What is the difference between polypeptides and proteins?

Polypeptides and proteins are related molecules, but there are key differences between them:

Size:

Polypeptides: Polypeptides are chains of amino acids linked together by peptide bonds. They can be relatively short or long, and the term "polypeptide" is often used to describe amino acid chains of various lengths.
Proteins: Proteins are larger and more complex molecules than polypeptides. They are typically made up of one or more polypeptide chains folded into specific three-dimensional structures. Proteins can have a wide range of sizes, from small proteins with just a few amino acids to large and complex proteins with thousands of amino acids.
Function:

Polypeptides: Polypeptides can have various functions in biological processes. Some act as hormones, neurotransmitters, or signaling molecules, while others may serve as structural components of cells and tissues.
Proteins: Proteins are highly diverse in function and can perform a wide range of roles in living organisms. They can function as enzymes, antibodies, transport molecules, receptors, structural components (e.g., collagen in connective tissues), and much more. Proteins are essential for almost all biological processes.
Structure:

Polypeptides: Polypeptides are linear chains of amino acids and do not have a well-defined three-dimensional structure. Their structure is relatively simple compared to proteins.
Proteins: Proteins have complex three-dimensional structures that are crucial to their function. These structures can be classified into four levels: primary (the linear sequence of amino acids), secondary (local folding patterns like alpha helices and beta sheets), tertiary (the overall 3D structure of a single polypeptide chain), and quaternary (the arrangement of multiple polypeptide chains in a multi-subunit protein).
Biological Significance:

Polypeptides: While polypeptides have biological significance, they are often considered as the building blocks of proteins. Proteins are typically the more biologically active and functional molecules.
Proteins: Proteins are central to the functioning of living organisms. They play crucial roles in metabolism, cell signaling, immune responses, and many other biological processes. They are often the targets of drug development and are of great importance in biochemistry and molecular biology.
In summary, polypeptides are smaller chains of amino acids, often considered as protein precursors, while proteins are larger, more complex molecules with specific three-dimensional structures and diverse biological functions. Proteins are essential for the proper functioning of cells and organisms, whereas polypeptides can serve various roles but are typically less complex in structure and function.