Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
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– mV +
voltmeter
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– mV +
plasma
membrane
of axon
electrode
potassium chloride
solution
fine glass capillary
portion
of axon
334
1 Before touching the axon, the two electrodes
2
are at the same electrical potential, so the
voltmeter shows a potential difference of zero.
Figure 15.9 Measuring the resting potential of an axon.
The membrane has protein channels for potassium
and for sodium which are open all the time. There are far
more of these for potassium than for sodium. Therefore,
some potassium diffuses back out again much faster than
sodium diffuses back in. In addition, there are many large,
negatively charged molecules inside the cell that attract
the potassium ions reducing the chance that they will
diffuse out. The result of these effects is an overall excess of
When one electrode is pushed inside the axon, the voltmeter
shows that there is a potential difference between the inside
and outside of about –70 mV inside with respect to outside.
negative ions inside the membrane compared with outside.
The membrane is relatively impermeable to sodium
ions but there are two things that influence the inward
movement of sodium ions during an action potential.
There is a steep concentration gradient, and also the inside
of the membrane is negatively charged, which attracts
positively charged ions. A ‘double’ gradient like this is
known as an electrochemical gradient.
a
cell surface membrane of axon
b
Sodium ions are constantly pumped out and potassium ions in.
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– –
– –
– –
–
–
+ –
– –– +
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cytoplasm of axon Na + K +
ATP
Na + –K + pump protein
ADP + P i
The inside of the axon is negatively charged in comparison
with the outside. The difference is about –70 mV.
axon cytoplasm
low Na +
high K +
tissue fluid
high Na +
low K +
Figure 15.10 a At rest, an axon has negative electrical potential inside. b The sodium–potassium pump maintains the resting
potential Sodium ions by keeping are constantly more sodium pumped ions out outside and potassium than there ions are in. potassium ions inside.
b
Na +
Na + –K + pump protein