15 September 2005
G436 Biogeochemistry
Assoc. Prof. A. Jay Kaufman
Bioenergetics of redox transformations
Reading: Bacterial Biogeochemistry chapter 1
Today we will explore the redox chemistry of biogeochemical interest, and define redox as the
coupled reductive and oxidative reactions that broadly result in the transfer of electrons between species.
In the global carbon cycle electrons flow from organic matter (reduced and electron rich) to carbon dioxide
(oxidized and electron poor).
The most important chemical property of an element is its combining capabilities, classically
related to the valence (number of bonds an element can form, or the number of electrons lost or gained by
an atom in an ionic compound or half the number shared for covalent compounds), but this is not
straightforward for all elements.
The rows in the periodic table are largely based on valence, but the non-metals that bind with
hydrogen are unique (dominated by carbon), and are the basis of biochemistry. Fixed valence and multiple
bonding in C above all allow for the building of complex but flexible frameworks that promote selective
interaction with other elements.
Species
CO2
CH2O
CH4
Oxidation Number
+4
0
-4
Structure
linear
tetrahedral
However, for more complex organic molecules we need a way to determine oxidation state of the
various carbon atoms, and this can be accomplished using the simple rules of electronegativity (the affinity
for electrons – high electronegativities are characteristic of oxidized forms since these are excellent
terminal electron acceptors).
N = oxidation number = n- - n+ where n- is the number of bonds to atoms that are more electronegative
than C, and n+ the number of atoms that are less electronegative than C.
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Rules for balancing redox equations:
C6H12O6 + 2SO42- = 2H2S + 6CO2 + 2H2O + 4H+
0
1.
2.
3.
4.
5.
6.
+6
-2
+4
write species with oxidized forms of element on the left side of the equation and the reduced forms
on the right.
balance the elements except for H and O on both sides
balance the number of O atoms by adding H2O
balance the number of H atoms by adding H+
achieve electronic neutrality by adding electrons to the left side
the stoichiometry of the two half reactions are adjusted to cancel out the electrons by subtraction.
Redox transformations and thermodynamics
Reactions characterized by changes in the valence state of reactants and products are also
characterized by thermodynamic relationships. Dissimilatory metabolism is entirely dependent on redox
reactions.
A(x) + B(y) → A(x-n) + B(y+n)
Where x and y are valence states and n = number of electrons transferred
These reactions are profoundly important in biological and biogeochemical systems, and in some abiotic
reactions as well. The energetics of redox reactions can be analyzed by separating them into half-reactions
A(x)  A(x
-n)
+ ne- -- oxidation
B(y) + ne-  B(y + n) -- reduction
Since each half reaction consumes or produces electrons, it is possible to establish, in principle,
electrochemical cells consisting of separate reductants and oxidants connected by a bridge that permits the
flow of current at an electrical potential or voltage, designated E, that is characteristic of the two half
reactions.
An electrochemical cell consists of two electrodes in a solution or two different solutions in
contact with each other. Across an interface where an electrode meets a solution, a difference in electrical
potential develops, called a potential.
emf = sum of various potential differences within a cell (like a car battery)
the KCl bridge provides a pathway for the migration of ions
the anode is where oxidation (loss of electrons) takes place
the cathode is where reduction (gain of electrons) takes place
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As voltage represents force acting through a distance, or energy, E for an electrochemical reaction
can be related to G as,
G = -nEF
Where F is the Faraday proportionality constant in JV-1
Recall that for a chemical reaction, the driving force, and thus the emf, depends on the activities
(concentrations) or reactants and products.
G = Go + RTln(aC)c(aD)d/(aA)a(aB)b
So solving for E by substitution and rearrangement yields the Nernst equation:
E = Eo - (RT/nF)ln (aC)c(aD)d/(aA)a(aB)b
And when products and reactants are in their standard states
Eo = (RT/nF)ln (aC)c(aD)d/(aA)a(aB)b
Thus the Eo (and Go) for reactions of biological interest can be calculated from compilations of half
reaction potentials by a simple addition process.
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