Cellular respiration may occur in environments where oxygen is absent such as the subsurface layers of the microbial mats depicted in Figure 26.7

Question

In such places, molecules
other than oxygen must be used as the terminal electron acceptor used to produce energy
for the cell.
As stated in Chapter 6, all biochemical reactions in a cell are subject to the laws of
thermodynamics. Like chemical reactions, by comparing the difference in free energy (G0
') of
the reactants and products in a biochemical reaction, you can determine how much energy is
released or consumed during the course of a reaction (Figure 6.9).
As discussed in Chapter 7, redox reactions are biochemical reactions that take place during
cellular respiration and result in the transfer of electrons from one molecule to another.
During the course of such reactions, one molecule gets oxidized, and another reduced.
Redox reactions require two half reactions between pairs of compounds (called redox pairs).
A list of some biologically relevant redox pairs is shown in the table to the right. The redox
pairs are arranged from the strongest electron donor pairs at the top to the strongest electron
acceptors at the bottom. The tendency of electrons to be passed between any two redox
pairs can be measured experimentally using a voltmeter. The voltage difference, defined as
the reduction potential (E0
'), has been determined for all redox pairs in the table using
hydrogen as a reference standard, which has a reduction potential of zero. The greater the
difference in reduction potential (E0
') between two half reactions, the more energy that is
released during a biochemical reaction. The difference in reduction potential is a direct
measure of the change in free energy (G0
'). Thus, the greater the change in reduction
potential between two redox pairs, the more free energy that is available for the reaction to
proceed.
Consider a respiring microorganism that uses pyruvate- / lactate- as an electron donor pair. If
this microorganism uses oxygen as a terminal electron acceptor under aerobic growth
conditions, we would calculate the change in reduction potential as follows:
O2 / H2O Eo'(V) = +0.82 (acceptor pair)
pyruvate- / lactate- Eo'(V) = -0.19 (donor pair)
The change in reduction potential (E0
') is calculated by subtracting the donor pair value from
the acceptor pair value.
E0
' = +0.82 - (-0.19) = 1.01 V
Now think about a respiring microorganism that uses the same electron donor pair, pyruvate-
/ lactate-, but produces energy and grows in an environment that lacks oxygen. In this case,
redox reactions occur with a terminal electron acceptor pair is not oxygen but another
acceptor pair shown in the table.
Calculate the change in reduction potential for each option below, then circle the best
acceptor pair.
AsO4
3- / AsO3
3- Eo'(V) = +0.139 - (-0.19) = 0.33 NO3
- / NO2
- Eo'(V) = +0.43 - (-0.19) = 0.62 Fe3+ / Fe2+ (pH 2) Eo'(V) = +0.77 - (-0.19) = 0.96
What will be an ideal response?

Answer 1

AsO4
3- / AsO3
3- Eo'(V) = +0.139 - (-0.19) = 0.33 NO3
- / NO2
- Eo'(V) = +0.43 - (-0.19) = 0.62 Fe3+ / Fe2+ (pH 2) Eo'(V) = +0.77 - (-0.19) = 0.96 The best acceptor pair is thus Fe3+ / Fe2+ (pH 2)