Carbohydrates are polyhydroxy aldehydes or ketones or compounds, which yield them on hydrolysis. Carbohydrates are classified into monosaccharides, disaccharides and polysaccharides, based on the number of sugar/saccharide units they possess. They could also be classified as aldoses and ketoses based on the functional group they possess. Common examples of monosaccharides include Glucose, Fructose, Galactose, and Mannose. Common examples of disaccharides are Sucrose, Lactose and Maltose. Monosaccharides exhibit stereoisomerism, optical isomerism, and anomerism and pyranose-furanose isomerism. All carbohydrates are considered to be derived from glyceraldehyde by successive addition of carbons. The penultimate carbon atom is thus the reference carbon atom for naming mirror images. The stereoisomers are prefixed as ‘D’ or ‘L’. D sugars are naturally occurring and human body can metabolize only D sugars. A carbon atom bound by four different groups on all its valencies is referred to as an asymmetric carbon. Anomers of monosaccharides are produced by the spatial configuration with reference to the first carbon atom in aldoses and the second carbon atom in ketoses. Two anomers of glucose are alpha-D glucose and beta-D glucose. Mutarotation is the result of anomerism. Optical isomerism is based on the rotation of plane polarized light by a pure solution of the sugar. The prefix ‘d’ or (+) and ‘l’ or (–) is used to indicate dextro and levorotatory compounds respectively. All reducing sugars form characteristic osazone crystals. Glucose and fructose form needle shaped crystals, maltose forms sun flower shaped crystals and lactose forms hedge-hog shaped crystals. Amino sugars form important components of mucopolysaccharides, e.g. galactosamine, glucosamine. Sucrose is formed from glucose and fructose linked by 1, 2 glycosidic linkage. Lactose is formed from galactose and glucose linked by beta 1, 4 glycosidic linkage. Maltose is formed from two glucose molecules linked by alpha 1, 4 glycosidic linkages. Starch is made of two components; straight chain amylose and branched amylopectin. The linkages are alpha-1, 6 types at branch points while alpha-1, 4 linkages form the straight chain. Action of amylase on starch yields limit dextrins. Mucopolysaccharides or Glycosaminoglycans (GAGs) such as Hyaluronic acid, Chondroitin sulfate, Keratan sulfate, and Dermatan sulfate are associated with connective tissue. Keratan sulfate is the only GAG that does not contain uronic acid. When the carbohydrate chains are attached to a polypeptide chain it is called a proteoglycan.