Red Blood Cells, Haemoglobin & Oxygen (Cambridge International A-Level Biology)

Spec point — 8.2.1 · describe the role of red blood cells in transporting oxygen and carbon dioxide with reference to the roles of: • haemoglobin • carbonic anhydrase • the formation of haemoglobinic acid • the formation of carbaminohaemoglobin

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

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

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 keeps the blood pH stable; binding of hydrogen ions also lowers haemoglobin's affinity for oxygen, so more oxygen is released in respiring tissues

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