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MCB3208Ch6lec - MCB32F08...

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MCB 32 F08 Chapter 6, Membrane transport of solutes, water and ions by diffusion, mediated transport, active transport, secondary active transport, filtration and osmosis. Generation of voltages. Outline Diffusion in solution Permeation across a membrane Water transport: filtration and osmosis Carrier mediated transport: Facilitated diffusion Exchangers Active transport: primary and secondary Ion channels and development of membrane voltages Body fluids As noted in previous lecture, intracellular and extracellular fluids have similar total salt concentrations (about 150 mM salt), but the specific compositions are quite different. Thus, extracellular fluid is high in [Na + ] and low in [K + ] and protein while the intracellular fluid is low in [Na + ], [Cl - ] and [Ca 2+ ] compared to the extracellular fluid. Intracellular fluid also contains about 1 mM ATP, which is found outside cells in much lower amounts. Membrane transport of ions and water The plasma membrane with its lipids and proteins determines what goes into and out of the cell. The movements occur by either diffusion through the lipids or mediated transport through the various specific proteins that are embedded in the membranes. Diffusion : property associated with any dissolved molecule, which has its own kinetic energy and vibrates randomly in solution, leading to the random movement throughout the solution. Diffusion of solutes occurs to equalize the concentration throughout the solution, from high to low concentration. Example : drop of dye at the bottom of a beaker. The rate of diffusional flux from one region to another in solution is directly proportional to the difference in concentration between two regions (C 1 and C 2 ) and the area available for exchange (A) and the diffusion coefficient of the solute (D, inversely proportional to the size of the molecule) and inversely proportional to the distance ( x) between the two points. Net diffusional flux = D x (C 1 – C 2 ) x A x ( x) -1 The rate of diffusional flux of lipid soluble substances like gases across a cell membrane is directly proportional to the difference in concentration between inside and outside the cell (C o and C i ) and the area of the cell membrane available for exchange (A) and the permeability coefficient of the solute (P, directly proportional to the lipid solubility of the molecule). Overall, most important determinants of flux into cells is permeability of the cell membrane to the molecule and the area of membrane. flux across a cell membrane = P x (C o – C i ) x area available for exchange
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Filtration and osmosis are processes determining movements of water across membranes. Filtration involves there being water pressure in one region being higher than in another region, and water flows from high to low pressure. Examples
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