A peptide is a chain of amino acids. In order to form the chain, two neighbouring amino acid molecules, condense with the elimination of water and covalently connected. The bond connecting the two amino acids is an amide linkage herein called a peptide bond. A peptide bond is formed upon the loss of hydroxyl group from carboxy-terminus of one amino acid and hydrogen from the amino terminus of the second amino acid. As a result, resulting carbonyl group’s carbon atom of one amino acid is directly bound to the nitrogen atom of a secondary amine of the second amino acid.
One amino acids loose hydrogen or and the other amino acid loose hydroxyl part. So, the leftover (residue) and not the complete amino acid is present in the peptide or protein. So, protein is said to contain not amino acids but only amino acid residues. Terminal amino acids of the peptides always have a free amino group (N-terminal) at one end and free carboxyl (C-terminal) group on the other end of the chain.
Depending on the number of amino acids that have condensed together, they may be called dipeptide, tripeptide, oligopeptide, and polypeptide. Peptides having molecular weights below 10,000 are called polypeptide, and those having still higher molecular weight are called proteins.