Question

Which of the following is a protein?
(A) Glycogen
(B) Amylopectin
(C) Keratin
(D) Lecithin

Answer 1

Living beings are made up of various complex biomolecules. Major biomolecules are carbohydrate, protein, lipid and nucleic acid. Proteins are polymers of $\alpha$-amino acids.

Complete step by step solution:
(A) Glycogen is a polysaccharide of glucose that stores energy in animals. Like starch, it is a polymer of glucose. Its structure is similar to amylopectin but it is extensively branched and compact.
Hence, it is not a protein.
(B) Amylopectin is a component of starch with the other component as amylose. It constitutes about eighty to eighty five percent of starch and is insoluble in water. It is a branched chain polymer of $\alpha$-D-glucose.
Hence, it is not a protein.
(D) Lecithin is a group of yellow-brown fatty substances found in animal and plant tissue. These tissues attract both water (hydrophilic) and fat (lipophilic) and thus are called amphiphilic. Lecithin is a mixture of glycerophospholipids.
Hence, it is not a protein.
(C) Keratin is one of a family of fibrous structural proteins known as scleroproteins. $\alpha$-Keratin is a type of keratin found in vertebrates. They form hair, wool, outer layer of skin, horns, nails, claws, hooves of mammals etc. $\beta$-Keratin is a type of keratin found in reptiles and birds.
Hence, option (C) is the correct answer.

Note: Lecithin is not to be confused with lectin which is a carbohydrate binding protein. They play a role in recognition at cellular and molecular level. Lectins are found in many foods including beans and grains.