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Monosaccharides are the simplest form of carbohydrate. They form the basis of larger, more complex molecules such as disaccharides and oligosaccharides, which are formed in a dehydration reaction between two or more monosaccharides. Examples of monosaccharides include glucose and galactose, and they all share the generic formula Cx(H2O)y. They contain two or more hydroxyl groups and are classfied by the type of carbonyl group they contain; this being either a ketose (ketone group), for example glucose or an aldose (aldehyde group) such as fructose.

Many monosaccharides may form disaccharides (e.g. maltose) and polysaccharides (e.g. starch & glycogen) by condensationreactions, which form glycosidic bonds[1].

In Metabolism, the body absorbs Monosaccharides as Disaccharides and Polysaccharides are too large to be absorbed into the intestinal Epithelium. Therefore many Sugars go through a series of digestive processes before being able to be absorbed and accumulated within the body using Enzymes. Salivary amylase, pancreatic amylase and intestinal disaccharidases all contribute to the breakdown of polysaccharides to produce monosaccharides.



Isomers of a molecule have the same molecular formula but have different structures. Stereoisomerism occurs when atoms are connected in the same order, but they differ in their spatial arrangement.


The formation of a ring of a sugar generates an additional asymmetric carbon at position 1 for aldoses. This stereochemical isomer at C1 results in an α form and a β form, not a new sugar. These sugar isomers are called anomers and a new anomeric carbon is formed. If within the sugar the C1 and C5 have the same stereochemistry it is classed as a β-anomer, if the C1 and C5 have different stereochemistry it is classed as an α-anomer. This also occurs with ketoses but the anomeric carbon formed is C2 instead of C1.


Epimers are isomers that differ at one of several asymmetric carbon atoms, in glucose, for example, this could be when C2, C3 or C4 would differ in configuration. These epimers would be classed as different sugars from one another. For example D-Glucose and D-Mannose, these would be epimers at C2 and β-D-Glucose and β-D-Galactose are epimers at C4[2].


  1. Alberts, Johnson, Lewis, Raff, Roberts, Walter, (2008), Molecular biology of a cell, 5th edition, New York, Garland Science.
  2. Berg JM, Tymoczko JL, Gatto GJ Jr, Stryer L. Biochemistry. 8th Ed, New York: Kate Ahr Parker. 2015. pg 336
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