Page 730 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Biological Reactions and Kinetics 685

CH 3 CH 3 molecular oxygen. In an ‘‘anoxic’’ reaction, nitrate may serve
but with a lower yield of ATP.
j j
CHOH $ CH¼O þ 2H þ 2e (22:1)
þ
22.2.1.1.2 ATP Energy Yields
j j
COOH COOH Table 22.3 compares reactions, moles ATP formed, DG8(reac-
lactic acid pyruvic acid tion), and DG8(captured in ATP) for combustion, aerobic
metabolism of glucose, and anoxic metabolism of glucose.
The electrons released are captured by nucleotide derivatives,
In aerobic respiration with oxygen as the electron acceptor, 38
especially nicotinamide adenine dinucleotide (NAD), which is
mol of ATP are generated, i.e., two in the Embden–Meyerhof
an electron carrier in all cells. Other electron carriers include
pathway, two in the citric acid cycle, and 34 in the respiratory
NADP (the same as NAD but with an extra phosphate group)
chain. The energy capture by ATP is thus, 38 mol ATP 7.0
and FAD (flavin adenine dinucleotide). Regarding NAD,
kcal=mol ATP ¼ 1113 kJ (266 kcal). In anoxic respiration,
which is illustrative, the nicotinamide portion of NAD accepts
only 26 mol ATP are formed per mol of glucose oxidized, i.e.,
the electrons and is reduced, e.g., as NADH. The NADH then
26 mol ATP 7.0 k cal=mol ATP ¼ 761 kJ (182 kcal) (Orhon
loses its electrons, via enzymes, to again form NAD. The
and Artan, 1994, p. 69).
reversible reaction is depicted,
22.2.1.1.3 Fermentation (Anaerobic Reaction)
NADH 2 $ NAD þ 2H þ 2e (22:2)
þ
If molecular oxygen is absent and if an inorganic oxidant is
reduced form oxidized form
not present to act as an electron acceptor, an organic molecule
In aerobic metabolism, the NAD carries the electrons to the may serve as both an electron donor and as an electron
final electron acceptor, molecular oxygen, or nitrate. In the acceptor; such a reaction is termed, ‘‘fermentation,’’ which is
case of molecular oxygen, the reaction is, after a sequence of anaerobic (Section 22.3.3.4). For a fermentation reaction, the
successive such reactions, free energy of reaction is less than for the anoxic reaction,
with correspondingly lower values of n(ATP) and cell yield.
1
NADH 2 þ O 2 ! NAD þ H 2 O (22:3)
2 22.2.1.2 Anabolism
The electron transfers occur in a sequence of reactions called Anabolism is also a sequence of reactions, with each reaction
the respiratory chain, through which ATP is formed. The facilitated by a specific enzyme. As with degradation, the
NAD and FAC molecules donate their electrons to a series particular pathway depends upon the substrate and the
of carriers called cytochromes. At each step in the electron microbe. The reaction sequence is, however, ‘‘endothermic,’’
transfer free energy is released, which permits the formation meaning that the standard free energy of reaction is positive,
of ATP. The final electron acceptor in an aerobic reaction is which means DG8 > 0, meaning also that the reaction is not

TABLE 22.3
Aerobic and Anoxic Reactions and Energy Captured ATP
o
o
DG (Reaction) DG (Captured in ATP)
ATP
Reaction (mol ATP) (kcal) (kJ) (kcal) (kJ)
1. Combustion
Oxidation of glucose (combustion)
C 6 H 12 O 6 þ 6O 2 ! 6CO 2 þ 6H 2 O
glucose oxygen
0 686 2870 0 0
2. Aerobic Reaction
Metabolism of glucose (EMP=citric acid cycle=respiratory chain=oxygen as electron acceptor)
C 6 H 12 O 6 þ38Pi þ 38ADP þ 6O 2 ! 6CO 2 þ 38ATP þ 44H 2 O
glucose oxygen
38 420 1757 266 1113
3. Anoxic Reaction
Metabolism of glucose (EMP=citric acid cycle=respiratory chain=nitrate as electron acceptor)

C 6 H 12 O 6 þ26Pi þ 26ADP þ 12NO 3 ! 6CO 2 þ 26ATP þ 32H 2 O þ 12NO 2
glucose nitrate
26 469 1962 182 761
Source: Adapted from Orhon, D. and Artan, N., Modeling of Activated Sludge Systems,
Technomic Publishing Co., Lancaster, PA, 1994, pp. 64–69.
Note: Pi is the symbol for phosphate.