IGCSE Biology Paper-2: Specimen Questions with Answers 190 - 190 of 203

Question 190



Which substance is transported by haemoglobin?


Choice (4)


Nitrogen and oxygen


Oxygen and water


Urea and uric acid


Oxygen and carbon dioxide




  • Most of the oxygen in the blood is contained within the red blood cells, where it is chemically bonded to hemoglobin. Each hemoglobin molecule consists of four polypeptide chains called globins and four iron-containing, disc-shaped organic pigment molecules called hemes. The protein part of hemoglobin is composed of two identical alpha chains, each 141 amino acids long, and two identical beta chains, each 146 amino acids long. Each of the four-polypeptide chains is combined with one heme group.
  • In the center of each heme group is one atom of iron, which can combine with one molecule of oxygen. One hemoglobin molecule can thus combine with four molecules of oxygen and since there are about 280 million hemoglobin molecules per red blood cell, each red blood cell can carry over a billion molecules of oxygen. Normal heme contains iron in the reduced form (, or ferrous iron) . In this form, the iron can share electrons and bond with oxygen to form oxyhemoglobin.
  • When oxyhemoglobin dissociates to release oxygen to the tissues, the heme iron is still in the reduced form and the hemoglobin is called Deoxy hemoglobin, or reduced hemoglobin. The term oxyhemoglobin is thus not equivalent to oxidized hemoglobin; hemoglobin does not lose an electron (and become oxidized) when it combines with oxygen. Oxidized hemoglobin, or Methemoglobin, has iron in the oxidized ( or ferric) state.
  • Methemoglobin thus lacks the electron it needs to form a bond with oxygen and cannot participate in oxygen transport. Blood normally contains only a small amount of Methemoglobin, but certain drugs can increase this amount. In carboxy hemoglobin, another abnormal form of hemoglobin, the reduced heme is combined with carbon monoxide instead of oxygen.
  • Because the bond with carbon monoxide is about 210 times stronger than the bond with oxygen, carbon monoxide tends to displace oxygen in hemoglobin and remains attached to hemoglobin as the blood passes through systemic capillaries.