Final answer:
The fibrous protein that is the primary structural component of hair and nails is keratin, which is insoluble in water and responsible for their hard, water-resistant properties.
Step-by-step explanation:
The fibrous protein that forms the basic structural component of hair and nails is keratin. Keratin is a type of structural protein that is insoluble in water and it provides skin, hair, and nails with their hard, water-resistant properties. Proteins like collagen and elastin are also fibrous proteins, with collagen being a major component in connective tissues and elastin being found in ligaments and elastic tissues of artery walls. However, it's keratin that is predominantly found in the hair, nails, and the outer layer of skin, which is produced by cells called keratinocytes. While nails are composed mainly of keratin, the nail bed actually refers to the skin beneath the nail plate, not the middle portion of the nail plate itself, making that statement false.