Same building blocks, different length
Peptides and proteins are made from the same units: amino acids linked by peptide bonds. The practical difference is size. Short chains are called peptides, and long chains that typically fold into a defined three-dimensional shape are called proteins. There is no universally fixed cut-off, but chains of roughly fifty amino acids or fewer are commonly described as peptides.
Structure and folding
Because proteins are longer, they usually adopt a stable folded structure that is central to how they are described. Shorter peptides may be more flexible and are often characterised primarily by their sequence rather than a complex fold. This is one reason peptides are frequently discussed in terms of their sequence and molecular weight.
Why the distinction is useful
For laboratory documentation, knowing whether a material is described as a peptide or a protein sets expectations for how it is characterised, stored and handled. Both are supplied and treated as research materials, with the same research-use-only boundary.
Frequently asked questions
Is there an exact size that separates them?
No. The boundary is conventional. Many sources use roughly fifty amino acids as a rough guide, but usage varies.
Do peptides and proteins get documented differently?
The same core documentation ideas apply — identity, batch and specification — though the analytical detail can differ with molecule size.