Environmental Engineering Reference
In-Depth Information
FIGURE A.1
Representation of the con-
version of chemical potential energy into
energy useable by organisms. The “stored”
energy in the electron donor (
D
) is released
as the electrons are transferred to the elec-
tron acceptor (
A
1
). Enzymes involved in
electron transport capture energy for use
in biosynthesis.
DONOR
D
→
D
+
and
e
-
BIOTA
Enzymes (electron transport)
are the “teeth” on the gears
ACCEPTOR
A
+
and
e
-
Overall Δ
G
is negative
→
A
More formally, redox reactions can be broken down into two
half-reactions
, one involv-
ing the loss of an electron and the other the gain of an electron. The half-reactions of ordi-
nary respiration are:
C
0
H
2
O
C
1
4
!
4
e
2
½
Δ
G
181 kJ
=
4 electron transfer
1
52
O
2
1
2O
2
2
4
e
2
!
½
Δ
G
313 kJ
=
4 electron transfer
52
where the superscripts indicate the oxidation state of the element and
e
2
indicates
an electron. Note that only the elements changing in oxidation state are shown for sim-
plicity and the reactions are not necessarily balanced. Summed, these two half-reactions
give the full equation given in the first paragraph of this primer. The terminology for
describing redox reactions can be confusing and the easiest thing to remember is that
if the oxidation state goes down the element/compound is being reduced, and con-
versely, if the oxidation state goes up it is being oxidized. In the previous example the
carbon in the carbohydrate (CH
2
O) is clearly being oxidized and the oxygen is being
reduced.
The total energy yield of this reaction is the sum of the energy yielded by the two
half-reactions, and for this example, both
G
values are negative and the overall
G
Δ
Δ
is
494 kJ per mole CH
2
O oxidized. If one of the half-reactions yields energy but the other
requires energy, the summed reaction can still proceed if the yield is greater than the
requirement. This allows for a wide combination of electron donors and electron acceptors
to be used in energy-yielding reactions by organisms. In the familiar reaction for aerobic
respiration, organisms use C as the electron donor and O
2
as the electron acceptor, but the
principles are just the same if a bacterium uses SO
4
as the electron acceptor for an electron
derived from the C in organic matter.
2
2C
0
H
2
O
2C
1
4
8
e
2
!
½
Δ
G
86
:
4kJ
=
8 electron transfer
1
52
S
1
6
O
2
2
S
2
2
8
e
2
!
½
Δ
G
40
:
4kJ
=
8 electron transfer
1
5
