Question

Living systems are made up of various complex biomolecules like carbohydrates, proteins, nucleic acids, lipids, etc. Carbohydrates are optically active polyhydroxy aldehydes or ketones or molecules which provide such units on hydrolysis. They are broadly classified into three groups — monosaccharides, oligosaccharides and polysaccharides. Monosaccharides are held together by glycosidic linkages to form disaccharides like sucrose, maltose or polysaccharides like starch and cellulose. Another biomolecule : proteins are polymers of α-amino acids which are linked by peptide bonds. Ten amino acids are called essential amino acids. Structure and shape of proteins can be studied at four different levels i.e. primary, secondary, tertiary and quaternary, each level being more complex than the previous one. Answer the following questions :

Carbohydrates are essential for life in both plants and animals. Carbohydrates are used as storage molecules as starch in plants and glycogen in animals. Chemically they are polyhydroxy aldehydes or ketones. On the basis of their behaviour on hydrolysis, carbohydrates are classified as monosaccharides, oligosaccharides and polysaccharides. All monosaccharides are reducing sugars, i.e., they are oxidized by Tollens' reagent and Fehling's solution. A monosaccharide like glucose is aldohexose and its molecular formula was found to be C₆H₁₂O₆. After reacting with different reagents like HI, H₂N – OH, Bromine water, (CH₃CO)₂O, etc. its structure was found to contain one aldehyde group, one primary alcoholic group, (– CH₂OH) and four secondary alcoholic groups (> CHOH). Despite having the aldehyde group, glucose does not give some of the reactions of aldehyde group like Schiff's test, NaHSO₃ addition. This explains the existence of glucose in two cyclic hemiacetal forms which differ only in the configuration of the hydroxyl group at C – 1.

Assertion (A) : Fructose is a reducing sugar. Reason (R) : Fructose does not reduce Fehling solution and Tollen's reagent.

Define the following terms :