Problem Set 4

1)                  Draw the following structures:

cis-9-docenoic acid


Saturated FA w/ Tm < 30oC

(any saturated FA with 9 or fewer carbons)


2)                  Membrane lipids from 4.74*109 erythrocytes form a monolayer of 0.89 m2 in water. The surface area of an average erythrocyte is ~ 100 mm2. Show that the membrane is a bilayer.


The surface are of this many erythrocytes works out to:

This is approximately the measured area, implying that the membrane of each erythrocyte must contain two layers of lipid.


3)                  How many moles of K+ will diffuse across the erythrocyte membrane in one minute under the following conditions:

[K+]in = 100 mM

[K+]out = 15 mM

Surface Area of erythrocyte: 100 mm2 = 100*10-12 m2

Permeability P of K+ = 2.4*10-10 cm-sec-1


Put everything in terms of centimeters (i.e. convert mols/liter to mols/cm3, m2 to cm2, etc.)



4)                  If the erythrocyte volume is 100 mm3, what percent of the internal K+ willl leak out in 1 minute?


Calculate the total amount of cellular K+:

What percent of this amount is represented by the answer to (6)?


5)                  A membrane has [NaCl] = 0.1 M on the right side and 0.01 M on the left side at a temperature of 37oC. What is the membrane potential when the membrane is permeable to:

a.       Na+?


b.      Cl-?


c.       Both?

The two ion fluxes offset completely; the net potential difference is zero.



6)                  In the nerve cells from a Roswell alien, you find the following cellular and extracellular monovalent ion concentrations:







55 mM

312 mM

88 mM


310 mM

80 mM

250 mM


Relative permeabilities:









Whats the membrane potential at equilibrium?