Question

5. Glycogen and starch both give a (+) test with I2-KI and a (-) test with Benedict's. If you hydrolyzed these two polysaccharides and tested the solutions with the same two reagents what results would you get? Explain these results.

Answer 1

Answer:

There will be no reaction between the product of hydrolysis and I2-KI (-ve). When the product of hydrolysis is tested with Benedict reagent, a brick-red precipitate is observed.

Explanation:

Benedict's reagent as a chemical reagent is a mixture of sodium carbonate, sodium citrate and copper(II) sulfate pentahydrate. It is often used in place of Fehling's solution to detect the presence of reducing sugars. A positive test with Benedict's reagent is shown by a color change from clear blue to a brick-red precipitate.

Glycogen and starch are both complex structures containing repeating units of glucose(a reducing sugar). The polysaccharides have non redusing ends and so cannot react with benedict reagent.

When they are hydrolysed, glucose which is a reducing sugar can then test positive with Benedict reagent.