Carbohydrates are relatively small molecules that are generally called as simple sugars (monosaccharides and disaccharides) as well as macromolecular substances (starch, glycogen, and celluloses). The word sugar emphasises the idea of energy in our minds. From the study of the structure–function relationship of biomolecules, it is apparent that each and every molecule differs structurally and also functionally. Table sugar, which is commonly used as food, is sucrose, whereas the sugar that is mainly present in fruits is fructose. A small change in the structural configuration can lead to changes in the functional properties as well. For example, when there is an α-linkage between glucose molecules, the resulting molecule is called a homopolysaccharide (starch/glycan). When the linkage is β, then the resulting molecule is called a cellulose, which is not digested by the human body. However, only the β-form, and not the α–form, is absorbed by the human body. Carbohydrates are not only considered the source of energy but also used for the synthesis of lipids, amino acids, glycoproteins, and proteoglycans in the system.