Monosaccharides are the simplest form of carbohydrates. They may be classified based on the number of constituent carbon atoms. For instance, hexose is a monosaccharide with six carbon atoms. They may also be classified based on the functional group present: aldose and ketose. Glucose, galactose, and fructose with the same chemical formula: C6H12O6 are hexose monosaccharides. Glucose and galactose are aldoses whereas fructose is a ketose.
Fructose is a ketonic monosaccharide since it has a reducing group (carbonyl) at carbon 2. This is in contrast to glucose (which is an aldose) that has its carbonyl group at carbon 1.
Fructose occurs naturally in plants, particularly in fruits, root vegetables, etc. It occurs freely or bonded to glucose to form sucrose. Sucrose (the common table sugar) is a non-reducing disaccharide that forms when glucose and fructose are linked together by an alpha linkage between the carbon 1 of glucose and the carbon 2 of fructose.
Fructose, glucose and galactose are regarded as the dietary monosaccharides since they are readily absorbed by the small intestines. When present as a component of dietary sucrose, the enzyme invertase in the small intestine cleaves sucrose into glucose and fructose. Too much fructose, though, could lead to malabsorption in the small intestine. When this happens, unabsorbed fructose transported to the large intestine could be used in fermentation by the colonic flora. This could lead to gastrointestinal pain, diarrhea, flatulence, or bloating.
Word origin: Latin fructus (“fruit”) + -ose (“sugar”)
- fruit sugar
- Sorbitol pathway
- Lobry de bruyn-van ekenstein transformation
- Resorcinol test
- Fructose intolerance
- Fructose metabolism inborn errors
- Fructose permease
- Hereditary fructose intolerance
- fructo- (prefix, denoting the fructose configuration)