The Pentose Phosphate Pathway
High-Yield Summary
- The pentose phosphate pathway (PPP, hexose monophosphate shunt) branches off glycolysis at glucose-6-phosphate. Its purpose is NOT ATP — it makes NADPH and ribose-5-phosphate.
- Oxidative phase (irreversible): glucose-6-phosphate dehydrogenase (rate-limiting) oxidizes G6P, producing 2 NADPH total and ribulose-5-phosphate, which becomes ribose-5-phosphate for nucleotide synthesis.
- Non-oxidative phase (reversible): rearranges pentose sugars, can feed fructose-6-phosphate back into glycolysis.
- NADPH is an electron DONOR for biosynthesis/antioxidant defense (fatty acid/cholesterol synthesis, respiratory burst, maintaining reduced glutathione) — opposite role from NADH (electron acceptor feeding the ETC for ATP).
- Red blood cells depend almost entirely on the PPP for NADPH, given their high oxidative stress exposure.
Key Terms
- Glucose-6-phosphate dehydrogenase (G6PD)
- Rate-limiting, irreversible first enzyme of the PPP's oxidative phase; deficiency causes hemolytic anemia under oxidative stress.
- Ribose-5-phosphate
- Pentose sugar product used to build nucleotides (DNA/RNA).
- Reduced glutathione
- Cell's primary antioxidant defense against reactive oxygen species; requires NADPH to stay reduced.
- Respiratory burst
- NADPH-driven production of reactive oxygen species by white blood cells to destroy pathogens.
Two-Phase Pathway
- 1Glucose-6-phosphate diverted from glycolysis into the PPP
- 2Oxidative phase (irreversible): G6P → 6-phosphogluconate → ribulose-5-phosphate, producing 2 NADPH
- 3Ribulose-5-phosphate → ribose-5-phosphate (for nucleotide synthesis) or into the non-oxidative phase
- 4Non-oxidative phase (reversible): sugar rearrangement can regenerate fructose-6-phosphate to re-enter glycolysis
NADH vs. NADPH
| Feature | NADH vs. NADPH |
|---|---|
| Role | Electron acceptor / Electron donor |
| Typical fate | Feeds ETC → ATP / Powers biosynthesis and antioxidant defense |
| Source pathway | Glycolysis, citric acid cycle / Pentose phosphate pathway |
Common MCAT Trap
- The PPP does not produce ATP — don't tally its NADPH into an ATP-yield calculation the way you would NADH.
- NADPH and NADH are chemically similar but functionally opposite: NADPH is a donor for biosynthesis, NADH is an acceptor headed for the ETC.
- The oxidative phase is irreversible; the non-oxidative phase is reversible — direction of sugar flow can go either way in the latter.
Quick Recall
What are the two major products of the PPP?
What is the rate-limiting enzyme of the PPP?
Why do red blood cells depend heavily on the PPP?
Which phase of the PPP is reversible?
Sign in to unlock this chapter
Biology chapters 1–3 are free. Sign up to unlock every chapter.