Proteins are made up of elements like carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus. They are formed through condensation reactions between amino acids and can be broken down through hydrolysis. There are four levels of protein structure: primary, secondary, tertiary, and quaternary. The primary structure is the linear sequence of amino acids, secondary structure involves alpha helices and beta sheets, tertiary is the overall 3D shape, and quaternary involves combinations of tertiary structures. There are essential and non-essential amino acids, with essential ones not synthesized by the body.