In the environment bacteria will preferentially use the available electron accep
ID: 532245 • Letter: I
Question
In the environment bacteria will preferentially use the available electron acceptor that will yield the greatest free energy change (i.e. most negative G’) in a coupled redox reaction. (i) In the respirations listed below, rank them from high to low based on their energetic favorability when each are coupled to formate oxidation to CO2 (CO2/formate (2e-), E’= -0.43V). Which one will be most preferred?
Type of Respiration
Ox/red (e-)
E’ (V)
Nitrate reduction
NO3-/NO2– (2e-)
+0.43
Tetrathionate reduction
S4O62-/2S2O32- (2e-)
+0.024
Arsenate reduction
HAsO42-/HAsO32- (2e-)
+0.14
Type of Respiration
Ox/red (e-)
E’ (V)
Nitrate reduction
NO3-/NO2– (2e-)
+0.43
Tetrathionate reduction
S4O62-/2S2O32- (2e-)
+0.024
Arsenate reduction
HAsO42-/HAsO32- (2e-)
+0.14
Explanation / Answer
Free energy change can be calculated by this formula
D G = -nFE0cell where n is number of electrons, F is Faraday constant, E0cell = E0cathode -E0anode
E0 is standard reduction potentials of corresponding electrodes
In the respirations listed below, the potential is calculated against formate oxidation to CO2, having E0 = -0.43 V
For Nitrate reduction E0 = +0.43 V, Arsenate reduction E0 = +0.14V and for Tetrathionate reduction E0 = +0.024V
E0cell = E0cathode -E0anode
In all cases, formate oxidation to CO2 takes place and it acts as anode,
Thus
E0cell= 0.86 V for Nitrate reduction
E0cell= 0.57 V for Arsenate reduction
E0cell= 0.454 V for Tetrathionate reduction
All the reactions are 2 electron reductions
Thus,
-D G is directly proportional to E0cell
Thus the decreasing order is,