Red Blood Cells
Different oxygen-carrying proteins
(a) In most vertebrates, hemoglobin delivers oxygen to the body and removes some carbon dioxide. Hemoglobin is composed of four protein subunits, two alpha chains and two beta chains, and a heme group that has iron associated with it. The iron reversibly associates with oxygen; in so doing, it is oxidized from Fe2+ to Fe3+. (b) In most mollusks and some arthropods, hemocyanin delivers oxygen. Unlike hemoglobin, hemolymph is not carried in blood cells, but floats free in the hemolymph. Copper, instead of iron, binds the oxygen, giving the hemolymph a blue-green color. (c) In annelids, such as the earthworm and some other invertebrates, hemerythrin carries oxygen. Like hemoglobin, hemerythrin is carried in blood cells and has iron associated with it, but despite its name, hemerythrin does not contain heme.
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