Page 372 - Physical chemistry understanding our chemical world

TRANSPORT PHENOMENA 339

If we say that a 1 is the analyte of known concentra- Care: we have assu-
tion (i.e. on the inside of the bulb), then the last term in
med here that the
the equation is a constant. If we call the term associated activities and concen-
with a 2 ‘K’, then we obtain trations of the solvated
protons are the same.
2.303RT
emf = K + log a 2
10
F
If a 2 relates to the acidic solution of unknown con- This derivation is based
centration then we can substitute for ‘log a 2 ’, by say- on the Nernst equation
10
ing that pH =− log [H ], so: written in terms of
+
10
ionic activities, but pH
2.303RT is usually discussed in
emf = K + ×−pH (7.52)
F terms of concentration.
which is the same as Equation (7.49)

7.6 Transport phenomena

How do nerve cells work?

Ionic transport across membranes

The brain relays information around the body by means of nerves, allowing us to
register pain, to think, or to instruct the legs to walk and hands to grip. Although the
way nerves operate is far from straightforward, it is nevertheless clear that the nerve
pathways conduct charge around the body, with the charged particles (electrons and
ions) acting as the brain’s principal messengers between the brain and body.
The brain does not send a continuous current through the nerve, but short ‘spurts’.
We call them impulses, which transfer between nerve fibres within the synapses of
cells (see Figure 7.16). The cell floats within an ionic solution called plasma. The
membrane separating the synapse from the solution with which the nerve fibre is in
contact surrounding the cell is the axon, and is essential to the nerve’s operation.
The charge on the inside of a cell is negative with respect to
the surrounding solution. A potential difference of about −70 mV No potential difference
forms across the axon (cell membrane) when the cell is ‘at rest’, i.e. forms along the mem-
before passing an impulse – we sometimes call it a rest potential, brane surface, only
which is caused ultimately by differences in concentration either across it.
side of the axon (membrane).
Movement of charge across the membrane causes the potential to change. A huge
difference in concentration is seen in composition between the inside of the axon
and the remainder of the nerve structure. For example, consider the compositional