On the basis of number of sugar units, there are three major classes of carbohydrates:
Monosaccharide (Greek: mono = one)
Monosaccharides are simplest carbohydrates and are often referred to as simple sugars as they cannot be further hydrolysed. They consist of a single polyhydroxy 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. They have general formula Cn(H20)n. The backbones of common monosaccharides are unbranched carbon chains in which all carbon atoms are linked by single bonds. In this open-chain form, one of e carbon atoms is double-bonded to an oxygen atom to form a carbonyl group; each of the other carbon atoms has a hydroxyl group.
If carbonyl group is at an end of carbon chain (i.e. in an aldehyde group) the monosaccharide is an aldose, e.g. glyceraldehyde, glucose, etc. If the carbonyl group is at any other position (in a ketone group) the monosaccharide is a ketose, e.g. dihydroxyacetone, fructose, etc.
Many of carbon atoms to which hydroxyl groups are attached are chiral centers, which give rise to the many sugar stereoisomers found in nature.
Based on number of carbon atoms, the monosaccharides are regarded as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C). There are aldoses and ketoses of each of these chain lengths: aldotetroses and ketotetroses, aldopentoses and ketopentoses, and so on. Glucose is an aldohexose while fructose is a ketohexose.
Oligosaccharides (Greek: oligo = few)
Oligosaccharides contain 2-10 monosaccharide molecules which are liberated on hydrolysis. Based on the number of monosaccharide units present, oligosaccharides are further subdivided to disaccharides, trisaccharides etc. They are short polymers of monosaccharides joined together by glycosidic bonds.
Disaccharides (such as maltose, lactose, and sucrose) consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar molecule, typically cyclic, reacts with anomeric carbon of another sugar. Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base. Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid.
Disaccharides can be:
- Reducing disaccharides: Have free aldehyde or keto group e.g. Maltose, Lactose.
- Non-reducing disaccharides: Lacks free aldehyde or keto group e.g. Sucrose, Trehalose .
Polysaccharides (Greek: poly = many)
Polysaccharides are polymers of monosaccharide units with high molecular weight (up to a million). They are usually tasteless (non-sugars) and form colloids with water. It consist of repeat units of monosaccharides or their derivatives, held together by glycosidic bonds.
It can be linear as well as branched polymers. Occurrence of branches in polysaccharides is due to the fact that glycosidic linkages can be formed at any one of hydroxyl groups of a monosaccharide units.
They are primarily concerned with two important functions: structural components and storage of energy.
Polysaccharides, also called glycans, differ from each other in identity of their recurring monosaccharide units, types of bonds linking the units, degree of branching and length of their chains.
Polysaccharides are of two types:
- Homopolysaccharides: These are composed of polymers of single type of monosaccharide units. E.g. Starch, Glycogen, Cellulose, Chitin.
- Hetropolysaccharides: Made up of two or more different monosaccharides or their derivatives. E.g. Hyaluronic Acid, Heparin.