Expert: John Locke - 12/4/2009

Question
Q: how many molecules of carbon dioxide would be released from the complete aerobic respiration of a molecule of sucrose, a disaccharide and why?

Answer
Thanks for using AllExperts. Sucrose consists of one glucose and one fructose molecule joined via a glycosidic bond; to utilize sucrose as a source of energy, that bond must be broken first to produce one free molecule each of glucose and fructose. The glucose molecule can then proceed through glycolysis and the citric acid cycle normally. A single glucose molecule produces two molecules of pyruvate, one molecule of which produce two CO2 molecules via one completion of the citric acid cycle. Accordingly, complete metabolism of one molecule of glucose in the CAC produces 4 CO2 molecules.

Fructose is metabolized entirely in the liver and is directed almost entirely toward the replenishment of glucose (gluconeogenesis) and synthesis of triglycerides, in which case no carbon dioxide is produced by the fructose molecule. However, it can be directed into the glycolytic pathway (as pyruvate) under the right conditions and from there into the CAC. In this case, one molecule of fructose would produce two molecules of pyruvate, and oxidation via the CAC would produce 4 CO2 molecules.  Aerobic respiration of one molecule of sucrose therefore produces a maximum of 8 molecules of CO2, but this will not occur under all circumstances.