# Homework Help: Electric field of a cell membrane

1. Jan 23, 2008

### throllen

1. The problem statement, all variables and given/known data

he fluid within a living cell is rich in potassium chloride, while the fluid outside it predominantly contains sodium chloride. The membrane of a resting cell is far more permeable to ions of potassium than sodium, and so there is a transport out of positive ions, leaving the cell interior negative. The result is a voltage of about -63.0mV across the membrane, called the resting potential. If the membrane is 7.85nm thick, and assuming the electric field E across it is constant, determine the magnitude of E.

Assume that the membrane's cytoplasmic (interior of cell) charge can be attributed to the presence of a certain fraction of negatively charged phospholipid molecules, each with a cross-sectional area of 0.550e-9m2. If each negatively charged lipid carries -1.60e-19C, how many such molecules are found in 1.0e-6m^2 of the inner surface of the membrane? What percentage of the membrane's inner surface do these molecules cover?

Q=CV
E=sigma/e0
sigma=Q/a

ok, I got E which is 8.03e6 N/C that's easy... but how do i start from here? I am so lost... I tried using Q=CV, and i got 1410 as the answer for how many of those molecules are in 1e-6 m^2 of membrane, but isn't the % 77.6%? I keep getting it wrong...

2. Jan 27, 2009

### cwoo123456

How did you get the answer for the molecules??