Chapter 19
Critical Thinking Exercise
Electrochemistry and the Quest for Clean Energy
When a neuron is not sending a signal, it is said to be "at rest." When the neuron is at rest,
the movement of ions across the membrane is at equilibrium. The electrochemical gradient force
balances the concentration gradient driving force. At this point the inside of the neuronal
membrane is negatively charged relative to the outside. The difference in voltage between the
outside and the inside of the neuron is the resting membrane potential. If the membrane is
permeable to a single ion, the Nernst equation can be modified to
Eion 
RT [ X ]o 26.72 [ X ]o
ln

ln
in mV
zF [ X ]i
z
[ X ]i
Eion = ionic equilbrium potential
z = charge on the ion
R= 8.314 J/K-mol
T= 37C = 310. K
F = 96, 485 C/mol
[X]o = extracellular concentration of the ion (M)
[X]i = intracellular concentration of the ion (M)
Given the following table of ion concentrations for a neuron, answer the following questions.
Ion
K+
Na+
Cl1.
What is Eion for K+?
a. +62 mV
b. +80. mV
[X]i, mM
100
15
13
[X]o, mM
5
150
150
c. -80. mV
d. +65 mV
e. -65 mV
2. What is Eion for Na+?
a. +62 mV
b. -62 mV
c. -80. mV
d. +123 mV
e. +246 mV
3. What is Eion for Cl-?
a. -65 mV
b. +65 mV
c. -73 mV
d. +73 mV
e. +80. mV
4. What happens to Eion if the concentration of the ion increases outside the cell?
a. increases
b. decreases
c. remains unchanged
Chapter 19
5. During the firing of a neuron, sodium ions flow into the neuron. What is the concentration of
sodium ions inside if the voltage rises to +35 mV and [Na+]o is 112 mM?
a. 21 mM
b. 24 mM
c. 30. mM
d. 32 mM
e. 40. mM
The neuronal membrane allows passage of more than one ion. The Nernst equation must be
modified to include the permeability of sodium and potassium ions. The new equation is
PK
[K  ]o  [Na ]o
26.72 PNa
Em 
ln
in mV
PK
z


[K ]i  [Na ]i
PNa
PK/PNa = 40 = permeability ratio of K+ to Na+
6. What is the resting membrane potential?
a. -80. mV
b. -73 mV
c. -65 mV
d. +62 mV
e. +123 mV
7. What ion has an Eion that matches the resting membrane potential?
a. Na+
b. K+
c. Ca2+
d. Cl8. Some diseases and drugs increase the extracellular concentration of potassium ions. What is
Eion if the concentration becomes 10. mM?
a. +62 mV
b. -62 mV
c. -80. mV
d. +123 mV
e. +246 mV
9. What is the resting membrane potential?
a. -73 mV
b. +73 mV
c. -84 mV
d. -52 mV
10. Does this increase or decrease the excitability of the neuron?
a. increase
e. +52 mV
b. decrease
2