3.2.11 The Role of Haemoglobin

Transport of oxygen

• The majority of oxygen transported around the body is bound to the protein haemoglobin in red blood cells
  • Red blood cells are also known as erythrocytes
• Each molecule of haemoglobin contains four haem groups, each able to bond with one molecule of oxygen
  • This means that each molecule of haemoglobin can carry four oxygen molecules, or eight oxygen atoms in total

Haemoglobin proteins are made up of four subunits, each of which contains a region called a haem group to which oxygen can bind

• When oxygen binds to haemoglobin, oxyhaemoglobin is formed

Oxygen + Haemoglobin  Oxyhaemoglobin

4O₂ + Hb  Hb4O ₂

• The binding of the first oxygen molecule results in a conformational change in the structure of the haemoglobin molecule, making it easier for each successive oxygen molecule to bind; this is cooperative binding
• The reverse of this process happens when oxygen dissociates in the tissues

Carbon dioxide transport

• Waste carbon dioxide produced during respiration diffuses from the tissues into the blood 
• There are three main ways in which carbon dioxide is transported around the body
  • A very small percentage of carbon dioxide dissolves directly in the blood plasma and is transported in solution
  • Carbon dioxide can bind to haemoglobin, forming carbaminohaemoglobin
  • A much larger percentage of carbon dioxide is transported in the form of hydrogen carbonate ions (HCO^3-)

Formation of hydrogen carbonate ions

• Carbon dioxide diffuses from the plasma into red blood cells
• Inside red blood cells carbon dioxide combines with water to form H₂CO₃

CO₂ + H₂O  ⇌  H₂CO₃

• • Red blood cells contain the enzyme carbonic anhydrase which catalyses the reaction between carbon dioxide and water
  • Without carbonic anhydrase this reaction proceeds very slowly
  • The plasma contains very little carbonic anhydrase hence H₂CO₃ forms more slowly in plasma than in the cytoplasm of red blood cells
• Carbonic acid dissociates readily into H⁺ and HCO^3- ions 

H₂CO₃  ⇌  HCO₃^– + H⁺

• Hydrogen ions can combine with haemoglobin, forming haemoglobinic acid and preventing the H⁺ ions from lowering the pH of the red blood cell
  • Haemoglobin is said to act as a buffer in this situation
• The hydrogen carbonate ions diffuse out of the red blood cell into the blood plasma where they are transported in solution

Carbon dioxide can be transported in the form of hydrogen carbonate ions

The Chloride shift

• The chloride shift is the movement of chloride ions into red blood cells that occurs when hydrogen carbonate ions are formed
• Hydrogen carbonate ions are formed by the following process
  • Carbon dioxide diffuses into red blood cells 
  • The enzyme carbonic anhydrase catalyses the combining of carbon dioxide and water to form carbonic acid (H₂CO₃)

CO₂ + H₂O ⇌ H₂CO₃ 

• • Carbonic acid dissociates to form hydrogen carbonate ions and hydrogen ions

H₂CO₃ ⇌ HCO₃^- + H⁺

 

• Negatively charged hydrogen carbonate ions formed from the dissociation of carbonic acid are transported out of red blood cells via a transport protein in the membrane
• To prevent an electrical imbalance, negatively charged chloride ions are transported into the red blood cells via the same transport protein
  • If this did not occur then red blood cells would become positively charged as a result of a buildup of hydrogen ions formed from the dissociation of carbonic acid