Question

A, B and C are three biomolecules. The results of the tests performed on them are given below:

| Molisch’s test| Barfoed Test| Biuret test
---|---|---|---
A| Positive| Negative| Negative
B| Positive| Positive| Negative
C| Negative| Negative| Positive

A, B and C are respectively:
A= Glucose, B= fructose, C= Albumin
A= Lactose, B= Glucose, C= Alanine
A= Lactose, B= Fructose, C= Alanine
A= Lactose, B= Glucose, C= Albumin

Answer 1

To answer this question, you must recall the tests performed in the question and the substances they identify. All these tests are used for checking the presence of carbohydrates and proteins in a given substance.

Complete answer:
We know that Molisch’s test is used to test all types of carbohydrates. That means both A and B are carbohydrates.
The Barfoed’s test is used for the identification of monosaccharide sugars. Thus, biomolecule B is a monosaccharide.
The Biuret’s test is used to detect the presence of a peptide bond which is present in an amino acid. Thus, C is a protein.

According to the above options, the correct answer is D.

Note:
Molisch’s test is a test for the identification of carbohydrates. In this test, carbohydrates are made to react with Molisch’s reagent and then with concentrated sulphuric acid. Molisch reagent is alpha naphthol dissolved in ethanol. It dehydrates the carbohydrate molecules forming furfural or its derivatives which on further reaction with the sulphonated alpha naphthol form a purple colored complex thus detecting the presence of carbohydrates. This test is given by all carbohydrates, i.e., monosaccharides, disaccharides and polysaccharides.
Barfoed test is used to distinguish between monosaccharide and disaccharide sugar. It involves the reduction of the sugar in acidic medium. In this test, a mixture of corrosive ethanoic acid and cupric acetate is added to the sample solution and is then boiled. Monosaccharides being reduced by nature react faster than disaccharides producing a red precipitate of cuprous oxide.
Biuret’s test involves the reaction of the given substance with cupric sulphate. The copper ions form a complex with peptide bonds of proteins, thus detecting their presence.