Scientists call amino acids the building blocks of proteins.
All amino acids contain carbon, hydrogen, oxygen, and nitrogen.
Some amino acids contain sulfur. Green plants and some microorganisms can make all the amino acids they need. But human beings and higher animals cannot make all of the 20 amino acids their bodies need to build tissues. Humans must get at least nine amino acids from their food. Protein foods, such as eggs, meat, milk products, and some vegetables, provide amino acids.