Peptide is a biochemical substance between amino acid and protein. It has a smaller molecular weight than protein, but a larger molecular weight than amino acid. It is a fragment of protein. That is to say, from more than two or up to dozens of amino acid peptide bond polymerization into a peptide, and then from multiple peptides with side chains polymerization into a protein. An amino acid can not be called a peptide, must be more than two amino acids linked by a peptide chain compound to be called a peptide; Many amino acids mixed together are not called peptides; Amino acids must be linked by peptide bonds, forming "amino acid chain", "amino acid string", string of amino acids can be called peptide. .