Carbohydrates are polyhydroxy aldehydes, ketones, alcohols, acids and their polymers. They are classified into monosaccharides, disaccharides (two monosaccharides), oligosaccharides (3 to 10 monosaccharides) and polysaccharides (more than 10 monosaccharides). Functions of carbohydrates : energy substrates, formation of vital metabolic intermediates; as components of nucleic acids (DNA, RNA), and coenzymes; modulation of protein structure and function; as essential constituents in the ground substance of connective tissue; as storage form of energy (glycogen).