Tutorial Answer 2

Hemoglobin is a protein composed of four distinct subunits (called polypeptide chains) that are normally in solution within red blood cells. Oxygen must diffuse from the air spaces in the lungs, into the blood plasma (largely water) and from the plasma through the membrane of the red blood cells before it can bind to hemoglobin. The bound oxygen must pass through the reverse pathway when it is released in the tissues. (See WU Chemistry Tutorial, "Hemoglobin")