Final answer:
A polypeptide is a chain of amino acids, while a protein is a functional biomolecule consisting of one or more polypeptides that have folded into a unique three-dimensional structure and undergone post-translational modifications.
Step-by-step explanation:
The important distinction between a polypeptide and a protein lies in their structure, function, and size. Although the terms are sometimes used interchangeably, a polypeptide is a polymer made up of amino acids, typically ranging from ten to a few hundred. These are formed when the amino group of one amino acid bonds with the carboxyl group of another, creating a peptide bond and a polypeptide chain with distinct N (amino) and C (carboxyl) terminals.
By contrast, a protein is a functional biomolecule that often consists of one or more polypeptides that have folded into a unique three-dimensional structure, essential for its function. Proteins generally have more than fifty amino acids, have a molecular weight above 5000 Daltons, and are immunogenic, which means they can prompt an immune response. They also undergo post-translational modifications after synthesis, which are critical for their proper function.
In summary, while all proteins are made up of polypeptides, not all polypeptides are considered proteins. Proteins are typically larger, have a structured shape, possess complex functions in the body, and become fully functional after undergoing post-translational changes.