The pentose phosphate pathway produces two main things: NADPH, a molecule cells use as reducing power for building reactions and antioxidant defense, and ribose-5-phosphate, the sugar backbone needed to make DNA and RNA. For every glucose molecule that enters the pathway, cells get two molecules of NADPH and one molecule of ribose-5-phosphate.
The Two Phases and Their Products
The pentose phosphate pathway runs parallel to glycolysis, branching off from the same starting molecule (glucose-6-phosphate). It has two distinct phases, each producing different things.
The first phase, called the oxidative phase, is where NADPH gets made. Two separate chemical steps strip electrons from glucose-6-phosphate and hand them to NADP+, converting it into NADPH. By the end of this phase, the original six-carbon sugar has been trimmed down to a five-carbon sugar called ribulose-5-phosphate, and two NADPH molecules have been generated. This phase is irreversible, meaning it only runs in one direction.
The second phase, the non-oxidative phase, reshuffles carbon atoms among sugar molecules. Ribulose-5-phosphate gets converted into ribose-5-phosphate (for nucleotide synthesis) or into xylulose-5-phosphate. From there, a series of carbon-swapping reactions can also produce erythrose-4-phosphate, a four-carbon sugar used to build aromatic amino acids. The non-oxidative phase also generates fructose-6-phosphate and glyceraldehyde-3-phosphate, both of which can feed back into glycolysis. Unlike the first phase, these reactions are reversible, so they can run in either direction depending on what the cell needs.
What Cells Use NADPH For
NADPH is often described as the cell’s “reducing currency” for building and repair work. Where ATP provides energy, NADPH provides electrons. Its roles fall into two broad categories: biosynthesis and antioxidant defense.
On the biosynthesis side, NADPH is essential for making fatty acids. The enzyme complex that assembles fatty acid chains requires a steady supply of electrons from NADPH at multiple steps. Cholesterol synthesis also depends on NADPH, as does the production of steroid hormones like cortisol and aldosterone. Cells that do a lot of fat or steroid manufacturing, such as liver cells and adrenal gland cells, run the pentose phosphate pathway at high rates to keep up with NADPH demand.
On the defense side, NADPH is the sole source of reducing power for the cell’s antioxidant systems. Reactive oxygen species (free radicals) constantly threaten to damage proteins, membranes, and DNA. The cell neutralizes these by maintaining a pool of reduced glutathione, a small molecule that converts hydrogen peroxide into harmless water. Once glutathione does its job, it becomes oxidized and inactive. NADPH is what recharges it back to its active form. Without enough NADPH, the glutathione system collapses and oxidative damage accumulates. Immune cells like macrophages also use NADPH from the pentose phosphate pathway to produce nitric oxide, a signaling molecule involved in fighting infections.
How Ribose-5-Phosphate Builds DNA and RNA
Every nucleotide in your body, whether it ends up in DNA, RNA, or energy carriers like ATP, contains a five-carbon sugar at its core. Ribose-5-phosphate is the starting material for that sugar. Cells convert it into a compound called PRPP (phosphoribosyl-pyrophosphate), which then enters the pathways that assemble both purine and pyrimidine nucleotides. Without a functioning pentose phosphate pathway, cells cannot make the building blocks they need to replicate their genetic material or produce new RNA for protein synthesis.
This makes the pathway especially important in rapidly dividing cells. Any cell preparing to divide needs to double its entire DNA content and ramp up RNA production, both of which require large amounts of ribose-5-phosphate.
Erythrose-4-Phosphate and Amino Acid Production
A less well-known product of the non-oxidative phase is erythrose-4-phosphate, a four-carbon sugar. In plants and microorganisms, this molecule combines with another metabolic intermediate to enter the shikimate pathway, a seven-step process that ultimately produces chorismate, the precursor for the aromatic amino acids phenylalanine, tyrosine, and tryptophan. Erythrose-4-phosphate also plays roles in histidine biosynthesis and vitamin B6 metabolism. Humans cannot synthesize aromatic amino acids (they’re essential and must come from food), but this connection matters in agriculture, microbiology, and understanding plant metabolism.
The Pathway Adapts to Cell Needs
One of the pathway’s most useful features is its flexibility. Cells don’t always need NADPH and ribose-5-phosphate in equal amounts, and the two phases can be tuned somewhat independently. When a cell needs lots of NADPH but not much ribose (as in a fat-synthesizing liver cell), the non-oxidative phase can convert excess ribose-5-phosphate back into glycolytic intermediates, effectively recycling the carbon while keeping the NADPH. When a cell needs ribose for DNA replication but has plenty of NADPH, the non-oxidative phase can run in reverse, pulling glycolytic intermediates in to make ribose-5-phosphate without generating additional NADPH.
The gatekeeper for the oxidative phase is the enzyme glucose-6-phosphate dehydrogenase (G6PD), which catalyzes the first and rate-limiting step. High levels of NADPH naturally slow this enzyme down, while a drop in NADPH signals it to speed up. This feedback loop ensures the pathway responds in real time to the cell’s redox state.
What Happens When the Pathway Fails
The clearest example of what the pentose phosphate pathway produces comes from seeing what goes wrong without it. G6PD deficiency is the most common enzyme deficiency in humans, affecting hundreds of millions of people worldwide. Because G6PD controls the first step of the oxidative phase, people with this deficiency cannot produce enough NADPH.
Red blood cells are hit hardest. Unlike most cells, red blood cells have no nucleus and no mitochondria, so they cannot make NADPH through any backup pathway. The pentose phosphate pathway is their only source. When NADPH runs low, glutathione can’t be recharged, and reactive oxygen species go unchecked. Hemoglobin denatures into clumps called Heinz bodies, and the red blood cell membrane stiffens and breaks apart. The result is hemolytic anemia, where red blood cells are destroyed faster than the body can replace them.
Triggers for hemolytic episodes include certain infections, fava beans, and specific medications like antimalarials and sulfonamides, all of which increase oxidative stress. Newborns with G6PD deficiency are particularly vulnerable to severe jaundice because the rapid breakdown of red blood cells floods the bloodstream with bilirubin.