Pentose Phosphate Pathway

Aside from glycolysis, another way that the body can oxidize glucose for use in the cells is through the pentose phosphate pathway. Like glycolysis, the pentose phosphate pathway occurs exclusively in the cytoplasm of cells. But, unlike glycolysis, NADP⁺ serves as the electron acceptor and is reduced to NADPH.

Note that NADP⁺ and NADPH are not the same as NAD⁺ and NADH. These two pairs of molecules actually serve opposite functions in the cell. Recall that NAD⁺ is used to oxidize glucose to release energy. This is because, within the cell, NAD⁺ is at high concentrations and NADH is at low concentrations. This concentration difference favors the reduction of NAD⁺ into NADH, providing the cell the power to oxidize other compounds. On the other hand, the relative cellular concentrations of NADP⁺ and NADPH provide reducing power to the cell. In the cell, NADPH is at high concentrations, and NADP⁺ is at low concentrations. This concentration difference favors the oxidation of NADPH into NADP⁺, allowing the cell to reduce other compounds. This reduction capacity of the cell is useful for a variety of anabolic processes, for example, fatty acid synthesis, cholesterol synthesis, and steroid hormone synthesis, as well as preventing oxidative damage from reactive oxygen species.

Another product of the pentose phosphate pathway is ribose 5-phosphate, which is where the pathway gets its name from. Ribose 5-phosphate is a pentose phosphate, consisting of the 5-carbon sugar, ribose, an essential component for nucleotides and nucleic acids. In other words, the pentose phosphate pathway also produces a vital precursor of DNA, RNA, and other coenzymes such as NADH, FADH₂, and coenzyme A.

There are two phases of the pentose phosphate pathway – the irreversible oxidative phase and the reversible nonoxidative phase.