Click here to
Components of Cells
The Macromolecules
Disaccharides

Glycosides

Glucose is a reactive molecule. In the presence of acid it will combine with any other molecule that has an alcohol (-OH) group as part of its structure. The resulting, larger, molecule is called a glycoside.

For example, ethyl alcohol (also called ethanol, a component of beer, wine or spirits) will combine with glucose in the presence of acid, thus:

The new molecule is no longer pure glucose, so the name is changed from glucose to a glucoside, and the part of the name coming from the alcohol molecule is placed first. So the above molecule made from combining ethyl alcohol with glucose would be ethyl glucoside.

This is a diether product and is stable to attack by alkaline oxidizers, but can be converted back to glucose and ethyl alcohol (ethanol) by acid hydrolysis - which is exactly the reverse of the reaction that formed it.

There are many natural and important glycosides in cells and various organisms. Attaching a sugar molecule to a lipid, for example, makes the lipid more soluble in water and hence changes it properties. Cell membranes contain a lot of molecules which are modified in this way.

Disccharides

Monosaccharide molecules have a number of alcohol (-OH) reactive groups as part of their structure, so it possible for two monosaccharide molecules to react in the way shown above so as to form a "double-saccharide" or disaccharide.

For example, two glucose molecules can react with one another to form a "double-saccharide" or disaccharide called maltose, which has the scientific name of 4-O-alpha-D-Glucopyranosyl-D-glucose. Watch this molecule being formed by rolling over the buttons in the diagram below.

Maltose is a natural product produced by breaking down starch. Although it is similar to sucrose (another disaccharide made from glucose and fructose) the human toungue does not think it is as sweet, but humans have no trouble digesting it. Yeast will also digest maltose, and in the absence of oxygen, will convert part of the molecule into ethyl alcohol (ethanol) and part to carbon dioxide. This is the basis of the bread rising and beer fermenting.


BIOdotEDU
© 2004, Professor John Blamire