Pentose phosphate pathway

Let’s say you just ate a carbohydrate loaded meal, like a bowl of rice.

A few hours after you’re done, those carbohydrates are broken down in the small intestine into their simplest chemical form; monosaccharides, the most important of which is glucose - a 6-carbon molecule that’s in the shape of a ring.

Glucose moves from the small intestine into the bloodstream, and blood glucose levels rise, which causes the pancreas to secretes a hormone called insulin.

Insulin makes more glucose enter cells through specific transporters called GLUTs.

Once glucose is in the cell, an enzyme called hexokinase attaches a phosphate group to its sixth carbon, creating glucose-6-phosphate.

From there, the cell has the option to take glucose through a metabolic pathway called glycolysis; which is the breakdown of glucose in order to generate ATP.

But if the cell doesn’t need ATP, glucose can be used to make some other useful products by entering an alternative metabolic pathway called the pentose phosphate pathway.

The pentose phosphate pathway is named for the products it ultimately generates; pentose refers to a five-carbon sugar called ribose, and phosphate refers to a molecule called nicotinamide adenine dinucleotide phosphate, or NADPH.

So the pentose phosphate pathway is an alternative pathway that glucose can enter when cells need to make more ribose and NADPH.

Ribose can be used to make nucleotides, which are the building blocks of our DNA and RNA.

And NADPH is rich in electrons, and can be used in various anabolic pathways.

Anabolic pathways are ones that synthesize molecules like fatty acids, from scratch, and require an electron donor - such as NADPH.

Like glycolysis, the pentose phosphate pathway happens exclusively in the cytoplasm and it doesn’t require any special organelles which means that all of our cells can use this pathway.

The pentose phosphate pathway can be divided into two phases: an irreversible oxidative phase that ultimately yields NADPH, and a reversible non-oxidative phase that yields ribose.

Irreversible means that the reaction can only go in one direction - that is, substrate to product.

On the other hand, reversible means that the reaction can go in both directions, and the substrate and product can be interconverted into one another, depending on what the body needs more.