A segment of 20 amino acids from a protein chain is called a polypeptide.
Peptides are biomolecules consisting of short chains of amino acids liked by covalent peptide bonds. When the chain has 2 amino acids, it is called a dipeptide ('di' meaning 'two'); when it has 3 amino acids it receives the name of tripeptide ('tri' meaning 'three').
A polypeptide, however, is much longer, as it has 20 or more amino acids in its chain and can be considered a polymer of amino acids.
When one or more polypeptides are also folded into a stable structure by non-covalent bonds, that is called a protein.
Proteins have many, many functions. The ones that can accelerate chemical reactions are called enzymes.
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