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Membrane transport mechanisms

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Western University
Physiology 3120
Tom Stavraky

Human Physiology Wednesday, September 16, 2009 “EC II” Membrane Transport Mechanisms • Diffusion through the cell membrane  Substance must be nonpolar/lipid soluble (oxygen, carbon dioxides, large alcohols)  Must be able to get through the inner portion of the bilayer  Driving force is the concentration gradient  Fick’s First Law of Diffusion predicts rate:  See equation in the notes  Negative sign because the concentration gradient produces a negative slope, so we need the negative sign to make the rate of diffusion positive  Rate is proportional to... • Area for diffusion • Temperature • Concentration gradient • Inverse of the molecular radius • Inverse of the viscosity of the medium • Diffusion through protein channels/pores  Water uses special pores called aquaporins  Polar molecules diffuse through protein channels  Integral transmembrane proteins  Factors affecting rate of movement: 1. Size of the molecule (<0.8nm)* 2. Charge (protein select for one charge over the other)* 3. Electrochemical gradient (move down both concentration & electrical gradient) 4. Pressure gradient (increases the kinetic energy of molecules) 5. Hydration energy* • Hille’s theory of closest fit (hydration shell must be strip
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