Answer
$(a)$Disufide bond is the molecular basis for the correlation between
disulfide-bond content and mechanical properties of the protein.
$(b)$Cystine residues (disulfide bonds) is the molecular basis for this property.
Work Step by Step
$(a)$Disufide bonds are covalent bonds, which are much more stronger than the noncovalent interactions that stabilize most proteins. They cross-link chains, increase their stiffness, mechanical strength, and hardness.
$(b)$Cystine residues (disulfide bonds) prevent the complete unfolding of proteins.
