Glycolysis occurs in almost all living organisms, but it does not generate very much energy in the form of ATP. The end product, pyruvate, still contains a good deal of chemical potential energy in its bonds. In the presence of oxygen, pyruvate can be further oxidized to release more energy, first to acetyl-CoA and then even further in the series of reactions in the citric acid cycle. Pyruvate oxidation is a key step that links glycolysis to the citric acid cycle. In eukaryotes, this is the first step that takes place inside the mitochondria.