Pentose Phosphate Pathway

Not all of a plant cell's glucose is broken down by glycolysis -- roughly two-thirds is processed that way, while the remaining third is oxidised through a parallel route called the pentose phosphate pathway (PPP), also known as the hexose monophosphate shunt (HMP shunt), the direct oxidative pathway, or the Warburg-Dickens-Lipmann pathway, after Otto Warburg, Frank Dickens and Fritz Lipmann who described it in 1938. It runs entirely in the cytoplasm of mature plant cells and provides an alternative, non-glycolytic way to oxidise glucose. The pathway has two phases. The oxidative phase converts six molecules of the six-carbon sugar glucose-6-phosphate into six molecules of the five-carbon sugar ribulose-5-phosphate, releasing 6 CO2 and generating 12 molecules of $NADPH+H^+$ (notably NADPH, not NADH -- a distinction that matters because NADPH is used for biosynthesis rather than for ATP generation via the electron transport chain) via the key regulatory enzyme glucose-6-phosphate dehydrogenase, which is itself inhibited whenever the NADPH-to-NADP+ ratio in the cell runs high. The non-oxidative phase then rearranges the resulting ribulose-5-phosphate molecules into a range of intermediate sugars of different chain lengths -- ribose-5-phosphate (5C), xylulose-5-phosphate (5C), glyceraldehyde-3-phosphate (3C), sedoheptulose-7-phosphate (7C) and erythrose-4-phosphate (4C) -- before finally regenerating five molecules of glucose-6-phosphate, closing the cycle. The overall reaction is $6\,\text{Glucose-6-P} + 12NADP^+ + 6H_2O \rightarrow 5\,\text{Glucose-6-P} + 6CO_2 + P_i + 12NADPH + 12H^+$, so the net result of completely oxidising one glucose-6-phosphate through this pathway is 6 CO2 and $12\,NADPH+H^+$. The pentose phosphate pathway matters for reasons that have little to do with ATP production directly: (1) it generates both NADPH and pentose sugars, both vital for anabolic (biosynthetic) reactions; (2) the NADPH produced d …