Carbohydrates are polyhydroxyl aldehydes or ketones, or substances that yield
such compounds on hydrolysis. Many, but not all, carbohydrates have the
empirical formula (CH O)n: some also contain nitrogen, phosphorus, or sulfur.
There are 3 major size classes of carbohydrates:
Also known as simple sugars, consist of a single polyhydroxyl aldehyde or ketone
unit. The most abundant monosaccharide in nature is the six-carbon sugar D-
glucose, sometimes referred to as dextrose. Monosaccharides of four or more
carbons tend to have cyclic structures.
Consists of short chains of monosaccharide units, or residues, joined by
characteristic linkages called glycosidic bonds. The most abundant are the
dissacharides, with two monosaccharide units. Typical is sucrose, which consists
of the six-carbon sugars D-glucose and D-fructose. In cells, most oligosaccharides
consisting of three or more units do not occur as free entities but are joined to
non-sugar molecules (lipids and proteins) in glycoconjugates.
Sugar polymers containing more than 20 or so monosaccharide units. Some
polysaccharides, such as cellulose, are linear chains; others, such as glycogen, are
branched. Both glycogen and cellulose consist of recurring units of D-glucose, but
they differ in the type of glycosidic linkage and consequently have strikingly
different properties and biological roles.
Monosaccharides and disaccharides:
The simplest of the carbohydrates, the monosaccharides, are either aldehydes or
ketones with two or more hydroxyl groups; the six-carbon monosaccharide
glucose and fructose have five hydroxyl groups. Many of the carbon atoms to
which hydroxyl groups are attached are chiral centers, which give rise to the
many sugar stereoisomers found in nature. Stereoisomerism in sugars is
biologically significant because the enzymes that act on sugars are strictly
stereospecific, typically preferring one stereoisomer to another by three or more
orders of magnitude, as reflected in Km values or binding constants.
The Two Families of Monosaccharides are Aldoses and Ketoses:
Monosaccharises are colorless, crystalline solids that are freely soluble in water
but insoluble in nonpolar solvents. The backbones of common monosaccharides
are unbranched carbon chains in which all the carbon atom