Question

Complete hydrolysis of starch gives:
A) glucose and galactose in equimolar amounts
B) galactose and fructose in equimolar amounts
C) glucose only
D) glucose and fructose in equimolar amount

Answer 1

Consider the monosaccharide units present in starch. Thy hydrolysis of a polysaccharide gives monosaccharides as the final product.

Complete answer:
We can represent starch with chemical formula ${\left( {{{\text{C}}_6}{{\text{H}}_{10}}{{\text{O}}_5}} \right)_n}$ . Starch is a polymer or a polysaccharide. The monosaccharide unit present in starch is $\alpha - D -$ glucopyranose unit.

Starch is insoluble in cold water, but We can prepare a colloidal solution of starch by grinding and soaking. On partial hydrolysis of starch and glycogen, We can obtain the disaccharide maltose and a low molecular weight dextran. Upon hydrolysis of starch We can get two fractions. The first fraction is around 20% of the hydrolyzed product. This fraction contains amylose that is water-soluble.

Amylose contains a linear chain that is made from several thousand glucose units. The second fraction that We obtain is called amylopectin. It contains around one million glucose units. Amylopectin is a branched polymer and is water-insoluble. Hence, when we subject starch to complete hydrolysis, We get $\alpha - D -$glucose as the sole product.

Let us write the structure of $\alpha - D -$ glucopyranose as shown below:

Note: Starch is a high molecular weight polysaccharide consisting of amylose and amylopectin. Amylose is a linear polymer and is around 20% of starch. Amylopectin is branched polymer and is around 80% of starch.