Enzymes and catalysts are the subject of Chapter 24, so at this point we shall introduce globular proteins by means of two molecules that perform other functions: hemoglobin, which carries 0₂ in the bloodstream from the lungs to the tissues; and myoglobin, which stores 0₂ in muscle cells until it is needed. Both myoglobin and hemoglobin are hemoproteins, with the protein chain enclosing a flat, planar iron-porphyrin ring complex called a heme group, shown at the left. The iron atom and the porphyrin ring together make up one large delocalized-electron system similar to the magnesium-porphyrin system in chlorophyll. Because of the delocalized electrons, both chlorophyll and the heme group absorb light in the visible spectrum, and are brightly colored. Chlorophyll is green because it absorbs strongly in the red end of the spectrum; hemoglobin and myoglobin absorb in the yellow-green and therefore have the red color familiar in blood and beefsteak.