Carbohydrates are made up of carbon (C), hydrogen (HE) and oxygen (O) atoms and almost all carbohydrates have the empirical formula Can (H20) n, where, n is the number of carbon atoms the carbohydrate contains. Structurally carbohydrates can be represented as polyhydroxy aldehydes or ketones.
Carbohydrates are also known as saccharides. The word saccharide is derived from the Greek word, saccharon, meaning sweet.
Carbohydrates are of three types: (i) monosaccharides, (ii) oligosaccharides and (iii) polysaccharides.
Monosaccharide is single polyhydroxy aldehyde (aldose) or ketone (ketose) units. Glucose is a single polyhydroxy aldehyde, while fructose is a single polyhydroxy ketone. A monosaccharide contains a skeleton, of carbon atoms. The minimum number of carbon atoms is three and it can go up to seven.
The monosaccharides are named as trioses, tetroses, pentoses, hexoses and heptoses based on the presence of three, four, five, six and seven carbon atoms, respectively. If we combine both the characters, the monosaccharides glucose and fructose are named as aldohexose and ketohexose, respectively. In monosaccharides are enlisted based on their form (aldehyde / ketone) and the number of carbon atoms they contain.
If you look at the, the prefix ‘D’ and ‘L’ are used to the names, glucose an fructose. Both the forms have the same molecular formula (same number of C, H and O atoms but different directions of the H atoms and OH groups.
The structure is three dimensional and the groups are changeable in space. Such compounds are known as stereo-isomers. Moreover, both forms are mirror images of each other. They are known as enantiomers or optical isomers. All optical isomers have a property of rotating the plane of the polarized light in a clock-wise or counter clock-wise direction.
This property is known as optical activity. When the plane of the polarized light is rotated in a clock-wise (right) direction, the prefix ‘D’ (Dextrorotatory) is used to denote the compound. Conversely, when the rotation is in a counter clock-wise direction, the prefix ‘L’ (Levorotatory) is used.
The central carbon atom of glyceraldehyde has four different groups attached to it. They are the aldehyde group (CHO), hydrogen atom (H), hydroxyl group (OH) and the hydroxymethyl group (CH2OH). The central carbon atom is the chiral (asymmetric) carbon atom. If the configuration around this carbon atom changes, i.e. the H atom and the OH group change sides, the new molecule will be the same glyceraldehyde. However, it will be a mirror image of the previous glyceraldehyde.
Therefore, glyceraldehyde may occur in two mirror image forms. We refer these two forms as to optically active forms /optical isomers / enantiomers. One form is D- glyceraldehyde, where the OH group is situated on the right side of the chiral carbon atom, while the other is L-glyceraldehyde, where the OH group is on the left side.
These two forms of the same compound constitute an enantiomeric pair. The monosaccharide having four carbon atoms will have two chiral carbon atoms and will exist in four optical isomers and hence, two enantiomeric pairs.
Oligosaccharides (oligos meaning few) are carbohydrates containing two to eight monosaccharides joined by glycosidic bonds. A glycosidic bond is formed when the hydroxyl groups attached to two adjacent monosaccharides react and eliminate a molecule of water.
Disaccharides are two monosaccharide containing oligosaccharides and trisaccharides three and so on. The disaccharide maltose is represented in the summarizes some common disaccharides found in nature.
Polysaccharides are carbohydrates that contain many (more than eight) monosaccharide units (monomers). The monomers join with each other by glycosidic bonds and form a polymer. Some polysaccharides are straight chain polymers, while some others are branched chain polymers.
Cellulose, the polysaccharide of the plant cell wall is a straight chain polymer of B-D-Glucose monomers, while starch and glycogen are branched chain polymenrs. When a polysaccharide contains similar monomelic units, it is a homopolysaccharide.
On the other hand, if it contains different monomeric units, it is called a heteropolysaccharide. Summarizes the properties of some commonly occuring polysachharide in nature.