What is the tough waterproof material that gives outer skin cells their protective quality?

The tough waterproof material that contributes significantly to the protective qualities of outer skin cells is keratin. Keratin is a fibrous protein found in the cells of the outer layer of the skin, known as the epidermis. It plays a critical role in forming a barrier against environmental damage, moisture loss, and pathogens. This protein provides strength and resilience to the skin, ensuring that it holds together and maintains its integrity despite exposure to various external factors.

Collagen, while essential for skin structure and elasticity, is primarily found in the deeper layers and does not provide the outermost waterproof barrier that keratin does. Elastin also contributes to skin elasticity but does not have the protective waterproof characteristics of keratin. Melanin is responsible for skin pigmentation and helps protect against UV radiation but is not involved in the structural waterproofing of skin cells. Thus, keratin is the key component that provides the tough, waterproof quality essential for the outermost skin cells, making it the correct choice.