Keratin is not an amino acid; rather, it's a type of protein that's rich in sulfur-containing amino acids, primarily cysteine. This protein is the key structural material making up the outer layer of human skin, as well as hair and nails. Proteins, including keratin, are composed of long chains of amino acids. Thus, while keratin itself is not an amino acid, it is intricately related to amino acids through its structure and function. Amino acids act as the building blocks for proteins, facilitating numerous biological functions, including the formation of keratin, which provides strength and protection to cells exposed to stress or damage.

Adhesive and cohesive forces are fundamental concepts in the study of material science, describing the attraction between different and same molecules, respectively. Adhesive forces depend on the types of materials in contact, the surface area, and surface roughness, determining how well substances like glues, tapes, or paints adhere to other surfaces. Cohesive forces, on the other hand, depend primarily on the molecular structure of the material itself and the intermolecular forces present, such as hydrogen bonding, Van der Waals forces, and ionic bonds. These forces play a crucial role in determining the mechanical strength, density, and melting or boiling points of materials. Factors like temperature and pressure can also influence both adhesive and cohesive forces by altering the molecular alignment or energy levels.

Polypropylene plastic, known for its lightweight, durable, and flexible properties, is indeed used in the production of shoes. Typically found in parts of the shoe that require additional support or structure, such as heel counters or toe boxes, polypropylene adds minimal weight while providing significant resilience and shape retention. Its resistance to water, stains, and chemicals makes it highly suitable for footwear, contributing to the longevity and maintenance ease of the shoes. Furthermore, polypropylene's flexibility allows for comfort and adaptability in fit, making it a popular choice among manufacturers for both performance-oriented and everyday footwear. The incorporation of this material aligns with the industry's ongoing search for innovative solutions that enhance the function and wearability of shoes without compromising environmental considerations, given polypropylene's recyclability.