Lysine is an essential amino acid required for protein synthesis, collagen formation, and calcium absorption in humans. It supports immune function, wound healing, and carnitine production for fatty acid metabolism. In food science, it is supplemented in cereal grains to improve nutritional quality of plant proteins. Chemically, its basic side chain carries a positive charge at physiological pH. It occurs naturally in high-protein foods like meat, fish, and dairy. Industrially produced via fermentation, lysine is used in dietary supplements and animal feed. Deficiency leads to symptoms including fatigue and impaired growth.