Carbohydrates: Types and Classification

Study Notes

Carbohydrates are defined as C:H:O in a 1:2:1 ratio, and can be represented by the general formula CH2OH.
They can be classified into three main categories: monosaccharides, disaccharides, and polysaccharides.

Monosaccharides are the simplest carbohydrates, and can be further classified into aldoses and ketoses.
Examples of monosaccharides include glyceraldehyde (a triose), erythrose (a tetrose), ribose (a pentose), and glucose, galactose, and fructose (hexoses).
Monosaccharides are optically active, contain at least one asymmetric carbon (chiral center), and can exist as stereoisomers.
Stereoisomers of monosaccharides can be classified into enantiomers, epimers, and anomers.

Enantiomers are non-superimposable mirror images of each other, and can exist in D or L conformation.
Epimers are sugar molecules that differ in configuration at only one of several chiral centers.
Anomers are cyclic ring structures that can be formed by aldhyde or ketone groups, and can exist in α or β configurations.

Disaccharides are formed by the combination of two monosaccharides through a glycosidic bond.
Examples of disaccharides include sucrose (glucose + fructose), lactose (galactose + glucose), and maltose (glucose + glucose).

Reducing sugars contain a reactive carbonyl group, are readily oxidized, and can reduce metal ions.
Non-reducing sugars, such as sucrose, have both anomeric carbon atoms fixed in a glycosidic linkage, and are not readily oxidized.

Polysaccharides are larger, containing hundreds of monosaccharides.
They can be classified into homopolysaccharides (composed of a single monosaccharide building block) and heteropolysaccharides (composed of different building blocks).
Examples of polysaccharides include glycogen, starch, and cellulose.

Glycogen is the major form of stored carbohydrate in animals, composed of α(1,4) glucose linkages, and highly branched by α(1,6) linkages every 8-10 residues.
Starch is the major form of stored carbohydrate in plants, similar in structure to glycogen but less extensively branched.

Cellulose is a structural polysaccharide composed of long, straight, non-branching chains of β-D-glucose linked by β-(1-4) glucosidic bonds.
It is insoluble in water, cannot be digested by humans, and plays an important role in the diet by increasing the bulk of stool and stimulating intestinal movement.