The basic building block of protein is the amino acid. Amino acids are organic compounds composed of carbon, hydrogen, oxygen, and nitrogen, and they link together to form proteins through peptide bonds. There are 20 different amino acids that can combine in various sequences to create a vast array of proteins, each with unique functions in living organisms. Some amino acids are considered essential because the body cannot synthesize them; they must be obtained through diet. Proteins play critical roles in nearly every biological process, including enzymatic reactions, cellular structure, and the immune response.