I am studying the Pentose phosphate pathway right now and I can't get my head around it. It starts off at
glucose-6-phosphate C6H13O9P
6-Phosphogluconolactone C6H11O9P (loses mass)
6-Phosphogluconic acid C6H13O10P (it gained mass??)
Ribose 5-phosphate C5H11O8P (loses mass)
I'm just wondering where the carbon molecule ran off to when ending up as Ribose, a C5 sugar. And why the Hydrogen and Oxygen molecules fluxuate so much in between the enzymatic steps.
A quick answer: the carbon goes to CO2.
More generally, a vast majority of metabolic pathways either add or subtract bits from the substrate, resulting in generation of small molecules, e.g. 6-phosphogluconate to ribulose-5-phosphate generates CO2 and tacks on some H+ ions onto NADP+, hence the mass loss from the substrate.
Wikipedia actually has a good explanation of the phases of PPP:
https://en.wikipedia.org/wiki/Pentose_phosphate_pathway#Oxidative_phase
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