In hemoglobin, the iron is enclosed by the porphyrin group, a flat grouping of carbon, hydrogen, and nitrogen atoms. (This is in turn encased in a protein.) When iron is in the form of the Fe³⁺ ion, it just fits into the space within the four N atoms, and the arrangement is flat. Speculate on what takes place to the structure when iron is reduced to the Fe²⁺ ion.