Examples in Biophysics
1. Classic biophysical studies by Hodgkin and Huxley explored the electrical properties of
the giant squid (Loligo pealei) axon. In these studies, the current density (Jm) of K+
(potassium ions) vs potential difference (Vm) across the axon cell membrane was
measured, producing the following data points:
Vm
(mV)
Jm
(mA/cm2)
-50 0.00
-40 +0.15
-30 +0.30
-25 +0.70
-12 +1.30
-3 +1.85
+5 +2.30
+25 +3.55
+40 +4.50
a. Is the surface membrane of the squid axon Ohmic (in whole or in part)? Explain.
b. Estimate the slope of Jm vs V. This is the K+ conductance per unit area, called the
specific conductance.
c. Estimate the resistance per unit area (or the specific resistance).
d. Estimate the drift velocity at +25 mV. Assume the charge carrier density (n) is 6.0x1028
m-3 and charge carriers are monovalent cations (i.e. q = +e).
