|MadSci Network: Biochemistry|
Hair is made of a protein called keratin. Keratin is not a water-loving protein, but rather a protein that tends to exclude water from its high- strength form, and the driving force for keratin folding up into this form is partially due to its desire to hide certain parts of itself away from water. The way it does this is to fold other parts of the protein up around the water-disliking parts and hide them from exposure to water. When you heat hair excessively, 2 different things probably happen. The first is that the normal structure of keratin is unfolded by the energy from the heat. This causes the water-disliking parts of keratin to be brought to the surface and exposed to water. But since they don't like to be near water, and water doesn't like to be near them, what happens is that water pulls away from those parts of the hair, and you are left only with air next to those parts, which you see as air bubbles.
The second thing that probably happens is that the hair gets burnt on the molecular level, and the building blocks of keratin (called amino acids) get modified by the combination of heat and oxygen. This might alter them into different molecules that are more water-disliking than the natural building blocks, and then water will avoid them, leaving only air, just like I described above.
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