1) Why do many proteins give positive result for Molisch test?

2) What is the composition of Barfoed’s reagent?

3) Of what use is Barfoed’s test in identifying unknown sugar?

4) From what other sources are pentoses obtained?

5) How can Seliwanoff’s test be used be distinguish fructose from sucrose?

6) What are the other source of glycogen? Of starch?

7) Give another name for glycogen and another name for starch.

Answer:

1. A positive reaction for Molisch's test is given by almost all carbohydrates (exceptions include tetroses & trioses). It can be noted that even some glycoproteins and nucleic acids give positive results for this test (since they tend to undergo hydrolysis when exposed to strong mineral acids and form monosaccharides).

2. Barfoed's reagent consists of a 0.33 molar solution of copper (II) acetate in 1% acetic acid solution.

3. Barfoed's test is a chemical test used for detecting the presence of monosaccharides. It is based on the reduction of copper(II) acetate to copper(I) oxide (Cu₂O), which forms a brick-red precipitate.

4. Pentoses are very important in biochemistry. Ribose is a constituent of RNA, and the related molecule, deoxyribose, is a constituent of DNA. Phosphorylated pentoses are important products of the pentose phosphate pathway, most importantly ribose 5-phosphate (R5P), which is used in the synthesis of nucleotides and nucleic acids, and erythrose 4-phosphate (E4P), which is used in the synthesis of aromatic amino acids.

5. Seliwanoff's test is used to distinguish aldoses from ketoses. On treatment with conc. Acid, ketoses are dehydrated more rapidly to give furfural derivatives and on condensation with resorcinol give cherry red complex. The test will be answered by fructose, sucrose and other keto containing carbohydrates.

Result Interpretation of Seliwanoff's test:

• Positive seliwanoff's test: Fructose and sucrose
• Negative seliwanoff's test: glucose, distilled water

6. Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants as energy storage. It is the most common carbohydrate in human diets and is contained in large amounts in staple foods like potatoes, maize (corn), rice, wheat and cassava (manioc).

7. Another name for Glycogen is animal starch. Another name of starch is amylum.