Figure 21.12 The glycerol 3-phosphate shuttle. Electrons from NADH can enter the mitochondrial electron-transport chain by reducing dihydroxyacetone phosphate to glycerol 3-phosphate. Electron transfer to an FAD prosthetic group in a membrane-bound glycerol 3-phosphate dehydrogenase reoxidizes glycerol 3-phosphate. Subsequent electron transfer to Q to form QH₂ allows these electrons to enter the electron-transport chain.