The glycerophosphate shuttle is composed of one cytosolic and one inner mitochondrial membrane enzyme. It is used to transport reducing equivalents from the cytosol to the mitochondrion, since cytosolic NADH cannot cross the inner mitochondrial membrane directly. The shuttle regenerates cytosolic NAD(+) to sustain cytosolic oxidative pathways such as glycolysis and provides reducing equivalents in the form of FADH2 to the electron transport chain to produce ATP. (Adapted from FBrf0210056 and FBrf0264740.)