A chain of amino acids with or without a defined secondary structure. A secondary structure consists of segments of polypeptides that form a characteristic pattern that extends in one dimension. The bonding of two or more polypeptides gives rise to proteins, which are then folded into a specific shape for a particular protein. Two example structures are called Alpha helix and Beta strand. All told, more than 7000 naturally occurring peptides have been identified, and they have important roles in human development involving the actions of hormones, neurotransmitters, and growth factors.
See Amino acids, Growth factors, Growth hormone (or somatotropin), Peptides, Protein-folding problem