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Beng 130 Handout-6 - Membrane Transport Transport across...

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Membrane Transport: Transport across, but not through, membranes Endocytosis Phagocyosis (cell eating) Pinocytosis (cell drinking) Exocytosis – i.e. release of neurotransmitters from presynaptic nerve endings Fusion of membrane vesicles Transport through membranes Diffusion Osmosis Protein mediated transport Facilitated transport Active transport (i.e. Na+/K+ pump) Membrane Potential: A membrane separates aqueous solutions in two chambers (A and B). The molecule X is at a higher concentration on side A than side B. Therefore, X tends to diffuse towards side B. Now consider the case in which A is negative wrt B and our molecule X is now an ion, X+. This electrical potential difference across the membrane will cause X+ to migrate toward A. The direction of net X+ movement depends on whether the effect of the concentration difference or the effect of the electrical potential difference is greater. We compare these two forces using the electrochemical potential The electrochemical potential μ of an ion allows us to compare the relative contributions of ionic concentration and electrical potential to the movement of the ion: ( ) ( ) ( ) [ ] [ ] ( ) B A B A B A E E zF X X RT X X X - + ° ° ± ² ³ ³ ´ µ = - = Δ + + + + + ln μ μ μ When ( ) + Δ X μ =0 we get the familiar Nernst potential: ( ) ( ) ( ) ° ° ± ² ³ ³ ´ µ - = - + + B A B A X X RT E E zF ln Gibbs-Donnan Equilibrium:
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