In our previous lesson on glycolysis, we saw that during glycolysis, 6-
carbon molecules of glucose are partially oxidized to produce two 3-carbon molecules of
pyruvate. This process releases some, but not all, of the energy stored in the bonds of the
glucose molecule, and results in a net production of 2 ATP molecules and 2 molecules of
reduced NADH + H+. For many organisms, however, glycolysis is only the beginning of
the story in terms of energy production. Organisms that are able to perform aerobic
respiration can fully oxidize glucose and utilize more of its energy through glycolysis and
three more pathways that occur after glycolysis. These three pathways, in the order that
they occur, are pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.
The complete oxidation of glucose through these pathways, as you might expect, releases
more energy than glycolysis alone. This energy, in turn, is ultimately captured in the
bonds of ATP. In fact, the pathways of aerobic respiration release around 34 more
molecules of ATP per molecule of glucose, resulting in a total of 36 molecules of ATP
per molecule of glucose – 18 times the amount of energy produced by glycolysis alone!
The pathways of aerobic respiration begin where glycolysis ended – with
pyruvate. Pyruvate oxidation occurs at the inner mitochondrial membrane of eukaryotic
cells, and at the cell membrane of prokaryotic cells. During this process, 3-carbon
pyruvate is oxidized to 2-carbon acetate, which is then bound to a second molecule,
called Coenzyme A. The energy-releasing oxidation reaction is coupled to the energy-
requiring reduction of NAD+ to NADH + H+. The energy and electrons thus captured in
NADH + H+ will play an important role later, so don’t forget this molecule.
In addition, note that one carbon molecule from pyruvate is completely oxidized and
released as carbon dioxide. What does this mean in terms of our original 6-carbon
glucose molecule? Remember tha