Page 373 - Physical chemistry understanding our chemical world

340 ELECTROCHEMISTRY

Cell membrane (‘axon’)
Cell exterior to separate the cell interior and exterior

Cell interior

Ions leaving Ions entering cell
cell interior interior from exterior
Figure 7.16 Schematic diagram showing a portion of a cell, the membrane (‘axon’) and the way
ions diffuse across the axon

Table 7.12 Concentrations of ions inside and outside nerve components
[Na ]/mol dm −3 [K ]/mol dm −3 [Cl ]/mol dm −3
+
+
−
Inside the axon 0.05 0.40 0.04–0.1
Outside the axon 0.46 0.01 0.054
Source: J. Koryta, Ions, Electrodes and Membranes, Wiley, Chichester, 1991, p. 172.

differences in Table 7.12. The data in Table 7.12 refer to the nerves
In this context, per- of a squid (a member of the cephalopod family) data for other
meable indicates that species show a similar trend, with massive differences in ionic
ions or molecules can concentrations between the inside and outside of the axon. These
pass through the mem- differences, together with the exact extent to which the axon mem-
brane. Themodeof
brane is selectively permeable to ions, determines the magnitude
movement is probably of the potential at the cell surface.
diffusion or migration.
The membrane encapsulating the axon is semi-permeable, there-
by allowing the transfer of ionic material into and out from the
axon. Since the cell encapsulates fluid and also floats in a fluid,
Some texts give the we say the membrane represents a ‘liquid junction’. A potential
name ‘diffusion poten- forms across the membrane in response to this movement of ions
tial’ to E j . across the membrane, which we call a ‘junction potential’ E j .If
left unchecked, ionic movement across the membrane would occur
until mixing was intimate and the two solutions were identical.
For a nerve to transmit a ‘message’ along a nerve fibre, ions traverse the axons and
transiently changing the sign of the potential across the membrane, as represented
schematically in Figure 7.17. We call this new voltage an action potential, to differ-
entiate it from the rest potential. To effect this change in potential, potassium cations