Proteins are the most abundant biomolecules of the living system. Proteins are the polymers of about twenty different $\alpha$-amino acids which are linked by peptide bonds. Ten amino acids are called essential amino acids. In zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases. On the basis of their molecular shape, proteins are classified into two types : Fibrous and Globular proteins. Structure and shape of proteins can be studied at four different levels i.e., primary, secondary, tertiary and quaternary, each level being more complex than the previous one. The secondary or tertiary structure of proteins get disturbed on change of pH or temperature and they are not able to perform their functions. This is called denaturation of proteins. Answer the following questions : (a) What are essential amino acids? (b) What is meant by zwitter ionic form of amino acids? (c) (i) Give one example each for Fibrous protein and Globular protein. (ii) What type of linkages hold monomers of proteins together? OR (c) (i) What is the structural feature which characterises a reducing sugar? (ii) What is the structural difference between nucleoside and nucleotide?