Oxygen Transport
In the loading and unloading of oxygen, there is a cooperation between these four haem groups. When oxygen binds to one of the groups, the others change shape slighty and their attraction to oxygen increases. The loading of the first oxygen, results in the rapid loading of the next three (forming oxyhaemoglobin). At the other end, when one group unloads it's oxygen, the other three rapidly unload as their groups change shape again having less attraction for oxygen. This method of cooperative binding and release can be seen in the dissociation curve for haemoglobin. Over the range of oxygen concentrations where the curve has a steep slope, the slighest change in concentration will cause haemoglobin to load or unload a substantial amount of oxygen. Notice that the steep part of the curve corresponds to the range of oxygen concentrations found in the tissues. When the cells in a particular location begin to work harder, e.g. during exercise, oxygen cincentration dips in that location, as the oxygen is used in cellular respiration. Because of the cooperation between the haem groups, this slight change in concentration is enough to cause a large increase in the amount of oxygen unloaded.
As with all proteins, haemoglobin's shape shift is sensitive to a variety of environmental conditions. A drop in pH lowers the attraction of haemoglobin to oxygen, an effect knownas the Bohr shift. Because carbon dioxide reacts with water to produce carbinic acid, an active tissue will lower the pH of it's surroundings and encourage haemoglobin to give up extra oxygen, to be used in cellular respiration. Haemoglobin a notable molecule for it's ability to tranport oxygens from regions of supply to regions of demand.