Question

Carbohydrates are optically active polyhydroxy aldehydes or ketones or molecules that provide such units on hydrolysis. They are broadly classified into three groups: monosaccharides, oligosaccharides and polysaccharides. Monosaccharide units are held together by glycosidic linkages to form disaccharides or polysaccharides. Starch and cellulose are polysaccharides. Starch is a polymer of $\alpha$-glucose and consists of two components: Amylose and Amylopectin. Amylose is water soluble, having 15--20% of starch, whereas Amylopectin is insoluble in water, having 80--85% starch. Chemically, amylose is a long unbranched chain of $\alpha$-glucose units held together by C$_1$--C$_4$ glycosidic linkage. Amylopectin is a branched chain of $\alpha$-glucose units held together by C$_1$--C$_4$ and C$_1$--C$_6$ glycosidic linkages. Write two differences between Amylose and Amylopectin.

Hint:

Amylose = Unbranched. Amylopectin = Branched.

Answer 1

Step 1: Context.
Starch is a storage polysaccharide made entirely of $\alpha$-glucose units. It has two structural components: amylose (~15 to 20%) and amylopectin (~80 to 85%).
Step 2: Structure of amylose.
Amylose is a linear, unbranched polymer of $\alpha$-glucose units linked exclusively by $C_1$-$C_4$ glycosidic bonds. It is water-soluble and forms a helical structure that gives a characteristic blue-black colour with iodine.
Step 3: Structure of amylopectin.
Amylopectin is a branched-chain polymer. The main chain has $C_1$-$C_4$ glycosidic linkages, but at every 24 to 30 glucose units there are branch points formed by $C_1$-$C_6$ glycosidic linkages. It is insoluble in water.
Step 4: Two key differences.
(1) Amylose is water-soluble; amylopectin is water-insoluble. (2) Amylose is an unbranched chain with only $C_1$-$C_4$ links; amylopectin is a branched chain with both $C_1$-$C_4$ and $C_1$-$C_6$ links.