12. Keratin is present in both skin and nail .But skin is more flexible than nail. Reason ?

a.  increased no of disulphide bonds

b. decreased no of water molecules

c. increased Na content

d. increased hydrogen bonds

More the Disulphide bond More tensile the protein,Hence harder.So answer is Increased Disulphide bond.

Reference

Protein Structure

Nails are made of a protein called keratin, which is also found in skin and hair. The difference is the sulfur content. Keratin contains an amino acid called cysteine. Cysteine contains the sulfur atom. Two cysteines combine to form a disulfide bond. The number of these disulfide bonds in the keratin is what makes the difference between skin, hair and nails. Lots of disulfide bonds means very little flexiblity, like a nail. Fewer disulfide bonds results in more flexibility, like hair and skin.

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