90 Introduction to Renewable Biomaterials
Glucose*
Ethanol
Ethanol
fermentation
Biomass
Aerobic respiration
Abe
fermentation
NADH
NAD+
NADH
NAD+
NADH
NAPD+
NADH
NAD+
NADH
AT P
AT P
+
14 ADP + 14 Pi
14 ATP + 14 H 2 O
Electron transport
chain and oxidative
phosphorylation
NADH
FADH 2
Citrate
Oxaloacetate
TCA cycle
Aceate Acetyl-CoA
CO 2
CO 2
2CO 2
CH 4
CH 4
CO 2
Pyruvate Acetaldehyde Butyraldehyde
Butanol
Butyrl-CoA**
Butyrate
AT P
Methano
Genesis
Methano
Genesis
Hydrogen dark
fermentation
H 2 Fdred
2 H+
H 2
2 H+
Fdox
Fdred
Fdox
NAD+
Figure 3.12Overview of major metabolic pathways used for bioenergy production and aerobic
respiration. Biomass used as a feedstock for fermentative production of bio-fuels and biochemicals is
decomposed into glucose and enters glycolysis or bio-fuel-producing organism. Note: Other hexoses
are converted into glucosein vivo, and pentoses will enter the pathway at later stages of glycolysis
through pentose phosphate pathway). Oily feedstocks can enter through훽-oxidation pathway as
acetyl-CoA. * Glucose is split into two pyruvate molecules during glycolysis; ** Butyryl-CoA is formed
through condensation of two Acetyl-CoA.
Aerobic respiration– In aerobic respiration, complete combustion of pyruvate is car-
ried out in a multi-step process. Pyruvate is converted into acetyl CoA and enters the
TCA cycle and oxidative phosphorylation – the series of enzymatic steps that perform
complete combustion of these molecules to CO 2 and H 2 O. This is a highly productive
route as aerobic respiration produces about 14 molecules of ATP* per molecule of pyru-
vate [32, 33], seven times higher that glycolysis itself.
C 3 H 4 O 3 +2.5O 2 +14ADP+14Pi↔3CO 2 +2H 2 O∗∗+14ATP+14H 2 O∗∗∗
* ATP synthesis occurs through a complex enzyme ATP synthase powered by proton
gradient.The estimation of exact number of ATP molecules per molecule of pyruvate
varies between organisms and does not need to be an integer.
** Biological combustion of pyruvate molecule.
*** Elimination of water from phosphate groups (ADP to ATP synthesis).
Aerobic oxidation provides another important function in addition to ATP synthe-
sis. The synthesis of energy carrier is coupled with regeneration of NAD+from NADH.
