13.1 Carbohydrates

Carbohydrates are macromolecules defined as polyhydroxy aldehydes or ketones. They have general formula ${\mathrm{C}}_{\mathrm{n}}({\mathrm{H}}_2\mathrm{O}{)}_{\mathrm{n}}$$\mathrm{C_n(H_2O)_n}$ .

Carbohydrates are synthesized by plants through photosynthesis process from carbon dioxide and water in the presence of sunlight and green pigment chlorophyll.

13.1.1 Monosaccharides

Monosaccharides are the simplest sugars which cannot be hydrolyzed. They consist of 3 to 9 carbon atoms. Therefore, they are classified according to the number of carbon atoms in their molecules as trioses, tetroses, pentoses, hexoses, and so on. The important monosaccharides are hexoses like glucose and fructose, etc. Glucose is a pentahydroxy aldehyde while fructose is pentahydroxy ketone having the open chain structures as follows and general formula ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$$\mathrm{C_6H_{12}O_6}$

Monosaccharides are white crystalline solids. They are soluble in water and have sweet taste. They cannot be hydrolyzed. They are reducing in nature, therefore, these are called reducing sugars.

13.1.2 Oligosaccharides

Oligosaccharides give 2 to 9 units of monosaccharides on hydrolysis. Therefore, they are classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of units they produce on hydrolysis. The most important oligosaccharides are disaccharides like sucrose. On hydrolysis, sucrose produces one unit of glucose and one unit of fructose.

These carbohydrates are white, crystalline solids easily soluble in water. They are also sweet in taste. They may be reducing or non- reducing.

13.1.3 Polysaccharides

Polysaccharides are macromolecular carbohydrates consisting of hundreds to thousands of monosaccharides. Examples of polysaccharides are starch and cellulose. They are amorphous solids. They are tasteless and insoluble in water. They are non- reducing in nature.

13.1.4 Sources and Uses of Carbohydrates

Carbohydrates range from simple to complex ones. They have varied sources and uses. Sources of simple sugars e.g. glucose, fructose and galactose are fruits, vegetables, honey and cereals.