Read the passage carefully and answer the questions:

Optically active polyhydroxy aldehydes or ketones or molecules which provide such units on hydrolysis are called carbohydrates. They can be broadly classified as monosaccharides, disaccharides and polysaccharides depending on their behavior towards hydrolysis. They are also classified as reducing or non reducing sugars. Another class of biomolecules is proteins which are polymers of twenty different amino acids linked by peptide bonds. Amino acids can be classified as acidic, basic or neutral depending on the relative number of amino and carboxyl groups in their molecule. The essential amino acids cannot be synthesized by our body and hence must be provided through diet. Nucleic acids are the polymers of nucleotides which in turn consist of a base, pentose sugar and phosphate. There are two types of nucleic acids namely, DNA and RNA which differ in the composition of sugar and a base.

Which of the following is non-reducing sugar?

Sucrose

The correct answer is Option (3) → Sucrose

Sugars are classified as reducing or non-reducing based on whether they possess a free aldehyde (–CHO) or ketone (>C=O) group that can reduce mild oxidizing agents such as Tollens’ reagent or Fehling’s solution.

If the anomeric carbon is free, the sugar behaves as a reducing sugar.

If the anomeric carbons are involved in glycosidic bond formation, the sugar becomes non reducing.

Sucrose

Sucrose is formed by the condensation of: Glucose + Fructose

The glycosidic linkage is between: C₁ of glucose and C₂ of fructose

Since both anomeric carbons are involved in the glycosidic bond, no free aldehyde or ketone group is available.

Therefore, sucrose cannot reduce Fehling’s or Tollens’ reagent, making it a non-reducing sugar.

Option 1 Maltose Incorrect. Maltose has one free anomeric carbon, so it behaves as a reducing sugar.

Option 2 Glucose Incorrect. Glucose contains a free aldehyde group and is a reducing sugar.

Option 3 Sucrose Correct. Both anomeric carbons are involved in bonding, so it is non-reducing.

Option 4 Fructose Incorrect. Although fructose is a ketose, it behaves as a reducing sugar due to tautomerism in alkaline medium.