Classification of Carbohydrates

Classifying Carbohydrates by Hydrolysis

Carbohydrates are sorted according to how they behave when hydrolysed — that is, by how many (if any) simpler sugar units they break down into on treatment with acid or water. This gives three broad groups.

1. Monosaccharides

A monosaccharide is a carbohydrate that cannot be hydrolysed any further into a simpler polyhydroxy aldehyde or ketone — it already is the simplest unit. Roughly 20 monosaccharides are known to occur naturally; glucose, fructose and ribose are common examples.

2. Oligosaccharides

Oligosaccharides are carbohydrates that, on hydrolysis, break down into two to ten monosaccharide units. They are further named by exactly how many units they yield:

• Disaccharides — 2 units (by far the most common oligosaccharides)
• Trisaccharides — 3 units
• Tetrasaccharides — 4 units, and so on

The two monosaccharide units released from a disaccharide need not be identical:

• Sucrose hydrolyses to one molecule of glucose and one molecule of fructose (two different units).
• Maltose hydrolyses to two molecules of glucose (two identical units).

3. Polysaccharides

Polysaccharides yield a large number of monosaccharide units on hydrolysis. Starch, cellulose, glycogen and gums are common examples. Unlike mono- and oligosaccharides, polysaccharides are not sweet to taste, which is why they are also referred to as non-sugars.

Reducing and Non-Reducing Sugars

Independently of the hydrolysis-based grouping above, carbohydrates are also classified by whether they can reduce two standard test reagents — Fehling's solution and Tollens' reagent:

• A carbohydrate that reduces these reagents is a reducing sugar.
• One that does not is a non-reducing sugar. …