Page 101 - Biofuels Refining and Performance

84 Chapter Three

Glucose (1 mole)
Media
Cytosol
Glucose

2 ADP

2 ATP
Glycerol Glycerol

1 NADH
2 NADH

Pyruvate
(2 moles) Acetaldehyde Ethanol Ethanol (2 moles)
1 NADH
CO 2

Figure 3.7 Central metabolic pathway in yeast under anaerobic conditions.

Three major interrelated pathways that control catabolism of carbo-
hydrate in most ethanol-producing organisms are

Embden-Meyerhof pathway (EMP) or glycolysis
Pentose phosphate pathway (PPP)
Krebs or tricarboxylic acid cycle (TCA)
In glycolysis, glucose is anaerobically converted to pyruvic acid and
then to ethanol through acetaldehyde. This pathway provides energy in
the form of ATP to the cells. The net yield in glycolysis is 2 moles of pyru-
vate (or ethanol) and 2 moles of ATP from each mole of glucose. This
pathway is also the entrance of other hexoses such as fructose, mannose,
and galactose to metabolic pathways. With only 2 moles of ATP formed
per glucose catabolized, large amounts of ethanol (at least 3.7 g of
ethanol per gram of biomass) must be formed [15, 38].
The PPP handles pentoses and is important for nucleotide (ribose-
5-phosphate) and fatty acid biosynthesis. The PPP is mainly used to