Measured permeabilities of plain phospholipid bilayers are: water, ; urea, ; glucose,…

PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Macromolecules, membranes and cell architecture · Molecular machines, transport and bioenergetics

Life at Thermal Energy, Ch The membrane: barrier, capacitor and pump

Problem

Measured permeabilities of plain phospholipid bilayers are: water, Ps=2.8×10−5 m s−1P_{\rm s} = 2.8\times10^{-5}\,\mathrm{m\,s^{-1}}; urea, 4.2×10−8 m s−14.2\times10^{-8}\,\mathrm{m\,s^{-1}}; glucose, 3.0×10−13 m s−13.0\times10^{-13}\,\mathrm{m\,s^{-1}}; sodium ion, 2.1×10−15 m s−12.1\times10^{-15}\,\mathrm{m\,s^{-1}}. ((a)) A patch of membrane of area 1.0 μm21.0\,\mathrm{\mu m^2} separates glucose at 5.0 mM5.0\,\mathrm{mM} inside from 0.5 mM0.5\,\mathrm{mM} outside. Using the glucose permeability above, find the rate at which glucose molecules cross this patch, in molecules per second. ((b)) By what factor is water's permeability larger than the sodium ion's? Give the answer as a number of decades (powers of ten). ((c)) Explain briefly, using the ideas of this section, why the sodium ion's permeability is so many decades below water's.

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