A peptide is a protein fragment in which the amino acids are linked by peptide bonds. The term is used to refer to some types of small proteins, which differ from polypeptides due to the presence of an internal sequence of amino acids (an internal segment). Peptides can be synthesized chemically or by recombinant DNA technology.
A polypeptide is a linear chain of amino acids, usually less than 1000 amino acids long. This length is limited by the maximum size that a given enzyme will polymerize (fold) into a single-chain molecule; thus, many polypeptides are shorter than their counterparts described above. A polypeptide typically has a characteristic fold or shape and is characterized by high molecular weight and stability.
A protein is a macromolecule consisting of one or more linear polymers of amino acids joined by peptide bonds between the carboxylate ion of each amino acid at one terminus and the amine group at the other terminus. Proteins perform vital functions in all known living organisms, such as catalyzing chemical reactions, DNA replication and transcription, energy production, signaling, immune responses, movement and growth. Proteins are composed of amino acids which are the fundamental building blocks of all living organisms.