Hollow woolen ball is _________ structure of protein
Proteins have three structural levels - primary, secondary, tertiary and quaternary.
The sequence of amino acids in polypeptide chain gives the primary structure of a protein. When the polypeptide chain is coiled or folded in a regular fashion, it gives the secondary structure of the protein. The long protein chain is also folded upon itself like a hollow woolen ball, giving rise to the tertiary structure.