Aerobic regeneration of NAD+ and further catabolism of pyruvate In aerobic eukaryotes, a complex mechanism has developed to use the oxygen in air as the final electron acceptor, in a process called oxidative phosphorylation. Aerobic prokaryotes, which lack mitochondria, use a variety of simpler mechanisms. Firstly, the NADH + H+ generated by glycolysis has to be transferred to the mitochondrion to be oxidized, and thus to regenerate the NAD+ necessary for glycolysis to continue. However the inner mitochondrial membrane is impermeable to NADH and NAD+. Use is therefore made of two "shuttles" to transport the electrons from NADH across the mitochondrial membrane. They are the malate-aspartate shuttle and the glycerol phosphate shuttle. In the former the electrons from NADH are transferred to cytosolic oxaloacetate to form malate. The malate then traverses the inner mitochondrial membrane into the mitochondrial matrix, where it is reoxidized by NAD+ forming intra-mitochondrial oxaloacetate and NADH. The oxaloacetate is then re-cycled to the cytosol via its conversion to aspartate which is readily transported out of the mitochondrion. In the glycerol phosphate shuttle electrons from cytosolic NADH are transferred to dihydroxyacetone to form glycerol-3-phosphate which readily traverses the outer mitochondrial membrane. Glycerol-3-phosphate is then reoxidized to dihydroxyacetone, donating its electrons to FAD instead of NAD+. This reaction takes place on the inner mitochondrial membrane, allowing FADH2 to donate its electrons directly to coenzyme Q (ubiquinone) which is part of the electron transport chain which ultimately transfers electrons to molecular oxygen , with the formation of water, and the release of energy eventually captured in the form of ATP. The glycolytic end-product, pyruvate (plus NAD+) is converted to acetyl-CoA, and NADH + H+ within the mitochondria in a process called pyruvate decarboxylation. The resulting acetyl-CoA enters the citric acid cycle (or Krebs Cycle), where the acetyl group of the acetyl-CoA is converted into carbon dioxide by two decarboxylation reactions with the formation of yet more intra-mitochondrial NADH + H+. The intra-mitochondrial NADH + H+ is oxidized to NAD+ by the electron transport chain, using oxygen as the final electron acceptor to form water. The energy released during this process is used to create a hydrogen ion (or proton) gradient across the inner membrane of the mitochondrion. Finally, the proton gradient is used to produce about 2.5 ATP for every NADH + H+ oxidized in a process called oxidative phosphorylation.
Aerobic regeneration of NAD+ and further catabolism of pyruvate In aerobic eukaryotes a complex mechanism has developed to use the oxygen in air as the final electron acceptor in a process called oxidative phosphorylation Aerobic prokaryotes which lack mitochondria use a variety of simpler mechanisms Firstly the NADH + H+ generated by glycolysis has to be transferred to the mitochondrion to be oxidized and thus to regenerate the NAD+ necessary for glycolysis to continue However the inner mitochondrial membrane is impermeable to NADH and NAD+ Use is therefore made of two "shuttles" to transport the electrons from NADH across the mitochondrial membrane They are the malate-aspartate shuttle and the glycerol phosphate shuttle In the former the electrons from NADH are transferred to cytosolic oxaloacetate to form malate The malate then traverses the inner mitochondrial membrane into the mitochondrial matrix where it is reoxidized by NAD+ forming intra-mitochondrial oxaloacetate and NADH The oxaloacetate is then re-cycled to the cytosol via its conversion to aspartate which is readily transported out of the mitochondrion In the glycerol phosphate shuttle electrons from cytosolic NADH are transferred to dihydroxyacetone to form glycerol-3-phosphate which readily traverses the outer mitochondrial membrane Glycerol-3-phosphate is then reoxidized to dihydroxyacetone donating its electrons to FAD instead of NAD+ This reaction takes place on the inner mitochondrial membrane allowing FADH2 to donate its electrons directly to coenzyme Q (ubiquinone) which is part of the electron transport chain which ultimately transfers electrons to molecular oxygen  with the formation of water and the release of energy eventually captured in the form of ATP The glycolytic end-product pyruvate (plus NAD+) is converted to acetyl-CoA and NADH + H+ within the mitochondria in a process called pyruvate decarboxylation The resulting acetyl-CoA enters the citric acid cycle (or Krebs Cycle) where the acetyl group of the acetyl-CoA is converted into carbon dioxide by two decarboxylation reactions with the formation of yet more intra-mitochondrial NADH + H+ The intra-mitochondrial NADH + H+ is oxidized to NAD+ by the electron transport chain using oxygen as the final electron acceptor to form water The energy released during this process is used to create a hydrogen ion (or proton) gradient across the inner membrane of the mitochondrion Finally the proton gradient is used to produce about 25 ATP for every NADH + H+ oxidized in a process called oxidative phosphorylation