IBDP Biology: Haemoglobin

• Haemoglobins are a group of chemically similar molecules found in many different organisms.
• It is a globular protein.
• RMM 68,000
• Quaternary structure:
• 4 polypeptide chains, each with a haem group (prosthetic groups).
• Each haem group contains iron, which can combine irreversibly with oxygen.
• Each haem unit can pick up one diatomic oxygen molecule.
• Oxyhaemoglobin therefore carries 8 oxygen atoms.
• Haemoglobin changes shape in different conditions, enabling it to have:
  • High affinity for oxygen in high concentrations.
  • Low affinity in low concentrations.
• Oxygen concentration is expressed as ‘partial pressure’ in kPa:
• Atmospheric pressure is approx. 100kPa.
• Oxygen is approx. 21% of atmospheric composition.
• I.E. its partial pressure (oxygen tension) is 21kPa.
• Summit of Everest, oxygen p.p is 7kPa

When it comes to IBDP Biology,

• Illustrated by an oxygen dissociation curve:
  • From 0% saturation, an increase in oxygen concentration results in a slow increase in saturation:
    • Haemoglobin has low affinity for the first oxygen molecule.
  • Rate of uptake increases as oxygen concentration increases further:
    • Oxygen binds cooperatively.
    • 1st oxygen distorts the shape of the haemoglobin.
    • 2nd, 3^rd oxygen molecules bind more easily.
  • At higher oxygen partial pressures, the rate of oxygen uptake decreases:
    • The remaining empty attachment sites on the haemoglobin are difficult to fill.
• In the lungs:
  • Oxygen partial pressure in the blood rarely exceeds 15kPa.
  • Haemoglobin rarely rises above 95% saturation in the lungs.
• From the lungs to the tissues:
  • The walls of arteries and veins are too thick to allow diffusion of oxygen.
  • Haemoglobin does not release its oxygen.
• In the tissues:
  • Oxygen partial pressure is low in the tissue fluid.
  • Oxygen dissociates from the haemoglobin.
  • Haemoglobin rarely falls below 50% saturation in the tissues.
  • However, supply is continuous.
  • In active muscles it falls below 45% saturation:
  • Oxygen partial pressure is as low as 4kPa.

When it comes to IBDP Biology,

• More soluble in water than oxygen
• Carbon dioxide is carried between respiring tissues and the lungs by 3 different methods:

• In respiring tissues CO₂ produced by respiration diffuses into the red blood cells.
• Carbon dioxide reacts with water to form carbonic acid.
• The pH drops.
• Carbonic acid dissociates to form a hydrogen carbonate ion and a hydrogen ion (a proton).
• This proton binds to haemoglobin.
• This changes the shape of the haemoglobin.
• This encourages oxyhaemoglobin to release its oxygen.
• In the lungs the reverse happens:
• Hydrogen ions react with hydrogen carbonate ions forming carbonic acid.
• More oxygen can be carried by the haemoglobin.
• CO₂ is formed.
• The pH increases.
• CO₂ diffuses into the plasma and into the alveoli.

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