Carbohydrates – characteristics, classification. Monosaccharides – structure, isomerism, physical properties chemical properties, representatives
.
Carbohydrates are a polyhydroxy aldehyde or polyhydroxy ketone, or a compound that hydrolyzes them. Depending on the number of elementary units, carbohydrates are divided into four chemical types: monosaccharide, disaccharide, oligosaccharide, and polysaccharide.
A monosaccharide or “simple sugars” is a carbohydrate that on attempted hydrolysis is not cleaved to smaller carbohydrates. Monosaccharides are the simplest carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The general chemical formula of an unmodified monosaccharide is (C•H2O)n, literally a "carbon hydrate". Monosaccharides are important fuel molecules as well as building blocks for more Complex carbohydrates. The smallest monosaccharides, for which the number of carbon atoms n = 3, are dihydroxyacetone and D- and L-glyceraldehyde.
A disaccharide on hydrolysis is cleaved to two monosaccharide units, which may be the same or may be different. For instance, sucrose – common table sugar – is a disaccharide that yields one molecule of glucose and one of fructose on hydrolysis.
An oligosaccharide (oligos is a Greek word meaning “few”) gives 3 to 10 monosaccharide units on hydrolysis. A general property of monosaccharides, disaccharides and most of oligosaccharides is their high solubility in water and usual, characteristic sweet taste.
Polysaccharide is hydrolyzed to more than 10 monosaccharide units. For instance, cellulose is a homopolysaccharide that gives thousands of identical glucose molecules when completely hydrolyzed.
when completely hydrolyzed. The natural biopolymers belong to the polysaccharide type. Polysaccharides (and oligosaccharides) are called also complex carbohydrates. They can be subdivided into two groups, depending on the nature of their monomeric units (which are the monosaccharides building the polymer):
- Homopolysaccharides contain as building block identical monosaccharide units;
- Heteropolysaccharides contain more than one type of monosaccharide.
Classification of monosaccharides is available according to three criteria:
- whether the monosaccharide contains an aldehyde or a keto group;
- the number of carbon atoms in their chain;
- The stereochemical configuration of the chiral carbon atom that is farthest from the carbonyl group.
Monosaccharides that are polyhydroxyaldehydes are called aldoses; those that are polyhydroxy Ketones are ketoses. Aldoses and ketoses are further classified according to the number of carbon atoms in the main chain.