AS Biology- transport of oxygen, carbon dioxide and the Bohr effect

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  • Created by: Heiwa
  • Created on: 27-11-12 13:54

Transport of oxygen

  • oxygen is carried in haemoglobin, which is found in erythrocytes
  • when oxygen reacts with haemoglobin it forms oxyhaemoglobin
  • 1 erythrocyte holds 250,000 Hb molecules, and each Hb molecule contains 4 subunits 
  • each subunit holds one Fe2+ ion, which can react with one 02 molecule.
  • when the first Fe2+ binds with oxygen it becomes easier for the rest of the Fe2+ ions to bond with oxygen, as the Hb molecule changes shape when each Fe2+ and O2 molecules react
  • the opposite occurs when the oxygen is being removed from Hb molecule, it becomes harder and harder to remove each oxygen molecule.
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Transport of carbon dioxide and HCO3-

  • 5% of CO2 is transported in the blood plasma, 10% in haemoglobin from carbohaemoglobin, and 85% in hydrogencarbonate ions (HCO3-)
  • Hydrogencarbonate ions are formed as CO2 diffuses into the blood, which then enters the erythrocytes 
  • CO2 then reacts with H2O to form weak carbonic acid, which is catalysed by the enzyme carbonic anhydrase
  • the carbonic acid then dissociates & releases H+ ions and HCO3- ions 
  • the HCO3- ions then diffuse out of the erythrocyte and into the blood plasma
  • to maintain the charge of the erythrocyte, Cl- ions move into the erythrocyte this is called the chloride shift
  • the H+ ions then are taken up Hb molecules to make haemoglobinic acid, this maintains the pH of the erythrocyte. Hb molecules acts as a buffer(a compound which maintains a constant pH)
  • as this is happening the oxygen tension (pressure of o2 in the cell) of the respiring tissue falls below that of the lungs (as the lungs are full of oxygen) so oxyhaemoglobin dissociates and releases oxygen to the tissues  HbO8 ->Hb + 4O2
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The Bohr effect

  • as we've just discussed, H+ ions are released from erythrocytes when they dissociate from carbonic acid. These H+ ions are then taken up by Hb molecules, where they have to compete for space
  • But when CO2 is present in the erythrocyte the H+ ions displace the oxygen in the haemoglobin
  • so, the oxyhaemoglobin releases MORE oxygen to the tissue. 

The Bohr effect states that: At any particular oxygen tension, the oxyhaemoglobin releases more oxygen when carbondioxide is present. 

  • so,when CO2 is present, Hb is less saturated with O2. So the O2 dissociation curve shifts downwards and to the right. (the Bohr shift)
  • the Bohr effect results in O2 being more readily released where more CO2 is produced from respirtion. this is coincidently exactly what muscles need to continue aerobically respiring!!!
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