What is the difference between a peptide and a protein?

The distinction is primarily structural and functional. Peptides are short amino acid chains (generally fewer than 50 residues) that typically lack a fixed tertiary structure, while proteins are longer chains that fold into defined 3-D conformations essential to their function. Peptides tend to act as signaling molecules — hormones, neuropeptides, or growth factors — binding receptors to modulate downstream cascades. Their smaller size also influences route of administration, half-life, and synthetic accessibility relative to full-length proteins.

Source: Biochemistry (Stryer, 9th ed.) · PubMed

Summarised from peer-reviewed literature for research reference. For laboratory research use only.