The basic building block of proteins is the amino acid. Amino acids are organic compounds composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. There are 20 standard amino acids that combine in various sequences to form proteins, which are essential for the structure and function of living organisms. Amino acids link together through peptide bonds to create polypeptide chains, which then fold into specific three-dimensional shapes, determining the protein's functionality. Some amino acids are classified as essential, meaning they must be obtained through diet, while others are non-essential, as the body can synthesize them internally.