8.2.1

Haemoglobin

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Haemoglobin

Haemoglobin is a globular protein found in red blood cells. Haemoglobin carries oxygen from the lungs around the body.

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Quaternary stucture

  • Haemoglobin is a protein made from four different polypeptide chains.
  • The four chains give haemoglobin a quaternary structure.
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Haem groups

  • Each polypeptide chain in a haemoglobin molecule has a haem group.
  • A haem group is a prosthetic group that is attached to the protein.
  • The haem groups contain an iron ion.
  • The iron ion is what makes haemoglobin red.
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Affinity

  • Haemoglobin has a high affinity (attraction) for oxygen.
  • When red blood cells reach the lungs, oxygen diffuses into the red blood cells and binds to haemoglobin.
  • Four molecules of oxygen bind to one molecule of haemoglobin.
  • When oxygen binds to haemoglobin, oxyhaemoglobin is formed.
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Dissociation

  • When the red blood cells reach the tissues in the body (e.g. muscle cells), oxygen is released from the oxyhaemoglobin in a process called dissociation.

Partial Pressure

Haemoglobin has the ability to bind to oxygen and dissociate from oxygen. This is controlled by the partial pressure (concentration) of oxygen in the surrounding cells.

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Partial pressure

  • Oxygen partial pressure (pO2) is the concentration of oxygen in the cells.
  • Carbon dioxide partial pressure (pCO2) is the concentration of carbon dioxide in the cells.
  • pO2 is important in determining whether oxygen binds to haemoglobin.
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Affinity

  • pO2 determines the affinity of haemoglobin for oxygen.
  • If pO2 is high, haemoglobin has a high affinity for oxygen and oxygen binds to haemoglobin.
  • If pO2 is low, haemoglobin has a low affinity for oxygen and oxygen dissociates from haemoglobin.
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Transport of oxygen

  • The effect of pO2 on the affinity of haemoglobin allows oxygen to be transported from the lungs (where there is lots of oxygen) to the respiring tissues (where oxygen is limited).
  • This allows oxygen to be transported to the cells where oxygen is needed most.

Role of Haemoglobin

Haemoglobin is vital for most vertebrates in carrying oxygen in the blood for respiration in the body’s cells. Haemoglobin also carries some of the carbon dioxide produced as a waste product in respiration and performs other important roles in the survival of many multicellular animals.

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Reversible binding of oxygen with haemoglobin

  • It is important that oxygen is able to release from haemoglobin after binding as it is required in tissue cells around the body.
  • 98% of haemoglobin is saturated with oxygen in lungs, whereas only 32% is saturated in respiring tissues. This illustrates how oxygen is able to dissociate from haemoglobin for use in cellular respiration.
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Affinity for carbon dioxide

  • Carbon dioxide must be transported to the lungs to be breathed out as a waste product of respiration.
  • Around 20% of carbon dioxide in the blood is bound to haemoglobin molecules. When carbon dioxide binds directly to haemaglobin, carbaminohaemoglobin is formed.
  • But, the majority of carbon dioxide is converted to carbonic acid by carbonic anhydrase.
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Carbonic anhydrase

  • Carbonic anhydrase is the enzyme that converts carbon dioxide into carbonic acid in the red blood cells then back to carbon dioxide when it reaches the lungs.
  • Carbonic acid dissociates into bicarbonate (hydrogen carbonate) ions and hydrogen ions in the blood.
  • pH is maintained by buffering agents such as haemoglobin itself that bind with the hydrogen ions to prevent a fall in pH.
    • When hydrogen ions bind to haemaglobin, haemaglobinic acid is formed.

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