Final answer:
Disulfide bonds in hair are covalent and stronger than hydrogen bonds, requiring harsh chemicals to break. Heat can easily disrupt hydrogen bonds, leading to temporary changes in hair structure. The resilience of disulfide bonds is essential for maintaining hair's shape and texture.
Step-by-step explanation:
The reason why disulfide bonds are harder to break than hydrogen bonds lies in their chemical nature. Disulfide bonds are a type of covalent bond, which means they involve the sharing of electrons between atoms. Specifically, these strong covalent bonds form between the sulfur atoms of two cysteine amino acids within a protein. Hydrogen bonds, on the other hand, are weaker because they are based on the attraction between a hydrogen atom, which carries a positive partial charge, and an electronegative atom like oxygen or nitrogen, which carries a negative partial charge.
Heat can disrupt weaker interactions like hydrogen bonds relatively easily through thermal motion, leading to changes in hair structure such as straightening when heat is applied. However, the breaking of disulfide bonds typically requires the use of harsh chemicals that can initiate a redox reaction, often in a process known as a 'perm' in which hair is curled or straightened permanently. These processes involve reducing agents to break the disulfide bonds and then oxidizing agents to reform them into a new shape. The resilience of disulfide bonds gives them the ability to maintain a protein's structure under physiological conditions, contributing to the characteristic texture and shape of hair.