7.1 Hair Structure & Chemical Bonds
Key Takeaways
- Hair is roughly 90% keratin protein built from amino acids made of the elements carbon, oxygen, hydrogen, nitrogen, and sulfur (COHNS)
- The three layers of the hair shaft are the cuticle (overlapping scales), the cortex (bulk, melanin, and strength), and the medulla (often absent in fine hair)
- Hydrogen and salt bonds are weak physical side bonds broken by water, heat, or pH changes and re-formed by drying; disulfide bonds are strong chemical side bonds broken only by chemicals such as perms and relaxers
- The dermal papilla is the small cone-shaped structure at the base of the follicle that supplies blood and nutrients needed for hair growth
- Porosity measures how well hair absorbs moisture; elasticity measures how far hair stretches and returns — about 50% of its length when wet
Hair Structure & Chemical Bonds
Trichology is the scientific study of hair, its structure, its growth, and its diseases. It is one of the most heavily tested theory areas on the PSI cosmetology written exam because nearly every salon service — shampooing, styling, perming, relaxing, coloring — is really an applied chemistry exercise performed on the hair's internal bonds. If you understand what hair is made of and which bonds each service breaks, most Hair, Scalp, Skin, and Nail Analysis questions become straightforward recall.
What Hair Is Made Of
Mature hair is composed of approximately 90% keratin, a hard, fibrous protein also found in skin and nails. Keratin is built from long chains of amino acids linked end to end by peptide bonds (also called end bonds). These amino acids are assembled from just five elements, a classic exam fact remembered by the acronym COHNS: carbon, oxygen, hydrogen, nitrogen, and sulfur. Sulfur is the element that makes permanent chemical changes possible, because it forms the disulfide bonds discussed below.
The Three Layers of the Hair Shaft
A strand of hair has up to three concentric layers:
| Layer | Location | Function | Exam Facts |
|---|---|---|---|
| Cuticle | Outermost layer | Protective overlapping scales, like shingles on a roof | Lies flat when healthy; raised by alkaline solutions so chemicals can enter |
| Cortex | Middle layer | About 90% of the hair's weight; contains melanin (pigment) and gives hair its strength and elasticity | All chemical texture and color services work here |
| Medulla | Innermost core | No known critical function | Often absent in fine or very light hair; always present in coarse hair |
A favorite exam trick is to ask which layer contains natural pigment — the answer is always the cortex. Another asks which layer is sometimes missing — the medulla, especially in fine hair.
The Hair Root: Structures Below the Scalp
The part of the hair inside the skin is the hair root, housed in a tube-like depression called the follicle. At the very base of the follicle is the hair bulb, the thickened, club-shaped structure of living cells that fits over the dermal papilla — a small cone of connective tissue containing blood and nerve supply. The papilla is frequently tested because it is the structure that nourishes the growing hair; no papilla, no growth. Attached to the follicle are two more structures: the arrector pili muscle, which contracts in response to cold or emotion and causes goose bumps, and the sebaceous gland, an oil gland that secretes sebum to condition the hair and skin.
The Three Side Bonds — and the Services That Break Them
The amino acid chains in the cortex are held in their natural shape by cross-links called side bonds. Knowing which bond a service breaks is the heart of this exam topic:
| Bond Type | Strength | Broken By | Re-formed By | Service That Exploits It |
|---|---|---|---|---|
| Hydrogen bond | Weak, physical | Water or heat | Drying/cooling into new shape | Wet setting, thermal styling |
| Salt bond | Weak, physical | Changes in pH (strong alkaline or acid solutions) | Restoring normal pH | Affected by alkaline chemical services; easily broken and rebuilt |
| Disulfide bond | Strong, chemical (sulfur-based) | Chemicals only — waving solution or relaxer | Neutralizer (perms) | Permanent waving, chemical hair relaxing |
Hydrogen bonds account for about one-third of the hair's strength. Because water breaks them, hair can be stretched on rollers while wet and will hold the new shape once dry — this is the entire theory behind wet setting. Disulfide bonds join two sulfur atoms across adjacent amino acid chains; they are not affected by water, which is why a perm survives shampooing. Only a chemical process — reduction by waving lotion, then oxidation by neutralizer — breaks and rebuilds them. Salt bonds are sensitive to pH, which is why exam questions link them to strongly acidic or alkaline solutions. And remember: peptide (end) bonds run along the amino acid chain itself; they are broken by the most aggressive chemical relaxers, not re-formed, and their loss is what causes severe hair damage.
Hair Analysis Properties
Four properties describe the condition and behavior of hair and are tested as definitions:
- Porosity — the hair's ability to absorb moisture. Low porosity means a tight, resistant cuticle; high porosity (raised or damaged cuticle) absorbs chemicals quickly and unevenly.
- Elasticity — the ability to stretch and return without breaking. Normal wet hair stretches about 50% of its length; dry hair stretches about 20%.
- Texture — the diameter of a single strand: coarse, medium, or fine. Fine hair is weaker and processes faster.
- Density — the number of strands per square inch of scalp: thin, medium, or thick.
A realistic exam scenario: a client with high porosity and fine texture wants a perm. The correct reasoning is that her hair will absorb the waving solution quickly and process fast, so a milder solution and close monitoring are required — the analysis properties directly drive the chemical choice.
Why This Matters for the Exam
Expect direct definition questions (COHNS, the three layers, the dermal papilla) and applied questions ("Which bond is broken when hair is wet and re-formed when dry?" — hydrogen). Master the bond table and you can answer the majority of texture-service chemistry questions on the written test.
A cosmetologist performs a wet set: hair is shampooed, wound on rollers, dried, and it holds the new curl pattern until the next shampoo. Which side bonds were broken by water and re-formed by drying to create this temporary change?
During a hair analysis, a client is found to have very high porosity. What does this tell the cosmetologist about how the hair will behave during a chemical service?