The secondary structure
   sections  of  a  polypeptide  may further be organized into a tertiary
   structure by these interactions:
<LI> Covalent disulfide bonds form between polar cysteine molecules.
<LI> Most of the polar hydrophilic R groups are located on the surface,
close to surrounding water molecules.
<LI> Most  nonpolar  hydrophobic  R groups are located on the interior,
avoiding water.

   This  diagram shows tertiary structure of myoglobin, an oxygen carrier
   molecule in muscle cells.