement - C...

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