Editing Biochemistry (section)

====Aerobic====
In [[aerobic glycolysis|aerobic]] cells with sufficient [[oxygen]], as in most human cells, the pyruvate is further metabolized. It is irreversibly converted to [[acetyl-CoA]], giving off one carbon atom as the waste product [[carbon dioxide]], generating another reducing equivalent as [[NADH]]. The two molecules acetyl-CoA (from one molecule of glucose) then enter the [[citric acid cycle]], producing two molecules of ATP, six more NADH molecules and two reduced (ubi)quinones (via [[FADH2|FADH<sub>2</sub>]] as enzyme-bound cofactor), and releasing the remaining carbon atoms as carbon dioxide. The produced NADH and quinol molecules then feed into the enzyme complexes of the respiratory chain, an [[electron transport system]] transferring the electrons ultimately to oxygen and conserving the released energy in the form of a proton gradient over a membrane ([[inner mitochondrial membrane]] in eukaryotes). Thus, oxygen is reduced to water and the original electron acceptors NAD<sup>+</sup> and [[quinone]] are regenerated. This is why humans breathe in oxygen and breathe out carbon dioxide. The energy released from transferring the electrons from high-energy states in NADH and quinol is conserved first as proton gradient and converted to ATP via ATP synthase. This generates an additional ''28'' molecules of ATP (24 from the 8 NADH + 4 from the 2 quinols), totaling to 32 molecules of ATP conserved per degraded glucose (two from glycolysis + two from the citrate cycle).<ref>[[#Voet|Voet]] (2005), Ch. 17 Glycolysis.</ref> It is clear that using oxygen to completely oxidize glucose provides an organism with far more energy than any oxygen-independent metabolic feature, and this is thought to be the reason why complex life appeared only after Earth's atmosphere accumulated large amounts of oxygen.