How are peptides different from proteins?

Proteins and peptides are both composed of amino acids linked by peptide bonds, but they are distinguished primarily by their size and structural complexity.12

• Size: Traditionally, peptides are defined as shorter chains consisting of 2 to 50 amino acids, while proteins are composed of 50 or more amino acids. Some definitions use molecular weight as a threshold, with some authorities setting an upper limit for polypeptides at approximately 100 amino acids.12
• Structure: Proteins are characterized by their ability to adopt complex, well-defined three-dimensional shapes, including secondary, tertiary, and quaternary structures. In contrast, peptides generally exhibit less defined structural conformations.1
• Relationship: Proteins are essentially large, complex molecules formed from one or more polypeptides. Because the distinction between a large polypeptide and a small protein can be imprecise, the terms are sometimes used interchangeably in academic contexts, though they remain distinct in their typical biological roles and structural properties.12