Protein is composed of amino acids and is the building block of muscle mass. It makes up 33% of muscles, 20% of bones and cartilages, and 10% of skin. Proteins are classified as simple, conjugated, or derived. Simple proteins like albumin and globulins are composed of one amino acid, while conjugated proteins contain amino acids and a prosthetic group. Protein deficiencies can cause kwashiorkor or marasmus in children.