Environmental Engineering Reference
In-Depth Information
generated by certain rearrangements of carbohydrate molecules. These often lead to a
more oxidized product (such as CO
2
) and a more reduced product, such as ethanol
(C
2
H
5
OH). Figure 13.1 shows the catabolic reactions in a yeast cell when carrying
out ethanol fermentation from glucose or fructose (both C
6
H
12
O
6
). The overall
catabolic reaction in this case is
C
6
H
12
O
6
+ 2ADP + 2P
i
!
2C
2
H
5
OH + 2CO
2
+ 2ATP
ð
RX
:
13
:
1
Þ
The formed ATP remains intracellular and is converted back to ADP and P
i
by anabolic reactions, which lead to conversion of carbohydrates into cells.
Substrates
Hexose
transporter
Glucose
Glucose
ATP
Fructose
Fructose
Glucose-6-phosphate
ATP
Fructose-6-phosphate
ATP
Fructose-1,6-phosphate
Glyceraldehyde-3-phosphate
Dihydroxyacetone-
1,3-phosphate
ATP
1,3-Diphosphoglycerate
NAD
+
NADH + H
+
3-Phosphoglycerate
Ethanol
Ethanol
2-Phosphoglycerate
Acetaldehyde
Products
Phosphoenolpyruvate
ATP
CO
2
CO
2
Pyruvate
FIGURE 13.1
Formation of ethanol in a yeast cell. Only the catabolic reactions are shown,
but thousands of other enzymatic reactions occur in the cell. “ATP” implies its synthesis from
ADP and phosphate, but for brevity, these are left out.
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