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Unformatted text preview: I.B.4. Two forces will act on ions. I.B.5. The chemical and electrical forces cause ions to flow across the CM. As a result of this ion movement, a charge difference (voltage) is created across the CM. Introduction to three beaker picture (next page) The membrane in the middle of the beaker is only permeable to potassium; i.e. it only allows potassium ions to flow through it The left side of the beaker is like the inside of the cell The potassium concentration approximates that seen in a typical neuron The large font (K+A-) represents a high concentration of potassium acetate Circles show a few of the ions in cartoon form A voltmeter (a device used to measure the voltage difference across the artificial membrane) is shown above each beaker In the figure, we are looking at three beakers at three different times as the voltage across the artificial membrane changes. Time 1) Initially, we add salt in the form of potassium acetate to both sides of the beaker and record the voltage across the artificial membrane before any ions can move. Time 2) In stage two, we look at a snapshot across the artificial membrane at an intermediate time after potassium ions are allowed to flow across the membrane. Notice the voltmeter now detects a voltage across the artificial membrane. Time 3) In the final stage, the voltage recorded by the voltmeter stabilizes at an equilibrium voltage of -83 mV. At this point, the voltage no longer changes across the membrane even though it is still permeable to potassium. Now lets use this overhead to write the size and direction of the chemical and electrical forces acting on the potassium ions at each time stage. Then we can use vector addition of the two forces to calculate the net force acting on the potassium ions. Arrows will represent the direction of forces. I.B.4. Two forces will act on ions Rules for determining the directions and relative strengths of the chemical and electrical forces acting on ions (i.e. the numbers associated with the arrows you drew in class). Also See Useful Rules and Useful Equations on course hyperlinks Rule 1a) The direction of the chemical force is from areas of high concentration of the ion you are interested in to areas of low concentration of that ion. Rule 1b) The chemical diffusion force acting on an ion only depends on the concentration difference of that ion across the cell membrane and the temperature. The chemical force acting on an ion can be approximated by using the following equation: (note: this is a modification of the Nernst equation). Equation 1) |F chemical | T | (ln [I out ]/[I in ])| = RT/F x |(ln [I out ]/[I in ])| which at 17 o C = 57.5 x |(log [I out ]/[I in ])|....
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- Fall '08