Explain the terms primary and secondary structure of proteins. What is the difference between $\alpha$-helix and $\beta$-pleated sheet structure of proteins?

(N/A) $1$. Primary Structure: The primary structure of a protein refers to the specific sequence in which amino acids are arranged in a polypeptide chain. Any change in this sequence results in a different protein.
$2$. Secondary Structure: The secondary structure refers to the regular folding patterns of the polypeptide backbone due to hydrogen bonding between the carbonyl oxygen $(C=O)$ and the amide hydrogen $(N-H)$ of the peptide bond.
$3$. Difference between $\alpha$-helix and $\beta$-pleated sheet:
- $\alpha$-helix: It is formed when the polypeptide chain coils into a right-handed spiral (helix) stabilized by intramolecular hydrogen bonds between the $C=O$ group of one amino acid and the $N-H$ group of the fourth amino acid residue.
- $\beta$-pleated sheet: It is formed when polypeptide chains are stretched out to nearly maximum extension and then laid side by side,held together by intermolecular hydrogen bonds,forming a sheet-like structure.