Red Blood Cells, Haemoglobin & Oxygen (Cambridge (CIE) AS Biology): Revision Note

Red blood cells, haemoglobin & oxygen

• The majority of oxygen transported around the body is bound to the protein haemoglobin in red blood cells

• 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

• When oxygen binds to haemoglobin, oxyhaemoglobin is formed

Transport of carbon dioxide

• Carbon dioxide produced in respiration is transported in the blood in three main ways:

  • A small proportion is dissolved directly in the plasma

  • Some binds to haemoglobin, forming carbaminohaemoglobin

  • The majority is carried as hydrogencarbonate ions in the plasma

The role of carbonic anhydrase

• Inside red blood cells, carbon dioxide combines with water to form carbonic acid

  • This reaction is catalysed by the enzyme carbonic anhydrase

• The carbonic acid then dissociates into hydrogencarbonate ions (HCO₃⁻) and hydrogen ions (H⁺)

• The hydrogencarbonate ions move out of the red blood cells into the plasma, where they are carried in solution

Haemoglobinic acid

• The hydrogen ions released in the dissociation of carbonic acid could lower blood pH

• To prevent this, haemoglobin acts as a buffer:

  • Haemoglobin binds with hydrogen ions to form haemoglobinic acid (HHb)

  • This maintains the blood at a stable pH while still allowing haemoglobin to transport oxygen

Carbaminohaemoglobin

• Some carbon dioxide binds directly to the amino groups of haemoglobin, forming carbaminohaemoglobin (HbCO₂)

  • This binding occurs more readily when haemoglobin has released oxygen in the respiring tissues