Cells need the five-carbon sugar ribose to manufacture nucleotides. This is made from glucose in an oxidative pathway called the pentose phosphate pathway (Fig. 13.6). This pathway also provides reducing power for biosynthetic reactions in the form of NADPH. Glucose-6-phosphate undergoes two oxidations generating a molecule of a pentose phosphate, CO2, and two molecules of NADPH. Like glycolysis, these reactions occur in the cytoplasm. The pentose phosphate pathway interacts with glycolysis, and this allows it to perform a different function. In a cell that does not need lots of ribose, but does need lots of NADPH for biosynthesis, the pentose is recycled by being combined with glycolytic intermediates. Going around the loop six times converts a glucose entirely to 6CO2, giving 12 NADPH for biosynthesis. Although the pentose phosphate pathway breaks up sugar to give CO2 and the strong reducing agent NADPH, it is not used for energy production: only in the mitochondria, where NADH can be used to power the electron transport chain, can reducing agents be converted to other energy currencies.
C> ribulose 5-phosphate + CO2
fructose-1,6-bis phosphate fructose-6-phosphate fructose-1,6-bis phosphate
dihydroxy- ^^^^ glyceraldehyde-3-phosphate acetone phosphate pyruvate
Figure 13.6. The reactions of the pentose phosphate pathway are shown in black with the reactions of glycolysis in green.
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