Protein
Definition
Large molecules built from amino acid chains, folded into precise shapes. They do most of the work in cells, from muscles to enzymes to antibodies.
Technical definition
A protein is a polymer of amino acids linked by peptide bonds, folded into specific three-dimensional structures from primary through quaternary levels. Structure determines function, spanning enzymes, transporters, signaling molecules, and structural materials.
Example
Hemoglobin, four folded amino-acid chains cradling iron-containing heme groups, carries oxygen in your blood.
Common confusion
Denaturation does not break a protein into amino acids. Heat or acid unfolds the 3D shape, destroying function, while the amino-acid chain itself stays intact.