The redox reactions catalyzed by oxidoreductases are vital in all parts of metabolism, but one particularly important function of these reactions is to enable nutrients to unlock the energy stored in the relatively weak double bond of oxygen. Here, reduced compounds such as glucose and fatty acids are oxidized, thereby releasing the chemical energy of O2. In this process, NAD is reduced to NADH, as part of beta oxidation, glycolysis, and the citric acid cycle. In eukaryotes the electrons carried by the NADH that is produced in the cytoplasm are transferred into the mitochondrion (to reduce mitochondrial NAD) by mitochondrial shuttles, such as the malate-aspartate shuttle. The mitochondrial NADH is then oxidized in turn by the electron transport chain, which pumps protons across a membrane and generates ATP through oxidative phosphorylation. These shuttle systems also have the same transport function in chloroplasts.