Following glycolysis or one of the alternative pathways, pyruvic acid can be further oxidized by a transitional step involving a process called decarboxylation.  In this step, an enzyme containing coenzyme A removes a carbon dioxide molecule producing a new two-carbon intermediate called acetaldehyde.  Coenzyme A binds to this molecule, forming the new intermediate acetyl coenzyme A or acetyl-CoA.

Though no ATP is generated, a molecule of NAD is reduced.  The hydrogens and electrons stored by reduced NAD will either be used in the final stages of respiration to generate new ATP, or will be given to an intermediate organic compound during fermentation.

The Kreb's Cycle

In the complete oxidation of glucose during respiration, acetyl-CoA releases acetaldehyde either into the cytoplasm of the bacterial cell or the mitochondrial matrix of the eukaryotic cell.  In each case, the two-carbon acetaldehyde is enzymatically joined to a four-carbon compound called oxaloacetic acid.  This produces a new six-carbon compound called citric acid.  Citric acid is decarboxylated twice and the four-carbon intermediate is ultimately converted back into oxaloacetic acid.

During this cyclic pathway, two molecules of CO2 are released as waste, three NAD molecules are reduced to NADH+H+, one FAD is reduced to FADH2 and one ATP molecule is generated by substrate-level phosphorylation.  Since two molecules of pyruvic acid are produced for each original glucose molecule, the Kreb's Cycle reactions occur twice.

After glycolysis, decarboxylation and the Kreb's Cycle reactions have occurred, all of the carbons and oxygens in the original glucose molecule have been released as CO2.  The hydrogens and and their electrons have been used to reduce NAD and FAD and will be delivered to the coupled reactions of electron transport and chemiosmosis.

Electron Transport

In electron transport, the reduced coenzymes NAD and FAD release their high-energy electrons and hydrogens to specialized molecules called flavoproteins, metal-containing ions and cytochromes found in the plasma membrane of prokaryotes or the inner membrane (cristae) of the eukaryote mitochondrion.  Electrons reduce and oxidize these molecules as they pass from one to the next, releasing small amounts of energy.  Ultimately the lower-energy electrons are attracted to singlet oxygen brought into the cell or mitochondrial matrix by diffusion.  Oxygen serves as the final electron acceptor in the process of complete aerobic respiration.


Hydrogens from reduced NAD and FAD are pumped outside the membrane and begin to accumulate, forming a concentration (proton) gradient.  This lowers the pH and results in protonmotive force that serves to drive the hydrogen ions through protein channels called ATP synthase (ATPase) molecules.  As hydrogens pass through ATPase, the enzyme links inorganic phosphates present in the cytoplasm or mitochondrial matrix to ADP molecules.  Once in the cytoplasm, each pair of hydrogens is bound to oxygen, forming water.  The complete breakdown of sugar and production of new ATP in the using oxygen as the final electron and hydrogen acceptor is called oxidative phosphorylation.

Summary of ATP Production via Oxidative Phosphorylation


ATP used to phosphorylate glucose                                                          -2

ATP generated by the substrate-level phosphorylation                             +4

Net ATP Gain                                                                                           +2

Molecules of reduced NAD          +2 (after chemiosmosis/e- transport)     +6

Decarboxylation of Pyruvic Acid

Molecules of CO2 released as waste  2

Molecules of reduced NAD          +2 (after chemiosmosis/e- transport)     +6

Kreb's Cycle

Molecules of CO2 released as waste  4

ATP generated by the substrate-level phosphorylation                             +2

Molecules of reduced NAD          +6 (after chemiosmosis/e- transport)   +18

Molecules of reduced FAD           +2 (after chemiosmosis/e- transport)    +4

TOTAL ATP                                                                                          +38

Anaerobic Respiration

In anaerobic respiration, the inorganic final electron acceptor is an ionic compound other than oxygen such as nitrate, nitrite or sulphate.  Anaerobic repiration generally produces less ATP than aerobic respiration.