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* Explain the adaptive advantage of haemoglobin

1 Structure of Haemoglobin: It is composed of four units of a protein called ‘globin’ and in the center of each globin is a ‘haem’ unit. The haem is a ring structure with iron in the center. It is the iron that binds the oxygen in a weak interaction that can easily broken. A RBC (red blood cell) is packed with haemoglobin molecules. The cell has no nucleus, so more haemoglobin molecules can be included.

2 Function of haemoglobin in Transporting Oxygen:
* transport of O2
* transport of some CO2 from body cells to the lungs
* conversion of CO2 to HCO3- by the action of the enzyme carbonic anhydrase in the RBC; HCO3- produces H+ * buffering of H+

Since O2 is not very soluble in H2O, and most of it is carried by Hb in the RBC’s. The interaction of iron ions with O2 binds O2 to the Hb, forming ‘oxyhaemoglobin’, (HbO2). This happens when the pressure (concentration) of O2 is very high for example, in the lungs. Each Hb can bind four O2 (one molecule per haem). The reverse occurs in the body cells, where O2 pressure (concentration) is low:

increase in oxygen concentration
Hb + O2 HbO2 decrease in oxygen concentration

Theoretically, one RBC can carry 4(280 x 106) molecules of O2 due to the large number of Hb in each RBC.

Note: (i) Hb increases the capacity of the blood to carry O2 ; and (ii) Hb carries O2 loosely therefore releases O2 freely in capillaries.

Other adaptive advantage is that Hb enclosed is a RBC and not simply dissolved in the plasma; otherwise it will upset the osmotic balance of the blood.

Haemoglobin, found in red blood...
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