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Answer The free energy required to transport 1 mol of H� from the interior of the cell,
where [H�] is 10�7
M, across the membrane to where [H�] is 10�1
M is
�Gt � RT ln (C2/C1)
� RT ln (10�1/10�7)
� (8.315 J/mol � K)(310 K) ln 106
� 36 kJ/mol
23. Standard Reduction Potentials The standard reduction potential, E��, of any redox pair is defined
for the half-cell reaction:
Oxidizing agent � n electrons 88n reducing agent
The E�� values for the NAD�/NADH and pyruvate/lactate conjugate redox pairs are �0.32 V and �0.19
V, respectively.
(a) Which redox pair has the greater tendency to lose electrons? Explain.
(b) Which pair is the stronger oxidizing agent? Explain.
(c) Beginning with 1 M concentrations of each reactant and product at pH 7 and 25 �C, in which
direction will the following reaction proceed?
Pyruvate � NADH � H� 88zy88 lactate � NAD�
(d) What is the standard free-energy change (�G��) for the conversion of pyruvate to lactate?
(e) What is the equilibrium constant (K�eq) for this reaction?
Answer
(a) The NAD�/NADH pair is more likely to lose electrons. The equations in Table 13–7 are
written in the direction of reduction (gain of electrons). E�� is positive if the oxidized
member of a conjugate pair has a tendency to accept electrons. E�� is negative if the oxi-
dized member of a conjugate pair does not have a tendency to accept electrons. Both
NAD�/NADH and pyruvate/lactate have negative E�� values. The E�� of NAD�/NADH
(�0.0320 V) is more negative than that for pyruvate/lactate (�0.185 V), so this pair has
the greater tendency to accept electrons and is thus the stronger oxidizing system.
(b) The pyruvate/lactate pair is the more likely to accept electrons and thus is the stronger
oxidizing agent. For the same reason that NADH tends to donate electrons to pyruvate,
pyruvate tends to accept electrons from NADH. Pyruvate is reduced to lactate; NADH is
oxidized to NAD�. Pyruvate is the oxidizing agent; NADH is the reducing agent.
(c) From the answers to (a) and (b), it is evident that the reaction will tend to go in the
direction of lactate formation.
(d) The first step is to calculate �E�� for the reaction, using the E�� values in Table 13–7. Re-
call that, by convention, �E�� � (E�� of electron acceptor) � (E�� of electron donor).
For
NADH � pyruvate 88n NAD� � lactate
�E�� � (E�� for pyruvate/lactate) � (E�� for NAD�/NADH)
� �0.185 V � (�0.320 V) � 0.135 V
�G�� � �n �E��
� �2(96.5 kJ/V � mol)(0.135 V)
� �26.1 kJ/mol
Chapter 13 Bioenergetics and Biochemical Reaction Types S-155
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