Question

Hairdressers use hot straightening irons to straighten curly hair. Temporarily straightened hair resumes its natural shape after the first wash, whereas permanently straightened hair retains its new shape until natural hair grows again. Explain this difference based on your knowledge about proteins.

Short answer

The difference between temporarily and permanently straightened hair lies in the protein structure and how it can be changed. Heating temporarily realigns protein structures in hair, but these can be reformed when hair is washed. Chemical treatments for permanent straightening break and reset these structures, making the change last until new hair grows.

Step-by-step solution

Understanding Hair Structure and Protein

Hair is composed of a type of protein called keratin. The structure of this protein molecule can determine whether hair is straight or curly.

Explanation of Temporarily Straightened Hair

In the case of temporarily straightened hair, the heating from the iron breaks the disulphide bonds in the hair's keratin, realigning the proteins which cause the hair to go straight. However, these bonds can be easily reformed when hair is washed, causing the hair to return back to its natural state.

Understanding Permanently Straightened Hair

For permanently straightened hair, the process involves chemicals that break the disulphide bonds and reset the protein structure permanently. Thus, the hair retains the straightened shape until new hair grows.

Key concepts

Keratin and Hair Composition

Understanding the basics of hair composition is crucial to comprehending the behavior of hair under different treatments. The primary building block of hair is keratin, a fibrous protein that provides the framework and strength to your locks. Keratin's unique structure, which includes various bonds such as hydrogen bonds and disulfide bridges, can greatly influence the texture and shape of your hair.

In its natural state, keratin proteins are organized in a way that can either lead to straight, wavy, or curly hair dependent on genetic disposition. The side bonds between individual keratin molecules play a pivotal role here: they can be temporary hydrogen bonds or much stronger disulfide bonds. It's the manipulation of these bonds that allows us to straighten or curl hair using different methods and technologies.

Temporarily Straightened Hair

The process of temporarily straightening hair is quite popular due to its simplicity and reversibility. When a hairdresser uses hot straightening irons, the heat temporarily breaks down hydrogen bonds and weaker disulfide bonds in the hair's keratin. This allows a realignment of the molecules, leading to hair that appears straight.

Reversion to Natural Form

It's important to understand that these changes are not permanent; the bonds can and do reform upon cooling and exposure to moisture. That's why after a wash, which introduces water and reduces the effect of heat, hair returns to its natural state. The temporary realignment is precisely that — temporary; a practical choice for those wanting straight hair for a short period without long-term commitment.

Permanently Straightened Hair

For those seeking a more lasting solution, permanently straightened hair is achieved through chemical treatments that alter the internal structure of the hair more drastically. Unlike temporary straightening, the chemicals used in permanent straightening break the hair's disulfide bonds and then restructure them to form a new, straighter shape.

The Permanent Change

Once these disulfide bonds are reset, they do not revert back to their original form even after washing. The resulting hair maintains its straightened state and will only display curly or wavy roots when new hair grows out, as the treatment does not alter the hair's natural genetic predisposition. This process offers a longer-lasting effect but also involves a greater risk of hair damage due to the strength of the chemicals used.