$\mathrm{D}$-Glucose is treated with an excess of acetic anhydride in the presence of pyridine. Identify the products formed.

(i) Name the type of linkages responsible for the formation of primary and secondary structures of proteins.

(ii) On electrolysis in acidic solution, $\alpha$-amino acids migrate towards cathode while in alkaline medium, they migrate towards anode. Explain.

(iii) What are essential and non-essential amino acids. Give two example of each.