Conversion Technologies 89
All energy
released;
none stored
Large activation
energy provided
by heat
Energy collected by
carrier molecules and
used by an organism
Free energy
Small activation energies
for each step
6 CO 2 + 6 H 2 O
+ 6 O 2
OH OH
OHO
OH
CH 2 OH
6 CO 2 + 6 H 2 O
OH OH
OHO
OH
CH 2 OH
+ 6 O 2
Figure 3.11Schematic representation of stepwise oxidation of glucose in an organism compared to
its combustion in oxygen. On the left biological decomposition of glucose into compounds of lower
energy and ultimately into CO 2 and H 2 O with most of the energy collected by carrier molecules such
as ATP and NADH. On the right complete combustion of glucose in oxygen with activation energy
from external heat source. Please note that substrates and products of both reactions are identical.
3.4.8.2 Central Metabolic Pathway under Anaerobic Conditions
Central metabolic pathway describes the energy generating flux of carbon in just about
all organisms. The conversion of energy starts with glucose and results in the produc-
tion of pyruvate, which can then enter other pathways that result in the production of
energy (TCA cycle, fermentation) or building cellular components like nucleic acids or
proteins. Glycolysis is the first step of energy release. During glycolysis, the six carbon
sugar, glucose, is split into two molecules of pyruvate containing three carbon molecules
each. During this process, chemical bond energy is released and transferred into two
molecules of ATP. It is beyond the scope of this book to provide the insight into the pro-
cess of glycolysis, and interested readers should refer to further reading section of this
chapter for more information.
The brief summary of glycolysis is presented here; please note that glycolysis
is a multi-step process and the equation does not reflect the complexity of this
process.
C 6 H 12 O 6 +2ADP+2Pi+2NAD+↔2C 3 H 4 O 3 +2ATP+2NADH+2H+
+2H 2 O∗+88 kJ
*PleasenotethatH 2 O comes from elimination of water from phosphate groups (ADP
to ATP synthesis) and not from glucose molecule.
The subsequent fate of pyruvate depends on the organism’s enzymatic machinery and
availability of oxygen. There are numerous possible routes a pyruvate molecule can take,
which are summarised in Figure 3.12. We introduce two most important processes,
respiration and fermentation, which could be paralleled to complete and incomplete
oxidation of biomass in thermochemical routes.
