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BIO 317 - Lec 3 - BIO 317 Lecture 3 Prof William Collins...

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1 Prof. William Collins Office: 534 Life Sciences Building Office Hours: Mondays, 4:00 – 5:00 PM or by appointment Recommended Reading: Matthews, Chapters 4 & 5 BIO 317: Lecture 3 Thought Question 1.0 M NaCl 0.1 M NaCl Na + Na + Cl - Cl - Permeable Membrane Assume there are two aqueous compartments with different concentrations of NaCl and separated by a membrane equally permeable to both Na + and Cl - . 1.What will happen to the concentrations of Na + and Cl - in each compartment? 2.Will there be a difference in the electrical potential between the two compartments (i.e., a membrane potential)? Hint: Cl - diffuses through water faster than Na + . 2 1
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2 Diffusion Potential Both Na + & Cl - will diffuse down concentration gradients (1 2) Cl - ions move faster in aqueous solutions than Na + Compartment 2 becomes more negative than compartment 1 - called Diffusion Potential This potential difference (E m ) influences diffusion of Na + & Cl - - Impedes diffusion of Cl - - Enhances diffusion of Na + Diffusion Potential stabilizes when potential difference balances the greater mobility of Cl - - Na + & Cl - cross membrane at same rate Diffusion Potential time The Diffusion Potential will continue as long as there is a concentration gradient between compartments 1 and 2. Eventually, the concentrations in compartments 1 and 2 will equilibrate and there will no longer be a difference in electrical potential between the compartments. E 2 is more negative than E 1 E 2 = E 1
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3 Equilibrium Potential • Cl - will diffuse down concentration gradient from 1 2 • However, Na + cannot cross membrane. • The diffusion of Cl - creates a separation of charge such that E 2 is more negative than E 1 • The negative E m opposes the diffusion of Cl - from 1 2 • Two forces : Diffusion: move Cl - from 1 2 E m : move Cl - from 2 1 • Equilibrium is achieved when E m is exactly sufficient to balance the force of diffusion and there is NO NET FLUX of Cl - across membrane E 2 is more negative than E 1 Nernst Equation
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