Question

Explain the following terms: (i) Zwitter ion
(ii) Peptide bond
(iii) Primary structure of protein
(iv) Polysaccharides

Hint:

Zwitter ions are typical of amino acids, peptide bonds link them, the primary structure is their order, and polysaccharides are analogous long chains of sugars. Remember: amino acids build proteins, and monosaccharides build polysaccharides.

Answer 1

(i) Zwitter ion:
A zwitter ion is a dipolar ion that contains both a positive and a negative charge within the same molecule but is overall electrically neutral. Amino acids in aqueous solution commonly exist as zwitter ions, where the amino group (\(-\mathrm{NH_2}\)) is protonated to \(-\mathrm{NH_3^+}\) and the carboxyl group (\(-\mathrm{COOH}\)) is deprotonated to \(-\mathrm{COO^-}\). \[ \mathrm{H_2N{-}CH(R){-}COOH \;\;\rightleftharpoons\;\; ^{+}H_3N{-}CH(R){-}COO^-} \] (ii) Peptide bond:
A peptide bond is a covalent bond formed between the \(-\mathrm{COOH}\) group of one amino acid and the \(-\mathrm{NH_2}\) group of another with the elimination of a water molecule (condensation reaction). It has partial double bond character due to resonance, making it rigid and planar. (iii) Primary structure of protein:
The primary structure of a protein is the linear sequence of amino acids in a polypeptide chain, linked together by peptide bonds. This sequence determines the higher levels of protein structure and ultimately governs its biological function. Example: the exact order of amino acids in insulin. (iv) Polysaccharides:
Polysaccharides are long-chain carbohydrates formed by the condensation of numerous monosaccharide units joined by glycosidic bonds. They can be storage polysaccharides (e.g., starch, glycogen) or structural polysaccharides (e.g., cellulose, chitin). They are usually insoluble in water and serve as energy reserves or structural materials in living organisms.